GB2258775A - Distinguishing independent radio transmissions in a specific frequency range - Google Patents

Abstract

In reconnaissance of radio transmissions occurring at arbitrary times within a specific frequency range, the amplitude and/or phase spectrum, or the real and imaginary parts of the complex spectrum, of a signal mixture received in the frequency range observed is sampled and averaged over several sampling cycles, and at the reception site a data reduction is effected by evaluation of the spectrum with respect to frequency occupancy and/or angle of incidence occupancy, and the reduced data are transmitted for further evaluation. <IMAGE>

Description

Patent Claims 1. Method of detecting, recognizing and distinguishing radio transmissions such as frequency happing (FH), burst, automatic channel selection (ACS) and single-channel (SC) transmissions occurring independent of one another and at arbitrary times within a specific frequency range, the received signals being evaluated over a specific time with respect to the radio trans mission method used, the angle of incidence and the transmis sion frequency, comprising: sampling and averaging over several sampling cycles of the amplitude and/or phase spectrum or the real and imaginary parts of the complex spectrum of a signal mixture received in the frequency range that is being observed; efferting data reduction at the receiving site by evaluation of the spectrum with respect to frequency occupancy and/or angle occupancy; and making available the reduced data for further evaluation.

2. Method as claimed in Claim 1 wherein the spectrum (transferred to the time axis) of the whole frequency range being observed is composed of partial spectra that are obtained using receiver modules containing dispersive delay lines.

3. Method as claimed in Claim 1 or Claim 2 wherein the amplitude spectrum, or the modulus values of the complex spectrum, is converted into a scheme which indicates the occupancy of a fre quency cellarray and is updated with each sampling cycle.

4. Method as claimed in Claim 3 wherein the phase spectrum, or the argument values of the complex spectrum, is assigned to the frequency cellarray and wherein, in the case of occupied fre quencies, the angle of incidency values derivable from the pha se spectrum are determined and entered into an angle cellarray.

5. Method as claimed in Claim 4 wherein spreads of angle values are recognized as such from the angle and frequency cellarrays, wherein the final angle of incidence is determined on the basis of the distribution of its occurrace, and wherein the final va lue of the angle of incidence and/or the spread of angle values are used for further evaluation.

6. Method as claimed in Claim 4 or Claim 5 wherein the contents of the cellarrays that has been periodically updated with respect to frequency and/or angle occupancy is transferred into a lar ger time scale and wherein the rate of occupancies within this time scale is used as the basis for evaluation.

7. Method as claimed in Claim 6 wherein, for controlling recon naissance, only those angle of incidence values are used for further evaluation which have been found to be essential.

8. Method as claimed in Claim 6 or Claim 7 wherein SC and/or ACS mode transmissions are recognized from the concentration of the spectral energy within a frequency cell or from its occupation density (= number of samples indicating occupied frequency sell) and wherein this recognition is based on a specific thre shold value that is characteristic of SC or ACS mode transmis sions.

9. Method as claimed in Claim 8 wherein the angle of incidence va lues determined averaed according to the formula

where k is the frequency cell, i the angle of incidence cell, nik the number of occupancies of k in i, and wherein the final angle of incidence value is determined.

10. Method as claimed in Claim 6 or Claim 7 wherein the FF3 and burst transmissions are recognized from their specific occupa tion densities, the distribution of the spectral energy sugges ting both a broad frequency range comprising several frequency cells and FH transmissions, while the short transmission time suggests burst transmissions, wherein, in the case of FM trans missions, the frequency occupancies belonging to one angle of incidence range are combined and wherein, by using a threshold value, SC and ACS mode transmissions are left unconsidered.

11. Method as claimed in Claim 10 wherein the transmission duration and/or the transmission end and/or the varying occupation den sity of the frequency cells are evaluated for such angle of in cidence ranges whose mode of occupancy suggests FH transmis sions.

12. Evaluation unit for implementing the method as claimed in any of the Claims 1 to 11 wherein at least two memory ranges (2, 3) are provided, in which the angle of incidence values and the rates of frequency occupancy are filed and which are addressed by a frequency cell counter (4) and wherein one adder (5, 6), respectively, is assigned to a memory range (2, 3), wherein a comparator (7) is provided which compares the rates of frequen cy occupancy with given threshold values (8) and determines the mode of radio transmission, wherein an analog-digital conver ter (10) for digitization of the phase spectrum as well as a threshold value switch (1) for taking binary decisions on the frequency occupancy are available, and wherein the data from the memory ranges (2, 3) can be used for further evaluation.

