Microwave Landing System for Use on a Back Course Approach

This invention generally relates to the subject of aircraft navigation and landing systems and, in particular, to an aircraft landing system employing microwave radio signals that are especially useful during take-off or a missed approach, including azimuth guidance on the back course.

For many years the airports have been equipped with the Instrument Landing System known as the ILS. However, this landing system is about to be replaced by a new Microwave Landing System (MLS) which has recently received virtually worldwide acceptance.

In view of the essential nature of aircraft landings, often considered to be the most critical of ordinary maneuvers, it is important to have a monitoring and back-up system that provides an independent check during in-flight approaches to insure the absolute reliability of the data being provided by the principal landing system. This would be in addition to the usual ground-based monitors which are used for checking course alignments, signal strengths, etc. of the signals radiated from the ground-based landing system components. Although other types of navigation techniques are constantly in use in an aircraft to check on the accuracy of the landing system, none provides the high degree of reliability required during final approach.

A detailed discussion of the means by which monitoring systems may be used for the critical landing operation is provided in my U.S. Pat. No. 4,635,064. Also included therein is an overview of MLS.

It is the principal object of the invention to provide an independent fixed guidance beam system for confirming the accuracy of the guidance data provided by an MLS system during take-off or a missed approach and, more particularly, to provide a guidance system that will use the same format and repeating time sequence that the MLS uses and that will be compatible and functional with airborne MLS receiving equipment, especially during take-off or a missed approach.

Another major object of the invention is to provide an independent monitoring system during take-off or a missed approach that can be implemented either by using independently radiated overlapping fixed beams from the ground, or alternatively, by using either the MLS sector clearance signal beams or the OCI beams which would be slightly modified and reoriented to make them overlap appropriately at the centerline of the approach path. Therefore, it is a specific object of the invention to provide an Independent Landing Monitor System (ILM) for monitoring the progress of each take-off or missed approach at an MLS site, whereby an aircraft can obtain truly independent confirmation of the MLS guidance data from ILM ground equipment which is integrated with the MLS ground equipment. In such a system, the airborne derived ILM data will be truly independent, but can be based upon the use of the already installed MLS airborne equipment without requiring added airborne equipment.

It is a further major object of the invention to provide a system of the type specified which lends itself to the convenient configuration of a scaled-down fixed beam guidance system which is compatible with airborne MLS equipment and which will provide guidance accuracy that approaches the guidance accuracy of the main MLS system, but at a small fraction of the cost thereof.

Still another object of the invention is to provide a simplified back azimuth guidance system for use either with a standard scanning beam MLS system or the simplified fixed beam MLS system. In addition, the present invention can provide monitoring of the standard MLS scanning beams guidance function as used for back azimuth guidance.

Other objects and advantages of the invention will become apparent during the following discussion of the drawings showing preferred embodiments of the invention.

This invention provides a method and guidance system having integrated therein an independent monitoring capability in which signals from two differently functioning guidance systems are integrated into a single system to provide both guidance monitoring and guidance during take-off or a missed approach. In the preferred embodiment of the system, the results of each of the differently functioning guidance systems are independently arrived at in the aircraft, compared in the aircraft, and the flight either continued or aborted depending on whether or not the two results are in acceptable agreement. The two systems respectively comprise: first, a well known MLS time sequence which is a part of the standard MLS system and during which guidance beams are scanned and the airborne receiver determines the position of the aircraft based on the times when the scanning beams pass over the aircraft; and second, a separate and differently operating system in which paired fixed guidance beams are radiated toward the aircraft at pre-determined MLS times and the aircraft receiver compares the relative intensities of the paired signals for position determining purposes. The latter amplitude-based system, for azimuth guidance, uses paired fixed guidance beams sequentially radiated at different available MLS times, and directed respectively to the left and right of the centerline, and overlapped at the centerline in such a manner that an aircraft flying along the centerline will intercept equal signal intensities to indicate on-course flight. Conversely, when the aircraft is off-course to one side of the centerline, it will intercept a stronger signal intensity at the time of transmission of the fixed beam which is directed to the other side of the centerline. As a result, an off-course condition is indicated by unbalance of the intensities of the two fixed guidance beam overlapping signals, the side to which the aircraft is off-course being identified by strengthening of the signal radiated by that beam and weakening of the opposite fixed guidance beam signal. The times in which these fixed beams are radiated will be MLS available times which are not necessarily assigned for any other purpose, and the signals radiated therein will be used for providing independent confirmation of the straight-line path and will be independent of the main scanned precision guidance signals of the MLS.

The present invention also provides fixed beam guidance which will be operative in the event of failure of the main MLS scanning system, or alternatively, which can provide a simplified MLS fixed beam guidance system that is suitable for use at small civil airfields that cannot afford MLS scanning beam guidance for reasons of cost, or for use at certain tactical landing areas that cannot employ MLS scanning beam guidance because of the size and weight of required scanning beam antennas.

The present invention also provides simplified back azimuth guidance for use with the standard MLS scanning beam landing system, or alternatively, with the simplified MLS fixed beam landing system. In addition, the present invention can provide guidance for monitoring of the standard MLS back azimuth guidance function.

