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Supercorrelation with Dr Ramsey Faragher - Webinar Follow Up Q&A

Posted By Administration, 01 July 2020

On 10th June 2020, Dr Ramsey Faragher presented a webinar on a new method for processing GNSS radio signals called Supercorrelation. There were so many questions during the webinar that we couldn't get to everything in the time we had so Ramsey has kindly answered the remaining questions for us below. If you didn't catch the webinar at the time you can watch it back here and then read the Q&A below.


Does supercorrelation help with low signal situations? I mean help with getting a lock in low-signal situations (only reflections available) as opposed to getting a very precise position when line of sight signal is available.

There are two parts to answering this question:
1. Supercorrelation boosts sensitivity by around 7-20dB depending on the existing capabilities and performance of the receiver that the technology is going into. We are able to confidently track signals right down to 4dBHz with a 1-second-long Supercorrelation. The longest Supercorrelators we have tested with real data were 5 seconds long. The boost in sensitivity can often allow a weak and obstructed line of sight signal to be found, even if it was not apparently present according to a standard receiver.
2. Since the question specifically states “only reflections available” I will clarify that if all of the signals detected are reflected and no LOS are available then standard Supercorrelation will not see those reflected signals at all if they are coming into the antenna from a direction that does not correspond to the location of the satellite in the sky. However performing a Skyscan reveals where all the energy is coming from, and allows you to still make use of the measurements from those reflected paths if you have the means to do so. For example you could match them to buildings around you if you have a 3D building model, and then employ shadow matching or 3DMA. If low-accuracy positioning is acceptable for the use case (e.g. confirming location indoors to simply update a weather report on a smartphone) then weak non-line of sight signals can still be used, and the receiver would be able to warn that the positioning accuracy is being limited by the use of non-line-of-sight signals.

What are the input parameters to generate the skyscan energy maps?

Skyscans are created using a bank of Supercorrelators, each tuned to couple strongly to energy coming from different azimuths and elevations. So you are solving the same problems that are described in the talk for creating a Supercorrelator, but for a whole range of possible positions for the satellite across the sky.

When will we see this in nav Receivers on ships?

We are engaged with a number of GNSS hardware manufacturers and OEMs to bring the benefits of our technology to their customers as quickly as possible. Please ask your current GNSS receiver manufacturer to let you know their current level of progress in integrating our technology into their chipset.


When will we see in likes of iPhone?

FocalPoint are currently working with a number of major smartphone chipset providers and handset manufacturers. We are hoping to see smartphone deployments of S-GNSS within the next 2-3 years depending on the different chipset cycles between different manufacturers. Apple acquired the Intel GNSS chipset at the end of 2019 so they did very recently become a company that can directly deploy S-GNSS technology themselves directly.


Is there any limitation for the supercorrelation, especially in some very poor skywindow like the urban area in Hong kong

The key requirement for Supercorrelation to function is that the receiver’s antenna must be moving through space. The minimum speed requirement is a function of the desired length of the Supercorrelator, but for our usual settings, the minimum speed required is 5cm per second, which is roughly 20x slower than walking pace.

Thank you Dr. Ramsey for the presentation, very exciting. Question about latency : what is the net effect on latency compared to conventional GNSS of longer correlation period and improved accuracy of Supercorrelation?

The current latency is 0.5 seconds for our typical settings. However in our next generation of the Supercorrelator we expect to bring this down to a few milliseconds. Note also that the latency in the position estimate for existing smartphone receivers is typically about 1 second, because they typically average together about a second’s worth of GPS measurements before providing an output to the user in an attempt to reduce errors in the navigation solution. Such averaging is not required for Supercorrelation.

In a mobile phone the clock source will not be perfectly stable due to the thermal dynamics, how does this affect super correlation over 1sec

Yes we have seen great variations in the performance of smartphone oscillators. Not just from thermal dynamics but from other unpredictable factors. In some smartphones we have tested our technology on there can be regular discontinuous jumps of 100Hz or more. We had to develop specific clock modelling and signal processing techniques to account for these problems, including what we call the Ultracorrelator, which we did not have time to cover in the talk, but the patent is in the public domain if you would like to read how that works.

