A microprocessor, to calculate the errors of the GPS system and to generate the structure of the message that is sent to the receivers.
Transmitter, to establish a one-way data link towards end-user receivers.
- User equipment, composed of a DGPS receiver (GPS + data link receiver from the monitored station).
According to abbreviationfinder, there are several ways to obtain DGPS corrections. The most used are:
- Received by radio, through a channel prepared for it, such as RDS on an FM station.
- Downloaded from the Internet, or with a wireless connection.
- Provided by a satellite system designed for this purpose. In the United States there is WAAS, in Europe EGNOS and in Japan MSAS, all compatible with each other.
Two types of corrections can be included in messages sent to nearby recipients:
- A correction directly applied to the position. This has the disadvantage that both the user and the monitoring station must use the same satellites, since the corrections are based on those same satellites.
A correction applied to the pseudo-ranges of each of the visible satellites. In this case, the user can make the correction with the 4 satellites with the best signal-to-noise ratio (S / N). This correction is more flexible.
The selective availability (SA) error varies even faster than the data transmission speed. Therefore, along with the message that is sent of corrections, the validity time of the corrections and their trends are also sent. Therefore, the receiver must do some kind of interpolation to correct the errors produced.
If it is desired to increase the coverage area of DGPS corrections and, at the same time, minimize the number of fixed reference receivers, it will be necessary to model the spatial and temporal variations of the errors. In such a case we would be talking about the wide area differential GPS.
With DGPS, errors due to:
- Selective availability (eliminated from the year 2000).
- Propagation through the ionosphere – troposphere.
- Errors in the position of the satellite (ephemeris).
- Errors caused by problems in the satellite clock.
For the DGPS corrections to be valid, the receiver has to be relatively close to some DGPS station; generally less than 1000 km. The accuracies handled by the differential receivers are centimetric, so they can be used in engineering.
Advantages of GPS over usual guidance systems
It is very important to understand that the calculation of position and altitude is not made from data from analog pressure, humidity or temperature sensors (or a combination of these) as in analog altimeters or altimeters-barometers, or even as in the most sophisticated digital altimeters, but it is made from the data sent by a constellation of satellites and orbit that, despite being simple as satellites, provide the reliability of making use of the most sophisticated and precise technology from which man currently has. It should also be noted that the evolution of those analog data, which, in effect, will be very useful to anticipate atmospheric changes and environmental conditions for the development of the activity to be carried out, they are relatively reliable in calculating exact position and altitude. In addition, all GPS’s incorporate really sophisticated navigation functions that will change the very concept of orientation. For example, routes can be drawn up on maps, registering on the device the points through which you want or must pass and, on the ground, activating that route, a graphic screen will indicate if you are on the correct course or if there is a deviation in some direction; or use the same function in reversible routes, that is, to record points through what is passed and then be able to return through those same points safely. With all these data, you can also deduce the speed at which you are traveling accurately, while maintaining a straight line course, or deduct the speed at which you are traveling if all course change points have been recorded… and a long etc. of very useful and interesting functions that can be discovered when using these devices.
Use the data obtained with GPS receivers with PC’s
If we need to export the data obtained with a GPS receiver to a computer to make the necessary calculations, it is good to remember that, usually, kits for data transfer between PC’s and GPS’s, as well as power supply kits, tend to be devices. optional when acquiring a GPS receiver, at least up to mid-range receivers, which are already beginning to incorporate functions that may make it necessary to include these kits in series. In addition, it cannot be forgotten that specific software is necessary to import this data in a more or less standard way, which allows it to be used in a versatile way. The most common interfaces with NMEA 0180, 0181 and 0183, so it is necessary a software that includes these interfaces, to make transfers through a serial port. It is also common to find interfaces with RS232 corrections that allow transfers through parallel ports. In addition, there are interfaces of many GPS manufacturers’ firms that create their own protocols. The software for these tasks is relatively cheap (if what you want is simply to obtain that data, of course), and there are even many applications of shareware and freeware that can be found.
