GLOBAL NAVIGATION SATELLITE SYSTEM

The Global Navigation Satellite System (Glonass) was designed to provide global real-time determination of the position and velocity of an unlimited number of moving objects at any point on the Earth’s surface, in the air, and in space. This system was developed in response to an order from the USSR (Russian Federation) Ministry of Defense. By instruction of the Russian Federation’s President, the Glonass system was put into operation on 24 September, 1993. A 7 March, 1995 decree of the RF Government put the system at the disposal of the world community in standard mode intended for civil, commercial, and scientific use without a user fee.
A RF Presidential decree of 18 February, 1999 designated the Glonass as a dual-purpose (military and civilian) space navigation system. The decree designated the Federal executive agencies responsible for its use, maintenance, and development as the Ministry of Defense and The Russian Aviation and Space Agency. Issues relating to developing and implementating the systems were to be coordinated by the Internavigatsiya Interagency Commission and an interagency task force set up in accordance with the Russian Federation government decree of 29 March, 1999.
Users are informed of system status by the Coordinating Scientific Information Center of the RF Ministry of Defense, as well as by the Information Analytic Center for the positional and time support of the Russian Aviation and Space Agency’s flight control center.
The system makes it possible to obtain highly accurate navigation location information with a maximum error less than or equal to 50-70 meters for position and 15 cm per second for velocity from any point on the globe or in near-Earth space. Information is received in real time after a two to three minute pause, when the user’s navigational system has just been turned on and continuously thereafter. At the same time, the system provides the capability to link the user’s timescale with the State unified timescale with an error of no more than one microsecond. If the user’s equipment can implement special methods for processing navigational information or differential operating modes are used, the accuracy with which the user’s position is determined may be improved significantly. Tests have shown that the maximum error in such cases does not exceed a few meters.

Concepts Involved in Positioning

Users of the system, who are in the continuous radio-navigation field created by the space navigation system, navigate by determining their position and velocity at any moment relative to the navigation satellites. Radio-navigational signals are continuously emitted by each navigation satellite in an orbital constellation. These signals are received by the user’s equipment, without interrogation, and are used to determine the relative pseudorange and pseudovelocity (rate of change in pseudorange). Pseudorange is determined on the basis of the time delay it takes the signal to go from the navigation satellite to the user (on the basis of phase shift of the signal obtained from the satellite’s relatively stable signal and the user equipment’s generator frequency and time store) and pseu-dovelocity on the basis of the Doppler shift of the received signal frequency. Measured values of pseudorange and pseudovelocity can be used to solve the navigational problem and determine the user’s position and speed. Because atomic standard clocks are used on each navigation satellite, the system synchronizes the radio-navigational signals emitted by the satellite with the ground-based highly stable atomic standard that functions as the system standard for frequency and time. Use of the ground-based frequency and time standard and the highly stable onboard frequency standard systems time is kept synchronized for each satellite.
The user’s intrinsic time differs from the satellite system time. For this reason, initially, during receipt of signals from the satellites, the frequency of the user’s generator is synchronized with the frequency of the signals received. The discrepancy between systems time and the user’s clock time is determined at the same time as the coordinates and the components of the user’s velocity vector when the navigational problem is solved.
Solution of the navigational problem requires, along with the measured values of pseudorange and pseudovelocity, the use of the satellite ephemeris. At each moment, they determine the satellite’s radius vector and velocity vector components with high accuracy. The ephemerides, along with the frequency-time corrections of the onboard generator of each navigation satellite relative to the system frequency and time standard, are transmitted in the radio-navigation signal and received by users.
To solve the navigational problem (i.e., to determine the user position and velocity), the pseudorange and pseudovelocity to four or more navigation satellites must be measured. Using the measured values of pseudorange and pseu-dovelocity, with known satellite position and velocity (from the ephemeris information), the navigational equipment estimates values of the measured parameters. As a result of comparing the measured and estimated values of the parameters, the navigational equipment determines the location, velocity, and displacement of its own timescale relative to the Glonass time-scale system.
The accuracy of the user’s measurement of position and velocity are influenced by the error associated with the characteristics of the parameters measured by the user’s receivers and the mathematical methods used to process them, that is, to solve the navigational problem.
The first group of errors results mainly from errors in the ephemerides and synchronization of each satellite’s onboard generator relative to the central synchronizer, which is the ground-based system standard for frequency and time. These errors occur as a result of imperfections in the procedures for comparing the frequency of the onboard and ground-based generators and maintenance of the onboard timescale. For example, a time shift of 10 nanoseconds can lead to an error of 3 meters in measuring pseudorange. The ground-based measurement complex corrects the onboard timescale so that the standard deviation of the shift relative to the timescale of the central synchronizer does not exceed 10 nanoseconds.
The error in the ephemerides is a result of the error in measuring the parameters of the satellite orbit based on trajectory measurements conducted by ground-based command tracking systems in an interrogating mode. The error in the ephemerides is a consequence of measurement errors made by the command tracking system, errors in their relative position in space, position relative to the common ground-based ellipsoid, and the appropriateness of the model of the forces acting on the satellite in flight, which are factored in when the parameters of the orbit and prediction of movement are estimated.
The second group of errors results from errors in the model of radio-signal propagation in the troposphere and ionosphere along the navigation satellite-user path. The magnitudes of tropospheric and ionospheric refraction change as a function of the length of the path that the signals must traverse and the state of the ionosphere and troposphere along this path. To diminish the effect of these errors, the user’s equipment uses navigational signals from those satellites that are above the plane of the user’s horizon by 5-10°. This curtails the length of the path that the signal must traverse through the troposphere and ionosphere. A method that involves measuring the pseudorange and Doppler shift at two frequencies is used to compensate for the error due to ionospheric refraction. This is the so-called two-frequency method. The remaining error using the two frequency method is proportional to [sin(g)]-1. This constitutes 1-2 m at an angle g = 10° between the plane of the user’s horizon and the flight direction to the navigation satellite. This method of computing ionospheric refraction is the most accurate; however, its use requires highly precise measurements at two frequencies, which increases demands on the user’s equipment and leads to a significant increase in the error component from radio interference.
The current error rate in measuring pseudorange is the result of re-reflected signals from Earth, the sea, and other nearby surfaces. These errors, in many respects, are a function of the spatial relationship between the navigation satellite, the receiving antenna, and the reflecting surfaces. The range of the errors is 0.5-2 m in the best case and reaches 100 m in the worst, for example, under urban conditions with high buildings.
The third group of errors is associated with the user’s equipment and is linked to errors in marking the moment that the radio signal is received. The most important contribution comes from the noise and dynamic error in the tracking circuit due to the lag in the bending and carrier signals. Typical error due to noise and quantization of the signal ranges approximately from 0.2-1 m.
The measurement accuracy of user location and velocity, which is affected by the error sources listed, depends substantially on the configuration of the constellation of navigation satellites used to solve the navigational problem. An important condition for achieving accuracy in navigational measures is the relative spatial location of the satellite constellation and the user. This is the basis for the concept of geometric dilution of precision, which is a measure of the diminished accuracy of navigational measurements resulting from the specific features of the relative spatial positions of the satellite and the user. In the ideal case, the most accurate positioning for a ground-based user is attained when the optimal constellation is used, that is, when the user is in the center of a regular tetrahedron. Then, the value of the geometric dilution of precision is equal to 1.5.
The Glonass system consists of three segments: the space segment comprising the constellation of navigation satellites; the control segment—the ground based control complex; and the segment represented by the user’s navigational equipment.

