Ionospheric effects on radio communication and ranging pulses

Transionospheric radio pulses used for communication and ranging purposes are modified by propagation effects arising from dispersion and scattering. To describe these effects quantitatively it is convenient to use the concept of temporal moments. The zeroth temporal moment is proportional to energy flow in the wave and is constant in a dissipationless ionosphere under the forward scatter approximation. The first temporal moment is related to the mean arrival time which can be arranged as a series consisting of terms proportional to
f_{c}^{-2n}, n = 0, 1, 2, ...
, where
f_{c}
is the carrier frequency. Here the
n = 0
term in this series representation of the first temporal moment is just the free-space transit time which can be related simply to the geometric distance, a quantity required with great precision in ranging applications. The
n = 1
term, which is proportional to the electron content, has been investigated by various investigators in connection with navigational satellite applications. The
n = 2
term in the expansion of the first temporal moment given in this paper comes about through dispersion, finite bandwidth effect, and scattering from random irregularities; the numerical values of each of these terms are computed for a Chapman layer. The second temporal moment is related to the mean square pulsewidth. It is shown that an amplitude modulated pulse at a carrier frequency of 100 MHz may be stretched up to several hundred fold in width by dispersion and scattering, but such an effect diminishes rapidly in importance as the carrier frequency is raised. These results provide useful information to designers of satellite-based communication and ranging systems.