GB2219707A

GB2219707A - Altitude control in fog

Info

Publication number: GB2219707A
Application number: GB8909774A
Authority: GB
Inventors: Dr Gunther Sepp
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1988-05-11
Filing date: 1989-04-28
Publication date: 1989-12-13
Anticipated expiration: 2009-04-28
Also published as: DE3816052C2; GB8909774D0; SE8900748D0; GB2219707B; DE3816052A1; SE8900748L

Description

2219707 ALTITUDE CONTROL SYSTEM FOR USE IN FOG The invention relates to an

altitude control system for use in fog for low-flying carriers, including missiles and submunition pods, which are equipped with a flight guidance system and an inertial navigation system.

The flight guidance of aeroplanes and missiles requires a weatherindependant measurement pf the actual flight altitude. Owing to good transmissivity of the atmosphere to radar waves even in bad weather conditions, radar altimeters are well-known and in general use. By contrast, laser radiation is strongly attenuated by fog and cloud, and laser range meters have so far been considered unsuitable for such altitude measurement. However, the applicants have found that, at least for low altitude missiles, laser ranging can come into practical consideration as altimeters, although difficulties do arise even at low altitudes, for example if the enemy generates artificial fog. Generally, artificial fog would prevent radiation of a laser altitude meter (LHM) from reaching the ground so that required flight altitude information cannot always be assured.

As it happens, artificial fogs may have extremely PKD170389 - 2 - short extent, say visual for less than one metre thickness. If the enemy covers its position with artificial fog, a laser altitude meter would detect the upper surface of the fog as a closer-located ground surface, albeit usually relatively weakly reflecting, but enough for a submunition pod or missile to fly high, i.e. over and missing its target.

It is an object of the present invention to provide a solution to such problems.

According to one aspect of this invention, there is provided an altitudecontrol system for low-flying carriers, for example missiles or submunition-pods, having a flight-guidance system and an inertial navigation system, wherein a laser ranger is employed as an altimeter, an intelligent signal-processing unit receives signals from said altimeter and serves to detect diminishing distance values and simultaneously reduced reception intensity and to control automatic switching of flight control from the altimeter to the on-board inertial navigation system when distance reduces by more than a preset amount.

According to another aspect of this invention, there is provided an altitude control system for low-flying carriers, for example missiles or submunition pods, PKD,1'70389 - 3 - having a flight guidance system and an inertial navigation system, comprising a laser ranger serving as a first laser altimeter and a second laser altimeter arranged for forward-looking operation, and signal processing means for comparing received signals from the first and second laser altimeters to determine any attenuation due to fog and means for correcting the measured flight altitude according to attenuation and, for greater attenuation, leaving flight control to the inertial navigation system.

Preferably, said altimeter or first laser altimeter comprises an AM-CW semiconductor laser ranger.

Ina further development, the signal processing means further stores or receives topographic data for overflown terrain for use in establishing said preset amount or said greater attenuation.

Such altitude control systems for missiles are difficult to locate or interfere with in recognizing artificial fog and initiating action to stabilize flight.

Specific embodiment of this invention will now be described by way of example with reference to the accompanying drawing which is an outline block circuit PKD170389 - 4 diagram for systems using a laser altitude meter cooperating with a flight guidance system.

In establishing that a laser range meter LEM can be utilized as a laser altimeter LHM 1, one embodiment was dimensioned to just achieve necessary measuring precision at visual ranges of about 50 metres in dense natural fog, making it usable for heights up to about 50 metres.

However, conditions can be somewhat different in artificial fogs. Such fogs can have extremely short extents, typically visual for less than one metre thickness. If the enemy covers its position with artificial fog, the laser altimeter LHM 1 would detect the upper surface of the fog as a closer located, though weakly reflecting, ground surface, so that a flying carrier, typically a missile or submunition pod, would automatically raise its flight path. To avoid this undesirable action, the laser altimeter LHM 1 needs to recognize the fog. This is possible by means of an intelligent signal processing unit SVA,-which processes signals from the laser altimeter LHM 1.

