What You Ought to Know about GPS

If someone asked the average guy on the street ten years ago what a GPS was, he would probably guess it is an obscure Television Station or perhaps a branch of the government. Today the average guy on the street could have one in his car or boat. Odds are that he will purchase one because he gets lost driving all too often and those pesky map quest directions rarely help. Does the average guy on the street know how a GPS works? More importantly, does he know why his GPS sometimes does not work? This article will answer your questions and more.

The Basics of GPS:
The GPS (Global Positioning system) consists of 24 networked, continually operating satellites placed in orbit around Earth that are in a fixed position above strategic points. They orbit the planet twice a day, are monitored, maintained and built by the United States Military. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites “visible” in the sky.

GPS Explained:
The primary role of a GPS receiver is to locate four or more of these satellites, calculate the distance to each and figure out its own location. This operation is based on a simple mathematical principle called trilateration. The following example is basic trilateration. I am typing this article in Seaford New York. If I didn’t know that I was in fact here, I would go about asking some people where I am and go from there.

If I wanted to be more accurate and find the street address I was on I could ask one or more people and draw more precise circles until I narrowed my location down to about 20 feet. Satellite positioning systems work in the same way but with spheres as their circles. Since the Satellites are in fixed positions, orbiting the earth and monitored at all times, the GPS receiver acts as the man asking his location and compares. A GPS receiver has the ability to ask up to 24 satellites, making the most accurate receivers the one with multiple channels.


So to get your exact location a GPS receiver must know the location of at least three satellites above you and the distance between itself and the satellites. A GPS receiver figures out both by analyzing low power messages emitted from the satellites in orbit. Since the waves travel at the speed of light (186,000 miles per second) and because the satellites emit these signals every day at a preprogrammed time (which the GPS receiver knows) the receiver measure the time delay from when the satellite is supposed to transmit its waves to when it is received. The length of this delay is exactly equal to the signals travel time. So the receiver, being ever so smart, multiplies this time by the speed of light to determine how far the signal has traveled. This high tech method is similar to the comparatively low tech method of counting the seconds between seeing lightning and hearing thunder to determine how far a storm is from the observer.

Another question arises out of this analogy. How do the satellites know what time it is? They aren’t on our planet!? Well the clocks in Satellites keep much better time then any Rolex. They go by Atomic clocks. Atomic clocks are accurate down to the .0000000009 second and carry a price tag from $10,000 to $20,000. A GPS receiver checks with the time value of 3 or more satellites and gets the accuracy of an atomic clock for a normal (quartz) clock price.

Location Location Location!:

How does a GPS receiver know where its big brothers in the sky are? Well, satellites travel in very predictable orbits because they are constantly monitored and adjusted by the U.S. department of Defense. The Dept. of defense transmits the new and exact location to all GPS receivers. The Gps receivers store these new locations in an electronic almanac which it updates often.

GPS does have some problems:

You are walking down Times Square, you look at your hand held Magellan and it tells you you’re on 50th street! How is this possible? Well for starters you’re surrounded by buildings hundreds and thousands of feet tall which GPS signals can bounce off of. The GPS receiver thinks the satellites are further away then they really are. Also, the earth’s atmosphere tends to slow down light. When a receiver sends out and then receives a signal its standard unit of measurement to determine the distance and location is the speed of light. Factors like smog, humidity and the Earths natural magnetosphere slow down light below the standard 186,000 miles per second. A GPS receiver can also completely lose its satellite link if it is taken underground or inside a building with an abundance of metal in its infrastructure.
Despite all these setbacks the Global Positioning System is thriving and growing with no end in sight. Recently the European Union, China, India, Israel, Saudi Arabia, Morocco and South Korea announced plans to cooperate in building Galileo, an alternative GPS system with more satellites for improved accuracy. With the advent of cellular phones being equipped with their own GPS receivers and a functional user interface which far exceeds any older models, the GPS is an infant technology with a bright and hopeful future.

Some common GPS terms:

3D Operating Mode: A three-dimensional GPS position fix that includes horizontal coordinates, plus elevation. It requires a minimum of four visible satellites.

