A technical discussion on the unique contribution of Supercorrelation™ to urban GNSS accuracy.

FocalPoint’s VP of Business Development shares his views on the importance of using Supercorrelation™ in combination with GNSS L1+L5 for the industry to finally be able to remove the errors caused by urban reflections.

The Unsolved Problem in GNSS

In open sky or suburban scenarios, all sources of GNSS error are typically well under control. However, in built up urban canyons, errors in the region of tens of meters are still very common. This is a result of Non-Line of Sight (NLoS) signals caused by reflections off buildings that corrupt the measurements of GNSS receivers and make inner city navigation unreliable.

L1 and the advent of L5

The L1 band (1575.42 MHz) is the legacy band that has been used to broadcast GNSS signals since GPS was born in 1973. In recent years, more complex signals were developed on L5 band (1176.45 MHz) and are currently also transmitted by most of the GNSS constellations. The advent of L5 GNSS in mass market receivers came with the promise of improved accuracy and multipath mitigation.

L5 and the “critical scenario”

The introduction of L5 in combination with L1, however, has not delivered on all of the promises that were made. L5 can be accurate to the decimeter level, providing it is tracking the Line of Sight (LoS) signal, but problems tend to occur in the “critical scenario” when the LoS is blocked and only reflections are available to the receiver. In this scenario, L5 will track the reflection extremely accurately, with the receiver taking the signal for the LoS. This is because it is the first available signal arriving at the receiver. The result can be a position fix with errors of up to tens of meters.

Let’s consider an example

Imagine you are in Times Square, New York City; there can be more buildings than sky visible – far from ideal for GNSS. Some satellites will be in Line of Sight from the receiver, but many others will be blocked and their signal will arrive to the receiver after being reflected off of the buildings. A standard GNSS L1+L5 receiver will measure the LoS signals from the unblocked satellites and the reflected signals from the blocked satellites and take all for clean LoS signals. However, the path length of the reflected signals will be longer than it should and can lead to position errors. The impact of these errors on end users are wide ranging, but include hindering critical to life safety applications , such as emergency services locating persons in need of assistance, or decreasing the efficiency of ride-hailing applications.

A standard GNSS L1+L5 receiver cannot cope alone with the challenging signal scenario that is found in a deep urban environment

The fundamental task in an urban scenario is to only lock onto and track the line of sight signals, and to ignore the non-line-of-sight signals. And for that reason, L5 alone will not improve accuracy.

In the telescope analogy, Supercorrelation™ acts as a ‘smart filter’ on the finder, showing you only LoS signals

GNSS L1+L5 receiver – the ‘telescope’ analogy

If we think of a GNSS receiver as a telescope, the L5 path and engine would be the main lens. It provides great detail, but unless you use a finder first, you will not know what it is that you are looking at through the main lens. In a standard GNSS L1+L5 receiver, L1 acts as a finder, and L5 as the main lens. L1 is very useful to acquire the signals with lower power consumption, and to tell the L5 engine where to track with full resolution. It sounds like a great plan, but the above-mentioned “critical scenario” is still not solved. The L1 finder will tell the L5 main lens to focus on the first arriving reflection wrongly assuming it is LoS, resulting in tens of meters of error. Neither L1 or L5 are able to tell LoS from NLoS.

The “critical scenario” is when LoS is blocked and only reflections are available to the receiver

Supercorrelation™ – the smart filter to only see LoS signals

These situations happen frequently in urban environments and can impact commonly used GNSS-based applications, such as navigating from A to B, adding uncertainty to already stretched emergency services, and causing problems with logistics and banking services.

FocalPoint’s innovative Supercorrelation™ provides the solution to the “critical scenario”. Supercorrelation™ is a software upgrade to existing GNSS receivers that allows them to detect when a signal is LoS, NLoS or a multipath combination and excludes the NLoS components. By upgrading your receiver with Supercorrelation™ software, you filter out the NLoS signals by angle of arrival, and are left with only LoS signals, which can be tracked with confidence. Using the telescope analogy, Supercorrelation acts like a smart filter on the finder, showing you only the objects you are looking for (LoS) and not undesirable artifacts (reflections).

Supercorrelation™ technology can be applied to both L1 and L5 signals – to effectively filter out the reflection components and leave the receiver with just the LoS. When the LoS is blocked, it would not report any signal, which avoids an error in the position fix caused by that particular satellite. Supercorrelation™ does this by using MEMs measurements and proprietary algorithms to create a ‘software antenna’ that takes advantage of the different doppler shifts between LoS and NLoS signals to boost gain in the direction of the LoS, while attenuating the NLoS signal.

Supercorrelation™ and GNSS L1+L5

With Supercorrelation™, GNSS L1+L5 receivers will filter out NLoS signals and enable confident, accurate tracking of true LoS signals. Not only will the receiver meet the promised performance improvements of L5; it will exceed them.

Click on the following links for more information on Supercorrelation and to request a demo of this innovative software.

Manuel (left) and Jez (right)

About the authors
Manuel has over 20 years experience in the GNSS industry including product line management with popular telco, smartphone and wearable brands. You can connect with Manuel on LinkedIn here.

Jez has been with FocalPoint for 2 years and has 10 years of experience in Product Development. You can connect with Jez on LinkedIn here.