GB2129643A - Means for and methods of homing under radar signals - Google Patents

Abstract

A radar or sonar guide homing system for aircraft, ships, etc., makes periodic distance measurements between a stationary transponder location (56) and a mobile interrogator location (50). An "on course" signal (90) is given when the difference DELTA R between successive measurement readings is maximized, in a negative direction (i.e., the gap between the stationary and mobile locations is closing). Otherwise an "off-course" signal (86) is given. An "S-turn" manoeuvre confirms the accuracy of the readings. <IMAGE>

Description

SPECIFICATION Means for and methods of homing under radar signals This invention relates to means for and methods of using radio navigation and distance measuring equipment, and more particularly to a use which enables a vehicle to home on astationarytransponder.

The invention is primarily described herein as being used on an airplane. However, it may be used on any of many other vehicles, also. Thus the word "airplane" isto be construed to include ships, boats, helicopters, and all similar vehicles. Also, the system is described in conjunction with distance measuring systems which use radar signals. However, it should be apparent that it may also be used with other systems, such as a sonar system. Accordingly, the word "radar" should be construed to mean all similar and suitable systems.

There are many different kinds of systems for using radar signals to measure distances and to navigate.

Usually, there are transmitters and receivers which send and receive radar pulses. Then, there are various data processing units for manipulating the information picked upbyorderivedfromthe radarsystem.

These data processing units drive display units and the system user is given a read out in terms of distances, azimuths, and the like. The resulting system is usually relatively expensive.

Most airplane or boat pilots do not really want or need all of the information which a conventional system gives. As a generality, most pilots merely want to have enough information to enable them to head theirvehicletoward a home base, and nothing more.

The fact that they are a certain number of miles out may be irrelevant because there is nothing much that can be done about it Likewise, most of the other information which a radar system gives relates to things that are out ofthe pilot's immediate interest or control.

Accordingly, an object ofthe invention is to provide means for and methods of homing, which requires a minimum amount of equipment. Another object isto provide meansfor and methods of using almost any radio distance measuring equipmentto home on.

Still another object is to provide a method of homing which is adapted to ruse a minimum display, which merely tells a pilot when he is headed in a correct direction.

A method of homing on a beacon in a system using electrical or sound signals for measuring distances, said methods comprising the steps of: (a) mounting a distance measuring transmitter on a vehiclewhich is to be guided to a home location; (b) mounting a transponder beacon atthe home location to which the vehicle isto be guided; (c) periodically transmitting an interrogation pulse from said vehicle toward and receiving a reply pulse from said transponder; (d) comparing the distance measured responsive to each interrogation and reply pulse with the distance last previously measured; and (e) directing said vehicle along a path which is indicated by said comparing means as causing the greatest negative change of distance measured between successive pulses.

While the inventive process may be used with many different kinds of radar, sonar, or like systems, it is here assumed that it is being used with a trilateral radar system disclosed inthefollowing U.S. patents: Merrick 3,810,179, Parker 4,275,398 and Dano 3,938,146, hereinafter called the "Merrick system".

The invention will be explained with help of information setforth in the attached drawings in which: Figure lisa schematic representation showing distance measurements along a straight path of an airplane relative, to the location ofatransponder station; Figure 2 is a graph setting forth information taken from Figure 1 which plots range vs. time, to show how the measurements peakwhen the vehicle is closest to the transponder; Figure 3 is a graph which shows the change in range values dRldt, taken from Figures 1,2, and which shows that as the airplane approached the transponder beacon a polarity difference appears at the point ofthe airplane's closest approach; Figure 4 graphically indicates howthe distance measurements occurwhen the airplane follows a curved path;; Figure 5 is a graph which shows that there is a peak in range differences when the vehicle in Figure 4 is headed directly toward the beacon; Figure 6 schematically represents an "S-turn" which may be followed byanairplanetoverifyits course, and explains how it corrects its path toward the beacon, if it drifts off course; Figure 7 is a representationofoneformofinforma- tion which may be given to a pilot in a digital format; Figure 8 schematically represents a system (such as the Merrick system) which incorporates the invention; and Figure 9 is a blockdiagram ofthe equipment used to practice in the invention in the system in Figure 8.

