Main types of satellite navigation systems - Differential-time-of-arrival systems

From INVESaTWIKI

Differential-time-of-arrival systems

In differential-time-of-arrival systems, the receiver measures the difference of the time of arrival of signals transmitted simultaneously from several satellites. Two such systems have been put into operation. The Global Positioning System (GPS) was developed by the United States. The former Soviet Union developed a similar system called Glonass, which is now operated by Russia and operates with some minor differences, similarly to GPS.
The Global Positioning System provides worldwide coverage with four satellites in each of six planes for a total of 24 satellites. The orbits are nearly circular and have an inclination of 55°. The ascending nodes of the six planes are equally spaced around the Equator. An altitude of 20,200 km (12,500 mi) gives the orbits a subsynchronous period such that they repeat their ground track every second orbit. The satellites are three-axis-stabilized by a combination of reaction wheels and magnetic torque applied against the Earth's magnetic field. The satellite is rotated so that the Earth panel with the antennas always faces the Earth and the solar panels are at right angles to the Sun. Each satellite broadcasts a ranging signal which is measured by all satellites in view. The satellites then broadcast a data message which contains the satellite ephemeris and all the measurements which it received. In this way, each member of a pair of ranging satellites has access to the pseudorange measurements in both directions (from the first satellite to the second, and vice versa). Each intersatellite range measurement is a combination of the geometric distance between the satellites and the difference of their clock biases. The two pseudoranges can be combined to yield an independent derived clock measurement and a derived true range measurement. The actual implementation of this requires additional terms to compensate for the satellite's motion and to account for the general-relativistic offset between the two satellites.
In a satellite navigation receiver, the antenna converts the radio signal from the satellite to electric current which can be filtered, amplified, and processed by the receiver. The phase center of the antenna is the position which the receiver locates. A preamplifier filters and amplifies the radio-frequency signal. A mixer shifts the L-band frequency to a lower intermediate frequency (IF) by, in effect, subtracting the frequency of a local oscillator from the incoming signal while preserving the signal modulation. Most newer receiver designs convert the intermediate-frequency signal to digital samples at this point.