Efficient support of planetary surface missions typically requires an orbiting asset that acts as a relay point to/from Earth. Orbital relay passes are normally 5 to 15 minutes in duration over any specific landed site. When multiple landed assets are co-located or near-located in the same coverage circle of a single relay orbiter, their telecom relay support opportunities will overlap. This will be the case with cooperative lander missions, a lander-rover operations pair, distributed intelligent lander missions, and future deployment of multiple equipment components for support of complex sample return or manned operations. In these situations, the capability of simultaneous support to multiple landers is very valuable for mission performance and operations flexibility. This technology work enables simultaneous telecom support to multiple landers (Mars, Titan, Europa), and provides single-radio, multi-mode support to Entry, Descent & Landing (EDL) and emergency operations (e.g., demodulation + Open Loop Recording).
The basic concept is a Frequency Division Multiple Access (FDMA) communications scheme wherein different users occupy non-overlapping signal bands all lying within the receiver’s Intermediate Frequency (IF) bandwidth. All user signals share the same wideband down conversion and wideband A/D (analog-to-digital) signal path. Data rate and modes of operation for each user are independent as long as the spectral content of all users fits within this down conversion and A/D bandwidth. The A/D is followed by user channelization, which is achieved via a polyphase, fast Fourier transform (FFT) filter bank that separates heterogeneous multi-user signals into single-user channels that are each sent to their own demodulation, decoding, and protocol downstream process. Thus, the remainder of the architecture comprises parallel digital modems (one per user). Each modem is similar to a single Electra modem and thus most of the Electra design can be used in the Multi-User Modem.
Each user is supported with a fixed data rate or data rate that automatically accommodates the corresponding link conditions of that user. Error correcting codes and protocol management for automatic retransmission are unique for each user. The combined supportable data rates depend on the chosen modulation schemes and processing throughput capabilities of the A/D converter as well as the processing capacity of the FPGA (field-programmable gate array).
A dual-user version of this technology was implemented on a Virtex-2 3000 FPGA that is used in the Electra software flight radio on the MAVEN and TGO Mars orbiters. This dual-user modem requires 73% of the Virtex-2 resources (slices).
The architecture requires a special two-step AGC (Automatic Gain Control). A wideband AGC, which “sees” all user signals, prevents saturation at the output of the A/D. After FFT channelization, individual digital AGCs are used to regulate the tracking loop and demodulation processes for each user. Furthermore, the digital AGCs can provide important metrics for individual user adaptive data rate (ADR) control.
The novelty of this technology is that it is developed for space applications (planet relay and near Earth missions), but it can be used directly to re-program existing space modems based on the Electra software defined radio.