Method for interferometrical radar measurement

IS 2 188 283 T3

DESCRIPTION

Procedure for interferometric radar measurement.

The invention relates to a method for interferometric radar measurement according to the generic concept of claim 1.

For constructional reasons, radar devices are precise distance measuring devices, which means that a radar device without special provisions can only determine the distance to a target from the antenna, but not its direction. It can only be established whether or not a target is within the antenna beam.

This problem is largely remedied in the ROSAR radar or heli-radar known up to now, by using, for example, 16 antennas staggered in height with an antenna aperture angle of for example 2.5 °. With this, the position of a raised obstacle, etc. can be determined. with an accuracy of approx. 2.5 ° in elevation. Obviously here too distant targets are represented by the same distance, on the same antenna and on the same picture element.

The azimuth resolution of the known heli-radar is, due to special signal processing, approx. 0.2^°. For this, reference is made to what is disclosed in DE 39 22 086 C1. However, the direction of an obstacle and thus its position in space, in which it is located, can only be determined with the aid of triangulation, two locally separated radar devices can be used for this in the simplest case.

However, it is also possible to take advantage of the characteristics of a coherent radar apparatus and, with the help of the phase of the emission signal, to carry out a type of triangulation. For this purpose a coherent radar apparatus is used, which coherently radiates a signal through one transmitting antenna and coherently receives the scattered echoes back through two locally separated receiving antennas. A coherent evaluation allows the calculation of the phase difference between the two reception signals. From the phase difference the direction from which the scattered echoes have been received is determined. If then the distance to and direction of an “obstacle” have been calculated, its position in space can also be determined. This type of three-dimensional position determination with the help of a coherent radar apparatus, with one transmitting and two receiving antenna, is generally called "radar interferometry" and has been known for a long time. It is already used for the creation of topographic plans with the aid of SAR systems in airplanes, for example by means of the DOSAR of the company Dornier GmbH.

Regarding the remaining state of the technique, we want to refer here to the following documents:

a) CT Allan, Review Article, Interferometric Synthetic Apertur Radar, in IEEE Geosciencie and Remote Sensing Society Newes Letter, Sept. 1995, p. 6 ff.

b) S. Buckreuss, J Moreira, H. Rinkel and G. Waller, - Advanced SAR Interferometry Study, communication DLR 94 - June 10, 1994, DLR, Institute for High Frequency Theory, Oberpfaffenhofen.

The current complete state of the art and mentioned above, including the ROSAR principle taken here as a basis, projects terrain elevations or other elevated obstacles on a plane, in such a way that if the terrain topography reproduced at that time is not known the height of the respective obstacle can be recognized. However, piloting aircraft requires a three-dimensional image.

The present invention has set itself the mission of outlining measures, based on the ROSAR principle, that make possible a radioaric representation of images of terrain and other obstacles.

This mission is solved in a surprisingly simple way by means of the measures proposed in claim 1. Configurations and improvements are indicated in the subordinate claims and in the example an exemplary embodiment is described, which is shown schematically in fig. 1. Here they show:

fig. 1 an example of execution in relation to the topical geometry for an interferometric ROSAR in schematic representation, fig. 2 a block connection diagram of the exemplary embodiment according to fig. 1,

ES 2 188 283 T3 FIG. 3 a diagram of the state of the art in relation to the ROSAR principle.

The general idea of ​​the invention foresees obtaining, in a helicopter that works according to the ROSAR system, a representation of quasi-three-dimensional radar images for flight piloting by means of two coherent receiving antennas are subordinated to the emitter arranged on the rotating crown. with reception channels.

The current ROSAR system shows, to obtain a three-dimensional image, for example 16 transmitters and receivers with their channels. However, these show a steering precision of approx. 2nd. If this ROSAR system is now expanded - as mentioned above - into a high-precision coherent reception channel, to obtain the high-precision three-dimensional radar image, only one emitter and two coherent receivers are required, instead of the current ones for example sixteen senders and receivers. Through the interferometric principle the steering precision is improved by approx. by a factor of 100.

The following description of an exemplary embodiment shown schematically in fig. 1 - wants to describe this in more detail.

A helicopter operating on the ROSAR principle flies at an altitude H above the earth's surface. At the end of the rotating antenna crown, a transmitting and two receiving antenna have been installed with corresponding coherent electromagnetic emission and reception. The received echoes are amplified, digitized and further processed.

The distance between this previously described arrangement, which is now called the INROSAR system, and the elevated point P, which is at a relative height h, is called R. The distance between the antenna A1 of the INROSAR and the elevated point P is R + ΔR and is therefore a quantity ΔR greater than the distance R to the antenna A2. The path difference ΔR of the two distances can be calculated from the known wavelength λ of the emitted radar signal and the measured phase difference Δφ of the reception echo of the two coherent reception channels.

This phase difference Δφ of the receive echo is calculated again from the images that have arisen by processing the receive echo. Each of the two images is presented in a complex, digital form, that is, it has a real and imaginary part or equivalent: amplitude and phase.

