Space segment composed of navigation satellites, which generate a group of radionavigational signals transferring a huge overhead information content simultaneously. Main functions of every satellite are- forming and beaming of radiosignals, which are necessary for users navigation determination and satellites onboard systems control.
Earth segment is composed of cosmodrome, command-measuring complex (CMC) and control centre. Cosmodrome provides taking the satellites to the required orbits, during the first deploying of the navigation system, and also periodical compensation of the constellation on the scope of satellites fails.
Command-measuring complex serves for the navigational satellites information supply, necessary for the navigation sessions implementation, and also for its control as the space crafts.
Control Centre, connected with the cosmodrome and command-measuring complex, coordinates the operation of all the satellites systems.
User's segment is composed of users equipment. It serves for the navigation signals reception, navigation parameters measuring and measurements processing. For solving of the navigation problems, users equipment has special embedded computer. The variety of user's equipment provides the needs of ground, sea, aviation and space users.
Modern satellite navigation is based on the use of no-request range measurements between navigational satellite and the user. It means that the information about the satellite's coordinates given to the user, is included into navigation signal. The way of range measurements is based on the calculation of the receiving signal time delay compared with the signals, generated by the users equipment.
In ideal case, when the measurements are precise and satellite time is identical with the user's equipment time the users positioning can be realized with 3 satellites.
Actually satellites time differs from the time on the users equipment. So, for users positioning one more coordinate is necessary- time drift between users equipment and the satellite time. That's why four satellites are needed for the solving of navigation problem.
Every satellite has calibration oscillator of the reference frequency f0 = 10,23 МHz, which forms the signals from its oscillation, named L1 and L2. They are used as encoded signals transmissitters and data senders, they are also used for the most precise ranging between the user and the satellite.
Precision conditions of navigation satellite systems are formed by the main failures level and geometrical disposition of the used satellites and the user.
Geometrical factor (or Dilution of Precision DOP) is a recalculation coefficient of the sporadic error of the radionavigation parameter into the error of the corresponding parameters determination.
Position DOP (PDOP) – means accounting the components of the vector in space.
Horizontal DOP (HDOP) - means accounting the components of the vector in the horizontal plane.
Vertical DOP (VDOP)-means accounting the components of the vector in the vertical plane.
Time DOP (TDOP)- means accounting the time.