Radio navigational aid with separate standard frequency signal

Abstract

radio guidance signals comprising a carrier having modulation patterns defining a predetermined guidance path plane for a craft to be guided are transmitted, preferably in a scanning beam, from a ground station. A separate standard frequency signal is also transmitted from the ground station which has a predetermined difference in frequency from the guidance signal carrier frequency, the standard frequency being employed to control a local oscillator at the receiver.

Description

This application is a contination-in-part of the prior U.S. patent application Ser. No. 54,510 filed July 13, 1970 now U.S. Pat. No. 3,715,757 issued Feb. 6, 1973, for a RADIO GUIDANCE SYSTEM WITH SEPARATE TRANSMISSION OF A STANDARD FREQUENCY SIGNAL TO ENHANCE THE DISCRIMINATION OF THE RECEIVER.

This invention relates to guidance systems, and to guidance systems which are particularly useful for aircraft and which may be operated at microwave frequencies. The systems of the present invention are particularly useful for the guidance of aircraft in descent towards an airport for landing. Accordingly, the invention is described in terms of this function. However, it will be understood that the invention is also very useful in systems providing other guidance functions for aircraft, land craft, or water craft.

Present aircraft instrument landing systems, sometimes referred to as ILS, are operable at very high frequencies (VHF) frequencies. These present systems represent a large existing investment in airport ground installations and also in aircraft equipment installations. However, at the VHF frequencies, there are many airport sites where the conventional ILS simply cannot be operated successfully because of the problems of reflections of signals which make the transmitter signals ambiguous and unusable to the aircraft. Furthermore, the VHF systems are very expensive, which further limits the number of installations, and prevents installation at many airports where they are needed.

Accordingly, it is one object of the present invention to provide an improved low-cost microwave aircraft guidance system which can be effectively used at otherwise difficult airports sites.

In carrying out the present invention, guidance function signals may be transmitted at microwave frequencies and converted in the receiver by a direct subtraction conversion to obtain signals at intermediate frequencies. In order to accomplish this purpose, it is necessary to provide for a local oscillator in the receiver which is extremely accurate, and therefore likely to be expensive.

Accordingly, it is another object of the present invention to provide a guidance system having improved means for automatically matching the receiver oscillator frequency to the transmitter frequencies.

In carrying out systems of the above type, it has been found to be very advantageous to radiate the radio navigation signals in the form of scanning beams, and preferably in the form of switched scanning beams. A system of that kind forms a portion of the subject matter described and claimed in a copending patent application Ser. No. 104,668 filed Jan. 7, 1971 by Donald J. Toman and Lloyd J. Perper now Pat. No. 3,774,214 issued Nov. 20, 1973 for a SCANNING BEAM GUIDANCE METHOD AND SYSTEM, and assigned to the same asignee as the present application. It has been found to be particularly advantageous to employ separate standard frequency signals for controlling a receiver local oscillator when the navigation signals are being transmitted in a scanning beam.

Accordingly, it is another object of the present invention to provide a radio guidance method involving the transmission of navigation signals by a scanning beam, and including the transmission of a standard frequency signal providing substantially uniform radiation over the entire sector covered by the scanning beam.

Other objects and advantages of the invention will be apparent from the following description and the accompanying drawings.

In carrying out the invention there is provided an improved guidance method for a craft to be guided including the steps of transmitting radio guidance signals from a ground station comprising a carrier having modulation patterns defining a predetermined guidance path plane for a craft to be guided, transmitting a separate standard frequency signal from the ground station having a predetermined difference in frequency from the guidance signal carrier frequency, and employing the standard frequency to control the local oscillator within a receiver in the craft to be guided to improve the discrimination function of the receiver in discriminating the desired guidance signals from other signals.

In the accompanying drawings:

FIG. 1 is a schematic circuit diagram showing the arrangement of a receiver system for installation in an aircraft for carrying out the present invention.

FIG. 2 is a chart illustrating a particular allocation of frequencies which may be employed in carrying out the principles of the invention.

FIG. 3 is a more detailed schematic circuit diagram illustrating preferred features of the microwave receiver and converter which may be employed in the system of FIG. 1.

FIG. 4 is a schematic circuit diagram showing details of the discriminator and threshold circuit which form parts of the receiver of FIG. 3.

And FIG. 5 is a schematic circuit diagram of a ground station transmitter to be used in carrying out one form of the invention.

In the specification, reference is made to airport ground stations. However, since the invention is usable for guidance functions for aircraft, for purposes other than for landing, and since the invention is usable also for water craft and land craft, it will be understood that the ground stations are not necssarily located at airports.