Hi Ramsey. Thanks for a very clear description. Do you have a rough estimate in MIPS (or similar) of the added processing needs. I am aware that we already use a huge amount in multipath mitigation, so I am looking for the difference between S-GPS and what we save from the conventional

Yes we have a variety of tools to answer this question for each chipset company, as they all have different existing capabilities before we add in our technology. In the best cases we can actually reduce the overall processing load because of the existing code and overhead that can be removed once Supercorrelation is added. For more thorough information about your chipset in particular please do get in touch.

Delighted that FPP have been given the DofE Award! My question is whether super correlation can be switched on/off by the user?

It is unlikely that this will be a user selectable option, but it is possible in principle.

Could the same method by applied to MMS GNSS receiver?

Supercorrelation can be applied to the ranging signals of all GNSSS, on all frequencies. It could also be applied to many terrestrial radio signals too.

How does the supercorrelation differ from autocorrelation?

Autocorrelation refers to when you correlate a signal with itself, in order to study particular properties of that signal. The supercorrelation process involves changing the correlator sequence stored locally to account for a set of error sources in order to provide a better estimate of the incoming radio signal from the satellite than you get by simply using the textbook description of what has been broadcast.

When do you expect this to become available in a product for the mass market?

FocalPoint are currently working with a number of major smartphone chipset providers and handset manufacturers. We are hoping to see smartphone deployments of S-GNSS within the next 2-3 years depending on the different chipset cycles between different manufacturers.

Tags:  gnss  gps  pnt  supercorrelation  technology 

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Controversy! I’ll explain the background to the Ligado situation…

Posted By John Pottle, 24 April 2020
I’m outraged! How could they! We’ve been living here happily for years, minding our own business. We’ve based so many decisions on things we thought we could rely on. We thought this was a quiet residential community out in the so-called Mobile Satellite Services (MSS) district. And then, all of a sudden, approval has been granted for these huge noisy “new age” developments next door. It’s going to affect us, for sure. We’re a bit concerned. No, actually we’re outraged! How could they!

I feel I’m sticking my neck into the lion’s den a bit here, so let me say up-front that what follows is simply my own personal perspective on a complex situation. This blog has but one intention – to try to explain, briefly, what all the fuss is about with respect to the 16 April 2020 news about the FCC granting an application from a company called Ligado (ref: https://www.fcc.gov/document/fcc-approves-ligados-application-facilitate-5g-and-iot-services).

You could say it all started back in 1971. The International Telecommunication Union (ITU) allocated spectrum then envisaged for maritime and aviation satellite services in the L-band. This became known as the Mobile Satellite Services or MSS band. Satellite phone and data services like Inmarsat started to use some of the MSS band. GPS started to operate in part of this band, then other global satellite navigation systems (GNSS), first the Russian GLONASS, then the European Galileo and Chinese Beidou.

The thing is, by the time these satellite transmissions reach the earth they are very low power. GNSS signals are below thermal noise, but receivers can lock onto these signals. GPS and multi-GNSS receivers also have a filter to make sure that adjacent signals are rejected, only admitting the wanted in-band signals.

The thing is that these filters are not all the same. Neither are the filters perfect, in that they will reduce, or attenuate, signal power from adjacent bands but only up to a point. Generically the more expensive, and power-hungry, the filtering the better it is. However, with the adjacent bands to GPS and other GNSS systems being occupied only with satellite signals, which are all low power when they reach the earth, the filtering being used to block adjacent band signals coming into the GPS receivers works just fine.

Then, in 2010, enter a company then called LightSquared. The intention was to provide 4G communications everywhere in the USA, and with more flexibility and agility than the incumbent networks. The ability to provide services everywhere was because LightSquared had satellite capabilities. However, the majority of the services were to be provided by thousands of terrestrial transmitters operating in … yes … the MSS bands.

