GB1605407A - Improvements in or relating to terrain following systems - Google Patents

Description

o (54) IMPROVEMENTS IN OR RELATING TO TERRAIN FOLLOWING SYSTEMS (71) I, Secretary of State for Defence, London, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to terrain-following systems for guiding aircraft including guided missiles in low-level flight with the aim of maintaining the altitude of the aircraft as far as possible close to a set height above the profile of the terrain under the flight path. Known terrain-following systems include forward-looking radar apparatus arranged to detect features of the terrain ahead of the aircraft and coupled to a control system for controlling the aircraft according to the profile of the terrain, for instance causing the aircraft to climb when approaching ridges or mountains and to descend into valleys. However stretches of water or sea can give inadequate or misleading signals to a forward -looking radar, and largely for this reason it has been found desirable to include radar altimeter equipment in terrain-following systems, with control equipment arranged so that in certain circumstances the radar altimeter equipment can exercise an overriding control on the system. The conditions in which the radar altimeter equipment is allowed to take control must be specified with some care to achieve good terrain-following over different possible configurations of the terrain without causing undesirably violent manoeuvres, especially on the occasions when control is transferred from the forward-looking radar to the radar altimeter or vice versa. In a known system of this kind the control units are arranged to prevent the altimeter equipment from taking control as long as significant return signals are being received by the forward-looking radar from terrain features in both of two specified regions of its field of view, these regions being specified by their range from the aircraft and having bounds dependent on the forward velocity of the aircraft.

It is an object of the present invention to provide an improved terrain-following system of this general kind.

According to the present invention there is provided a terrain-following system for guiding an aircraft in low-level flight comprising a forward looking terrain-sensor equipment, altimeter equipment, aircraft guidance equipment controllable either by the terrain-sensor equipment or the altimeter equipment, inhibiting means for preventing the altimeter equipment from controlling the guidance equipment whenever the terrain sensor equipment receives signals of amplitude greater than a predetermined amplitude from both or two regions AA and BB, the region AA comprising terrain at a range not less than RA but not more than RB ahead or the aircraft and the region BB comprising terrain at a range not less than Re but not more than RD ahead of the aircraft , a velocity transducer, a climb or dive angle transducer, and range limit means responsive to outputs from the velocity transducer and from the climb or dive angle transducer for varying RB. RC and RD according to prescribed functions of the velocity and climb or dive angle of the aircraft.

To enable the invention to be more thoroughly understood, some features of the prior art will be more fully described with reference to Figure 1 of the drawings accompanying the provisional specification, and an embodiment of the invention will be described, by way of example, with reference to Figures 2, 3 and 4 of the drawings accompanying the provisional specification. In these drawings Figures 1 and 2 are diagrams showing the field-of-view of the terrain-sensor equipment and parameters relevant to the operation of the systems concerned, Figure 3 is a schematic block circuit diagram of an embodiment of the invention, and Figure 4 is a graph showing how the specified conditions in the embodiment of Figure 3 are made to depend on the dive angle as well as the forward velocity of the aircraft.

Figure 1 shows an aircraft 1 equipped with a known terrain-following system which includes forward-looking radar and a radar altimeter. The limits of the field-of-view scanned by the forward-looking radar are indicated by broken lines, and the path of the altimeter signals is approximately indicated by arrows. Broken arcs indicate the loci of points at various ranges RA, RB, Rc, and RD from the aircraft. The region within the field of view of the forward-looking radar and between the ranges RA and RB is designated AA and shaded; the similar region between the ranges Re and RD is designated BB and shaded by lines of the opposite slope.

Control units (not shown) in the aircraft 1 are arranged to prevent the radar altimeter from taking control of the aircraft's terrain-following system as long as the forward-looking radar is receiving some returned signals from a terrain feature within the region AA and some returned signals from a terrain feature within the region BB. This is a prior art arrangement with values for RA and Rc, and RB and RD given by prescribed linear functions of the aircraft's forward velocity.

Figure 2 is a similar drawing showing an aircraft 10 which has an improved terrain-following system incorporating an embodiment of the present invention. The field-of -view of its forward-looking radar and its altimeter signals are indicated as in Figure 1, but Figure 2 also shows the additional parameters and the overlapping arrangement of regions AA and BB used in the improved system.

The broken line 11 represents a horizontal drawn through the aircraft, and broken line 12 represents the direction of flight of the aircraft.

The angle y between the lines 11 and 12 is the climb or dive angle; if the aircraft is descending as illustrated y will be a dive angle, considered negative. If the flight direction were above the horizontal y would of course be a positive angle of climb.

Figure 3 indicates the arrangement of the improved terrain-following system in the aircraft 10. It comprises a forward-looking radar 20, velocity transducer 21, climb or dive angle transducer 22, and radar altimeter 23 ( all of which may be conventional ), with a control system represented within the broken line 24, and an autopilot or flight director apparatus 25.

It will be readily appreciated by persons skilled in the art that the control system 24 may be realised either as an arrangement of special-purpose units represented by the boxes within the broken line 24, or by an appropriately programmed computer. In the latter case the part of Figure 3 within the broken line 24 may be taken as a flow chart representing the functions performed and actions effected within the computer; this may indeed be the preferred realisation of the invention.

Within the control system 24, terrain-following guidance signals are derived from the output signals of the forward-looking radar 20, by a technique or apparatus used in prior art terrain-following indicated by box 26.

