WO1990015343A1

WO1990015343A1 - A system for detection, localization and classification of target objects

Info

Publication number: WO1990015343A1
Application number: PCT/NO1990/000099
Authority: WO
Inventors: Dagfin Brodtkorb
Original assignee: Miros A/S
Priority date: 1989-06-08
Filing date: 1990-05-29
Publication date: 1990-12-13
Also published as: NO892064L; NO892064D0; NO169267C; AU5820490A; NO169267B

Abstract

A system for detection, localization, and classification of targets, which preferably are located on or close to the surface of the earth, comprises an interrogator, comprising a transmitter, a receiver, and an antenna and is designed for emitting electromagnetic pulses, and one or a plurality of transponders which respond to an emitted pulse by emitting a response signal, each target being provided with at least one transponder, each of which comprise an antenna, an impedance matching network, and a pulse generator. The transponders are designed to emit a response signal as a response to a detected interrogator pulse within a period of time, which is within the time interval between the detected interrogation pulse and the subsequent interrogation pulse. The response signal is coded, so that the code is preferably characteristic of a particular transponder or the category of the target associated with the transponder. The interrogator is designed to detect the response signal and comprises a system processor, which is connected with the navigational system and is, furthermore, connected with a display device. A method for use of the system is also disclosed.

Description

A System for Detection, Localization and Classification of Target Objects .

The invention relates to a system for detection, location, and classification of targets, which are preferably located on or close to the surface of the earth, which system is connected with an external navigational system for position finding, or comprises a position finding navigational system, and which, furthermore, comprises an interrogator device, comprising a transmitter, a receiver, and an antenna, and is designed to transmit electromagnetic pulses, and one or more transponders, which respond to a transmitted pulse by emitting a response signal, each target being provided with at least one trans¬ ponder.

The invention especially relates to a system of the above- mentioned kind which may be used in connection with a search and rescue system at sea and on land.

A similar system is, e.g. known from GB-PS No. 1 295 566 relating to a lightweight rescue transponder which responds, e.g. to pulses transmitted by radar, and which may be carried by a person for identification and location of that person. The transponder comprises a pulse receiver and a pulse transmitter and it responds to a received pulse by transmitt¬ ing pulses which may be detected by a receiver, if desired, for position finding and identification. GB-PS No. 1 552 044 shows a radar transponder for use with a seeking radar of a pulse modulated kind to indicate the position of a person to be rescued in case of a shipwreck or the like. The radar transponder has a receiver unit for receiving and demodulating a pulse modulated radar signal, as well as a controlled transmitter unit which transmits a response signal which is frequency modulated to sweep a frequency range including the carrier frequency of the searching radar. GB-PS No. 1 584 937 shows a radar beacon or a radar transponder which may also be used, e.g. in rescue operations in connection with a shipwreck. GB-PS No. 2 070 894, DE published application No. 27 122 595, and US-OS No. 4 047 171 also show radio beacons or radar transponders for use in navigation or position finding, e. g. at sea. For transponders partly passive units based on SAW technology may be used.

Furthermore, use of completely passive radar reflectors was known for a long time. They are, inter alia, used for search and rescue in connection with, e.g. shipwrecks and other accidents at sea. A well-known problem in connection with such passive reflectors is that the receiver does not only record the reflected signal from the reflector, but also background noise in the form of ground reflections, or so called clutter from the surroundings, e.g. the surface of the earth, which may render detection of the reflector signal difficult. It is especially difficult to detect reflections from such passive reflectors when they are in heavy or high sea, since echo signals or clutter from the surface of the sea commonly mask the reflection signal from the reflector.

Also, problems with ground reflections and clutter are not avoided by use of transponders. The problem was, however, recognized in the above mentioned US-PS 4 047 171, in which it is stated that the response signal will have an increasing amplitude level when the distance between the searching radar and the transponder is reduced. The signal received by the radar is, thus, stronger at short distances, since the level of the output signal from the transponder is correspondingly increased. To some extent, this will contribute to cancel the increasing background echo observed by the radar at short distances, but the background echoes will still coincide with the response signal in time and, thus, reduce the possibility of detection.

