through only 2/5ths (40%) of the time. It takes 25 full data frames to get a full almanac. Each full
frame takes 30 seconds, so 25 frames takes 12.5 minutes, which is why your manual gives 12.5
minutes as the download time. In cas! e you’re wondering, here’s what an almanac (or at least a
portion of one) looks like: Epoch: 48871.0000 MJD (almanac reference time 9-6-1992 0h
UTC); ID# Type smaxis(km) eccentri inclina rt.ascen arg.peri mean-ano Hlth 02 GP
26560.0520 0.011080 54.9026 342.9035 194.5554 224.6108 0 03 GP 26560.2633
0.013058 64.3151 063.1001 142.6658 053.7576 0 11 GP 26560.3892 0.013453 63.8026
062.4385 231.0716 209.1055 0 12 GP 26560.3892 0.012450 62.7486 299.5745 340.7176
015.4047 0 13 GP 26559.9161 0.004059 63.5554 061.4368 214.5911 099.5112 0 14 GP
26559.7802 0.004146 55.0626 165.4253 067.8533 134.7840 0 15 GP 26559.8959
0.007275 55.1120 106.2742 109.0210 264.1008 0 Got that? Once the receiver’s got it, it can
locate other satellites in the sky, download the ephemeris and other data and tell you where you
are, within a few feet or so. CLOSE ONLY COUNTS IN HORSESHOES So how accurate is
the GPS? You hear all kinds of incredible claims about the GPS being accurate enough to locate
a gnat’s ass while others say it’s only good for about 100 yards, give or take. Which is true?
Well, it depends. GPS is generally said to be available in two forms, PPS and SPS. Depending
on whose figures you want to believe, PPS or precision positioning service is accurate to about a
six-foot CEP (circular error of probability), but this mode is proprietary to the military and is
crypto-coded to keep it that way. SPS or standard positioning service is actually capable of the
same accuracy, but the Pentagon can invoke something called selective availability (SA), which,
in military jargon, can “adjust SPS resolution to any degree necessary.” SA currently degrades
the SPS accuracy to about 100 yards CEP, and the GPS usually delivers on that promise. SA,
by the way, is an intentional “dithering” of the clock accuracy and perhaps a contamination of the
ephemeris dat! a. Since the armed forces paid much of the GPS research, development and
launch costs, they insisted on having some sort of strictly military function for the system in order
to get their money’s worth. One last note about errors: I mentioned something called a
single-frequency receiver. That’s a bit misleading because I didn’t explain that GPS satellites
broadcast on two frequencies, called L1 and L2. L1 is at 1575 MHz, L2 is 1227 MHz. Military
receivers generally receive both L1 and L2, then compare the results from each to greatly reduce
the ionospheric errors affecting GPS signals passing through the atmosphere. Single frequency
receivers used by civilian aircraft and maritime traffic use a fixed mathematical model to allow for
ionospheric errors. In the proverbial nutshell, that’s how the Global Positioning System works. It’s
a great system now, and with projected improvements should be an integral part of aircraft
navigation for the next fifty years or so. “Everything You Always Wanted To Know” back issues
available: Number 1: Airborne Radar Principles Number 2: Interrogation Friend or Foe (IFF)
Number 3: Radar Fairings and Radomes Number 4: Electronic Countermeasures Number 5:
Tactical Air Navigation (TACAN) Number 6: Missile Guidance Techniques Phone the author on
(44) 86923-2222 or on Autovon 263-4612. Comments welcome. Ich speche deutsch Hablo
espa?ol