DOI: 10.48577/jpl.zvrshv
title: Antenna Beam-Sharing: Progress Toward Multiple Uplinks Per Antenna

description: Within a decade, the number of spacecraft requiring support from NASA’s Deep Space Network (DSN) is expected to increase by a factor of 2 to 3. Key drivers behind this expectation include NASA’s increasing reliance on more affordable, targeted smallsat missions, a more robust science program in general, and the emergence of multi-element human lunar exploration missions. Given that a proportional increase in the number of antennas needed to communicate with all of these missions might prove prohibitively expensive, NASA has been exploring ways to gain increased capacity from its existing antennas. One way to do this is through antenna beam-sharing. While successfully employed for simultaneous downlink, beam-sharing for simultaneous uplink has not been an option, within the same frequency band, due to the severity of the intermodulation products. An alternative approach to achieving near-simultaneous uplink involves multiplexing the Forward Communications Link Transmission Units (FCLTUs) intended for all of the participating in-beam spacecraft onto a single uplink frequency, which they all receive. Each spacecraft then identifies which of the transfer frames within the received FCLTUs are intended for it on the basis of spacecraft ID. Known as Multiple Uplinks Per Antenna (MUPA), this technique has been under study for several years and is now at the point where key components are being prototyped and implementation pathfinding is occurring. This paper describes the progress in these efforts to date. These efforts fall into three broad categories: the FCLTU multiplexer, ground system interfaces, and the spacecraft radio. The FCLTU multiplexer efforts include developing and testing the software needed to: receive FCLTUs from disparate Mission Operations Centers (MOCs), assess whether their destination spacecraft are in view or not, queue them accordingly, multiplex the in-view FCLTUs, send them on to the DSN under a virtual spacecraft ID for radiation, and feedback the appropriate radiated FCLTU statistics to each of the originating MOCs. The ground system interface efforts include identifying the modifications to DSN systems needed to successfully schedule MUPA sessions, distribute appropriate support data products to the uplink and tracking data systems, and ensure that the coherent turnaround of the shared uplink carrier frequency to each of the individual spacecraft downlink carrier frequencies is executed and communicated in ways that fully support simultaneous 2-way Doppler and ranging. The spacecraft radio efforts include designing, simulating, and/or testing radio features on the JPL Iris radio that will enable it to support variable turnaround ratios, selectable uplink frequencies, onboard Doppler compensation when trying to acquire and lock onto the shared uplink frequency, and filtering of the received transfer frames on the basis of spacecraft ID. These efforts have also involved examining MUPA’s compatibility with different types of uplink encryption, coding, and modulation and how these factors would enter into the scheduling of a MUPA session. Together, all of these efforts and the underlying system engineering for them, are “setting the stage” for the next key step on the path to implementing MUPA: flight demonstrations. Preparations with the SunRISE and EscaPADE projects for a multiplexer demonstration are already underway, and the team is working to identify other flight projects amenable to demonstrating the Iris radio’s MUPA-related capabilities.