GB2251749A - Ionospheric sounding equipment - Google Patents

Abstract

Ionospheric sounding equipment receives and measures the intensity of a chirp signal generated at a remote location, generates a similar, replica chirp signal, mixes said chirp signals to generate a difference frequency signal indicative of the propagation time of the remotely-generated chirp signal, spectrally analyses the difference frequency signal over a succession of timed intervals, and plots as an ionogram the frequency of said difference frequency signal against the frequency of the replica chirp signal; the variation in intensity with frequency of said frequency difference signal being indicated. The intensity of said frequency difference signal is conveniently indicated by coloration, in which specific colours correspond to specific signal strengths. The ionogram may usefully provide a key to indicate to users the correspondence between colour and signal strength. As described, the chirp signal is swept from 2 to 30 MHz in 280 seconds and each spectral analysis is made on a chirp sweep of 1.36 seconds, the analyses being repeated 3 times per second at a resolution of 0.73Hz over a range of 500Hz.

Description

IONOSPHERIC SOUNDING EQUIPMENT This invention relates to equipment with which the characteristics of the ionosphere can be studied, in order to determine how variations in the ionosphere affect communications in the high frequency (HF) radio band, to improve short term ionospheric forecasting and to aid the design of robust communications equipment.

Techniques have been developed in the past which have made use of radio sounding techniques in order, for example, to enable an operator to select the appropriate operating frequency for a particular HF communications link. These systems, known as ionosondes, are essentially radars which emit pulses or chirp waveforms and which measure the group delay of a return signal bounced back from the ionosphere. The layered structure of the ionosphere gives rise to propagation via separate layers, resulting in different time delays corresponding to different propagation modes. The receiver may be co-located with the transmitter, in which case the ionosonde is a vertical sounder, or the transmitter and receiver may be separated by distances of up to several thousand kilometres, such a system being known as an oblique sounder.

One particular, commercially-available receiver includes a display, useful to an operator in that it provides a simple ionogram, ie a plot of signal delay as a function of signal frequency, together with either amplitude, automatic gain control (AGC) or signal quality information.

The receiver operates by mixing the chirp signal received from the transmitter with a replica of the transmitted signal and generating a difference frequency which is a measure of the signal time delay. This difference signal lies between 0 and 500Hz which corresponds to a variation in propagation time of 5ms.

The information supplied on amplitude, AGC and signal quality is the integrated value across all the propagation modes at a particular frequency, and since in any frequency sweep bin, the AGC of the receiver is designed to optimise the display brilliance to make the propagating mode clear, the equipment cannot show the user whether, for example, the signal amplitude relates to two modes of equal amplitude or one high amplitude mode and one weak mode.Such information is of scientific interest and is relevant when deciding whether multipath effects are likely to be significant but the equipment was not originally designed to supply such detailed information and this invention seeks to process the signals generated in the receiver (especially the difference frequency signal) further, in order to extend the information which can be displayed or supplied to a user.

This invention consists of ionospheric sounding equipment comprising: means for receiving a chirp signal generated at a remote location; means for measuring the intensity of said received chirp signal; means for generating a similar, replica chirp signal; means for mixing said chirp signals to generate a difference frequency signal indicative of the propagation time of the remotely-generated chirp signal; means for spectrally analysing the difference frequency signal over a succession of timed intervals; and and means for plotting as an ionogram the frequency of said difference frequency signal against the frequency of said replica chirp signal, the variation in intensity with frequency of said frequency difference signal being indicated.

The intensity of said frequency difference signal is conveniently indicated by coloration, in which specific colours correspond to specific signal strengths. The ionogram may usefully provide a key to indicate to users the correspondence between colour and signal strength.

By way of example, one embodiment of the invention will now be described with reference to the drawing, which is a typical ionogram obtained by use of the invention.

The invention utilises as its main component an RCS-4 oblique chirp ionospheric sounder receiver manufactured by the Barry Corporation, which is used to process a received chirp signal, swept linearly from 2 to 30 MHz in 280s (ie at 100 kHz/s). The chirp signal, received from the antenna of the receiver, is mixed with a replica of the transmitted signal to obtain a difference signal output which is a measure of the relative signal propagation time.

This signal output is fed to a processing and control unit (PCU) where it is spectrally analysed to give delay information, the results of which are then combined with signal power levels derived from the RS232 output of the receiver to determine the variation of power with respect to difference signal frequency for the instantaneous chirp signal frequency.

