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the observer's position. Conversely, if the observer's position is
precisely known, then the orbit of the satellite can be determined and
used to study the Earth's gravity. Examples: Transit, DORIS
Global navigation satellite systems (GNSS) are dedicated radio

positioning services, which can locate a receiver to within a few
meters. The most prominent system, GPS, consists of a constellation
of 31 satellites (as of December 2013) in high, 12-hour circular orbits,

distributed in six planes with 55 inclinations. The principle of location
is based on trilateration. Each satellite transmits a precise ephemeris
with information on its own position and a message containing the
exact time of transmission. The receiver compares this time of

transmission with its own clock at the time of reception and multiplies
the difference by the speed of light to obtain a "pseudorange." Four
pseudoranges are needed to obtain the precise time and the receiver's

position within a few meters. More sophisticated methods, such as
real-time kinematic (RTK) can yield positions to within a few
millimeters.

In geodesy, GNSS is used as an economical tool for surveying
and time transfer. It is also used for monitoring the Earth's rotation,
polar motion, and crustal dynamics. The presence of the GPS signal in

space also makes it suitable for orbit determination and satellite-to-
satellite tracking.

Task 3. Answer the questions to the text.

1. What are the main methods used in satellite geodesy?
2. What is the most advanced method in satellite geodesy and why?
3. What do you know about satellite laser ranging?

4. What is the Doppler positioning?
5. What is the most prominent geodetic satellite system and why?

Task 4. Define whether the following statements are true or
false and translate them. Correct the false ones according to
the text.

1. In Earth to Space methods a satellite may be observed with

ground-based instruments.
2. A satellite may or use its instruments to track or be tracked by
another satellite in space to Earth methods.
3. Radar altimetry contributes to the definition of the

international terrestrial reference frames by providing the information

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