13. Evaluation unit as claimed in Claim 12 wherein the memory ran ges (2, 3) are linked with a processor in which further data reduction is effected.

Method of Detecting, Recognizing and Distinguishing Radio Transmissions Occurring Independent of One Another in a Specific Frequency Range The invention relates to a method of detecting, recognizing and distinguishing radio transmissions such as frequency hopping (FH), burst, automatic channel selection (ACS) and single-channel (SC) transmissions occurring independent of one another and at arbitrary times within a specific frequency range, the received signals being evaluated over a specific time with respect to the radio transmission method used, the angle of incidence and the transmission frequency. In addition, the invention relates to an evaluation unit for implementing this method.

For reconnaissance in a specific frequency range, it is necessary to detect and locate an arbitrary number of radio transmissions, an important criterion being the distinction of the mode of radio transmission applied. SC and burst mode transmissions, for example, differ only in the duration of transmission. In the case of ACS, several groups of radio network subscribers use the same group of channels, so that it becomes difficult for opposing reconnaissance to identify the networks.

Application of the FHmode implies rapid switching between a major amount of frequencies within a (hopping) band. The systematics of this quasi-arbitrary selection of frequencies isonly known to the subscribers to the FH radio network. A reconnaissance receiver monitoring this radio network therefore must determine the respective actual frequency of the FHradio network in a search run; in the case of rapid hopping sequences and simultaneous activity of several radio networks within the receiving range and within the frequency band to be observed, however, this is normally not possible.

For detecting and locating SC mode transmissions, both manual and automatic methods are known. In the case of a short-time transmission, i.e. a burst transmission, however, systematic recognition is possible only with receivers which permit simultaneous observation of the whole frequency range to be monitored. Burst transmissions, on the other hand, cannot be monitored by means of conventional direction-finding (DF) methods, as the transmission is terminated before the DF command is given, i.e. because the time required for direction finding is longer than the transmission time. Recognition of ACS mode transmissions, just as burst mode transmissions, require continuous monitoring of the allocated group of frequencies by means of special receiver modules, which contain dispersive delay lines as essential components.

Direction finding of the individual channels can be effected by conventional methods. FH mode transmissions can at present only be detected manually. Conventional locating methods fail for the reasons given for burst mode transmissions.

All information obtained by conventional automatic methods have so far been reported in full to an evaluation center. In the case of FH methods, the information rate is so high that transmission by conventional methods is not possible.

Therefore, it is the aim of the present invention to develop a method which permits automatic recognition, distinction and at the same time direction finding of radio transmissions. In addition, transmission of the resultant data should be possible by conventional means.

It has been found that this aim can be achieved by a method of the type described in the foregoing if the amplitude and/or phase spectrum, or the real and imaginary parts of the complex spectrum, of a signal mixture received in the observed frequency range are sampled and averaged over several sampling intervals, and if at the receiving site data reduction is effected by evaluation of the spectrum with respect to frequency occupancy and/or angle occupancy and the reduced data are made available for further evaluation. Favorable embodiments of the method according to the invention are described in Subclaims 2 to 11. An evaluation unit for implementing the method according to the invention is described in Claims 12 and 13.

To permit detection, direction finding, reception and processing of signals originating from opposing radio transmissions, sensitive receivers are required according to the invention, which offset the drawback of the unfavorable receiving site with respect to the radio network observed. In addition, quasi-simultaneous observation of the whole frequency range to be monitored is necessary. Both requirements are satisfied by application of a receiver, module which contains dispersive delay lines (compressive receiver). Such a receiver has, for example, an analysis bandwidth of 6 MHz, a frequency resolution (3-dB values) of 15 kHz, a sampling cycle duration of 200 us. Its sensitivity is comparable to that of a conventional reconnaissance receiver.In the case of the example specified, the time between two sampling cycles is so short that FH mode transmissions with hopping rates up to 5,000 s can be detected. Simultaneous monitoring of the whole reconnaissance band of, e.g., 20 to 88 MHz can be achieved according to the invention by division into sub-bands to each of which a receiver module is allocated. To improve the dynamic behaviour of the receiver, it is possible to eliminate emissions with high incoming signal levels by using in addition narrow-band filters of conventional design.

