Peter Kinman

Range Measurement as Practiced in the Deep Space Network

Range measurements are used to improve the trajectory models of spacecraft tracked by the deep space network. The unique challenge of deep-space ranging is that the two-way delay is long, typically many minutes, and the signal-to-noise ratio is small. Accurate measurements are made under these circumstances by means of long correlations that incorporate Doppler rate-aiding. This processing is done with commercial digital signal processors, providing a flexibility in signal design that can accommodate both the traditional sequential ranging signal and pseudonoise range codes. Accurate range determination requires the calibration of the delay within the tracking station. Measurements with a standard deviation of 1 m have been made.

Carrier synchronization of offset QPSK for deep space telemetry

Offset quadri-phase shift keying will be the telemetry modulation scheme for many future deep space missions, in accord with new CCSDS standards. The Block-V Receiver of the Deep Space Network has a carrier synchronization loop for offset QPSK, but this loop has four stable lock points within each cycle of carrier phase. In this paper, a method is proposed for reducing this four-fold phase ambiguity to two fold. This can be accomplished with the addition of simple signal processing at both the transmitter and receiver. The two-fold phase ambiguity is resolved by a unique synchronization word, just as is commonly done already with binary phase-shift keying (BPSK). This strategy of reducing the four-fold phase ambiguity to a two-fold phase ambiguity does require periodic bit inversions at the modulator. So this solution is not available for the general offset QPSK modulator that does not include the necessary periodic bit inversions.

Dynamic telemetry bit rates for deep space communications

A new telemetry playback scheme promises to maximize telemetry return for deep space missions. For a given effective isotropic radiated power from the spacecraft, the received signal-to-noise spectral density ratio and hence the supportable bit rate vary during a tracking pass as the elevation angle changes. In the past, spacecraft would use just one bit rate or perhaps a few different bit rates during a pass. However, large bit rate changes sometimes cause the ground receiver to go out of lock. The new scheme, which is examined here, allows the spacecraft to change its bit rate in frequent, small steps to match the signal-to-noise spectral density ratio profile. Because the rate changes are small, the ground receiver will be able to remain in lock.

Turbo-code performance with imperfect carrier synchronization

The threshold performance of deep-space telemetry is characterized for four turbo codes. The mathematical models given here are based on simulations that account for imperfect carrier synchronization. These models are valid for coherent detection of binary phase-shift keyed (BPSK) telemetry when the carrier synchronization is based on either a residual-carrier loop or a (suppressed-carrier) Costas loop. The required Eb/N0 depends on the code, the threshold frame error rate, the bit rate, and the signal-to-noise ratio and bandwidth of the carrier synchronization loop. For residual-carrier tracking, these mathematical models are used to calculate the optimum modulation index.

Detection of very weak transmissions from deep space

Abstract Designers of future planetary missions often reduce spacecraft transmitter power and oscillator stability requirements to decrease mission cost. Unfortunately, these reductions can make it impossible to detect weak signals from deep space using conventional demodulation techniques. The new Block V receiver being installed in the Deep Space Network (DSN) can recover suppressed carrier signals and can utilize very narrow loop bandwidths — as narrow as 0.1 Hz. However, operations at very narrow tracking loop bandwidths are quite sensitive to spacecraft oscillator stability. Low-cost oscillators planned for future missions can force the use of wide tracking loop bandwidths, leading to reduced carrier tracking performance. This reduced carrier tracking performance can, in turn, lead to a significant increase in required spacecraft transmitter power. This paper presents the theory behind coherent detection of very weak telemetry from deep space. It then briefly recounts tests characterizing Block V performance for the NEAR spacecraft in two-way coherent operations and presents recommendations for mission designers.