Summary Of Marek Ziebart Talk

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.

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: φi = x r − x s +δt r c +δt s c + N + I +T + M φ + ε

Where φi is the phase difference,

xr −xs

is the position difference between the

receiver and the satellite, c is the speed of light and δt r and δt s are the receiver and satellite clock offset from system time. Saud Saadi 200300844