According to the invention, the signals received, e.g., in the range between 20 and 88 MHz, are distributed in a junction box among about 10 to 12 sub-band detectors, each of which processes a bandwidth of about 6 MHz. A band-pass filter is used for rough preselection of the respective sub-band. An adjustable band-stop filter permits close high-power transmitters to be eliminated or at least attenuated. The sub-band is converted in a mixer and passed on to a self-adjusting amplifier which adapts the signal mixture contained in the sub-band to the volume range of the receiver.

The spectrum of the signal mixture is transferred by the receiver to the time axis after each sampling cycle (e.g. after every 200 /us). It can be averaged over several sampling cycles and then is available at a lower repetition rate. The timing for the output of the spectrum (100 /us), however, remains unchanged, so that it is possible to represent the whole frequency band to be observed on the time axis by lining up several, e.g. 10, sub-band spectra.

Finally, a distinction is made between "occupied" and "non-occupied" states of the individual channels on the basis of the spectral power density.

In the case of a frequency band of 60 MF3z to be observed, a channel spacing of 25 kHz and a read/cancel cycle TL of 1 ms, the detector generates an information flow of 2.4 Mbit/s.

In order to reduce the effort required to process data rates of this magnitude, preprocessing by hardware aimed at a reduction of irrelevance is useful, which is described below in more detail. An important characteristic of the transmitter to be observed is its position, which is normally determined by multibeam direction fin ding and subsequent position-finding calculation. In the case of burst and FH mode transmissions, the direction finder must meet much higher demands with respect to time because of the short channel dwell time involved, in particular as the next transmission channel triggered by the frequency hopping transmitter normally cannot be determined in advance. If applied to low-rate frequency hopping, time delays between signal detection and completion of direction finding, therefore, should not exceed 5 ms.Accurate carrier direction finding is not possible, when FH systems are operated by frequency shift keying. Direction finding for FH transmissions cannot be achieved by conventional DF command and reply structures, because the time required for commanding already exceeds the acceptable DF reaction time. Rather, it is necessary to command the DF unit directly by the detector.

According to the invention it is possible, for example, to use a DF antenna with four outputs for phase comparison. Each of the four outputs is connected to one junction box, from which the incoming power is distributed to band-pass filters tuned to the further frequency bands to be processed. The high-frequency signals of the four DF channels are fed, via four equally tuned band-pass filters for sub-band selection and via band-stop filters for blocking the signals received at high levels, to four mixers, to which a common oscillator frequency is supplied in phase, in addition to the radio-frequency received. The signals derived from the mixer, whose mutual phase relations represent the phase relations at the DE antenna, are passed on to four sub-band amplifiers which are jointly controlled. The signals of the individual processing chains are fed to a separate DFT module for each DF channel. Each of these DFT modules converts the signal of a DF channel into its complex spectrum (amplitude and phase) . The DFT modules are timed in phase by a coinmon clock, in order to obtain a result that can be evaluated with respect to the phase differences.

As in the case of the above-described detector unit, the phase spectra supplied by the DFT module can be averaged and combined into a phase spectrum covering the whole frequency band to be observed.

The combination of detector unit and direction finder supplies a data rate of 19.2 Mbit/s. As these data have a high degree of redundancy and irrelevance, they are forwarded to automatic preprocessing within the acquisition component, where they are reduced to an amount that is reasonable for remote post-processing and brought into a format that is appropriate for the message.

The amplitude spectrum obtained from the detector can in principle by converted into a scheme which indicates the occupancy of the VHF range for a 25-kHz channel spacing and which is updated at a 1-ms cycle. In the same way, the angles of incidence of the signals can be derived from the phase information supplied by the direction finder and allocated to the occupancy scheme. Only those angle values are relevant which correspond to occupied channels; the other angle values would only indicate the angles of incidence of noise and therefore are sorted out.

