Radio Transmitter Development to Support Multi-Gbps Satellite Downlinks in Ka-Band

Advancements in spaceborne remote sensing technology have significantly increased the amount of data generated onboard spacecraft, necessitating a robust downlink system capable of much higher rates than previously required. For the NASA/ISRO Synthetic Aperture Radar (NISAR) mission the Jet Propulsion Laboratory (JPL) has developed a Ka-band Modulator (KaM) variant of the Universal Space Transponder (UST) product line to support 2000 Msps downlink from a single radio using Offset QPSK (OQPSK) modulation in the Earth Exploration Satellite Service (EESS) band from 25.5 to 27.0 GHz. The KaM has successfully completed its protoflight development campaign and flight models have been integrated onto the NISAR spacecraft, awaiting its launch. The KaM is a Software-Defined Radio (SDR) that offers the flexibility to change all aspects of the RF modulation to channel coding without altering the underlying hardware. This paper describes aspects of the design/implementation trade space to achieve higher data rates on the KaM while meeting transmission band spectral limitations. We explore the use of higher-order modulation schemes such as 8PSK and 16APSK, which can substantially increase data throughput within the available 1500 MHz bandwidth of the EESS band. Performance simulations and prototype radio-based characterization tests demonstrate that with firmware updates, the KaM can be upgraded to support data rates up to 3000 Msps using 8PSK modulation and 4000 Msps using 16APSK modulation. By employing a dual polarization downlink scheme using 16APSK, the updated radio could enable downlink rates of up to 8000 Msps for future missions with much greater downlink volume requirements. Link analyses are presented for Low Earth Orbit science satellites comparing the data rates against station coverage time to NASA's Near Space Network (NSN) and possible daily downlink volumes. A feasibility study is presented that evaluates high-rate lunar downlinks, pairing KaM technology with proposed Lunar Exploration Ground Sites (LEGS) considering current high-power amplifiers and antenna technology. Additionally, future Mars surface terminal links are analyzed to explore the viability of high-rate relay communications from the Martian surface to future Mars relay orbiters.