A GNSS receivers’ function is to output a set of position coordinates after processing the reception of the signal broadcast by a constellation of GNSS satellites. Unofficially, the receiver’s grade used to be delineated by the accuracy that it could produce. A chipset would be capable of 5m CEP at best, while a geodetic class card could be tuned to achieve less than 1cm R95. With the advances in IC manufacturing and other innovations, however, that line is beginning to blur. In this blog post we compare these receiver grades along the signal processing chain, from reception at the antenna to a final position output.

Let’s begin with the antenna. In the geodetic class, the antenna element of the antenna connected to the receiver matters. This is the starting point where “garbage in – garbage out” is most applicable for RF designers. If you ever get the chance to look inside a geodetic class antenna, you may notice a whirlpool or vortex pattern in the element. While the fractal pattern is visually dazzling, the distance between the lines of the swirls is tuned and trialed painstakingly by RF engineers and designers to ensure the most stable phase center (the physical position in space at which the signals are received from the various azimuths and elevations of the satellites). This is the specific point of the coordinate. Manufacturers of such antennas are judged by the repeatability of the PCB routing with low tolerances.  

As for the GNSS antenna in the mobile handset, its design is dictated by the amount of space available in the handset device, trading off phase center stability for meeting signal sensitivity requirements. Stable phase center variation is predominantly wishful in this use case due to radio wavelength physics.  Furthermore, there are additional antennas in the handset that are connected to other transceivers that could interfere with GNSS reception, further fueling the chagrin of antenna designers already constrained by industrial design requirements.

Then there is the ability to track GNSS satellites. Correlation is the technique used to identify and lock onto a satellite. (Interestingly, it works very similarly to a song Identification service like Shazam and Soundhound.) In geodetic receivers, patented techniques are used singularly to lock onto satellites rejecting multipath signals to ensure a direct line of sight measurement. Consumer grade chipsets don’t have the computational resources to perform those advanced techniques and may even use a multipath signal. The shortcomings are mitigated with connectivity. Connecting to GNSS assistance services, like Location.io, to prime the receiver with information about the GNSS satellite’s location to increase its sensitivity in ranging to it.

The next part of the signal processing chain is measuring the distance to those satellites, correcting those measurements and, finally, determining an accurate location.  In the geodetic case, typically two of the exact make and model GNSS receivers with very high measurement fidelity are paired together. One is configured as a reference base station receiver and situated at an optimal vantage point to observe as many satellites as possible and generate corrections for them. These corrections are then delivered via radio modem directly to the rover receiver. The rover applies corrections from the reference base receiver based on the common satellites between them and outputs an accurate corrected position.  

The application of corrections also applies to consumer receivers. Both Geodetic and consumer grade rover GNSS receivers’ position accuracy benefits from corrections. But for consumer grade receiver use a typical user will not bother surveying the position of the base station reference receiver, or finding a survey monument to place the base station over,  let alone configure the radio modem connection. Nor will they buy a reference base station receiver, nor check the matching make and model. The device just needs a source of corrections and, similarly to how GNSS assistance is applied, what the receiver lacks in measurements fidelity is made up for in connectivity. This time, though, they have access to reference networks that supply observations to generate corrections at a wider area scale. Corrections services like Truepoint.io are ready for this.  A single base to single rover pair scales up to a reference receiver network serving many connected GNSS rover receivers.  

At the top of this post, I mentioned the delineation between consumer grade and geodetic grade is getting blurred. Developments in reference network coverage, RF ASIC design, positioning algorithms, network bandwidth and connectivity, have all made it possible for accurate positioning to almost be taken for granted. GNSS chipsets now have comparable measurements to Geodetic grade receivers when connected to assistance and corrections services to deliver an accurate position to whichever app on the handset needs it. This is not to suggest that geodetic grade receivers are outdated. In fact, they play a vital role in reference networks with their configurability and dedicated computational horsepower for the different aspects of tracking a GNSS satellite to ensure integrity. The accuracy produced by consumer grade chipsets will always rely on the observations the geodetic grade receivers produce. Albeit a little blurry, the line delineating a geodetic and consumer grade GNSS receiver turns out to be a connection between them.

Dual Frequency GNSS is considered a giant leap forward in the evolution of GNSS technology. It overcomes ranging errors generated in the Ionosphere, where a storm of electrons bend a single frequency line of sight to the satellite, adding length to the range (for the spatial thinker) or delay (for the frequency time thinker). When you observe two frequencies from the same satellite, the difference between their measured behavior and their known theoretical behavior in the vacuum of space  indicates the ranging error being caused by the Ionosphere.