The invention is described in more detail on the basis of the attached schematic drawings: Fig. 1 shows the scheme of occupancy and DF values; Fig. 2 shows the principle of data reduction; Fig. 3 shows the matrix of short-term statistics; Fig. 4 shows an example of derivation of the matrices for frequency occupancy (f,t) and angle occupancy (omit); Fig. 5 shows the matrix of frequency occupancy versus time; Fig. 6 shows the matrix of angle occupancy versus time; and Fig. 7 shows the block diagram of the unit for determining the short-term statistics.

As results from Fig. 1, it is possible to indicate the channel occupancy for each cycle of 1 ms, and the angles of incidence for the occupied frequency cells, or channels. This instantaneous image is regenerated in each cycle and represents a data rate of 19.2 Mbit/s. As only the half space located in front of the direction finder is of interest and all the other values obtained by direction finding are left unconsidered, the data rate decreases to 9.6 Mbit/s.

If, according to Fig. 2, a larger time scale is used, the data rate is further reduced to 3.07 Mbit/s. To this end, the occupancy and DF values are entered into a matrix according to Fig. 3.

The combination, which in the following is designated as short-term statistics, is derived from the occupancy and DF matrix according to Fig. 1 and regenerated every second. It indicated for each channel and for each angle of incidence cell, or sector, how often it has been found to be occupied. The short-term statistics permits the characters of the transmissions received to be derived. All transmissions received will extend over a more or less extended angular range, depending on the spread of DF values. This spread of angle values will decrease with increasing DF accuracy. The mean value of the DF values corresponds to the probable DF value that is obtained if it is assumed that, in the time interval under consideration, the channel concerned was occupied only by one transmitter. This condition will in general be satisfied by SC or ACS mode transmissions.

At the same time, another useful characteristic for further processing can be derived, because in the c ase of speech transmission (analog or digital) the communication times, on the average, range between 4 and 6 s and thus can probably be observed during the whole time interval of 1 s. For this time interval, the number of occupancy states of the channel observed, which are reported by the detector, will be large.

With the designations given in Fig. 3, the following relation-ships are obtained: Most probable DF value:

where k indicates the channel (frequency cell), i the sector (angle of incidence cell), nik the number of occupancies of channel k in sector i.

SC or ACS mode transmissions:

NE = minimum number of detections fo SC and ACS mode transmissions FE or burst mode transmissions:

NF = maximum number of indication errors Non-occupied channel:

The determination of the decision thresholds NE (e.g. 100) and NF (e.g. 4) must be substantiated by empirical investigations.

The result of the decision and the mean DF value are entered into a matrix according to Fig. 5 (see also Fig. 4), so that one matrix line is obtained every second. If the decision 5k is coded with 2 bit and the probable DF value OLk with 7 bit, this corresponds to a data rate of 21.6 kbit/s.

To distinguish between SC or ACS mode transmissions and FM mode transmissions, the following steps are necessary: SC and ACS mode transmissions: The times of start of transmission (transition from another mode into that of SC or ACS mode transmission) and end of transmission (transition into another mode) are recorded. All mean DF values between start and end of transmission are averaged again. The spread of mean DF values is determined.

SC transmission is described by a data set according to the following table: Mode of transmission (SC/ACS) 2 bit Channel No. k 12 bit Sector No. i (mean DF value) 7 bit Spread of DF values 3 bit Start of transmission within the minute concerned 6 bit End of transmission within the minute concerned 6 bit Hour and minute of detection 11 bit Length of data set: 47 bit In addition, those bit strings of channel coding have to be taken into account which are dependent on the coding method.

Even higher traffic densities will not give rise to problems with respect to the data rate to be transmitted.

FH and burst mode transmissions: In the case of SC and ACS transmissions, the spectral energy is concentrated on one channel, and the duration of transmissions in general is sufficiently long, so that the detector reports a high channel occupation density. These conditions are not satisfied in the case of FE and burst mode transmissions. In the case of FH mode transmissions, the spectral energy is distributed over a wide frequency range. In the case of burst mode transmissions, the durations of transmission are extremely short. The data sets to be reported to the special processing unit for technical analysis can be derived from the matrix of frequency occupancy versus time (Fig. 5). They have the format described in the following table.

A format according to this table is suitable for reporting by the technical analysis unit. The respective hopping band limits are forwarded to the acquisition station together with the reconnaissance order.

