GB2219167A

GB2219167A - Laser altimeter for low-flying bodies

Info

Publication number: GB2219167A
Application number: GB8910109A
Authority: GB
Inventors: Wolfgang Knauer; Gunther Sepp
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1988-05-11
Filing date: 1989-05-03
Publication date: 1989-11-29
Anticipated expiration: 2009-05-03
Also published as: SE8900747D0; GB2219167B; GB8910109D0; DE3816053C1; SE8900747L

Description

HN170489 - 1 LASER ALTIMETER FOR LOW-FLYING FLYING BODIES The invention

relates to an altimeter for lowflying flying bodies, such as submunition containers, which are equipped with a flight guidance system and an inertial navigation system.

The flight guidance of aircraft and flying bodies requires weatherindependent measurement of the actual flying height. On account of the high reliability of radar transmission through the atmosperhic irrespective of poor weather conditions, radar altimeters have been the general rule in all prior art guidance systems. Laser radiation, on the other hand, is severely impaired by mist and clouds, so that laser rangefinders have hitherto been considered as unsuitable for use as altimeters.

Mist and clouds are the most important causes of damping of laser radiation of near -IR wave length, as provided by semiconductor lasers. Rain, snow and hail are of less importance in this respect. The degree of laser beam damping can be ascertained from meteorological measurements of the visibility in mist or in cloud. The failure rate, owing to poor weather conditions, of a "laser altimeter" has now been estimated by making use of the regional weather statistics with respect to ground mist and cloud base. These now indicate that, on average, in Central Europe in 98 out of 100 hours the range of visibility 221,9167 HN170489 - 2 notwithstanding ground mist is greater than ab out 200 m and that visibility only falls below 50 m at the most in one but of 100 hours.

These above considerations lead to the conclusion that, contrary to the general view of the prior art, laser range-f nders are fit for use as altimeters for low-flying submunition containers with adequate reliability, since, for the lower measuring ranges which are necessary for such aircraft, the damping of the laser radiation even in the case of bad weather and mist does not become too great. Failures of such a laser altimeter would occur only in the event of extremely poor visibility conditions which are, as the statistics show, sufficiently rare.

An object of the present invention is to provide an altimeter for lowflying flying bodies which is no longer susceptible to interference and notwithstanding the occurance of natural mist or poor visibility conditions is capable of functioning effectively.

In accordance with the invention, an altimeter arrangement for low-flying flying bodies which are equipped with a flight guidance system and an inertial navigation system, is characterised by comprising a laser altimeter in the form of a semiconductor-laser laser range-finder which, even upon rolling motion of the flying body, constantly ensures a downward measurement, the laser altimeter or a deflecting mirror associated with its optical system being arranged 1 il,k 1 HN170489 - 3 sluably or swivellably and being controlled by the inertial navigation system of the flying body.

Exemplified embodiments are explained in the following description and shown in the figures of the drawings.

Fig. 1 is a diagrammatic view of a laser altimeter for terrain hugging flight having a deflecting mirror and a rotatable wedge plate for beam deflection; Fig. 2 is a diagrammatic view of a laser altimeter having a deflecting mirror for the roll compensation; Fig. 3 is a diagrammtic view of an arrangement of individual laser altimeters on the underside of the flying body and viewed from the tail side of the flying body, each altimeter having a fixed direction of vision; Fig. 3a is a diagrammatic view of the arrangement shown in Fig. 3 viewed from the longitudinal side of the flying body; Fig. 4 is a diagrammtic sketch illustrating the height-measuring procedure with several immovable laser altimeters, the scanning beams thereof being represented diagrammtically; Fig. 5 is a graph indicating the relationship between range of vision V and necessary laser transmitting power PL at a height of 200 m and a reduced signal noise ratio (SIN) requirement of 20 dB; Fig. 6 is a graph indicating the relationship between measuring range (mid-point range) R and range of HN170489 - 4 vision in the case of a transmitting power of 30mW and S/N = 28 dB.

The critical flight phase of a low-flying body FK, such as a munitions dispenser or guided missile, is the terrain hugging phase and/or the final approach at about 50 m height. At least at this altitude range a laser range-finder LEM is usable as laser altimeter (LHM), if it is so dimensioned that at this flying height and in the case of severe mist with ranges of vision reduced to around 50 m it still achieves the necessary accuracy of measurement. Greater flying heights without correspondingly better visibility lead, however, to measurement failure. However, as the aforementioned statistics show, the likely failure rate is under 2%.

The proposed laser altimeter LHM is, of course, immune to high-energy radar interference (jamming radiation). Jamming lasers can blind the LHM at the most only briefly so long as their interference radiation during the flying-body overflight falls into the extremely small LHM field of vision. Permanent destruction of the LHM optical system by high-energy lasers is not to be expected on this flying-body or dispenser scenario, so that functional disturbances of the flying-body height-holding by interference radiators can be considered practically as precluded.

