Wherever You Go, There You Are.

location based services

That just happens to be the title of Jon Kabat-Zinn's 1994 book on Buddhist meditation. However, you could also apply that description to the U.S. Air Force's Global Positioning System (GPS). This technology knows exactly where you are, even if you don't.
Also referred to as Navstar, GPS has been in operation since the early 1990s. It proved invaluable initially in the Gulf War and in all military endeavors since then many times over. Not only that, GPS has become an essential part of many commercial and personal products that rely on location, position, and navigation.
Every year, GPS goes through constant updates, making it more accurate than ever before. And many low-cost GPS radio chips and navigation receivers have become available to everyone. In fact, one standout trend is to add GPS to as many handheld products as possible.
GPS IN ACTIONThe GPS system comprises a constellation of 24 operational satellites, plus at least three spares, that orbit the earth at 12,548 miles or 10,898 nautical miles (20,200 km) up with an inclination of 55° to the equator. There are six orbits with four satellites each. The rotational period is just two minutes short of 12 hours per orbit. With this arrangement, at least five to eight satellites are always "in view" anywhere on earth. On the ground in Colorado, the USAF maintains a station that monitors and controls the satellites. It ensures that each satellite maintains its position in the constellation and gets the correct position data it needs to transmit back to earth.
Each satellite carries four atomic clocks (two cesium-based and two rubidium-based) that generate dead-on accurate timing pulses. These are used as the basis for generating the signals sent to receivers on earth. Each satellite contains its own unique pseudorandom code (PRC) for differentiating itself from its neighbors.
Also, each satellite transmits what is called ephemeris information, which defines precisely where it is in orbit. Such information is translated into a ground track on earth that will identify its latitude and longitude, providing the requested location. The earth station updates the ephemeris data daily.
Each satellite transmits its PRC and ephemeris data in the microwave L band at 1575.42 MHz. This is called the L1 signal. The receiver can recognize each individual satellite by its unique PRC, just as in other direct-sequence spread-spectrum systems. The PRC is transmitted at a 1-Mbit/s rate using binary phase-shift keying (BPSK). Repeating every 1023 bits, this is called the coarse-acquisition (C/A) code.
Figure 1 shows how this code is used as the chipping code for the navigation data, which occurs at a 50-bit/s rate—yes, 50 bits per second! The navigation code contains the ephemeris data. The overall L1 signal occupies a bandwidth of about 1 MHz.
Each satellite additionally transmits an L2 signal at 1227.6 MHz. The L2 uses another 1023-bit PRC called the P-code. It occurs at a 10.23-Mbit/s rate and is used to chip the 50-bit/s navigation data. The P-code also may be encrypted, in which case it's called the Y-code. The resulting signal then modulates the 1227.6-MHz carrier and the L1 signal as well. The L2 signal is strictly for military use.
Back on Earth, a receiver picks up the signals, does a tricky triangulation calculation, and spits out time, altitude, and position data. The position information is in the form of latitude and longitude. Since time is available, it also can figure velocity. Receiver manufacturers call it PVT, or position-velocity-time output. Using fancy software and map overlays, you can generate a display that shows where you are on a detailed map, much like that used in those 1960s James Bond movies.
The most important issue in getting a GPS fix is being able to "see" the satellites. Given that they're over 12,000 miles away, you need all the signal you can get plus a good antenna and a super-sensitive radio. The only real way to get a signal is to have the antenna in clear view of the satellites. If you go inside, you'll lose the signal. That's why GPS radios only work outside or in a vehicle with a window.
Once you have a good view of the satellites, the receiver takes several minutes to lock on to one of the satellites. It then extracts the data and is passed off to another satellite in view, and again the data is taken. Next, it locks onto a third satellite, and so on. Latitude and longitude data requires three satellites. Altitude and speed calculations require four satellites.
The receiver measures the signal's time of travel from the satellite to the receiver. Knowing the speed of light or radio waves in space (slightly less than 300 million meters/s) and the precise time, it can calculate the distance to the satellite. That distance value is used in the calculations along with the other data from the satellite.
The receiver itself is the usual superhet or direct-conversion type with DSP demodulation and baseband recovery in an on-chip or external CPU. The processor, usually quite powerful, typically is a 32-bit CPU with floating point so it gets the required accuracy.