Indication of FH mode transmission: Mode of transmission (FH) 2 bit Channel No. k 12 bit Sector No. i (mean DF value) 7 bit Hour and minute of detection 11 bit Length of data set: 32 bit Report on hopping band limits: Mode of transmission (fah) 2 bit Lower band limit (channel No.) 12 bit Upper band limit (channel No.) 12 bit Sector No. i 7 bit Hour and minute of analysis 11 bit Length of data set: 40 bit Reconnaissance of FE mode transmissions again starts from the short-term statistics according to Fig. 3 but, in contrast to SC mode transmission where the elements of a matrix column are combined into one sector, the reported occupancies corresponding to a matrix line are combined.

By elimination of the numbers of occupancies n ik of those matrix elements of the short-term statistics which exceed a threshold value n5 that remains to be determined, or by restriction of the range of occupancy numbers to this threshold value as the upper limit, it is possible to exclude or strongly reduce the effect of SC mode transmissions::

ik (nik d n ) n ik ik = no or O (n ik > n5) By forming the sums

k , k = channel numbers of the 1 2 hopping band limits over the restricted occupancy numbers n' ik of the matrix elements of the short-term statistics, the occupancy numbers of the sectors i with FH mode transmissions are obtained for each second.

Shape and contents of the resultant matrix are shown in Fig. 6 (example of the derivation in Fig. 4). The mode of transmission in sector i, expressed by ri, is determined according to the following consideration: If only noise or SC mode transmissions are received from the sector observed, the sum N. corresponds to the indication error rate or to the threshold value of the occupancy numbers for SC mode transmissions, multiplied by the number of SC transmitters that are active at this point of time in the sector observed. As the whole FH band is observed and as the FH transmitter always transmits du ring its times of activity on any frequency of this band, FH made transmissions supply substantially higher values for N. than SC or ACS mode transmissions.

FH made transmission is assumed to occur exactly at that time when exceeds a decision threshold For the occupancy matrix shown in Fig. 6, one matrix line is generated in each second, corresponding to a data rate of 1.79 kbit/s, if the decision r. and the number of occupancies N. are coded with 1 1 1 bit and 13 bit, respectively. Assuming that 10 of such matrices are necessary for covering the whole VHF range, the total data rate amounts to 17.92 kbit/s or, if the rear half space (own area) is omitted, to 8.96 kbit/s.

Prior to the delivery of the message, this data rate is again reduced by another preprocessing step. In principle, only such sectors are reported whose mode of occupancy suggests FF3 mode transmissions. Besides, messages are delivered only when an FH mode transmission starts or ends, or when the number of occupancies varies by more than a specific factor. Simultaneous equidirectional changes in adjacent angle sectors suggest that, because of the spread of DF values, the same transmitter has been detected in several sectors. For reasons of redundancy reduction, only that sector is reported in which the transmitter is most probably located.

This sector is characterized by the comparatively largest additive change of the rate of occupancies. The DF accuracy (spread of DF values) can be estimated from the influence exerted on the adjacent sectors. The data set to be reported may have the form indicated in the following table. The following types of changes are possible: start of FH mode transmission, end of FH mode transmission, increase in number of occupancies and decrease in number of occupancies.

Mode of transmission 2 bit Frequency range 3 bit Sector No. 7 bit Spread of DF values 3 bit Type of change 2 bit Value of change in number of occupancies 10 bit Time of change within the minute concerned 6 bit Hour and minute of detection 11 bit Length of data set: 44 bit The frequency range indicates the highest and lowest frequencies observed.

Data preprocessing reduces the amount of measured values obtained to the frequency and angle occupancy schemes. Because of the high velocity required, a solution to this problem may consist in the implementation of a special hardware circuitry for high-speed processing, while a processor may be used for processing at a lower rate.

The basic principle is shown in Fig. 7. A high-speed preprocessing unit supplies in each second a data block containing short-term statistics. In the preprocessing units 1 and 10, the amplitude and phase spectra, inspectively, are converted into binary values.

These values are supplied every 417 ns. Processing is synchronized by the corresponding clock of 2.4 MHz.

