Test finds LightSquared plan interferes with GPS

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Federal regulators have renewed allegations that the mobile Internet service LightSquared interferes with military and aviation operations in what experts say is a blow to the fledgling business.

The fresh test results had been a last-ditch chance for Reston, Va.-based LightSquared to prove that its satellite service was safe. But the results confirmed findings that the network would interfere with key Global Positioning System technology used to steer planes and operate sensitive construction and military equipment, The Washington Post reported.

The Federal Communications Commission is a key regulator of the telecommunications industry and plays an important role in shaping U.S. technology policy.

Later today, the company is expected to announce its business plans in the wake of the report.

Many in the recreational boating industry had feared that LightSquared’s satellite service also would be a hindrance to boaters.

This summer, BoatUS delivered more than 15,000 comments from concerned boaters, sailors and anglers to the FCC, asking the agency to protect the future reliability of GPS across the United States.

“We hope these 15,000 comments indicate to the FCC the critical need of having a reliable navigation system, not just for boaters and anglers, but for pilots, drivers, outdoor adventurers and first responders. It is unimaginable that the federal government — the guardian of the bandwidth — would consider approving a proposal with so many problems and grave public safety consequences,” BoatUS president Margaret Podlich said at the time the comments were delivered.

Some government officials said LightSquared’s problems didn’t seem fixable.

“There appear to be no practical solutions or mitigations that would permit the LightSquared broadband service, as proposed, to operate in the next few months or years without significantly interfering with GPS,” deputy secretary of defense Ashton Carter and deputy secretary of transportation John Porcari wrote in a letter. The officials head the interagency National Space-Based Positioning, Navigation and Timing group.

Their conclusion, after several months of testing, will put off the company’s attempts to gain license approval by the FCC to light up its satellite network and begin selling broadband Internet service that would compete with AT&T, Verizon and T-Mobile, the newspaper reported.

In a recent statement, LightSquared urged the FCC and the National Telecommunications and Information Administration “to retake the lead on government testing for GPS filtering solutions after a series of actions by federal agencies have demonstrated bias and inappropriate collusion with the private sector as reported by numerous media outlets, including Politico, PC World and Reuters.”

Test results must be re-evaluated by unbiased officials and engineers, the company said. Testing must proceed in cooperation with all parties to ensure that effective and appropriate guidelines are in place.

“LightSquared intends to protect its legal rights in order to ensure that fairness, transparency and the rule of law are guiding the testing process,” it said in a statement. “LightSquared has faith that, in the end, a fair process will prove that the technological solutions it has put forward will clear the way for hundreds of millions of Americans to get the wireless broadband competition they crave.”

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4 comments on “Test finds LightSquared plan interferes with GPS

  1. Keith Peshak

    LightSquared, or any competitor using near frequency strong terrestrial transmission, will produce an off-band signal that will be received by the GPS receiver because of the bandwidth of the GPS receiver “patch” antenna. This is called “quality factor” or “Q” and is a measure of center frequency divided by 3db bandwidth. Unfortunately, GPS uses a high frequency, and Q diminishes with frequency (why radios and TVs all use a lower frequency IF stage, to get better transmitter station selectivity, and do not count on the antenna or RF stage for much help there).

    GPS receivers can use a DSP computer to, and we will use the analogy of a noise canceling headset, make an interference signal “anti-noise” to add into the desired signal with noise, to remove the noise from the desired signal. Unfortunately, the signal must also be heard in the signal plus noise coming from the antenna to the DSP computer input.

    Design assumption: the filter between the patch antenna and the first LNA RF amplifier inside the GPS antenna must reduce the interference signal amplitude down to equal to or lesser than the desired GPS signal before the DSP computer can be effective at eliminating the noise.

    If you have zero loss at the GPS bandpass, how much LightSquared signal attenuation do you need over the entire noise spectrum bandpass? Let’s do a “back of the envelope” engineering guesstimate:

    You need to reduce the power spectral density of the noise to or below the power spectral density of the signal. Power spectral density is measured in watts per square foot.

