Performance of Stand-Alone GPS - Understanding GPS: Principles and Applications - page 345

Number of GPS satellites visible

Location
12/23/94Z
42.35N
71.08W
0.00km
7 deg 3D
PDOP 6.0

10

9

8

7

Availability

6

5

4

Dilution of
precision

3

2

1

0

GPS

0 2 4 6 8 10 12 14 16 18 20 22 24

Time of day (hours UTC)

Figure 7.20 Number of visible GPS satellites over a 24-hour period.

7.5.1 Discussion of Criticality
Anomalies can occur, caused by either the satellite or the CS, which result in unpre-
dictable range errors above the operational tolerance. These errors are different from
the predictable degraded accuracy resulting from poor satellite geometry, which was
discussed in the previous section. Integrity anomalies should be rare, occurring only a
few times per year [17, 40], but can be critical, especially for air navigation.

7.5.2 Sources of Integrity Anomalies
There are four main sources of integrity anomalies: system-allocated signal-in-space
(SIS) aberrations, space segment-allocated SIS aberrations, control segment-allo-
cated SIS aberrations, and user segment SIS aberrations [41]. Satellite clock anoma-
lies are due to frequency standard problems such as random phase run-off, a large
frequency jump, or a combination of both. The MCS has reported clock jumps
when the beam current or temperature of the frequency standard has varied greatly.
Clock anomalies are the most prevalent source of space segment anomalies and the
most common source of major service anomalies. These anomalies can result in
thousands of meters of range error.
The first generation Block I satellites experienced many more clock anomalies
than the Block II generation of satellites [40] and did not have the radiation harden-
ing against the space environment that has been built into the Block II satellites.
Consequently, Block I satellites were subject to bit hits , which affect the navigation
message, as well as C-field tuning word hits . The C-field tuning register that aligns
the cesium beam is affected by solar radiation. Changing the bits that account for
the alignment/direction of the cesium beam has in some instances resulted in rang-
ing errors of thousands of meters in only a few minutes.