Software-Defined Radio (SDR) Creates New Possibilities for Satellite Navigation

by sales@dgaequipment.com | Sep 18, 2025 | Classroom

Software-Defined Radio (SDR) Creates New Possibilities for Satellite Navigation Systems
The Global Navigation Satellite System (GNSS) refers to a system that uses orbiting satellites to assist terrestrial devices in determining navigation information. Receivers typically use multilateration algorithms to infer their position relative to the orbiting satellites. This information generally consists of various timing and orbital parameters, from which receivers can deduce their location relative to the satellites. Although initially developed for defense purposes, the practicality of this technology has now led to its deployment in a wide range of consumer, commercial, and industrial products.

Key criteria for evaluating receiver performance include spatial accuracy, sensitivity, and integrity. This is important because GNSS satellites orbit the Earth at an altitude of approximately 20,000 kilometers, transmitting with a power between 20-240W. This corresponds to a received signal strength measured on the Earth's surface of about -130dBm (or roughly 0.05% of a cell phone signal strength). Furthermore, since signals are also transmitted on the same frequency, receivers on Earth not only need to detect the signal but also recover the encoded information to process the data.

This requires GNSS receivers to balance the competing demands of high sensitivity for weak signals while actively filtering out signals outside the specified range. The sensitivity of a receiver is a key performance indicator, relating to the minimum signal strength that can be received while still ensuring the encoded data can be acquired and decoded. Although high sensitivity is crucial for high performance, the receiver must also incorporate a method to filter incoming data. These filters are necessary to ensure the receiver is not damaged by unwanted interference and can be used to enhance the desired signal. Once the signal is received and filtered, the encoded data needs to be decoded for specific applications; this requires the receiver to have processing capabilities.

Each of the aforementioned functions is typically handled by dedicated, application-specific integrated circuits (ICs). These ICs can be used anywhere GNSS is required - from vehicle navigation to mobile phones, to logistics applications requiring location tracking. Traditional GNSS receivers are designed using these ICs, but as a result, they are often inflexible and non-upgradable, catering only to the needs of specific constellation frequencies, such as GPS L1. This presents multiple challenges and costs for those who require flexibility across multiple constellations and frequencies and wish to be able to upgrade their receivers as technology advances.

Traditional GNSS receivers are usually limited to specific constellations and, by extension, tuning ranges. However, multi-GNSS capabilities offer significant advantages when using multiple frequencies and/or constellations. More satellites not only improve the continuity and availability of the system but also reduce the time to first fix and better support operations in challenging areas, such as polar or mountainous regions, where terrain can cause visibility issues between the receiver and satellites.

The integrity of GNSS systems is far from guaranteed - these systems can be affected not only by natural sources of interference and atmospheric phenomena but also by radio interference from artificial sources. This interference can impact single or multiple frequencies due to either spurious or intentional emissions. In the case of spurious interference, receiver redundancy helps ensure proper operation.

However, traditional receivers face significant limitations when operating in intentionally contested environments, such as situations where specific frequency bands might be jammed or provide false or misleading information. These scenarios often require the receiver to identify and distinguish between spurious or spoofed emissions and the actual underlying signal. For mission-critical applications, the ability to recognize when operating in a contested environment is a fundamental requirement.

In such contexts, receiving data from multiple constellations and frequencies and cross-checking the results between expected and actual positions is an important attribute. Since traditional GNSS receivers are typically developed to operate in uncontested environments, upgrading these systems to meet this need involves significant cost and downtime. Increasingly, Software-Defined Radio (SDR) offers the capability to flexibly implement robust algorithms that can not only identify various contested environments but also successfully maintain lock and navigation information.

Software-defined radio receivers are inherently flexible and allow traditionally hardware-defined functions to be changed via software. Software-defined receiver hardware has two parts that make it an attractive GNSS receiver solution. The first is a flexible radio front end that allows users to tune to different frequencies and, in many cases, to do so simultaneously. These radio front ends can also provide analog filtering to reduce interference caused by nearby sources. If the SDR receiver has sufficient radio channels, this can be done across multiple frequencies and constellations at the same time. The second part that makes SDR receivers an attractive solution is their onboard digital signal processing (DSP) capability. Many SDRs have some form of onboard DSP that can process the received signals. This DSP can also perform additional digital filtering on the input signals to further improve quality.

Together, these capabilities provide a platform that can economically deliver the functions of traditional GNSS receivers while allowing for the use of larger bandwidths. In summary, they enable the implementation of more complex algorithms on the receiver and also provide a means to rapidly upgrade them as new processing techniques and technologies develop. These software-defined systems create a whole new set of possibilities for global navigation satellite systems and should be considered for any GNSS project.

Software-Defined Radio (SDR) Creates New Possibilities for Satellite Navigation

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