This topic deals with the specifications of the dual GPS/Galileo RF front-end. As the starting point for the design of the RF front-end, a study of the technical issues related to the signals of the GPS and Galileo standards is shown. This is employed to obtain the specifications of an interoperable dual GPS/Galileo RF front-end, which is explained in the second part of this topic.
Global Navigation Satellite Systems
In this topic, the GPS and Galileo standards are explained in more detail. Specifications of the RF front-end must be determined in response to the signals transmitted by the satellites.
Global Positioning System (GPS)
The development of the Navstar GPS took nearly 20 years and cost more than $10 billion. It is the first and currently the only fully operational Global Navigation Satellite System (GNSS). The GPS project began in 1973 and attained full operational capability (FOC) in 1995, although it was already in use at the beginning of the 1980s. Developed by the U.S. Department of Defense (DoD), GPS is intended to serve as primary means of radio navigation well into the twenty-first century. GPS replaced less-accurate systems such as LORAN-C, OMEGA, VOR, DME, TACAN, and Transmit.
GPS has become much more than a military navigation platform since it has been opened to civilian use. Many new civil applications have appeared over the last few years in response to the decreasing cost, size, and power consumption of GPS receivers. Moreover, receiver capabilities continue to improve and small multichannel receivers with sophisticated tracking, filtering, and diagnostic features are making even advanced applications possible.
The civil uses of GPS include, but are not limited to, marine and aviation navigation, precision timekeeping, surveying fleet management (rental cars, taxis, delivery vehicles), aircraft approach assistance, geographic information systems (GIS), wildlife management, natural resource location, disaster management, meteorological studies, and recreation (hiking and boating)[HP AN1272].
Architecture Any GNSS consists of three different parts, namely the space segment, ground segment, and receiver. The space segment is composed of the satellites in space, whereas the ground segment controls the operation of the system from the Earth. Varying degrees of accuracy and services can be obtained depending on the receiver in use.
The GPS space segment is comprised of 24 Navstar satellites (and one or more in-orbit spares) distributed throughout six orbital planes. It takes 12 hours for the satellite to orbit the Earth, during which time it will have travelled 10900 nautical miles (approximately 20200km) orbits, meaning that each satellite passes over the same location on the Earth roughly once a day. Normally, five satellites are within range of users worldwide at any given moment. During the last 28 years, four different generations of GPS satellites have been developed: Block I, Block IIA, Block IIR (replenishment), and Block IIF (follow-on). The average lifespan of the first three generations of satellites is from 7 to 10 years, while the last generation is expected to last 15 years.
First launched in 1997, Block IIR satellites make up the majority of the current constellation of satellites. Block IIR satellites are equipped with an auto-navigation capacity (AUTONAV) that allows each spacecraft to maintain full positioning accuracy for at least 180 days without Control Segment support. The latest satellites in this series (Block IIR-M) carry a new military code or M-code. The M-code will be more jam-resistant than the current military GPS code (also known as P-code). In addition, these satellites will offer a second civil signal on the L2 band. Beyond the Block IIR-M, there are also plans to upgrade the system through the introduction of the GPS IIF programme (its first launch is planned for 2008). The Block IIF programme will transmit a third civil signal on the L5 band. A fifth generation of GPS satellites, Block III, is expected to dramatically enhance the performance of the system. It will add a third signal on the L1 and L2 bands (for which the first launch is planned for 2012). These satellites will provide a more resistant, accurate, and reliable signal through increased transmission power.
The ground segment consists of different stations scattered throughout the world. A master control station in Colorado Springs controls the space segment. In addition to operating the master control station, the United States operates five unmanned monitor stations and four ground antennas to pick up GPS satellite signals. The data collected by the monitor stations are used to calculate positioning corrections for the satellites. This process ensures the synchronisation of the satellites and the accuracy of the signals sent to the Earth.
GPS Signals and Services There are two types of GPS terminals, categorised according to the code they can acquire. The services available depend on the code received. Therefore, depending on the receiver, the offered services are as follows:
■ Standard Positioning Service (SPS) This service is available freely to the general public for civilian applications. Position is set by using the coarse acquisition (C/A) signal.
■ Precise Positioning Service (PPS) PPS receivers are used exclusively by authorised government agencies. Military receivers do not have to go through the C/A signal to track the P(Y) signal. Then, once military personnel is equipped with PPS receivers, C/A signal can be switched off on the battlefield without fear of repercussions for friendly military forces[Kaplan96].
Currently Transmitted Signals A GPS satellite currently sends signals L1 and L2 with a central frequency of 1575.42MHz for L1 and 1227.6MHz for L2, as shown in Figs. 2-1 and 2-2. There is a third signal, defined as L3, sent by satellites with a central frequency of 1382.05MHz. This signal will not be covered in this topic due to the fact that it is employed by the Nuclear Detonation Detection System (NUDET) and has no navigation finality.
Figure 2-1 shows how L1 and L2 are built. The L1 signal is a QPSK signal modulated in phase by the C/A-code and the information of the navigation message, and in quadrature by the precision code (P-code) and the navigation message. The L2 signal is a BPSK or QPSK signal modulated by a single signal, the C/A-code, the P-code, or the P-code and the information of the navigation message, depending on the selector position.
Figure 2-2 shows the baseband spectrum of the signals for a normalised transmitted power of 1W while Eq. 2-1 and Eq. 2-2 express the signals analytically.
