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Per Vices Team - Per Vices
Introduction
The satellite industry is one of the giants of the modern world, with applications ranging from cutting-edge orbital telescopes for space exploration to telecommunications and GPS signaling, used on a daily basis by the average person. One integral part of satellites is the radio system, which is responsible for generating, transmitting, receiving, and processing electromagnetic signals. Modern satellites cannot rely on simple analog-based radio systems anymore, as they introduce significant limitations in terms of flexibility, upgradability, and information processing. Therefore, software-defined radios (SDRs) are one of the fundamental building blocks in any satellite. They are responsible for not only basic radio communications, but also more complex tasks, such as wideband spectrum monitoring for space situation awareness (SSA) and space domain awareness (SDA) applications, jamming and interference detection/avoidance, common clock sources, and ground station control/communication. This article will discuss the role of SDRs in the satellite industry, covering from the basic structure of SDRs to the applications mentioned above. This is the 12th article in a series of SDR use cases. Click here to see more articles in this series.
Before delving into the SDR technology and its applications, let’s take a step back and focus on the current state of the satellite industry, and what demands the global satellite system must fulfill in modern applications. This includes the satellite orbits and their applications, the exponential increase in the number of satellites over the years, and the role of international collaboration in these matters. Furthermore, this article will briefly discuss how our entire modern economy is dependent on the satellite system due to GNSS/GPS.
Evolution of the Satellite Industry
A satellite system consists of the satellite itself and one (or more) ground stations. The satellite can be defined as a self-contained communication device orbiting Earth, able to receive and transmit radio signals to and from ground stations by using a transponder RF module. These signals can then be used in telecommunications, telemetry, and geolocation. The satellite receives communication and command signals from the ground station. Historically, satellites are designed to operate in three different orbits:
GEO satellites are a remarkable breakthrough in technology, as they allow the stationary positioning of the satellite relative to a point in the surface of the Earth. These satellites were first proposed by Arthur C. Clarke in 1945, and are applied today in communications, meteorology, and navigation. Twelve years later, in 1957, the first satellite was launched by the Soviet Union: the famous Sputnik 1. However, the first voice signal transmission via satellite is credited to the US, with the SCORE (Signal Communication by Orbiting Relay Equipment) project. Commercial satellite communication was then developed through the pioneer works of John Pierce (Bell Laboratories) and Harold Hosen (Hughes Aircraft).
Modern Satellite Communication
The most basic communication link in modern satellite applications is the connection between the satellite and the tracking telemetry and control (TT&C) in the ground station. This communication link provides position tracking of the satellite and control over its functionality, such as the propulsion system and thermal manager. Moreover, the TT&C is used to monitor several variables of the satellite, including temperature, electrical signals, and radiation incidence. The hardware requirements for the communication link also change with the type of orbit: LEO and MEO satellites need tracking antennas to ensure a consistent connection between devices, which is not a requirement in GEO satellites. Moreover, while GEO systems can provide global coverage using only three satellites, LEO and MEO systems require more than 20 and 10 devices, respectively. This is the reason why GEO satellites are the preference in GPS, as they provide huge coverage and consistent positioning/navigation. However, LEO and MEO satellites are more adequate for mobile communications, as the signal delay of GEO satellites is unacceptable in telephony applications. Therefore, orbit selection is highly dependent on the application.
In general, satellites are applied in three sectors: telecommunications, including mobile phones and wireless networks; broadcasting of radio and television signals; and data transference. Satellites are also the ideal option for applications with limited ground resources, such as isolated areas with small and dispersed communities. Future developments in space technology can further improve the performance of satellites in terms of on-board computing capabilities, bandwidth power, and lifespan. Furthermore, the advent of nanosatellites is bringing a new perspective into satellite communications, with mega-constellations of devices operating in a coherent network for a variety of applications.
Modern Ground Stations
In terms of ground station technology, the hardware required depends mainly on the frequency range of the satellite transmission. The operation frequencies, also called bands, are divided into seven different categories, defined by letters. The L, S, and C-bands are lower frequency bands typically transmitted with low power, requiring larger antennas for reception. The ground station typically consists of a large antenna, a feed horn, a waveguide, the main receiver, and often a radome for protection. Higher frequency bands, including the X, Ku, Ka, and V-bands, are known for transmitting more power to the ground stations, thus the antenna dishes can be made very small. Ku and Ka-bands are typically used by Internet service providers (ISPs). Parabolic antennas are a popular choice in high-frequency ground stations, as they provide high directional capabilities with a small volume (see Figure 1). They use the dish geometry to amplify the incoming signal passively, reducing the overall noise. As shown in Figure 1 below, the actual antenna is the feed horn at the focal point of the parabolic dish, where the electromagnetic energy is concentrated for amplification.
