GLOBAL POSITIONING SYSTEMS, OR GPS AND RELATED TOPICS
(as presented to the Del Norte Amateur Radio Club at their monthly meeting January 3, 2002)
By Lori Bennett-Tetrick

Man has always wanted to know where he is, where he is going and precise ways to get there. How nice of the Department of Defense to spend 12 billion dollars to allow us mere mortals to be able to pinpoint our location to within a few meters.

What is GPS?
GPS is a worldwide radio navigation system formed from a constellation of 24 satellites and their ground stations. In 1970, Rockwell began researching satellite technologies for the military that would allow a single soldier the ability to autonomously determine their position within 10 to 20 meters of truth. In 1977, NTS-2 was the first space-based satellite platform to send a GPS signal to Earth. Thirteen Block I (first iteration) satellites were launched in the late '70's and early '80's (only one of which is still operational) and more than 20 Block II satellites have been launched since. As those deteriorate they will be replaced by more advanced forms of satellites by the Department of Defense. This satellite system is called NAVSTAR for NAVigation System by Timing And Ranging.

How does a GPS work?
GPSR's use triangulation, trilateration or resection to determine position. This is done by the GPS checking the time it takes a satellite signal to reach the GPS and back. Satellites and their timing methods are monitored very carefully by that satellite's Earth-side tracking station. That means that those satellites have to have extremely accurate clocks onboard and these are monitored (and corrected) by each satellite's ground station. They also have "180-day autonomy" - the capability to provide navigation data without uploads from ground stations for as long as six months.

The GPS unit on the ground is designed to make corrections for any signal degradation occurring during its passage through the atmosphere. Triangulation is achieved when the GPS unit receives solid signals from three or more individual satellites. With three signals, a GPS can pinpoint its location anywhere on Earth.

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Doing the Math
Mathematically, only three satellites are required to pinpoint a three-dimensional position. The mathematical equation required to determine one's distance from any given satellite is: Velocity multiplied by Time = Distance. Since GPS uses radio signals, the velocity is the Speed of Light (186,000 miles per second).

This also means that the clocks need to be extremely accurate. If one of the satellites being used has a clock that is only 1/1000 of a second off, that satellite's calculation would be 200 miles off. Therefore, each satellite carries a $100,000 atomic clock. Since the average mortal cannot afford that kind of technology, our GPS receivers need to get a fourth satellite reading to be truly accurate. Usually, this is not difficult in an open area (read: few trees or buildings above/around).

The microprocessors within GPSR's recognize drastic differences in signals, and are programmed to trim the signals down (subtracting or adding time) until they get an answer that allows all of the time "ranges" to intersect at one "point". This slows the operation of triangulation down. Therefore, adding one more satellite's information to the mix reduces the possibility of errors and the need for alteration of time readings.

Neither the GPS nor the satellites utilize numbers to make these calculations. They generate a very complex digital code called "Pseudo-Random Code". This makes the satellites nearly impervious to being "hacked" by anyone with hostile or mischievous intent.

Scientific and Commercial uses for GPS
GPS is no longer the exclusive bailiwick of the military. Now, geological and hydrographical assessors utilize complex GPS units, allowing extraordinarily accurate surveys of both land and sea. The recent discovery of volcanic activity off of the Northern California/Oregon shore was discovered using advanced GPS technology. There are units called "geo-caters" that are complex GPS receivers, allowing not only tracking of ocean-going vessels and large transport vehicles, but enabling communications via voice and email from a base unit to the receiving vehicle. GPS receivers are used in aviation guidance systems and in tracking endangered wildlife.

Private-use GPSRs
The average Joe can utilize GPS technology to navigate a boat on large bodies of water and avoid pre-programmed or self-programmed hazards like reefs or rocks and plot a course around them. Hunters and hikers can use a GPS to plot a path to travel, mark their better hunting/hiding spots and get back to their vehicles without being hopelessly lost, using a GPSR's "trail of crumbs", the line it draws on the display screen.

GPSR units are now built into high-end consumer products such as car navigation systems, portable PCs, cellular phones, entertainment systems, hand-held devices, even bracelets that can help recover lost children.

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APRS - Automatic Positioning and Reporting Systems
Developed by an Amateur Radio Operator, Bob Bruninga (WB4APR), APRS uses GPS-equipped stations with two-way radios to communicate, whether mobile or stationary. This seems to be a very proprietary technology.

From the Northwest APRS Website:
"The Automatic Position Reporting System (APRS) is a packet radio system used for tracking objects, including people, vehicles, boats, aircraft, manned/unmanned balloons, even cows; weather tracking systems or just about anything, using an Amateur Radio mode called Unconnected Packets, or UI."

"An APRS system typically consists of a computer (DOS 286 or better, Win95/98/NT/Me, or Macintosh color system), a TNC, and a radio (HT, mobile or desktop) operating on VHF, HF or both. A mobile APRS system is complemented with a GPS that has data output capabilities. Routine updates in position are transmitted over the air, and show up as moving symbols on a map. This is similar to commercial systems found in rental or high-priced cars, yet different because the information is transmitted over the air and displayed on computers running APRS. Fixed APRS stations with internet connectivity can also log onto one of several regional or worldwide APRS databases to see APRS in action around the globe."

Uses for APRS include routine mobile tracking (most popular among amateurs), boat, manned handheld, race and parade support, high-altitude balloon and manned balloon tracking and disaster reporting, which should be very popular with ARES members.

Manufacturers such as Kenwood have recognized the popularity of APRS and are building radios that have the TNCs and APRS software already onboard. With the popularity of PDA's (personal data assistants aka: handheld computers), an HT, GPS and computer all rolled into one unit isn't too far over the horizon.

About Geocaching
See additional article HERE

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