Brad Parkinson, the first director of the GPS Joint Program Office, has created a presentation to educate government officials and personnel about the differences between communication and navigation, and why the GPS community is so adamantly opposed to any integrity threats — and why those in government should be similarly opposed. That presentation is available here. Parkinson concludes concludes "It is inconceivable to many who understand the GPS applications and the national dependencies that the Federal Government: would seriously consider authorizing jammers that could do such harm to the country."
And yet . . . .
The presentation points out that "Integrity is the linchpin for most of the high-productivity applications, e.g.: aviation, heavy vehicle control, automatic mining, surveying, auto-farming." LightSquared threatens GPS availability, accuracy, and, most important, Integrity.
Parkinson proceeds to elucidate the difference between digital communication and digital navigation. "They are superficially similar, but there are fundamental differences that must be understood."
In digital radio communications:
• Incoming message is not known – finding it is the whole point
• Must determine whether each signal “bit” is a one or a zero
• Use sophisticated methods to correct errors
In contrast, in digital radio navigation:
• Incoming signal sequence (ones and zeros) is totally known by user
• The goal of the user is to precisely time the transition from one to zero (and zero to one)
Without noise, a narrow-band receiver would give consistent results. Unfortunately the GPS band is dominated by natural radio noise. Natural radio noise causes much uncertainty in zero crossings for narrow-band receivers
To achieve the maximum accuracy, the full-band GPS receiver has “sharper transitions," reducing the effect of noise and allowing a more precise timing measurement. Thus, the full-band GPS receiver enables sub-meter accuracy and the significant U.S. productivity gains in agriculture, construction, and machine control.
Why does high-performance GPS need full-band GPS receivers?
Using full-band, the timing uncertainty of the basic GPS ranging measurement is greatly improved. This is essential for the sub-meter accuracy that is the basis for many of the productivity-enhancing applications credited with 10s of billions of dollars in annual savings. Full-band GPS is similar to a fine telescope: without Full-Band, the signal is not well focused
Why is the GPS community so adamant
that 1 dB of additive noise (26% more noise power)
is all that can be tolerated? Because LSQ’s suggested 6dB means four times the noise power: This affects every satellite in view. Th result is significant corruption in timing the code transitions and much-reduced position accuracy. Another result may be complete loss of lock on GPS signals. It would also greatly lengthen the time to attain “First Fix.”
With double noise amplitude (6 dB more noise power),
full-band GPS receiver have significantly degraded accuracy. A recreational GPS user (cell phone) could tolerate some degradation — provided he can receive at all. The precision user has no such latitude.
Parkinson goes on to explore difference views of propagation models, and concludes: to assure navigation in the face of interference, GPS must assume the least attenuation of the interfering signal. This is contrary to a communications designer who must assume greatest attenuation. These differences are frequently at least 10 times and can be 20 times or more
Parkinson's overall conclusions:
It is inconceivable to many who understand the GPS applications and the national dependencies that the Federal Government:
• Would seriously consider authorizing jammers that could do such harm to the country
• Would place the burden on the GPS community to prove they are being harmed
• Would tolerate a private company repeatedly changing their intent and forcing the GPS user community to again re-prove the harm
Our banner should be "Do not destroy the availability and integrity of GPS. It is essential to the GPS infrastructure."
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