Overview
The ionosphere, which is that part of the Earth's upper atmosphere that
is ionised by radiation from the sun, effects radio propagation from the
extremely low frequencies (<3kHz) to super high frequencies (30GHz). Below
30 MHz the ionosphere is an essential part of the propagation, whereas
above 30MHz the ionosphere is a source of band pollution particularly
at night in the LF and MF bands (30kHz to 3MHz) and system disruption
for Earth-space communications such as navigation systems. The nature
of the ionosphere is intimately linked with the fluctuations in the emissions
from the sun as well as the state of the Earth's atmosphere. This makes
it highly variable and level of disruption to communications difficult
to predict at the moment.
The changing state of the ionosphere is generally monitored by networks
of vertical ionosondes. These radars transmit and receive HF (1- 30MHz)
radio waves to and from the ionosphere directly above the monitoring station.
The result is real-time information on the state of the ionosphere needed
by communications users allowing them to adapt their operational systems
accordingly. The oblique ionospheric sounder extends this idea. The transmitter
and receiver of the oblique sounder are not co-located like the vertical
but are generally hundreds thousands of kilometres apart. So the instrument
is able to study how the radio signals of real communications (below 30MHz)
propagate via the ionosphere under a variety of conditions. However the
interpretation of the oblique propagation is also significantly harder
than the case of the vertical.
Project Rationale
Many radiowave applications require real-time determination of the 2-dimensional
structure of the ionospheric electron density along the path, or at least
of the mid-path electron density profile. Since it is often impractical
to deploy a vertical ionosonde at the point of interest, such as across
oceans, the idea of reconstructing an average mid-path profile from oblique
incidence measurements is very appealing. Oblique incidence sounding also
provides an excellent test of ionospheric forecasting tools, since ray
tracing through the predicted electron density structure can be compared
directly to the measured group delays on the oblique path. In addition
the measurements can be used to improve absorption modelling, which is
of particular interest to broadcasters wishing to limit spectrum pollution.
Through knowledge of the state of the ionosphere oblique sounding can
be used to improve the accuracy of single frequency GPS (Global Positioning
System) navigational information.
A recently developed oblique sounding system, providing absolute ionospheric
group delays using GPS-timing, is used to carry out experimental campaigns
at mid and high latitudes. The project involves collaboration with the
Radio Science and Propagation Group at DERA Malvern and with a number
of British universities. This work fits in well with the objectives of
European project COST 251, and is part of collaborative activities with
several participating countries.
Objectives
- Organisation of experimental campaigns including suitable vertical
incidence control measurements. At some stage this will involve other
European countries, probably within the framework of COST 251.
- Development
of inversion algorithms to obtain vertical electron density profiles from
oblique soundings.
- Development of ionospheric ray-tracing algorithms.
- Testing of inversion and ray-tracing techniques using the oblique
sounder measurements and the control vertical measurements.
The Contents of This Report
This report contains a description of the nature of the oblique ionospheric
sounding and the measurement campaigns conducted in the UK and abroad.
Also included is description of the ray tracing and other modelling used
in the analysis of the propagation along with a presentation of some of
the results. Included as Appendices are a full listing of the dates and
propagation paths recorded during the period of the project.
Contents
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