AU651663B2

AU651663B2 - Back-scatter sounding

Info

Publication number: AU651663B2
Application number: AU11495/92A
Authority: AU
Inventors: George Frederick Earl
Original assignee: Commonwealth of Australia
Current assignee: Commonwealth of Australia
Priority date: 1991-03-11
Filing date: 1992-03-06
Publication date: 1994-07-28
Anticipated expiration: 2012-03-06
Also published as: AU1149592A

Description

40512 HKS:MAH:LL P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

o n-> a o+ Name of Applicant: THE COMMONWEALTH OF AUSTRALIA Actual Inventor: GEORGE FREDERICK EARL o Address for Service: COLLISON CO.,117 King William Street, Adelaide, S.A. Invention Title: BACK-SCATTER SOUNDING Details of Associated Provisional Applications: No. PK5008 dated 11th March 1991 The fellowing statement is a full description of this invention, including the best method of performing it known to us: BACKGROUND OF THE INVENTION This invention relates to a method of reduction of radio frequency interference in ionospheric sounding measurements.

Ionospheric sounding is one of the techniques employed to obtain information essential to the operation of an Over-the-Horizon radar (OTHR) or HF Skywave radar system.

Unlike the traditional ground-based radars which can only see as far as the horizon, an OTHR can make observations over 1000 km away, unconstrained by the earth's surface. This is made possible by refracting radar beams from *000 10 the ionosphere which is usually about 100 to 300 km above the earth's "surface.

roa The OTHR concept is straightforward. A signal is beamed from a transmitter and then refracted down from the ionosphere to illuminate a target; the echo from the target travels by a similar path back to a receiver which is somewhat 1 5 remote from the transmitter. The coverage of an OTHR is potentially a million square kilometres for a single installation.

o a Although the OTHR concept is simple, using the ionosphere for the radar requires a sophisticated understanding of the ionosphere's complexity. It changes with geographic location, time of day, season of year, sunspot activity 0 20 and other influences.

One particular ionospheric sounding technique is ground back-scatter sounding. The ionospheric ground back-scatter technique involves measuring signals reflected from a distant region of the earth's surface, with the p! ionosphere serving as the propagation medium for both transmission and 2 5 reception paths. This is in contrast to conventional vertical incidence sounding in which signals are reflected from regions of the ionosphere close to vertically above the sounder. The back-scatter technique has been used in remote sensing applications involving studies of both the distant ionosphere as well as the land and sea scattering sources, and is a key sensor in frequency 3 0 management systems used to support over.,*the-horizon radar.

Ili, I- 3 In common with all wide-band ionospheric sounding systems, the ground back-scatter technique is susceptible to contamination by radio frequency interference (RFI) as a result of congestion in the HF band. In practice, typical back-scatter signal amplitudes are considerably weaker than those characteristic of other forms of ionospheric sounding, thereby compounding the susceptibility to RFI. RFI is manifest in the back-scatter ionograms in the form of vertical noise stripes. The problem is particularly serious at night when HF congestion produces RFI severe enough to obscure even the basic structure of the desired back-scatter ionogram.

1 0 The eradication of such interference is imperative in systems required to automatically interrogate the back-scatter sounder data base. The purpose of io °00 this invention is to provide a method of producing high quality ionograms 000 under a wide range of conditions, in real time and without operator intervention.

:"oao 15 In order to comprehend the method, it is first necessary to understand the signal processing and associated parameters employed within the backscatter sounder. These will be described with reference to a known Over-the- Horizon radar system.

A typical back-scatter sounder system operates in Frequency Modulated 20 Carrier Wave mode (FMCW), sweeping at a rate of 100 kHz s-l. In common too.

too with all FMCW sounders, the received signal is mixed with a replica of the transmitted signal in a process known as deramping. A spectral analysis of "0o the deramped signal is made, with frequency being :roportional to group 0000 delay, and power spectral density being the measure of received power as a function of group delay. Thus, the distribution of received power as a function of group delay at a specific operating frequency (a vertical slice through an ionogram) is the power spectral density function of the deramped signal at that i operating frequency.

