Global Positioning - GPS

GPS

GPS, the Global Positioning System, is the only system today able to show you your exact position on Earth at any time, any where, and in any weather. GPS satellites orbit 11,000 nautical miles above Earth. They are monitored continuously at ground stations located around the world. The satellites transmit signals that can be detected by anyone with a GPS receiver.

The first GPS satellite was launched in 1978. The first 10 satellites launched were developmental satellites, called Block I. From 1989 to 1997, 28 production satellites, called Block II, were launched; the last 19 satellites in the series were updated versions, called Block IIA. The launch of the 24th GPS satellite in 1994 completed the primary system. The third-generation satellite, Block IIR, was first launched in 1997. These satellites are being used to replace aging satellites in the GPS constellation. The next generation, Block IIF, is scheduled for its first launch in late 2005.

The Elements

GPS has three parts: the space segment, the user segment, and the control segment. The space segment consists of a constellation of 24 satellites plus some spares, each in its own orbit 11,000 nautical miles above Earth. The user segment consists of receivers, which you can hold in your hand or mount in a vehicle, like your car. The control segment consists of ground stations (five of them, located around the world) that make sure the satellites are working properly. The master control station at Schriever Air Force Base, near Colorado Springs, Colorado, runs the system.

A Constellation of Satellites

An orbit is one trip in space around Earth. GPS satellites each take 12 hours to orbit Earth. Each satellite is equipped with an atomic clock so accurate that it keeps time to within three nanoseconds-that's 0.000000003, or three-billionths of a second-to let it broadcast signals that are synchronized with those from other satellites.

The signal travels to the ground at the speed of light. Even at this speed, the signal takes a measurable amount of time to reach the receiver. The difference between the time when the signal is received and the time when it was sent, multiplied by the speed of light, enables the receiver to calculate the distance to the satellite. To calculate its precise latitude, longitude, and altitude, the receiver measures the distance to four separate GPS satellites.

Receivers

GPS receivers can be carried in your hand or be installed on aircraft, ships, tanks, submarines, cars, and trucks. These receivers detect, decode, and process GPS satellite signals. More than 100 different receiver models are already in use. The typical hand-held receiver is about the size of a cellular telephone, and the newer models are even smaller. The commercial hand-held units distributed to U.S. armed forces personnel during the Persian Gulf War weighed only 28 ounces (less than two pounds). Since then, basic receiver functions have been miniaturized onto integrated circuits that weigh about one ounce.

Ground Stations

The GPS control segment consists of several ground stations located around the world:

  • a master control station at Schriever Air Force Base in Colorado
  • five unstaffed monitor stations: Hawaii and Kwajalein in the Pacific Ocean; Diego Garcia in the Indian Ocean; Ascension Island in the Atlantic Ocean; and Colorado Springs, Colorado
  • four large ground-antenna stations that send commands and data up to the satellites and collect telemetry back from them

How It All Works

The principle behind GPS is the measurement of distance (or "range") between the satellites and the receiver. The satellites tell us exactly where they are in their orbits. It works something like this: If we know our exact distance from a satellite in space, we know we are somewhere on the surface of an imaginary sphere with a radius equal to the distance to the satellite radius. If we know our exact distance from two satellites, we know that we are located somewhere on the line where the two spheres intersect. And, if we take a third and a fourth measurement from two more satellites, we can find our location. The GPS receiver processes the satellite range measurements and produces its position.

GPS uses a system of coordinates called WGS 84, which stands for World Geodetic System 1984. It produces maps like the ones you see in school, all with a common reference frame for the lines of latitude and longitude that locate places and things. Likewise, it uses time from the United States Naval Observatory in Washington, D.C., to synchronize all the timing elements of the system, much like Harrison's chronometer was synchronized to the time at Greenwich.

What That Means to Us

The GPS system was developed to meet military needs, but new ways to use its capabilities for everyday life are continually being found.

GPS is helping to save lives and property across the nation. Many police, fire, and emergency medical-service units use GPS receivers to determine the police car, fire truck, or ambulance nearest to an emergency, enabling the quickest possible response in lifeor-death situations. GPS-equipped aircraft can quickly plot the perimeter of a forest fire so fire supervisors can produce updated maps in the field and send firefighters safely to key hot spots.

