The global satellite navigation system (Global Navigation Satellite system - GNSS) provide pilots and other aircraft systems of coordinate information about the location of the aircraft and the reference time. GNSS Global Navigation Satellite System measured coordinatesof aircraft (latitude, longitude, height), three components of theaircraft vector velocity and attach the hardware of the aircraft the exact time. Currently deployed and used by two powerful GNSS: GPS (USA) and GLONASS (Russia). Both GNSS Global Navigation Satellite System created as a military system for precise positioning of objects for military purposes. Services available GNSS Global Navigation Satellite System for civil use [2]. It is also expected deployment of European civil GNSS - GALILEO. China also began to develop GNSS - BEIDOW.
Segments of GNSS Global Navigation Satellite System
In general GNSS Global Navigation Satellite System consists of three segments:
- Segment spacecraft;
- Segment management and control;
- Segment of users.
- derermining aircraft coordinates;
- calculation of geographic coordinated location;
- evaluation of geodetic flight altitude of aircraft;
- determination of the 3 components of the velocity vector of aircraft;
- determining of track angle;
- calculation of ground speed of aircraft;
- ensuring the system of aircraft exact time UTC;
- indication of coordinates.
Spacecraft segment consists of a number of artificial earth satellites (AES) (Fig. 106 They serve as beacons that emit navigation signals by which the satellite signals determines the location.
Fig. 106. Spacecraft Segment of GNSS
Segment management and control consists of ground stations located in different parts of the earth's surface in such a way as to ensure communication with all GNSS Global Navigation Satellite System of AES. Ground station is control the position and the parameters of each AES. To determine the coordinates of the user is needed information about the exact location of each AES. Ground stations monitor the precision of the radar equipment to determine the position of each AES and through the substation download pass them on AES.
User segment consist of unlimited number of satellite signal’s receivers.
The principle of operation of GNSS Global Navigation Satellite System is long-range positioning method. The principle of determining the coordinates of a user using GNSS Global Navigation Satellite System is shown on fig.107.
The determining of user coordinates is based on finding the distance from user antenna to navigation satellire.
User coordinates are determed by solving the system of equations: where R – distances from satellite to user antenna; x, y, z – satellites coordinates; x0, y0, z0 – user coordinates.
Fig. 107. Determinig of user coordinates with the help of GNSS
Since turning GNSS Global Navigation Satellite System receiver it begins to receive signals from navisfation satellites. Moreover almanac is received and downloaded from navigation satellite. GNSS Global Navigation Satellite System almanac contains accurate information about each satellite, including its position at a particular time. The exact position of the satellite at a current time is calculated using satellite trajectory equation according to known coordinates at certain time of day. In this way GNSS Global Navigation Satellite System receiver can obtaine actual information about each navigation satellite.
The range from satellite to user is determined by measuring of time passing of navigation signal. Each of satellite emmites certain navigation signals in strictly defined time. GNSS Global Navigation Satellite System receiver “knows” when the satellite signal will be emmiting. At this time receiver with the help of built-in clock is generate corresponding navigation signal and submit it to the delay line. After receiving of navigation signal from satellite received signal are to compare with delayed signal by calculating autocorrelation function. In case when both signals will be compared at the same time the value of autocorrelation function will be equal to one. Simultaneosly from delaty line receives time t for which thenavigation signal received from the satellite to receiver.
In whis way distance to navigation satellites is calculated like:
where с – speed of propagation of radiowave in space.
As internal clock in navigation satellite receiver cannot exactly determine time for synchronizing then determining the range you must consider user clock error. For range determining fro different sattelites one clock is used, and time error can be calculated with adding one more equation:
where Δt – error, which takes into account the inaccuracy of course the internal clock of the GNSS Global Navigation Satellite System receiver.
Accordingly to this fro determong user position it is necessary to receive signals not less than from 4 navigation satellites.
Accuracy of GNSS Global Navigation Satellite System positioning depends on numerous error. Some of them connected with geometry location of satellite above user and local errors caused by passing of adisognal through the atmosphere.
One of the ways of decreasing impact of this errors is application of local stations determine differential GPS corrections (Differential Global Positioning System – DGPS). Nowadays GPS develops a netwotk of functional additions to GNSS Global Navigation Satellite System providing user reports about GNSS error in a particular region.
In USA creates and develops network of functional additions Wide Area Augmentation System (WAAS) [60] and Local Area Augmentation System (LAAS) in Europe – European Geostationary Overlay System (EGNOS), in Asia-Pacific– Multifunction transport Satellite System (MTSAS) (fig. 108).
Fig. 108. Areas of action of functional additions
Principal of operation WAAS, EGNOS, MSAS is the same (fig. 109).
Fig. 109. Functional addition WAAS
Station of differential corrections is placed of the Earth’s surface with accurately known coordinates. GNSS receiver which placed on the station determines location. While determining difference between exact location and coordinated obtained with the hepl of GNSS Global Navigation Satellite System on preset time determines corrections which transmitted to the user via satellite Differential corrections are received by GNSS receiver via the same antenna loke the signals from navigation satellites.
In the construction of LAAS differentian corrections are transmeitted not by satellites but via radiostation DGPS (fig. 110). In this building coverage is much lowers than the radio coverage satellite, hence the name Local.
Fig. 110. Functional addition LAAS
Corrections from specific DGPS are actual only for certain zone borders of which are determined according to atmospheric composition and placement of DGPS.
Using of differential corrections for determining aircraft location gives ability to provide necessary accuracy of positioning forn navigation needs.
Modern navigation GNSS equipment calculates location coordinates by results of estimation of coordinates from 2 GNSS: GPS and GLONASS followed by a more precise location.
Functions of GNSS Global Navigation Satellite System
GNSS Global Navigation Satellite System board equipment performs the following functions:
GNSS equipment is often integrated with other systems of aircraft such as FMS (RocwellColins – FMS-5000; Universal Avionics – UNS-1Ew, UNS-1Fw, UNS-1Lw; Honeywell – or with indicator of light air conditions for aviation GNS-XLS), (fig. 111) [67].
Fig. 111. GNSS Global Navigation Satellite System GNS-430W receiver [67]
Some manufactures of GNSS Global Navigation Satellite System board equipment integrate base of aeronautical information in GNSS receiver and provide possibility of programming of flight plan. It gives the ability. This allows the pilot to provide information on the magnitude of the deviation from the specified aircraft trajectory and point to the air navigation facilities, located near the PC (CH-3301).