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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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MSF Radio Time Signal – What has happened?

Posted By John Hasselgren , 06 April 2020

I have three radio controlled clocks in my house, all quite old, which have performed well over many years. However, earlier this year I noticed that they were all showing slightly different times. One has an analogue display and a button that allows me to force it to check itself against the time signal (other than on the hour) and, if necessary, reset itself. The others are small digital clocks which include in their display an indication of the strength of the received radio signal. Both these clock showed that they hadn’t picked up the signal. Replacing the batteries made no improvement and it wasn’t until I placed the small digital display clocks in the garden that they were able set themselves correctly. For years all three have worked well indoors. Why the change?

Move on to today, 29th. March. Last night before going to bed I reset my wristwatch and the bedside alarm clock, both just quartz crystal mechanisms, to British Summer Time. This morning I found that none of the radio controlled clocks had reset themselves to BST, something they had always done in the past. Pressing the button on the analogue display clock made it start searching as normal, but when it stopped and reset itself it showed a time of 12:22 when the time was actually 10:38. A second attempt, and a third, again showed an incorrect time. Eventually it did manage to get the time right.

The only clock in my house to move to BST was the one controlling the heating and hot water. This isn’t radio controlled, working on neither the radio time signal nor on GNSS, because it drifts over time. Instead it has an internal calendar and once the date and time has been set manually it knows when to change between UTC and BST.

Again, placing the clocks in the garden enabled them to reset themselves. Does all this mean that the radio transmission from Anthorn has been reduced in strength?

Luckily my wristwatch agrees with internet time on my computer or I would be uncertain of the time. It tells me that it is now time to get my Sunday lunch into the oven.

Update on 18 May 2020:

Further to my post on 6th April on this subject I now have to report that my radio controlled clocks have started to behave as they should.

For a while, even placing them in the garden didn’t allow them to pick up the signal broadcast from Anthorn. Setting them manually allowed all three of them to agree but they gradually drifted and were in disagreement by a few seconds.

Today, 18th May, I suddenly noticed that all three were in agreement, although the two that have a signal strength indicator were showing no signal. I decided to check them against the pips at midday on the BBC, using FM to avoid the digital delay that makes the pips useless on other transmissions.

All three clocks and the pips coincided with one another, and then I noticed that the signal strength meters were indication a strong signal. All was now well.

But why had there been a problem? The NPL web site gave a list of scheduled maintenance times when the signal would be off air, but none of these coincided with 29th March when none of my clocks re-set themselves to BST, although one sentence states that work carried out between 28th April and 13th May had been completed early and that the service was operating normally from 4th May.

John Hasselgren.

Tags:  radio time signal 

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Note on a meeting of IMO NCSR: 15 – 24 January 2020

Posted By Administration, 27 January 2020

by Kim Fisher

The meeting of the sub-committee on Navigation, Communications, Search and Rescue was chaired by Ringo Lakeman of the Netherlands. This meeting competed his 5 year term as chairman but under exceptional arrangements it was agreed to extend his term for a further year in order to provide continuity for the proposed revision of SOLAS/IV. The meeting was also the second of the exceptional 8 day meetings agreed due to the extensive workload of this sub-committee, but it was further agreed to allow an 8 day meeting again next year.

The next meeting will need to complete the revision of Chapter IV (Radiocommunications) of the International Convention on the Safety of Life at Sea (SOLAS) and associated documents if the intended entering into force of 2024 is to be achieved. This work had been ongoing since 2009. Considerable progress was achieved and it seems hopeful that completed texts will be ready for next year. The new SOLAS facilitates the use of the Iridium satellite system as an alternative to Inmarsat but issues of interoperability between the systems and monitoring of and charging for Maritime Safety Information (MSI) broadcasts require more work.

Revised guidelines for the shore-based maintenance and annual testing of satellite emergency position indicating radio beacons (EPIRBs) were agreed and also a revised version of the SafetyNET manual. It was announced that Peter Doherty who has chaired the international SafetyNET coordinating panel for the past 17 years was standing down.

The developing ICAO work for a Global aeronautical distress and safety system (GADDS) led to a new circular on interim guidance for search and rescue services regarding implementation of autonomous distress tracking of aircraft in flight. pending revision of the International Aeronautical and Maritime Search and Rescue (IAMSAR) Manual.

The Indian regional navigation satellite system (IRNSS) was agreed to be recognised as part of the worldwide radionavigation system and performance standards were agreed for the Japanese regional satellite system (QZSS). A proposal for generic performance standards for satellite navigation systems was carried forward to the next meeting.

Revised guidelines for vessel traffic services (VTS) were agreed to bring them up to date. Work on guidelines for maritime services descriptions remains ongoing.

Safety measures for non-SOLAS ships operating in Polar waters was discussed and will be progressed further in a correspondence group to be coordinated by New Zealand. Revisions of guidelines on places of refuge for ships will be progressed further in a correspondence group to be coordinated by the UK.

A detail anomaly in the revised IMO Circular on navigation-related symbols, terms and abbreviations (SN.1/Circ.243) agreed at the last meeting led to a corrigendum removing the symbol for MSI.

Proposals to modify ECDIS requirements to accept new S-100 charts will be discussed again at the next meeting.

Amendments were agreed to the traffic separation schemes in Norway, Slupska Bank Poland and Off Ushant France together with amendments to the two way route in the Great Barrier Reef and Torres Strait Australia.

The next meeting of NCSR is planned for 10 to 19 February 2021. A meeting of the Maritime Safety Committee is planned for 13 to 22 May 2020. A meeting of the Joint IMO/ITU Experts Group is planned for 6 to 10 July 2020. A meeting of the ICAO/IMO Joint Working Group is planned for 12 to 16 October 2020 in London.