The applicant previously developed an automatic, so called "hands free, walk through" system for detection and ident¬ ification of personnel, which is substantially used in connection with oil activity at sea. The main elements of the mentioned system are a passive identification transponder based on SAW technology which is carried by all personnel, electronic interrogation gates which are able to detect, and identify the identification transponders, and a data processing and presentation system. Such a system, however, might also be a starting point for the development of similar systems for other applications. Such an application is, e.g. detection and localisation of personnel in the water. In connection with emergency situations which, e.g. require oil rigs to be evacuated, persons may fall overboard. Search and rescue operations may have to be conducted in darkness and under unfavourable conditions precluding search by the aid of optical means. It is, however, an essential problem with common radar that the return echo from small targets (e.g. a person) will be completely masked by echo from the surface of the sea (clutter) , even if the targets are provided with passive radar reflectors as mentioned above. The application of a passive SAW-based transponder known per se, thus, represents an ideal solution to this problem. The response signal from the target may be filtered off from the sea reflection background in the time domain, and then reliable detection is possible and is only limited by noise from the radar receiver.

It is, thus, an object of the present invention to provide a system of the above mentioned kind. With said system pulses are transmitted by an interrogator or the like and they hit a transponder and the background surrounding the transponder (e.g. the ground or the surface of the sea) approximately at the same time, so that a strong echo is generated which is completely dominated by reflection from the background.

Another object of the invention is, thus, that power which is received by the transponder antenna is to be delayed until the background echo has subsided, and that a response signal is transmitted from the transponder after a delay of suitable duration, but before the next interrogation pulse is received by the transponder. The delay which is generated according to the invention will contribute to separate the relatively weak response signal transmitted by the transponder from the background echo or clutter signals. In this manner the system will show high sensitivity for detecting the response signal. This sensitivity is not limited by the background echo, but only by thermal noise in the interrogator receiver.

A third object of the invention is to provide a simple code of the response signal to permit distinction between various kinds of targets (e.g. a survival suit, a safety jacket, a life raft, a lifeboat, etc) .

A fourth object is to combine detection of the response signal with position finding.

According to the invention the above-mentioned objects are achieved by the fact that the transponders comprise an antenna, an impedance matching network, and a pulse generator, that the transponders are designed to emit a response signal in response to a detected interrogation pulse within a period of time lying within the time interval between the detected inter¬ rogation pulse and a subsequent interrogation pulse, the frequency of the response signal preferably being equal or approximately equal to the frequency of the interrogation pulse, that the response signal is coded, the code preferably being characteristic of a certain transponder or the category of the target associated with said transponder, that the interrogator is designed to detect the response signal, that the interrogator comprises a system processor which is connected with the navigational system, and that the inter¬ rogator is connected with a display means. A further object is to provide a method for use of a system which is distinguished by the above features, said method being characterized by the fact that a response signal, which is triggered by an inter¬ rogation impulse from the transponder is delayed by a period of time which is longer than the duration of an echo signal caused by ground reflections of the triggering interrogation impulse, but shorter than the time interval between said interrogation pulse and a subsequent interrogation pulse, the duration and rate of the interrogation pulse being adapted to the current conditions of ground reflection and the geometry of measure¬ ment. Further features and advantages of the system and the method for use of the system will appear from the following dependent claims.

The invention is disclosed in more detail below with reference to an embodiment of the system and its application in con¬ nection with the attached drawing.

Figure 1 shows a diagrammatic view og a transponder according to the invention; Figure 2 shows a time-dependency diagram of an interrogation pulse and a response signal; Figure 3 shows a pulse generator used in the transponder of

Figure 1 and based on SAW technology; Figure 4 is a block diagram of an interrogator according to the invention; Figure 5 is a time-dependency diagram of received background echo and response signal; Figure 6 shows sequential sweeping of a region A; Figure 7 shows the geometry of the sweeping system; Figure 8 is a diagrammatical view of an operational scenario; Figure 9 illustrates the search radar system; Figure 10 is a diagrammatical view of the design of a baseline system according to the invention.