Afirst observation is that, when an aircraft or a boat is headed for a particular place, it moves at a remarkably constant speed. All of the same en viromental factors (wind direction, velocity, etc.) tends to be stable over a substantial period of time. The throttle is placed at some position and not moved thereafter. Consequently, if the airspeed appears two change relative to some fixed point, it probably means that the vehicle has changed its direction. From elementarytrigonometry, it is seen that: Velocityz ceived) = Velocitytrne cosine g where: pr is the angle between the actual flight path and the most direct path to a home base.If the angle isO0, the cosine is one; if it is 20, the cosine is 0.99939, which is less than one part in a thousand. However, if the readings are repeated every one second, for example, the difference between the readings is relatively large and change is easily detected. Therefore, even a slight change in course is easily detected when the periodic difference is analyzed.

These principles may be applied to and explained by the graphical presentations in the attached figures.

More particularly, Figure 1 shows the straight path followed by an airplane travelling along the line 20.

The transponder beacon is located at point B. At regulartime intervals, a distance measuring unit aboard the airplane sends a radar pulse and takes a distance reading to the transponder beacon B (seven readingsA(1)-A(7) being shown in Figure 1).

At reading A(1 ) the airplane receives a distance indication represented bythe length of line 22. During each ofthe succeeding readings A(2), A(3), A(4), the distance to beacon B is found to be getting shorter.

Between the readingsA(4) and A(5), the airplane has reached the closest point to transponder beacon B.

Then, the distance becomes progressively longer on each ofthe succeeding readings A(6) -A(7).

If the ranges R or lengths ofthe various lines A(1 ) to A (7) are plotted on a uniform time scale (Figure 2), there is a U-shaped curve with a minimum at the location between points4and 5which is closestto the transponder beacon B. By comparison, it is seen that the Figure2 minimum between times "4", "5", corresponds to the shortest distance in Figure 1. Thus, it is found that an aircraft will be headed straight for the beacon if it always flies at the minimum or point of curve inflection between the time periods "4" and "5".

If the Y-axis positions or coordinates of each of the points on curve 26 is subtracted from the Y-axis position or coordinate ofthe next succeeding point, one can plot the difference between successive measurements, which is seen in Figure 3. Thus, point 28,forexample, represents the length of the line 30 in Figure 2, which is the difference between the distances measured attimesA(1) and A(2). This difference may be called the "AR" or change in range. As long as the aircraft is approaching the beacon B (Figure 1), the AR distances are reducing or progressively moving from a more negativeto a less negative value as shown in Figure 3. When the aircraft passes its closest point and begins to move away from the beacon, the AR distances start to increase and become progressively more positive.The AR crosses the zero axis between time periods "4" and "5," which conforms to the teachings of Figures 1 and 2.

Figures 1 - 3 considerthe manner in which the ranges change when the airplane moves along a straight line. Next, to be considered is the manner of range changes, when the aircraftfollowsa curving path 32 (Figure 4).

If the same reasoning is followed thatwas explained above with respectto Figures 1 - 3, the airplane periodicallytakesa distance reading to the beacon B (Figure 4). Common sensetells onethatthe maximum velocity between the airplane and the beacon occurs when the airplane is flying directly toward the beacon between time periods A(3) and A(4). This observation is confirmed in Figure 5 when the differences in range AR is plotted againsttime. The Y-axis position of point 34 represents the distance 36 in Figure 4 (i.e., the difference in measured distances in time periods A(1) and A(2). Again,there is a peak in Figure 5 when the airplane is proceeding directly toward the beacon B.

Hence,the conclusion is drawn that the airplane should always be flown to maintain it at the peak point on the AR/dtcurveofboth Figure2 and Figure 5.

Therefore, all that is required is to provide a distance measuring device which operates periodically (say once every second) and a memory which enables a recognition of whetherthe AR value is increasing or decreasing and which gives a signal when the AR is a maximum value as compared to preceding values.

The pilot needs to have a way of verifyingthe of verifying the maximum readings, which is shown in Figure 6. For example, unknown to the pilot, he encountersastrong side wind which drives the airplane offcourse in path segment 42. Accordingly, despite the fact that the airplane is always headed inthe same direction, itis no longer moving directlytowardthe be-acon. There- fore, the pilot occasionally wants to reorient his airplanetodeterminewhetherhe istrulyfollowing a path which gives a maximum AR/dtreading.

In greater detail, some point before the graph of Figure 6 begins, the pilot flies along some course and receives readings which put him on a course leading toward the beacon. When Figure 6 opens, the airplane is on a course in path segment 40 which is headed toward the beacon. Nearthe end of thins straight path (42),thewind blowstheairplaneoffcourse. Accor- dingly, to verify the course in path segment 44, the pilot turns the airplane either left (dashed line) or right (solid line). During this turn, the distance measure ments are as shown in Figures4and 5. By seeking the maximum A R/dt (Figure 5), the airplane is brought back on course in path segment 46.