Each analysis is based on a chirp sweep of 1.36s, providing sufficient processing power, repeated 3 times per second, ie 840 analyses in every sweep of the chirp signal to give an overall resolution corresponding to 840 pixels in the horizontal direction of an ionogram. The spectral analysis of the difference signal is carried out with a resolution of 0.73Hz over its 500Hz range, corresponding to 682 pixels in the vertical direction of an ionogram.

The resulting composite of signal power, difference signal frequency and chirp signal frequency is then built up, as the chirp signal carries out its sweep, into a colour-coded ionogram generated by a computer, the colour of the plot corresponding to the received power at the particular position.

In terms of hardware, the PCU consists of a rack with slide-in modules containing printed circuit cards. The PCU is based around three Texas Instruments TMS32020 digital signal processors each of which is mounted on its own circuit board which also contains local memory enabling the processor to perform its calculations. The processors can communicate with each other and the outside world via two buses which run along the backplane of the rack.

The first bus has some global memory attached to it which each of the processor boards can access when they wish to pass data between each other. The other bus has attached to it all the interface devices, such as the RS232 and parallel computer interfaces, an analogue-to-digital (A/D) converter and also all the display devices for showing the status of the PCU.

All the software within the PCU is written in TMS32020 assembly language and its basic function is to perform a series of Fast Fourier Transforms (FFTs) on the incoming difference signal to extract the frequency information. The information is then scaled and sorted for subsequent display.

The software in the PCU is divided into three modules corresponding to each of the three processors and the whole system is driven in a pipelined fashion. The incoming data is passed to the A/D converter and is then analysed in 2048 point blocks at a sampling rate of 1500Hz over the 1.36s input data period, three of which blocks are analysed per second.

The first processor in the PCU reads in the data from the A/D converter and applies a window function to the data, the purpose of which is to weight out the edges of each 2048-point data block before the FFT is carried out in order to minimise spreading or leakage. In practice, a Kaiser-Bessell window function is used, which attenuates the sidelobes of the signal peaks sufficiently without excessively reducing the signal resolution.

Following the application of the window function, the data is passed to the second processor which carries out the FFT process. The results of the FET are then transferred to the third processor where the data is sorted and finally sent to the computer for display. The third software module also controls the PCU displays.

An example of an ionogram generated by the invention is illustrated in the drawing. The fixed features of the display include axes indicating the signal frequency from 2MHz to 30 MHz and the relative delay up to 5ms. Also included is a colour code by which the signal strength of a displayed point may be assessed from its colour.

In operation, data is displayed on the ionogram (on the screen of the computer or built up on a photographic plate) from left to right as the chirp frequency increases during its 280s sweep. The information remains displayed on the screen until overridden by the information gathered on the subsequent chirp sweep.

The lines shown on the display correspond in known manner to the various ionospheric propagation modes and whilst in existing equipment the relative strength of the lines would not be determinable, the colours of the lines on the display indicate at a glance the intense parts of each mode and their relative strengths.

As an optional feature, the embodiment just described may incorporate absolute propagation time information by including a Navstar clock, conveniently in the third software module. Whilst in the absence of timing information the equipment provides adequate relative propagation time information, by synchronisation of the replica signal with the transmitted chirp signal the frequency resulting difference signal is proportional to the propagation time of the transmitted signal. A GPS receiver needs to be incorporated in the chirp transmitter to enable this synchronisation to be carried out.

Claims (6)

1. Ionospheric sounding equipment comprising: means for receiving a chirp signal generated at a remote location; means for measuring the intensity of said received chirp signal; means for generating a similar, replica chirp signal; means for mixing said chirp signals to generate a difference frequency signal indicative of the propagation time of the remotely-generated chirp signal; means for spectrally analysing the difference frequency signal over a succession of timed intervals; and and means for displaying as an ionogram the frequency of said difference frequency signal against the frequency of said replica chirp signal, the variation in intensity with frequency of said frequency difference signal being indicated.

2. Ionospheric sounding equipment according to Claim 1 in which the intensity of the frequency difference signal is indicated by coloration.

3. Ionospheric sounding equipment according to either preceding claim in which the length of the timed intervals over which the frequency difference signals are spectrally analysed is substantially greater than the time between the commencement of successive timed intervals.

4. Ionospheric sounding equipment according to any preceding claim in which a window function is applied to the frequency difference signal data collected during each timed interval.

5. Ionospheric sounding equipment according to Claim 4 in which a Kaiser-Bessell function is used as said window function.

6. Radio oblique sounding equipment substantially as hereinbefore described with reference to the drawings.