SDR in Satellite Ground Stations
by Brandon Malatest, Cofounder and Chief Operating Officer, Per Vices Corp.
Satellite ground stations (SGS) are fundamental components of space-based systems designed for communications, navigation, remote sensing, and scientific research (Figure 1). They are used to communicate with satellites in space, acting as gateways and modems between the orbiting devices and the network architectures on land. With the increasing demand for satellite services, the need for high-performance SGS has become more pressing, especially considering the cutting-edge technological shifts that come with 5G and IoT, such as ground stations as a service (GSaaS). This article will explore the latest technologies and applications used in SGS, focusing on the role of software-defined radios (SDRs).
To meet this demand, ground stations must implement RF transceivers capable of adapting to different communication protocols, modulation frequencies, waveforms, and huge amounts of data with minimum hardware modification. Software-defined radios (SDR) revolutionized the design and operation of satellite ground stations, combining the high performance of state-of-the-art radio front end (RFE) with flexible FPGA-based digital backends, which can be reprogrammed using software without any hardware modifications. This flexibility enables SDRs to tune to a wide range of frequencies, capture large amounts of data, and perform various digital signal processing (DSP) operations on board.
As the RF transceiver is responsible for transmit and receive operations between SGS and satellites, it should come as no surprise that the specifications of the RF device dictate the SGS performance. Technologies used in ground stations are constantly evolving, with advancements in antenna design, digital signal processing, and data analysis enabling new applications and capabilities. As the demand for satellite communications and remote sensing continues to grow, novel approaches for RF transceivers are required.
SGSs are used for a wide range of applications, from satellite communications to remote sensing and scientific research, so the specific equipment needed for each application can vary significantly. Some ground stations require many independent radio chains that are used to receive and process signals from multiple satellites. This is particularly important for global navigation systems, where several satellites (organized in constellations) must be tracked and processed simultaneously.
Other ground stations require both transmit and receive functionality, allowing them to not only receive data but also send commands and data to satellites (Figure 2). These stations are typically used for remote sensing applications, in which satellites are used to capture data about the Earth’s surface, atmosphere, and oceans. Some SGSs must be able to tune across a very wide bandwidth, such as those applied in deep space communications or military applications. Consequently, they must implement specialized high-frequency equipment, including wideband receivers and transmitters.
In any case, SGS radio receivers must be capable of handling high throughput of data at increasingly high frequencies to address industry requirements, considering the growing trend towards using ground station services such as Software as a Service (SaaS). This approach enables users to access ground station functionality over the Internet without the need for specialized hardware or software. This paradigm shift aims to make SGS more accessible to a broader range of users, including small businesses, researchers, and even hobbyists.
SDR for Satellite Ground Stations
The basic SDR consists of a radio front end (RFE) and a digital back end that is typically implemented using an FPGA or ASIC. The RFE handles both receive and transmit functions across a wide tuning range, while the digital back end is responsible for all on-board digital signal processing (DSP) functions, including modulation and demodulation, up- and down-conversion, Ethernet networking, and serial JESD204B communication with the RFE.
One of the major advantages of SDRs for ground stations is their flexibility in terms of performance, working frequency, and communication protocols, as the FPGA can be reprogrammed online and repurposed without any hardware modifications. The most advanced SDRs can process signals with 3 GHz of instantaneous bandwidth over multiple channels with independent ADC and DACs, which allows for a wide simultaneous coverage of spectrum through several transmit and receive chains (Figure 3). This makes them well suited for applications that require processing of high-speed data, such as remote sensing or Earth observation. Overall, SDRs have become a valuable asset for ground stations due to their flexibility, high bandwidth processing capabilities, and adaptability to changing requirements.
Several key features make SDRs useful for satellite ground stations. One important characteristic is their wideband tuning capabilities, which allow for the reception and transmission of signals across a broad range of frequencies. This is particularly important for General-purpose SGSs and GSaaS architectures, which need to be able to communicate with a wide variety of different satellites that may be operating on different frequencies and waveforms.
With an SDR, operators can quickly and easily tune the radio to the correct frequency and begin receiving data without having to manually tune the device. In addition to their wideband tuning capabilities, SDRs often have multiple independent radio chains, which enable the capture of multiple signals simultaneously and under different conditions. This is important for ground stations that need to receive data from multiple satellites at once, where each channel of the multiple-output (MIMO) SDR can be automatically tuned to a specific satellite or constellation.
SDRs also typically have high sample rates with an adjustable RF bandwidth, which is important for capturing wideband signals with high resolution and to reduce overall noise through oversampling. The high sample rates and adjustable bandwidth allow for the capture of a large amount of data, which is critical for many ground station applications. The high dynamic range of SDRs is achieved through high-bit-resolution ADCs and linear amplifiers, which allow for the capture of weak signals without distortion or interference from strong signals.
