2.

Reduced accuracy of the GPS receiver Kalman filter operation due to

non linear HV dynamics during each Kalman filter calculation cycle.

All oscillators are sensitive to accelerations.  The requirement for a military GPS

receiver s crystal oscillator is in the order of a maximum rate offset of  2 nano  

seconds/second/g on two axes and a maximum of 3 nanoseconds/second/g on the third

axis.  If the receiver s measurement cycle is one second, a 4.5 g acceleration will

therefore result in 9 nanoseconds of time error on one of the "better" axes.  The

Kalman filter in the receiver will also contribute with a time error due to the less

accurate satellite tracking and therefore PVT dilution under dynamic conditions.  This

error is about 2 nanoseconds/second/g.  The total time error under dynamic conditions

is assumed to be approximately 20 ns greater than in the static mode.

11.4.4.2  Reduced Time Accuracy Due to SA

When SA is used by GPS, the  pseudorange errors and therefore the position, velocity,

and time errors will increase for a SPS receiver.  This reduced pseudorange

measurement accuracy will degrade the horizontal position to 100 m (95%).  

The relevance for the time accuracy available from a SPS receiver is as follows:

1.

100 m (95%) horizontal position error is equivalent to 31.3 m UERE for

each of the pseudoranges used in the navigation solution in the receiver.

2.

A user in a known location using only one satellite for the time transfer will

experience a time error of:

31.3

[

m

]

= 104 ns (1

) = 204 ns (95%)

3x10

8

[

m/s

]

If the user is at an unknown location and uses 4 satellites, then his total time error due

to geometry, SA and receiver errors will be:

104 ns * TDOP = 104 * 1.7 = 175 ns (Typical TDOP value)

SA will reduce the accuracy for SPS precise time dissemina tion to users of GPS, but

the effect can be reduced by smoothing the time measurements.  Also for SPS users,

the relative timekeeping accuracy normally realized by observing the same satellites

would be adversely affected by SA.  The timing provided to them would be irregular,

and their clocks would not be able to track it well.  An alternative is to perform coordi  

nated time transfer operations with USNO or another laboratory that maintains

adequate traceability to UTC (USNO); because of the time  varying nature of SA, the

coordination may need to be closer than it would under non  SA conditions.

11 13