Analysis of nonlinear time-varying channel effects on the differential carrier phase delay for same-beam interferometry

Same-beam interferometry (SBI) is a kind of interferometric tracking technique. It involves the simultaneous coherent tracking of two or more deep space probes, separated by a small line-of-sight angle, by a single ground antenna. Due to its inherent high precision characteristics, the differential carrier phase delay measurements acquired in SBI contain a wealth information of the relative distance between pair of probes, which could be used to determine the tides and rotational state of a planetary body, or to enhance the knowledge of the planetary gravity field. In the future deep space missions, Code Division Multiple Access (CDMA) provides the ideal signal structure, because it maximizes the bandwidth commonality for SBI, and it is also a priori modulation option for Multiple Spacecraft Per Antenna (MSPA) operations. This paper has made a detailed investigation on the performance evaluation of reducing the nonlinear time-varying effects of the communication channels on the final differential carrier phase delays for CDMA based SBI. Phase error equations have been established on mathematical analysis, and simulation has been made to evaluate the effects of nonlinear time-varying effects in the communication channels for both CDMA mode and non-CDMA mode. The results show that the CDMA modulation could definitely reduce the nonlinear time-varying effects and improve the measure accuracy of the differential carrier phase delay in SBI.