In message <m0wjQUE-0002uZC@wa4mei.radio.org> Bill Coleman AA4LR writes:
> You'll need TWO GPS antennas, since a typical GPS receiver is only
> accurate to around 300 feet. Having two antennas allows you to correct
> for the variation in GPS position that occurs from time to time. (A
> technique known as differential GPS)
Although the GPS approach to aligning the pointer on a rotator
indicator was a fine joke, I'd still like to prevent any
misunderstandings of this technology from becoming accepted wisdom.
Having two antenna's won't help as the error is not related to poor
reception but to deliberate variations (dithering) applied by DOD to
the clock time sent as part of their Commercial Service.
Real differential GPS uses a beacon station which already knows where
itself is with great precision sending out corrections (say over an
FM broadcast station's auxiliary data sub-carrier) for all the
various satellites within its own receiving range. These corrections
allow your own receiver (if it's equipped for differential working)
to correct the errors in the ranges from the three or four satellites
that were used in the calculation of its own position solution.
Differential beacons are operated by the US Coast Guard and also by
commercial firms as subscription services.
Last year VP Gore announced that these deliberate efforts to dilute
GPS's C/S precision would be removed, but he gave no date. I've seen
estimates of 2005 to 2010. So hang on to your Garmins, Magellans and
Trimbles as they're going to improve in accuracy over time.
BTW I align my bearings from received signal strength. So even if
the beam squints, the indicator points correctly to the major lobe.
On the matter of the ham on the other end of the barrel with tools, I
heard from Martin VK5GN that the prototype from which this was
plagerised was from a speech given by Gerard Hoffnung to the Oxford
Union (the debating society of Oxford University) in December 1958.
It has certainly aged better than some of us.
Regards,
--
Tony - G3SKR / W2TG email: tgold@panix.com
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