1. What is GPS?
1.1 GPS Ground Control Stations
1.2 GPS Satellites
1.3 GPS Receivers
2. How does GPS signals transfer?
3. What is WAAS?
4. What is map datum?
5. Introduction to Car Navigation System

1. What is GPS?

The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. It was initiated in 1973 and the first GPS satellite was launched on 21 February 1978. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS provides accurate, three-dimensional position, velocity, and time, in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.

Currently there are two main systems in the world used for Global Positioning and navigation, that of the United State’s NAVSTAR GPS and that of the Russians, called GLONASS (Global Navigation Satellite System). Due to the inherent limitations and relatively inaccurate positioning obtained from the GLONASS, all commercial and most military use is based on the United State’s GPS system.

The GPS Global Positioning system consists of three main components:
GPS Ground Control Stations (Operated by the US Defense Force)
The ground control component includes the master control station at Falcon Air Force Base, Colorado Springs, Colorado and monitor stations at Falcon AFB, Hawaii, Ascension Island in the Atlantic, Diego Garcia in the Indian Ocean, and Kwajalein Island in the South Pacific. The control segment uses measurements collected by the monitor stations to predict the behavior of each satellite’s orbit and atomic clocks. The prediction data is linked up to the satellites for transmission to users. The control segment also ensures that GPS satellite orbits remain within limits and that the satellites do not drift too far from nominal orbits.

GPS Satellites (The space component)
The space segment includes the satellites and the Delta rockets that launch the satellites from Cape Canaveral in Florida, United States. GPS satellites orbit in circular motion at 17,440 km altitude, each orbit lasting 12 hours. The orbits are tilted to the equator by 55° to ensure coverage in Polar Regions. The satellites are powered by solar cells that continuously orientate themselves to point the solar panels towards the Sun and the antennas towards the Earth. Each satellite contains four atomic clocks.

GPS Receivers (what you buy and use to determine your position and support your personal navigation)
When you buy a GPS, you are actually buying only the GPS receiver and get free use of the other two main components, worth billions of dollars - compliments of the Government of the United States. The ground stations send control signals to the GPS satellites; the GPS satellites transmit radio signals and the GPS receivers receive these signals and use it to calculate its position.

The calculations used to determine your GPS receiver’s position is based on very small time differences, from when the satellite transmitted the signal, to when the GPS receiver received the signal. These small differences are then used to calculate the distance from the receiver to the satellite. However, when receiving only one signal, we can only calculate how far away from the satellite we are. When receiving two signals, we can determine two likely positions where we are. We need three satellite signals to determine our exact position on the earth’s surface. When more than three satellites are “visible” to the GPS receiver, it will also calculate the altitude of the receiver.

Your GPS receiver requires signals from at least three satellites to determine your exact position on the earth’s surface. With a fourth signal, your altitude can also be determined. Receiving signals from more than four different satellites, the position of the GPS receiver can more accurately be determined.

The GPS satellite constellation is designed in such a manner as to guarantee that at least 4 satellites are visible from any place on earth at any moment in time. Most of the time however, you should have at least 6 satellites visible. Many commercial GPS receivers can receive and process signals from 12 satellites for increased reliability and accuracy. GPS satellites carry atomic clocks that measure time to a high degree of accuracy. The time information is placed in the codes broadcast by the satellite so that a receiver can continuously determine the time the signal was broadcasted. The signal contains data that a receiver uses to compute the locations of the satellites and to make other adjustments needed for accurate positioning. The receiver uses the time difference between the time of signal reception and the broadcast time to compute the range to the satellite. The receiver must account for propagation delays caused by the ionosphere and the troposphere. With three ranges to three satellites and knowing the location of the satellite when the signal was sent, the receiver can compute its three-dimensional position.

To compute ranges directly, however, the user must have an atomic clock synchronized to the global positioning system. By taking a measurement from an additional satellite, the receiver avoids the need for an atomic clock. The result is that the receiver uses four satellites to compute latitude, longitude, altitude, and time.

2. How does GPS signals transfer?

GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.

A GPS signal contains three different bits of information — a pseudorandom code, ephemeris data and almanac data. The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information. You can view this number on your GPS unit’s satellite page, as it identifies which satellites it’s receiving. Ephemeris data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits ephemeris data showing the orbital information for that satellite and for every other satellite in the system. Almanac data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position.

3. What is WAAS?

WAAS (Wide Area Augmentation System) is an extremely accurate navigation system developed for civil aviation. Before WAAS, the U.S. National Airspace System (NAS) did not have the ability to provide horizontal and vertical navigation for precision approach operations for all users at all locations. With WAAS, this capability is becoming a reality. WAAS provides service for all classes of aircraft in all flight operations - including en route navigation, airport departures, and airport arrivals. This includes precision landing approaches in all weather conditions at all locations throughout the NAS.

4. What is map datum?

A map datum is a reference surface which is defined mathematically and approximates the shape of the earth in particular areas. At different areas across the world, different map datums were used due to the differences in the earth’s general surface shape at different places. Specific map datums are more applicable to particular areas or regions than others.

A particular map datum is physically represented by a group of surface based trigonometric stations, whose positions have been accurately measured and is used to determine the reference surface (or map datum). A map datum enables us to calculate the position of a specific location accurately and consistently. However, there are significant deviations, when a waypoint (a longitude and latitude set) located with a particular map datum, is used with a different map datum as reference. So it might appear that specific points on the earth’s surface, have different latitudes and longitudes, depending on the map datum that was used, or alternatively, one waypoint (a longitude and latitude set) may seems to point to different physical positions.

Currently there are many, many map datums in use. Some kinds of GPS products support more than 100 different map datums to reduce the inaccuracy caused by the deviations of different map datums. Theoretically this implies that one waypoint can locate more than 100 different places, all depending on the map datum that was used as reference. All GPS and map users should be aware of this peculiarity and take note of the most applicable map datum to be used. If you use waypoints from any source, or use a map with longitudes and latitudes, make sure that your GPS is using the same map datum as used originally to determine the waypoint.

5. Introduction to Car Navigation System

In this age of ever more complex road systems and increasingly heavy traffic, it is not always easy to stay on course. Road maps are all but obsolete. What we need today are innovative solutions that make life easier for drivers by automatically selecting the right route and guiding the vehicle reliably to the desired destination.

You will know exactly where you are and where you are heading, because the Car Navigation System operates by GPS (Global Positioning System), and a CD-ROM with a digital road map holds all the information the system needs to show you the way. The Car Navigation System always knows which route is best for you. Three of the 24 GPS satellites orbiting the earth report the current position to your navigation system. At the same time, state-of-the art sensors record every movement, registering even the slightest change of direction. Most roads in the country are stored on CD ROMs. This means the computer always knows your exact position, and so do you.

Before leaving, you simply enter your destination in the control unit. All the computer needs to map out the best route is the name of the street and town you are traveling to. All you need to do is follow the instructions it gives you. The Car Navigation System promises to be a valuable aid for travelers, sales people, delivery personnel, real estate agents, drivers of emergency vehicles....anyone and everyone who needs to reach a variety of destinations as quickly and efficiently as possible.

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