Tags:  IMO  International Maritime Organization  Maritime  Report 

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Urban Air Mobility

Posted By Administration, 11 December 2019


Graham Purchase reports on news from the Commercial UAV Show held in London in November, so standby for some more drone acronyms!

Among several short talks given at the UAV Show at the ExCel Centre, one that stood out for me was actually about ground infrastructure. It was given by Duncan Walker, MD of Essex-based Skyports Ltd. Skyports have produced the first ‘Vertiport’ comprising a small building for check-in and boarding/leaving an Autonomous Air Vehicle (AAV) and for swapping the batteries. This is combined with a short ‘taxiway’ and a platform for take-off/landing, like a helipad. The modular structure was erected by a team of 150 people in only 1 week in Singapore, for the 26th Intelligence Transport World Conference, held at Marina Bay in October.


Left: Voloport showing the taxiway and take-off landing pad, right: Inside the Vertiport

Skyports had teamed up with Florian Reuter, CEO of Volocopter GmbH, to demonstrate the concept of UAM at the conference. The vertiport, named ‘Voloport’ was opened by the EC Transport Commissioner, Violeta Bulc. About 8000 people, including representatives from EASA, the CAA and NATS, visited the Voloport during the conference. There were also 2 solo demonstration flights of the ‘Volocopter’ AAV by a test pilot, seen here near the giant Sands Hotel. The vertiport structure was dismantled after the show, ready for re-use at similar events in other ‘mega-cities’.


Left: Vertiport at night, right: Over Marina Bay

Skyports is targeting other large cities, and is working to secure sites in London, Los Angeles, Melbourne and elsewhere; using, for example, the rooftops of offices, multi-storey car parks or railway stations. They are planning both passenger ‘air-taxi’ and also cargo-delivery services. To establish these services will require buy-in from politicians and detailed work with the regulators, as well as the development of Unmanned Traffic Management (UTM) services. UTM is a new type of ATM needed for these large drones, which will have to be integrated with existing ATC systems. A number of companies are working on UTM, including Altitude Angel, whose ‘GuardianUTM’ system was demonstrated, in conjunction with NATS and other partners, during ‘Operation Zenith’ at Manchester Airport in November 1018 (For more details, see the RIN website News pages). For parcel delivery, Skyports envisages much smaller infrastructure, probably sited around the periphery of cities, or at airports, and there would be a variety of final delivery methods. AAVs will be much quieter than helicopters, but still 3-4 times faster than road travel, with prices similar to using a taxi. However, pubic acceptance of the technology will be key to its success.


Left: There are 18 electrically powered motors, right: Volocopter top view

The Volocopter demonstrator is a 2-seat aircraft with 18 rotors, and 3 independent batteries, and has a ballistic-parachute in case of emergency. The company is based in Bruchsal, Germany, and has plans involving its partner, Mercedes Benz, for mass production. Also, Volocopter has received a $55M investment from a Chinese company, and is working with a number of firms on developing delivery-drones
There are several other significant players in the UAM marketplace; here are some examples:
Airbus has developed 2 demonstrators, the ‘Vanhana’ single seat cargo delivery AAV with tandem tilting wings; it is capable of 100 kts with a payload of 50 kg. Their ‘CitiAirbus’ is a 4-seat flying-taxi AAV capable of 70kts with a duration of about 15 min.


Left: AirBus Vanhana delivery drone, right: CityAirbus

Not to be outdone, the Beijing Yi-Hang Creation Science & Technology Company has produced the Ehang air-taxi and the Falcon delivery & public security management (zoom & infra-red camera equipped) AAVs, in conjunction with ‘smart-city’, ‘smart-logistics’ and ‘aerial media’ solutions. The Falcon is already in trial service with DHL-Sinotrans in China! One can post an item in a large roadside box; the top of the box then slides open to allow a drone to take off and carry the package to a similar box near the delivery address, then a courier collects the package and completes the delivery. A control centre monitors the whole delivery process and drone operations, possibly involving 5G communications. A fully automated warehouse and sorting centre is also under development. Aerial Media includes the performance of most spectacular light shows involving hundreds of their ‘Egret’ drones flying in formation. (Videos are available on the Ehang website).


Left: Ehang air-taxi, right: Ehang Falcon DHL

Back in the UK, Bristol-based Vertical Aerospace is developing a passenger-carrying AAV, called Seraph. The Seraph prototype had its maiden flight at Cotswold Airport (Kemble) in October, and it is due to be operational by 2022.

Vertical Aerospace SERAPH

These much-hyped personal air-taxis may grab the headlines but the industry will have to overcome huge regulatory obstacles which will delay their introduction, eg in UK. However, they will be much easier to introduce in more sparsely populated and deregulated places, perhaps like Dubai or parts of the US and Australia. Meanwhile, global parcel delivery companies are lobbying hard for permission to start operations in out-of-town locations and are already calling for access to UK airspace. This was discussed at a recent GATCO-BALPA conference, where DfT and CAA officials made it clear that we can expect the lowest levels of, initially controlled, airspace to be set aside for the use of drones, within a few years. To enable the drones/AAVs to detect and avoid other traffic, both the drones and existing conventional aircraft are likely to be required to continuously transmit their GNSS-derived position using Automated Dependent Surveillance (ADS-B) technology. The AAVs will have to ‘file’ their flight plans and operate under UTM control.
On the domestic front, please note that all drones over 250 grams now have to be registered annually with the CAA, and the owners have to undertake an online test of their knowledge of drone operating laws. Also, as the final picture demonstrates, it seems that the advent of drone add-ons, or perhaps replacements for, delivery vans are closer than most people think!

UPS Van & Drone

Tags:  AAV  autonomous air vehicle  drone  UAV  unmanned aerial vehicle 

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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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