The block diagram of the transponder is diagrammatically illustrated in Figure 1. The transponder comprises an antenna, an impedance matching network, and a pulse generator.

When the transponder receives an interrogator pulse with length Tj__p on its antenna, the pulse generator is triggered and generates a response signal which is transmitted from the antenna after a certain time Tp^. The response signal is comprised of RF pulses at mutual intervals T_x as shown in Figure 2. By using different values of T_x for different categories of target objects the category of the target object may be determined by measuring the time delay T_pd in a receiver which is provided in the interrogator.

In principle the pulse generator of the transponder may be implemented by an active unit, based on battery drive, or by a passive system whithout any battery. A passive system without any battery has a number of operative advantages, and it may advantageously be implemented by the aid of SAW technology (Surface Acoustic Wave), as diagrammatically shown in Figure 3. Such SAW components are well-known to those skilled in the art and, in principle, consist of a crystal, e.g. of lithium niobate (with a surface pattern of metal which constitutes transducers, reflectors, etc. The received interrogation pulse from the interrogator is fed to the transducer. Here, the electromagnetic energy is converted to an acoustic surface wave, which will move along the crystal. A reflector is placed at a distance providing a propagation time equal to half of the desired time delay _pcj. Another reflector has a distance from said first reflector corresponding to half of the desired delay T_x. When the acoustic wave reaches both reflectors current reflection waves are generated, which will move back towards the transducer. The transducer converts both acoustic re¬ flection waves to electromagnetic pulses which are transmitted, via the transponder antenna. The technique used here is in principle like techniques known per se with radar transponders based on SAW technology, since SAW components well known to those skilled in the art are used for delay components in RF communication and detection systems. The transponder which may be designed to be an integrated, encapsulated chip, is coded by selecting a delay T_x being an integral multiple of the length of interrogation pulse T__p. This means that T_x = n • T__p. Obviously, the pulse generator may comprise more than two reflectors, which may be used to provide a large number of different codes.

Figure 4 shows a block diagram of the interrogator. It should be mentioned that the interrogator is a dedicated inter¬ rogator, i.e. particularly designed to be used in the system according to the invention and not, e.g. a conventional air or ship's radar. Pulse generator PG generates an interrogator pulse in the form of a pulse train with centre frequency f_x, pulse width T_j__p, and repetition frequency f_r. The pulse train is amplified in output amplifier Al, bandlimited in transmit¬ ter filter BP1, transmitted through circulator SIRK and, via antenna ANT. The transmitted interrogation pulses hit the transponder and the background surrounding the transponder, i.e. the ground or surface of the sea, approximately simul¬ taneously, and an echo is generated which is completely dominated by background reflection.

After the given time delay T_pcj, and when the background echo has subsided, the transponder reacts by transmitting a response signal. The reception of background echo and response signal in the interrogator receiver is shown in Figure 5. The response signal is caught by antenna ANT and fed to receiver filter BP2, via circulator SIRK. The received response signal is amplified in input amplifier A2 and fed to frequency mixer Ml. Here, the signal is mixed with a local oscillator signal of a frequency f2, down to a suitable intermediate frequency fj_f = fι_ - 2 _' The signal is filtered in signal filter Pb3, amplified in an intermediate frequency amplifier A3, and the modulation curve of the signal is detected in detector ENVDET. The detected modulation curve is video integrated in integrator VIDEO INT, until the desired signal/noise ratio is achieved. Then threshold detection occurs in detector TERSK DET, and classi¬ fication of the target category is achieved by measuring the^' distance between transponder pulses in identification circuit KLAS.