The Space Segment

The configurations of orbital groupings of navigation satellites are selected so that the user, at any point on the Earth’s surface and at any moment, may work with a constellation close to the appropriate optimum that permits navigational measurements with the prescribed characteristics.
In accordance with this requirement, a fully deployed constellation of the Glonass space navigation system includes 24 navigation satellites located in three orbital planes, separated by 120° of longitude from each other. There are eight satellites in circular orbits in each orbital plane. The altitude of the satellites’ circular orbits is about 19,132 kilometers, a satellite’s period of rotation around Earth is 11 hours 15 minutes and 43 seconds. The plane of the orbit is inclined by 64.8° to the plane of the equator. The satellites are regularly spaced at 45° from each other in an orbital plane. The phase shift ofa satellite’s position in one plane with respect to the position of a satellite in another plane is 15°. This orbital configuration makes it possible for a user, at any point on the surface of Earth, in the air, or in space up to an altitude of 2000 km, to receive radio-navigational signals from between 5 and 11 navigation satellites at the same time, depending on the area where the user is located, and to process the measurements from all of the satellites or to select a grouping closely approximating the optimum.
The value of the geometric dilution of precision Kg, associated with an orbital constellation and the probability PN of seeing a given number of satellites by users on the surface of Earth are shown in Table 1.
The Glonass satellite period of revolution around Earth differs from the geosynchronous period. Because of this difference, the path of each satellite in the constellation on the surface of Earth is displaced approximately 21° from the equator every 24 hours. The interval of repetition of a satellite’s path in its zone of radio-visibility to ground-based facilities is 17 passes (7 days, 23 hours, 27 minutes and 28 seconds). Because of this, the contribution of resonant disturbances resulting from the eccentricity of Earth’s gravitational field is diminished by a factor of 8-10 compared to the disturbance of orbits that have a geosynchronous period of revolution. This leads to decreased displacement of each satellite in an orbital plane relative to a given point. The orbital structure in this case is deformed less over time, is more stable, and requires less energy to maintain a given configuration in the orbital grouping.
Table 1. The Geometric Dilution of Precision Kg and the Probability PN of Seeing a Given Number of Satellites

Satellites that have system numbers 1-8 are located in the first orbital plane, those that have numbers 9-16 in the second plane, and those numbered 17-24 in the third plane. The system numbers of the satellites in an orbital plane increase in the direction opposite to the satellites’ motions. The nominal values of absolute longitude of the ascending node of orbital planes, which is fixed at 0:00 Moscow standard time on 1 January, 1983 is equal to 251.4° +120° (k-1), where k is the number of the orbital plane. The nominal distance between satellites in one plane is 45° latitude.
The argument of latitude of satellite No. 9 (the first satellite in the second orbital plane) is 15° greater than the argument of latitude of satellite No. 1. The latitude of satellite No. 17 (the first satellite in the third orbital group) is 30° greater than the latitude of satellite No. 1.

The Navigation Satellite

The satellite is the main component of the Glonass system. The satellite consists of a cylindrical pressurized housing containing the instrument module, the solar array panels with controls, the framework of the antenna-feeder devices, the instruments ofthe attitude control system, the modules of the propulsion system, and the louvers of the thermal regulating system with its controls. Optical angular reflectors are mounted on the satellite. These are used for calibrating the measurement systems by measuring distance in the optical range obtained by ground-based quanto-optical systems (Fig. 1).
The satellite’s equipment provides high-quality navigational measurements and emits highly stable navigational radio-signals of two types—standard accuracy (SA) and high accuracy (HA) in the decimeter bandwidth; receives, stores, generates, and transmits navigational information; generates, codes, stores, and transmits time signals; receives and processes codes for correcting and phasing onboard timescales; retransmits and emits signals for radio-monitoring of the satellite orbit and determines adjustments to the onboard timescale; analyzes the status of onboard equipment and generates control commands; receives, confirms receipt, decodes, and processes one-time commands; receives and processes programs controlling satellite modes of operation in orbit; generates and transmits signals ”Calling Ground Control” when there is a breakdown or important monitored parameters exceed the limits of the norm; and generates telemetry data on the status of onboard equipment and transmits them to the program control office.

Figure 1. Glonass navigation satellite.
The functions listed are performed by the satellite onboard navigational transmitter; the onboard timescale generator; the onboard control system; the attitude control and stabilization system; the systems responsible for orbital correction, electric power supply, and thermal regulation; the onboard fuel and environmental maintenance devices; the structural components; and the cable network. This equipment is continually being improved to lengthen the active service life of the satellite, increase the accuracy of onboard frequency standards, increase the duration of autonomous operation, increase the level of automation of control, and expand functional capabilities. The most important elements of the onboard equipment have backups.
The onboard navigational transmitter generates and emits highly stable radio-navigational signals in two frequency bands, L1 and L2. The Glonass system uses frequency separation of navigational satellite radio signals in both bands. Each satellite transmits navigational radio signals at its own particular frequency in the L1 and L2 bands. Satellites at opposite points of the orbital plane (antipodal satellites) may transmit navigational radio signals at the same frequencies without causing interference for users on the ground.
Each satellite emits radio-navigational signals in the direction of Earth using a transmission antenna. The operating portion of its directional pattern is 2 j0 = 38° wide. The axis of the pattern is aimed at the center of Earth. Thus, each satellite’s radio signal covers Earth’s disk up to an altitude of 2000 km.
The navigation satellite transmits navigational radio signals of two types, standard and high precision along the radio links of the L1 and L2 frequency bands. The standard precision signal with a cycle frequency of 0.511MHz is intended for use by Russian and foreign citizens. The high precision signal with a cycle frequency of 5.11 MHz is modulated by a special code and is not recommended for use without coordination with the RF Ministry of Defense. The standard signal is available to all users who are equipped with the appropriate user equipment and have Glonass satellites in their visibility zone. The characteristics of the standard navigational signal are not intentionally degraded. In subsequent versions of the navigational satellites (Glonass-M), plans call for offering consumers a standard precision code in the L2 band.
The nominal values of the carrier frequencies for satellite navigational radio signals for each band are defined by the following expressions:

where k = 0, 1…24—the number of the carrier frequency designators for the radio signal and

are the carrier frequencies for L1 and L2, respectively. For each satellite, the effective frequencies L1 and L2 are coherent and are generated from the common frequency standard. The nominal value of the frequency of this standard, from the standpoint of an observer on Earth’s surface, is equal to 5.0 MHz. The interface control document stipulates a stage by stage change in the frequency range of the Glonass system in the direction of decreasing the number of designators to 12. The distribution of frequencies for the L1 band, in accordance with numbers of designators of radio signal carrier frequencies in the program phases, is shown in Fig. 2.
The navigational signal in the L1 band contains the range code, the onboard timescale and navigational data (ephemeris information, correction of the time, frequency and phase of the onboard frequency standard). The navigational signal in the L2 band contains the range code. It is used to diminish the effect of ionospheric refraction through the two-frequency method on the accuracy of navigational parameter measurements and is intended for special users.
The onboard clock assures continuous transmission of highly stable synchronized frequencies in the navigation satellite system and generation, storage, and transmission of the onboard timescale. It generates signals of standard precision and networks of synchronized frequency impulses. The main component of the clock is the atomic frequency standard.
All navigation satellites are equipped with cesium frequency standards. The precision of mutual synchronization of the onboard timescale is 20 nanoseconds (standard deviation). The basis for generation of the system timescale is the hydrogen standard of frequency of the central system synchronizer, whose diurnal instability is 5 x 10 ~14. The discrepancy between the system timescale and the scale of the State standard for the Universal Time Code UTC (SU) should not exceed 1 ms.

Figure 2. Distribution of frequencies in the L1 band by designator numbers.
The timescale of each Glonass satellite is periodically checked against the timescale of the central synchronizer. Corrections of the timescale of each satellite with respect to the system timescale are computed at the System Control Center and are transmitted to each satellite twice a day. The error involved in checking the satellite timescale against the system timescale does not exceed 10 nanoseconds when it is measured.
The system timescale is corrected at the same time as the correction of the whole number of seconds in the UTC scale, which is regularly conducted by the Universal Time Service. This correction occurs at 00 hours, 00 minutes, 00 seconds during the night between June 30 and July 1 or from December 31 to January 1. The users of the Glonass system are informed ahead of time of the regular implementation of the second correction of the system timescale. Thus, between Glonass system time and UTC(SU), there is no discrepancy by a whole number of seconds. However, between Glonass system time and UTC (SU), there is a constant shift of three hours, corresponding to the system time at the Ground Control Complex.
The onboard control system includes the onboard command equipment, the onboard computer system, the onboard telemetry system and the control module. The onboard computer system stores and processes navigational information and generates and transmits navigational images.
The attitude control and stabilization system is used to stabilize the satellite and provide its initial orientation to the Sun and Earth, orient the satellite’s vertical axis to the center of Earth and the solar array to the Sun, orient the thrust vector of the engines to maintain the satellite in its position, and provide orbital correction. The satellites use an active three-axis system for attitude control and stabilization that has a control flywheel and a jet momentum unloading system.

Launch Facilities

A Proton-K launch vehicle with a 11C861 upper stage (the Proton-KM launcher with the Briz-M upper stage developed at the Khrunichev Space Center) is used to insert the navigation satellites into orbit. These launches take place from the Baikonur spaceport. Groups of three satellites at a time are inserted into the given plane directly into one of the points of the orbit that needs to be filled. At this point, all three satellites separate. One of the three remains at that point. Its orbit is corrected to compensate for possible errors in insertion. The other two satellites are then moved to their assigned points in the orbital plane.
Positioning of a satellite at its assigned point in the orbital plane occurs in stages. These include measurement of insertion orbit parameters and generation of a positioning program. In accordance with the program and using the onboard orbital correction engines, the satellite is decelerated or accelerated. After acceleration (deceleration) a phase of unpowered flight begins, during which the satellite is moved to its assigned point. Once it approaches this point, the correction engine is again turned on to stop the motion of the satellite in the plane and to adjust the orbital parameters in accordance with the assigned position. After each satellite has been positioned at its assigned point, the orbital parameters are precisely determined and included in the system. The duration of the spacecraft positioning at its assigned point can vary and may take from one week to one month, depending on the distance between the insertion point and the assigned point.
Possible variants for filling out an orbital constellation with single satellites using Rus’ type launch vehicles, such as converted launchers of the Rokot type, and the newly developed Angara type launch vehicle, are being considered.

Ground Control Complex

The Ground Control Complex tracks the satellites and provides them with the information they need to maintain the operation of onboard equipment and to generate radio-navigational signals and navigational messages. It also monitors the technical status of the satellites and the accuracy of the navigation time field generated by the system.
The main tasks of the ground control and monitoring complex are maintaining the necessary trajectory and other measurements for computing the predicted values of the ephemerides and other auxiliary information, sending ephemeris information and frequency and time corrections to the satellite, monitoring the navigational and time field, generating a unified system timescale, telemetry monitoring, transmitting command and program information, supporting orbital correction, and positioning the satellite in its assigned orbit.
The complex consists of the System Control Center, a network of monitoring and measuring stations located throughout the territory of Russia. The monitoring stations are centers for tracking satellite signals and collecting the information needed to determine the ephemerides, time corrections relative to the system timescale, and frequency corrections relative to the system standard of frequency and time. The System Control Center collects and processes data to determine the predicted ephemerides and model parameters for onboard clocks, as well as other data to send to the onboard satellite equipment. Communications between the ground command and control complex and the satellites uses the radio channels of the Command Tracking Stations.
The Ground Control Complex contains the following interconnected stationary spatially dispersed components: the Systems Control Center, the Central Synchronizer, the Command Tracking Stations, the Phase Monitoring System, the Quanto-Optical Stations, and the equipment for monitoring the navigational field. All components of the Ground Control Complex are located on Russian territory close to the following geographical points: Krasnoznamensk in the Moscow Oblast (System Control Center), St. Petersburg (Command Tracking Station 9), Shchelkovo in the Moscow Oblast (Command Tracking Station, Phase Control System, Central Synchronizer, Navigational Field Monitoring Equipment), Yeniseysk (Command Tracking Station 4), Ulan-Ude (Command Tracking Station 13), Komsomolsk on the Amur (Command Tracking Station 20, Quanto-Optical Station, Navigational Field Monitoring Equipment).
The Ground Control Complex controls the flight and operation of the satellite’s onboard systems by generating control commands and transmitting them to the satellites; making trajectory measurements for determining and predicting orbital parameters; and computing ephemerides for all satellites; measuring time and determining the divergence of all satellite onboard timescales from the system timescale, synchronizing the onboard timescale of each satellite with the central synchronizer time scale and that of the Universal Time Service; generating files of auxiliary information, navigational messages containing the predicted satellite ephemerides, and of the Glonass system almanac; correcting timescales of each satellite and other data for developing a navigation framework; transmitting the files of auxiliary information to each satellite and monitoring their receipt; monitoring the operation of satellite equipment using the telemetry channels and diagnosis of their status; monitoring of information in the navigational messages of the satellites and receipt of ground control call signals; monitoring the characteristics of the navigational field; determining the phase shift of the distance measurement navigational signal with respect to the phase of the central synchronizer; planning the work of all the technical equipment for the Ground Control Complex; and automated processing and exchange of data among all Ground Control Complex components.

