In a proposed improved technique for pseudonoise turnaround ranging of a radio transponder, the pseudonoise modulating signal would be regenerated in the transponder. The net result of the regeneration would be an increase in the effective return ranging power. This increase would provide some margin for decreasing ranging time, decreasing the power of the return ranging signal transmitted by the transponder, or increasing the power of another (e.g., telemetric) signal transmitted from the vehicle that carries the transponder. The technique was conceived for use in measuring the distance between a master station on Earth and a spacecraft that carries a transponder; with modifications, it may also prove useful in other applications that involve ranging and/or communications in both outer-space and terrestrial settings.
In turnaround ranging, one determines the distance between the master station and the transponder by measuring the round-trip travel time for a known ranging signal modulated onto a carrier signal (denoted the uplink carrier signal) transmitted by the master station. The receiver in the transponder locks onto the uplink carrier signal, demodulates the ranging signal, and remodulates the ranging signal onto another carrier signal (denoted the downlink carrier signal), which is coherently related to the uplink carrier. A receiver at the master station locks to the downlink carrier signal and demodulates the ranging signal. Finally, the received ranging signal is correlated with the transmitted ranging signal, and the offset between the two signals that yields the maximum correlation amplitude constitutes the estimate of the round-trip signal travel time.
In this case, the ranging signal would be a pseudonoise binary sequence that would be phase-modulated onto the uplink carrier. Pseudonoise binary sequences have been used before in turnaround ranging of spacecraft because they offer a desirable combination of high ranging resolution, low ranging ambiguity, and no need for receivers to "know" when pseudonoise sequences started.
The need for regeneration of the pseudonoise sequence (or, for that matter, any other ranging signal) in a transponder arises as follows: In the transponder, in the absence of regeneration, uplink noise is received along with the ranging signal, so that unavoidably, the uplink noise is modulated onto the downlink carrier along with the ranging signal. In a typical deep-space application, the retransmitted uplink noise power can exceed the retransmitted ranging power by as much as 30 to 40 dB. Thus, a considerable amount of power that could otherwise be used for telemetry or other purposes is wasted retransmitting the uplink noise, and the retransmitted noise degrades the ranging signal received at the Earth or other master station. If the pseudonoise sequence could be regenerated in the transponder with proper timing instead of being simply turned around along with the uplink noise, then it could be retransmitted without the uplink noise, thereby increasing the ranging-signal-to-noise ratio.
The major task for the transponder at any given time is to determine the current position in the pseudonoise sequence in order to regenerate and retransmit the sequence with proper timing. Part of this task is to determine the current phase of the ranging signal. Assuming that the ranging signal is frequency-coherent with the uplink carrier signal, following standard practice in turnaround ranging, the proposed technique would involve locking to the received uplink carrier signal to obtain a timing signal for a numerically controlled oscillator in a pseudonoise-sequence-tracking loop (also called a "chip-tracking" loop).
Because the ranging signal would look like a square-wave signal except for an occasional flip in polarity, the phase of the signal would be tracked by use of a first-order square-wave phase-locked loop. The nature of the pseudonoise sequence would be such that the tracking phase error would average out to zero in the long term, and the effect of the tracking phase error could be diminished through low-pass filtering. Because the phase-error output of a phase-locked loop is proportional to the signal amplitude, which can vary widely, an automatic gain control (AGC) would be necessary for normalizing the amplitude before input to the loop filter. The reference amplitude signal for the AGC would be the amplitude of the signal in the in-phase channel of the carrier-tracking loop.
The chip-tracking loop would contain correlators instead of a traditional lock detector. The correlators would integrate over a desired number of pseudonoise-repetition periods (chips), and the current position relative to the pseudonoise sequence would be determined from the correlator outputs. The timing of the transmitting pseudonoise-sequence generator would be set accordingly, and the numerically controlled oscillator in the chip-tracking loop would be used to clock the binary modulation sequence to the downlink transmitter. After each integration period, the current positions in the transmitted and received pseudonoise sequences would compared; agreement of these positions would signify lock.
This work was done by Jeff Berner, James M. Layland, Peter Kinman, and John R. Smith of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.