If a transponder is detected, an emergency signal is trans¬ mitted to system processor SYSPROC , which reads the position of the detection from an external navigational system. The navigational system, however, may also advantageously be integrated with the interrogator. Emergency information stating the category and position of the target is displayed on system display DISPL, which is connected with system processor SYSPROC. A typical embodiment of the system according to the invention may utilize a radiated power from the interrogator of 40 dBm, interrogation pulses with a frequency of 850 MHz, a pulse length of 2 μs, and a pulse repetition frequency of 25 kHz, and an antenna gain of the interrogator of 19 dB, whereas the antenna gain used for the transponder is 0 dB. Those skilled in the art will understand that different parameters may obviously be chosen. Instead of electromagnetic RF pulses an interrogator which transmits pulses in the optical region, e.g. infrared pulses may thus be used. The transponder used must then be designed to respond to an optical signal, and the response signal may correspond to an optical signal, but it must not be such a signal. Utilization of optical channels, however, will provide less probability of detection than use of RF signals, and it will also require a more complicated transponder and detection system, especially if interrogation pulses and response signals, respectively, are used on very different frequencies. The most simple and elegant concept is thus utilization of RF signals of the same frequency both for interrogation pulses and response signals.

To provide an example of an application of the present system an operative scenario is presented below, which is based on the system according to the invention, and on the basis of said operative scenario a set of advantageous functional specifications, will be derived for a system for detection, localisation, and classification of targets according to the invention. Also, an embodiment of a suitable system concept will be presented.

In case of a blowout or another accident that may occur on an oil rig, all personnel has to leave the rig. Evacuation is carried out by the aid of lifeboats and helicopters. All personnel commonly use survival suits. During evacuation it may happen that persons fall overboard and drift away from the rig due to sea currents. This will cause a search and rescue operation. Search must be completed within a period of time T. A reasonable upper limit of T must be stated to provide fair chances of finding survivors. Under unfavourable conditions, e. g. in the winter, time T may be assumed to be 30 minutes. Furthermore, if a maximum current of 2 rns^-1 is assumed, a person might drift up to 3.6 km in 30 minutes. If the direction of the surface current is known, it is thus reasonable to assume that an area A of approximately 4 x 4 km, i.e. A = 16 km² has to be searched. We assume that the search is made by a helicopter, i.e. that the interrogator according to the invention is installed on board of the helicopter and is connected with a navigational system, which may be integrated with the interrogator and is, thus installed in the helicopter, although this must not be so. We also assume that an area W x D, called footprint, is covered by each search. In order to achieve a reliable detection an observation period _]_ of each search is required. Coverage of the entire area provides the formula:

^{τ =} —ΪTT ^{' T}l ⁽¹⁾

In order to cover a region A = 16 km² during T = 30 min. an area of 8900 m²/second has to be covered. Flying speed is assumed to be v (ms^-1) . This provides

_X = d/v (2)

with reference to Figure 6. By use of expressions 1 and 2 the following relation results between flying speed v and width of footprint w:

w = T-v (3)

A reasonable flying speed of a helicopter is c = 120 km/h ~ 33 ms^-1. If we assume that A = 16 km², and T = 30 min., equation 3 yields that width w of the footprint must be larger than w = 269 m. In order to ensure 100% coverage, 100% overlap should be used. A reasonable width of footprint is thus approximately w = 500 m. The probability of detection is set at a minimum of 99%.

The system according to the invention should, thus, preferab¬ ly satisfy the following requirements:

* Speed of search v = 33 ms^-1 (64 knots)

* Width of footprint w = 500 m

* Probability of detection < 99%

Below, an embodiment of a system according to the invention is disclosed, which meets all the above specifications, and, most important of all, preferred system parameters are derived.

As mentioned above, the system according to the invention comprises the following elements:

* Each survival suit is equipped with a miniaturized passive transponder chip based on SAW technology (Surface Acoustic Wave) and a small antenna. The transponder is mounted so that the antenna is above water when the target, i.e. a person is floating in the sea.

* The helicopter performing the search is equipped with special radar equipment.

