Multiple GPS satellites

The reason why so many satellites are in orbit is to provide GPS coverage all over the world. Also, a GPS receiver needs information from several satellites to tell you where you’re located:

• 2-D: A minimum of three satellite signals is required to determine your location in two dimensions: latitude and longitude.
• 3-D: Four satellite signals are required to determine your position in three dimensions: latitude, longitude, and elevation. Based on the orbits of the satellites — and if the sky is unobstructed — at any given time, your receiver should be able to get signals from at least 6 satellites (and up to 12).

Types of NAVSTAR Satellite data

GPS units receive two types of data from the NAVSTAR satellites.

• Almanac: Almanac data contains the approximate positions of the satellites. The data is constantly transmitted and is stored in the GPS receiver’s memory.
• Ephemeris: Ephemeris data has the precise positions of the satellites. To get an accurate location fix, the receiver has to know how far away a satellite is. The GPS receiver calculates the distance to the satellite by using signals from the satellite.

Using the formula Distance = Velocity X Time, a GPS receiver calculates the satellite’s distance. A radio signal travels at the speed of light, which is 186,000 miles per second. The GPS receiver needs to know the amount of time that the radio signal takes to travel from the satellite to the receiver in order to figure out the distance. Both the satellite and the GPS receiver generate an identical pseudo-random code sequence. When the GPS receiver receives this transmitted code, it determines how much the code needs to be shifted (by using the Doppler-shift principle) for the two code sequences to match. The shift is multiplied by the speed of light to determine the distance from the satellite to the receiver. Both the satellite and the GPS receiver clocks must be synchronized for accurate measurements to take place. Because putting a $50,000 atomic clock in each GPS receiver wouldn’t make much sense, receivers use a much cheaper quartz clock that is kept up-to-date and synchronized by the satellite signals.

The GPS 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.

Understanding GPS radio signals

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. 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 (described next) and is easier to jam and spoof (to prevent GPS units from receiving signals and to broadcast false signals to make a receiver think it’s somewhere else when it’s really not). The one 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/Acode and then switch over to P-code.

Precision (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).

Satellites in GPS World

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 Earth —that is, if nothing obstructs the signals.
Three important pieces of hardware are on each GPS satellite:

• Computer: An onboard computer that controls its flight and other functions
• Atomic clock: Keeps accurate time within 3 nanoseconds (around 3-billionths of a second) A super-accurate clock is required for correctly measuring the distance between the satellite and a GPS receiver based on the amount of time it takes for a satellite signal to reach the Earth.
• Radio transmitter: Sends signals to Earth The solar-powered GPS satellites have a limited lifespan (around ten 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 don’t just help you stay found. All GPS satellites, starting with NAVSTAR 8 (in 1980) carry NUDET sensors. No, this isn’t some high-tech, pornography-detection system. NUDET is an acronym for NUclear DETonation, and GPS satellites are equipped with sensors designed to detect nuclear-weapon explosions, assess the threat of nuclear attack, and help evaluate nuclear strike damage.