Both the MLS scanning beam guidance generation, and the fixed beam guidance generation furnishing the monitoring capability, can be performed by receivers of the MLS scanning beam type since such receivers are already designed to perform both timing and amplitude measurements. In addition, the data processing required for ILM monitoring is well within the capability of modern microprocessors already designed for MLS system use (i.e., the required additional ILM data processing beyond that required for conventional MLS purposes alone does not raise any serious problems in the airborne receiver design).

FIG. 1 is a diagram showing a typical radiation sequence for the various functions of a known MLS system;

FIG. 2 is a diagram showing typical radiation antenna patterns of a known MLS system;

FIG. 3 is a diagram showing the sequence of transmissions within an MLS guidance function, and showing the corresponding signals as received at the aircraft;

FIG. 4 shows a first embodiment including azimuth radiation patterns for a fixed beam precision guidance system;

FIG. 5 shows the relative amplitudes of signals as received at an aircraft which were radiated from the fixed beam precision guidance patterns of FIG. 4.

FIGS. 6A and 6B show azimuth antenna radiation patterns respectively from a conventional MLS system, and from a modified MLS system wherein the OCI patterns have been altered to provide a second embodiment of the invention, wherein both MLS scanned beam and fixed beam precision guidance are provided at the same ground location;

FIGS. 7A and 7B show azimuth antenna radiation patterns respectively from a conventional MLS system and from a modified MLS system, wherein the left and right clearance patterns have been altered to provide an alternative form of the second embodiment of the invention wherein both MLS scanned beam and fixed beam precision guidance are provided at the same ground location;

FIG. 8 is a diagram showing, for a typical MLS system, the angular positions at which clearance beam signals are transmitted relative to the beginning and end positions of the TO and FRO scanned MLS beams;

FIGS. 9(A-C) is a diagram showing a third embodiment of this invention wherein fixed beam precision guidance signals are tramsitted during the scanning intervals of the TO and FRO scanned MLS beams;

FIG. 10 is a block diagram showing an azimuth embodiment of ground and airborne MLS equipment, modified to implement the present invention which combines both MLS scanning beam guidance and fixed beam precision guidance at the same integrated ground location;

FIG. 11 shows ground-station azimuth radiation patterns for a system as shown in FIG. 10; and

FIGS 12 and 13 respectively are diagrams illustrating the outputs of an airborne receiver in response, respectively, to guidance signals received thereat from conventional MLS scanning beam guidance ground equipment, and from a system employing fixed beam precision guidance signals radiated from the ground.

While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail, several specific embodiments, with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention and not intended to limit the invention to the specific embodiments illustrated.

The typical MLS system is a sequentially operating system which radiates both guidance functions and data functions at different exclusively-occupied time intervals in a repeating sequence. Up to 15 different functions can be radiated; these include:

Basic Data Word 1: Approach Azimuth Guidance

Basic Data Word 2: High Rate Azimuth Guidance

Basic Data Word 3: Approach Elevation Guidance

Basic Data Word 5: Back Azimuth Guidance

Basic Data Word 6: Future-System Growth Guidance

Auxiliary Data: A, B and C

These functions can be divided into two separate categories, one category providing guidance, and the other category providing, to the aircraft, data relating to that particular MLS installation (i.e., location of MLS equipment with respect to the runway, equipment status, type of services provided, etc.). The MLS signal format currently includes both a flare guidance function, a 360 degree auxiliary guidance function, and an auxiliary data function (all not yet fully specified).

The number of guidance functions that are radiated by any particular MLS installation depends on the guidance services which must be provided at that ground location. There is also certain basic data that must be radiated from that installation. The above listed Auxiliary data and Future-System Growth Guidance functions and their associated time intervals are reserved to accommodate future needs not yet fully specified.

It should be noted that, while a Flare Guidance function and a Future-System Growth function, normally intended t provide 360 degrees azimuth coverage are presently listed as MLS Guidance functions, there is a high probability that they will never be implemented and, hence, the time intervals currently assigned to this use will be, in all probability, available for future MLS system growth as, for example, to include my invention. The possibility also exists that additional time intervals will be created to incorporate improvements to MLS, such as, for example, my invention.

Each of the above listed data or guidance functions has its own identification code within the preamble associted with that function, which code uniquely identifies the nature of the associated function to the airborne equipment. Since the individual functions are thus identified by their codes, it is not necessary that these functions be performed in any particular sequence.

Specific sequences are, however, recommended in the ICAO SARPS (Standards and Recommended Practices) for installations that provide a particular combination of functions. In addition, each particular function must be radiated at a certain minimum repetition rate consistent with the service that function performs (i.e., azimuth approach guidance must be provided at a rate consistent with aircraft/pilot response for a desired guidance performance). The aircraft receiver and processor include capability for recognizing the function being radiated to it, using the preamble which precedes it, whereby the aircraft receiver is enabled to utilize the information appropriately.

The precision guidance functions of the MLS are provided by means of a narrow beam that scans the region in which precision guidance is being provided. The time between successive passage of the scanning guidance beams past the airborne receiving antenna is precisely measured by the airborne precision timing circuit and used to provide the desired angular guidance data.

The purpose of using scanned beams for localizer and elevation determinations in MLS, as distinguished from fixed beams as used in ILS, is to permit the approach and landing of aircraft along nonlinear courses having greater flexibility than straight-line paths (i.e., permitting curved azimuth and elevation approaches which are deemed especially useful at high traffic airports). Although a curved approach path may be useful at some distance from touchdown, in order to be more certain of a safe landing, the aircraft will usually fly the last, most critical portion of the approach to touchdown, along the usual non-maneuvering straight-line centerline course. Moreover, during most approaches, the aircraft will still follow a relatively standard straight-line glidepath, typically a 3 degree glideslope which is the same as used in ILS landings, prior to touchdown.

