I dealt with an outfit many years ago that worked with "differential GPS",
where an outfit in Houston (John Chance) would determine the difference and
then broadcast a correction on FM broadcast stations. One of the applications
was for crop duster aircraft, so that they could fly swaths without the need
for the flag waver on the ground.
It was a nice system until they were overseen by a bunch of arrogant
Honeywell rejects who decided to remove the overage in the ground links. One
day the entire system failed in Nebraska and the venture flopped. I
encountered those Honeywell rejects later and they forced that place into
failure as well.
The flag wavers on the ground are far more reliable.
>
>The antenna on that GPS was my brother's design. They are still using his
>antenna designs 11 years after his death as no one at Garmin has designed
>anything better. (he died of cancer at age 63). I remember him describing
>to me how he designed that particular helical antenna. We were both EEs
>and both took the same fields and waves class from the same hard professor,
>me 7 years later.
>
>Back when my brother first started designing GPSs, 1980s, it was his
>third startup with the other two founders - Garry and Ming = GarMin, the
>military dithered the signals to limit the accuracy to something like
>100m (or 100Feet ??). They have since relaxed that. The military systems
>have 10X the bandwidth and were impressed Garmin could get better
>accuracy on the commercial system. So back then, to get survey level
>accuracy, even with the dither, Garmin had two units, used the relative
>distance between them, and was accurate within inches, if my memory
>serves me correctly. But it could only do relative accuracy, not
>absolute. Not sure what could be done today with relative distance
>accuracy without the induced signal dither. Might still be the same.
>
>I am not sure if there is still some level of dither imposed by the
>military. With the military system, at 10X the bandwidth, I suspect they
>can get pretty accurate. I'm also not sure if Garmin still makes the
>survey units.
>
Chris
When the going gets weird, the weird turn pro
- Hunter S. Thompson
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