Ephemeris Support

Satellite motion parameters are measured and predicted in the ballistic sector for ephemeris-time support based on the results of trajectory measurements obtained by the Command Tracking Center in the interrogating mode. Ephemeris is measured in routine control operations, during which preliminary processing of trajectory measurement occurs and motion parameters are measured; ephemeris information is computed, and the system almanac is generated; a posteriori estimate of the precision of ephemeris information takes place; and the parameters of Earth’s rotation are processed and predicted.
These tasks start with the processing of trajectory measurements obtained during the past day. After preliminary processing of newly received measurements, the parameters of satellite motion are determined with more precision. During precise determination of motion parameters, the constant components of measurement error are determined for each measurement station. Trajectory information obtained during an 8-day interval of motion is used to solve this problem. The problem is solved by the least squares method, and the full sample of measurements during the 8-day satellite motion interval is used.
The more precisely determined initial conditions determining satellite motion parameters are used for computing ephemeris information and the system almanac. This data is in turn used to generate auxiliary information, which is transmitted to the Command Tracking Station for relay to the onboard satellite equipment.
A system of differential equations for satellite motion, solved numerically, is used to define orbital parameters and compute ephemerides precisely. The system of differential equations takes account of the disruptive factor caused by Earth’s gravity, including anomalies of Earth’s gravitational field to the eighth order of magnitude inclusively; the gravitational field of the Moon and Sun; the effect of lunar-solar tidal disturbances on Earth; light pressure factoring in the specific features of changes in the reflecting properties and the magnitude of the satellite surface turned toward the light.
During computation, the precision characteristics of the ephemerides, which are computed daily for each satellite, are evaluated. Considering the more precise ephemerides determined during the current day, the standard parameters (averaged over the measurement interval) are computed, and the maximum deviation of the evaluated ephemerides from the standard area is derived. The vector of maximum deviations is entered into the database and used to compute sampled evaluations of the precision of ephemeris support for a given time interval for individual satellites or the whole system. At the end of every month, the quality of the measurement system’s operation is evaluated. The standard deviations of the satellite ephemerides in the orbital system of coordinates for daily prediction are shown in Table 2.
During the computation of the ephemerides, the coordinates of Earth’s pole and changes in the duration of a day are also derived. Specially developed methods make it possible to determine universal time as well. The accuracy of the results obtained are evaluated at the level of 15-20 cm for the pole coordinates, 0.5 ms for the duration of Earth days, and 1ms for universal time. Glonass has been regularly deriving parameters of Earth’s rotation based on data from satellite observations in real time since 1984.
Table 2. Error in Navigation Satellite Ephemeris

The derivation of these data is methodologically and administratively associated with the information support hardware of the Glonass systems, which determines the reliability and precision (acceptable for practical purposes) of the values obtained for Earth’s rotational parameters. However, the regional location of the Command Tracking Stations exclusively on Russian territory and the features of the orbital structure of the system introduce certain issues into the technology for deriving Earth’s rotational parameters.
Derivation of these parameters occurs during the technical phase of controlling the satellites, which involves daily computation of the satellites’ orbits and Earth’s rotational parameters based on observational data for each satellite during the previous 8 days. Each such derivation generates three values for Earth’s rotational parameters—two coordinates for the pole, Xp and Yp, and the rate of rotation. The current values of the polar coordinates and rotational rate are made more precise during processing of observations using the least squares method for the 8-day interval.
Universal time is derived by comparing the results of the ongoing determination of orbits and their ephemerides, computed using data for the parameters of Earth’s rotation, which were coordinated at the start with data from the International Earth Rotation Service. Thus, when each daily technical cycle of ephemeris information for each satellite is computed, the parameters of Xp, Yp, and (UT1 — UTC) are evaluated. Averaging the data obtained for all of the satellites (with outlying values excluded) makes it possible to obtain more precise evaluation of the daily values for Earth’s rotational parameters, which comprise the series of data measured at the ballistic center of the Glonass system. The daily Earth rotational parameter measurements are processed weekly. The results obtained are transmitted to the State Center of Measurement of the Earth’s Rotation Parameters, where they are used to derive immediate and ultimate values.

Differential Methods of Navigation

The use of different variants of the differential method of navigation provides significant capability for obtaining highly accurate characteristics and improving the reliability of navigational measurements. The essence of the differential method involves using monitoring stations to identify and compute, in the form of corrections, strongly correlated components of the errors in navigational measurements. They determine the coordinates of their location on the basis of measurements of pseudorange and compare them with known results of their geodesic alignments. They use the results of the comparison to compute the appropriate corrections, which are transmitted along the communications channels to users of the Space Navigation System in the relevant region. Corrections allow users to make navigational measurements with enhanced precision.
There are four versions of the differential navigation method using the Space Navigation System’s radio-navigation field: correction of position coordinates, correction of range, pseudosatellite system, and time correction.
In the location correction method, the monitoring station receiving equipment is used to determine corrections of the results of the navigational problem. The users themselves correct their navigational positions by the magnitude of these corrections. In this version, both the monitoring station and the users must operate with the same constellation of satellites, which requires that their work is coordinated.
The method of range correction involves the receiving equipment of the monitoring station to determine and transmit to the users corrections of the measured values of pseudorange for all visible navigation satellites. The users correct their measurement values of pseudorange by the appropriate correction factor obtained by the monitoring station.
The pseudosatellite system includes a ground-based transmitting device in the constellation of navigational satellites. The signals from this device are used for the user’s navigational positioning. The receiving equipment of the differential station (in this version, the pseudosatellite) measures the correction factor for pseudorange of all of the satellites in the radio-visibility area and transmits them as part of the navigational message to the users. The user receives the signals from the pseudosatellite (differential station) along with the signals from the actual satellites and determines its coordinates considering the correction factor obtained from the pseudosatellite.
The time correction method involves having both the monitoring station and the user measure and track the time that signals arrive from the satellites. In this version, the user determines his location relative to the monitoring station using data on time and phase of signals received from the satellites at his own location and at the monitoring station, and also on geodesic coordinates of the station and the location of the satellites relative to it. It is assumed that if the coordinates of the monitoring station are precisely known, it is then possible to recompute the relative location of the user in geographic coordinates with enhanced precision.
It is hypothesized that the use of differential modes of navigation makes it possible to improve the accuracy of coordinate determination to 5-10 m and to increase the reliability of receiving navigational information by providing rapid transmission of signals containing information on incorrect functioning of the space system and the reliability of coverage of the working area by, in effect, increasing the number of satellites when the monitoring station functions as a pseudosatellite.