Mapping, construction, and surveying companies use GPS extensively. During construction of the tunnel under the English Channel, British and French crews started digging from opposite ends: one from Dover, England, and one from Calais, France. They relied on GPS receivers outside the tunnel to check their positions along the way and to make sure they met exactly in the middle. Otherwise, the tunnel might have been crooked. GPS allows mine operators to navigate mining equipment safely, even when visibility is obscured.

Remember the example of the car with a video display in the dashboard? Vehicle tracking is one of the fastest-growing GPS applications today. GPS-equipped fleet vehicles, public transportation systems, delivery trucks, and courier services use receivers to monitor their locations at all times.

Automobile manufacturers are offering moving map displays guided by GPS receivers as an option on new vehicles. The displays can be removed and taken into a home to plan a trip. Several Florida rental car companies have GPS-equipped vehicles that give directions to drivers on display screens and through synthesized voice instructions. Imagine never again getting lost on vacation, no matter where you are.

The future of GPS is as unlimited as your imagination. New applications will continue to be created as technology evolves. GPS satellites, like stars in the sky, will be guiding us well into the 21st century.

What is WAAS?

You've heard the term WAAS, seen it on packaging and ads for Garmin® and Magellan® products, and maybe even know it stands for Wide Area Augmentation System. Okay, so what the heck is it? Basically, it's a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy. How much better? Try an average of up to five times better. A WAAS-capable receiver can give you a position accuracy of better than three meters 95 percent of the time. And you don't have to purchase additional receiving equipment or pay service fees to utilize WAAS.

The US Federal Aviation Administration (FAA) and Transport Canada are developing the WAAS program for use in precision flight approaches. Currently, GPS alone does not meet the FAA's navigation requirements for accuracy, integrity, and availability. WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite.

WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.

Currently, WAAS satellite coverage is only available in North America. There are no ground reference stations in South America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system. Another benefit of WAAS is that it does not require additional receiving equipment, while DGPS does.Other governments are developing similar satellite-based differential systems. In Asia, it's the Japanese Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS users around the world will have access to precise position data using these and other compatible systems.

WAAS Update and Links

PRN-135 (INTELSAT(PanAmSat)/Galaxy-15) was taken out of "Test Mode" and placed in normal mode at 08:00 UTC on 11/9/06. PRN-135 ranging quality is set to "not monitored". PRN-135 ranging integrity will be improved to NPA quality, then PA quality as operational experience is gained and planned WAAS software upgrades are fielded during the 2nd half of 2007

PRN-138 (Telesat/ANIK-1fR) was taken out of "Test Mode" and placed in normal mode at 15:21:00 UTC on 7/13/07. PRN-138 is operating as a NPA quality ranging source.

GPS/NAVSTAR Frequencies

Each satellite transmits its navigation message with at least two distinct spread spectrum codes: the Coarse / Acquisition (C/A) code, which is freely available to the public, and the Precise (P) code, which is usually encrypted and reserved for military applications. The C/A code is a 1,023 chip pseudo-random (PRN) code at 1.023 million chips/sec so that it repeats every millisecond. Each satellite has its own C/A code so that it can be uniquely identified and received separately from the other satellites transmitting on the same frequency. The P-code is a 10.23 megachip/sec PRN code that repeats only every week. When the "anti-spoofing" mode is on, as it is in normal operation, the P code is encrypted by the Y-code to produce the P(Y) code, which can only be decrypted by units with a valid decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user.

Frequencies used by GPS include

  • L1 (1575.42 MHz): Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code, plus the new L1C on future Block III satellites.
  • L2 (1227.60 MHz): P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites.
  • L3 (1381.05 MHz): Used by the Nuclear Detonation (NUDET) Detection System Payload (NDS) to signal detection of nuclear detonations and other high-energy infrared events. Used to enforce nuclear test ban treaties.
  • L4 (1379.913 MHz): Being studied for additional ionospheric correction.
  • L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life (SoL) signal (see GPS modernization). This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2009.
  • Telemetry on 2227.5 MHz.

GeoCaching

Geocaching is an entertaining adventure game for gps users. Participating in a cache hunt is a good way to take advantage of the wonderful features and capability of a gps unit. The basic idea is to have individuals and organizations set up caches all over the world and share the locations of these caches on the internet. GPS users can then use the location coordinates to find the caches. Once found, a cache may provide the visitor with a wide variety of rewards. All the visitor is asked to do is if they get something they should try to leave something for the cache.

In The Beginning

 On May 2, 2000, at approximately midnight, eastern savings time, the great blue switch* controlling selective availability was pressed. Twenty-four satellites around the globe processed their new orders, and instantly the accuracy of GPS technology improved tenfold. Tens of thousands of GPS receivers around the world had an instant upgrade.