Figure 1: Uplink and downlink with parabolic dish antennas are shown.
Modern Challenges
Modern applications are pushing the boundaries of satellite technology. High-frequency applications beyond the V-band are already being employed in space: in 2021, the ARTES Advanced Technology program launched the first W-band satellite, operating with a 75 GHz transmitter onboard an LEO satellite. Using such high frequencies, powerful radio receivers with high-end modulation/demodulation systems are required to extract the information from the signal with minimum noise. Moreover, rain fading is much higher in these cases, which increases the noise figure at the receptor. Another bottleneck in modern satellite technology is the monitoring of thousands of devices in nanosatellite constellations, which is significantly burdensome for the ground station. Multiple-input multiple-output (MIMO) radio equipment is fundamental to accomplish simultaneous connection to the whole constellation network. Moreover, the monitoring of space debris and satellites is becoming increasingly necessary as the number of debris increases in the earth’s orbit, especially in the context of satellite constellations. Finally, there is a crucial need for tight timing synchronicity between satellites and the ground stations, particularly in multiple satellite networks for navigation, maritime, and positioning systems. This issue is a major challenge in modern satellite technology, so let’s dive a bit further into this matter.
Because satellites are way above the ground, they provide a pathway for radio signals with virtually no obstacle, which results in a much more reliable and deterministic transmission of information. To achieve time precision, each satellite must be equipped with precision onboard atomic clocks, with multiple ground stations being responsible for monitoring and synchronizing the time sources of the network. Precise modeling of the transmission delay is fundamental in a variety of applications, including navigation, positioning, and telecom synchronization. For instance, time-related data is extremely important in TT&C systems, where both position, velocity, and time information about the target need to be resolved. Also, the global navigation satellite system (GNSS) would certainly not be feasible without reliable time-tracking of signals to obtain the propagation delay, which is essential to resolve the distances with enough accuracy for target positioning. The performance of GNSSs can be significantly improved by using satellite-based augmentation systems (SBAS). In an SBAS, the GNSS transmissions are constantly being monitored by reference ground stations, that are precisely located across a large area. These stations are used to correct the time error and improve the accuracy, availability, integrity, and continuity of the GNSS service. The SBAS then broadcasts the calculated error and corrections using GEO satellites, with the augmented signals overlaying the basic GNSS message.
Breakdown of SDRs for the Satellite Industry
SDRs are composed of three basic stages: the radio frontend (RFE), the digital backend, and the mixed-signals interface. The RFE contains the receive (Rx) and transmit (Tx) functions of the SDR, managing signals over a wide tuning range. The highest-bandwidth SDR in the market right now, the Cyan SDR (Per Vices, Figure 2 below), uses an RFE with a tuning range between 0-18 GHz (upgradable to 40 GHz), as well as an instantaneous bandwidth of 3 GHz over 16 independent Tx/Rx channels. The digital backend contains an FPGA with onboard DSP capabilities. The backend is responsible for modulation, demodulation, up/down-converting, signal processing, and data transferring. The SDR is based on FPGA technology, which means the backend can be easily reprogrammed to accommodate different radio protocols, DSP algorithms, and even artificial intelligence, without any hardware replacement. This increases the flexibility, adaptability, and upgradability of the system. The mixed signals interface connects the RFE to the digital backend, and it is performed by powerful DACs and ADCs. Furthermore, precise timing can be obtained through dedicated clock sources, based on internal time boards. High-end SDRs use oven-controlled crystal oscillators (OCXO) to obtain extremely stable and accurate clock frequencies for the FPGA, the ADCs/DACs, and channel synchronization.
Figure 2: Per Vices Cyan is a high bandwidth SDR.
The first radio systems were completely based on analog components. With the advent of digital electronics, the fast evolution of digital signal processing, and the invention of the FPGA, software-defined radios (SDRs) started to dominate over their analog counterparts. Traditional analog radios are bulky and defined by hardware, so they require a lot of dedicated electronics tuned to a particular application. This means that any small upgrade or modification will require hardware replacement, which is onerous and expensive. SDRs, on the other hand, are defined by software, so modifications can be easily implemented by reprogramming the digital backend, without any hardware replacement. Moreover, the modular nature of SDRs allows device customization based on the size, weight, and power (SWaP) requirements, which is critical in the satellite industry – in particular nanosatellite applications. Finally, SDRs can integrate advanced signal processing algorithms and even artificial intelligence within onboard devices, which is not possible on purely analog radio systems.
