From this point forward, you're going to need a fight card to keep up with events taking place over in the LightSquared Arena. Here's an update:
December 9th, 2011: Bloomberg News reports a second round of Federal Communication Commission (FCC) mandated interference tests confirmed LightSquared's broadband system would disrupt the signal to 75% of GPS units. Go here.
December 10th, 2011: The Wall Street Journal reports LightSquared's financier, Minnesota youth hockey prodigy, Philip Falcone, is being threatened with possible civil-fraud charges by the U.S. Securities and Exchange Commission (SEC). Go here.
December 12th, 2011: Bloomberg News article fills in the details of the SEC investigation into Mr. Falcone and Harbinger Capital Partners. Go here.
December 12th, 2011: CEO of LightSquared, Sanjiv Ahuja, fires off a letter to Department of Defense Deputy Under Secretary Ashton Carter and Department of Transportation Under Secretary John Porcari (interference testing leads), demanding an investigation into the information leaked to Bloomberg News on December 9th. Go here for the letter.
December 14th, 2011: The report on the second round of interference testing is due to the Executive Committee overseeing testing.
Comment: Get Reeeaddy to RUMMMBLLLEEEE! More background information on this situation can be found in the following previous posts on this blog:
- September 19, 2011: LightSquared Turns Into a Political Dogfight
- September 13, 2011: Congressional Hearings Into LightSquared Interfernce with GPS
- July 20, 2011: Comments From the LightSquared Discussion Panel at the ESRI Survey Summit
- July 12, 2011: The LightSquared Battle Heats Up
- July 8, 2011: LightSquared GPS Update
- June 16, 2011: More on LightSquared
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).
ReplyDeleteGPS 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 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
You can adjust the number down as LightSquared offers new lower power limits, and you can adjust the number up as you decide on lower effective range limits; since 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 better.
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, will easily destroy this topology option.
Anyway you approach the problem of separating a small signal, like a flashlight beam from 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. Good luck, because we need a PNT system, and Loran has been terminated.
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