Arrangement for the interferometric radar measurement according to the rosar principle

[0002] Applicants claim-priority under 35 U.S.C. §119 of German Application No. 199 02 008.6 filed Jan. 21, 1999. Applicants also claim priority under 35 U.S.C. §120 of PCT/DE99/04065 filed Dec. 22, 1999. The international application under PCT article 21(2) was not published in English.

[0003] The invention relates to an arrangement for the interferometric radar measurement based on the ROSAR principle, as defined in the introductory part of claim 1.

[0004] A radar method comprising a synthetic aperture based on rotating antennas (ROSAR) is known from DE 39 22 086.

[0005] A method for the interferometric radar measurement in conjunction with a helicopter radar system (Heli-Radar) operating according to the ROSAR principle is been made known by the application firm in DE 199 02 007, which holds the same position in time as the present application. Said method permits a quasi-three-dimensional representation of the radar image of terrain or other obstructions. The invention described in the following is based on said method.

[0006] A method for interferometric radar measurements with a synthetic aperture is admittedly known from GRIFFITHS, H., “Interferometric Synthetic Aperture Radar”, Electronics and Communication Engineering Journal, GB, Institution of Electrical Engineers, London, Volume 7, No. 6, Dec. 1, 1995 (12-01-1995), pages 247 to 256, XP000545120; ISSN: 0954-0695). Said method, however, requires a radar platform that moves along a substantially linear track. A possibility for such radar measurements without a radar platform moving on its own cannot be realized in conjunction with said method. Furthermore, the corresponding processing algorithms of said method have to be adapted to the linear movement as well. The movement of the antenna is in this way given in a Cartesian system of coordinates. However, contrary to said method, in conjunction with the ROSAR method, processing algorithms are preset in a polar system of coordinates in accordance with the rotational motion of the antennas. This means that it is not possible to apply the processing algorithms of said prior art to ROSAR-systems.

[0007] The problem, is solved with an arrangement for interferometric radar measurement having one coherent transmitting antenna for emitting a radar signal, and two coherent receiving antennas that receive a radar signal. The two conerent receiving antennas each have a receiving channel. The antennas are disposed on the Brotating arm of a rotating device. The difference in radar signal path distance between the two coherent receiving antennas and the measured points of impact is then calculated using the wavelength λ of said emitted radar signal and the measured phase of the receiving echo of the two coherent receiving channels. The arrangement is embodied as an interferometric ROSAR system, and positioned in such a way that each relevant area of the field to be measured can be detected. To obtain an image of the overall situation, ROSAR systems can form a data network when linked together.

[0008] Said problem is solved with the help of the measures specified in claim 1. Embodiments and advanced developments of the invention are specified in the dependent claims and explained in greater detail in the following description with the help of an exemplified embodiment. Said explanations are supplemented by the figures of the drawing, in which:

[0009] FIG. 1 is a schematic representation of an exemplified embodiment of the arrangement for the interferometric radar measurement comprising a second antenna carrier arm (Rag);

[0010] FIG. 2 is a schematic representation of an interferometric radar measuring system shown in greater detail, which is comprised of only one rotating antenna carrier arm (RA); and

[0011] FIG. 3 is a schematic representation of an interferometric radar measuring system having a circular array of antennas.

[0012] Defined system requirements have to be met for a modern taxi- and runway radar monitoring system. Such requirements have to be satisfied, for example as follows:

[0013] However, it is necessary to point out that the radar system, for example for monitoring taxi- and runways, can be operated at other frequencies as well, in particular at even higher frequencies starting,with 20 GHZ and higher.

[0014] Now, in order to be able to satisfy the specified system requirements in accordance with the interferometric ROSAR principle, the calculations are based on the following statement:

[0015] The angle resolution in azimuth α_AZachievable with the ROSAR method is computed according to DE 43 23 51 C1 based on the following equation: $\begin{array}{cc}{\alpha }_{\mathrm{AZ}}=\frac{\lambda }{5·L·\sin \left(\gamma /2\right)}.& \left(0.1\right)\end{array}$

[0016] With an opening angle of the real antenna in azimuth of γ=45° it is possible to compute the associated rotor length L at which the required angle resolution α_AZ=0.25° is satisfied. The angle of γ=45° has to be viewed as an exemplified embodiment only and can be enlarged without any problem if any higher angle resolution is required. The associated rotor length L is computed based on the following equation: $\begin{array}{cc}L=\frac{\lambda }{5·{\alpha }_{\mathrm{AZ}}·\sin \left(\gamma /2\right)}.& \left(0.2\right)\end{array}$

[0017] With λ=C/fs0.01875 m; γ=45°, and α_AZ=0:25°, it follows that the rotor length amounts to L=2.2465 meters. Said rotor length L is capable of rotating without problems at 1 r/s. If a second rotor arm (RAg) having an identical configuration in terms of radar is installed quasi as a counterweight, the rotation frequency can be halved, i.e. the rotation now takes place at a speed of 0.5 rps.