Use of the Glonass System

Space navigation became one of the first areas of applied cosmonautics that led to the extensive use of space navigation systems in the interests of society. Many current interesting projects implemented on Earth or in space would be unthinkable today without their use. Space navigation systems make it possible to develop and begin to use new navigation technologies for military purposes, for aviation and ships at sea; monitoring the orbits of satellites, launch vehicles, and upper stages and control of their motion; geodesy; geology; fishing; mining; land management; construction; transport; science; and other sectors.
Because positioning is achieved through uninterrogated measurements of pseudorange and pseudovelocity, the number of users of the Glonass space navigational system is unlimited and may be as large as possible. Users of the Glonass system are equipped with specially developed user equipment. At present in Russia, we are implementing a Federal program to develop various equipment prototypes for use by the Ministry of Transport and the Russian Aviation and Space Agency.
The use of the space navigation system by civilians makes it possible to improve safety and decrease the cost of operating air, sea, ground, and space vehicles by increasing the accuracy and reliability of navigational and time support. Plans call for using the space navigation system to solve all navigational problems, aside from those involving high precision aircraft landing and ship navigation on internal waterways. However, the use of the system in the differential mode enables solution of these problems, too, as well as many others, for example, those that arise in hydrographic, geodesic, and topographic work.
For air transport, global coverage of all airports in Glonass’ operational zone makes it possible to improve local and general navigation, increase the density of air traffic, decrease the strain on major airports, and use backup airports more efficiently. Given the appropriate communications channels, Glonass can be used to prevent collisions and control air traffic. In particular, simply decreasing the length of each flight by several kilometers as a result of improved navigational support can save significant amounts of fuel for each aircraft.
At present there are proposals to use Glonass as an additional navigational system in air transport. It may be employed in instances where other systems could not be used. Here, it will be necessary to increase the reliability of navigational information receipt, that is, achieve virtually immediate detection and user notification of system malfunction, to ensure a reliable system work zone with the required geometric dilution of precision and low cost of user equipment. This type of use would increase the importance of employing the differential method.
The high accuracy of the system’s navigational determinations makes it possible to solve all of the necessary problems of route navigation, seagoing navigation in the open sea and also sailing in narrows and in ports, as well as mining, fishing, and laying pipes and cable. The future use of Glonass in near-Earth space for navigational support of satellites and launch vehicles is promising. Russia has conducted successful tests of user equipment mounted on the Proton-M launcher equipped with a Briz-M upper stage.
The use of the Space Navigation System on land vehicles that have compatible communications receiving and transmitting equipment would make it possible to monitor traffic automatically along roads and exercise control over freight and passenger traffic when needed. The Space Navigation System can replace the majority of currently used radio-navigational systems for supporting industrial and scientific work in geodesy, geology, cartography, and other branches of science and technology. The system supplies highly accurate time information to stationary and moving objects on land, sea or in the air. We should also add less traditional uses of the Space Navigation System: for determining the orientation of objects when radio signals are received at distributed antennas, for synchronizing communications and power systems, and for monitoring deformations of Earth’s surface in geodesy.
The main Russian developers of Navigational User Equipment are the Russian Scientific and Research Institute for Space Instrument Building, the Russian Institute of Radio-Navigation and Time, the Moscow Kompas Design Bureau, and others.
The use of the Glonass’ radio-navigational field, in addition to solving major navigational problems, makes it possible to determine the location and velocity of the user in space, use the differential mode to perform high-accuracy local navigation relative to a ground-based correction station, perform high-precision geodesic alignment of ground-based objects that are far from each other, synchronize standards of frequency and time at remote ground-based facilities; and determine the orientation of an object based on radio-interferometric measurements received by distributed antennas.

