When centimetres matter, high-precision GNSS receivers are the solution

High-precision receivers aren’t just a want these days by customers. It’s now a need for those depending on position accuracy down to the centimetre. Advances in receiver and antenna technology, and correction services like real-time kinematics (RTK) and precise point positioning (PPP), now mean you can be almost anywhere in the world and still have highly accurate positioning, navigation, and timing (PNT) at your fingertips.

How does a GNSS receiver work?

The foundation of achieving high-precision PNT lies in the receiver platform. Receivers are devices that receive radio frequency signals transmitted from global navigation satellite systems (GNSS) and process those signals to determine the user’s position, velocity, and precise time.

Satellite signals are extremely weak and travel over 20,000 km through space and the atmosphere before reaching a GNSS antenna. The antenna detects, amplifies, and converts the radio frequency signal to an electronic signal, which is then processed by the receiver to unpack, or demodulate, the navigation and timing information encoded in the signal. The receiver then uses algorithms that use the signal information to calculate the distance, or range, to the satellite and output position, velocity, attitude, and time (PVAT). Episode three of our Intro to GNSS on-demand webinars, “Reception through antennas to receivers,” explains how receivers demodulate signal data for processing.

Receivers need to track a minimum of four satellites to calculate PVAT (three signals for the x, y, and z directions and one for time). The more signals used, the more accurate the calculation becomes. So to be high-precision, receivers must be able to identify and track signals across multiple frequencies and constellations. This type of high-precision receiver is called a multi-constellation, multi-frequency (MCMF) receiver. When more satellites and signals are tracked, more data is available to use in calculations. Ultimately, more data leads to a more robust and precise PNT.

The complex math behind high-precision GNSS receivers

There is more than one way a GNSS receiver can calculate the range to the satellite. The method depends on the positioning solution type, which can be code-based or carrier-based.

Code-based ranges are calculated by multiplying the time it takes for the signal to travel from the satellite to the antenna by the speed of light. Carrier-based takes this a step further by determining the number of carrier cycles (signal waves) between the receiver and the satellite and multiplying this number by the carrier wavelength (e.g., 19 cm for L1). This enables higher precision positioning because the carrier-based range is significantly more precise than the code-based range.

Correction services close the gap for high-precision positioning

We’ve looked at the hardware and the math. Now, let’s briefly talk about the software element that ensures high-level accuracy—receivers compatible with correction services. GNSS signals have inherent errors that limit positioning to metre-level accuracy. These errors can be caused by the satellite clocks and orbits, and atmospheric delays. Correction services mitigate these errors by sending information to the receiver in real time. There are several ways these errors can be corrected, with satellite-based augmentation systems (SBAS), RTK and PPP being the leading methods.

SBAS use reference stations geographically distributed throughout the SBAS service area. These stations receive GNSS signals and forward them to the master station. Since the locations of the reference stations are accurately known, the master station can calculate corrections for the area. Corrections are uplinked to the SBAS satellite and then broadcast to GNSS receivers throughout the SBAS coverage area. User equipment receives the corrections and applies them to range calculations.

RTK is a technique that uses carrier-based ranging, which is orders of magnitude more precise than code-based positioning. RTK requires local base station infrastructure that calculates and sends corrections directly to the receiver via ultra-high radio frequency.

PPP is also a carrier-based technique that models GNSS errors to provide a high level of position accuracy from a single receiver. A PPP solution depends on a set of corrections generated from a network of global reference stations. Once the corrections are calculated, they are delivered to the end-user via satellite or over the Internet.

You can learn more about GNSS corrections in our blog “What are Corrections?”