Figure 2-1 Signals sent by a GPS satellite
In Eq. 2-1 and Eq. 2-2, the square root is the signal amplitude
is the navigation message,
> are the acquisition codes (spread spectrum codes), and ,
is the carrier frequency.
The coarse acquisition code C/A is also a pseudorandom noise (PRN) code with a clock frequency of 1.023MHz. It is the basis for the code-division multiple access (CDMA), used to send the signal from the satellites to the receivers. Every satellite has a unique C/A-code. These signals are detected and then separated through these codes, which have high-quality cross-correlation properties. This acquisition code constitutes the basis for the service used by the civilian SPS.
The P-code is a 10.23MHz frequency PRN code It is unique for each satellite and used for codification purposes. As PPS bases its service on this code, it is exclusively for military use.
This topic focuses on the civilian use of the receiver. Thus, the frequency band considered is the one due to the C/A-code, plus the navigation message, which is 2.046MHz centred on the 1575.42MHz frequency. The signal located on the L2 band will not be considered for the dual GPS/Galileo RF front-end dealt with in this topic.
Figure 2-2 (a) L1 signal spectrum; (b) L2 signal spectrum
Commercial receivers only employ the L1 band. Before 1 May 2000, employing the C/A-code on L1 and with the Selective Availability switched on, the 3D accuracy was around 25-100m, 95 percent of the time. Nowadays, employing the C/A-code on L1, and with the SA set to 0 the 3D accuracy, it is around 6-11m 95 percent of the time. New civil signals will offer an improved accuracy, integrity and continuity of service.
Planned Signals for the Future The U.S. DoD is planning to renew the GPS satellite constellation. Twenty-nine new satellites are being launched between 2003 and 2012. These satellites will send not only current signals but also additional ones that will allow more accurate and reliable positioning.
Four new signals are expected to be used: two for military purposes on the L1 and L2 bands, and two for civil use on the L2 band and the new L5 band. Since this topic is not focused on military applications, only those signals used by civilians are discussed in this section.
Figure 2-3 L2 sent by GPS satellites
The L2 signal sent by new satellites can be expressed analytically as follows:
where the first term represents the signal being currently sent as shown in Eq. 2-3. The second term represents the signal for civilian use. The generation of the signal, a 1227.6MHz (fL2) frequency signal modulated by two codes, is shown in Figure 2-3. Navigation messages are coded by Forward Error Correction (FEC) techniques. The last term represents the new signal for military purposes, the details of which are unknown.
The new signal uses C/A-code or another different one from what it is used now, the replacement code (RC), as acquisition code. The C/A is a PRN-code with a clock frequency of 1.023MHz. Compared to C/A, the new RC-code is significantly longer.
By means of this signal and the one on the L1 band, a civilian receiver will be able to correct delays caused by the ionosphere and troposphere, offering more accurate positioning than they do now.
Figure 2-4 shows the spectrum for the new signal of the L2 band. Signal bandwidth is 2.046MHz, the same as the civilian signal of the L1 band.
The L5 signal will be transmitted by the new satellites (from Block IIF) and can be expressed analytically as follows:
Figure 2-4 Spectral power of the L2 band’s new signal
It is a QPSK signal phase modulated by
codes and in quadrature by
codes. The codes
and
are PRN codes with a clock frequency of 10.23MHz. Thus, the bandwidth of the RF-modulated signal is 20.46MHz.
and
are Neumann-Hoff codes with a clock frequency of 10.23MHz. They increase the size of the
from 10230chips to 1023000chips and
from 10230 chips to 204600chips. The carrier frequency
is 1176.45MHz.
Figure 2-5 shows how to obtain the new L5 signal and Figure 2-6 illustrates the spectrum of the signal.
As in the case of the L2 band signal, the signal on the new L5 band can be used together with the L1 band signal to eliminate the effect of the delays caused by the ionosphere and troposphere, thus obtaining more accurate positioning.
Figure 2-5 Signal L5 sent by GPS satellites
Figure 2-6 Spectral power of the new signal on the L5 band
Moreover, Block III satellites will add a new civilian signal, called L1C, which will be transmitted on the L1 carrier frequency in addition to the C/A-code signal. The development of L1C represents a new stage for GNSS; the signal is not only designed for GPS transmission, it will also be interoperable with Galileo’s Open Service signal centred on the same frequency[Betz06].
Description of GPS Signals Table 2-1 briefly shows the current signals of GPS satellites and the signals of the new generation, planned to be in use starting in 2012.
TABLE 2-1 Signals sent by the new GPS satellite constellation [ARINC00]
|
Frequency band |
L1 |
L2 |
L5 |
|
Channels |
A B C |
A B C |
I Q |
|
Frequency |
1575.42MHz |
1227.6MHz |
1176.45MHz |
|
Modulation type |
 |
 |
 |
 |
 |
 |
|
|
Bit rates |
 |
 |
 |
 |
 |
 |
|
 |
 |
 |
|
|
Minimum received power @ elevation 10° |
 |
 |
 |
 |
 |
 |
|
 |
 |
 |
In the future, civilian users will be offered three kinds of receivers depending on their location accuracy[ARINC00]:
■ Current receivers will still work with the same accuracy as now tens of metres, and should be sufficient for certain applications.
■ Dual receivers that receive and process two signals, L1 and L2 or L1 and L5, will be more accurate, metre level. This is achieved by correcting delays caused by the ionosphere and troposphere, offering more accurate positioning.
■ High-accuracy receivers will make use of signals L1, L2 and, L5. These kinds of receivers will offer centimetre-level accuracy and will be required to be differential.