Referring more particularly to FIG. 1, a preferred system in accordance with the present invention is illustrated to include as ILS localizer receiver 10 and an ILS glide slope receiver 14. Both of these receivers feed signals to an ILS indicator 12. The localizer receiver 10 is operable in the normal VHF localizer frequencies in the band from 108 to 112 MHz. Such signals may be received from a localizer antenna 16 through a switch schematically shown at 20. The glide slope receiver is operable to receiver signals at the usual glide slope frequency band from 328.6 to 335.4 MHz. Such signals may be received from a glide slope antenna 18 through a switch element schematically shown at 22 and operable together with the switch element 20. With the exception of the switch elements 20 and 22, the components of the system thus far described may be conventional ILS components.

In accordance with the present invention, a microwave receiver and converter 24 is provided and connected to receive microwave ILS signals through a microwave antenna 26. The microwave signals include signals which are ocillator of the DC level of the combination of signals that are received.

The meaning of the term "dispersed sequence," as it relates to the above description, is more fully defined as follows: The idea is that the sequence involves the selection of successive antenna elements which are in relatively dispersed spacial positions. Thus, the selection of different elements involves a positional skipping around in the selection of sucessive elements, until all of the elements have been selected to accomplish a complete scan. Examples of suitable dispersed sequences, assigning the numbers 1 through 8 respectively to the antenna elements 122-136, are given as follows:

Sequence I: 1 3 5 7 2 4 6 8

Sequence II: 1 4 7 2 5 8 3 6

Sequence III: 1 5 2 6 3 7 4 8

It was stated above that in a dispersed sequence, the radiation does not usually occur in two successive switched pulses from two adjacent antenna elements. Sequence III above accomplishes this by having only one intervening switched pulse between the two switched pulses from two adjacent antenna elements. Thus, in the 1, 5, 2 portion of the sequence, there is only one pulse from antenna element 5 which intervenes between the successive pulse from the adjacent antenna elements 1 and 2. In some instances, it is preferred to disperse the sequence even more than this, assuring that there are two or more intervening pulses between the successive pulse from two adjacent elements. Sequences I and II accomplish this. It is often preferred also to provide a sequence in which there is a skip of more than one beam position for successive pulses. Sequence II accomplishes this. In going from 1 to 4, the positions 2 and 3 are both skipped, and this pattern of skipping at least two positions is continued for each successive pulse period. This arrangement provides for the advantage that no two successively received pulses will ever be of substantially equal amplitude. That condition can occur with the other sequences listed above if, for instance, the receiver is located at the center line of beam 2. Then a sequence which starts off as 1,3,5...., for instance, will produce equal amplitude signals at 1 and 3 (assuming the 1 beam is radiated at an energy peak equal to the 3 beam). This is avoided with sequence II. Accordingly, sequence II represents what appears to be an optimal sequence for an eight element array. It has been found that it is not always absolutely necessary to provide that no two successive switched pulses are radiated from two adjacent antenna elements in order to achieve a satisfactory dispersed sequence. For instance, a completely random sequence may be employed, and repeated, and if the random sequence is long enough to give a truly random distribution, the occurance occurrence of two immediately successive switched pulses from two adjacent antenna elements is infrequent enough so as to avoid serious error. Another exception is illustrated in the following sequence:

Sequence IV: 1 3 5 7 8 6 4 2.

In this sequence, 7 and 8 are together, and 2 and 1 are together when the sequence is next repeated. In the system as presently disclosed in connection with FIG. 5, this exception does not seriosly seriously impair the operation because the beams radiated from antennas 122 and 136 (in the 1 and 8 positions) are the so-called "cover" beams covering wide angles and having lower peak energies than the beams from the other positions.

While this invention has been shown and described in connection with particular preferred embodiments, various alterations and modifications will occur to those skilled in the art. Accordingly, the following claims are intended to define the valid scoppe of this invention over the prior art, and to cover all changes and modifications falling within the true spirit and valid scope of this invention.

Claims

1. An improved guidance method for a craft to be guided including

transmitting radio guidance signals from a ground station comprising a carrier having modulation patters patterns defining a predetermined guidance path plane for a craft to be guided,

transmitting a separate standard frequency signal from the ground station having a predetermined difference in frequency from the guidance signal carrier frequency,

and employing the standard frequency to control a local oscillator within a receiver in the craft to be guided to improve the discrimination function of the receiver is discriminating the desired guidance signals from other signals.

2. A method as claimed in claim 1 wherein the standard frequency is employed in the receiver to generate control signals to control the local receiver oscillator to thereby lock the frequency of the oscillator in a fixed frequency relationship to the standard frequency.

3. A method as claimed in claim 2 wherein the frequency lock condition is detected and used to gate guidance signals through the receiver to provide guidance information to the operator of the craft to be guided.

4. A radio guidance method comprising

transmitting a pattern of radio carrier signals in a scanning beam with different portions of the scanning beam being positioned on opposite sides of a guidance path plane and carrying modulation to define the guidance path plane for a craft to be guided,

transmitting a standard radio frequency signal providing substantially uniform radiation over the entire sector covered by the scanning beam while the scanning beam is being transmitted,

the standard frequency signal having a predetermined difference in frequency from the radio carrier frequency of the scanning beam for controlling a receiver local oscillator in the craft to be guided for enhacing enhancing the signal discrimination function.