The phase difference Δφ is now obtained, up to a multiple of π (π module), by complex multiplication of the pixel elements of one of the images by the conjugated complex pixel elements of the other image, and the formation then of the arctan. of the respective real and imaginary part. In this way the phase difference Δφ is obtained and, by including Δφ in (equation 1), then ΔR.

<img file="ES2188283T3_D0001.tif" />

The phase centers of the two receiving antennas A1 and A2 are far apart along the length B, the so-called baseline. From the cosine theorem and some simple angular relations we obtain:

<img file="ES2188283T3_D0002.tif" />

Once the visual angle θ has been calculated in equation (2), the relative height h can now be determined:

h = HR- cos (0) (3)

In order to represent the image elements on the graphic screen with the INROSAR, the height h is not really needed, but the visual angle θ is used exclusively to calculate the coordinates of a high point P on the graphic screen. It is also not necessary to know the angle of inclination

ES 2 188 283 T3 of the antenna, since the representation on the screen is exclusively a relative representation of the image elements in relation to the perpendicular to the baseline B of the two antennas A1 and A2. Although the representation of images depends on the position of the helicopter - for example due to pitch - the antennas of the INROSAR system and the center of the image are always in a fixed relationship with each other. The height h and the angle of inclination α of the antennas are only needed if, with the help of this INROSAR, a topographic plane with an absolute altitude H of the terrain flown is wanted to be elaborated. These above formulas are also useful for an error study, as described below.

The errors that affect INROSAR are the phase noise δφ and the variation of the base line B between the phase centers of antennas A1 and A2. Phase noise is made up of a sum of parts of different components. The greatest contributions come from the emitter, the receiver, the system cadencemeter and the noise of the A / D converter. A topical magnitude for all δφ phase noise from an INROSAR system is approx. 5th. The variation of the baseline between the phase centers of the antennas A1 and A2 can be produced, for example, by heating with solar radiation. As topical value 0.001 m is assumed. The different influences produce a dispersion δh of the height of the high point P and thus a dispersion of the observation angle δθ.

&lt;⁴&gt;

4ττ B

Λ = -Λ (5)

With this, a dispersion of the observation angle δθ is obtained, as follows:

¿&gt; ^ = Arcsen ^^ (6)

In an exemplary embodiment according to FIG. 1 the helicopter flies in the normal situation. This means that antennas A1 and A2 are positioned perpendicular to each other. From equation (1) we obtain ΔR. The value of the phase difference Δφ of the echoes from antennas A1, A2 is ambiguous and can only be determined up to a value between 0 and 2π. This 2π ambiguity must be resolved by additional measurements. For this, an additional transmitter / receiver is suitable to the INROSAR system with a superdirectional transmitter / receiver antenna in elevation, which covers the sector of the observation angle. Because of its superdirectionality in elevation, the distance to the elevated point on the ground can be clearly determined from the receiving echoes. The INROSAR system uses this distance as the base value and calculates the other ambiguities based on the increasing distance from the continuous phase transitions. A calculus example offers the most detailed explanations.

It starts from the mode that the helicopter flies in the normal situation. This means that antennas A1 and A2 are arranged perpendicular to one another.

As parameters apply:

IS 2 188 283 T3

<td>Parameter</td><td>Meaning</td><td>Value 1 / Value 2</td>

<td>H</td><td>INROSAR flight altitude</td><td>100 m</td>

<td>R + ΔR</td><td>Distance between high point P and antenna A1</td><td>Ex. 1: 500.009 m Ex. 2: 500.09 m</td>

<td>R</td><td>Distance between high point P and antenna A2</td><td></td>

<td>B</td><td>Baseline between the phase centers of the antennas</td><td>0.15 m</td>

<td>δΒ</td><td>Baseline length error B</td><td>0.001 m</td>

<td>δφ</td><td>INROSAR phase noise</td><td> 5^or</td>

<td>α</td><td>Angle of inclination of antennas A1 and A2</td><td>90 ° (perpendicular)</td>

<td>λ</td><td>Radar wavelength</td><td> 0,0090909</td>

From equation (2) we obtain:

0 = arcs (R + Δτ)² -R² - B²

2RB (7)

Example 1

01 = trees (500, 009² - 500, OOP² - 0,15² · 500,000 · 0, 15 .

= arceae (0.05985) = 86, 57 °

Λ1 = 300-500.00 cos (86.57 °) = 70.09m

Example 2

<img file="ES2188283T3_D0003.tif" />

From equations (4) and (5) it follows for the dispersion δh of the height h of the elevated point P:

ES 2 188 283 T3, 0.00909-500.00 * 4 0.15 ^(5/57’³⁾ = 0.21 / 77 exact. 0.210401168 m = -500.00-tan (53.14 °)

0,001

0.15? (2): 2.035-0.048 / w = 4.45 m

With this, a dispersion of the observation angle δθ is obtained, as follows: because of the phase noise δφ = 5 °:

<img file="ES2188283T3_D0004.tif" />

and because of the error of the length of the baseline B and δΒ = 0.001 m

<img file="ES2188283T3_D0005.tif" />

In fig. 2 shows a block connection diagram of the exemplary embodiment shown in fig. 1, which is equipped with the modules necessary for the proposed interferometric radar procedure and which does not require any further explanation for the technician.