Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 465

then require three gyroscopes and three accelerometers to inertially determine its
position and velocity in free space.
One of the significant factors related to the quality of an inertial system is the
drift of the gyroscopes, measured in degrees/hour. The drift of a gyro is a false out-
put signal caused by deficiencies during the manufacturing of the sensor. In inertial
sensors, these are caused by mass unbalances in a gyroscope's rotating mass and by
nonlinearities in the pickoff circuits, as is seen in fiber-optic gyroscopes (FOGs).
This false signal is in effect telling the navigation system that the vehicle is moving
when it is actually stationary. The manufacturing of gyros with low drift is very
costly. A gyroscope with drifts of greater than 100°/hour would cost less than
$1,000 (all costs are in 2005 U.S. dollars). Inertial units with drift from 1 to
100°/hour are currently priced from $1,000 to $10,000. Accuracies of less than 1
degree/hour are available at prices ranging from $10,000 to $100,000. The quality
of the inertial sensors has a large role in the cost effectiveness of a navigation system.
If 0.0001°/hour gyroscopes were to cost less than $100, GPS may not be needed
today. But in actuality, inertial sensors are expensive, and a significant result of the
integration of GPS with inertial sensors is the ability to use lower performing, more
cost-effective sensors. This is shown in Figure 9.4, where the upper curves show the
performance of three classes of inertial sensors. (Note that CEP is an indicator of
delivery accuracy. It is the radius of a circle in which 50% of the projectiles are
expected to fall within the given radius—see Section 7.3.2.) When these systems are
integrated with GPS, the lower curve dictates the performance of the integrated
GPS/inertial (GPSI) system. Therefore, during operation of a navigation system
when both GPS and inertial components are operational, the inertial navigation
errors are bounded by the accuracy of the GPS solution.
One significant contribution the GPS receiver makes to the operation of the
inertial subsystem is the calibration of the inertial sensors (see Figure 9.5). Note that
mean radial error (MRE) is another indicator of delivery accuracy. (MRE is the
mean of the miss distance of all projectiles.) Inertial instruments are specified to
meet a turn-on to turn-on drift requirement. (Each time a gyro is powered up, its ini-

100 deg/hr(11,100 km/hr)

1 deg/hr (111 km/hr)

0.1 deg/hr (11 km/hr)

GPSI systems (< 46 m)

Time

Figure 9.4 Comparison of navigation accuracies.