Global navigation satellite system (GNSS) technology has come on in leaps and bounds in recent decades. Shortly after the turn of the century, for example, we saw the time it takes to achieve an accurate initial position reading slashed from minutes to less than 30 seconds. Later that decade, we saw receiver sensitivity improve, going from 130 dBm to 167 dBm. And where the American Global Positioning System (GPS) was the only worldwide positioning satellite constellation at the turn of the millennium, it’s since been joined by the European Galileo system, as well as China’s BeiDou and Russia’s GLONASS. And that’s before you add India’s NAVIC and Japan’s QZSS regional systems to the mix. This proliferation has enabled GNSS chip-makers to build receivers that work with multiple constellations. What’s more, the satellite signals have been modernized, and this year, for the first time, multi-band GNSS is becoming affordable. All of this provides the foundations for the next key GNSS focus topic: achieving accuracy down to the decimeter- or even centimeter-level.
The accuracy challenge
To identify its position, a GNSS receiver uses triangulation, picking up its distance from four or more satellites. This distance is calculated based on the time it takes for the satellite’s signal to get to the receiver. The difficulty is that an error of even a couple of billionths of a second can have a big impact on the accuracy of the reading. If there’s an error in the orbit position of the satellite, accuracy drops by up to 2.5 meters. A satellite clock error can lead to a further drop of up to 1.5 meters. Perturbations in the troposphere and ionosphere add further accuracy losses of 1 meter and 5 meters respectively – and more when the satellite is near the horizon, or when solar activity is especially intense. However, the biggest error occurs when signals from the satellite reach the receiver indirectly, or multiple times, as can happen when they bounce off building walls. This is known as the ‘multipath’ effect.
If you’re somewhere with open skies, a standard GNSS receiver will give you a reading with accuracy down to 2 meters.