Ground based observations and service
Radio signals travelling through the Earth's ionosphere are delayed due to the interaction of the electromagnetic wave field with the plasma in particular with the free electron gas. Since the plasma resonance frequency in the ionosphere is generally below 20 MHz, this interaction reduces with increasing radio frequency (~ 1 / f2). Nevertheless, the effect is still significant in the L- and/or S-band or even higher. The net effect upon radio waves is proportional to the integrated electron density (Total Electron Content - TEC) along the line-of-sight signal path between a satellite to a receiver. TEC is a key parameter that describes the major impact of the ionized atmosphere on the propagation of radio waves.
Due to the dispersive nature of the ionosphere mentioned above, differential code and carrier phases at L1/L2 frequencies can be computed. From these measurements the TEC may be derived assuming that L1 and L2 signals travel at nearly the same ray paths.
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