Performance of GMSK for telemetry and PN ranging under realistic conditions

In the frame of CCSDS activities, a system capable of simultaneously transmitting high rate telemetry and ranging has been studied in the last years. In this system the telemetry is transmitted through a GMSK modulator with the PN (Pseudo Noise) ranging sequence included as an additional phase shift. The receiver first estimates the transmitted telemetry bits, regenerates and removes the estimated GMSK signal from the received signal, and then estimates the ranging chips and, through a bank of correlators, the round trip delay of the received ranging signal. Ranging is an interfering signal which degrades the performance of the telemetry subsystem, while errors in the estimation of telemetry bits compromise the correct detection of the ranging chips. The first simulation results obtained by ESOC were presented at TTC 2010 in the paper “Analysis of GMSK for Simultaneous Transmission of Ranging and Telemetry” and were limited to ideal synchronization and to the case of a telemetry bit rate equal to the ranging chip rate. In this new paper we describe additional results obtained from the simulation of the complete system, including realistic synchronization, and for telemetry rates which are different from the chip rate. The paper will: 1) consider the effects of the receiver telemetry clock jitter on the regenerated GMSK signal and on the subsequent ranging receiver: it will be shown that regeneration through the Laurent OQPSK approximation or through a look-up table, which directly stores the GMSK phase for each combination of input bits, achieve good performance with low complexity; 2) discuss the effects of perfect synchronization between the transmitted telemetry and ranging signals: in this case, depending on the relative delay between the two signals, the recovered ranging clock may suffer from a bias, which corresponds to an error in the range estimation (lack of accuracy); 3) estimate the system losses when the telemetry bit rate is different from the ranging chip rate; 4) estimate the loss due to phase noise. The analysis will be limited to the case of GMSK with BTb=0.5 and ranging code T2B, which is the suggested scheme for deep space missions with demanding acquisition time requirements.