5.2: What is GPS?
Description
This article is from the Geographic Information Systems FAQ, by Lisa Nyman lnyman@census.gov with numerous contributions by others.
5.2: What is GPS?
More information is available from:
http://www.utexas.edu/depts/grg/gcraft/notes/gps/gps.html
http://www.einet.net/editors/john-beadles/introgps.htm
Richard Langley <lang@unb.ca> maintains a
list of GPS informations sources.
From: Ed Keller <ewk@icf.hrb.com>
Greg Holmberg <greg@surfgear.com>
Don Cooke <don_cooke@gdt1.com>
GPS consists of three parts: the 24 satellites, a portable receiver, and the
control center on Earth. Each satellite carries a computer
and a very accurate atomic clock.
The control center calculates each satellite's orbit a week or
so into the future, predicts ionospheric conditions over that time, and then
uploads this information into the satellite's computer. This information is
called the "ephemeris". By consulting its clock and the ephemeris, the
satellite can tell where it is in the sky at any given microsecond during
the week. It transmits its position and the current time continuously.
Each receiver has a less accurate clock. When activated, it listens for
satellites that are scheduled to be above the horizon each satellite has its
own assigned radio frequency).It then subtracts the first received time from
the time on its internal clock. This gives a distance, which can be pictured
as a sphere around the satellite several hundred miles in diameter. The next
signal will define a similar sphere, and the intersection of the two is a
circle which passes through the Earth.
If the system were ideal, a second time and location signature from another
satellite would give a precise triangulation. In reality, three signals are
needed simultaneously for latitude and longitude, and a fourth is required
for fixing altitude in steps of less than a few hundred feet.
The reason for this apparent gross inaccuracy is a DoD policy called
"Selective Availablity". he satellites are instructed to "dither" both their
times and their locations, that is, they round off their transmissions into
less accurate steps. This is done mainly to prevent a foreign military power
from using the GPS as a free,super-accurate,preinstalled targeting computer.
There are two ways to use GPS to find your position. The first technique
(discussed above) gives instant results anywhere with just one receiver. The
penalty is the decreased accuracy. You can expect a single receiver
GPS-calculated position to be within 100 meters of where a surveyor would
place it. The best measurements are made when three satellites are 120
degrees apart around the horizon, and the fourth satellite is directly over-
head. If you desire greater accuracy, the arrival time of the signal at the
receiver may be compared with that of signals received simultaneously
at another location whose exact coordinates are known.The difference between
these two sets of data yields a location accurate to within 0.5 to 20
meters, and is known appropriately as "Differential GPS".
If you desire even greater accuracy, the underlying carrier wave of the
signals can be analyzed after capture, and compared with signals received
simultaneously at another known location. The difference between
these two sets of data yields a location accurate to within a centimeter.
Why does the GPS signal include ionospheric data when timing
data would suffice?
The ionosphere is a high-altitude layer of the Earth's atmosphere that
receives punishing amounts of solar radiation. This radiation ionizes the
gases, causing the layer to have varying permeability to radio waves. As the
thickness of the ionosphere changes, the GPS signal (coming from outside the
atmosphere is slowed briefly by a predictable amount. If this deceleration
is accounted for in the receiver, the timing skew can be subtracted, and
greater accuracy is achieved.
Further reading:
GPS Satellite Surveying by Alfred Leick (John Wiley & Sons, 1990) or
Guide to GPS Positioning by David Wells et al.
"Poor Man's Cruise" front page,Wall Street Journal, August 26, 1993
GPS World Magazine GPS Report (pricey)
P.O. Box 10460 7811 Montrose Road
Eugene, OR 97440 Potomac MD 20854
503-343-1200 301-340-2100
and an excellent primer on GPS "the new utility" from Trimble:
Trimble Navigation
645 North Mary Ave
Sunnyvale CA 94086
408-481-2994
art_lange@trimble.com
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