Don’t mess with my GPS


The FCC had granted a conditional waiver to LightSquared, for them to continue developing their nationwide high-speed cellphone network. Apparently LightSquared never could prove that their system wouldn’t interfere with GPS and even went so far as to suggest that it was GPS’s problem, not theirs. From here:

Testing showed that LightSquared’s signal did not bleed into the GPS band. Instead, the problem was that GPS receivers were too sensitive to filter out LightSquared’s powerful cell towers operating on nearby frequencies.

LightSquared argued that it was the GPS industry’s responsibility to build receivers that only listened to their own designated frequencies, but GPS companies argued that LightSquared was trying to build a cellphone network relying on frequencies that should only be used by satellites, which transmit much fainter signals.

I can’t imagine what it would take to develop, test, and replace every GPS system out there. The FCC is revoking the waiver, and I’m sure a number of pilots and military personnel are breathing a sigh of relief. Any thoughts from the electrical engineers out there about the fight over frequencies?

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3 Comments

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3 responses to “Don’t mess with my GPS

  1. elizzar

    i’m sure it’s obvious the whole reason for splitting the RF spectrum into frequency bands is to prevent cross-over interference. if the bands are well defined, then surely transmit and receive equipment should be well designed to only function in its designated band(s)? is it more the case that the explosion of communications demand ie. in mobile phones etc. was totally unforeseen and early GPS equipment wasn’t particularly stringently built, the problem being now ultra-high-frequency spectrum is money-making? according to wikipedia (i’m sorry!) GPS operates in a 1.1 – 1.6 Ghz band, with mobile phones generally operating below and above this band in the GHz range, depending on technology (3G, 4G etc), country / carrier, but with around 0.2 GHz difference at least.
    ………………
    ok, being curious i’ve done a little more reading and totally changed my post – it seems lightsquared are planning to operate around 1.525-1.559 GHz, or at 1.526-1.536 in a late modification. one of the main (L1) GPS signals is at 1.575 GHz transmitted from satellite. not sure the bandwidth of that signal; will be much lower power than a terrestrial transmitter. i guess it goes back to how sharply can you define this L1 frequency and make the equipment cut-off / filtered to designation.
    does seem like this could the tip of the iceberg in future as spectrum becomes more desirable and more capacity is needed, very interesting subject. sorry for waffling!

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  2. mrmacs

    I’m not an EE but…

    What I read about this issue several months ago amounts to this: LightSquared leased a section of radio frequency right next to GPS. GPS is low-powered, and LightSquared band wouldn’t interfere if used for ground to space and back as the lease required.
    But LightSquared decided that they wanted to use the band for terrestrial communications (think cell phone) as well, which required higher power, and this is what bled over into GPS territory.
    Apparently, the lease rate for ground to space is much cheaper than terrestrial, and LightSquared was trying to rig the system in their favor. And got their pee-pee whacked for their effort.
    Oh, and don’t look at the politics behind the company, that’ll really get your dander up.

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  3. LSQ had access to the upper part of the band they were assigned, and if they had adhered to that, they would have been fine. GPS operates on in two bands, L1 and L2. Both are important and would have been messed with some, with L1 being the worst affected, if I recall correctly.

    As a Surveyor, I’m glad LSQ lost this battle. an Attorney was at the American Congress On Surveying and Mapping’s conference in San Dog this last July, and he had his spin down pretty well, but a guy from Trimble Navigation nailed his hide to the wall. There was no way LSQ’s scheme was not going to interfere with GPS.

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