Viavi 87339, Operation Manual

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Testing
4 -12
4.1.7 Measuring Time to First Fix (TTFF) and Position Accuracy
TTFF and position accuracy measurements are important parameters in GPS receiver
testing. In many GPS applications, the time it takes for the receiver to return its actual
location can significantly affect the receiver’s usability. In addition, the accuracy with
which a receiver returns its reported location is important.
For a receiver to obtain a position fix, it must download the almanac and ephemeris
information from the satellite through a navigation message. Because it takes 30 seconds
for a receiver to download an entire GPS frame, a “cold start” TTFF condition can take
anywhere from 30 to 60 seconds.
Signal strength also is a factor in correlation lock time and hence TTFF.
Fig. 4-4 TTFF vs. Signal Strength
Many receivers specify several TTFF conditions, including Acquisition (cold and warm
start), Reacquisition (hot start) and Positional Accuracy.