7.4.1  Clock Aiding

A GPS receiver uses its own internal clock or may use a more accurate external clock as

time reference.  If only three (instead of four) satellites are available, then the GPS receiver

can assume that its time reference is correct ( Dt = known) and treat the three available

satellite range measurements as actual ranges instead of  pseudoranges.  In this case, the

accuracy of the position derived from the  pseudorange measurements will correspond to

the equivalent time reference error.

If the GPS receiver clock or the external clock can be monitored during a previous period in

which the receiver navigates with four satellites, then the clock phase bias and drift can be

calculated.  The resulting corrections for clock errors can be used to provide very accurate

GPS time during a satellite outage and an accurate GPS position can be maintained for

several minutes.

The method of using a clock instead of a satellite is not recommended as a permanent

solution, but rather to help the GPS receiver operate during short periods when only a

limited set of satellites is available.  A GPS receiver should be capable of receiving (and

providing) precise time via a dedicated PTTI interface or via the 1553  bus.

7.4.2  Altitude Aiding

Similar to the clock aiding discussed in the previous paragraph, an airborne GPS receiver

can use a barometric altimeter as aiding to replace a satellite measurement.  Long term

altimeter errors are calibrated during periods of four satellite operation.  Subsequently,

when less than four satellites are being tracked, the calibrated  baro altimeter data are used

as a known U

z

 value in the 4 unknowns of (U

x

, U

y

, U

z

) and Dt.  Conceptually, the barometric

altitude added to the earth radius provides a range measurement from a satellite with

position at the center of the earth.  An accurate GPS position can be maintained for as long

as the estimated baro altitude errors are valid. Since the barometric altitude errors are

generally slowly varying, both in time and distance, reasonable position accuracy can

usually be maintained for 10 15 minutes, or within a radius of roughly 10  nmi of the position

of the last 4 satellite solution.  A gradual loss of position accuracy, especially in the vertical

channel, can be expected.  Depending on the algorithm used to compute altitude from

pressure, the loss of accuracy may be hastened by altitude changes in a nonstandard

atmosphere, particularly if no temperature compensation is used.

7.5  AIDING TO MAINTAIN SATELLITE TRACK

In normal receiver operation, the code and carrier tracking loops are both being tracked

in phase lock.  There is a symbiosis between the code and carrier tracking loops where

each loop aids the other.  In a high jamming environment, the receiver may lose its

ability to track the carrier.  Subsequent accelerations will cause the carrier frequency of

the received GPS signal to vary due to a change in the Doppler  shift.  The Doppler shift

of the frequency of the received carrier signal is proportional to the relative velocity of

the receiver with respect to the satellite along the line of sight from the receiver to the

satellite. 

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