Navigational System Integrity and Joint use of Glonass and GPS

The study of the requirements of various navigational system users suggests the need to identify and take account of integrity, an important characteristic of a navigational system. One measure of a navigational system’s integrity is the probability of detecting a failure in less than threshold time. The Radiotechnical Commission on Aeronautics (RTCA) has introduced a definition of the integrity of a space navigational system that is either the user’s primary means of navigation or an auxiliary means. If a navigational system is used as an auxiliary navigation aid, then integrity can be defined as the capability of the system to provide timely warning that it cannot be used for navigation in its current state. This means that the inadequacy of the system’s function must be determined before errors in the output of navigational parameters exceed a specified threshold.
The integrity of a space navigation system that serves as the primary means of navigation requires that the system exclude inaccurate information from subsequent processing before errors in the output parameters exceed a stipulated threshold. Recently a great deal of emphasis has been placed on navigational system integrity, making this characteristic comparable in significance to the accuracy of systems and complexes. Situations associated with malfunctions of navigational systems can be either easy or difficult to detect. Easily detectable situations include signal loss or distortion and the occurrence in the navigational message of an indicator forbidding the use of the navigational signal. Situations that are difficult to detect include failures whose external manifestations are relatively indistinguishable from reliable performance. Failures of this type may have the following external manifestations: shift of the onboard timescale of the navigational signal, drift in the frequency of the onboard generator, drift of the carrier frequency of the output signal, and distortion in ephemeris information.
Various methods are used to monitor a space navigation system’s integrity. Integrity monitoring occurs directly on board the navigation satellite or uses a ground-based monitoring segment. Monitoring stations receive signals from all visible system satellites and use them to determine whether one of the satellites has failed. Because it is not very efficient, this approach may not satisfy many users. Special equipment incorporating automated methods of system integrity monitoring has been developed for such users. Such equipment is based on the use of redundant information obtained from other navigation aids, as well as redundant information from the space system itself. To implement these methods, various types of combined navigation devices have been developed, including altimeters that have a highly stable frequency standard and inertial navigational system combined with other radio-navigational systems. Incorporation of navigational equipment into a single functional, structural, and integrated system allows fuller use of the redundant information available onboard the moving object. To increase the level and degree of integration of navigation equipment as part of a unified navigational complex, the functions of various navigation and other radiotechnical systems have been combined, leading to the development of combined systems and multifunctional integrated complexes, as well as to integration of different technical devices measuring the same or functionally associated navigational parameters.
The development and use of combined receivers operating simultaneously with Glonass and GPS signals provides a significantly higher level of navigation service to air, sea and other users by integrating radio-navigational fields; increasing the number of radio signals received from all visible satellites, which may be 10-21; and improving the accuracy and integrity of the navigation system. To obtain the maximum effect from the integration of the Glonass and GPS systems, user receiving equipment at the level of radio signal processing system has undergone in-depth structural and functional integration.
The Glonass and GPS systems are similar, but from the standpoint of developing user equipment, they have a number of significant differences. For example, the two systems use close but not identical navigation signal frequencies. Thus, in the Glonass system, multichannel access to the navigation signals of each satellite is based on frequency separation; in GPS, this entails code-based distinction among signals. The systems also differ with respect to the navigational and auxiliary information that is transmitted in a navigational framework. Glonass systems time does not differ unintentionally from UTC(SU) by even a whole second. However, there is a constant difference of three hours, corresponding to the systems time of the ground control complexes between Glonass and UTC(SU). There is a few seconds difference between GPS systems time and UTC(SU) (13 seconds in 2001). This difference is measured during regular corrections of UTC(SU) times at the level of seconds. At the same time, there is no constant hour difference in the GPS system.
Both systems use similar, but not identical, systems of coordinates and time. GPS uses the world geodesic system, WGS-84, of the U.S. Defense Mapping Agency. This system has undergone several modifications to improve its accuracy, and at present, it virtually coincides with the Standard Terrestrial System (CTS) defined by the International Time Bureau (BIN) for the epoch of 1984. Glonass uses the geocentric system of coordinates, PZ-90. This system has its origin at the center of Earth. The 0Z axis is aimed at the mean location of the North Pole for the epoch 1900-1905, as determined by the International Geodesic Union and International Geodesic Association. The 0X axis is parallel to the equator for the epoch 1900-1905; the X0Z plane is parallel to the mean Greenwich meridian. The 0Y axis completes the coordinate system to the right.
The reference systems differ in that the WGS-84 system defines the 0Z axis as the axis passing through the current location of the pole, and the PZ-90 system defines the 0Z axis as passing through the mean location pole of the epoch from 1900-1905. This difference is recognized by including a transition matrix developed by the International Time Bureau, which may be used effectively to rectify the results of navigational measurements.
Another difference results from the fact that the system of coordinates, determined for each system by its network of observation points relative to which the “exact” ephemeris of each system is defined, has not been rectified, that is, there is no reliable information concerning the relationship between the two systems. Special studies would be required to establish the nature of this relationship.
Preliminary matrices for this rectification were obtained from experiments conducted by using the unpowered Etalon satellites launched as part of the program to ensure Glonass accuracy. The results showed that the following equations may be used to convert between the WGS-84 and PZ-90 coordinate systems:

The equations were submitted to ICAO as preliminary proposals for developing a standard model of Earth for future systems of astronavigation and other uses. The discrepancy shows that the error resulting from the difference in the systems of coordinates is several meters. This must be considered in developing precision integrated user equipment that makes simultaneous use of Glonass and GPS signals. The equations cited are tentative and subject to further refinement.