Of particular significance to this invention is the straight path followed during take-off or a missed approach procedure. In this procedure, an aircraft (for example, an aircraft making an ILS missed approach) flies a straight path determined by paired fixed overlapping azimuth guidance beams similar to the fixed overlapping azimuth guidance beams used for landing. These beams are termed "back azimuth beams" and are a standard ILS feature. The MLS system includes the use of a back azimuth scanning beam guidance system essentially identical to the landing scanning beam azimuth guidance system, and of the same complexity.

The following discussion considers approach azimuth and back azimuth, interchangeably, since operation of each is essentially identical. The invention is, however, focused on the azimuth flight regions flown in a take-off or missed approach procedure.

In addition to the use or radiation of the scanning beam to provide precision guidance, the MLS guidance function may also include the use of sequentially radiated fixed beams. These fixed beams serve two separate purposes.

One purpose, termed OCI (out of course indication) is to suppress false courses outside the established MLS guidance region. These false courses might be caused by side lobes of the precision scanning beams. False course suppression is accomplished by radiating one or more fixed beams that provide greater signal strength, by a prescribed amount, than the side lobes of the scanning beam in the area in which it is desired to suppress possible false guidance courses. Up to six false course suppression beams can be radiated within the azimuth guidance function, and up to two within the elevation guidance function.

The second purpose of using radiated fixed beams is to provide a clearance capability. Clearance beams are used in the MLS installation where the azimuth scanning beam does not scan the entire, normally prescribed, precision azimuth guidance region of .+-.40 degrees about the runway centerline, but scans only a portion of that region. In such installations, these clearance beams are radiated left and right of the scanning beam precision coverage, but within the specified guidance coverage region. Measurement of the amplitudes of such beams will provide a fly/left, fly/right signal for use in the aircraft for intercepting the region in which precision proportional guidance is provided by the scanning beam. Both the OCI and the clearance beams are radiated at prescribed times within the time allocated to the guidance function within which they might be utilized.

It can be noted therefore that MLS is a sequentially operating system that can provide many different guidance functions in a very flexible building block configuration. In addition, this flexibility is enhancedby providing auxiliary functions for unspecified future growth potential. Precision guidance data is provided by accurate measurements of the times when the scanning beam passes over the aircraft. In addition, airborne amplitude measurements are also being made to determine the intensitites of sequentially radiated fixed beams that may be utiltized at some MLS installations to prevent false courses (OCI beams) and to aid in the acquisition of the precision guidance beams (clearance beams).

Referring now to the drawings, FIG. 1 shows a function radiating sequence, both for data and for azimuth and elevation guidance, for a simplified MLS system. Appropriate basic data words are radiated as represented by data function blocks labeled 1, at times available between the times of radiation represented by other function blocks, such as blocks 2 for azimuth and blocks 3 for elevation guidance functions, respectively; these functions all being radiated at repetition rates specified by the MLS ground equipment. A suitable preamble, such as the preamble 4, is shown in each function block at the start of the function being radiated, and the preambles for the various functions are radiated by the antennas at the guidance installations 5 and 6, the elevation installation 6 being located near the approach end of the runway, and the azimuth installation 5 being located along the centerline at the far end of the runway. Back azimuth equipment 5', similar to approach azimuth equipment, would be located at the approach or opposite end of the runway. Specifically, the preamble for the aximuth guidance function 2 is radiated from an antenna with a pattern 8', FIG. 2, at the azimuth installation that provides coverage throughout the guidance coverage region 8 of .+-.40 degrees azimuth. The basic data words are generally radiated, together with their preambles, using this same angular coverage. If the preamble associated with a particular function indicates that it is an azimuth guidance function, then the TO and FRO scanning guidance signal associated with that preamble will be radiated by a narrow beam antenna with a pattern 9 which scans that same sector 8. The actual guidance data are obtained within the aircraft receiver by measuring times between successive passages of the scanning beam pattern 9 past the aircraft, as is well known in the MLS art.

FIG. 3, in the vicinity of (a) shows in more detail the organization of the radiation during an azimuth scanning function of the MLS ground system, including a preamble 11, sector signals 15, and TO and FRO scanning time intervals 20 and 20'. The preamble 11 is radiated first and includes a continuous wave 12 used for acquisition by the airborne receiver of the radio frequency carrier. This carrier is followed by a receiver reference time synchronization code 13. This is followed by a function identification code 14, in this case identifying the function as azimuth approach scanning guidance. These radiations constitute the preamble 11 of duration shown by the arrow 21.

The next radiated signals are the sector signals 15, which begin by transmitting a ground station identification code 16, which lasts for the period identified by the arrow 22. Following the identification code, there is radiated a constant level test signal 17, with duration as shown by arrow 23. This signal is used in the airborne equipment by switching airborne antennas during this internval 23 to determine which airborne antenna provides maximum signal and, hence, should be utilized thereafter. Subsequently, OCI signals 18 are radiated, which are processed in the receiver to determine whether the aircraft is on a false course (i.e., following a scanning beam side lobe as indicated by a received OCI signal being stronger than the false scanning beam signal). Next, a TO scan test pulse 19 is radiated, followed by a TO scanning beam, and a FRO scanning beam, followed by a FRO scan test pulse 19' is radiated, the test pulses being radiated immediately preceeding and following the times allocated to the TO scanning beam time 20 and the FRO scanning beam time 20'.

