SPECIAL FEATURE
COTS in Space: Software Defined Radio (SDR) for Satellite Communication By Brandon Malatest, Founder, Per Vices Corporation Introduction One of the most critical devices in any satellite is the radio frequency (RF) communication module. It receives and transmits information that is used for receiving control from ground stations, sending status from onboard experiments and machine conditions, routing data packages for internet access, and navigation applications, such as GNSS/GPS. In modern satellites, almost all RF communication functions can be performed using a commercial off-the-shelf (COTS) system. In this context, software-defined radios (SDRs) are the best choice for COTS radio systems, given the variety of size, weight, and power (SWaP) options in the market. However, not all COTS SDRs are suitable for satellite applications, as they have to withstand the harsh conditions of space, such as drastic temperature fluctuations and intense ionizing radiation.
Figure 1: Image of the components of an SDR 22
COTS Journal | May 2022
In this article, we will discuss the role of COTS SDRs in satellite systems, the main advantages of their use in onboard applications, and why they are the perfect choice for the job. Furthermore, it is crucial to take into consideration the impact of the harsh space environment on the analog and digital components of the SDR, so we will also discuss how satellite based SDRs require specific types of RF components to withstand the space conditions and work reliably. What is SDR? Before we can discuss anything related to the space applications of SDRs, we need to properly define an SDR. Software-defined radios are the product of a paradigm shift in radio technology that pushed most of the communication functions and signal processing to the digital realm, leaving only the essential RF signaling int the analog circuitry. Thus, SDRs
are composed of three main blocks: the radio front-end (RFE), the digital backend, and the mixed signal interface (Figure 1). The RFE is the analog portion of the SDR responsible for the receive (Rx) and transmit (Tx) functions, used to receive the signals over a wide tuning range of 0 to 18 GHz, and in state-of-the-art technology this can be upgraded to 40 GHz. Moreover, the highest-bandwidth SDRs in the market can reach up to 3 GHz of instantaneous bandwidth per channel, with MIMO operation offering several channels in one RFE. The RFE is also responsible for filtering, amplification, and impedance matching. The digital backend, on the other hand, is responsible for performing most of the signal processing functions (such as DSP algorithms, modulation/demodulation, upconverting, and down-converting) and communication functions, including the radio protocols, artificial intelligence (AI) algorithms, and packetization. The digital backend typically consists of a high-performance FPGA with onboard DSP capabilities. Finally, the mixed-signals interface performs the connection between the RFE and the FPGA, and it is composed of multiple channels of ADCs and DACs. Both backend and ADC/DACs require high synchronicity to work properly, so a powerful time board is crucial. The implementation in high-end SDRs consists of an oven-controlled crystal oscillator (OCXO), that provides a stable (5 parts/billion) and accurate 10 MHz signal for clock, with low noise floor and phase noise. One fundamental aspect of satellite electronics are that they are designed for worst case scenarios, as hardware replacements, maintenance, and adjustments are, almost always, impractical. This makes the design significantly expensive and overcomplicated. Moreover, the space conditions are highly variable due to the atmospheric and ionospheric effects, particularly in Low Earth Orbit (LEO)