System History and Developmental Phases

Modern space navigation was developed through the efforts of scientists and experts of many countries. The most decisive contributions were made by Soviet and U.S. specialists who developed the current low-orbital navigational systems for the USSR (Tsikada) and United States (Transit) and the mid-orbit USSR-Russian Glonass system and the U.S. GPS. The Soviet and U.S. systems were created through parallel development at virtually the same time, independently of each other. Each of these nations conducted development using their own scientific and technical facilities that had evolved in the process of previous development of radio-navigation technology, particularly during the period when first- and second-generation space navigation systems were being developed. The difference in the scientific traditions and approaches and in the level of development of technical principles led to differences in the designs and characteristics of the space navigation systems.
The first scientifically grounded proposal to use satellites for navigation was made in the USSR before the launch of the first satellite (10/4/1957).
Scientific research on the potential use of radio-astronomical methods for aircraft navigation, directed at defining technologies for creating aircraft radio-direction finders using intrinsic radiation from the Sun, Moon, and remote discrete sources was being conducted at the A.F. Mozhayskiy Leningrad Military Air Engineering Academy (LMAEA) between 1955 and 1957 under the direction of V.S. She-bshayevich. This research was limited by the small number of celestial sources of power sufficient to be received by antennas small enough to be installed onboard aircraft. In this context, the idea arose of using artificial celestial bodies equipped with radio transmitters of the required power. Rocket technology by this time had developed the potential for placing an artificial satellite in Earth orbit. In the research conducted, it was proposed to use such satellites as orbital radio-navigational points. It was emphasized that observation of orbiting points makes it possible to obtain a sufficient selection of variable navigational parameters (which would not be provided by the Sun and the Moon which move slowly in the sky). The conclusion of this work was that the use of satellites as carriers of radio-navigational signals was very promising. In October 1957, an interagency scientific and technical conference on the problem of radio-astronomical navigation supported proposals for the navigational use of satellites. They recommended comprehensive scientific research to develop rationales for various technical methods of developing satellite-based radio-navigation systems. The first publication defining ways to use satellites for solving aviation problems appeared in the USSR in the departmental (official bulletin) Information Collection No. 33, based on material presented at a scientific workshop held at the LMAEA in December 1957 (3).
The scientific and technical principles underlying low-orbital space navigation systems were developed significantly through comprehensive scientific research in a project entitled Satellite (1958-1959), which was performed by five Leningrad institutions (LMAEA, the Institute of Theoretical Astronomy of the USSR Academy of Sciences, the Institute of Electromechanics of the USSR Academy of Sciences, and two Naval Scientific Research Institutions) and the Gorkiy Scientific Research Institute for Radiophysics.
Over the course of 2 years, the scientific and technical foundations for the first generation Space Navigation System were laid. These included general structural principles for the system; methods of measuring (telemetry, radial-velocity, differential telemetry, goniometric-radio distance finding); selection of the frequency band to be used; the effect of conditions of radio-wave propagation; power for the radio links; a priori evaluation of the accuracy of positioning and flight altitude; the effect of instability of frequency and time standards; selection of satellite orbital parameters; evaluation of the accuracy of ephemeris prediction; a method for supplying users with ephemeris; techniques for coding the transmitted information; and specifics for using Space Navigation Systems for marine and aviation navigation.
Thus, from 1955-1959, Soviet researchers developed an independent scientific and technical foundation for satellite-based navigation and by the start of 1960, were prepared for practical development of a low orbital Space Navigational System, the implementation of which was turned over to the Leningrad Scientific Research Radiotechnical Institute (LSRRI)—currently the Russian Institute of Radio-Navigation and Time (RIRT). Here during the first half of the 1960s, the preliminary project was developed for the first Soviet low orbit Space Navigation System, Tsiklon.
Actual development work on the equipment for the first Soviet Space Navigation System was allocated as follows: the Research Industrial Association for Applied Mechanics in the city of Krasnoyarsk was the head organization and satellite developer under the direction of M.F. Reshetnev; the Moscow Research Industrial Association of Space Instrument Building was responsible for development of the onboard and ground-based radiotechnical complex and user equipment (directors, M.I. Borisenko and N.Ye. Ivanov); LSRRI was responsible for development of the user equipment and onboard frequency standards (directors, P.P. Dmitriyev and A.F. Smirnovskiy); and the Naval Scientific Research Institutes # 9 was responsible for development of applications for use of the system by the Navy. The Moscow Area Central Scientific Research Institute # 4 (subsequently the M.K. Tikhonravov Central Scientific Research Institute for Space Systems # 50) creatively interacted with all of the development organizations and worked on issues of control and ephemeris support of low-altitude Space Navigation Systems (V.A. Korobkin and A.V. Tsepelev).
The first navigational satellite, Kosmos-192, was inserted into orbit on 23 November, 1967. During this period, the developers faced numerous issues demanding theoretical interpretation and specific solution. Control of the satellite was performed in the Main Scientific Research Testing Center of Space Systems of the USSR Ministry of Defense (currently the Main Center for Testing and Controlling Space Systems of the RF Ministry of Defense). Starting in 1969, the Main Center was assigned the mission of increasing the accuracy of ephemeris support. By that time, it had become clear that the major source of error in navigational positioning was errors in the ephemeris. At that time, such errors were as high as 1-2 km. Multilevel work on this problem was directed by the head of the center’s ballistic administration, V.D. Yastrebov. Under his direction, this problem was solved by E.V. Mesropov, V.M. Makarov, and G.M. Solovyev. V.I. Kudymov and B.I. Sukhikh of the Applied Mechanics Design Bureau also participated in this work. Success depended on many factors. First and foremost was that trajectory measurements by Doppler radio systems had to have stable accuracy, as well as highly accurate geodesic alignment. Further, an appropriate model of navigation satellite motion had to be developed to support accurate ephemeris prediction. Developers of radio-technical measurement systems from the Space Instrument Building Scientific Production Association were recruited to help in this work, as were geodesic specialists from Scientific Research Institute # 29 from the Military Topographic Administration and ballistic experts from Central Institute # 4 and the Krasnoyarsk Design Bureau for Applied Mechanics. As a result of their joint efforts, precision ephemeris prediction was attained that exceeded by a factor of 2-3 the precision stipulated in technical specifications for the Space Navigation System.
The first-generation space navigation system, Tsikada built and put into operation in 1979, consisted of four satellites inserted into circular orbits 1000 km high at an inclination of 83° and an even distribution of orbital planes along the equator. The use of the system enabled users, on the average of every hour and a half to two hours, to receive navigational signals in a session lasting for 5-6 minutes and determine their positions. The system used the Doppler principle of positioning, in accordance with which during the session the Doppler shift of a highly stable satellite signal was determined and used to compute the user’s coordinates.
Subsequently, Tsikada was equipped with instruments to detect vessels in distress that were equipped with radio-buoys emitting SOS signals at frequencies of 121 and 406 MHz. These signals were relayed to special ground stations where they computed the coordinates of the distressed ship. Equipping Tsikada satellites with equipment to detect such ships enabled development of the Kospas system. Jointly with the U.S.-French-Canadian Sarsat system, they established a united search and rescue system that saved several thousand lives.
Because of the discrete nature of the navigational sessions and their significant duration (5-10 minutes), the low-orbit space navigation systems could be used successfully as highly accurate navigation aids only for objects moving relatively slowly, especially ships at sea. In the late 1960, the problem arose of expanding the capabilities of the system to navigational support of faster moving objects and simultaneously increasing system efficiency and accuracy. It became necessary to create a universal navigation system, meeting the needs of all potential users: aviation, oceangoing vessels, ground transportation, spacecraft, ballistic and winged missiles, and space launch vehicles.
The Glonass system as a universal Space Navigation System for different types of users was developed during the 1970s. Experience gained in working to develop the low orbital system was fully used to develop high-accuracy intermediate-orbit space navigation systems. Scientific research accompanied development at all stages, starting with derivation of parameters during evaluation of technical proposals and ending with updating of software on the basis of test results. Research continued after the system went into use and is still continuing, now focusing on the further improvement of the system, increasing its accuracy and integrity, and expanding its functional capabilities.