As can be seen across the top of FIG. 3, in the vicinity of (b), the receiver in the aircraft develops appropriate signals, based on the ground-radiations, as just discussed. Specifically, as shown in FIG. 3 for the azimuth scanning function, the preamble transmissions produce, in the receiver, a preamble sequence 21, followed by the ground station identification code 22, followed by the airborne antenna selection radiation 23. There are shown two OCI radiations 24 and 25 (out of six OCI radiations that could be transmitted), followed by the ground radiated TO scan test pulse 26. The TO and FRO scanning guidance beams 28 and 29, respectively, are then received in the aircraft at times depending on the loation of the aircraft with respect to the centerline of the runway, and finally, the FRO scan test pulse 27 is radiated. The clearance pulses are not shown in FIG. 3 because they would not be used in the FIG. 2 configuration because the scanning beam scans the full .+-.40 degrees region, not a sector of reduced width.

In order to implement the present invention so as to provide both scanned beam guidance and fixed beam precision guidance integrated therewith in a common system, the fixed beam precision guidance signals must be incorporated into the just-described MLS system in a compatible manner, so that a monitoring capability can be provided by comparing, in the airborne equipment, the scanning beam guidance results and the fixed beam guidance results.

It is an important object of the invention to integrate the fixed beam guidance system into the MLS in such a way that future MLS ground installations which are modified to implement this invention, by incorporating fixed beam precision guidance capability, will not provide incorrect data to earlier model MLS airborne systems which are not so modified. Even though achieving this objective is desirable, it is not absolutely necessary because the added capability in later built equipment could be restricted, at least for a time, to special applications which would not be utilized with the older MLS airborne equipment.

For example, the modified ground systems could initially be restricted to certain tactical ground installations for the battle environment, the use of which would be restricted to aircraft equipped with updated tactical MLS type receivers that would be configured in accordance with this disclosure. Such military receivers could be programmed to recognize that an auxiliary data word, (such as that reserved for an individual country' s special needs, i.e., the Auxiliary National Data word), normally considered to provide only data, could also be radiating guidance data, such as that described in my invention, and appropriately process such data for guidance generation purposes.

For the purpose of illustrating the present inventive concepts which integrate fixed beam precision guidance into an MLS-type scanning beam system, three different embodiments will be discussed.

The first of these embodiments is shown and described with reference to FIGS. 4 and 5 and invovles the radiation of overlapping azimuth fixed beams, from additionally installed antennas which function within various time intervals provided in the MLS sequence, to accommodate future growth of the system. The second embodiment shows the modification of currently radiated MLS systems beams to provide fixed beam precision guidance, the modifications being of existing MLS OCI beams as shown in FIG. 6, or being of existing left and right clearance beams as shown in FIG. 7. The third embodiment shows the radiation from additionally installed antennas of my fixed beam precision guidance signals, during unused portions of intervals, already assigned for other MLS functions, for example as shown in FIG. 9, during TO and FRO scanning intervals of the MLS system.

Considering now the first embodiment, this embodiment uses available Future Growth functions that can accomodate future growth needs, and radiates, in its associated time interval, my fixed beam guidance signals. While this embodiment has reference to the landing path, it is equally applicable to the take-off or missed approach azimuth path. This embodiment uses an existing or a Future Growth data function to radiate data appropriate to accompany the fixed beam precision guidance signals. This embodiment also employs paired separate antennas, additionally installed at the MLS ground location, for radiating paired fixed precision guidance beams along the flight path to provide guidance which is independent of the guidance provided by the MLS TO and FRO scanning beam identified by the reference numeral 9 in FIGS. 2 and 4. These added fixed precision guidance beams are properly identified by their own preamble which defines their function to the airborne receiver, and by added data transmitted therewith to enable proper processing of the signals received at the aircraft from the fixed guidance beams. In this embodiment, the fixed beam precision guidance function is provided by added antennas 80 and 81, FIG. 10, driven by the ground equipment sequentially to radiate a left pattern 31 and a right pattern 32 with respect to the desired azimuth centerline course 36. Note that the azimuth beams 31 and 32 partially overlap along the centerline 36 in FIG. 4.

The fixed beam guidance function signals, as received in the aircraft, are shown in FIGS. 5A and 5B and include, in each case, a preamble 37 containing information, including function identification, etc., followed by paired fixed beam guidance signals representing azimuth information for two different locations 35, and 36 of an aircraft. The fixed beams 31, 32 in FIG. 4 are all transmitted with equal intensity along the azimuth flight path. Since the paired beams respectively overlap equally along the centerline 36, an aircraft which is precisely located therealong will receive equal intensities of paired signals from both of these beams. Thus, FIG. 5B shows the resulting signals 38 and 39 in the aircraft receiver to be of equal amplitudes. However, if the aircraft strays from the desired path, the signal from the beam, which is directed more toward the side to which the aircraft has strayed, will be strengthened, while the signal from the beam directed away from that side will be weakened. Thus, FIG. 5A shows that, for an aircraft position 35 left-of-center, the left signal 38' is stronger than the associated right signal 39'.

