ERSDEM-3: A Gigabit and Multi-channel Modem for Earth Observation
Pieter van Duijn, Jasper Schoenmaker, Balanchander Ramamurthy, Bill Cowley, Ricky Luppino, Mark Mirza · SpaceOps 2008 Conference · 2008
As the resolution of satellite imagery improves, requirements for high-speed transmission of image data from low-earth-orbit satellites to ground stations continue to grow. This paper describes the most recent design in the Earth Resource Satellite DEModulator (ERSDEM) series. Novel features of this third generation design are discussed and include either single channel operation to 500 Msym/s or multiple channel reception of independent downlinks, flexible receiver signal processing implemented in high-speed gate arrays, extensive test and diagnostic features that may be remotely accessed and an integrated high-rate baseband interface than provides additional data processing advantages. I. Introduction During the last few decades the number of remote-sending satellites in low orbit has increased dramatically and the range of applications requiring satellite imagery continues to grow. Improved image resolution and the use of on-board data storage have led to significant increases in the downlink transmission requirements from space to ground stations. Most remote sensing systems are currently using X Band transmission at data rates from tens to hundreds of Mbit/s. Spectrum constraints and the need for higher data rates have prompted the consideration of Ka Band downlinks and new transmission standards to meet future remote-sensing requirements (e.g. see [1]). This paper describes a recent project to design a flexible high-rate demodulator that copes with Gbit/s data rates, either via high-rate single carrier systems, or by simultaneously demodulating up to 4 downlinks. The aim of the ERSDEM-3 project has been to achieve a flexible Gbit receiver and transmitter, through the use of the latest reconfigurable gate-array technology. Given that sample rates of at least 1 GHz are required in the digital modem to receive/transmit 500 Msym/s signals, but that practical processing within field programmable gate arrays (FPGAs) is limited to approximately 250 MHz clock rates, a major emphasis has been to develop algorithms with the required parallelism and pipelining. This has been achieved through time domain processing, unlike the frequency domain approach used in [2]. The present design follows a series of earlier Earth-Resource Satellite DEModulators (ERSDEM) projects (see [3], [4]). Apart from the features mentioned above, this third generation design includes a larger range of modulation options, new synchronisation approaches to cope with multi-channel operation, a (CCSDS) trellis-coded decoder, additional diagnostic facilities and an integrated baseband interface. A proof-of-concept design was completed in 2007 and is shown in Fig. 1. At present an updated version of the design, being the commercial production version, is in its final phase of implementation and testing. This paper is arranged as follows. In the next section the overall architecture of the demodulator and test modulator is described, plus the technology used for hardware implementation. Section 3 gives an overview of the receiver signal processing, particularly in the multi-channel mode of operation, including some test results from the prototype. Section 4 describes the control, status and test features, while section 5 provides details of the software interfaces and integrated high-speed baseband interface. Conclusions are given in section 6.