Method for interferometrical radar measurement

The invention relates to a method and an arrangement to interfe rometrischen radar measurement according to the respective preamble of An claims 1 and 4. FIG.

By design are radars precise distance measuring what stating that a radar device without special measures only the Ab was an objective of the antenna but not determine its direction can. One can only determine whether a target within the antenna beam or not.

This issue is the previously known ROSAR- or helium Radar largely resolved by z. B. 16 staggered in height antennas with an antenna aperture angle, for example 2.5 ° used who the. This can be used to place a raised obstacle etc. inside egg determine ner accuracy of about 2.5 ° in elevation. However, be also equidistant targets in the same antenna at the same image point shown.

Angular resolution of the famous Heli-radar is due to a special signal processing about 0.2 °. Reference is made to the disclosure in DE 39 22 086 C1 referenced. The direction of an obstacle and thus the However, location in space where this is located, one can only by means of a determine triangulation, for which purpose two locally ge in the simplest case separated radars can be used.

However, it is also the characteristics of a coherent radar make use and using the phase of the transmission signal a kind Triangula Make tion. For this purpose one uses a coherent radar, coherently which emits a signal through a transmitting antenna, and the back scattered echoes two locally separated receiving antennas again kohä rent receives. A coherent analysis allows the calculation of Pha sendifferenz between two received signals. From the phase difference the direction is determined from the received the scattered echoes are. If one has the distance and direction of an "obstacle" is calculated, so can also be the place determined in the room. This type of dreidimensio dimensional position determination by means of a coherent radar with a Sen Deactivating and two receive antennas is generally "radar interferometry" ge called and has long been known. It is already for creating topographical phic cards using SAR systems on aircraft used in for example through the DOSAR the Fa. Dornier GmbH.

For further prior art this was ver to the following documents reported:

<ul><li>a) CT Allan, Review Article, Interferometric Synthetic Aperture Radar, in IEEE Geoscience and Remote Sensing Society Newes Letter, Sept. 1995 p. 6 ff.</li><li>b) S. Buckreuß, J. Moreira, H. Rinkel and G. Waller, - Advanced SAR Interfe rometry Study, DLR release 94-10, June 1994, DLR, Institute of High frequency technology, Oberpfaffenhofen,</li></ul>

The entire existing and above-mentioned prior art, Finally, the underlying principle here ROSAR projected Geländeer increases or other elevated obstacles in a plane so that at Un knowledgeable of the present imaged topography of the terrain, the height of the respective obstacle is not recognized. For flight control but is a three-dimensional image is required.

The present invention is based on the object based on the Principle of the interferometric radar measurement to an arrangement and Ver go to show that of a quasi-three-dimensional radar image representation Terrain and other obstacles make.

This object is proposed by the claims 1 and 4 Measures achieved in a surprisingly simple manner. In the subclaims are refinements and developments indicated and in the descrip tion is explained an embodiment in the <b>Fig.</b> 1 outlines. It show:

<b>Fig.</b> 1 shows an embodiment relating to the typical geometry for a interferometric ROSAR in schematic representation,

<b>Fig.</b> 2 is a block diagram of the embodiment of <b>Fig.</b> 1,

<b>Fig.</b> 3 is a diagram of the prior art with respect to the ROSAR- Principle.

The general idea of ​​the invention provides, in one after ROSAR- System working helicopter a quasi-three-dimensional to obtain radar image representation for flight guidance in that one on the rotating hub mounted transmitter two coherent Receiving antennas are associated with receiving channels.

The previous ROSAR system has to obtain a three-dimensional image For example, 16 transmitters and receivers with their channels on. These have However, a direction inaccuracy of approximately 2.5 °. If now this ROSAR system - as mentioned above - a high-precision extended coherent reception channel, so are for obtaining the highly accurate three-dimensional radar image only one transmitter and two coherent Receiver instead of the previous example, sixteen transmitters and receivers required. By the interferometric principle is the previous Directional inaccuracy improved by a factor of about one hundredth

The following description of an embodiment - in <b>Fig.</b> 1 outlines - to explain this in more detail:A functioning after ROSAR principle helicopter flies in a Height H above the ground surface. At the end of the rotating antenna Cross are a transmitting and two receiving antennas with associated coherent Transmit and receive electronics attached. The received echoes amplified, digitized and further processed.

