GPS System Segments - Understanding GPS: Principles and Applications - page 100

90

Rms URE
Max URE
Max RSS

80

70

60

50

40

30

20

10

0

15 20 25 30 35 40 45

NAVSTAR number

Figure 3.19 MCS upload message fit errors.

compliance with IS-GPS-200, with navigation bits populating the subframes. Addi-
tionally, the MCS-ground antenna-satellite uploads are checked, after the naviga-
tion data is locked into the satellite's memory and prior to authorizing the L-band
transmission. Error protection codes exist along the entire path of navigation service
for integrity. The satellite upload communication protocol is enforced to assure
proper and error-free data content onboard the satellite before its use is authorized.
The NAV Data is based on predictions of the CS Kalman filter estimates, which
degrade with age of data. The 2SOPS monitors the navigation accuracy and per-
forms contingency uploads when the accuracy exceeds specific thresholds. Unfortu-
nately, the dissemination of the NAV Data message is a tradeoff of upload frequency
to navigation accuracy. Various upload strategies have been evaluated to minimize
upload frequency while maintaining an acceptable navigation service [6, 27]. Figure
3.20 shows the basis tradeoff curve: an increase in upload frequency reduces the pre-
diction age of data and thus improves the signal-in-space URE (see Section 7.2). GPS
navigation accuracy depends on many factors, including performance of the satellite
AFSs, the number and placement of the monitor stations, measurement errors,
ephemeris modeling, and filter tuning.

3.3.2 CS Planned Upgrades
Over the next several years, the CS will field two major upgrades: the L-AII and the
AEP. The L-AII upgrade modifies the existing MCS mainframe implementation to
support additional monitor stations and satellites in a partitioned Kalman filter.
Since the 1980s, the MCS has used a partitioned Kalman filter consisting of up to six
satellites and up to six monitor stations per partition. This partition filter design was
due to computational limitations and hindered CS navigation accuracy. The L-AII
upgrade will enable the MCS to support up to 20 monitor stations and up to 32 sat-
ellites in a partition. (Note: The CS Kalman filter will maintain the partitioning and