Itshould now be obvious that only a distance measuring radar device and asimplecalculatortype of device is required to practice the invention. Since the invention may be practised with almost any distance measuring device, very often all that is required is a small add-on unit coupled to a distance measuring part of another system. That other system may still perform its primaryfunction.

For most needs, the calculator type device merely subtracts one value from anotherand recognizesthe highest value among the substractions. It may simply be connected to operate visual indicators which indicateeitherthe highestvalue ornotthe highest value. For example, a light may be green when the unit indicatesthata maximum AR"dtis being received and red when less than a maximum is being received.

Thus, in the "S-turn" of Figure 6, the light appears green during path segments 40,42. It is green in segment 42 because the preceding readings do not change enough to create a new maximum difference.

When the pilotturns offcourse in the "S-turn" maneuver of path segment 44, the light becomes red until the new maximum is read, and then it again becomes green.

Without adding too much cost, the calculator part of the system may be connected to give a digital read out. Figure7 exemplifies one such read out. Arbitrary ly, Figure 7 is drawn to showthatthe lasttimethat a maximum reading was set, the aircraftwas ona compass course of 3210. The aircraft was then travelling awayfromthe beacon ata rateof+110 meters/second (note the "+" mark which indicates that the gap is opening). The greatest (most negative) rate of change occurred when the aircraft was travelling at a peak rate of -125 metersisecond (the "-" mark indicates thatthe gap is closing). The apparent rangeto the transponder is 34.526 kilometers.

Wheneverthepilotwantsto go forward the transponder, he pushes the "Start Button" and follows the procedures outlined above. He flies a course which makes his "present rate" the most negative numberthat he can achieve. For example, as he circles the airplane, the highest number achieved is -125 meters/second. If he brings his "present rate" to -125 metersisecond, he is headed directly toward the beacon. As he corrects his course, it is conceivable that the peak rate number could increase in the negative direction. If so, he would keep his "present rate" meter on that new and more negative number.

The main value of the digital read out is that it becomes easier to program a path toward the beacon since the user knows the actual peak rate which is found while the airplane is circling. Therefore there can be lessforturning in the S-turn maneuver to test for path accuracy, as explained with Figure 6.

Figure 8 shows an exemplary installation using parts of the above-identified Merricktrilateral radar system, although the invention is not necessarily limited to use with this pa rticu lar system. As here shown, the distance measuring parts 50 of a mobile master station are mounted on either an airplane 52 or boat 54. A stationary transponder beacon 56 is mounted at a remote location 58, which may be an airport building or a boat house, for example. The vehicle mounted mobile equipment 50 periodically transmits a pulse (say about once a second) and the stationarytransponder 56 sends a reply pulse in response thereto.

The term "transmits a pulse" must be construed in the light ofthe system being used. In Merrick's above identified trilateral radar system, literallythousands of pulses might be sent to receive one "good distance reading." Some times a number of good readings are taken to ensure reliability through redundancy. Atthe electronic speeds ofthe system, this reading corres ponds to one ofthe readings A(1) - A(7) in Figures 1,4.

In any event, each good reading is herein described by the term "transmits a pulse" orthe like. The object of the system is to guide the mobile equipment on airplane 52 orthe boat 54to the stationary equipment 56 at location 58. The master station 50 comprises a transmitter 60, receiver 62, and distance measuring unit 64, which are parts of the above identified Merrick trilateral radarsystem.Themasterstation50causes the transmitter 60 to send an interrogation pulse to the transponder 56, while banking receiver 62 to prevent it from responding directly to thetransmitted pulse. The transponder beacon 56 receives the pulse and introduces a turn-around delay ("TAD") period, during which the masterstation 50 removes the blanking from the receiver 52.AftertheTAD period,the transponder 56 sends a reply pulse to the receiver 62.

The distance measuring unit 64 measures the duration ofthe time period required for the round trip ofthe interrogation and reply pulse, subtracts the TAD period and indicates the distance between the two stations 50,56.

The remainder ofthe equipment in Figure 9 is added to the Merrick system, according to the invention. To initiate the system, a "Start Button" (Figure 7) is pushed to mark wires 66,68 in Figure 9. The marking on wire 66 resets to zero a one-pulse period memory 70, a peak AR memory72, and a counter 92. The - marking on wire 68 starts a free running clock 74, which measures about a second, which is the interval between the successive readings A(1) -A(7). Thus, everytimethatthe clock 74 releases a clock pulse, a different one of the readings A(1 ) -A(7) is taken.

Meanwhile, the Merricktrilateral orothersystem may be carrying on any other function which it is designed to perform.

