Software Defined Radio Use Case for GPS/GNSS

Software defined radios can be used for several different purposes: this article explores GPS/GNSS and how SDR can be used to great benefit.

Introduction

In this article we discuss how software defined radios (SDR) provide the radio communication capabilities of global positioning systems (GPS) and global navigation satellite systems (GNSS), and how different signals originating from the satellite constellations are used. With the increasing number of satellite constellations in orbit around the Earth, it is becoming particularly challenging to maintain radio communications.

Other articles in this series feature methods of applying SDR in RADAR, spectrum monitoring, defense, test & measurement, low latency, interoperability, and satellite industries.

What is GPS/GNSS?

GNSS refers to any satellite constellation providing positioning, navigation and timing (PNT) information to users on the Earth. An example is GPS, a market leading radio based PNT service, designed for military and civilian use.

There are other GNSS constellations, such as BeiDou, Galileo, GLONASS, NavIC, and QZSS. Having multiple satellite constellations and bands increases accuracy, redundancy, and availability. Efforts were initiated by the International Committee of GNSS Provider’s Forum to make GNSS services interoperable using L1C signals and reduce receiver costs for end-users and complexity.

Each GPS satellite in the constellation carries a set of atomic clocks to maintain high-precision time. This helps maintain the reference clock and synchronization necessary for transmitting/receiving several ranging codes and navigation messages.

What are GNSS receivers? What do they do?

One needs a GNSS receiver to interpret radio signals transmitted by GPS satellites. Advanced algorithms are applied to determine current location (eg: latitude, longitude) from the signals. The information provided by a generic receiver can be used in a wide range of applications. Most of them rely on the receiver’s navigation solution ie, receiver computed position, velocity, and time. Nowadays, receivers have been widely expanded to miniaturized platforms, chipsets, microprocessors, integrated chips, FPGAs, and handheld devices, including integration in most mobile phones.

Each GPS satellite transmits a unique pseudo random noise (PRN) code used to estimate the distance between the satellite’s antenna and the receiver’s antenna. In the most common GNSS receiver architecture, there are single-frequency receivers and multi-frequency receivers (ie, capable of receiving PNT signals from various GNSS constellations), like the diagrams shown in Figure 1.

Figure 1: Diagrams representing single and multi constellation receivers are shown.

Many chipsets and modules currently support multiple satellite constellations that offer better resilience and availability of signals in urban environments, where obstruction of signals is common.

The GNSS receivers are continuously estimating and correcting, primarily for three observable parameters: Code Delay quantifies misalignment between local PRN code replica in the receiver and incoming PRN code in radio signal from satellite. Carrier Phase measures apparent distance between satellite and the receiver based on circular polarization of electromagnetic signals. Doppler Shift reflects relative motion between satellite’s antenna and the receiver’s antenna plus a common offset proportional to receiver’s clock frequency error.

What are GNSS receivers used for?

GPS radio navigation is used in different industry sectors, including:

1. Transportation: To track rolling stock, locomotives, and rail-cars, maintenance vehicles, wayside equipment, etc.
2. Aviation: On-board aircraft to exchange exact position information with ground air traffic control. Airports leverage GPS to aid take-off, approaching and landing of aircraft.
3. Telecommunications: To locate remote assets, field force, and drive testing for improvement of coverage. GPS is also used by equipment for time synchronization.
4. Defense: To navigate drones/UAVs used in military surveillance/reconnaissance and combat, and munitions guidance for missiles, etc.

What are GNSS Augmentation Systems?

GPS signals are prone to different kinds of errors. While real-time error correction using known ground-based reference stations is common, the absolute or relative accuracy of positioning information could be enhanced further. The following categories of augmentation services are provided to accomplish this:

1. Information Augmentation by a ground-based tracking network that calculates signal error corrections and integrity information. It then broadcasts the data to users through the Internet or satellite communication channels.
2. Signal Augmentation refers to systems that provide additional ranging signals to complement those already provided by GPS.