A logical step forward on the path to ruling out Ionospheric ranging errors further is the introduction of a third frequency into the mix. At the time of writing, GNSS hardware manufacturers are betting on triple frequency GNSS technology lending even more clarity to the Ionospheric effect. If economics were not a constraint in building an ideal infrastructure focused solely on supporting absolute positioning performance, a gold standard like octuple GNSS frequencies would make ranging to a satellite through the Ionosphere equally as accurate as ranging to that satellite through a vacuum….Though the challenges of getting all parties to agree on how to accomplish this could dwarf the economic constraints.

But that is not the focus of this blog post…exactly. What about all of the fielded L1-only receivers deployed since GPS hit the commercial mainstream? Does Single Frequency still make sense today?  

Dual and triple frequency GNSS shipments are projected to have double-digit compound annual growth into 2025.  But the vast majority of GNSS Chipsets out there are Single Frequency L1, and dual frequency L1L2 or L1L5. Triple frequency GNSS shipments are a comparative 1.8 % sliver of the more than 2.2 billion Receivers forecasted to ship in 2025.   

Granted, these forecasts were made before the Covid pandemic and the ensuing supply chain instability that continues to plague the manufacturers of all things electronic. Certainly, there is good reason to make the most of what we have in hand now. Namely, the single frequency receiver. So, let’s examine the case for this humble yet widely fielded positioning device.

A single frequency receiver calculating an autonomous single point position can achieve an accuracy of 5m CEP, and DGNSS accuracy 1m CEP.  With PPP corrections supplied to innovative positioning algorithms, 50 cm accuracy can be established easily in under 30 seconds. With RTK Corrections—within 3km of your local base station—2 cm is possible. Comparing these figures to a multi–Frequency receiver, the expected Single point, DGNSS, PPP and RTK accuracies that are achievable are respectively 1.2m, 50 cm, 20 cm and 1 cm (with the part per million RTK baseline not degrading accuracy until you were 40 km away).  There is no question that including more frequencies helps with positioning performance. And with more satellite observations, positioning becomes more robust.

As a caveat to any GNSS performance specification, there is no way to account for every adversity in the environment. Therefore, every manufacturer of GNSS receivers can only post test results obtained in perfect conditions, with no obstructions or reflective surfaces from horizon to horizon, in all directions, using a geodetic grade antenna, and a perfectly tuned RF cable network connected to the receiver.  Testing is repeated and monitored over months to satisfy the savviest in statistical position punditry. 

The critical point here is that across all positioning techniques single frequency receivers also produce better positioning accuracy when fed corrections their positioning algorithms can consume. Obstructions to direct line of sight affect all receivers, regardless of frequency capabilities, and are mitigated with the ability to track more constellations, increasing the probability of a satellite tracking in a direct line of sight. While justifiable for some applications and even negligible to implement multi frequency, a good bet for products with integrated single frequency receivers could be an update to their firmware for better positioning by connecting to the best corrections possible.  

If the humble single frequency GNSS receivers out there can consume corrections and can be connected to the internet, there is still positioning performance to be realized.  Should your product or service fall within this category, please contact us for an expert consultation on how Rx Networks’ assistance data services can boost the performance of your current positioning engine.

Hello (again).  We’re Back!

It is 2022 and we thought it was high time we restarted our blog, beginning with an explainer video on how our GNSS data services, Location.io and TruePoint.io, enable the best positioning performance on the GNSS Chipset integrated with your device. 

Some examples of devices containing GNSS chipsets include:

• Smart phones
• Asset Trackers
• Sports and outdoor Wearables
• Drones
• Robotic Lawn Mowers
• Micro-Mobility devices
• Ride sharing and Delivery mobile phone Apps
• Telecom infrastructure
  
  …etcetera!
   

The video below expresses the process of positioning in simplified visual geometry, and highlights many of the benefits of adding GNSS data services to your connected device. While we do make reference to complex algorithms and mathematical formulas, fear not, there won’t be a test at the end of the video   

Be sure to visit again mid month when we plan to explore and simplify additional GNSS data services topics and definitions.  And, as always, please do not hesitate to contact us for more information on this video, or on any of Rx Networks’ services.