This unbalance in the signals 38', and 39' is processed in the aircraft receiver to deliver an appropriate output signal whose amplitude indicates non-centered position of the aircraft to the left in azimuth, because when the aircraft strays from the desired path, the signal strnegthens on the side to which it has strayed, and weakens on the side from which it has moved away. Moreover, the degree to which straying off-course has occurred is proportionally indicated by the degree of unbalance of the amplitudes of the paired signals 38, and 39 to achieve proportional-guidance.

These output signals may be used either to provide auxiliary monitoring signals which, in the MLS system, are used for comparison with similar outputs based on MLS scanning beam guidance signals or, alterntively, the fixed beam guidance can replace the MLS scanning beam signals entirely for small airport installations in order to provide, at such airports, less complex and expensive guidance capability which is still compatible with the receiver systems in aircraft having full MLS capability. Alternatively, the fixed beam guidance can provide back azimuth guidance, either in conjunction with MLS scanning beam or fixed beam landing guidance. Radiation of this fixed beam precision guidance and associated data utilizes only a very small percentage of the time allocated in the MLS system to accommodate as yet undefined future growth of the system. Since only a minor amount of this available time allotment is needed for the fixed beam guidance function, time available for future growth is scarcely diminished.

Consideration will now be given to the second embodiment concept which uses a somewhat different approach to the fixed beam precision guidance concept. The different approach is illustrated by two different forms shown, respectively, in FIGS. 6 and 7. The second embodiment differs from the concept discussed in connection with FIGS. 4 and 5 in that, instead of providing additional antenna means for performing the fixed beam precision guidance functions as related above, the second embodiment alters the radiation patterns of existing MLS antennas to overlap their beams along the centerline and thereby achieve the desired fixed beam precision guidance, without diminishing the normal functions of those already-existing antennas. FIG. 6A shows unmodified radiation patterns including left and right OCI beams, while FIG. 6B provides a first form of the embodiment which uses these OCI (Out of Course Indication) antenna beams of the MLS system by overlapping the beams at the centerline 36. FIG. 7 provides a second form of the embodiment which uses the right and left clearance antenna beams of the MLS system, described hereinafter.

In the first form, FIG. 6A shows standard MLS OCI beams 43 and 44 used to suppress the possibility of false courses by an aircraft which might be following a side lobe 45 of a scanning beam antenna instead of the main beam 9. When the aircraft is outside the .+-.40 degrees scanning sector, it is considered to be outside of the guidance region 8. These OCI beams 43 and 44 are radiated at such intensities that, for an aircraft located outside the guidance beam region 8, their received amplitudes must be greater than the signals from any scanning beam lobe or clearance beam guidance lobe received, but for an aircraft located within the region 8, the OCI signal intensity must be at least 5 db less than the guidance signals. After an airborne receiver has sequentially received signals respectively identified by their times of occurence within the guidance function as being OCI signals, and, in addition, has received clearance signals (from installations employing them) and scanning beam signals, then if the clearance or scanning beam guidance signals are not greater by at least 5 db in intensity than the OCI signals, the airborne signal processor warns the pilot that he is out of the guidance coverage region 8. Of course, the OCI signals are of greater intensity than the scanning beam signals 9 or clearance signals (if radiated) at all locations outside the azimuth coverage of the guidance region 8. Up to six azimuth OCI beams are provided for in various MLS systems. Nothing in the MLS OCI specification criteria, therefore, precludes OCI beams from being used to provide the present fixed beam precision guidance during the assigned OCI radiation times.

As shown in FIG. 6B, the OCI beams have been modified to provide the overlapping contour shown as beams 46 and 47 which overlap the azimuth centerline 36 at 48. When thus configured, the beams 46 and 47 can still perform their normal OCI functions, while, at the same time, the portions of the beams overlapping the centerline at 48 can give the same kind of fixed beam precision guidance function as was discussed above with respect to FIGS. 4 and 5. Such dual function OCI and fixed beam guidance radiations would not be incorrectly interpreted by present day unmodified MLS receivers since their fixed beam guidance capabilities would simply be disregarded thereby, while, at the same time, the receivers would properly utilize the OCI information. On the other hand, future MLS receivers which are appropriately programmed by radiated data words to take advantage of the dual OCI beam capabilities, would also obtain fixed beam precision guidance from these OCI beams.

It is recognized that in some airport locations having severe lateral multipath problems, it may be difficult to utilize the wide coverage fixed OCI beams 46 and 47 shown in FIG. 6B to provide precision fixed beam guidance having adequate definition and freedom from lateral multipath effects by overlapping the beams along the centerline 36. Nothing in the MLS specified criteria for generation of OCI beams, however, precludes the radiation during unused OCI time intervals of narrow precision fixed guidance beams 31 and 32, as shown in FIG. 4. Such beams would be ignored by presently existing MLS receivers, but would be properly utilized by future MLS receivers having the proper programming enabled by data words, for example, to process their precision fixed beam guidance data.

