DE2633760C3 - Line scanning system for area scanning in aircraft - Google Patents

Description

The invention relates to optical scanning and film recording systems in general and to the Use of special detector aperture shapes to improve their spatial Sa; rip! Ing (sampling) rates and in addition to improving the subjective quality of the movie recording in particular.

Aircraft infrared line scan equipment for scanning and making a record of the terrain below Aircraft usually use discrete detectors and scan the field of the object in one direction while they are on the plane to generate the scan in the other direction (perpendicular) support. Such systems suffer from problems with the room sampling rate.

It is common practice with line scanners to use square detectors. The sensor system is located in an aircraft and scans the object field in the direction of the flight path of the aircraft perpendicular direction (across the track scanning). The aircraft movement creates the other on its path Scan or raster component (along the track scan). The detector's object field sampling rate in that of the cross-track scanning direction is continuous while that of the longitudinal track direction is discontinuous. Further the longitudinal track rate is variable if the ratio of aircraft speed to altitude changes (V / H).

In systems with square detectors, it is common to determine the ratio of the limiting speed to the Height (VH / H) to be taken into account, such as the one where each subsequent transverse track strip is edge to edge with the previous strip collides. How the V / H ratio is limited, the sampling

b ^r > rate is reduced to one per resolution element in the long track section (here a resolution element is defined as the respective field of view, which results from the size of the respective detector element).

Sampling theory dictates that a minimum of two samples per pixel must be faithfully reproduced the original waveform is necessary In terms of spatial sampling, it is required that the sampling interval is spatially separated by half of a pixel. That The result of meeting this critical criterion would be the V / H capability of the system have to cut in half.

In the case of a film recorder that matches the detector, i. H. using a square light source scanning aperture To draw the lines on the film, the raster effect is applied to the V / H boundary (after the collision of the scan lines) obviously. If the V / H ratio decreases, either overlap the following line the previous one, what because of the built up on the overlap portion Film density results in a further raster effect, or the longitudinal track width of the line is reduced by to compensate so that the primary screen effect remains evident.

The above explanations about the sampling rates for the detector also apply to the recording scanning aperture.

The object of the invention is:

a) to improve the longitudinal track detector sampling rate and

b) to improve the recording raster effect and the sampling rate without affecting either the spatial To have a detrimental effect on the dissolution or the V / H limitation of the system.

According to the invention, a line scanner is used for Use in aircraft for terrain mapping provided, the detector arrangement of the scanning system contains distributed Dctcktorelemente, which in Rcilu-ii in reciiicni angles to the Abiastrichiung and are offset from one another in such a way that the elements of a subsequent row are opposite columns are in a previous row. The system is now characterized in that the individual Detector element areas are rhombus-shaped, the rhombuses having diagonals parallel to the Scan direction and the distribution and the dimensions of the elements so in the vertical direction to the scan direction that they result in a considerable sampling overlap during the scan.

In infrared line scanners, for a given infrared optical aperture, the thermal sensitivity is directly proportional to the square root of the detector area. For a given optical aperture and for a given thermal system sensitivity, the detector area is kept constant. At a In the preferred embodiment, the rhombuses are actually squares rotated by 45 °, and that Detector area is unchanged.

An embodiment of the invention is described with the help of the drawing. In these is:

Fi g. I the representation of three possible detector forms,

FIG. 2 shows the relative corresponding curves for the detector shapes according to FIG. 1,

F i g. 3a and 3b show the effect of changing V / H for diamond-shaped detectors, and

F i g. Figure 4 compares an array of diamond detectors with the same number of the same size Square detectors.

In Fig. 1 three opening shapes are considered - Square, rhombus and circular. One dimensionalc ΐ Fourier transform can be written as follows:

For the square opening:

. sin (i-ii /) κι

where d is the length of the opening sides and ω is the reduced spatial frequency 2jr / "(where / is the spatial frequency); Γι For the circular opening:

2J, (»<r)

where / i is the Bessel function of the first kind.
For the diamond opening:

_ 21 -cosl .. ,, /)

where c / is the half diagonal in the scan direction and c is the half diagonal perpendicular to the scan direction

With a frequency corresponding to about half a revolution per opening mode, this is relative Address the square opening about 3% smaller than that of the circular system, which is about 3% smaller than that the diamond opening is. The high frequency response for the circular opening is better than the edge that of the square opening (zero occurs first at a frequency 15% higher than that of the square opening), while the high frequency response of the diamond opening further enhances that of the square opening (first, zero occurs at a frequency 40% higher than that of the square opening).

