8/14/2019 Summary of Marek Ziebart Talk

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Summary of Marek Ziebart's talk which was titled "GPS/GNSS for

Geodesy and Geophysics"

In this IGT talk, Ziebart M. has described the use of the two well-known navigation

systems, the Global Positioning System (GPS) and the Global Navigation Satellite

Systems (GNSS). Firstly, the GPS and GNSS were fairly depicted. Then, the lecturer

went on to generally speak about the number of satellites and orbits that the satellites

are bounded by. After that, the whole concept behind positioning was described and

was divided by three segments. Those segments are; (1) the control segment, (2) the

space segment and (3) the user segment. The first segment is the transmitter (or

satellite), the second is the atmosphere and its processes that affect the

electromagnetic waves (EM) as they travel from satellites and picked up by the

receiver (or GPS) and the third segment is the receiver system.

The ranging concept was then introduced. It basically encloses the idea of

determining the range (or distance) between the satellite and the receiver (GPS).

Using at least three satellites is fairly adequate to obtain the position on the Earth's

surface with a few metres or centimetres as percentage of error. There are number of

errors in the measured position that could arise and hence degrade the measurements.

Two important sources of errors are (1) the Clock Synchronisation Error (CSE) and

(2) the error that arises as a consequence of atmospheric processes. Since the

atmosphere has different layers, each with different density value, the EM waves are

expected to be refracted as they travel through those layers according to Snell's law.

Therefore, some transmitted EM signals might not arrive at the same time to the GPS.

In fact, they are not expected to be picked at the same time, so there will be a delay in

time.

8/14/2019 Summary of Marek Ziebart Talk

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According to the lecturer Ziebart M., the CSE is highly expected to arise when only

three satellites are used for positioning. However, this error is diminished to about a

few millimetres level precision when one more satellite is added. Using four satellites

would make the CSE limited. Finally, the lecturer emphasized the role of phase

difference calculations in reducing the CSE which is given by:

+++++++= M T I N ct ct x x sr sr i

Where i is the phase difference, sr x x is the position difference between the

receiver and the satellite, c is the speed of light and sr t and t are the receiver and

satellite clock offset from system time.

Saud Saadi

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