The beam-width of the radar antenna is adapted to the search geometry, i.e. the search footprint width. The radar antenna is directed vertically down towards the surface of the earth, or slightly ahead of the helicopter. The search geometry is shown in Figure 7. The transponder will be detected when it is illuminated by the radar antenna, i.e. is located within the radar footprint. The transponder location is then deter¬ mined by the aid of the helicopter position, the position of the radar footprint relative to the helicopter, and the time elapsed from the first to the last detection. The search radar or interrogator preferably uses a frequency of approxi¬ mately 850 MHz. The same frequency is used by the transponder. It should be mentioned that SAW technology may be utilized for slightly more than 2 GHz.

The beam-width of the antenna may be derived from the search geometry of the system. Hence the antenna gain will also be given. Other parameters that should be determined are polari¬ zation, centre frequency, and bandwidth. Said parameters are all determined by the signal or waveform properties of the system. For a given footprint width (or rather a footprint area) there is a trade off between antenna beamwidth and the flying height of the helicopter. Assuming that the transpond¬ er range varies proportionally with flying height, we have for the received average echo or response power:

The antenna gain G may be expressed in terms of the antenna beamwidth or flying height h and footprint dimensions w and d. The received effect may then be written:

i.e. the received effect is inversely proportional to the square of the footprint area and independent of flying height, The selection of an optimum flying height may thus be based on other premises. A reasonable flying height is h = 1000 m. From Figure 2 it will appear that the following equations apply: θ « Arctan ( /R₀) (6)

Ri = h/sin(Φ - φ/2) (7)

R₂ = h/sin(Φ + φ/2) (8)

ΔR = Ri - R2 (9)

D_x = h/tan(Φ - φ/2) (10) D₂ = h/tan(Φ + φ/2) (11)

d = O₁ - D₂ (12)

Assuming that the antenna depression angle is Φ = 60°, for a flying height of 1000 m R_Q = 1155 m is obtained. To get a footprint width w = 500 m, the antenna beamwidth θ must be θ = 23,4°. For simplicity φ = θ = 23° (symmetrical antenna) is selected, providing an antenna gain of

41 000 G = _θ._φ « 77 « 19 dB (13)

The maximum and minimum ranges Rl and R2 are found to be:

Rl = 1335 m R₂ = 1054 m

hence the range difference Δ R = R^ = R₂ = 281 m.

The surface distances D^ and D₂ are found to be

Oi = 885 m D₂ ^'= 335 m

hence the footprint depth d is d = Dι - D₂ = 550 m.

The antenna centre frequency is equal to the system frequency, 850 MHz. The antenna bandwidth should be slightly larger than the signal bandwidth, i.e. a few MHz. The horisontal component of the electrical field will be short circuited close to the surface of the sea, and the system should, thus, use vertically polarized signals to reduce the effect of the surface of the sea.

For the rescue transponder a simple code of the response signal is sufficient to identify said signal and to disting¬ uish between response signals from the transponder and possible echoes from other man made objects or signals from other transponders. There is, indeed, no requirement for identifi¬ cation of individual transponders.

The waveform of the interrogation pulse will be a pulse-train. The pulse-length should be selected to be as long as possible in order to maximize the power of the transmitted waveform. An upper limit of the pulse-length is given by the properties of the transponder. The maximum total transponder delay is in the order of 12 - 15 μs. In order to ensure that the transponder signal may be distinguished from ground reflect¬ ion or background echo, i.e. the echo from the surface of the sea, a signal delay of 5-10 μs or slightly more is neces¬ sary. With four reflectors being used the maximum pulse-length is, thus, 1-2 μs.

Preferably the entire footprint should be covered by one pulse. This is achieved if pulse-length τ is selected to be

X λ 2 ΔR/c = 2-281/3-10⁸s = 1,9 μs (14)

A reasonable choice, thus, seems to be τ = 2 μs.

The maximum pulse repetition frequency PRF is determined by the maximum signal propagation delay given by the free space distance from radar antenna to transponder R_Q, and the total delay τg of the transponder:

2-R_x PRF ! ( + τ₀ )^_1 (15)

By using R^ = 1335 m and τ = 15 PRF must be less than 41,8 kHz. To ensure reasonable margins and to avoid folding of sea clutter echoes due to range ambiguities a smaller value of 25 kHz may be selected.