As an alternative form of this second embodiment of the invention which, in either form, alters exiting MLS radiated beams and uses them for fixed beam precision guidance, FIG. 7 serves to illustrate the concept of modifying the standard MLS azimuth clearance beams 50 and 51 shown in FIG. 7A, by extending them to overlap along the centerline 36 of the approach path to assume the beam shapes shown at 52 and 53 of FIG. 7B. As discussed with reference to FIG. 2, in the usual MLS system, the scanning beam 9 provides coverage over the whole guidance region 8 which extends .+-.40 degrees of the centerline 36. However, in the scanning beam embodiment depicted in FIG. 7, the scanning beam 9' only provides precision scanning beam proportional-guidance over a sector 49 extending .+-.10 degrees each side of the centerline 36, such restricted angular coverage being appropriately noted in the radiated basic data words and being an existing standard alternative form in the MLS system. The left and right sector arcs 8" and 8"' which are located just outside the scanned region 49, and which extend to the MLS specified limits of .+-.40 degrees, are according to present MLS specifications covered by MLS clearance beams 50 and 51 as shown in FIG. 7A. Measurements of the signal intensities of the clearance beam signals are used in the aircraft to provide appropriate fly/left or fly/right indications to the pilot to guide him to intercept the main scanned guidance beam 9' in the sector 49. The criteria set forth in the MLS specifications for clearance beams are: that the signal intensity of clearance beam 50 for an aircraft located in sector arc 8" must exceed the signal intensity of clearance beam 51 by 15 db, and must exceed the side lobes of the scanning beam signal 9' by 5 db; and in addition, that the signal intensity of clearance beam 50 must be 5 db below the scanning beam signal intensity for an aircraft located at -10 degrees along line 49a (i.e., at the negative edge of the scanning beam coverage region 49). Similar specifications apply to clearance beam 51 in sector 8"'. It is therefore apparent that nothing in the MLS specification regarding the clearance beams prevents these clearance beams from being overlapped along the centerline 36, as shown at 52 and 53 in FIG. 7B, for the purpose of providing fixed beam precision guidance for aircraft located within the .+-.10 degrees guidance region 49

FIG. 8 shows the standard MLS format for the clearance beams relative to the .+-.10 degrees scanning beam 9' which, in the figure, are shown four times adjacent to the four outermost angular positions of the scanning beam 9'. The four clearance beam pulses 55, 56, 57 and 58 are radiated at times when the scanning beam 9' has reached its outermost TO and FRO scan limits. For an aircraft position which is to the left of the centerline 36, for instance as shown at 49a in FIG. 7A, the fly/right pulse intensities 56 and 57, as received at the aircraft, would be the same as each other, but less (not illustrated) than the fly/left pulse intensities 55 and 58. For a centerline aircraft location 36, the intensities of all four pulses would be equal, as shown in FIG. 8. It should be noted that in MLS systems where the scanning beam scans the full 40 degrees each side of center, so that clearance beams are not employed (FIG. 4A), they can still be added to the system for fixed beam precision guidance purposes, and will be used for guidance monitoring purposes by up-dated and properly programmed MLS receivers, while being ignored by presently existing unmodified MLS receivers.

The third embodiment of the present invention, as described with reference to FIG. 9, radiates from newly added fixed beam precision guidance antennas during unused time intervals within the existing guidance function signal format, as distinguished from using available function time intervals as provided in the MLS system to accommodate future growth of the system, as exemplified by the first embodiment, or from using presently existing radiated beams within already existing guidance functions which are also used for other purposes such as OCI or clearance beams.

This third embodiment employs, for instance, additional fixed beam precision guidance antennas oriented according to FIG. 4, but which radiate their fixed guidance beams at unused suitable times during the over-all interval of time allocated for the scanning beam function. The use of these times is somewhat along the lines suggested in U.S. Pat. No. 4,306,239. Such unused suitable times would be outside the times of actual beam scanning and/or clearance beam radiation, since the interval of time allocated for the scanning beam is much longer than is required for normal scanning beam operation and/or clearance beam radiation. Normal scanning beam operation does not exeed .+-.40 degrees, whereas sufficient time is included in the interval to permit up to .+-.62 degrees of scanning. The time allocated for the scanning operation between the .+-.40 degrees and the .+-.62 degrees is, thus, normally available for other uses, such as the radiation of fixed beam precision guidance pulses.

FIG. 9A shows the interval of time allocated to the scanning function within the MLS time sequence, and FIGS. 9B and 9C show corresponding signals received in the aircraft for azimuth guidance, assuming the radiation of azimuth fixed beam guidance pulses of the type discussed in connection with FIG. 4. As illustrated in FIG. 9B, the left fixed guidance beam is radiated at a time corresponding with -50 degrees of the TO scan, as shown at 60, and again at a time corresponding with -50 degrees of the FRO scan, as shown at 63. Likewise, the right fixed guidance beam is radiated at a time corresponding with +50 degrees of the TO scan, as shown at 61, and again at a time corresponding with +50 degrees of the FRO scan, as shown at 62. The angular scan is shown in FIG. 9A. The relative intensity of the left fixed guidance beams 60 and 63 with respect to the intensities of the right fixed guidance beams 61 and 62, as shown in FIG. 9B, when compared in the processor of the MLS receiver, serve to indicate that the aircraft is off the centerline 36 to the left. The differences of these intensities indicate how far off the centerline the aircraft is located. Equal amplitudes of the intensities (i.e., of all four pulses 60', 61', 62' and 63' as shown in FIG. 9C), indicate that the aircraft is exactly on the centerline 36. Data words radiated in association with the fixed beam guidance pulses inform the aircraft receiver of the portion of its program which should be used to process the signals received in the aircraft from that ground location. Furthermore, the signals radiated according to this third embodiment are radiated at times different from the times normally used for radiating clearance beam signals, or test pulses, or scanning beam signals, so that presently existing MLS receivers will not erroneously interpret these fixed beam precision guidance signals. Ample time is allocated in the MLS scanning beam scanning time interval for this purpose.