This was a massive change of plan from the perspective of the Positioning, Navigation and Timing communities using GNSS signals with quiet neighbours. The hitherto quiet adjacent band just below the GNSS frequency allocations was about to get very noisy indeed, with very high power terrestrial signals. With these high power signals adjacent to GPS / GNSS the filtering assumptions that had worked hitherto, with quiet neighbours, were no longer adequate. Although the GPS / GNSS receivers were attenuating the lower adjacent band spectrum, the filters were neither sharp nor deep enough. The LightSquared terrestrial MSS transmissions would be admitted to the GPS / GNSS receivers and, effectively, act as a jammer to GPS and other GNSS systems. The effect of this was to either cause problems with performance or, in many cases, stop the GPS / GNSS receivers working completely.

LightSquared was relying on a 2004 approval from the Federal Communications Commission (FCC) in the United States that authorised use of ground-based transmitters as so-called auxiliary terrestrial components (ATC) in the MSS band. The MSS band had always allowed for the possibility of fill-in ATC’s, which was the basis of the FCC’s 2004 approval.

The purpose of this article is not to take any view on the rights and wrongs of this case. Suffice it to say that they are extremely complex and involve large investments and lawyers as well as test results and lobbying groups. What I will say is that it’s readily possible to make strong and compelling cases for or against on all sides of this debate.

It’s no surprise, then, that what happened next in this situation was also quite complex. In January 2011 LightSquared received a conditional authorisation to operate. One year later, in February 2012, the FCC suspended the authorisation, citing interference and disruption concerns to satellite navigation services (a LOT of testing had been done and results presented on both sides). Later in 2012 LightSquared filed for Chapter 11 bankruptcy.

However, three years later the company was re-formed, with new financing, and, from February 2016, a new name: Ligado.

Fast forward to 16 April 2020 and the FCC unanimously voted to grant Ligado a licence modification that allows them to operate. An internet search on “Ligado FCC decision” will reveal the reactions. You will see words like “misguided” and “harm” and “rebuke” along with conspiracy theories and related allegations.

Irrespective of where this all ends up in future, I hope this blog provides some context, particularly to the nature of the adjacent band interference problem which is at the heart of the past and ongoing issues.

John Pottle / 24 April 2020

Tags:  GNSS  GPS  L-band  ligado  Mobile Satellite Services 

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What is spoofing and how to ensure GPS security?

Posted By Administration, 03 October 2019

As technological advances make GPS/GNSS* devices more affordable, our lives are becoming increasingly dependent on precise positioning and timing. Industries such as survey, construction and logistics rely on precise positioning for automation, efficiency and safety. GNSS time provides the pulsating heartbeat for the backbone of our industry by synchronizing telecom networks, banks and the power grid. A single day of GNSS outage is estimated to cost 1 billion dollars in US alone (1). GNSS is a reliable system, and to keep it as such professional GNSS receivers need to be wary of all possible vulnerabilities which could be exploited. Using GNSS receivers which are robust against jamming and spoofing is key for secure PNT (Positioning, Navigation and Time).

*GNSS refers to the constellations of satellites broadcasting signals from space that transmit positioning and timing information to GNSS receivers on Earth. The receivers then use this information to determine their location. These systems include the American GPS, European Galileo, Russian GLONASS, Chinese BeiDou, Japanese QZSS (Michibiki) and the Indian NAVIC system. "

What is GPS/GNSS spoofing?

Radio interference can overpower weak GNSS signals, causing satellite signal loss and potentially loss of positioning. Spoofing, is an intelligent form of interference which makes the receiver believe it is at a false location. During a spoofing attack a radio transmitter located nearby sends fake GPS signals into the target receiver. For example, a cheap SDR (Software Defined Radio) can make a smartphone believe it’s on Mount Everest!

Figure 1: cheap SDR (Software Defined Radio) can overpower GNSS signals and spoofs a single-frequency smartphone GPS into believing it is on Mount Everest.


Why GPS spoofing?

Imagine a combat situation. Clearly, the side which uses GPS/GNSS technology would have an advantage over the side which does not. But what if one side could manipulate GPS receivers of their adversary? This could mean taking over control of autonomous vehicles and robotic devices which rely on GPS positioning. For example, in October 2018, Russia accused the US of spoofing a drone and redirecting it to attack a Russian air base in Syria(2).