It is therefore to be seen that the advantages of the individual sampling opening shapes in terms of the The response only exists to occur at spatial frequencies that are significantly higher than half a revolution per opening width is what makes it difficult in an edition to evaluate in terms of the resolution limits to be carried out relatively alone.

The response of the three forms of opening to higher fractions is because of the first zero following Response characteristics very different (Fig. 2). This can be viewed as a "resonance" phenomenon. The first "resonance" for a square opening takes place at about 1.5 rounds per opening, the next at 2.5 rounds, etc. Both the square and the circular openings have a significant response (more than 10%) at the first response and the Square opening at around 10% at the fourth "resonance". This effect leads to a »wrong Resolution «and can be used as a source of ambient noise can be viewed in the annex.

The response of the diamond opening is less than 10% at the first »resonance«.

In terms of the "edge" approach of the various Detector shapes on a stretched track target, the diamond-shaped opening has one on the edge faster rise time than the square and circular openings (because of the high frequency response this opening).

For a given detector area and a speed-to-altitude ratio, the Rhombic and circular openings have advantages over the square opening in terms of number of the longitudinal track samples per pixel. F i g. 3a therefore shows diamond openings with 50% overlap. The sampling theory suggests that an absolute minimum of 2.0 samples per pixel should be faithful Reproduction of the original image is necessary. This condition can be with approximately 40% diamond openings overlapping can be approximated (Fig. 4).

When using a film recording device, e.g. B. light emitting diodes as line writing Element for the detector array, apply with respect to the frequency response and the application of the Sampling rate similar arguments. If the diamond-shaped openings overlap by 40%, the subjective quality of the film recording by simply allowing the photo diode scan lines to Overlapping on the recording film improved. The result is a smooth recording characteristic with very low "raster effect". As the scan lines increasingly overlap (Fig. 3b), such as film speed to correct the scale of the recording is decreased as V / H decreases, the resulting raster effect will be of high frequency and comparatively small amplitude. The mean brightness of the light emitting diode may change around the mean video level hold, and no information is lost.

4 shows that with an arrangement of square detectors, in which the following detectors are offset in the next row to cover the column there is no overlapping and each pixel is checked only once. Only through reorganization of the detectors by 45 °, their number, shape and size remaining the same, so that they now have diamonds are diagonals parallel to the scanning direction, there is an overlap so that each pixel is checked 1.4 times will.

The invention thus enables:

1) Increasing the longitudinal track sampling scale while maintaining resolution, sensitivity and scan line density to be kept;

2) Improvement in writing and recording rasterization to reduce the visual raster effect in the recording shown without the detrimental influence of the spatial resolution of the system on the minimum value;

3) Arrange the detector scan and the playback scan both to match the raster sampling rate as large as possible, as well as to make the visual raster effect as small as possible.

The invention also includes a detector assembly in which the detectors are mounted so that the V / H ratio can be set to a particular flight pattern or broadcast or which Detector arrangement can optionally be simulated to follow these requirements.

For this purpose 2 sheets of drawings

Claims (5)

Patent claims: 1. Line scanning system for use in an aircraft for scanning a terrain, the ^r > detector arrangement of the scanning system contains distributed detector elements in rows, which run at right angles to the scanning direction and are offset from one another so that the elements of a subsequent row columns in a previous' Opposite row, characterized in that the individual detector element areas are rhombus-shaped, the rhombuses having diagonals parallel to the scanning direction and the distribution and dimensioning of the elements at right angles to the scanning direction so that they result in a sampling overlap during scanning . 2. Installation according to claim 1, characterized in that the sampling overlap is practically 40% - '» amounts to. 3. Plant according to claims 1 and 2, characterized in that the rhombuses so rotated Squares are that each has a diagonal that is parallel to the direction of scan. 2 "> 4. System according to the preceding claims, characterized in that a recording raster system Has writing openings that are similar to the detector elements of the scanning system. 5. Plant according to the preceding claims, ji> characterized in that the detector elements are adjustably attached and the detector arrangement can be selectively simulated.