Accordingly, it can be seen that there are many different practical embodiments including those herein described, as well as others not described, that can be used to radiate fixed beam precision guidance pulses for azimuth guidance within the framework of the specified MLS format, which format is very flexible and has vast unused and/or unassigned time periods, leaving ample time in which to radiate the fixed beam guidance signals, as well as data words, to appropriately programmed MLS receivers, while avoiding erroneous responses by presently existing unmodified receivers.

It may also be true that some presently existing unmodified MLS receivers do not have the precision needed for utilizing the present fixed beam guidance signals to generate precision landing guidance. The problem is that they may lack adequate capability for accurately measuring the relative amplitude intensities of the fixed beam guidance signals with such precision as would be required to match the accuracy of the scanning beam guidance, in view of the fact that existing receiver amplitude measuring capability may be only suffficient to provide false course suppression based upon signal strengths received from MLS OCI beams, or to provide fly/left, fly/right guidance based upon measurements of signal strengths from clearance beams. In addition, signal processing software required for use in the receiver in conjunction with the fixed beam guidance function is not currently programmed into present models of MLS receivers. However, proper modification of current receivers to use the present fixed beam guidance features is easily accomplished, including improvement of their capability for signal intensity measuring and comparing, and improvements to add the appropriate software. New receiver models can readily incorporate the needed precision amplitude measuring capability and the appropriate software.

FIG. 10 shows an integrated system operative to provide both scanning beam and fixed beam guidance according to this invention. The specific detailed embodiment described herein is for a conventional back azimuth or azimuth system similar to that of FIG. 1, but having fixed beam precision guidance integrated thereinto, using OCI allocated time intervals, and providing either monitoring of the scanning beam guidance function or, alternatively, providing for independent fixed beam landing guidance for use at small airports. In FIG. 10, an airborne receiver and data processing means is illustrated. The azimuth ground equipment shown in FIG. 10 includes a transmitter 70 which selectively feeds, via a suitable switch unit 72, multiple antennas 74 through 81 which respectively have different radiation pattern shapes, depending on the function that each is desired to perform. Although switch 72 is shown schematically as a rotary switch, it would comprise an electronic switching unit in a practical installation. The position of the switch 72, and its dwell time at each selectable position, is controlled by a programmed Radiation Control Logic Unit (RCLU) 7. The control unit 71 also controls whether the transmitter 70, when connected to a particular antenna, transmits a continuous wave output or whether it transmits a data encoded function (for example, the reference time code or appropriate data words). Such encoding is provided by the data encoder 73 when called for by the RCLU 71.

The beams radiated by the various antennas provide signals received in the air by airborne equipment which includes an antenna 92 coupled to an airborne receiver 93. The output signals from the receiver are delivered to an airborne data processor 94 which performs all the programmed functions necessary to provide guidance and which delivers guidance signals to drive guidance means. Here, the guidance means is depicted as a guidance indicator 95 which is of standard form, except that the glideslope needle B would be inoperative during back azimuth usage.

The system shown in FIG. 10 provides the standard functions comprising the currently accepted MLS system but, in addition, can be utilized as set forth hereinafter to provide either scanning beam guidance compatibly combined with my novel fixed beam guidance monitoring system or, alternatively, to provide guidance using only my fixed beam guidance system (i.e., at airports which are small and have low traffic levels and therefore do not require the more sophisticated flight paths that the scanning beams of the full MLS system can provide.)

As discussed previously, MLS is a very flexible system that can be installed in a wide variety of configurations, depending on the terrain at a particular site and the traffic volume, etc. The particular detailed MLS embodiment discussed below, with reference to FIG. 11, was chosen as representative of a typical installation, and also as an illustration of previous discussions within this disclosure. The MLS system shown in FIG. 10 is configured to provide azimuth or back azimuth scanning beam coverage using the beam 9' shown in FIG. 11 to provide a precision guidance region 49 of .+-.10 degrees. Beams 50 and 51 provide clearance sector beams outside this .+-.10 degrees region and extending guidance to the limits of .+-.40 degrees, defined by the sector 8. OCI beams 43 and 44 provide false course the guidance limits of .+-.40 degrees. Beams 31 and 32 provide the fixed beam precision guidance of my invention.

A sequence of conventional MLS beam radiation from the ground antennas of FIG. 10 is normally as follows. As shown in FIG. 10, a preamble is first transmitted from antenna 74 with antenna pattern coverage corresponding to .+-.40 degrees toward approaching aircraft in the guidance region 8, as shown in FIG. 11. The preamble 11, FIG. 3, includes an identification of the MLS function 14 being radiated which operates in the receiver to call up the proper processor program to process the data being radiated within that same guidance function from the ground installation. Sector beams 15 are then radiated from the antenna 74, commencing with the ground installation identification code 16 (station identity) and the airborne antenna selection signal 17. Then the out-of-course OCI beams 18 are transmitted from the azimuth antennas 75 and 76 to provide the beams 43 and 44, FIG. 11, to warn the pilot when he is outside the guidance region 8. The TO test pulse 19 is then radiated via the antenna 74, followed by radiation of the left clearance beam 50 via antenna 78, which is then followed by the TO scanning beam 9' via the antenna 79, after which radiation of the right clearance beam 51 occurs via the antenna 77. After a pause, a second right clearance beam is radiated via the antenna 77, followed by the FRO scanning beam 9' using the antenna 79, which is then followed by radiation of a left clearance beam 50 by the antenna 78. Radiation of the FRO test pulse 19' via the antenna 74 terminates the MLS azimuth guidance function.

