The determination of the position refers to the acquirement of satellite observations (measurements) carried out in points of interest, coordinates (absolute or relative) of these points in a well-specified reference system. Satellite observations consist of various types of measurements performed between the satellite receiver located on the ground or near it and one or more satellites evolving on circumterrestrial orbits. The position determining methods are based on observations made using satellite signals broadcasted in the microwaves field. The emission flow of the satellite signal is usually continuous or can be pulses at regular intervals. The reception of these signals is done similarly.
Satellites play an active role by broadcasting signals that are received by specialized devices (receivers) that decode this signal. After decoding the signal from it, the information needed to determine the position of the receiver is extracted.
Global Navigation Satellite Systems (GNSS) are systems that allow high precision determination of position in a geocentric reference system at any point located on the earth surface, near or outside, using Earth's artificial satellites.
Currently the most well-known GNSS systems are NAVSTAR-GPS (USA) and GLONASS (Russia). There are no significant differences between the two GNSS systems in terms of operating principles and technology used. Each system includes three segments: the spatial segment (satellites), the control segment (Monitoring and control stations) and the user segment. GNSS satellites transmit time information, navigational information, and system status messages to users. The control segment is responsible for maintaining the satellite constellation, the attached time system and the determination of satellite orbit. Currently there are other countries intending to create GNSS systems (complementary).
NAVigation Satellites with Time and Ranging – Global Positioning System (NAVSTAR – GPS)
The GPS space segment comprises a number of 31 satellites positioned on 6 orbital planes angled at 55° at an altitude of 20230 km. The satellite revolution period is 11 hours and 56 minutes. An identical satellite constellation can be observed after a sidereal day 4 minutes earlier. The GPS constellation is composed of the GPS Block IIR-Replacement satellites (replacing the GPS Block II/IIA satellites from 1997), GPS Block IIR-M (modernized-introducing a new military signal-code M, on both carrier waves and C/A code on the second carrier wave called L2C), GPS Block IIF-Fallow On (introducing a new signal on a third frequency, L5) and GPS Block III (a new civilian signal, C/A code on the L1 carrier wave). The positioning accuracy for the civilian segment has improved from about 100m to 13m by suspending ITS (Selective Availability) technique. Major progress is expected by modernizing the spatial segment (the third L5 carrier wave, C/A code on the L2 carrier wave, etc.). The reference system used is WGS84 (World Geodetic System 1984).
GLObalnaya NAvigatsionnaya Sputnikovaya Sistema - GLObal NAvigation Satellite System (GLONASS)
The GLONASS space segment comprises a number of 27 satellites positioned on 3 orbital planes angled at 64.8° at an altitude of 19100 km. The satellite revolution period is 11 hours and 16 minutes. From the 27 designed satellites a total of 22 are operational. Each satellite has an atomic clock that generates a frequency from which the two carrier waves are formed. The signals transmitted are similar to the GPS system, including the C/A code on the L2 carrier. The precision level of the Russian GLONASS system is comparable to that of the NAVSTAR-GPS system. In Romania, the GLONASS system can be used, complementing the constellation of the GPS system. The reference system used is PZ90.
European Union (EU) and the European Space Agency have developed the GNSS European system, named Galileo. The constellation of the Galileo system will consist of a number of 30 satellites positioned on 3 orbital planes angled at 56° at the altitude of 23616km. The revolution period of a satellite will be about 14.4 hours. In terms of positioning accuracy, availability and integrity, Galileo will be superior to the other systems. GALILEO will be interoperable with NAVSTAR-GPS and GLONASS. The reference and coordinate system used will be ETRS type (European Terrestrial Reference System). For positioning ROMPOS® uses the NAVSTAR-GPS and GLONASS global systems. When commissioning the new European system Galileo, ROMPOS® will also implement the use of this new GNSS system.