time that the measurements were taken.  Alternatively, measurements can be

deweighted in the integrated solution and latency errors added to the system error

budget.  If implemented correctly, a GPS sensor can still contribute to a navigation

solution when less than four satellites are being tracked.  Such a system is capable

of incorporating a single satellite measurement into the system Kalman filter, thus

bounding the navigation system solution in one dimension.  The disadvantage of

using GPS as a sensor in an integrated positioning solution is the high level of

complexity involved in integrating such a system.

In general, stand alone GPS receivers do not allow corrected pseudorange and

delta range data out of the receiver since it is classified data.  Therefore, some

receivers provide the capability to process the integrated solution within the GPS

receiver.  Many GPS sensors are now small enough to be embedded as a card or

module into another system, such as an INS or Flight Management System (FMS).

In such cases, the corrected pseudorange and delta range data may be permitted

off the card or module since the classified data would be contained within a single

unit.  However, GPS technology now allows most of the GPS receiver functions to

be performed by a single semiconductor chip or small chip set.  Consequently,

future security/processor devices such as the Selective Availability Anti Spoofing

Module (SAASM) may return to the "stand alone" architecture, providing the

capability to process the integrated solution aboard the device, while not allowing

the corrected pseudorange and delta range data out of the device.  For additional

discussion of GPS integration architectures and related issues, refer to Chapter 8.

For security design guidelines refer to Navstar GPS PPS Host Application

Equipment (HAE) Implementation Guidelines.

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