What to look for in an antenna for High Precision GNSS.
The Secrets of High Precision
When it comes to GNSS (Global Navigation Satellite System) technology, accuracy is everything. But different applications interpret accuracy differently, and interpretation is what will make the selection process for the right antenna all the more important. If you’re reading this, it’s likely because you’re embarking on a new project where you’ve been tasked with improving the location accuracy of a product, and you have questions around understanding what is different about an antenna for high precision GNSS, and why it’s important. While standard GNSS antennas are great for everyday navigation apps (and are somewhat trivial to develop products around), high-precision applications like surveying and agriculture require a completely different breed.
Standard GNSS Antennas: The Stalwart of Navigation
Without boring you with the maths, in its simplest form a standard precision GNSS receiver needs to simply decode a “message” from each satellite, and ensure it sees enough of them to allow it to triangulate a location. The primary distinction being the receiver only cares about the contents of the “message”. As long as the antenna can see the satellites (i.e. there is adequate signal strength) of the various constellations (there’s a lot of them), the very best precision achievable from a standard receiver is going to approach ~2.5m (precision approaching ~1m can be achieved with some augmentation services, but beyond the scope of this article). This “poor” precision can be attributed to many factors, but the biggest influence comes from the Earth’s atmosphere, with delays and distortions occurring as the GNSS signal passes through our ionosphere and troposphere. As such, any additional improvements in antenna performance is unable to correct for them, and is unlikely to yield any improvement in the precision of the receiver.
High Precision GNSS Antennas: Enhanced for Centimetre Precision
At the get-go, for applications demanding pinpoint accuracy, you’re likely to encounter terms and acronyms like RTK (Real-Time Kinematic), PPP (Precise Point Positioning), SSR (State Space Representation), SBAS and GBAS (Satellite and Ground Based Augmentation Systems), and many more. These acronyms define types of correction or augmentation services that can be used to improve precision, and you’ll choose one of these to deliver the desire level of precision. While understanding them is an important part of your product development, explaining how they facilitate improving precision falls outside the scope of this article, suffice to say that the antenna characteristics described here are equally beneficial for any correction service methodologies.
To the question posed by this article though, and to achieve High Precision, an antenna needs some special features:
Multi-Frequency Reception: Antennas used for standard precision GNSS typically observe a single frequency band. Antennas used for High Precision GNSS by contrast, typically observe multiple frequencies, and the essence of this article is to convey why this is important. I mentioned earlier in this article how the Earth’s atmosphere distorts the satellite signals. The peculiarity here (and the importance of multiple frequencies) is that the distortion of the signals at different frequencies is a well understood phenomenon. Using this phenomenon, the various signals can be compared with each other, calculations (or corrections) can be applied to remove errors, and precision can be improved. The same peculiarity can be applied to multi-path corrections, where the behaviour of reflected signals can be compared and used to mitigate inaccuracies that it induces.
Phase Centre Variation Minimization: Imagine the antenna as the bullseye on a dartboard. The exact point where the satellite signal is received is called the phase centre, and would be the reference point that identifies the object’s location. In any antenna, this point will shift slightly depending on the angle of the incoming signal, and the frequency of the signal. A high quality antennas is meticulously designed to minimize this variance, often achieving a variance of 3mm or less. A low PCV is important in measuring or resolving the “phase” of the carrier signal of each satellite, and is one of the critical aspects that differentiates an antenna for high precision GNSS projects.
Low Axial Ratio: Axial Ratio correlates with how the antenna performs in the presence of reflections (often referred to as multi-path). In combination with the multi-frequency aspect described above, an antenna with lower Axial Ratio will perform better at ignoring the reflected signal.
Other antenna parameters also determine applicability for use in high precision GNSS applications, and other articles published on this site go into a lot of detail to explain their significance. But the premise of this article is to articulate how the methodology differs to achieve high precision, and this difference in methodology predicates the need of high quality GNSS antennas. As such, and if your next GNSS project is looking for precision approaching 1cm, please ensure you’re selecting an antenna that creates the opportunity for this to occur.