The distance between these above-described arrangement below INROSAR system is called, and the point P, which located in a relative height h R is called. The distance between the Antenna A1 of INROSAR's the point P is R + .DELTA.R and is therefore by a small amount .DELTA.R greater than the distance R to the antenna A2. Of the Path difference .DELTA.R of the two distances can be made of the known Wavelength λ of the transmitted radar signal and the measured Phase difference Δφ of receiving echoes of the two coherent Receiving channels are calculated.

This phase difference Δφ of receiving echoes in turn is from the Images calculated, which resulted from processing of the received echoes are. Each of the images is in the complex, digital form, ie has real and imaginary - or equivalently: amplitude and phase.

The phase difference Δφ now follows up to a multiple of π (Modulo π) by complex multiplication of the pixels of an image with the complex conjugate pixels of the other image and subsequent formation of the arctan of the respective real and imaginary. This yields the phase difference Δφ and by inserting in Δφ (Eq. 1) then .DELTA.R.

The phase centers of the two receiving antennas A1 and A2 are the Length B, the so-called baseline removed. From the law of cosines and some simple angle relations result:

After the viewing angle θ is calculated in equation (2) can now the relative height determined h are:

h = H - R.cos (θ) (3)

To display the pixels on the graphics display in which INROSAR the amount actually h not required, but it is only the Viewing angle θ to calculate the coordinates of a reference point P on the graphics display used. Nor does it matter to know the Inclination angle of the antenna, because the display on the screen only a relative representation of the pixels with respect to the Senkechte is the base line B of the two antennas A1 and A2. Although the Image display depends on the position of the helicopter - for example, by the nod - but are the antennas of INROSAR Sytems and the center one another always in a fixed relationship. The height h and the inclination angle α of the antennas are only required when using this INROSAR's a topographic map with an absolute height H of overflown area should be created. These foregoing are formulas also useful for a fault analysis, as will be explained below.

The relevant INROSAR errors are the phase noise δφ and the Change in baseline B between the phase centers of the antennas A1 and A2. The phase noise is made up of a sum of shares various components together. The largest contributions provide the Transmitter, the receiver, the system clock and the A / D converter noise. A typical order of magnitude for the overall phase noise δφ a INROSAR system is approximately 5 °. The change in the baseline between the phase centers of the antennas A1 and A2 can be, for. example, by heating caused by sunlight. A typical value is 0.001 m adopted. The various influences result in a scattering of .delta.h Height of the reference point P and thus δθ a scattering angle of view.

This results in a scattering of the viewing angle δθ as follows:

In an embodiment according <b>Fig.</b> 1 flying the helicopter in the Normal position. This means that the antennas A1 and A2 perpendicular are positioned above each other. From equation (1) is determined .DELTA.R. Of the Value of the measured phase difference Δφ of the echoes from the antennas A1, A2 is ambiguous and can only up to a value between 0 and 2π be determined. This ambiguity of 2π must by additional measurements be determined. For this is a suitable for INROSAR conception additional transmitter / receiver with sharp narrow beam in elevation transmitting / Receiving antenna covering the lowest range of viewing angles. by virtue of its sharp focusing in elevation may be made to receive the echoes Distance to be uniquely determined reference point on the ground. The INROSAR system takes this distance as a fundamental value and calculates the further ambiguities due to the increasing distance from the continuous phase transitions. A calculation example is the closer Remarks:The starting point is the fact that the helicopters in the Normal position flies. This means that the antennas A1 and A2 perpendicular are arranged above one another:The parameters are:

From equation (2) follows:

<imgref idrefs="7/2" />example 1

<imgref idrefs="7/3" />example 2

From the equations (4) and (5) it follows for the scattering .delta.h the height h of Reference point P:

This results in a scattering of the viewing angle δθ as follows:Because phase noise δφ = 5 °:

and because of error of the length of the baseline B to .DELTA.b = 0.001 m

In <b>Fig.</b> 2 is a block diagram of the in <b>Fig.</b> 1 illustrated brought embodiment, the proposed with the Interferometric radar method is provided required building blocks and for the specialist needs no further explanations more.