Digital frequency tracker

Claim 2

A digital frequency tracker for use in a doppler navigation system comprising: a. means to clip the input to the tracker to provide an output which is a series of pulses; b. a clock providing an output pulse train; c. means to sample and store the output of said clipping means at the occurrence of each clock pulse; d. means to compute from the sampled data a plurality of autocorrelation function values; e. means to store a value respresenting a frequency F0; f. means to store a constant value; g. means to multiply said value representing F0 by said constant; h. means to add the output of said multiplier to F0 and store the result as F1; i. first means to subtract the output of said multiplier from F0 and to store the result as F2; j. first means to compute and store the power at F1 comprising means to compute a Fourier series wherein the stored value of F1 is the frequency input and said autocorrelation function values the coefficients; k. second means to similarly compute and store the power at F2; l. second means to subtract the computed values of power at F1 and F2 and to provide an output indicative of the magnitude and direction of the difference therebetween; m. first means responsive to said output to change the stored value representing F0, so as to tend to make to power at F1 and F2 equal; and n. timing means responsive to inputs from said clock providing control inputs to each of said means to sample, means to compute, means to store, means to add, means to subtract and means to change the stored value representing F0.

Claim 3

The tracker according to claim 1 and further including means to acquire a frequency F0 which lies in the frequency range of the doppler signal and to provide a value representative thereof to said means to store said value representing F0.

Claim 4

The tracker according to claim 2 wherein said means comprise: a. means to generate first and second timing signals; b. means to cause said first and second computing means to compute power in response to said first timing signal and to compute power dentiy from zero to F1 and F2 in response to said second timing signal; c. means to store a value representing an average noise power density; d. a comparator having said noise as a first input and providing a first output if a second input is greater than said first input and a second output if said second input is equal to or less than said first input; e. means to couple the output of said subtracting means to said first responsive means in response to said first timing signal and as second input to said comparator in response to said second timing signal; f. second means responsive to the second output of said comparator to change the value of F0; and g. third means responsive to said second output to maintain the presence of said second timing signal as long as said second output is present.

Claim 5

The tracker according to claim 3 and further including meanS to provide an output indicative of altitude.

Claim 6

The tracker according to claim 4 wherein said doppler navigation system includes at least a doppler transmitter and a receiver and said altitude output means comprise: a. means to generate an FM reference for use in the doppler transmitter; b. means to generate a first sample pulse train at the frequency of said FM reference; c. means to generate a second sample pulse train having pulses shifted 90* from those on said first pulse train; d. means to alternately provide said first and second pulse trains as the clock input to the frequency tracker, each for a period of one power density computation; e. first storage means coupled to said second subtracting means to store a first power density obtained with said first pulse train as a clock input; f. second storage means coupled to said second subtracting means to store a second power density obtained with said second pulse train as a clock input; g. means to difference said first and second power densities; h. means to shift the phase of said first and second pulse trains with respect to said FM reference in response to the output of said differencing means such as to tend to make said first and second power densities equal; and i. means to store and provide as an output the difference in phase between said FM reference and said first pulse train, which difference will be proportional to altitude.

Claim 7

The tracker according to claim 5 wherein said means to generate an FM frequency comprise: a. a modulo N/2 counter; b. a high frequency clock providing an input to said counter; and c. a flip-flop obtaining its input from said counter and providing its output as said FM frequency; said means to generate a first sample comprises: d. a register; e. a second counter; f. a comparator having as inputs the outputs of said register and said second counter and providing an output when the two inputs are equal said output being a reset input for said second counter; g. a flip-flop obtaining its set input from the output of said modulo N/2 counter and its reset output from said comparator; h. an AND gate having as inputs said clock and the output of said flip-flop and providing its output as a count input to said second counter; and i. a one shot multivibrator having as an input the output of said comparator and providing said first pulse train at its output; said means to generate said second pulse train comprises: j. a modulo N/4 counter; k. a second flip-flop obtaining its set input from said comparator and its reset input from the output of said modulo N/4 counter; l. a second AND gate having as inputs said clock and the output of said flip-flop and providing its output as a count input to said modulo N/4 counter; and m. a second one-shot multivibrator having the output of said module N/4 counter as an input and providing said second pulse train as its output; said means to alternately provide said first and second pulse trains comprise: n. a third AND gate having said first pulse train as an input; o. a fourth AND gate having said second pulse trains as an input and having its output tied to that of said third AND gate and providing the sampling clock input to said tracker; and p. means to alternately enable said third and fourth AND gates; and said means to shift the phase of said pulse trains comprises: q. means to add a function of the difference from said differencing means to N/2 + 1 and to provide said sum to said register whereby said register will have stored therein and may provide as an output the difference in phase between said FM reference and said first pulse train.

Claim 10

The method according to claim 7 wherein said frequency F0 is selected by steps comprising: a. scanning across the frequency spectrum in discrete F0 steps; b. computing at each F0 step, with a Fourier series and said function values, the power density between two frequencies FA and FB on either side of F0; c. comparing said computed power density with a power density representative of the system noise between FA and FB; and d. stopping at the F0 step where the computed density exceeds said noise.

Claim 11

The invention according to claim 8 and further including steps to provide an output indicative of altitude comprising: a. generating an FM frequency for use in the Doppler transmitter; b. generating a first pulse train at said FM frequency; c. generating a second pulse train with pulses 90* out of phase with those of said first pulse train; d. computing a first power density from the tracker input signal using said first pulse train as a sampling input; e. computing a second power density from the tracker input signal using said second pulse train as a sampling input; f. subtracting said first and second power densities; and g. shifting the phase of said first and second pulse trains with respect to said FM reference to cause said two power densities to be equal whereby the amount of phase shift will be proportional to altitude.