The technique for including the radiation of my fixed beam precision guidance functions in the FIG. 10 MLS system is as follows: First, there are six azimuth OCI time intervals available for radiation of OCI beams in the present MLS specification. It is conventional to radiate only two OCI beams 43 and 44, FIG. 11, in just two of these six intervals, leaving four unused OCI intervals available. The present fixed beam precision guidance beams can therefore be radiated during unused OCI intervals by programming the logic control unit 71 to transmit sequentially via the switch 72 and antennas 80 and 81, my fixed beam precision beams having beam patterns 31 and 32 which overlap along the centerline 36, as shown in FIG. 11. In addition, the logic control unit 71 initiates the transmission of data words and the antenna 74 and switch 75 at appropriate times in the overall MLS radiating sequence, which data words identify the nature of the fixed beam precision function.

FIG. 12 shows the output response by an MLS receiver and processor to azimuth MLS scanning beam guidance beam received in response to scanning beam 9' as shown in FIG. 11. The illustrated airborne response to the MLS scanned proportional guidance system includes a curve P of the processor output voltage for various different angular locations of the aircraft on both sides of the 0 degrees centerline 36, plotted horizontally. For an aircraft position at 0 degrees, on the centerline, the output will be zero, meaning that the azimuth needle A of the indicator 95, FIG. 10, will be centered. As displacement of the aircraft off the centerline increases, the output on curve P will linearly increase needle movement for displacements up to 10 degrees on each side of center, although the over-all curve is shown for deviations up to 40 degrees each side of the centerline. The response of the FIG. 12 curve beyond the linear portion P, which is the limit of scanning beam proportional coverage, is controlled by the clearance beams 50 and 51 to provide constant amplitude fly/left and fly/right signals PL and PR.

FIG. 13 shows a curve K which is similar to curve P in FIG. 12 in over-all appearance, but represents my fixed beam guidance system response. Less of the contour of the curve K beyond the centerline 36 is linear, the linear portion encompassing less than about 5 degrees each side of the centerline, for instance, for aircraft approaching between the points 96 and 97, as shown in FIG. 11A. However, useful guidance is still given in the zones between the 5 degrees and the 10 degrees displacements off centerline 36. The clearance beams are also provided by the antennas 77 and 78 of FIG. 10 to produce fly/left or fly/right indications as shown at KR and KL, just as in the case of FIG. 12. A comparison of FIGS. 12 and 13 shows that a scaled-down system which uses only my fixed beam guidance system, and eliminates scanning beams, gives results which are comparable with those of the full MLS system, and which are quite adequate for small airport use. It is pertinent to note that the ILS landing system, as used for many years at large airports, provides linear guidance in its proportional display for only about 21/2 degrees each side of the centerline, with the needle of the cockpit display pegged for positions beyond 21/2 degrees from the centerline to provide fly/left and fly/right instructions to the pilot.

The above discussions show that the standard MLS system, the MLS system augmented to include my fixed beam monitoring system, and the scaled-down system using my fixed beam guidance in place of scanning signals, are all mutually compatible, and can all be interchangeably used at airports around the world, the different systems merely requiring appropriate programming of the ground radiation control logic 71 and the ground data encoder 73 to indicate which type of system is at a particular location, and requiring appropriate programming in the airborne data processor 94 to cooperate with the different ground installations. The discussions also show that my fixed beam system, while using the same ground transmitter 70 as the MLS system, uses different antennas and operates differently and at different times in the MLS radiation sequence, and, therefore, operates independently, to a large extent, of the MLS scanning beam system when serving to monitor the latter.

This invention should not to be limited to the exact embodiments shown in the drawings, for obviously, changes may be made within the scope of the following claims. For example, it can further be noted that my invention does not preclude the possibility, and indeed enhances the possibility, of an MLS compatible landing system with a signal format, as outlined in this disclosure, but operating at another microwave frequency. For example, such an MLS compatible system, as outlined in this disclosure, could be implemented at a higher frequency, such as 10,000 or 15,000 MHz, with a much smaller ground station. Specifically, the use of MLS compatible fixed guidance beams, at a higher frequency, would result in such a ground system not only being smaller because of the use of a higher frequency, but also smaller because of the reduced size of the fixed beam antennas in contrast to the scanning beam antennas. This small size would be particularly attractive for certain applications, such as some unique military applications where an extremely small ground station is vital. In the aircraft, all that would be additionally required would be a simple fixed frequency converter to convert the higher frequency to the accepted MLS frequency of about 5,000 MHz, with all other aspects of the airborne equipement being as described in this disclosure. In this manner, such an airborne equipment could be utilized, not only to receive the signal from a

small unique military requirement ground station, but also the conventional 5,000 MHz MLS signal radiated from civil airports, for example. These and other variations of my invention are intended to be within the spirit and scope of the claims which follow. Thus, it should be understood that no limitation with respect to the apparatus and method described herein should be inferred, and all such modifications are to be covered which fall within the scope of the claims.