Alternative guidance signals are derived from the radar altimeter 23 by known techniques or apparatus symbolised by the box 27. A conventional arrangement symbolised by box 28, is used to select and pass to the autopilot 25 whichever of these guidance signals is going to command the most nose-up attitude. The remaining part of the control system represented by box 29 is arranged to inhibit the altimeter-derived guidance signals in certain circumstances. It receives signals from the velocity transducer 21 and climb or dive angle transducer 22, representing the velocity V and angle y respectively, and derives the range limits from these signals according to equations of the following form: RA = rl RB = Re + r2 Re = aV if y > 0 Rc=aV-y(bV-c)ify < 0 R=dV-eif > 0 RD=dV-e-y(fV-g)ify < O where rl, r2, a, b, c, d, e, f and g are constants.

In the present embodiment these constants have the values ri = r2 = 1000, a = 4.48, b = 1.09, c = 490,d=8.355,e= 131,f= 1.2525andg=452.5, assuming that V is measured in feet per second and y is measured in degrees; the equations will then give the range limits in feet. The optimum values for these constants will depend on the control laws used to derive the guidance signals in boxes 26 and 27. Clearly the range limits RA, RB, Rc, RD can be derived by quite simple computing circuits or procedures.

Figure 4 shows these limits plotted for the two cases y 2 0 and y = - 10 degrees. Having derived these limits, the control system compares the return signals received by the forward-looking radar from terrain features within the regions AA and BB with predetermined threshold levels.

Then if and only if the comparisons show that return signals of amplitude exceeding the predetermined threshold levels have been received from both the regions AA and BB, the control system will apply an inhibiting signal to suppress or obstruct the altimeter-derived guidance signals. Clearly this may be achieved by two range gate circuits feeding threshold circuits which control a simple logic circuit, for instance a NAND gate, or alternatively by a program sequence of simple comparisons and logical decisions.

The object of the arrangement is to increase the size of the AA and BB regions and thereby inhibit the altimeter control as much as possible on flights over undulating terrain where its intervention tends to produce an unnecessarily high flight path and to cause extra vertical acceleration transients. The AA and BB regions must however be restricted to allow the altimeter to take control where terrain surfaces such as water or snow give insufficient signals to the forward-looking radar.

The effects of this are that when the aircraft approaches a coastline over land, going offshore, the radar altimeter is allowed to take charge sufficiently before the aircraft reaches the coast to ensure that it will not drop significantly below the prescribed height over the water. In this case the radar altimeter is enabled to take charge earlier according to the dive angle. When the aircraft approaches a coastline over water, the radar altimeter is forced to relinquish control before the coastline is reached, the change of control being advanced by an amount which increases in accordance with the dive angle, if the aircraft is still diving in these circumstances.

These features give better terrain following on flight paths over undulating terrain, while still ensuring satisfactory control when crossing lakes, estuaries and islands. Clearly the invention may be applied to any terrain following systems, in which the forward-looking terrain-sensor equipment need not necessarily be a forward-looking radar, but in which an altimeter control loop is also used.

WHAT I CLAIM IS: 1. A terrain-following system for guiding an aircraft in low-level flight comprising a forward looking terrain-sensor equipment, altimeter equipment, aircraft guidance equipment controllable either by the terrain-sensor equipment or the altimeter equipment, inhibiting means for preventing the altimeter equipment from controlling the guidance equipment whenever the terrain-sensor equipment receives signals of amplitude greater than a predetermined amplitude from both of two regions AA and BB, the region AA comprising terrain at a range no less than RA but not more than RB ahead of the aircraft and the region BB comprising terrain at a range not less than Re but not more than RD ahead of the aircraft, a velocity transducer, or climb or dive angle transducer, and range limit means responsive to outputs from the velocity transducer and from the climb or dive angle transducer for varying RB, RC and RD according to prescribed functions of the velocity and climb or dive angle of the aircraft.

2. A terrain-following system as claimed in claim 1 wherein the range limit means is arranged to calculate RB and Re such that RB is always greater than Rc.

3. A terrain-following system substantially as hereinbefore described with reference to any one or more of the Figures 2,3 and 4 of the drawings accompanying the provisional specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. aircraft approaches a coastline over water, the radar altimeter is forced to relinquish control before the coastline is reached, the change of control being advanced by an amount which increases in accordance with the dive angle, if the aircraft is still diving in these circumstances. These features give better terrain following on flight paths over undulating terrain, while still ensuring satisfactory control when crossing lakes, estuaries and islands. Clearly the invention may be applied to any terrain following systems, in which the forward-looking terrain-sensor equipment need not necessarily be a forward-looking radar, but in which an altimeter control loop is also used. WHAT I CLAIM IS:

1. A terrain-following system for guiding an aircraft in low-level flight comprising a forward looking terrain-sensor equipment, altimeter equipment, aircraft guidance equipment controllable either by the terrain-sensor equipment or the altimeter equipment, inhibiting means for preventing the altimeter equipment from controlling the guidance equipment whenever the terrain-sensor equipment receives signals of amplitude greater than a predetermined amplitude from both of two regions AA and BB, the region AA comprising terrain at a range no less than RA but not more than RB ahead of the aircraft and the region BB comprising terrain at a range not less than Re but not more than RD ahead of the aircraft, a velocity transducer, or climb or dive angle transducer, and range limit means responsive to outputs from the velocity transducer and from the climb or dive angle transducer for varying RB, RC and RD according to prescribed functions of the velocity and climb or dive angle of the aircraft.

2. A terrain-following system as claimed in claim 1 wherein the range limit means is arranged to calculate RB and Re such that RB is always greater than Rc.

3. A terrain-following system substantially as hereinbefore described with reference to any one or more of the Figures 2,3 and 4 of the drawings accompanying the provisional specification.