A plurality voting method for acquisition of regenerative ranging measurements

Sequential tone ranging and regenerative ranging may both be considered to consist of two parts: acquisition of the ranging clock phase, from which the range accuracy is derived, and a resolution of the range ambiguity that remains when only the clock period is considered. With sequential tone ranging, the ambiguity is resolved by dividing the frequency of the ranging tone in steps until the remaining ambiguity is resolved by knowledge of the spacecraft position. With regenerative ranging, the ambiguity is resolved by a determination of the phase of a long ranging code. This code phase is determined by both the spacecraft, which then regenerates the code, and by the ground station. It is possible to have sufficient signal-to-noise ratio to provide good range measurement accuracy with regenerative ranging and yet have insufficient signal-to-noise ratio (SNR) to acquire the proper range code phase at the ground station for each individual range measurement. A set of range measurements may be processed together to determine the proper range ambiguity resolution even when none of the individual measurements is reliable. An approach of processing together a set of measurements has been developed and applied to data from the New Horizons mission while the spacecraft was in excess of 22 Astronautical Units from Earth. The value of T*Pr/No was less than 30 dB, making normal processing of the data unreliable. Consistent and correct results have been demonstrated even when the initial range phase values were scattered over the entire possible range of values. This paper will present the method of processing together a set of range measurements and show the application of this method to New Horizons data. The predicted probability of success as a function of the number of available points and the signal-to-noise ratio of the individual points is presented. The SNR required for correct acquisition of the ranging code can be reduced by 15 dB relative to the requirement for indi- idual range measurements when 120 range measurements are processed together. This approach expands the conditions over which regenerative ranging will be successful in deep space missions.