RADAR APPARATUS

Abstract: The invention relates to an FMCW type of radar apparatus, provided with a transmitter for the periodic transmission of a transmitter signal, an antenna, a mixer stage, an analog-to-digital converter, a fourier transformer and an interference suppressor for suppressing pulsed interference signals. During the presence of a pulsed interference signals, the values measured at the output of the analog-to-digital converter are substituted by measured values obtained from previously or subsequently transmitted signals.

Description: Radar apparatus The invention relates to a radar apparatus designed for the periodic transmission of a frequency-swept transmitter signal and the subsequent reception of return signals of this transmitter signal, provided with a receiver unit comprising a mixer stage for generating heterodyne signals from received return signals and a reference signal derived from the transmitter signal, an analog preamplifier, an analog-to-digital converter and a fourier transformer, for the conversion of the heterodyne signals into target- representing signals. A radar apparatus of this type is for instance the FMCW radar, well-known in the art. A drawback of the FMCW radar is its susceptibility to pulsed interference signals, these signals usually being stronger by many orders of magnitude than genuine return FMCW signals. In this respect, the fourier transformer plays a prominent part, since it effectively determines the spectrum of the pulsed interference signals which, in this application, causes the noise floor of the FMCW radar to be increased with, typically, 10-50 dB. This then may cause relatively weak targets to disappear from the screen during the occurrence of interference signals of this type, whereas peaks in the fourier spectrum of the pulsed interference signal may appear as spurious targets. The invention at least substantially obviates this drawback and is characterised in that the receiver unit is provided with a detector for detecting pulsed interference signals and an interference suppressor connected to the detector, for suppressing received pulsed interference signals, such that these pulsed interference signals do, at least substantially, not reach the fourier transformer. An advantageous embodiment of the radar apparatus according to the invention is characterised in that the detector is of the analog type and is connected to the analog preamplifier, and that the interference suppressor is designed to suppress digital values generated at the output of the analog-to-digital converter for the duration of the pulsed interference signal. An alternative solution is to detect the presence of the pulsed interference signal after analog-to-digital conversion, although this entails a considerable investment in hardware and/or software. In suppressing pulsed interference signals, a number of measured values are in fact substituted by values which may be chosen at random. These values are usually selected to be equal to the last value measured just prior to the occurrence of the pulsed interference signal, in the course of which the analog-to-digital converter only briefly emulates a sample-hold circuit well-known in the art. For that purpose, it is also possible to incorporate a sample- hold circuit. In keeping a certain number of measured values at a constant level, an error is in fact introduced into the measurement, which error will, after fourier transformation, still cause an increase of the noise floor and the occurrence of spurious targets, although to a far lesser degree. In order to at least substantially preclude an increase of the noise floor and the occurrence of spurious targets, a further advantageous embodiment of the radar apparatus according to the invention is characterised in that there are provided reconstruction means for the generation of substitute measured values for measured values lost during suppression. In generating substitute measured values, the invention is based on the assumption that strong return signals are usually produced by nearby objects and that in this type of radar apparatus, these strong return signals manifest themselves as low-frequency signals which, moreover, hardly change for successively transmitted signals. A further advantageous embodiment of the radar apparatus according to the invention is thereto characterised in that the reconstruction means are designed to generate substitute measured values on the basis of measured values obtained from at least one other transmitter signal. A still further advantageous embodiment of the radar apparatus according to the invention is characterised in that there is provided a buffer storage device, incorporated between the analog-to-digital converter and the fourier transformer for storing measured values obtained from at least two successive transmitter signals and that the reconstruction means are designed to generate substitute measured values for a measured value obtained from a transmitter signal on the basis of measured values obtained from the previous transmitter signal and the subsequent transmitter signal. Such a reconstruction process minimizes an adverse effect of gradually changing measured values for successive transmitter signals. Assuming that the process of gradually changing measured values, obtained from transmitter signals transmitted in succession is a continuous one, the reconstruction means may be advantageously implemented as interpolation means. The inventive radar apparatus is then preferably characterised in that, for each measured value missing, the reconstruction means perform a linear interpolation on a corresponding measured value obtained from the previous transmitter signal and a corresponding measured value obtained from the subsequent