In many geodetic problems or applications, it is required to determine, from a detection point, the direction to an object point, such as, for example, the azimuthal angle and angle of elevation to a further reference point or the compass direction. Such problems are classical tasks of geodesy.
In order to make an object point or an object to be surveyed detectable and surveyable, this object point is distinguished from other points in space, for example by virtue of radiation being actively emitted by it.
Another possibility for distinguishing an object point is to increase the directed reflectivity in the object point, for example by mounting one or more reflectors, for example a corner cube with its inversion centre on the point or in a defined environment of the point.
A further example for distinguishing an object point is its definition as a position relative to a known object form, such as, for example, a fixed target, or relative to an edge/corner/centre/centre of gravity of an object.
From the detection point, a defined solid angle element or field of view of a detector, which contains or should contain the object point, is detected and recorded by a sensor so that monitoring is possible. If the object point is present within the monitored solid angle element, the distinguishing of the object point leads to a pattern on the sensor by virtue of an image. This pattern specific to the object is focussed on the detector in a direction-dependent manner with a certain bearing or position. This position of the pattern on the sensor permits a calculation of the direction of the object point relative to the detection point, it being possible, if required, to include additional information.
An example of such an image which can be used for direction determination is the focused image of the object point and its defined environment on a position sensitive device (PSD) or image sensor with the use of an objective or of a diffractive optical system. Another example is imaging with infinite focus, which directly assigns a direction-dependent position on the sensor to received object rays. In this example, the divergent radiation emitted by an object point is focussed to give a pattern having approximately circular symmetry on the sensor.
The position of the pattern is determined by the sensor or evaluation electronics and converted into the sought direction of the object point relative to the detection point, it being possible, if required, to use additional information about object properties, object distance and detector properties.
As a suitable sensor which permits position determination, it is possible to use, for example, a PSD as an individual sensor or an image sensor as a matrix of individual sensors, so-called pixels or image points. The latter has the advantage that any troublesome stray light is distributed over the individual sensors or pixels of the image sensor, and the utilisation of the sensor dynamics and the signal/background ratio are more advantageous than with the use of only one individual sensor.
However, a disadvantage of the use of image sensors is the considerably increased time requirement for reading out and evaluating the pixels in comparison with the use of only one individual sensor. For example, a VGA image sensor having 640×480 pixels requires a time which is 307,200 times greater in comparison with the use of an individual sensor.
In the determination of the direction to an object or an object point, problems due to an increased time requirement for reading out and processing the sensor signal are encountered with the use of two-dimensional sensors, which is advantageous because of the stability to interfering radiation, so that a comparatively low measuring frequency of the direction determination results.
The direction determination can be divided into two problems depending on the application:
Static measuring task—Here, the object point is immobile or has a change of direction relative to the detector which is negligible with respect to required accuracy and measuring frequency of the direction determination.
Dynamic measuring task—Here, the change of direction from the object point to the detector is not negligible. In the dynamic measuring task, problems arise if the change of the direction to the object point during the evaluation of the measurement is so great that the object point is outside the field of view of the detector during the subsequent measurement. If a plurality of measurements follow one another, the direction from the object point to the detector may change in the course of the measurements, for example by a random or involuntary movement of the object point. Such changes, which may be repeated, give rise to problems in the direction determination if the object point leaves the field of view of the detector.
In this case, tracking of the field of view, possibly also performed automatically, for example for target tracking, becomes more difficult. Under unfavourable circumstances, tracking based on the direction measurement and with the aim of detecting the object point again can no longer be carried out, so that the measurement may have to be stopped under certain circumstances.
Optimization of the stability of the direction measurement to rapid changes in the direction is therefore advantageous. However, a specified accuracy of measurement of the direction measurement must be reached.
A special case of the direction measurement considers accuracies of measurement which are greater than or equal to the field of view angle of the detector. The measuring task therefore now consists in the decision or verification that the object point is within the field of view of the sensor. This is sufficient, for example, for tracking the object point.
A high measuring frequency—adapted if required—leads to a higher tolerance of the regulation to rapid changes of direction and is therefore also advantageous in this special case.
High measuring frequencies are also advantageous in the case of the static measuring task, since, in the case of the rapid measurement, a plurality of individual measurements can be gathered within the time determined by the application and an increase in the accuracy of the measurement is thus possible. Moreover, brief strong disturbances, which can be eliminated in the case of rapid measurement, occur in the event of a disturbance of the measurement by turbulent air flows (heat striae).