Understanding GPS Signals

GPS satellites transmit two types of radio signals: C/A-code and P-code.
Briefly, here are the uses and differences of these two types of signals.

Coarse Acquisition (C/A-code)
Coarse Acquisition (C/A-code) is the type of signal that consumer GPS units receive. C/A-code is sent on the L1 band at a frequency of 1575.42 MHz. C/A broadcasts are known as the Standard Positioning Service (SPS). C/A-code is less accurate than P-code (see the following section) and is easier for U.S. military forces to jam and spoof (broadcast false signals to make a receiver think it’s somewhere else when it’s really not). The advantage of C/A-code is that it’s quicker to use for acquiring satellites and getting an initial position fix. Some military P-code receivers first track on the C/A-code and then switch over to P-code.

Precision (P-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 (Y-code) so only special receivers can access the information. The P-code signal is broadcast on the L2 band at 1227.6 MHz.

P-code broadcasts are known as the Precise Positioning Service (PPS).

The GPS 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.

A short history of GPS

Military, government, and civilian users all over the world rely on GPS for navigation and location positioning, but radio signals have been used for navigation purposes since the 1920s. LORAN (Long Range Aid to Navigation), a position-finding system that measured the time difference of arriving radio signals, was developed during World War II.

The first step to GPS came way back in 1957 when the Russians launched Sputnik, the first satellite to orbit the Earth. Sputnik used a radio transmitter to broadcast telemetry information. Scientists at the Johns Hopkins Applied Physics Lab discovered that the Doppler shift phenomenon applied to the spacecraft — and almost unwittingly struck gold.

A down-to-earth, painless example of the Doppler shift principle is when you stand on a sidewalk and a police car speeds by in hot pursuit of a stolen motorcycle. The pitch of the police siren increases as the car approaches you and then drops sharply as it moves away. American scientists figured out that if they knew the satellite’s precise orbital position, they could accurately locate their exact position on Earth by listening to the pinging sounds and measuring the satellite’s radio signal Doppler shift. Satellites offered some possibilities for a navigation and positioning system, and the U.S. Department of Defense (DoD) explored the concept. By the 1960s, several rudimentary satellitepositioning systems existed. The U.S. Army, Navy, and Air Force were all working on independent versions of radio navigation systems that could provide accurate positioning and allweather, 24-hour coverage. In 1973, the Air Force was selected as the lead organization to consolidate all the military satellite navigation efforts into a single program.

This evolved into the NAVSTAR (Navigation Satellite Timing and Ranging) Global Positioning System, which is the official name for the United States’ GPS program. The U.S. military wasn’t just interested in GPS for navigation. A satellite location system can be used for weapons-system targeting. Smart weapons such as the Tomahawk cruise missile use GPS in their precision guidance systems. GPS, combined with contour-matching radar and digital image-matching optics, makes a Tomahawk an extremely accurate weapon. The possibility of an enemy using GPS against the United States is one reason why civilian GPS receivers are less accurate than their restricteduse military counterparts.

The first NAVSTAR satellite was launched in 1974 to test the concept. By the mid-1980s, more satellites were put in orbit to make the system functional. In 1994, the planned full constellation of 24 satellites was in place. Soon, the military declared the system completely operational. The program has been wildly successful and is still funded through the U.S. DoD.

What Is 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.
Other satellite Global Positioning Systems are either in orbit or planned, but this book uses the term GPS for the Global Positioning System operated by the United States government.

Digital Orthophoto Quadrangle (DOQ)

Digital Orthophoto Quadrangle (DOQ) data consists of a computer-generated image of an aerial photograph. The image is corrected so that camera tilt and terrain relief don’t affect the accuracy. DOQs combine the image characteristics of a photograph with the geometric qualities of a map. The USGS has DOQs available for the entire United States. Most are grayscale, infrared photos; there are higher-resolution color photos for a few large U.S. metropolitan areas.

A booming business provides high-resolution, color aerial photographs to individuals, government agencies, corporations, and educational and nonprofit organizations. Companies like AirphotoUSA (www.airphotousa.com), Keyhole (www.keyhole.com) and DeLorme’s TopoBird subsidiary (www.topobird.com) provide imagery with quality and resolution that’s close to what was only available to intelligence agencies. If you want aerial photographs for business or government purposes, check these commercial sources.

Who is Mr. Sid?

This question is more accurately stated as what is MrSID? MrSID is the Multi-Resolution Seamless Image Database. It’s a file format used for distributing large images over networks, originally developed by a company called LizardTech. Graphics in MrSID format are compressed with a lossless compression algorithm (a method of compressing data that guarantees the original data can be restored exactly) designed to produce relatively small, high-resolution images. The file format is perfect for aerial and satellite images that have large file sizes, and the government is increasingly using it for distributing data. (The Library of Congress is even using it for electronic versions of paper documents.) A number of free viewers support MrSID; use Google to find download sites. (One of my favorites is IrfanView, which is available at www.irfanview.com.)

Digital Raster Graphics (DRG)

Digital Raster Graphics (DRG) data is a scanned image of a USGS topographic map. These digital maps are available for free on the Internet or are sold commercially in collections on CDs or DVDs.
These digital maps are scanned at 250 dpi (dots per inch) and stored in a TIFF file format, using embedded GeoTIFF (geographic information) tags for location data.
You can view the map by itself or both the map and its location data. Use one of the following methods:

• View the map by opening the DRG file with any current graphics program that supports large TIFF files.
• Use the DRG file with a mapping program that supports GeoTIFF to view the map and access its location data.

For more technical details about USGS digital map data, check out the agency’s product Web site:
mapping.usgs.gov/products.html