The Glonass System was developed through a great deal of cooperation among organizations centered around the enterprises that had developed the first-generation Space Navigation System. The RF Ministry of Defense (the Military Space Force), the lead system customer, monitored its development and further improvement, and also deployment, support, and control of its orbital constellation. The M.F. Reshetnev Scientific Production Association of Applied Mechanics was the lead developer of the system, the navigational satellite, and the automated system for satellite control and its software. The Russian Scientific Research Institute for Space Instrument Building was the lead developer of the ground control system, the onboard equipment for navigation and command-measurement radio links and user equipment. The Russian Institute for Radio-Navigation and Time was the lead developer of the system synchronizing onboard and ground-based timekeeping devices and the user’s navigational equipment. The Polet Production Association was the developer and manufacturer of the Glonass satellite. The Tikhonravov Scientific Research Institute for Space Systems # 50 and Scientific Research Institute for Military Topographic Management # 29 of the RF Ministry of Defense, RF Naval Scientific Research Institute # 9, and RF Air Force Scientific Research Institute # 30 all participated in developing the system, working on the statement of work, the scientific and technical supervision of testing, and validating the effectiveness and principles for using the system. M.K. Tikhonravov Central Institute # 50 of the Military Space Force was assigned to improve and maintain the accuracy of ephemeris support of the system.
The Space Navigation System and the ground facilities of the Glonass system were developed by working groups of these enterprises led by Academican M.F. Reshetnev and Doctors of Technology Y.G. Guzhva, L.I. Gusev, A.G. Gevorkyan, N.Ye. Ivanov, A.V. Karpov, and A.V. Tsepelev. V.A. Korobkin was responsible for the development of the high accuracy model for computing ephemeris.
During the design and flight-test phase, the focus was on attaining the required accuracy characteristics for the navigation system. Specifically, developers were attempting to meet demands for levels of accuracy that had never yet been attained in any Russian space system. The low accuracy of ephemeris and frequency-time support that existed at the start ofsystem design were the result of the following factors:
* a low level of geodesic and geodynamic support. The parameters of Earth’s rotation were determined basically from data of the International Time Bureau, which failed to meet requirements for effectiveness and made the system dependent on the work schedule of the International Time Bureau.
* the level of stability of ground-based and small onboard frequency generators was more than an order of magnitude lower than that required.
* there were no measurement devices that could provide the required accuracy of trajectory measurements.
* the low precision of mathematical models of satellite motion for determining orbits and predicting ephemeris, resulting from uncertainties in defining the power of light pressure and computing active forces and force of the eccentric gravitational field.
* low precision of geodesic alignment of tracking devices.
The solution of the problem of attaining the requisite accuracy was based on implementing the following set of scientific and technical design and manufacturing measures:
* development of radio-technology measurement devices in the interrogating mode of the slant range with a potentially attainable accuracy of 1-3 m;
* development and deployment of quanto-optical systems for measuring slant range and angular coordinates with a maximum error of 1 m and 1 angular second, respectively;
* development of a radio-technology system for monitoring the phases of the navigational signal, linked with system frequency created by a highly stable ground-based standard generator;
* development of highly stable onboard generators of radio-signal frequencies;
* development of the Etalon passive satellite. Launching of passive satellites into orbit (Kosmos-1989 and Kosmos-2024) established that the forces acting on such satellites were approximately an order of magnitude lower than the forces acting on actual satellites of the system. The use of passive satellites in the navigation satellite orbit made it possible to develop a reference point for increasing the level of geodesic and geodynamic support of the navigation satellites and identifying the nature of the effects of the unmodeled forces acting on them, as well as for conducting the first evaluation of the divergence between the system of coordinates used in Glonass and that used in GPS for computing ephemeris.
In addition to introducing new components into the system, a set of fundamental problems had to be solved, including developing and incorporating models of navigational satellite motion possessing minimal methodological error, although a significant number of disturbing factors remained unmodeled; development of methods and technical systems for determining (refining) orbital parameters and the parameters of the coordinated models of motion, including geodesic alignment of measurement devices, refinement of the parameters of Earth’s gravitational field and the forces comprising light pressure; development of high accuracy methods for mathematical interpretation of measurements of current navigational parameters, techniques for standardizing highly accurate measurement systems and thus attaining the requisite accuracy characteristics; development of principles for monitoring the frequency-time characteristics and synchronization of onboard generators and methods for predicting their changes; development of techniques for geodesic and geodynamic support when using the ground control center’s own devices and the potential of the developed software; and refining the coordinated parameters of the model of satellite motion and the geodesic alignment of the Command Tracking Stations and the quanto-optical stations based on the use of highly accurate measurements of the orbits of the navigation satellite and the Etalon passive satellite.
During the final development of the system, these problems were solved successfully. Theoretical principles and methods were developed that made possible efficient determination of the parameters of Earth’s rotation from current satellite navigational parameters measured by the tracking stations. These methods provided more accurate determination of the kinematic parameters of satellite movement, with systematic errors of measurement by the Command Tracking Station and parameters of the model of light pressure forces and three parameters of Earth’s rotation (change in duration of days and two component coordinates of Earth’s pole). Comparison of the parameters thus obtained for Earth’s rotation to analogous data published by the International Time Bureau showed that the discrepancy between them was less than one meter for the coordinates of the Earth’s pole and less than one millisecond for the irregularity of Earth’s rotation.
The data obtained for Earth’s rotational parameters are used for extrapolating their values a month ahead. The error in extrapolation does not exceed two meters for the coordinates of the pole and 10 milliseconds for universal time. This level of accuracy satisfies currents requirements for accuracy.
The next factor characterizing the precision of user positioning is the accuracy of synchronization of satellite navigational signals, which is performed using ground-based devices by aligning onboard timescales to the scale of a Central Synchronizer. Accuracy of synchronization is attained by creating and using a highly stable ground-based generator—the major component of the Central Synchronizer with relative instability of 5 x 10 ~14 and the onboard standards of frequency with relative instability ofl x 10 ~13- 3 x 10 ~13 per day.
At the same time, the discrepancy between the phase of navigational signals must not have an error exceeding 10 nanoseconds, and prediction of changes in frequency-time corrections must be highly accurate. The use of generators with this level of instability is proposed in accordance with the program for developing Glonass systems.
Flight tests of the Glonass system were begun in October 1982 with the launch of the Kosmos-1413 satellite. By Order of the RF President on 24 September, 1993, the system, consisting of12 navigational satellites, was adopted by the RF Armed Forces.
Figure 3 is a photograph of the group that participated in developing and testing the system after the final meeting of the State Commission in May 1991, at which the decision was made to compile documents for the President’s signature to adopt the Glonass system for the Armed Forces. In the first row from left to right are Chairman of the Federal Commission, G.S. Titov, General System Designer, Deputy Chairman of the Federal Commission, M.F. Reshetnev, and Deputy General Designer, Yu.M. Knyazkin.
In 1995, the orbital constellation of the system was fully deployed. Further work on the space navigation system was conducted to increase the systems accuracy, ensure its integrity, and increase its service life. However, because of the sharp reduction in funding of new developments and expenditures on
replenishing the system, the size of the orbital groups was decreased. In 2001, the orbital constellation consisted of six functioning satellites. They were part of a Glonass/GPS joint use mode and methods were developed to use it in various areas. At present, resources have been allocated to supply the system and develop satellites with 5- and 7-year service lives. It is proposed that the system will be restored to its full complement by 2010. Measures have been mandated to increase accuracy by using onboard clocks with instability of 1 x 10 ~13 and a ground-based hydrogen standard of 1 x 10 ~14, and also devices for comparing timescales of onboard clocks with the ground-based standard whose error is 3-5 nanoseconds. The potential of this system is far from exhausted. Its potential is in the continuous updating of the system and the development of in-depth integrated systems for controlling the motion of moving objects. It is considered that the Glonass system must function as an independent system and also in conjunction with GPS and other navigational systems being developed.

Figure 3. The Glonass team.