Figure 2: GNSS spoofing could be used to manipulate movement of aerial drones.

In the last 3 years over 600 incidents of spoofing have been recorded in the seas near the Russian border. These ships appeared to be “transported” to nearby airports (3). This type of spoofing might have been introduced as a defense mechanism to ground spy drones. Most semi-professional drones on the market have a built-in geo-fencing mechanism which lands them automatically if they come close to airports or other restricted areas (4).

Some of the most enthusiastic spoofers are Pokémon GO fans who use cheap SDRs (Software Defined Radios) to spoof their GPS position and catch elusive pokémon without having to leave their room.

Types of Spoofing

Spoofers overpower relatively weak GNSS signals with radio signals carrying false positioning information. There are two ways of spoofing:

Rebroadcasting GNSS signals recorded at another place or time (so-called meaconing)
Generating and transmitting modified satellite signals

Spoof-proof: how to protect your receiver against spoofing?
In order to combat spoofing, GNSS receivers need to detect spoofed signals out of a mix of authentic and spoofed signals. Once a satellite signal is flagged as spoofed, it can be excluded from positioning calculation.

There are various levels of spoofing protection that a receiver can offer. Let’s compare it to a house intrusion detection system. You can have a simple entry alarm system or a more complex movement detection system. For added security you might install video image recognition, breaking-glass sound detection or a combination of the above.

Like a house with an open door, an unprotected GNSS receiver is vulnerable to even the simplest forms of spoofing. Secured receivers, on the other hand, can detect spoofing by looking for signal anomalies, or by using signals designed to prevent spoofing such as Galileo OS-NMA and E6 or the GPS military code.

Advanced interference mitigation technologies, such as the Septentrio AIM+, use signal-processing algorithms to flag spoofing by detecting various anomalies in the signal. For example, a spoofed signal is usually more powerful than an authentic GNSS signal.

AIM+ won’t even be fooled by an advanced GNSS signal generator: Spirent GSS9000. With realistic power levels and with actual navigation data within the signal, AIM+ can identify it as a “non-authentic” signal.

Other advanced anti-spoofing techniques such as using a dual-polarized antenna are being researched today, read more about this method here.

Satellite navigation data authentication

Various countries invest in spoofing resilience by building security directly into their GNSS satellites. With OS-NMA (Open Service Navigation Message Authentication), Galileo is the first satellite system to introduce an anti-spoofing service directly on a civil GNSS signal.

OS-NMA is a free service on the Galileo E1 frequency. It enables authentication of the navigation data on Galileo and even GPS satellites. Such navigation data carries information about satellite location and if altered will result in wrong receiver positioning computation. While currently in development, OS-NMA is planned to become publicly available in the near future. Also GPS is experimenting with satellite based anti-spoofing for civil users with their recent Chimera authentication system.

Figure 3: European Galileo satellites provide an open authentication service on the E1 signal and a commercial authentication service on the E6 signal. Picture, courtesy of the European Space Agency.

Recently, within the scope of the FANTASTIC project led by GSA, OS-NMA anti-spoofing protection was implemented on a Septentrio receiver.

The strongest shield: signal-level GNSS authentication

The Galileo system will be offering Commercial Authentication Service (CAS) on the E6 signal with the highest level of security for safety-critical applications such as autonomous vehicles. The signal level encryption will be based on similar techniques as the military GPS signals. Only the receivers who have the secret key are able to track such encrypted signals. The secret key is also needed to generate the signal making it impossible to fake. CAS authentication techniques are currently being prototyped at Septentrio in collaboration with the European Space Agency.

Spoof-resilient GNSS means reliable precise positioning and timing, and a peace of mind for everyone touched by this indispensable technology.