Critical system design objectives for a back-scatter sounder are that data 3 0 should be recorded between 6 and 30 MHz at a resolution of 200 kHz and cover group ranges to 6000 km at a nominal resolution of 50 km. In the current system, as a consequence of the single receiver being time multiplexed across 8 receiving beams, data are derived from each beam for a nomirzal 25 kHz in each 200 kHz of the HF spectrum. In the 250 msec _p.

4 available to each beam, 6 independent data sequences each of 30 msec duration are acquired and subjected to power spectral analysis and calibration. In the absence of the RFI clean-up method, all 6 available spectr I from a 200 kHz band are used to produce a final averaged spectrum for that band.

The fundamental reason for RFI contamination is that in the process of unconditionally accepting all spectra into the averaging process, some spect are derived from data characterised by the unwanted additive RFI being stronger than the wanted back-scatter signal. It was recognised that the i 1 0 contaminating effect of RFI would be reduced if a procedure could be developed whereby spectra generated from the raw signal processing could be classified as either immune from, or contaminated by, interference.

Contaminated spectra could then be rejected. The essential requirement wa the generation of a reference with which candidate spectra could be 1 5 contrasted in order to make the accept/reject decision.

It appeared that a useful reference might be generated by calculating (independently for each range cell) the minimum spectral density across the a tra

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00 0 o 0 o So o spectra available for a 1 MHz band. Range cells corresponding to background noise will produce a minimum value consistent with the statistics of background noise, while those range cells due to back-scatter clutter will produce a value consistent with the statistics of clutter. Critically, the reference would be free of the effects of RFI, since even under the most hostile conditions, significant RFI would not exist in all 30 spectra available from a 1 MHz band. It also seemed profitable to simultaneously reduce both the 25 variance of the spectral estimates used in establishing, e minima, and the computational load, by compressing the number of range cells from 128 to 32 by simple arithmetic averaging, the original data being retained at the full resolution of 48.8 km. The range averaged data are thus used solely for the purpose of deciding which of the original data should be retained and which 3 0 should be rejected on the basis of being corrupted by RFI.

SUMMARY OF THE INVENTION Therefore, according to perhaps one form of this invention, although this need not be the only or indeed the broadest form, there is proposed a method of reduction of radio frequency interference in ionospheric ground back-scatter oo 0 0I 00t t

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sounding measurements comprising the steps of: recording a plurality of back-scatter sounding spectra by transmitting a signal from a transmitter 1 means and receiving at a receiver means signals back-scattered from a remote location; converting the back-scatter sounding spectra to back-scatter range signals; apportioning the range signals into a plurality of range cells, each cell j containing a signal proportional to the strength of the signal back-scattered from a corresponding range; converting the range signals to spectral density signals; determining a minimum spectral density for each of the range cells across the plurality of back-scatter sounding spectra; summing the minimum spectral densities to produce a reference; summing the spectral density signals for each of the plurality of back-scatter spectra across the plurality of range cells to produce a single spectral density 1 5 value for each spectrum; comparing the single spectral density value for each spectrum with the reference and rejecting any spectrum for which the spectral density value exceeds the reference by more than a predetermined amount; and producing a back-scatter sounding ionogram from the remaining spectra.

In preference the transmitted signal is a frequency modulated carrier wave signal and the received signal is deramped by mixing the received signal with a replica of the transmitted signal.

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6 In preference the back-scatter sounding spectra are converted to back-scatter range signals by performing a Fourier transform and applying a Hanning window function.

In preference the range signals are converted to spectral density signals by taking the logarithm of the range signals.

In preference the predetermined amount is in the range 7db to 12 db and preferably is 9db.

It is possible for a spectrum to be corrupted by RFI only in the vicinity corresponding to the maximum and minimum ranges and yet pass a test 10 based on the average behaviour across all range cells. Such spectra may be o" rejected by applying this invention to guard bands located adjacent to the minimum and maximum group delays.

Thus, in order for a spectrum to be accepted as RFI free it must pass a three way test based on its spectral content across all range cells as well as in the 1 5 vicinity of the maximum and minimum ranges.