On the digital side, the capability of performing heavy digital signal processing functions using parallel chains embedded into an FPGA is advantageous when compared to analog approaches. This feature allows ground station operators to customize the SDR to their specific needs without hardware replacement or modification, including custom filtering, beamforming/beam-steering, pulse compression, frequency hopping, and even machine-learning/artificial intelligence algorithms.
The FPGA can be reprogrammed entirely online and repurposed remotely, which provides added flexibility and adaptability to any conditions. SDRs are available in various form factors, including rack-mountable units, making them easy to integrate into existing SGS equipment. This is important for ground stations that need to upgrade their equipment or add new capabilities without having to completely replace their existing infrastructure.
Finally, the digital backend offers a native and ready-to-use host interface through high-speed optical links (qSFP+), which allows for seamless integration with the host architecture, network, or storage system. By enabling agnostic host communication, the SDR can work with virtually any host solution, including proprietary software, open-source systems (such as GNU-radio), and custom programs in common languages, such as C++ and Python.
SDRs are commonly applied in satellite ground stations to enable the reception and processing of signals from a wide range of satellites and easily transmit high-throughput data to the host system. One of the main advantages of using SDRs is their ability to work with a variety of input sources, including different feed horns, antennas, and external downconverters, making it possible to receive signals from many different sources simultaneously, each with different frequency ranges, bandwidths, and waveforms.
This is not only due to the extremely flexible RFE, but also because of high sample-rate ADCs and DACs and the FPGA-based digital backend that is able to perform heavy DSP computations through several parallel channels with minimum latency. These transceivers can also easily be integrated into existing networks through Ethernet connections, making it easier to interface with other SGS equipment and external networks.
The FPGA provides high throughput with very low latency, which enables the transmission of a huge amount of information to the host with minimum data loss. In addition to their flexibility in terms of input sources, SDRs also provide significant benefits in terms of signal processing capabilities.
For example, they can perform custom DSP operations from pulse compression and data packaging to beamforming and specialized filtering, allowing the processing of complex signals that may require very particular processing techniques. This is extremely important in the context of SGSs for electronic warfare, where SDRs can be used to detect and analyze signals from hostile sources and counteract them automatically.
The high dynamic range of SDRs provided by their high-resolution ADC and linear amplifiers allows for the capture and processing of weak signals that may be difficult to detect using other technologies without saturating from interference and jamming.
Overall, the use of SDRs in satellite ground stations provides significant advantages in terms of flexibility, signal processing capabilities, and ease of integration with other ground station equipment when compared to traditional approaches. As such, they have become an essential tool for satellite communications in general, and their importance is likely to grow as the demand for satellite communication and data transmission increases.
Several key parameters should be evaluated when considering SDRs for satellite ground stations. One critical feature is the tuning range, which refers to the range of frequencies that the SDR can receive and transmit without manual tuning. A wide tuning range is important to allow for communication with a variety of satellites that operate in different frequency bands. Another important factor is the number of independent radio chains or channels that the SDR can handle simultaneously, which is crucial for ground stations that need to communicate with multiple satellites at the same time.
Instantaneous bandwidth is also a fundamental parameter, as it determines the amount of data that can be captured and processed at once and relates directly to spectrum coverage. High dynamic range and low noise figure are necessary to handle RF signals with a wide range of amplitudes, which varies greatly in satellite communications. Finally, the onboard DSP resources are critical for performing the necessary processing on the captured data and guarantee proper host connection and communication protocol. This includes basic operations, such as channelization, filtering, mixing, and data packaging, and complex algorithms, such as frequency hopping, beamforming/beam-steering, and neural networks. By evaluating these features, SGS operators can select the most appropriate SDR for their needs.
Satellite ground stations are critical components of the global communication infrastructure, supporting a wide range of applications that vary from basic SATCOM to remote sensing, scientific research, and EW. The equipment used in SGSs can vary significantly depending on the specific application, with some requiring many independent radio chains, others requiring both transmit and receive functionality, and others requiring high bandwidth radio receivers.
As ground station services become more accessible through software as a service (SaaS), even greater innovation and new applications will occur in the near future. The use of advanced technologies, such as SDRs, has revolutionized the way satellite ground stations operate. The wideband tuning capabilities, high sample rates, and on-board DSP resources of SDRs have made them an essential tool in modern SGS systems.
These transceivers can also be integrated into existing equipment, which significantly reduces deployment time and cost of new technologies, even on legacy architectures. As the demand for high-speed data transmission and reliable communications continues to increase, the importance of SDRs will become more and more apparent, as they have not only improved the efficiency and accuracy of satellite communications but also are paving the way for new and innovative solutions in the SGS industry.