GPS For Dummies

Getting into GPS

GPS stands for Global Positioning System. A special radio receiver measures the distance from your location to satellites that orbit the earth broadcasting radio signals. GPS can pinpoint your position anywhere in the world. Pretty cool, huh? Aside from buying the receiver, the system is free for anyone.
You can purchase an inexpensive GPS receiver, pop some batteries in it, turn it on, and presto! Your location appears on the screen. No map, compass, sextant, nor sundial is required. Just like magic. It's not really magic, though, but has evolved from some great practical applications of science that have come together over the last 50 years.

How GPS works

The intricacies of GPS are steeped in mathematics, physics, and engineering, but you don't need to be a rocket scientist to understand how GPS works. GPS is composed of three parts:

• Satellites

• Ground stations

• Receivers

Eyeing satellites

In GPS jargon, a satellite is the space segment. A constellation of 24 GPS satellites (21 operational and 3 spares) orbits about 12,000 miles above the Earth. The satellites zoom through the heavens at around 7,000 miles per hour. It takes about 12 hours for a satellite to completely orbit the Earth, passing over the exact same spot approximately every 24 hours. The satellites are positioned where a GPS receiver can receive signals from at least six of the satellites at any time, at any location on the Earth (if nothing obstructs the signals).
A satellite has three key pieces of hardware:

• Computer: This onboard computer controls its flight and other functions.

• Atomic clock: This keeps accurate time within three nanoseconds (around three-billionths of a second).

• Radio transmitter: This sends signals to Earth.

GPS satellites don't just help you stay found. All GPS satellites since 1980 carry NUDET sensors. No, this isn't some high-tech pornography-detection system. NUDET is an acronym for NUclear DETonation; GPS satellites have sensors to detect nuclear-weapon explosions, assess the threat of nuclear attack, and help evaluate nuclear strike damage.

The solar-powered GPS satellites have a limited life span (around 10 years). When they start to fail, spares are activated or new satellites are sent into orbit to replace the old ones. This gives the government a chance to upgrade the GPS system by putting hardware with new features into space.
GPS satellites transmit two types of radio signals: C/A-code and P-code. Coarse Acquisition (C/A-code) is the type of signal that consumer GPS units receive. Precision Code (P-code) provides highly precise location information. P-code is difficult to jam and spoof. The U.S. military is the primary user of P-code transmissions, and it uses an encrypted form of the data so only special receivers can access the information.

Covering ground stations

Ground stations are the control segment of GPS. Five unmanned ground stations around the Earth monitor the satellites. Information from the stations is sent to a master control station — the Consolidated Space Operations Center (CSOC) at Schriever Air Force Base in Colorado — where the data is processed to determine each satellite's ephemeris and timing errors.

An ephemeris is a list of the predicted positions of astronomical bodies such as the planets or the Moon. Ephemerides (the plural of ephemeris) have been around for thousands of years because of their importance in celestial navigation. Ephemerides are compiled to track the positions of the numerous satellites orbiting the earth.

The processed data is sent to the satellites once daily with ground antennas located around the world. This is kind of like syncing a personal digital assistant (PDA) with your personal computer to ensure that all the data is in sync between the two devices. Because the satellites have small built-in rockets, the CSOC can control them to ensure that they stay in a correct orbit.

GPS receivers

Anyone who has a GPS receiver can receive the satellite signals to determine where he or she is located. Consumers can buy practical GPS receivers in sporting goods stores. They're easy to use and are mostly targeted for recreational and other uses that don't require a high level of location precision. The Big Three manufacturers in the consumer GPS market are Garmin, Magellan, and Lowrance. Consumer GPS receivers are reasonably priced, from less than $100 to $400 in the U.S.

How accurate is a GPS receiver?

According to the government and GPS receiver manufacturers, expect your GPS unit to be accurate within 49 feet (that's 15 meters for metric-savvy folks). If your GPS reports that you're at a certain location, you can be reasonably sure that you're within 49 feet of that exact set of coordinates.

GPS receivers tell you how accurate your position is. Based on the quality of the satellite signals that the unit receives, the screen displays the estimated accuracy in feet or meters. Accuracy depends on

• Receiver location

Obstructions that block satellite signals

Although survey-grade GPS receivers can provide accuracy of less than two centimeters, they are very specialized and expensive, require a lot of training, and aren't very portable. Their accuracy is achieved with DGPS and post-processing collected data to reduce location errors. The average GPS user doesn't need this level of precision.

Clouds, rain, snow, and weather don't reduce the strength of GPS signals enough to reduce accuracy. The only way that weather can weaken signals is when a significant amount of rain or snow accumulates on the GPS receiver antenna or on an overhead tree canopy.