In principle, the number of spectra rejected may be such that no spectra remain in a given band. In this case it is preferable that adjacent spectra are :used to interpolate across the missing spectra.

According to a further form of this invention, there is proposed a method of 0 20 reduction of radio frequency interference in ionospheric ground back-scatter °oosounding measurements comprising the steps of recording a plurality of back-scatter sounding spectra by transmitting a signal from a transmitter means and receiving at a receiver means signals back-scattered from a remote location, whereby the transmitted signal is a frequency modulated 2 5 carrier wave signal and the received signal is deramped by mixing the received signal with a replica of the transmitted signal; converting the back-scatter sounding spectra to back-scatter range signals by performing a Fourier transform and applying a Hanning window function; apportioning the range signals into a plurality of range cells, each cell 3 0 containing a signal proportional to the strength of the signal back-scattered from a corresponding range; linearly averaging the range signals in the range cells to produce a reduced CL.- 7 set of range cells; converting the range signals to spectral density signals by taking the logarithm of the range signals; determining a minimum spectral density for each of the range cells across the plurality of back-scatter sounding spectra; summing the minimum spectral densities to produce a reference; summing the spectral density signals for each of the plurality of back-scatter spectra across the plurality of range cells to produce a single spectral density value for each spectrum; comparing the single spectral density value for each spectrum with the reference and rejecting any spectrum for which the spectral density value exceeds the reference by more than 9db; and producing a back-scatter sounding ionogram from the remaining spectra.

In a still further form of this invention there is provided an apparatus for the 1 5 reduction of radio frequency interference in ionospheric ground back-scatter sounding measurements including transmitter means for the transmission of a plurality of signals into a region of the earths surface; receiving means for receiving signals back-scattered from the said region; analysis means for deriving back-scatter sounder spectra from the received signals; calculating means for generating a reference with which each back-scatter sounding spectrum is compared; comparing means for comparing each back-scatter sounding spectrum with i: 25 the reference and rejecting back-scatter sounding spectra which exceed the 0 reference by a predetermined amount; means for producing a back-scatter sounding ionogram from the remaining spectra; and display means for displaying the resultant back-scatter sounding ionogram.

3 0 In preference the calculating means generates a reference by converting the 0000back-scatter sounding spectra to back-scatter range signals, apportioning the range signals into a plurality of range cells, converting the range signals to 0,,il spectral density signals and summing the minimum spectral densities for each of the range cells across the plurality of back-scatter sounding spectra.

8 BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of this invention a preferred embodiment will now be described with reference to the attached drawings in which FIG. 1 shows a back-scatter ionogram exhibiting RFI.

FIG. 2 shows a back-scatter ionogram with clean-up enabled, The method has been trialed on an Over-the-Horizon radar system and will be described with reference to that system. The method is implemented in real time as back-scatter spectra are recorded.

A typical Over-the-Horizon radar system acquires 6 spectra in a 200 kHz band 1 0 of the HF spectrum and scans at the rate of 100 kHz per second. The method is implemented on data in 1 MHz operating bands and there are therefore back-scatter spectra available in each 1 MHz band. The spectral data is Fourier transformed and convolved with a Hanning window function. The resultant data is in the form of intensity as a function of range. The data is 1 5 apportioned into 128 range cells, each cell representing 48.8 km on the ground. The data can be depicted as a matrix of 30 columns (corresponding to the 30 spectra recorded for the 1 MHz operating band) by 128 rows (corresponding to the 128 range cells) and where Sjk is the spectral density in cell jk.

20 S1,1 S1,2 Sl,k S1,30 S2,1 Sj,1 Sl,k S128,1 S128,k S128,30 Using simple averaging the number of range cells is reduced from 128 to 32 and converted to logarithmic form using the following method 4m Cmk 10 Loglo- 41 Sj 4 j=4m-3 m=1,32 k= 1,30 thereby yielding U, 00 QO~ 0 0o o C1,1 C2,1 Cm,1 C32,1 C1,2 C1,k C1,30 Cm,k C32,k Cm,3o C32,30 0 008 0 00 So a o o 0 00 0 a o a o tt 00 0 o <6 o The averaging step is not essential to the method but has advantages in data 1 0 handling by reducing the computational load.

