Satellite transmission capturing method for gps receiver

Claim 1

A satellite transmission capturing method for GPS receivers wherein a reference carrier is reproduced by a demodulator comprising a first multiplier for inverse diffusing said satellite transmission, a second multiplier for in-phase channel signal demodulation, a third multiplier for orthogonal channel signal demodulation, a first low-pass filter corresponding to said second multiplier, a second low-pass filter corresponding to said third multiplier, a fourth multiplier for detecting the phase difference of said satellite transmission and said reference carrier, a phase-locked loop (PLL) filter, a voltage controlled oscillator for generating said reference carrier, a demodulation intensity calculating circuit, a control circuit and a switch for switching to said PLL loop when said reference carrier is synchronized with said satellite transmission, said method comprising the steps of: inverse diffusing said satellite transmission and applying the resulting received satellite transmission to said second and third multipliers; generating the reference carrier; demodulating said received satellite transmission by multiplying said received satellite transmission by an in-phase component of said reference carrier with said second multiplier and multiplying said received satellite transmission by an orthogonal component of the reference carrier with said third multiplier to obtain a demodulated in-phase channel signal I(t) and a demodulated orthogonal channel signal Q(t); incrementing a frequency of said reference carrier within a predetermined Doppler-shift frequency range beginning at a starting frequency in frequency increments greater than a capture frequency range required for PLL operation to lock onto a captured transmission; calculating a demodulation intensity based upon said in-phase channel signal I(t) and said orthogonal channel signal Q(t) corresponding to a particular frequency obtained following each frequency increment, said frequency increments being determined by a bandwidth corresponding to a predetermined threshold level of said demodulation intensity; comparing said demodulation intensity obtained at each frequency increment to said threshold level; reducing said frequency increments to correspond to said capture frequency range when said demodulation intensity of said particular frequency reaches said threshold level; determining whether said particular frequency is located substantially at the peak of demodulation intensity corresponding to a received frequency of said received satellite transmission; and initiating said PLL operation when said particular frequency is located substantially at the peak of demodulation intensity corresponding to said received frequency of said received satellite transmission to phase synchronize said reference carrier with said received frequency.