Two Sets of GPS Almanac on Time-to-first-fix Influence

As almanac data effects the acquisition of satellites signals,two sets of almanac,high almanac and midi almanac were investigated.High almanac,mid almanac and GPS broadcast ephemeris data were compared in satellite position and velocity,code phase and Doppler frequency drift search space ambiguity and time-to-first-fix(TTFF).Simulation analysis was done in many cities with the actual almanac data.Results show that high almanac degrades little but mid almanac degrades in accuracy over the time span of one week.Over the first two d high almanac and mid almanac shall be very close in TTFF;over the subsequent five d,high almanac will be superior to midi almanac to second level in TTFF.