The preferred design of the transponder is a reflective device with a size of approximately 0,7 x 20 mm, and a simple antenna with a matching network.

An insertion loss in the order of 25 dB or better may reason¬ ably be expected. A small vertically polarized resonant whip antenna will provide an expected antenna gain in the order of 0 dB or higher.

The received response signal power may be expressed as follows:

P_t • G_r ² • G_t ² • l⁴ ^pr = (4 • π)4 . R₀ ⁴ • L ^(lβ)

in which

Pt = radiated power (40 dBm)

G_r = radar antenna gain (19 dB)

G_ξ= transponder antenna gain (0 dB)

1 = system wavelength (0.35 m for f = 850 MHz)

R_Q = distance to transponder (1155 m)

L = transponder insertion loss (25 dB)

The equation for the response signal power may also be written in a logarithmic form:

P_r = P_r(dBm)+2-G_t+2-G_r+40-lg(l)-43,96-40-lg(R₀)-L (17)

Using the above values P_r = -132 dBm.

The radar receiver noise level may be expressed as

P_N = k-T-B-F (18)

in which k • T = 4 • 10~²¹ Ws

B = receiver intermediate frequency or IF bandwidth

(0,5 MHz for 2 μs pulse-length) F = receiver noise factor (5 dB is a reasonable value)

The receiver noise power may also be expressed in a logarith- mic form:

P_N(dBm) =-174 + 10-lgB + F (19)

Using the above values the noise level PN = -112 dBm. Hence the expected intermediate frequency signal/noise ratio SNR = -132 -(-112)dB = -20 dB.

To ensure reliable detection the signal/noise ratio for predetection should be at least +10 dB, hence an improvement of the signal/noise ratio by 30 dB is necessary.

The signal/noise ratio may be improved by increasing the transmitted power and/or (incoherent) pulse integration. With a flying speed of 33 m/s the response signal of the trans¬ ponder will be within the radar footprint (d = 550 m) for about 16 sec. Pulse repetition frequency PRF is 25 KHz, which means that the transponder is interrogated approximately 4oo.00 times.

Incoherent integration of N pulses will result in an inprove- ment of the signal/noise ratio of

I = 10-lg(N°/8) = 8-lg(N) dB (20)

Consequently, an integration of 5000 pulses will result in an improvement of integration of the signal/noise ratio of I ~ 30 dB, which is exactly what is needed. The signal/noise ratio may be further increased to 15 dB by a slight increase of the radiated power from 40 to 45 dBm (30 W) .

A signal/noise ratio of 15 dB will result in a detection probability of 70% assuming an unknown target phase and a false alarm rate of 10^-6.

During a search a transponder will be observed 80 times with this probability. After 10 observations the cumulative proba¬ bility of detection will be in the order of 0.99999. Reliable detection should thus be expected.

The requirement of 99% probability of detection within a search area of 16 km² within 30 min. is, thus met. The above calculations show examples of the effect of a system according to the present invention for detection of targets or persons in the sea by use of passive miniaturized transponders, and a simple reliable radar system for interrogation. Such trans¬ ponders are very inexpensive in production, even in moderate numbers. It should, however, be understood that the system according to the invention is not limited to detection, localization, and classification of targets in the sea, but may be used generally to find targets on or close to the surface of the earth. The targets, thus, must not be persons, and the transponders may indeed be installed on quite different categories of objects. It should be mentioned that expensive lost vehicles and pleasure craft constitute a great problem to insurance companies. Installation of a transponder according to the invention would permit application of the invention for search and localization, e.g. of stolen or abducted objects, possibly persons, and may thus be used in combating various kinds of criminality. Furthermore, the system may be used to find lost domestic animals - e.g. the loss of grazing sheep is a great problem to Norwegian sheep owners. If individual animals were marked with a transponder, e.g. coded with an owner's code, losses as well as costs would be reduced in connection with search. In fact, the application of the system is not so much limited by technical possibilities, as by the imagination of potential users.