1. arstechnica.com/science/2019/06/study-finds-that-a-gps-outage-would-cost-1-billion-per-day

2. rntfnd.org/2018/10/26/russia-claims-us-spoofed-drones-to-attack-base/

3. gps.gov/governance/advisory/meetings/2018-12/goward.pdf

4. gpsworld.com/spoofing-in-the-black-sea-what-really-happened/

5. Technical paper by Septentrio - Authentication by polarization: a powerful anti-spoofing method

6. insidegnss.com/new-report-details-gnss-spoofing-including-denial-of-service-attacks/


This blog is courtesy of Septentrio. See more at www.septentrio.com

Tags:  gnss  gps  resilience  spoofing 

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An iPad Nightmare

Posted By David Broughton, 28 August 2018


Fly/Sail is always the most-fun weekend of the year, and this year it was going to be at Solent Airport and the Hornet Services Sailing Club, Gosport. A handful of fliers and sailors gather at Saturday lunchtime, with the sailors being flown around the local area in the afternoon and then, after a very sociable evening, the sailors accommodating the fliers overnight and taking them for a sail the next morning. All head homewards with huge grins after Sunday lunch.

But, flying from Conington, Peterborough, meant that I had a good hour’s flight to Solent, having to pass by Luton and Heathrow airports amongst a handful of other restricted areas. The aircraft I was to hire was a beautiful-looking and modernised Piper PA-28, with drooping wingtips. I had previously looked carefully at its navigation fit: it had a couple of VOR receivers and an elderly GPS with no graphics.


So I bit the bullet and decided that, as an almost-octogenarian, I would treat myself to an iPad and navigation software. I needed one with GPS, so checked with the experts at the Mac store in Cambridge – yes, all new iPads have GPS they confirmed. I decided to buy it on offer from the largest department store in Cambridge, whose expert also confirmed that the iPad 6th Generation 32GB for £319 ‘has GPS’.

I had also had an offer of free SkyDemon for a month, so I took that up; it works well on the iPad but, sadly, not on my Mac desk- or lap-tops. So I put the track (Conington-Cranfield-Woodley NDB-Solent) directly into the iPad; it was incredibly easy to do and let me play with height to avoid airspace infringements. As well as a very usable chart, it produced an excellent flight-plan, complete with many useful frequencies, for viewing or printing.


By now I had been to AeroExpo and bought a smart knee-pad from Pooley’s to hold the iPad. So, to convince myself that all was working, I tried it in the car, with my wife driving of course. But as soon as I went into SkyDemon navigate mode, I received a warning that I could only undertake a 30-minute flight under the free trial. Wow, thank goodness I had given it a try – had I only read the instructions with the trial I would have known that. But the navigation in the car seemed to work well; the aircraft symbol tracked us around and aligned itself with heading. So I paid £12 for a month of trial that would allow SkyDemon to work as a useful navigator.

On the Saturday morning, with the aircraft full of fuel and overnight kit, we taxied out at Conington. My co-pilot was a lapsed PPL who, between take-off and landing, was happy to hold height and heading, allowing me to devote time to the iPad strapped to my knee. As soon as we headed south, I opened-up the iPad on my knee – to be greeted with the message ‘Current Location Not Available’. The beast obviously had no intention of navigating, so I threw it onto the back seat and scrambled for my chart and printed pilot-log. Thankfully, I had prepared a 250k chart with the track and timing marks and the flight continued as an unexpected and sweaty map-reading exercise. Thankfully, we made it to Solent with no infringements, thanks in part to a very helpful Farnborough Lower Airspace Radar Service (LARS).

It turns out that the iPad that I had bought, in spite of assurances from Mac and department store staff, has no GPS (or any other GNSS). I can only assume that it worked during my car trial by using the car’s Bluetooth and inbuilt navigation. I spoke to a handful of fliers at Solent, who confirmed what I had just discovered about my iPad; a couple showed me their small Bluetooth GNSS receivers which, at around £90, had resolved the problem for them completely by pairing with the iPad and allowing it to navigate.

So a lesson or two learnt: make sure that free software trials fully do what you need; don’t glibly believe what sales staff assure you about iPads; and, most importantly, ensure that you have a properly-prepared paper chart and hard-copy flight-plan at your fingertips... and brush-up by reading the Institute’s booklet on ‘Infringement Avoidance’.

Tags:  Aircraft  Fly/Sail  GPS  Navigation Software 

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