5. A method as claimed in claim 4 wherein

the radio carrier signals are transmitted in a switched scanning beam with different individual and distinct switched portions of the scanning beam carrying unique modulation to define the guidance path plane.

6. A method as claimed in claim 5 wherein

the individual switched portions of the scanning beam are modulated by controlling the duration of the pulse of carrier frequency energy radiated for each portion of the scanning beam.

7. A method as claimed in claim 4 wherein

the standard frequency signal is transmitted on a higher duty cycle than any one of the different portions of the scanning beam.

8. A method as claimed in claim 7 wherein

the standard radio frequency signal is transmitted as a continuous wave.

9. A method for transmitting radio guidance signals comprising

transmitting a pattern of signals in a switched scanning beam with individual and distinct switched portions of the scanning beam being positioned on opposite sides of a guidance path plane by switching the radio frequency energy in a dispersed position sequence from one switched portion to another so that the signal from each individual switched portion is intermittent in nature,

and modulating the scanning beam with two different modulation signals by providing different ratios of the respective amounts of modulation by said two different modulation signals for said distinct switched portions of the scanning beam with the variation of the modulation ratio being substantially symmetrical about the guidance path plane so that said modulation signals are respectively dominant on opposite sides of the plane and the value of the ratio is one at the plane to thereby define the guidance path plane.

10. A method as claimed in claim 9 wherein

the dispersed position sequence is one in which successive switched portions of the scanning beam are always radiated from positions mutually displaced by at least two scanning beam switched portion positions.

11. A method as claimed in claim 9 wherein

the dispersed position sequence is one in which successive switched portions of the scanning beam are always radiated from positions mutually displaced by at least three scanning beam switched portion positions.

12. A radio guidance signal transmitter comprising means for transmitting a pattern of radio carrier signals in a scanning beam with different portions of the scanning beam being positioned on opposite sides of a guidance path plane,

means for modulating the portions of the scanning beam positioned on opposite sides of the guidance path plane to thereby define the guidance path plane in terms of guidance signals for a craft to be guided,

and means for transmitting a standard radio frequency 10 signal in a substantially uniform pattern of radiation over the entire sector of space covered by the scanning beam,

the standard frequency signal having a predetermined difference in frequency from the radio carrier frequency of the scanning beam for controlling the local oscillator of a receiver.

13. A transmitter as claimed in claim 12 wherein

said means for transmitting a pattern of radio carrier signals comprises a plurality of antenna elements and switching means for switching radio carrier signals from one antenna element to another to thereby provide individual and distinct switched portions of the scanning beam to form a switched scanning beam,

and means for individually modulating the different switched portions of the scanning beam to define the guidance path plane.

14. A transmitter as claimed in claim 13 wherein said modulating means is a pulse modulation means operable in synchronism with the switching of the different portions of the scanning beam to said different antenna elements and operable to control the duration of each period of transmission of energy from each of said antenna elements to thereby provide modulation information in each pulse of carrier energy transmitted from each antenna element.

15. A transmitter as claimed in claim 13 wherein

said means for transmitting said standard frequency signal is operable upon a higher duty cycle than any one of the different switched portions of the scanning beam.

16. A transmitter as claimed in claim 15 wherein

said means for transmitting said standard frequency signal comprises an antenna element separate from said different antenna elements comprising said means for transmitting said switched portions of said scanning beam,

said means for transmitting said standard frequency signal being operable on a continuous basis to transmit a continuous wave signal.

17. A method for transmitting radio guidance signals comprising

transmitting a pattern of signals in a switched scanning beam with individual and distinct switched portions of the scanning beam being arranged in different positions on opposite sides of a guidance path plane by switching the radio frequency energy in a dispersed position sequence from one switched portion to another so that the signal from each individual switched portion comprises intermittent pulses of carrier frequency energy,

and modulating the individual switched portions of the scanning beam to define the guidance path plane by controlling the duration of each pulse of carrier frequency energy radiated for each portion of the scanning beam.

18. A method as claimed in claim 17 wherein

the scanning beam is modulated with two different modulation signals by providing different ratios of the respective amounts of modulation by said two different modulation signals for said distinct switched portions of the scanning beam with the variation of the modulation ratio being such that said modulation signals are respectively dominant on opposite sides of the guidance path plane and the value of the ratio is one at the plane to thereby define the guidance path plane.

19. A method as claimed in claim 17 including

transmitting a standard radio frequency signal providing substantially uniform radiation over the entire sector covered by the scanning beam while the scanning beam is being transmitted,

the standard frequency signal having a predetermined difference in frequency from the radio carrier frequency of the scanning beam for controlling a receiver oscillator in the craft to be guided for enhancing the signal discrimination function.