Claims

CLAIMS :

1. A system for detection, localization, and classific¬ ation of targets, which preferably are on or close to the surface of the earth, which system is connected with an external navigaational system for position finding or comprises a position finding navigational system, and which system also comprises an interrogator comprising a. transmitter, a receiver, and an antenna, and is designed for transmitting electro¬ magnetic pulses, as well as one or a purality of transponders, which react to a transmitted pulse by transmitting a response signal, every target being provided with at least one trans¬ ponder, c h a r a c t e r i z e d i n that each trans¬ ponder comprises an antenna, an impedance matching network, and a pulse generator, that the transponders are designed to. send off a response signal as a response to a detected inter¬ rogation pulse within a period of time which is within the time interval between said detected interrogation pulse, and the subsequent interrogation pulse, the frequency of the response signal preferably being equal to or at least approxi¬ mately equal to the frequency of the interrogation pulse, that the response signal is coded, said code preferably being characteristic of a particular transponder, or the category of targets associated with said transponder, that the inter¬ rogator is designed to detect the response signal, that the interrogator comprises a system processor connected with the navigational system, and that the interrogator is connected with a display.

2. A system according to claim 1, c h a r a c t e r i z e d i n that the transponder is an active transponder.

3. A system according to claim 1, c h a r a c t e r i z e d i n that the transponder is a passive transponder.

4. A system according to claim 3, c h a r a c t e r i z e d i n that the passive transponder is a SAP device.

5. A system according to claim 1, c h a r a c t e r i z e d i n that the interrogator is a dedicated interrogator.

6. A system according to claim 1 or 5, c h a r a c t e r i z e d i n that the interrogator is mounted in an aircraft.

7. A system according to claim 6, c h a r a c t e r i z e d i n that the antenna comprised by the interrogator is substantially directed vertically towards the surface of the earth.

8. A system according to one of the preceding claims, c h a r a c t e r i z e d i n that the response signal consists of two RF pulses at a given interval.

9. A method for use of a system for detection, localiz¬ ation, and classification of targets which are preferably located on or close to the surface of the earth, which system is connected with an external navigational system for position finding or comprises a position finding navigational system, said system also comprising an interrogator comprising a transmitter, a receiver, and an antenna and being designed to transmit electromagnetic pulses, and one or a plurality of transponders which respond to a transmitted pulse by transmit¬ ting a response signal, each target being equipped with at least one transponder, c h a r a c t e r i z e d i n that a response signal from the transponder, which is triggered by an interrogation pulse is delayed by a period of time which is longer than the duration of an echo signal caused by ground echoes of the triggering interrogation pulse, but shorter than the interval between said interrogation pulse and a subsequent interrogation pulse, the length and rate of the interrogation pulse being adapted to the current ground reflection conditions and the geometry of measuring.

10. A method according to claim 9, in which the interrog¬ ator is mounted in an aircraft, c h a r a c t e r i z e d i n that the antenna comprised by said interrogator is substantially directed vertically towards the surface of the earth.

11. A method according to claim 9, c h a r a c t e r i z e d i n that the mutual interval between two RF pulses constituting the response signal is selected so as to characterize a particular transponder or the category of a particular target, and that said interval is measured in the interrogator, and that classification of a particular transponder or a target is made on the basis of said measured pulse interval.

12. A method according to one of claims 9 - 11, c h a r a c t e r i z e d i n that in case of detection of a response signal the position of the detection is read off the navigational system by the aid of the system processor and a message providing information on the category and position of the target is displayed on the display of the system.

13. A method according to one of claims 9 - 12, c h a r a c t e r i z e d i n that in the interrogator a radiated pulse power of 40 dBm is used, that the interrogator emits pulses with a frequency of 850 MHz, that a pulse-length of 2 μs and a pulse repetition frequency of 25 kHz, as well as an antenna gain of 14 dB for the interrogator and 0 dB for the transponder are used.