The accumulated sums of angle values and occupancy rataes are filed in two memories 2 and 3 which are addressed by a counter 4 whose contents correspond to the number of the actual, analyzed channel. Eac step of addressing the memories 2 and 3 starts with reading out of the old values and adding of these values to the actual measured values by adders 5 and 6. The sums are filed in a subsequent write cycle. In the calculation of the rate of frequency occupancies, the new result is subsequently compared by a comparator 7 with four threshold values (T1...T4) 8. The result of this comparison indicated the transmission mode and is also filed. In this way, the measured values are compressed into short-term statistics within one second.The contents of memories 2 and 3 and transferred into the memory range of the processor with highspeed DMA transfer via the processor bus 9. By means of software routines, the frequency and angle occupancies are determined from the short-term statistics by an appropriate sorting process. It may be necessary to use two independent parallel processors for this purpose. Further reduction of the data by determination oif the points of time of transmission as well as of the duration of transmission and averaging of the DF values is also effected by means of software routines.

Recognition of burst mode transmissions is possible, as these do not differ from FX mode transmissions with respect to their channel occupancy figure in the short-term statistics, but normally occupy only one single channel. Therefore, they are observed and recognized by the technical analysis unit in the same way as the hopping bands of FH mode transmissions. In the case of burst mode transmissions, it is recommended to file the data for an extended period of time, in order to be able possibly to draw conclusions from the site, from frequency and from the rate of occurrence.

The hopping rate can be derived from the short-term statistics. The dwell time is proportional to the number of occupancies "i' In the case of mixed frequency hopping/time hopping operation, the dwell time is variable.

The information reduced according to the invention is reported to an evaluation center. The center supplies a result of tactical value by allocating the events reported by an acquisition station among one another or to events reported by other acquisition stations, in addition using appropriate basic material.

First, the events reported to the evaluation center-by various acquisition stations can be allocated among one another, which serves in particular to determine the transmitter location by multibeam direction finding. Suitable allocation parameters are exclusively those parameters which are reported by the acquisition stations.

Parameters suitable for allocating identical SC or ACS mode transmissions include, in particular, frequency and time expressed by channel number, start of transmission within the minute concerned, end of transmission within the minute concerned, and hour and minute of detection. In the allocation of FH mode transmissions, the time criteron is of special importance, whereas the frequency is only of minor importance. Suitable criteria are frequency range, type of change, value of change of the number of occupancies, times of change within the minute concerned and hour and minute of detection.

The amount of reported results allocated among one another and made less redundant is combined with the result of the position-finding calculation and can be stored in a file, using the following format SC/ACS transmissions: Mode of transmission (SC/ACS) 2 bit Channel number 12 bit Sector number 4 x 7 bit Spread of DF values 4 x 3 bit Start of transmission within the minute concerned 6 bit End of transmission within the minute concerned 6 bit Hour and minute of detection 11 bit Transmitter location determined by position finding 56 bit Position finding confidence 7 bit Length of data set: 140 bit FH mode transmissions: Mode of transmission 2 bit Frequency range (highest and lowest frequency) 2 x 4 bit Number of occupancies 2 x 10 bit Sector number 4 x 7 bit Spread of DF values 4 x 3 bit Start of transmission within the minute concerned 6 bit End of transmission within the minute concerned 6 bit Hour and minute of detection 11 bit Transmitter location determined by position finding 56 bit Position finding confidence 7 bit Length of data set: 156 bit These data are collected and used in further processing steps which serve for distinguishing between SC and ACS mode transmissions and for determining the group frequencies and recognizing alternate frequencies.

Thus, it is possible to recognize specific SC mode communication circuits by repeating communication relations on the same channel and by typical network structures, using conventional methods. A characteristic feature of ACS transmissions is that a transmitter of a specific location transmits on different channels of a group of frequencies. In addition, there is a difference in spatial distribution and in the apparent number of subscribers to SC networks. The individual channels of the ACS frequency group can also be identified on the basis of their higher rate of occupancies.

If it has been recognized that specific network groups communicate in the ACS mode, the individual networks can be determined on the basis of their spatial distribution and by taking into account the time sequence of transmissions in terms of a traffic analysis.

In the case of FH mode transmissions, the individual networks differ both in the occupied frequency band and in the hopping code. The only decision criterion for reconaissance is the frequency band, so that - similar to ACS mode transmissions - groups of networks are obtained which occupy the same frequency bands. Decomposition of the network group into individual networks cyn be achieved by conventional methods of traffic analysis.