The 32 averaged logarithmic values for each back-scatter spectfum are then summed for each of the 30 back-scatter spectra to produce a value proportional to the integral of the logarithmic power spectral density function Ik for each back-scatter spectra. Thus 32 Ik= Cmk k= 1,30 m=1 yielding Treating each of the 32 rows sequentially and independently, the 30 spectra are scanned to determine the minimum spectral density for each of the 32 2 0 range cells, thus yielding an array of 32 minimum spectral density points.

These minimum spectral density points are then summed to produce a value proportional to the integral of the logarithmic spectral density function, N.

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The spectral density Ik for each spectrum is then compared to the minimum spectral density N and any spectra differing from the minimum by greater than 9 db are rejected as being contaminated by RFI. The only parameter controlling the accept/reject decision is the 9 db threshold. Data passing the acceptance test are averaged in order to yield final estimates at 200 kHz resolution. The method is implemented in real time during the data acquisition phase so the method does not inhibit the rate at which back-scatter data may be acquired.

The effectiveneoo v; the clean-up method can best be appreciated by 1 0 observing its effect on data acquired under conditions of known severe RFI.

The data of FIG. 1 and FIG. 2 were collected at night specifically for the purpose of demonstrating the effectiveness of the clean-up method. FIG. 1 '"oo shows data collected under conditions of severe RFI. FIG. 2 shows similar data processed according to this invention. Furthermore, if the 9 db threshold were 15 relaxed RFI would reappear.

The table below shows the signal processing parameters applicable to the spectra shown in figure 1 and 2.

FMCW sweep rate 100 kHz/sec A/D converter sampling rate 16.6 kHz/sec "4 20 Number of samples in FFT 5i 2 °o Coherent integration time 30 msec Number of range cells retained 128 Range cell resolution 48.8 km Total range coverage 620 km 2 5 The effectiveness of the clean-up method in rejecting RFI is readily apparent.

Data of such poor quality that even a skilled operator wou i;nd difficult to interpret are decontaminated to such an extent that automated interpretation is possible.

The method described above can be implemented in real time without 3 0 operator intervention by employing an array processor. The method produces high quality ionograms under a wide range of conditions.

11 Throughout this specification the purpose has been to illustrate the invention and not to limit this. For example, if an Over-the-Horizon radar system is operated without time multiplexing then a single receiver is available to collect data from a single antenna beam. In this case each 1 MHz band is characterized by 240 spectra and the accept/reject threshold becomes 12 db.

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Claims

1. A method of reduction of radio frequency interference in ionospheric ground back-scatter sounding measurements comprising the steps of recording a plurality of back-scatter sounding spectra by transmitting a signal from a transmitter means and receiving at a receiver means signals back- scattered from a remote location; converting the back-scatter sounding spectra to back-scatter range signals; apportioning the range signals into a plurality of range cells, each cell containing a signal proportional to the strength of the signal back-scattered from a corresponding range; converting the range signals to spectral density signals; U determining a minimum spectral density for each of the range cells across the plurality of back-scatter sounding spectra; summing the minimum spectral densities to produce a reference; summing the spectral density signals for each of the plurality of back-scatter spectra across the plurality of range cells to produce a single spectral density value for each spectrum; comparing the single spectral density value for each spectrum with the reference and rejecting any spectrum for which the spectral density value exceeds the reference by more than a predetermined amount; and producing a back-scatter sounding ionogram from the remaining spectra.

2. The method of claim 1 in which the transmitted signal is a frequency modulated carrier wave signal and the received signal is deramped by mixing the received signal with a replica of the transmitted signal.

3. The method of claim 1 or 2 in which the back-scatter sounding spectra are converted to back-scatter range signals by performing a Fourier transform and applying a Hanning window function.