The International Civil Aviation Organization (ICAO) classifies augmentation systems as either Satellite-based (SBAS), Ground-based (GBAS) or Airborne-based (ABAS). The diagram in Figure 2 represents different forms of augmentation systems.

Figure 2: Different types of augmentation systems are used for different purposes.

Augmentation services provide differential GPS (DGPS) corrections and integrity verification near airports that do not have instrument landing system (ILS). For instance, the Wide Area Augmentation Service is a navigation aid developed by FAA to augment GPS for higher accuracy, integrity, and availability. It enables aircraft through all phases of flight. Local Area Augmentation Service is an all-weather landing system based on DGPS corrections provided through omnidirectional VHF Data Broadcast in critical areas to the Terminal, Approach, Surface segment of the flight path.

Other places have their own augmentation systems, such as China’s BDSBAS, EU’s EGNOS, Russia’s SDCM, India’s GAGAN, Japan’s MSAS, Canada’s CDGPS, Australia’s SouthPAN, South Korea’s KASS, and Africa’s ASECNA.

How are GNSS performance metrics evaluated using Augmentation Systems?

Civil aviation has produced requirements for GNSS in terms of Required Navigation Performance (RNP) standards. It includes:

1. Accuracy: expressed in terms of NVE as difference between real position of the aircraft and position provided by airborne GPS equipment. SBAS assures compliance with respect to accuracy requirements by providing the user corrections to the satellite orbit and clock errors as well as the ionospheric propagation error.
2. Integrity: expressed in User Differential Range Error (UDRE), clock ephemeris covariance matrix, and Grid Ionospheric Vertical Error (GIVE). The Dual Frequency Range Error (DFRE) is a critical parameter in SBAS.
3. Continuity: expressed in probability that operation performance is kept over a certain period. It is the ability of the total system to perform its functions without interruption during intended operation.
4. Availability: expressed as a percentage of time that the services of the GPS are usable by the navigator within the specified coverage area. It is dependent on the physical characteristics of the environment and technical capabilities of the transmitters.

How does an SDR fit into GNSS systems?

Because SDR equipment can support receiving radio signals originating from any GNSS satellite constellation, it is flexible enough to be utilized for building a multi-constellation receiver.

What is Software Defined Radio?

An SDR platform, like the one shown in Figure 3, is a radio communication system where software performs many functions, such as mixing, filtering, modulation, etc. that were traditionally in analog hardware, and now done using digital components and embedded system on a chip (SoC) technology. It offers a longer lifespan of equipment and adaptability to changing GPS/GNSS transmission encryption standards, waveforms, protocols and more.

Figure 3: Cyan by Per Vices is an example of a software defined radio.

SDR can be thought of as programmable hardware built on low-cost general-purpose processors, various microwave chips and FPGAs. The system contains a radio front end (RFE) for a wide range of bandwidth coverage and a digital backend for processing functions. Figure 4 shows the components of an SDR platform.

Figure 4: The different components of an SDR platform will help GNSS systems function.

1. The RFE comprises multi-input multi-output (MIMO) signal transmit (Tx) and receive (Rx) functionality boards including amplifiers, modulators and demodulators, filters, ADC and DAC signal converters.
2. The digital backend comprises FPGA for various digital signal processing, such as: decoding, demodulating, error correction of GPS signals, etc.

Compared to traditional GNSS receivers, which are bulky, hardware defined, and have limited functionality, the SDR platforms are modular, compact, easy to upgrade or re-configure, and provide for much better accuracy.

Challenges of receiving GPS / GNSS RF signals

A GNSS receiver capturing RF signals must perform the following tasks continuously:

1. Acquisition determines satellites visible in the range. The receiver must estimate radio wave propagation delay and Doppler shift associated with each satellite. DFT and IDFT techniques are applied.
2. Tracking involves refining radio wave propagation delay and Doppler shift estimates and tracking changes in these features over time. DLL and PLL techniques are used. The tracking of each satellite is handled in the DSP domain. Each RF signal source is treated as a chain. This RF signal at the antenna is amplified, down converted to baseband frequency or intermediate frequency, filtered, and digitized into samples continuously and sent to a host system where tracking is possible in software.
3. Navigation is measurements from all the visible satellites in the range to estimate the receiver's position, velocity, and timing.

