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Iowa farmer crusades on LightSquared-GPS issue

The battle between GPS manufacturers and users, and wireless broadband company LightSquared, continues. 

In the meantime, Cleghorn, Iowa farmer Tom Oswald is doing what he can to inform his fellow farmers about the implications of LightSquared’s plan and the impact it could have on auto-guidance and precision farming.

Oswald, who is a director at large and treasurer of the Iowa Soybean Association, discussed those concerns in an interview with Brownfield’s Ken Anderson.

AUDIO: Tom Oswald (8:12 MP3)

 

  • Tom Oswald is right. Wish we had FAA and DHS and FCC people with his level of understanding. Let’s do some engineering to aid everybody’s understanding (it’s a dry read, but worth the effort):

    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
    http://gpsinformation.net/main/gpspower.htm

    Which limits have been exceeded on the majority of satellites
    http://www.glonass-center.ru/en/GPS/

    Which is why it is not working well
    http://www.raimprediction.net/ac90-100/summaries.php?id=npa_nobaro

    But we knew well in advance that GPS was cratering
    http://www.gpsworld.com/gnss-system/block-iif-follow-or-failure-7265

    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 LightSquare 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).
    http://www.loran.org/ILAArchive/ArchiveIndex.htm

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