a minor overall detriment to weapon effectiveness when compared to the alternative

loss of weapon availability.

6.1.5.5  Summary of the Commentary

If there are significant differences from the "model" receiver implied by the performance

standards, different constraints applicable to the application, or different mission

requirements, an independent assessment of the performance standards or similar

parameters is probably warranted via computer simulation.  In addition, real time

integrity checking and calculation of a figure of merit can significantly reduce the

uncertainty associated with global averages and long term statistics, and significantly

improve the probability of success of a given military mission.

6.2  Satellite Selection Criteria

6.2.1  Introduction

The criteria used for satellite selection is a very important factor in GPS receiver

design.  Different receivers perform satellite selection using different algorithms.  The

important satellite criteria to be considered include:

a. Satellite health

b. Geometric dilution of precision

c. User range accuracy

d. Elevation angle

e. Availability of external aids.

6.2.2  Satellite Health

The NAV msg contains satellite health information for all the satellites in the GPS

satellite constellation.  Each satellite broadcasts health summaries for all (up to 32)

GPS satellites, in page 25 of subframes 4 and 5.  Each summary consists of 1 bit

indicating the health of the NAV msg and 5 bits indicating the health of the satellite

signals.  (Refer to "Technical Characteristics of the Navstar GPS" or ICD GPS 200PR

for additional details).  A satellite should never be used in a Nav solution if its Nav 

message is indicated to be unhealthy.  If the NAV msg health is good, the five bit signal

status message should be compared against valid operating modes for the receiver to

determine if the satellite can be used.  For example, a P code receiver could use a

satellite broadcasting L1 only, if an ionospheric model can be  used instead of dual

frequency measurements to make the ionospheric corrections.

The NAV msg also contains a health message in subframe 1 which indicates the health

of the broadcasting satellite.  Since the data in subframes 4 and 5 are updated less

frequently than subframe 1, subframe 1 may be used to indicate short term health

problems or may be updated before subframes 4 and 5.  Therefore, after a satellite is

acquired, the health data in subframe 1 should also be checked to deter mine if the

satellite can be used.

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