GB2315629A - Infra-red altimeter - Google Patents

Abstract

The altitude of an aircraft is derived using a modified infrared line scanner adapted to scan regions of terrain below the aircraft alternately in forward and rearward directions. The results of the forward and rearward scans are correlated to determine the time delay between scanning of the same swathe of terrain in forward and rearward directions. This time delay can be used together with the velocity of the aircraft to calculate its altitude.

Description

Apparatus for Measuring Altitude This invention relates to apparatus for measuring the altitude of a platform, such as an aircraft, travelling above a surface, such as the ground or sea.

Various techniques exist for estimating the altitude of aircraft, of which the most common are the radar altimeter (which measures instantaneous ground clearance) and the anaroid barometer (which estimates the height relative to a given dartum). The radar altimeter is an active device; it radiates energy, and relies upon some of that energy being reflected from the terrain in order to measure the distance. It is also a very accurate device, having a minimum typical error of 3 feet below a thousand foot altitude. By contrast, the barometer is passive and inaccurate, depending as it does both on mechanical systems for its measurement, and on the weather for its accuracy.

This invention arose in an endeavour to provide an altimeter which is both passive and accurate.

The invention provides apparatus for measuring the altitude of a platform travelling above a surface at a known velocity relative thereto comprising: infra-red sensing means designed to remove radiation from first and second directions so that, in use, it may be arranged on the platform to view areas of the surface spaced apart in the direction of flight so that an area viewed once by a scan in the first direction is viewed again, after a time delay, in the second direction; means for correlating outputs of the sensing means dervived from viewing in the two directions to determine the said time delay; and means for calculating, from the time delay, and the known velocity of the aircraft, the altitude thereof.

Calculation of the altitude (h) is a simple matter.

For example, where the first and second directions each make equal angles e with the vertical forwardly and rearwardly with respect to the direction of flight, the calculation can be performed using the equasion h = dt/2 tan 8 where h is the altitude and t is the time delay.

The said areas are preferably swathes extending laterally with respect to the direction of flight. This is convenient since it enables a sensible correlation to be performed even when the flight path deviates from a straight line. Also, the scanning of such swathes can easily be performed using a known infra-red sensing device. It would be possible however, in an alternative system, for the scanning to be performed in the direction of flight by the movement of the aircraft.

Where a line scanner is used each swathe will be scanned: but such a scanning action is not necessary. The linear array of sensors could be used to inspect different points along the swathe.

The sensing means can be a single sensor (or a single group of sensors) in which case an optical system will be needed to cause the sensor or the group of sensors to receive radiation from the different directions at respective different times. Alternatively two sensors (or two groups of sensors) can be devoted respectively to receiving radiation from the respective different directions.

One way in which the invention may be performed will now be described with reference to the accompanying drawings in which Fig. 1 is a schematic perspective illustration of an altitude measuring device constructed in accordance with the invention travelling over an area of ground; and Fig. 2 is a schematic block diagram of a signal processing facility forming part of the device shown in fig. 1.

Referring firstly to figure 1, an aircraft (not shown) follows a flight path 1 over an area of terrain 2.

The aircraft carries an altimeter which will now be described.

The altimeter comprises a unit 3 having mirrored faces 4,5,6 and 7. The unit 3 is mounted for rotation about an axis 1 which is the same as the flight path.

An infra-red sensor 8 receives, at the time illustrated in figure 1, infra-red radiation from a swathe I of ground which it sweeps longitudinally to produce an output signal on line 9 characteristic of any texture or features on that swathe. The swathe 1 is located forward of the aircraft at an angle 8 with respect to the vertical.

The speed of rotation is such that a very short time later, before the aircraft has moved a significant distance, a further sweep is produced, this time by the mirrored surface 5. The surface 5 is so angled that it sweeps a swathe IX which is arranged at an equal angle 6 rearwardly with respect to the flight path. The speed of rotation of the unit 3 is adjusted so that the swathes do not overlap but this is not essential to the functioning of the device.

Referring to figure 2, an optical sensor 10 cooperates with the unit 3 (driven by a motor 11) to produce switching signals on line 12. These cause a switch 13 to direct outputs from the sensor 8 to lines 13A and 13B depending on whether infra-red radiation is being received from a forward or a rearward direction by mirrored surfaces 4,6 or 5,7 respectively.

The sensor output for each forward swathe is fed into a tapped delay line 14 which holds recordings of a large number of such swathes. The sensor output for each rearward scan is fed via a buffer 15 into a store 16.

A correlator 17 correlates each recording of a swathe from the store 15 in turn with each of the large number of recordings in the delay line 14. The correlator hence determines the number of scan periods that have passed since the swathe currently recorded in store 16 was previously scanned in the forward direction. The resulting time delay t is used in a calculator 18 to derive the altitude h using known values e and V, the latter being the velocity of the aircraft.

Claims (6)

1. Apparatus for measuring the altitude of a platform travelling above a surface at a known velocity relative thereto comprising: infra-red sensing means designed to receive radiation from first and second directions so that, in use, it may be arranged on the platform to view areas of the surface spaced apart in the direction of flight so that an area viewed once by a scan in the first direction is viewed again, after a time delay, in the second direction; means for correlating outputs of the sensing means dervived from viewing in the two directions to determine the said time delay; and means for calculating, from the time delay and the known velocity of he aircraft, the altitude thereof.

2. Apparatus according to claim 1 in which the said areas of terrain are swathes extending laterally with respect to a direction of flight.

3. Apparatus according to claim 1 in which the sensing means is designed to scan in the direction of each swathe.

4. Apparatus according to claim 1,2 or 3 in which the sensing means comprises a single sensor or group of sensors and an optical system for causing it to receive radiation from the different directions at respective different times.

5. Apparatus according to claim 4 when dependent on claim 3 in which the optical means comprises an element having a plurality of reflective faces and means for rotating the element, the said element being arranged so that such rotation causes the said scanning, and alternate faces of the element being angled to cause the sensing means to receive radiation alternately from the first and second directions.

6. Apparatus substantially as described with reference to the accompanying drawings and substantially as illustrated therein.