3.5  TIME TO FIRST FIX

Time To First Fix (TTFF) is a measure of the elapsed time required for a receiver to

acquire the satellite signals and navigation data, and calculate the first position

solution. TTFF begins when initialization of the receiver is complete (including self 

test, loading of PPS keys, and any required operator input) and the receiver is

commanded to begin the positioning function.  Some source material (U.S. DoD in

particular) may refer to TTFF1 and TTFF2.  TTFF1 is based on C/A code

acquisition with hand over to P code tracking.  TTFF2 is based on direct P code

acquisition.  REAC (reaction time) is the term typically used to include both the

initialization process and TTFF.  Since initialization may necessitate operator

action, REAC specifications or require ments may require assumptions of operator

response times.  TTFF is a function of the initial receiver state as well as receiver

design.  The following paragraphs describe the satellite acquisition and initial

positioning processes in more detail.

Table 3 2.  Time Error Budget

Error Component

Error (ns, 95%)

 US Naval Observatory Measurement Component

137

 Control Segment Measurement Component

 59

 GPS Time Predictability

 92

 Navigation Message Quantization

  6

 Satellite Orbit

 22

 Satellite Clock

 63

 Satellite Group Delay

 12

 Downlink and User Equipment

 65

Total (RSS) Time Transfer Error Budget

199

Table 3 3.  Precise Time Output Accuracy (95%)

for a Typical PPS P code Receiver

Receiver Mode

Receiver Output

S/A On

S/A Off

Stand Alone, Stationary, or

Instrumentation Port

2 ms

2 ms

Low Dynamic

PTTI Port

127 ns

127 ns

3 8