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GPS Receiver

About GPS Receiver and how it works.

  1. Binu
    The main function of the GPS receiver is to receive the exact position of the receiver located on the Earth's surface. For this purpose,24 satellites are revolving around the outer space. The GPS receiver receives the data in NMEA format. The data includes the information regarding the latitudinal and longitudinal data, number of satellites, Greenwich Mean Time etc.

    The 3 Segments of GPS

    The NAVSTAR system (the acronym for Navigation Satellite Timing and Ranging, the official U.S. Department of Defense name for GPS) consists of a space segment (the satellites), a control segment (the round stations), and a user segment (you and your GPS receiver).Now let's take the three parts of the system and discuss them in more detail. Then we'll look more closely at how GPS works.
    The Space Segment

    The space segment, which consists of at least 24 satellites (21 active plus 3 operating spares) is the heart of the system. The satellites are in what's called 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 the solar energy fails (eclipses, etc.), they have backup batteries on board 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 L1, L2, etc.). Civilian GPS receivers "listen" on the L1 frequency of 1575.42 MHz in the UHF band. 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.

    To give you some idea of where the L1 signal is on the radio dial, your favorite FM radio station broadcasts on a frequency somewhere between 88 and 108 MHz (and sounds much better!). The satellite signals are also very low power signals, on the order of 20-50 watts. Your local FM radio station is around 100,000 watts. Imagine trying to listen to a 50-watt radio station transmitting from 12,000 miles away! That's why it's important to have a clear view of the sky when using your GPS.

    L1 contains two "pseudorandom" (a complex pattern of 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 in order 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 the 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.

    How does it Work?

    The GPS receiver has to know two things if it's going to do its job. It has to know WHERE the satellites are (location) and how FAR AWAY they are (distance). Let's first look at how the GPS receiver knows where the satellites are located in space. The GPS receiver picks up two kinds of coded information from the satellites. One type of information, called "almanac" data, contains the approximate positions (locations) of the satellites. This data is continuously transmitted and stored in the memory of the GPS receiver so it knows the orbits of the satellites and where each satellite is supposed to be. The almanac data is periodically updated with new information as the satellites move around.

    Any satellite can travel slightly out of orbit, so the ground monitor stations keep track of the satellite orbits, altitude, location, and speed. The ground stations send the orbital data to the master control station, which in
    turn sends corrected data up to the satellites. This corrected and exact position data is called the "ephemeris" (pronounced: i-'fe-me-res) data, which is valid for about four to six hours, and is transmitted in the coded information to the GPS receiver. So, having received the almanac and ephemeris data, the GPS receiver knows the position (location) of the satellites at all times.

    Time is of the Essence

    Even though the GPS receiver knows the precise location of the satellites in space, it still needs to know how far away the satellites are (the distance) so it can determine its position on Earth. There is a simple formula that tells the receiver how far it is from each satellite.

    Calculating position

    Your distance from a given satellite object equals the velocity of the transmitted signal multiplied by the time it takes the signal to reach you (Velocity x Travel Time = Distance).

    GPS works on the principle, called "Time of Arrival". the same basic formula to determine distance, the receiver already knows the velocity. It's the speed of a radio wave - 186,000 miles per second (the speed of light), less any delay as the signal travels through the Earth's atmosphere. Now the GPS receiver needs to determine the time part of the formula. The answer lies in the coded signals the satellites transmit. The transmitted code is called "pseudo-random code" because it looks like a noise signal. When a satellite is generating the pseudo-random code, the GPS receiver is generating the same code and tries to match it up to the satellite's code. The receiver then compares the two codes to determine how much it needs to delay (or shift) its code to match the satellite code. This delay time (shift) is multiplied by the speed of light to get the distance.

    Your GPS receiver clock does not keep the time as precisely as the satellite clocks. Putting an atomic clock in your GPS receiver would make it much larger and far too expensive! So each distance measurement needs to be corrected to account for the GPS receiver's internal clock error. For this reason, the range measurement is referred to as a "pseudo-range". To determine position using pseudo-range data, a minimum of four satellites must be tracked and the four fixes must be recomputed until the clock error disappears.

    Now that we have both satellite location and distance, the receiver can determine a position. Let's say we are 11,000 miles from one satellite. Our location would be somewhere on an imaginary sphere that has the satellite in the center with a radius of 11,000 miles. Then let's say we are 12,000 miles from another satellite. The second sphere would intersect the first sphere to create a common circle. If we add a third satellite, at a distance of 13,000 miles, we now have two common points where the three spheres intersect. Even though there are two possible positions, they differ greatly in latitude/longitude position AND altitude. To determine which of the two common points your actual position is, you'll need to enter your approximate altitude into the GPS receiver. This will allow the receiver to calculate a two-dimensional position (latitude, longitude). However, by adding a fourth satellite, the receiver can deter-mine your three-dimensional position (latitude, longitude, altitude). Let's say our distance from a fourth satellite is 10,000 miles. We now have a fourth sphere intersecting the first three spheres at one common point.

    The unit stores data about where the satellites are located at any given time. This data is called the almanac. Sometimes when the GPS unit is not turned on for a length of time, the almanac can get outdated or "cold". When the GPS receiver is "cold", it could take longer to acquire satellites. A receiver is considered "warm" when the data has been collected from the satellites within the last four to six hours. When you're looking for a GPS unit to buy, you may see "cold" and "warm" acquisition time specifications. If the time it takes the GPS unit to lock on to the signals and calculate a position is important to you, be sure to check the acquisition times. Once the GPS has locked onto enough satellites to calculate a position, you are ready to begin navigating! Most units will display a position page or a page showing your position on a map (map screen) that will assist you in your navigation.

    GPS Receiver Technology

    Most modern GPS receivers are a parallel multi-channel design. Older single-channel designs were once popular, but were limited in their ability to continuously receive signals in the toughest environments - such as under heavy tree cover. Parallel receivers typically have from between five and 12 receiver circuits, each devoted to one particular satellite signal, so strong locks can be maintained on all the satellites at all times. Parallel-channel receivers are quick to lock onto satellites when first turned on and they are unequaled in their ability to receive the satellite signals even in difficult conditions such as dense foliage or urban settings with tall buildings.