PHYSICS · Multi-part · Fermi University Physics · Plasma Physics · Waves in plasmas · Laboratory, industrial and space plasmas
Plasma Physics, Ch Plasma oscillations and light in an unmagnetised plasma
Problem
The GPS L1 signal has frequency f=1.575GHz. The F2 layer of the ionosphere has a peak electron density ne,max=1.0×1012m−3 (its critical frequency is 9.0MHz).
((a)) Calculate the critical density nc for the L1 frequency.
((b)) Using ne,max=nccos2θ0 for a flat layer, find the angle θ0 from the vertical at which an L1 signal would just be reflected by this layer, and hence the elevation above the horizon (measured from 90∘−θ0) below which reflection would occur.
((c)) State whether GPS satellites, which are tracked down to elevations of a few degrees above the horizon, ever have their L1 signals reflected by the ionosphere, and explain why the Earth's curvature, which keeps a ray that reaches the ground from crossing a layer 300km up at more than about 73∘ from the local vertical, makes this conclusion firmer still.