4. The method of any of claims 1 to 3 in which the range signals are converted to spectral density signals by taking the logarithm of the range 30 signals. i f~ The method of any of claims 1 to 4 in which the predetermined amount S VR u. is in the range 7db to 12 db. 13

6. The method of claim 5 in which the predetermined amount is 9db.

7. The method of any of claims 1 to 6 further characterized by the additional step of linearly averaging the range signals in the range cells to produce a reduced set of range cells.

8. The method of claim 7 in which the number of range cells is reduced by a factor of four.

9. A method of reduction of radio frequency interference in ionospheric ground back-scatter sounding measurements comprising the steps of recording a plurality of back-scatter sounding spectra by transmitting a signal 1 0 from a transmitter means and receiving at a receiver means signals back- scattered from a remote location, whereby the transmitted signal is a frequency modulated carrier wave signal and the received signal is deramped by mixing the received signal with a replica of the transmitted signal; converting the back-scatter sounding spectra to back-scatter range signals by 1 5 performing a Fourier transform and applying a Hanning window function; apportioning the range signals into a plurality of range cells, each cell containing a signal proportional to the strength of the signal back-scattered from a corresponding range; linearly averaging the range signals in the range cells to produce a reduced 2 0 set of range cells; converting the range signals to spectral density signals by taking the logarithm 00 of the range signals; 000 0 determining a minimum spectral density for each of the range cells across the o plurality of back-scatter sounding spectra; o 25 summing the minimum spectral densities to produce a reference; summing the spectral density signals for each of the plurality of back-scatter hspectra across the plurality of range cells to produce a single spectral density value for each spectrum; comparing the single spectral density value for each spectrum with the S3 0 reference and rejecting any spectrum for which the spectral density value exceeds the reference by more than 9db; and producing a back-scatter sounding ionogram from the remaining spectra. 04 An apparatus for the reduction of radio frequency interference in ionospheric ground back-scatter sounding measurements including transmitter means for the transmission of a plurality of signals into a region of 14 the earths surface; receiving means for receiving signals back-scattered from the said region; analysis means for deriving back-scatter sounder spectra from the received signals; calculating means for generating a reference with which each back-scatter sounding spectrum is compared; comparing means for comparing each back-scatter sounding spectrum with the reference and rejecting back-scatter sounding spectra which exceed the reference by a predetermined amount; 1 0 means for producing a back-scatter sounding ionogram from the remaining spectra; and display means for displaying the resultant back-scatter sounding ionogram.

11. The apparatus of claim 10 in which the calculating means generates a reference by converting the back-scatter sounding spectra to back-scatter 1 5 range signals, apportioning the range signals into a plurality of range cells, converting the range signals to spectral density signals and summing the minimum spectral densities for each of the range cells across the plurality of back-scatter sounding spectra.

12. A method of reduction of radio frequency interference in ionospheric sounding measurements as herein described with reference to the attached figures. Dated this 19th day of May 1994 00 THE COMMONWEALTH OF AUSTRALIA o ~By its Patent Attorneys S 25 COLLISON CO i 4 0 0 SI T r. /IQ L Tj i ABSTRACT A method of reduction of radio frequency interference in ionospheric ground back-scatter sounding measurements comprising the steps of: recording a plurality of back-scatter sounding spectra by transmitting a signal from a transmitter means and receiving at a receiver means signals back-scattered from a remote location; converting the back-scatter sounding spectra to back- 3catter range signals, apportioning the range signals into a plurality of range cells, each cell containing a signal proportional to the strength of the signal back-scattered from a certain range; converting the range signals to spectral 1 0 density signals; determining a minimum spectral density for each of the range cells across the plurality of back-scatter sounding spectra; summing the 00 °0 minimum spectral densities to produce a reference; summing the spectral density signals for each of the plurality of back-scatter spectra across the Splurality of range cells to produce a single spectral density value for each 1 5 spectrum; comparing the single spectral density value for each spectrum with the reference and rejecting any spectrum for which the spectral density value exceeds the reference by more than a predetermined amount; and producing a back-scatter sounding ionogram from the remaining spectra. o I o o0 o o 4.V il