[0018] The angle resolution in elevation ε follows directly according to the principle of the real aperture based from the opening angle of the antenna of ε=2.00. The values γ=45° and ε=2.0° have to be understood to be 3 dB-values of the directional characteristic of the antenna, i.e. as the half-value width in terms of output. With the opening angles of the antenna in azimuth and elevation of γ=45° and ε=2.0°, it is possible to estimate the antenna measurements based on the general formula applied in antenna theory, notably $\begin{array}{cc}\gamma \approx {70}^{°}·\frac{\lambda }{D_{\mathrm{azimuth}}}& \left(0.3\right)\end{array}$

[0019] and $\begin{array}{cc}ϵ\approx {70}^{°}·\frac{\lambda }{D_{\mathrm{elevation}}},& \left(0.4\right)\end{array}$

[0020] in order to make a final statement with respect to the mechanical dimension or stress. The following follows from equations (0.3) and (0.4):

[0021] Dazimuth=0.029 m, i.e. about 3 cm,

[0022] and

[0023] Delevation=0.656 m, i.e. about 66 cm.

[0024] The antenna thus measures 3 cm×66 cm (azimuth×elevation) and is capable of rotating without problems with said proportions. According to FIGS. 1 and 2, the antennas themselves are protected also by corresponding radomes (R1, R2) and favorably influenced aerodynamically by such radomes.

[0025] In conjunction with the pulse radar employed in the present case, the resolution of the distance is computed by the compressed transmitting pulse duration τ and the speed of light C as follows: $\begin{array}{cc}\Delta R=\frac{C·\tau }{2}& \left(0.5\right)\end{array}$

[0026] The required distance resolution ΔR of 3 m is satisfied with the compressed transmitting pulse duration of π=20 ns.

[0027] Now, in order to be able to detect and cover the entire area of the taxi- and runways of an airport, it will be necessary in most cases to position several radar devices or installations on the airport. Their arrangement in terms of location has to be selected in this conjunction in such a way that each relevant area of the,airport can be monitored. Such an arrangement naturally will be airport-specific, i.e. it will be different from case to case. It is naturally understood that the information supplied by the radar system always has to be processed in such a way that a picture of the overall situation is obtained. So that the individual radar installations will not exert any influence on each other, provision is made that the transmitting frequencies around the respective transmission bandwidths are selected not in an overlapping, but rather in an offset manner.

[0028] The arrangement proposed herein for the interferometric radar measurement in conjunction with a radar system operating based on the ROSAR principle comprises one coherent transmitting antenna and two coherent receiving antennas A1, A2 with their receiving channels, which are integrated in or on a rotating arm RA of a rotating system and position in such a way that each relevant area of the field to be measured or monitored can be detected. Antennas A1, A2 are protected aerodynamically shaped radomes R1, R2. The rotating device can comprise a circular array of antennas A1, A2 with an electronic control C for the cyclic activation of areas of the array of antennas A1, A2 as shown in FIG. 3. Transmitting frequencies of individual interferometric radar insallations are offset in relation to each other by respective transmission bandwidths. The interferometric arrangement for radar measurement is linked with a radar arrangement B for detecting moving targets. An evaluation device D detects the direction and path of objects moving in a monitored area. Evaluation device D also determines the size and type of objects detected.

[0029] According to another embodiment of the invention, provision is made that a-second rotor arm (RAg) is installed. Said second rotor arm has an identical configuration in terms of radar and quasi represents a counterweight or imbalance compensation, which offers the benefit that the rotation frequency can be halved.

[0030] The present system is particularly important at airports because it assures optimal detection of fighting means after a military attack and detection of openly scattered, exposed ammunitions, bombs or other explosive devices, as well as detection of the damage caused after a military attack by measuring the altitude profile of the surface of the airport.

[0031] Another embodiment of the invention increases the possibilities for using and applying the system, notably by linking it with radar methods known per se for detecting moving targets. In this way, it is possible to detect all aircraft or land-based vehicles or marine vessels moving within the area of an airport to be monitored.

[0032] In case the system is operated at lower frequencies in the range of from 1 GHz to about 3 GHZ, it can be employed without any problems as an omnidirectional radar installation on airports, and as an omnidirectional radar installation in the medium ranges as well.

[0033] The proposed invention can be applied not only for monitoring airports and seaports, but for monitoring traffic junctions and other important sections of traffic lanes such as surface roads or railway installations as well.

[0034] Furthermore, it is possible to detect the direction and path of detected aircraft, ships, motor vehicles and other objects. Their path can be followed, so that a so-called driving track can be formed. The disposition of the path is thus dependent on the driving track, which can be exploited for traffic control systems or traffic disposition.

[0035] Furthermore, an object classification can be carried out, i.e. a distinction can be made on an airport, for example between aircraft, motor vehicles, tank trucks and follow-me vehicles. The same applies to the distinction of ships or other objects in general object monitoring operations.