Different Segments of GPS
The Space Segment
The space segment, which consists of at least 24 satellites (21 actives plus 3 operating spares), is the system’s heart. The satellites are in a “high orbit” about 12,000 miles above the Earth’s surface. Operating at such a high altitude allows the signals to cover a greater area. The satellites are arranged in their orbits so a GPS receiver on Earth can always receive from at least four of them at any given time.
The satellites are traveling at speeds of 7,000 miles an hour, which allows them to circle the earth once every)’ 12 hours. They are powered by solar energy and are built to last about 10 years. If solar energy fails (eclipses, etc.), they have backup batteries onboard to keep them running. They also have small rocket boosters to keep them flying in the correct path.
Each satellite transmits low-power radio signals on several frequencies (designated LI, L2, etc.). Civilian GPS receivers “listen” to the UHF band’s LI frequency of 1575.42 MHz. The signal travels “line of sight”, meaning it will pass through clouds, glass, and plastic, but will not go through most solid objects such as buildings and mountains.
LI contains two “pseudorandom” (a complex pattern of a digital code) signals, the Protected (P) code and the Coarse/Acquisition (C/A) code. Each satellite transmits a unique code, allowing the GPS receiver to identify the signals. “Anti-spoofing” refers to the scrambling of the P-code to prevent its unauthorized access. The P-code is also called the “P (Y)” or “Y” code.
The main purpose of these coded signals is to allow for calculating the travel time from the satellite to the GPS receiver on Earth. This travel time is also called the Time of Arrival. The travel time multiplied by the speed of light equals the satellite range (distance from the satellite to the GPS receiver). The Navigation Message (the information the satellites transmit to a receiver) contains the satellite orbital and clock information and general system status messages and an ionospheric delay model. The satellite signals are timed using highly accurate atomic clocks.
The Control Segment
The “control” segment does what its name implies — it “controls” the GPS satellites by tracking them and then providing them with corrected orbital and clock (time) information. There are five control stations located around the world — four unmanned monitoring stations and one “master control station”.
The four unmanned receiving stations constantly receive data from the satellites and then send that information to the master control station. The master control station “corrects” the satellite data and, together with two other antenna sites, sends (“uplinks”) the information to the GPS satellites.
The User Segment
The user segment simply consists of you and your GPS receiver. As mentioned previously, the user segment consists of boaters, pilots, hikers, hunters, the military, and anyone else who wants to know where they are, where they have been, or where they are going.