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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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