transmitter signal. The invention will now be further explained with reference to the following figures of which: Fig. 1 represents a block diagram of an embodiment of the radar apparatus; Fig. 2A represents a heterodyne signal containing a pulsed interference signal; Fig. 2B represents this heterodyne signal containing a suppressed pulsed interference signal; Fig. 2C represents this heterodyne signal containing a suppressed pulsed interference signal after reconstruction. Fig. 1 shows a block diagram of a feasible embodiment of the radar apparatus according to the invention in which a radar transmitter 1 periodically generates a RF frequency- swept transmitter signal. Radar transmitter 1 may for instance be a YIG oscillator controlled with a sawtooth- shaped variable current, possibly followed by a RF amplifier. The signal generated by radar transmitter 1 is transmitted by an antenna system 2. Antenna system 2 also receives returns of the transmitted signal. These return signals are applied to a mixer stage 3 to be mixed with a reference signal derived from the transmitter signal, e.g. an attenuated transmitter signal. Mixer stage 3 may be preceded by a low-noise preamplifier, which enhances the sensitivity of the radar. The transmitter signals can be transmitted continuously, as a result of which the radar apparatus operates as an FMCW radar well-known in the art; an alternative solution is to transmit the signals intermittently. An essential aspect is that mixer stage 3 delivers a heterodyne signal which represents targets as periodic signals whose frequencies are a measure for the target range and whose amplitudes are a measure for the target cross-section. The heterodyne signal produced by mixer stage 3 is subsequently applied to a first amplifier 4 and a second amplifier 5 after which it is digitised in an analog-to-digital converter 6. To prevent pulsed interference signals from degrading the proper functioning of the radar apparatus, there is provided an analog detector 7, connected to the output of first amplifier 4, followed by a filter 8 and a pulse generator 9. Depending on the radar apparatus parameters and the anticipated interference pulses, the gain of first amplifier 4 and the filter characteristic of filter 8 are chosen in combination such that pulse generator 9 only produces an output signal if antenna system 2 receives an interference pulse. Filter 8 is for instance a high-pass filter that blocks an interference signal, but admits the leading edge of a pulsed interference signal. This edge may then serve to trigger pulse generator 9, which produces a switching pulse with a predetermined duration. The switching pulse is applied to a multiplexer 10, along with output signals of analog-to-digital converter 6. These output signals continuously appear at the output of multiplexer 10, unless pulse generator 9 produces a switching pulse. If such is the case, a flag value generated by a value generator 11 will be present at the output of multiplexer 10, a value that does not correspond to values supplied by analog-to- digital convertor 6 so that, in processing the digitised heterodyne signals, the need for reconstruction becomes apparent . It frequently occurs that a ship-mounted FMCW radar experiences severe interference from a pulse radar likewise disposed on the ship. Generally speaking, it will then be possible to derive a second switching pulse from the pulse radar, which pulse is active during the transmission of a pulse by the pulse radar. This second switching pulse may then be logically combined with the switching pulse to obtain a combined switching pulse. To enable reconstruction, the output signal of the multiplexer 10 controlled by the switching pulse is written to a buffer storage device 12 which may comprise digitised heterodyne signals related to at least two signals transmitted in succession. Buffer storage device 12 is connected to an interpolator 13 which also receives digitised heterodyne signals related to the most recently transmitted signal. On the basis of available signals, interpolator 13 reconstructs the digitised heterodyne signal related to the next-most recently transmitted signal by means of linear interpolation for each value and subsequently passes this reconstructed digitised heterodyne signal to a fourier transformer 14 which, in a manner known in the art, converts this signal into a target-representing signal suitable for presentation on a radar display. For the reconstruction, this implies that if for a sample x(s,k) , the flag is set, this value is substituted by the value X(s,k) , with: X (s,k) = % {x(s-l,k)+x(s+l,k) } in which x(s,k) represents the k-th digitised value related to the s-th transmitter signal, x(s-l,k) is the k-th digitised value related to the s-l-th transmitter signal, and x(s+l,k) is the k-th digitised value related to the s+i-th transmitter signal. If, for a sample x(s,k) the flag has not been set, it is obvious that: X(s,k) = x(s,k) It is possible to involve more than three transmitter signals in the interpolation, although this is only useful if corresponding samples for two successive transmitter signals show interference, a situation that will hardly ever occur. A state-of-the-art data processor usually only contains a general-purpose RAM device and a digital signal processor (DSP) . If a data processor of this type is used, the buffer storage device 12 constitutes part of the RAM device, which enables the interpolation and the fourier transformation to be performed by the DSP. Fig. 2A represents a heterodyne signal containing a pulsed interference signal. The duration of the heterodyne signal related to one transmitter signal is 500 micro seconds, the duration of the pulsed interference signal is 10 micro seconds. The heterodyne