The announcement a day before came as a welcome surprise to everyone who worked with GPS technology. The government had planned to remove selective availability - but had until 2006 to do so. Now, said the White House, anyone could "precisely pinpoint their location or the location of items (such as game) left behind for later recovery." How right they were.

On May 3, one such enthusiast, Dave Ulmer, a computer consultant, wanted to test the accuracy by hiding a navigational target in the woods. He called the idea the "Great American GPS Stash Hunt" and posted it in an internet GPS users' group. The idea was simple: Hide a container out in the woods and note the coordinates with a GPS unit.

The finder would then have to locate the container with only the use of his or her GPS receiver. The rules for the finder were simple: "Take some stuff, leave some stuff."

On May 3rd he placed his own container, a black bucket, in the woods near Beaver Creek, Oregon, near Portland. Along with a logbook and pencil, he left various prize items including videos, books, software, and a slingshot. He shared the waypoint of his "stash" with the online community on sci.geo.satellite-nav:
N 45 17.460 W 122 24.800

 Within three days, two different readers read about his stash on the Internet, used their own GPS receivers to find the container, and shared their experiences online. Throughout the next week, others excited by the prospect of hiding and finding stashes began hiding their own containers and posting coordinates. Like many new and innovative ideas on the Internet, the concept spread quickly - but this one required leaving your computer to participate.

Within the first month, Mike Teague, the first person to find Ulmer's stash, began gathering the online posts of coordinates around the world and documenting them on his personal home page. The "GPS Stash Hunt" mailing list was created to discuss the emerging activity. Names were even tossed about to replace the name "stash" due to the negative connotations of that name. One such name was "geocaching."

The Origins!

Geocaching, first coined by Matt Stum on the "GPS Stash Hunt" mailing list on May 30, 2000, was the joining of two familiar words. The prefix geo, for Earth, was used to describe the global nature of the activity, but also for its use in familiar topics in gps such as geography.

Caching, from the word cache, has two different meanings, which makes it very appropriate for the activity. A french word invented in 1797, the original definition referred to a hiding place someone would use to temporarily store items. The word cache stirs up visions of pioneers, gold miners, and even pirates. Today the word is still even used in the news to describe hidden weapons locations.

The second use of cache has more recently been used in technology. Memory cache is computer storage that is used to quickly retrieve frequently used information. Your web browser, for example, stores images on disk so you don't have to retrieve the same image every time you visit similar pages.

The combination of Earth, hiding, and technology made geocaching an excellent term for the activity. However the "GPS Stash Hunt" was the original and most widely used term until Mike Teague passed the torch to Jeremy Irish in September 2000.

Geocaching and Radio

Is there a FRS/PMR channel to find out if other Geocachers are in the area?

Yes. Geocachers have decided on channel 2 as the primary for both FRS and GMRS, and 12 as the alternate FRS (Family Radio Service) channel and 8 for the alternate GMRS (Europe). FRS and GMRS radios are longer distance walkie talkies, like the Motorola Talkabout.

Geocach.com

For the first few months, geocaching was confined to existing experienced GPS users who already used the technology for outdoor activities such as backpacking and boating. Most users had an existing knowledge of GPS and a firm grasp of obscure lingo like datums and WGS84. Due to both the player base and the newness of the activity, players had a steep learning curve before going out on their first cache hunt. Tools were scarce for determining whether a cache was nearby, if one existed at all.

As with most participants, Jeremy Irish, a web developer for a Seattle company, stumbled upon Mike Teague's web site in July while doing research on GPS technology. The idea of treasure hunting and using tech-gadgets represented the marriage of two of his biggest interests. Discovering one was hidden nearby, Jeremy purchased his first GPS unit and went on his first hunt the following weekend.

After experiencing the thrill of finding his first cache, Irish decided to start a hobby site for the activity. Adopting the term geocaching, he created Geocaching.com and applied his professional web skills to create tools to improve the cache-hunting experience. The cache listings were still added by hand, but a database helped to standardize the listings. Additional features, like searching for caches around zip codes, made it easier for new players to find listings for nearby caches.

With Mike Teague's valuable input, the new site was completed and announced to the stash-hunting community on September 2, 2000. At the time the site was launched there were 75 known caches in the world.

Other Related Sites

The following are links leading to GPS - GeoCaching related information.


Scanner and Radio Communications
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