X-Band PN Delta DOR Signal Design and Implementation on the JPL Iris Transponder

Abstract

This paper describes the design and implementation of the Pseudorandom-Noise (PN) Delta Differential One-way Ranging (DDOR) signal format on the JPL Iris Cubesat Software Defined Radio. The Iris radio is now in-flight use in Deep Space onboard the MARCO spacecraft and will soon be utilized to multiple EM-1 missions. The spread spectrum Delta DOR format enables more accurate differential ranging measurements over the classical DOR tone format, and it is applicable to deep space missions that require accurate navigation or require accurate angular position measurements for another purpose such as determining the ephemeris of a planet or small body. The classical Delta-DOR technique makes time delay measurements of spacecraft and quasar signals to determine spacecraft angular position in the radio reference frame defined by the quasar coordinates. The measurement system is configured to provide common-mode error cancellation as nearly as possible. In the PN DOR mode, instead of a spacecraft modulating their downlink, with a sinusoidal signal, referred to as a DOR tone as done in the classical DOR mode, the sinusoidal tone is replaced with a spread spectrum signal. Using a spread spectrum DOR signal instead of a DOR tone enables cancellation of effects due to phase dispersion across the channels used to record quasar signals. This error source, referred to as phase dispersion or phase ripple, is currently the dominant measurement error for the classical DOR format. PN spreading will improve on classical DOR performance accuracy because by choosing the PN spreading code and shaping filter carefully, the spacecraft signal can be made to closely resemble the quasar signal resulting in reduction of the Delta-DOR error due to phase dispersion by 80% to 90% over the classical DDOR approach. The article will describe the choice of a Gold code sequence that possesses good autocorrelation properties as well as excellent cross-correlation properties that was used to spread the DOR tone, and the choice of root-raised-cosine (RRC) chip-shaping filter to reduce the amount of excess-bandwidth of the output waveform. The paper will also describe the path that was taken for the FPGA implementation on the Iris radio including a Simulink model of the PN DDOR module that is used for automatic HDL code generation. This module was then integrated into the Iris firmware and verified. Future work includes the integration of the designed subsystem into the Universal Space Transponder and extension of the derived waveform to the KA-band. We hope that the outlined PN DDOR implementation can be integrated into an upcoming mission that utilizes the Iris radio (such as EM-l missions).