Question

a)Describe how observing two satellites from two receivers can allow clock errors

to be removed.

b). Describe how ionospheric effects may be removed from GPS signals

Answer 1

Ans(a) : The clock error in satellites is caused due to the oscillator not being synchronized to the GPS time. It is one of the main errors int the satellite as it would affect the accuracy of position of the satellite. The clock error is being provided at every two hours by the broadcast navigation message. It is very important to remove this error to obtain better accuracy of the position of satellite. It can be removed by providing differencing between the satellites or it may be added as an unknown parameter in the estimation. By observing the two satellite from two recievers the error is being eliminated as the position is obtained relatively and error from one point is being eliminated by the other receiving point and hence this method is used for removing clock error in the satellite GPS system.

Ans(b) : The ionospheric effect is being observed in relatively unhindered travel of GPS through the Vacuum of space. By this effect, there is a delay in transit of signal from satellite to the reciever. This effect is needed to be removed so that there is no delay in transit and accurate readings are being obtained by the GPS. To remove these effect from a single reciever, we need to obtain ionospheric correction available in some another part of navigation message. The higher frequency carrier is less affected by the ionosphere so it is adviced to have satellite in GPS system that have a Carrier of high frequency and due to that the error would be eliminated upto some extent.