GeReLEO-MODULOS - A Versatile LEO-Modem for Data Relay Satellite Systems

Kurzfassung

Earth observation satellites – typically low Earth orbit (LEO) satellites – require both data links for telemetry, tracking and command, and for download of mission data. Maintaining a contact to a passing LEO satellite for more than half of its orbit is possible with geostationary Earth orbit (GEO) satellites using suitable antennas onboard. Main objective of the GeReLEO-MODULOS project is to develop an engineering model of a modem for LEO satellites, which supports both bidirectional telemetry/telecommand (TM/TC) and unidirectional high-rate download of data via a GEO relay satellite. Key features of the modem are (i) support of several LEO satellites at the same time, and (ii) both high/low rata data transmission to/from big and small (inexpensive) LEO satellites. The software/firmware for the physical layer and for the higher layer protocols including interface(s) to the satellite bus allows flexible adaptation to missions’ needs. Apart from size and power consumption, bandwidth efficiency is an important requirement. Because of the timevariant free space loss on the inter-satellite link, varying antenna gain caused by relative movement between LEO and GEO, and due to changing atmospheric conditions on the feeder link the modem supports different modulation and coding (ModCod) schemes. Switching between ModCod schemes is either triggered via the return channel, or is determined by a pre-computed deterministic pattern. The modem hardware consists mainly of Ka-band up/down-converters and a Zynq-7000 XC7Z045 all programmable (AP) system on a chip (SoC). This SoC integrates a processing system (PS) containing two ARM c CortexTM-A9 MPCoreTM application processors, and a programmable logic (PL) for high performance applications. The former is used for the higher layer protocol implementation; the latter takes care of the digital signal processing and the physical layer. In this paper we will introduce the GeReLEO-MODULOS hardware and software concept. The chosen SoC approach allows both efficient modem software development and high-rate data transmission.