Both satellites and ground stations are completely dependent on radio systems, so the SDR is a crucial component in the satellite industry. However, the selected SDR must withstand the orbital conditions while still providing sufficient performance to satisfy the satellite’s requirements. For instance, onboard SDRs must be designed to resist the extreme temperatures and radiation levels of space. Furthermore, SDRs are needed during the deployment of nanosatellite constellations after launching, to properly distribute the satellites into position by tracking the constellation configuration using RF communication and feeding control information to the onboard maneuvering system. Ground stations also employ SDR systems in the TT&C, receiving data, issuing commands, and uploading software to the satellite. In multi-satellite networks, like nanosatellite constellations, high data rates and high throughput are crucial in ground stations, to avoid losing data and ensure that all satellites are accounted for. SDRs can support high data throughput by using 40GBASE and 100GBASE optical links. Furthermore, very fast data processing can be obtained through parallel computing using FPGAs, which gives high performance at low power consumption. Finally, SDRs provide high sensitivity, which improves the detection of weak signals.
SDRS for Modern Satellite System Applications
SDRs can operate normally with all the satellite bands mentioned in the previous sections (L, S, C, X, Ku, Ka, and V), thus there is a large range of applications in the satellite industry. For instance, SDRs play a major role in inter-satellite tracking: by performing a variety of DSP computing on the received signals, they can evaluate important parameters that are used in distance estimation, including received signal strength (RSSI) and Doppler shift. Also, the high SFDR and high SNR in modern SDRs are crucial to measure the weak signals received in RSSI-based tracking systems. Furthermore, in the context of SSA/SDA, high throughput data links with very low latency are required, to ensure that the data packages are transmitted/received in time. The lowest latency SDRs in the market, like the Cyan model from Per Vices, should be implemented in critical SSA/SDA applications. Finally, SDRs can be used to simulate and test satellite networks, protocols, and modulation/demodulation schemes, improving the efficiency in system design and optimization.
In the ground station context, MIMO SDRs are capable of providing multiple independent Rx/Tx sources that are essential in phased-array radars, typically used in satellite tracking. The MIMO SDR must not only provide a multiple-channel RFE, but also needs to be capable of handling the vast amount of data coming from several sources simultaneously, so high data throughput and fast signal processing are also mandatory. The FPGA-based digital backend allows the SDR to perform beamforming/beam steering algorithms, optimizing antenna directionality, efficiency, and gain. Furthermore, ground stations often need to control or communicate to multiple satellites, with different frequencies to avoid interference. In these cases, MIMO SDRs are the best choice, as they can handle multiple channels independently. Finally, SDRs can implement different filtering and digital processing techniques to improve the RF performance of ground receivers, including noise rejection, bit-error rate, co-channel interference, and signal sensitivity.
Another important function of SDRs in satellites is the generation of a common clock signal. The clock board presented in most high-end SDRs provides the time stability required for critical time and frequency applications, making it a great choice for GNSS systems. The use of the internal SDR clock as the main system clock for the satellite increases system flexibility and reduces the engineering time, by using resources that are already available in the device. However, if another external clock is being used as a reference, the SDR can be easily implemented in a “slave mode”, where the internal clock is replaced by the external reference clock for synchronization.
Finally, SDRs can perform several DSP calculations that are fundamental in any satellite application, such as decoding/encoding, modulation/demodulation, up/down-converting, and multiplexing. With an FPGA, several different modulation schemes can be implemented using the same hardware, including BPSK, QPSK, and QAM. These features allow the development of satellite modems, also called SATCOM modems, on the embedded FPGA of the SDR. The FPGA allows modification of the modulation scheme and the RFE parameters, including gain, frequency, and sample rate, enabling estimation and adjustment of Doppler shifts. This reconfigurability allows the SDR to support complex communication standards, such as the Digital Video Broadcast Satellite Second Generation Extension (DVB-S2X). DVB-S2X systems can be used HDTV broadcast services, internet access, and cellular backhauling. Figure 3 below shows the main differences between an FPGA-based modem and a traditional modem.
Figure 3: Traditional and FPGA-based modems are compared.
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