Claims (15)

Amendments to the clans have been filed as follows

1. A method of detecting, recognising and distinguishing radio communication transmissions which may be of different modes (for example frequency hopping (FH), burst, automatic channel selection (ACS) and single channel (SC) mode transmissions), which occur independently of one another and at arbitrary times within a specific frequency range, and which can occur simultaneously with one another, in which received signals are evaluated over a specific time to ascertain the radio transmission mode used, the angle of incidence and the transmission frequency, the method comprising:: sampling and averaging, over a plurality of sampling cycles, (i) the amplitude and phase spectrum or (ii) the amplitude spectrum and the angle of incidence or (iii) the complex spectrum of a signal mixture received in the frequency range that is being observed; effecting data reduction at the receiving site by evaluation of the spectrum with respect to frequency occupancy and angle occupancy; and making available the reduced data for further evaluation.

2. A method as claimed in claim 1, wherein partial spectra of the spectrum (transferred to the time axis) of the whole frequency range being observed are obtained using receiver modules containing dispersive delay lines.

3. A method as claimed in claim 1 or claim 2, wherein the amplitude spectrum, or the modulus values of the complex spectrum, is converted into a scheme which indicates the occupancy of a frequency cell array and is updated at each sampling cycle.

4. A method as claimed in claim 3, wherein the phase spectrum, or the argument values of the complex spectrum, is assigned to the frequency cell array, and wherein, in the case of occupied frequencies, angle of incidence values are determined and entered into an angle cell array.

5. A method as claimed in claim 4, wherein spreads of angle values are recognised as such from the angle and frequency cell arrays, a final value of the angle of incidence is determined from the distribution of its occurrence, and the final value of the angle of incidence and/or the spread of angle values is/are used for further evaluation.

6. A method as claimed in claim 4 or claim 5, wherein the contents of the cell arrays that periodically have been updated with respect to frequency and/or angle occupancy are transferred to a larger time scale and the number of occupancies within this time scale is used as the basis for evaluation.

7. A method as claimed in claim 6, wherein, for controlling reconnaissance, only angle of incidence values which have been found to be essential are used for further evaluation.

8. A method as claimed in claim 6 or claim 7, wherein SC and/or ACS mode transmisions are recognised from the concentration of the spectral energy within a frequency cell or from its occupation density (= number of samples indicating an occupied frequency cell), and this recognition is based on a specific threshold value that is characteristic of SC or ACS mode transmissions.

9. A method as claimed in claim 8, wherein the angle of incidence values are averaged according to the relationship:

where k is the frequency cell, i is the angle of incidence cell, njk is the number of occupancies of k in i, and wherein the final angle of incidence value is determined.

10. A method as claimed in claim 6 or claim 7, wherein; FH and burst mode transmissions are recognised from their specific occupation densities, the distribution of the spectral energy suggesting both a broad frequency range comprising several frequency cells and FH transmissions, while a short transmission time suggests burst transmissions; in the case of FH transmissions, the frequency occupancies belonging to one angle of incidence range are combined; and by using a threshold value, SC and ACS mode transmissions are ignored.

11. A method as claimed in claim 10, wherein the transmission duration and/or the end of transmission and/or the varying occupation density of the frequency cells are evaluated for angle of incidence ranges whose mode of occupancy suggests FH transmissions.

12. An evaluation unit capable of implementing a method as claimed in any one of claims 1 to 11, the evaluation unit comprising at least two memories in which, in use, angle of incidence values and number of frequency occupancy values are stored, a frequency cell counter for addressing the memories, a respective adder associated with each of the memories, a comparator operative to compare the number of frequency occupancy values with given threshold values and to determine the mode of radio transmission, and an analog-to-digital converter and a threshold value switch for taking binary decisions on the frequency occupancy, the data from the memories being usable for further evaluation.

13. An evaluation unit as claimed in claim 12, wherein the memories are linked with a processor in which further data reduction can be effected.

14. A method of detecting, recognising and distinguishing radio communication transmissions which may be of different modes, which occur independently of one another and at arbitrary times within a specific frequency range, and which can occur simultaneously with one another, the method being substantially as herein described with reference to the accompanying drawings.

15. An evaluation unit capable of implementing a method according to claim 14, the evaluation unit being substantially as herein described with reference to the accompanying drawings.