signal reveals the presence of two periodic signals, both originating from relatively nearby objects. Small-sized targets of interest are imperceptible as they lie below the noise level, their presence is detected only after fourier transformation of the heterodyne signal. In the presence of the pulsed interference signal, small-sized targets remain imperceptible; these are usually 10-20 dB below the fourier spectrum of the pulsed interference signal. Moreover, the fourier spectrum of the pulsed interference signals gives rise to the occurrence of spurious targets which exceed the thermal noise level by typically 20 dB. Fig. 2B represents the same heterodyne signal in which, according to the invention, the pulsed interference has been suppressed, i.e. during the presence of the pulsed interference signal, the measured values have been substituted by the latest valid measured value. After fourier transformation, the resulting imperfection again results in a fourier spectrum in which targets may be lost and spurious targets may occur, the effect of which however is far less noticeable than that produced by the pulsed interference signal originally present. Fig. 2C represents the same heterodyne signal in which, according to the invention, the pulsed interference signal has been suppressed; moreover, the measured values determined during the presence of the pulsed interference signal have been substituted by an interpolation of measured values obtained from the previous and the subsequent transmitter signal. It should be taken into account that the heterodyne signals produced by nearby, strong objects undergo little change for successive transmitter signals. For this type of heterodyne signals, the interpolated measured values are near-perfect. The interpolated measured values for small-sized, distant objects may on the other hand not be perfect, although these objects will after fourier transformation not cause any problems because the fourier spectrum related to the error made is below the thermal noise level. In actual fact, it is particularly the clutter component of the heterodyne signal that is subjected to reconstruction, because an imperfection in that component will, after fourier transformation, cause an increased noise floor and the occurrence of spurious targets.

Claims: Claims : 1. Radar apparatus designed for the periodic transmission of a frequency-swept transmitter signal and the subsequent reception of return signals of this transmitter signal, provided with a receiver unit comprising a mixer stage for generating heterodyne signals from received return signals and a reference signal derived from the transmitter signal, an analog preamplifier, an analog-to-digital converter and a fourier transformer, for the conversion of the heterodyne signals into target-representing signals, characterised in that the receiver unit is provided with a detector for detecting pulsed interference signals and an interference suppressor connected to the detector, for suppressing received pulsed interference signals, such that these pulsed interference signals do, at least substantially, not reach the fourier transformer. 2. Radar apparatus as claimed in claim 1, characterised in that the detector is of the analog type and is connected to the analog preamplifier. 3. Radar apparatus as claimed in claim 1, characterised in that the interference suppressor is designed for suppressing digital values generated at the output of the analog-to-digital converter during the pulsed interference signal. 4. Radar apparatus as claimed in claim 3, characterised in that reconstruction means are provided for the generation of substitute measured values for measured values lost during suppression. 5. Radar apparatus as claimed in claim 4, characterised in that the reconstruction means are designed for at least substantially keeping the digitised values generated at the output of the analog-to-digital converter at a constant level for the duration of the pulsed interference signal. 6. Radar apparatus as claimed in claim 5, characterized in that the reconstruction means are designed to maintain the last digital value measured prior to the occurrence of the pulsed interference signal. 7. Radar apparatus as claimed in claim 4, characterised in that the reconstruction means are designed to generate substitute measured values on the basis of measured values obtained from at least one other transmitter signal. 8. Radar apparatus as claimed in claim 7, characterised in that a buffer storage device is provided, incorporated between the analog-to-digital converter and the fourier transformer, for storing measured values obtained from at least two successive transmitter signals and that the reconstruction means are designed to generate substitute measured values for a measured value obtained from a transmitter signal on the basis of measured values obtained from the previous transmitter signal and the subsequent transmitter signal. 9. Radar apparatus as claimed in claim 8, characterised in that the reconstruction means comprise interpolation means. 10. Radar apparatus as claimed in claim 9, characterised in that, for each measured value missing, the interpolation means perform a linear interpolation on a corresponding measured value obtained from the previous transmitter signal and a corresponding measured value obtained from the subsequent transmitter signal.

Inventors: TEULINGS WILHELMUS ANTONIUS, THOMASSEN SIERK MICHAEL, TEULINGS, WILHELMUS, ANTONIUS, THOMASSEN, SIERK, MICHAEL
Applicants: HOLLANDSE SIGNAALAPPARATEN BV, TEULINGS WILHELMUS ANTONIUS, THOMASSEN SIERK MICHAEL, HOLLANDSE SIGNAALAPPARATEN B.V., TEULINGS, WILHELMUS, ANTONIUS, THOMASSEN, SIERK, MICHAEL
Application year: 1998
Main classification: G01S7
IPC codes: G01S7, G01S13