    Watts (the undesired is stronger, goes proportionally):

    db = 10 log Pi/Pr
    How much does LightSquared transmit – don’t know, keeps changing.
    How much is the effective radiated power of a GPS satellite – there is a spec for that.

    db = 10 log 15,750/300
    db = 17.2

    Square Feet (the undesired is closer, goes inverse square proportionally):

    db = 10 log Rr**2/Ri**2
    How far away is LightSquared – assume 1000 feet
    How far away is a GPS satellite – assume 11,000 miles x 5,280 feet per mile
    Surface of a sphere is 4 x Pi x radius squared
    The 4 and the Pi factor out

    db = 10 log (11,000 x 5,280)**2 / 1,000,000
    db = 95.3

    Attenuation required = 17.2 + 95.3 = 112.5db

    If you have zero loss at the GPS bandpass, how much LightSquared signal attenuation do you need over the entire noise spectrum bandpass? Let’s do this another way, using “the other side of the envelope” engineering numbers, but where do we get those:

    Power level and orbital lifetime limit

    Which limits have been exceeded on the majority of satellites

    Which is why it is not working well

    But we knew well in advance that GPS was cratering

    GPS is supposed to be at around -130dBm on the ground, might want to expect less.

    LightSquared’s lower, re-re-revised (they’re going to turn it way up later, they admit) power level is -30dBm “near” their tower. Don’t expect a definition of “near”, and keep in mind that LightSquared keeps changing numbers.

    Attenuation required = -30dbm – (-130dbm) = 100db

    But what does this mean:

    db is a log scale so 10db is a factor of 10, 20db is 100, 30db is 1000, 40db is 10,000, 50db is 100,000…

    100db is, everybody get, 10**10 = LightSquared 10,000,000,000 times stronger than GPS, might want to expect stronger

    112.5db is, everybody get, 10**11.25 = LightSquared 177,827,941,000 times stronger than GPS, by geometery ratio watts/foot**2

    Really really really really big! Somehow, LightSquared accusing the PNT committee of being off by 32 times doesn’t seem like much (only 15db)!

    You can adjust the number down as LightSquared offers new lower power limits, and up as they increase the power as they claim they will. And you can adjust the number up as you decide on lower effective range limits (how far from the LightSquared tower your GPS should not work). You get the idea how this is done.

    Hint, the antenna design will buy you something, which is not accounted for here, but not that much.

    Hint, the DSP ability might be better than assumed, and this is the dimension where discovery and invention might make things considerably eventually maybe conceivably a little better far into the future.

    Hint, the configuration might be changed to allow multiple lesser attenuation filters in series, perhaps separated by amplifiers, but no amplifier can be allowed to saturate from the off-band interference signal, and “sneak paths” for the strong interference signal around filter segments, such as in the power circuitry for the amplifiers, or the printed circuit board dielectric, will easily destroy this topology option.

    Anyway you approach the problem of separating a small signal, like a flashlight on the moon, from a large signal, like a nuclear bomb detonation from 1000 feet away, you are going to have a big engineering challenge ahead of you. With LightSquared and every one else who wants to repurpose spectrum from satellite transmission to terrestrial transmission. Good luck, because we need a PNT system, and Loran has been terminated. It will probably be some other country’s navigation satellite system which their military controls (Glonass (Russia), Compass (China), Galileo (European)).

    eLoran was better than GPS for accuracy and for availability, only cost $12M/yr to operate, could not be jammed, worked where GPS did not anywhere within the confines of the 50 states and coastal waters.

    That was an M, not a B.

    A billion seconds ago, it was 1959.

    A billion minutes ago, Jesus was alive.

    A billion hours ago, our ancestors were living in the stone age.

    A billion days ago, no bi-ped walked on the earth.

    A billion dollar infusion is immediately necessary to restore the GPS satellite constellation to have all satellites become within service life limit constraints.

    eLoran should be turned back on again (tell the Department of Homeland Security, they are demonstrably not paying attention), because our modern infrastructure cannot operate without a PNT system – everything from telephones to the electric power grid to banking.

    Government, unfortunately, is run predoninently by two groups of professionals, MBAs and lawyers. Since when has any MBA or lawyer ever had a decent detailed understanding of how anything works?

  2. tigerpilot

    LightSquared needs to pack up their tent and go away. They bought a piece of spectrum that was intended for satellite use, where signals reaching ground would be weak and not interfere with navigation signals. After buying the frequency band the venture capital firm tried to bribe Obama and the FCC to allow the use of earth based transmitters.
    Hopefully the FCC will put this issue to bed once and for all. With the Defense Department opposing the licensing there is little chance any ground based use will ever be permitted.


    I’m so glad you clarified all the discussion. Now everyone will understand exactly what the problems are including the govenrnment MBAs and lawyers.

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