Each time that the distance measuring unit 64 gives a distance reading responsive to clock 74, that distance reading is stored in one-pulse period memory 70 and simultaneously sentto a compare circuit 76. The compare circuit 76 finds the different ("E R") between the present distance reading and the immediately preceding reading, after which memory 70 is resetto store the present reading. Any suitable buffer storage may be used to preclude a possible loss of the present reading pending the reset.

The difference reading AR is stored in memories 72, 80. The memory 72 can increase its stored reading in a negative direction, but it cannot reduce that stored reading prior to a system reset. The negative reading means that the gap is closing between the mobile and stationary stations 50,56. A positive reading indicates thatthe gap is opening.

The outputs ofthe AR circuit 80 and the peak- AR memory circuit 72 are fed into a difference amplifier 82, which may be adapted to accept any predetermined tolerance of differences. If there is an output from the difference amplifier 82, a signal feeds through an inhibit gate 84to activate any suitable off-course display 86 (such as a red light, for example).

An inverter 88 is turned off by the output from the difference amplifier. If there is no differencein the outputs from the circuits 80,72, there is no output from the difference amplifier 82, and inverter 88 turns on. Gate 84 is inhibited to prevent any off course display at 86. On course display 90 (e.g. a green light) isturned on and the pilot knows that he is on course.

An optional counter 92 counts a number of pulses from clock74 and then resets itself and the memories 70, 72. When the peak- AR memory 72 resets, the difference amplifier 82 gives an output signal which causes the off-course display 86 to come on. The pilot seeing this, changes his course slightly, thereby instinctively going through an S-turn maneuver of Figure 6. As he does so, the peak- AR circuit 72 takes a new maximum reading, thereby correcting for any drift in the airplane course (as when a side wind deflects the airplane from its apparent course).

The important thing forthe pilot to do is keep his airplane on one side of the beacon. Therefore, he is always closing the gap between his airplane and the beacon. If he overflies the beacon, the gap begins two open, in what appears two be an almost random manner. Then, he will have to start the procedure all over, pushing his start button and circling to find the best course.

Those who are skilled in the art will readily perceive howto modify the inventive method. Therefore, the appended claims are to be construed to cover all equivalents falling within the scope and the spirit of the invention.

Claims (11)

1. A method of homing on a beacon in a system using electrical or sound signals for measuring distances, said method comprising the steps of: (a) mounting a distance measuring transmitter on a vehicle which is to be guided to a home location; (b) mounting a transponder beacon atthe home location to which the vehicle isto be guided; (c)periodicallytransmitting an interrogation pulse from said vehicle toward and receiving a reply pulse from said transponder; (d) compa ring the distance measu red responsive to each interrogation and reply pulse with the distance last previously measured; and (e) directing said vehicle along a path which is indicated by said comparing means as causing the greatest negative change of distance measured between successive pulses.

2. The method of claim 1 and the added step of periodically maneuvering said vehicle through an S-turn to verify its course heading.

3. A homing system comprising mobile means for sending distance measuring interrogation signals, stationary means for returning a reply signal responsive to said interrogation signals, means responsive to timing an interval between said interrogation and said reply signalsforindicating the distance between said mobile and stationary means, means for periodically initiating said sending of said interrogation signals, whereby there are periodic distance measurements, means responsive to each successive periodic distance measurement for detecting the range difference between each successive one of said distance measurements, and means responsive to the detected difference for indicating a maximum range difference when said mobile means is travelling toward said stationary means.

4. The homing system of claim 3 and means for storing a memory ofthe maximum difference which is indicated while the mobile means is circling to find the best path toward the stationary means for indicating whetherthe present range difference is equal to or less than said maximum difference.

5. The homing system of claim 3 and means responsive to said range difference for giving a + AR signal when said mobile means is moving awayfrom said stationary means and a - AR signal when said mobile means is moving toward said stationary means.

6. The homing system of claim 5 and means for storing a memory of the peak- AR signal, and means for matching a present AR signal with the stored peak - AR signal.

7. The homing system of claim 6 and means for giving an off-course signal when said present AR signal does not match said stored- AR signal.

8. The homing system of claim 6 and means for giving an on course signal when said present A signal matches said stored- A signal.

9. The homing system of any one ofthe claims 6 - 8 and means for periodically resetting said stored memory of said peak- AR signal.

10. A homing system substantially as described herein with referenceto and as illustrated in Figure8 of the accompanying drawings.

11. A homing system substantially as described herein with reference to and as illustrated in Figure 9 ofthe accompanying drawings.