Satellite navigation and Software Defined Radio

Article By : Victor Wollesen

Global Navigation Satellite Systems (GNSS) refers to systems that use satellites in orbit to assist earth bound devices in determining navigation...

Global Navigation Satellite Systems (GNSS) refers to systems that use satellites in orbit to assist earth bound devices in determining navigation information. Receivers typically use multilateration algorithms to infer their location relative to the orbiting satellites. This information typically consists of various timing and orbital parameters, from which a receiver can infer their position with respect to the orbiting satellites. While originally developed for defense purposes, the utility of this technology has now seen it deployed across a variety of consumer, commercial, and industrial products.

The original, and most well known, GNSS system is the Global Positioning System, which is owned and operated by the United States government. The impact, utility, and benefit of GPS, has spanned everything from personal navigation via cell phones, to air plane navigation, to construction surveys, and logistic. The strategic and economic importance of the system has also motivated other countries and alliances to develop their own, alternative systems, such as Galileo, Global’naya Navigatsionnaya Sputnikovaya Sistema (GLONASS) and BeiDou.

The critical criteria used to assess the performance of receivers includes spacial accuracy, sensitivity, and integrity. This is important as GNSS satellites orbit the Earth at an altitude of approximately 20,000 km with a transmission power of between 20-240W; this corresponds to measured received signal strength on the surface of the Earth of around -130dBm (or around 0.05% of the strength of a cell phone signal). Moreover, the signals are also being transmitted on the same frequency and the receivers on Earth need to not only detect the signal but they also need to recover the coded information in order to process the data.

This requires GNSS receivers to simultaneously balance the competing requirements of high sensitivity to weak signals and also aggressively filtering out of signals outside the specified range. The sensitivity of a receiver is a key metric in performance and relates to the minimum signal strength that can be received while still ensuring the encoded data can be captured and decoded. Although high sensitivity is key to high performance, receivers must also contain a method of filtering the incoming data. These filters are required to ensure the receiver is not damaged by unwanted interference and can be used to enhance the signals that are desired. Once the signal is received and filtered, the encoded data needs to be decoded for the specific application; this requires the receiver to have processing capabilities.

Each of the above functions are usually accomplished through dedicated, application specific, integrated circuits (ICs). These ICs are used everywhere GNSS is required; from vehicle navigation, to cell phones, to tracking logistics applications requiring location tracking. Traditional GNSS receivers are designed using these ICs but as a consequence are typically inflexible and not able to be upgraded resulting in the ability to only meet the needs for a specific constellation frequency, for example GPS L1. This presents multiple challenges and costs for those that require flexibility across multiple constellations and frequencies and would like the ability to upgrade their receivers as the technology advances.

Traditional GNSS receivers are often limited to specific constellations, and, by extension, tuning ranges. There are however significant benefits to multi-GNSS capabilities where multiple frequencies and/or constellations are used. Not only do more satellites improve the continuity and availability of the system, they also improve the time to first fix, and better support operation in challenging areas, such as polar or mountainous regions, where the topography causes visibility issues between the receiver and the satellite.

The integrity of GNSS systems is far from assured – not only are these systems subject to natural sources of interference and atmospheric phenomena, they are also subject to radio interference from artificial sources. This interference may affect a single or multiple frequencies, and due to spurious or intentional emissions. In the case of spurious interference, receiver redundancy helps ensure correct operation.

However, traditional receivers face serious limitations when operating within intentionally contested environments, such as those in which specific bands may be jammed, or provided with false or misleading information. These cases often demand receivers to identify and discriminate between spurious or false emissions and the actual underlying signal. For mission critical applications, being able to identify when operating in a contested environment, is an essential requirement.

In such cases, receiving data from multiple constellations and frequencies, and checking results between expected and actual position is an important attribute. As traditional GNSS receivers are generally developed for operation in uncontested environments, there is a non-trivial cost and down time associated with upgrading these systems to meet this need. Increasingly, Software Defined Radios (SDR) are providing a capable of providing the flexibility to implement robust algorithms that can not only identify various contested environments, but also successfully maintain lock and navigational information.

Software defined radio receivers are inherently flexible and allow for traditionally hardware defined functions to be now changed using software. There are two parts to the software defined receiver hardware that make them an attractive solution as a GNSS receiver. The first is the flexible radio front end, which allows users to tune to different frequencies and, in many cases, at the same time. These radio front ends can also provide analog filtering to reduce the interference caused by nearby sources. This can be done across multiple frequencies and constellations concurrently, provided the SDR receiver has enough radio channels. The second part of SDR receivers that make them an attractive solution is the on board digital signal processing (DSP) capabilities. Many SDRs have some form of DSP on board which allows for the processing of the received signals. This DSP also enables additional digital filtering to be on the incoming signal to further improve the quality.

Together, these capabilities provide a platform capable of economically providing the functionality of traditional GNSS receivers, while permitting the use of substantially larger bandwidths. Together, they allow for more sophisticated algorithms to be implemented on receivers, and also provide a means for them to be rapidly upgraded as new processing techniques and technologies are developed. These software defined systems create an entire new set of possibilities for GNSS and should be considered for any GNSS project.

— Victor Wollesen is the CEO and co-founder of Toronto-based Software Defined Radio company Per Vices Corporation.

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