When a low frequency signal travels through the atmosphere, its velocity changes due to atmospheric disturbances. The GPS command center applies atmospheric models to assess frequency errors and update them from time to time. A receiver cannot distinguish direct signals from several multipaths signals. This problem must be addressed in the tracking loop using SNR, correlation methods, and wavelet filter techniques.

Different impairments can happen in GPS signal acquisition, tracking, and navigation due to the following factors:

1. Orbital Eccentricity
2. Clocks
3. Ionosphere
4. Troposphere
5. Code Multipath
6. C/A Code Noise
7. Carrier Multipath
8. L1 Carrier Noise
9. Antenna Phase Centre Variation

GPS signals are extremely weak and susceptible to interference. It can be blocked using inexpensive jammers or interference from the Ionosphere. False GPS signals can be created using simulators, spoofing a receiver into calculating wrong position.

There is also a lack of signal availability due to obstruction or signal weakness or dilution of precision, which introduces errors that occur due to relative position of satellites in 3D space. Moreover, arrangement of satellites in space affects accuracy of positioning. There is an ideal arrangement of satellites to transmit accurate signals. Typically, it involves a satellite directly overhead with others equally spaced near the horizon. For instance, overhead satellites are favorable near high rise buildings in urban areas.

Using multi-frequency GPS signal acquisition helps more efficiently with error correction. Multi-constellation GNSS signal acquisition offers redundancy, but at the cost of adding more complexity.

Where is SDR used in GPS/GNSS systems?

An SDR platform is particularly suited for the following:

1. Augmentation systems, since it is an extremely high-performance device and offers a great deal of phase stability and components that can detect the faintest signals due to high SNR and SFDR.
2. Testing of a GNSS receiver to see:
   • how SDR copes with jamming or spoofing attacks. It is vital when designing new systems. This is achievable through GPS signal simulations.
   • the simulation of a multi-constellation receiver applications (eg: self-driving cars, long-range drones) or ways to prevent spoofing/jamming attacks, etc.
3. Track & Trace solutions such as location tracking services or in safety critical navigation systems.

GNSS systems are continuously being upgraded by operators to increase accuracy and reliability. The augmentation systems undergo regular updates. Typical hardware receivers are at a disadvantage in this multifaceted, ever-changing context, whereas SDR platforms can be adapted to the new requirements.

Benefits of SDR platforms in GPS/GNSS applications

SDR can tune to multiple satellite constellations, deploy different protocols and upgrade quickly as new ones are available. Since signal processing is handled in the software instead of hardware, digital down/up conversion and filtering is possible on FPGA which provides robust and accurate reception of GNSS signals. SDR supports multiple latency and reliability configurations, easy-to-use web interface, and compatibility with USRP hardware drivers. SDR offers channel adjustable capacity. It can use many DSP channels to communicate with multiple satellites simultaneously. For example, Per Vices’ Cyan SDR offers flexibility to extend capacity and support a maximum 64 channels. SDR supports filtering/adjacent channel rejection.

Real-Time Kinematic (RTK) positioning is applied in the field of surveying which requires higher precision. RTK uses a single fixed base station and rover mobile stations to reduce position error. The base station transmits correction data to rovers. Instead of code-based positioning, it relies on carrier-based ranging techniques. To correct common errors in the current satellite navigation system, it relies on carrier-phase corrections. CORS is a network of RTK base stations that broadcast corrections usually over the Internet to improve over false initialization of a single base station.

Conclusion

Using flexible, programmable, and relatively low-cost hardware, the SDR offers benefits to application builders and users based on PNT services offered by GNSS constellations. One gains full control of innovations and adaptation since the changes in software is completely in your hand and the RF hardware provided in a single piece of equipment that is powerful enough to manage any level of complexity and digital signal processing. You can make almost everything possible in radio communication using SDR.

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