What is GPS

What is GPS?

Satellite navigating system GPS (Global Positioning System) or the Global system of positioning, is more exact - its-her space segment, represents constellation from 24 satellites. System GPS (the official name - NAVSTAR) is developed under the order and is under the control of the Ministry of Defence the USA. In 1980 system have opened for civil use. GPS works under any weather conditions worldwide 24 hours per day. With its help it is possible to determine with a high degree of accuracy coordinates and speed of mobile objects. For using GPS services GPS it is not raised neither a user's payment, nor a payment for connection. Everything, that is necessary for using system GPS is to get the GPS-receiver.

System GPS works

Satellites GPS rotate around of the Earth and transfer navigating radio signals. GPS-receivers accept these signals and calculate a location. The receiver compares time of emission of a signal to time of reception of this signal. The difference between these sizes allows to calculate distance up to the satellite. Knowing distance up to several satellites, the GPS-receiver can determine the location and display it on an electronic map.

Accepting the information at least from three satellites, the GPS-receiver can determine two-dimensional coordinates of the user (breadth and a longitude). "Having grasped" four and more satellites, the device can determine three-dimensional coordinates (breadth, a longitude and height). Having determined a location of the user, the receiver can calculate such parameters as speed, a travelling corner, a trajectory, the gone distance, distance up to a terminal point, time of rising and a sunset and many other things.

Accuracy of GPS

Modern multichannel GPS-receivers provide high enough accuracy. So, 12-channel GPS-receivers GARMIN trace up to 12 satellites GPS simultaneously, providing fast and confident definition of a location, including in city conditions or under dense crones of trees. The more GPS satellites sees the receiver - the its indications are more exact. On the average, accuracy of GPS-receivers GARMIN makes 15 m.

Accuracy of GPS-receivers can be increased by reception of differential amendments. The most perspective sources of differential amendments - the global differential subsystems transmitting the amendment to signals GPS from geostationary satellites. For their use it is not stipulated any payment. To them concern American system WAAS, European EGNOS and Japanese MSAS. They improve accuracy of GPS-receivers locations definition up to 1-3m.

American system WAAS already works. Now a cover zone of this system - territory of Northern America. The big number of models of GPS-receivers GARMIN can use WAAS at calculation of a location without any additional equipment.

European analogue WAAS - system EGNOS - is in a testing stage. Signals from its two satellites are accepted from the Ukraine. It is more about system EGNOS you can find out on a site of the European space agency www.esa.int

Accuracy of the GPS-receivers established on sea courts, can be increased also up to 1-5 m by reception of the differential amendments transmitted by a network of radio beacons, located on sea coasts. In the USA these beacons are in conducting Service of a coast guard. For reception of signals DGPS the payment is not raised, however for their use it is necessary to get receiver DGPS, for example GARMIN GBR 21, and to connect it to the GPS-receiver.

Orbital grouping of satellites

24 satellites making a space segment of system GPS, rotate around of the Earth on circular orbits at height about 20000 kms. Each satellite does two full revolutions less than for 24 h. Satellites move with speeds about 11000 km/h. A meal of satellites GPS is provided with solar batteries, onboard there are also reserve storage batteries. Each satellite has small rocket engines intended for correction of orbital trajectories.

Some more interesting facts, concerning orbital grouping GPS:

* The first satellite has been started in 1978.
* The number of satellites has reached necessary amount of 24 - in 1994.
* Average time of a life of each satellite GPS makes about 10 years.
* The weight of satellite makes about 900 kg, and its width - about 5 m with the unwrapped solar batteries.
* Capacity of the transmitter makes no more than 50 Wt.

Structure of transmitted signals

Satellites GPS transfer two low-power signals on frequencies L1 and L2. Civil GPS-receivers work on frequency L1 equal 1575,42 MHz. Reception of signals is possible only from the satellites which are taking place within the limits of direct visibility. Clouds, glass and plastic are not barrier to a signal while the majority of dense objects, such as buildings, the lay of land, metal subjects and people - are.

The signal transmitted by satellites GPS, contains three important making items - a pseudo-casual code, Ephemeris Data and the almanac. The pseudo-casual code contains number of the satellite transmitting the information. GPS-receivers can display it on page of satellites for example.

Ephemeris Data constantly transmitted by each satellite, contain the important information on the status of the satellite (working or non-working), and also the current date and time. This part of a signal is necessary for calculation of a location by the GPS-receiver.

The almanac contains the information on where there should be satellites. Each satellite transfers the almanac with orbital information for the given satellite, and also all other satellites GPS.

The reasons of GPS mistakes

Accuracy of a fixing by GPS is influenced with the following factors:

* ionosphere and tropospheric delays
In process of passage of an atmosphere the signal is slowed down. System GPS uses the built - in model which determines average size of a delay for partial correction of a mistake of this type.
* Multibeam reception
It occurs, when signal GPS is reflected from objects, such as high buildings or rocks and gets in the GPS-receiver. The increase in time of passage of the reflected signal results in occurrence of a mistake.
* A mistake of hours of the receiver
The built - in hours of the GPS-receiver concede in accuracy to the nuclear hours which are taking place onboard satellites. It can cause small mistakes in definition of time of passage of a signal.
* Orbital mistakes
Are known as well as Ephemeris mistakes, there correspond discrepancies in a transmitted location of satellites
* Number of seen satellites
The more satellites "sees" the GPS-receiver, the above accuracy. Buildings, elements of a relief, and sometimes dense foliage can interfere with reception of signals GPS, resulting to mistakes in a fixing or to its impossibility.
* Geometry of seen satellites
It is determined by a relative positioning of satellites during each moment of time. Such geometry of satellites, when corners between directions on them big is ideal. Bad count such geometry when satellites settle down on one line or it is close to it.
* The going distortion of signal GPS
The program of selective availability (Selective Availability, SA) the Ministries of Defence the USA provided the going entering a mistake into signal GPS. The purpose of this program was prevention of possible use of civil GPS-receivers in the military purposes. In May 2000 the government of the United States has switched off mode SA that has increased accuracy of civil GPS-receivers about 100 m up to 15 m.