In order always to measure the height above ground, the narrow field of vision of the LHM even upon rolling motions of the flying body (curving flight) has to be t HN170489 - 5 directed perpendicularly downwards. This is ensured by a swivelling of the laser altimeter itself or of an associated deflecting mirror Sp under control of the flying-body navigation system.

The altimeter apparatus should comply with the following design parameters:

Measuring range for the launching and end phase should be about 200 m and for the flight phase about 50 m. The accuracy of measurement should be below 1 m and the line of sight of the laser altimeters must be vertically downwards and this should be maintained even upon rolling motion of the flying body to around +/-750.

A further design requirement is provision of a further laser altimeter LHM2 which has a direction of sight obliquely forwards at an angle of 450 up to 601 which makes possible a quasi-contour flight.

A ground scanning rate of about 10Hz is sufficient.

The receiving optical system 10 of the laser altimeter is a Cassegrain mirror telescope of short type of construction into the so-called "dead" centre of which the laser beam from an AM/CV semiconductor laser 11 having a microscope objective 12 is imaged by means of reflective prisms 14, 13. In order, despite the small dimensions, to obtain an adequate power rating, the transmitting and receiving field of vision have to remain restricted to a few mrad. As a result of the rolling motions of the flying body a wide-angle coverage of +/-750 is not statically realisable, except with the

HN170489 - 6 aid of a deflecting mirror Sp which is controlled by a drive unit 16. As illustrated in Figure 2, the scanning beam of the laser altimeter strikes the mirror Sp horizontally and is deflected at a right angle downwards or, depending on the mirror setting, to the side.

Fig. 1 shows an exemplified embodiment of a receiving optical system be means of which the direction of vision obliquely forwards can be adjusted and the requirement for a second altimeter thereby dispensed with. In this embodiment a rotatable wedge plate 17 is arranged between the optical system 10 and the mirror Sp, which plate 17 optionally directs the scanning beam onto the mirror Sp or past it. To achieve this rotations of the wedge plate 17 through 1800 are necessary, which are undertaken by a drive (not shown). The transmitting/receiving unit 10, 11, 12 has to be installed in this case - as shown in the figure slightly at an angle so that the beam deflection by the prism 13 can be effective in both directions. A single wedge plate 17 is sufficient for this purpose.

Figures 3, 3a and 4 show an exemplified embodiment without moving parts in which three or more complete altimeter transmitting/receiving units LHM1, LHM2, LHM3 are arranged fixedly with specific directions of vision in a flying-body dome 20. From the items of range information from the individual laser range-finders well as the inertial information of the artificial t h ti HN170489 - 7 horizon the flying height as well as the terrain contour are ascertained by means of trigonometrical calculation.

Two arrangements are possible for signal processing. On the one hand each channel may have its own range evaluation means, or, in the other case, a single evaluation unit may be provided which sequentially interrogates the individual channels. In either case, a single, simple computer is necessary to process the trigonometrical algorithms in order to ascertain the actual flying height.

The critical relationships between range of vision V and necessary transmitting power P or, respectively, range R are illustrated graphically in Figures 5 and 6. The required minimum range of vision of 50 m for flight at 50 m height corresponds to the visibility in cases of dense mist or fog, and is thus fallen below only in extreme cases, such as cases of artificial fog or smoke.

Claims

HN170489 CLAIMS 1. An altimeter arrangement for low-flying flying bodies

which are equipped with a flight guidance system and an inertial navigation system, characterised by comprising a laser altimeter in the form of a semiconductor-laser laser range-finder which, even upon rolling motion of the flying body, constantly ensures a downward measurement, the laser altimeter or a deflecting mirror associated with its optical system being arranged sluably or swivellably and being controlled by the inertial navigation system of the flying body.

8 -
2. An altimeter arrangement as claimed in claim 1, characterised in that the laser altimeter has as its receiving optical system a Cassegrain mirror telescope of short type of construction into the "dead" centre of which the laser beam from a semiconductor laser having a microscope objective is imaged.
3. An altimeter arrangement as claimed in claim 2 including a deflecting mirror arranged at a 450inclination on the horizontal axis of an adjusting unit.
4. An altimeter arrangement as claimed in claim 3, characterised in that a rotatable wedge plate is arranged between the receiving optical system and the mirror.

HN170489 - 9
5. An altimeter arrangement as claimed in any preceding claim comprising a combination of several laser altimeters, each a complete transmitting/receiving unit having a specific fixed aiming direction, and an associated computer for processing the trigonometrical algorithms with respect to the actual flying height.
6. An altimeter arrangement for low-flying flying bodies substantially as hereinbefore described with reference to Figs. 1 to 4 of the accompanying drawings.

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