If it is assumed that the last part of the flight path of a flying carrier is over terrain that does not vary greatly in "surface roughness", the processing unit SVA can be arranged to recognize sudden encountering of artificial fog, usually also natural fog, by means of the intelligent signal processing. Suitable criteria can include an apparently diminishing distancemeasurement value, which is typical using AM-CW methods, and further include simultaneous reduction of reception intensity, i.e., effective "smoothing" of the ground roughness by the fog often, of course, artificial fogs are laid only directly in front and Over the target position, so altitude control of such relatively short terminal flight phase can be taken over by the inertial navigation system (INS) of the flying carrier.

In another embodiment, it is proposed to couple with the first laser altimeter LHM 1 a second and forwardlooking laser ranger or altimeter LHM 2, say as for terrain-following control purposes, and to determine attenuation due to fog by comparison of the reception intensities of the two laser altimeters LHM 1 and LHM 2. Account can be taken of angledependent back-scattering according to Lambert's Law, and the attenuation value can be used for correction of apparently shortened measured distance representing flight altitude. Thus, received signals for the vertically measuring laser altimeter LHM 1 can be delayed by time of flight for the carrier between the "laser-footprints", of the two PKD170389 - 6 altimeters LHM 1 and LHM 2, i.e. so as to be operative for the same point on the ground and thus have, at least approximately, the same relation or remission ratio. Required delay is dependent on time flight speed over the ground, and temporal correlation of the two received signals can be used in continuous calibration of the inertial navigation system INS In thin fog there will be automatic correction of laser altimeter values. In dense or artificial fog, switching of control can be made to the inertial navigation system of the missile. Where altimeter dependence for control purposes is abandoned (in favour of control by the inertial navigation system) affects of fog are substantially negated.

Reliability of the above measures can be quite considerably increased if the ground which is to be overflown by the missile is known well, at least in simplified or coarse data form that is stored at the signal processing unit SVA, which will conveniently be of program-controlled computer type. Then, development of threshold or encountered difference or change values can be aided for processing purposes in switching the laser altimeters and the signal processing means out of and into controlling operation.

Claims

PED170389 - 7 CLAIMS

1. An altitude-control system for low-flying carriers, for example missiles or submunition-pods, having a flight-guidance system and an inertial navigation system, wherein a laser ranger is employed as an altimeter, an intelligent signal-processing unit receives signals from said altimeter and serves to detect diminishing distance values and simultaneously reduced reception intensity and to control automatic switching of flight control from the altimeter to the on-board inertial navigation system When distance reduces by more than a preset amount.

2. An altitude control system for lowflying carrier, for example missiles submunition pods, having a flight guidance system and an inertial navigation system, comprising a laser ranger serving as a first laser altimeter and a second laser altimeter arranged for forward-looking operation, and signal processing means for comparing received signals from the first and second laser altimeters to determine any attenuation due to fog, and means for correcting the measured flight altitude according to attenuation and, for greater attentuation, leaving flightbontrol to the inertial navigation system.

3. An altitude-control system according to Claim 1 PKD170389 - 8 or Claim 2, wherein said altimeter or first laser altimeter comprises an AM-CW semiconductor laser ranger.

4.. An altitude-control system according to any proceding claim, wherein the signal processing means further stores or receives topographic data for overflown terrain for use in establishing said preset amount or said greater attenuation.

I

5. An altitude-control system arranged and adapted to operate substantially as herein described with reference to and as shown in the accompanying drawing.

1 -1 Pubhshed.1989 at The Patent Office. State House, 66/71 High Holborn, London W9IR 4TF. Further copies Maybe obtainedfrom The PatentOfnce. Sales Branch, St MW Cray, Orpington, Kent BR5 3ED.,Printed by Multiplex techniques ltd, St Mary Cray, Kent, COM 1/87