Ambiguity: The unknown number of whole wavelengths of the carrier signal contained in an unbroken set of measurements from a single satellite at a single receiver.

Acquisition Time: The time it takes a GPS receiver to acquire satellite signals and determine the initial position.

Active Leg: The segment of a route currently being traveled. A “segment” is that portion of a route between any two waypoints in the route.

Atomic Clock: A very precise clock that operates using the elements cesium or rubidium. A cesium clock has an error of one second per million years. GPS satellites contain multiple cesium and rubidium clocks.
Azimuth: The horizontal direction from one point on the earth to another, measured clockwise in degrees (0-360) from a north or south reference line. An azimuth is also called a bearing.

Beacon: Stationary transmitter that emits signals in all directions

Bearing: The compass direction from a position to a destination, measured to the nearest degree (also call an azimuth)

Carrier Frequency: The frequency of an unmodulated output of a radio transmitter.

Cartography: The art or technique of making maps or charts.

Code Division Multiple Access (CDMA): A method whereby many radios use the same frequency, but each one has a unique code.

Clock Bias: The difference between the indicated clock time in the GPS receiver and true universal time (or GPS satellite time).
Control Segment: A worldwide chain of monitoring and control stations that control and manage the GPS satellite constellation.

Coordinated Universal Time (UTC):
Replaced Greenwich Mean Time (GMT) as the world standard for time in 1986. UTC uses atomic clock measurements to add or omit leap seconds each year to compensate for changes in the rotation of the earth.

Course Over Ground (COG): Your direction of movement relative to a ground position.

Differential GPS (DGPS): An extension of the GPS system that uses land-based radio beacons to transmit position corrections to GPS receivers. DGPS reduces the effect of selective availability, propagation delay, etc. and can improve position accuracy to better than 10 meters.

Elevation: The distance above or below mean sea level.

Ellipsoid: A geometric surface, all of whose plane sections are either ellipses or circles.

Estimated Position Error (EPE): A measurement of horizontal position error in feet or meters based upon a variety of factors including DOP and satellite signal quality.

Estimated Time Enroute (ETE): The time it will take to reach your destination (in hours/minutes or minutes/seconds) based upon your present position, speed, and course.

Estimated Time Of Arrival (ETA): The estimated time you will arrive at a destination.

Geocaching: A high-tech version of hide-and-seek. Geocachers seek out hidden treasures utilizing GPS coordinates posted on the Internet by those hiding the cache.

Geosynchronous Orbit: A specific orbit around where a satellite rotates around the earth at the same rotational speed as the earth. A satellite rotating in geosynchronous orbit appears to remain stationary when viewed from a point on or near the equator. It is also referred to as a geostationary orbit.

Latitude: A position’s distance north or south of the equator, measured by degrees from zero to 90. One minute of latitude equals one nautical mile.

Liquid Crystal Display (LCD): A display circuit characterized by a liquid crystal element sandwiched between two glass panels. Characters are produced by applying an electric field to liquid crystal molecules and arranging them to act as light filters.
Multipath Error: An error caused when a satellite signal reaches the GPS receiver antenna by more than one path. Usually caused by one or more paths being bounced or reflected.

Nautical Mile: A unit of length used in sea and air navigation, based on the length of one minute of arc of a great circle, especially an international and U.S. unit equal to 1,852 meters (about 6,076 feet).

Position: An exact, unique location based on a geographic coordinate system.

Position Fix: The GPS receiver’s computed position coordinates.

Pseudo-Random Code: The identifying signature signal transmitted by each GPS satellite and mirrored by the GPS receiver in order to separate and retrieve the signal from background noise.

Selective Availability (SA): The random error, which the government can intentionally add to GPS signals, so that their accuracy for civilian use is degraded. SA is not currently in use.

Triangulation: A method of determining the location of an unknown point, as in GPS navigation, by using the laws of plane trigonometry.

User Interface: The way in which information is exchanged between the GPS receiver and the user

Wide Area Augmentation System (WAAS): A system of satellites and ground stations that provide GPS signal corrections for better position accuracy. A WAAS-capable receiver can give you a position accuracy of better than three meters, 95 percent of the time. (At this time, the system is still in the development stage and is not fully operational.) 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.

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