WO1995006339A1

WO1995006339A1 - High reliability fail-soft microwave landing system having high continuity of service and integrity

Info

Publication number: WO1995006339A1
Application number: PCT/US1994/009380
Authority: WO
Inventors: Richard F. Frazita
Original assignee: Scan Tek, Inc.
Priority date: 1993-08-20
Filing date: 1994-08-18
Publication date: 1995-03-02

Abstract

A high reliability fail-soft MLS having high continuity of service and high integrity. The MLS is designed so that signal point failures of the system are almost eliminated so as to enhance the reliability of the system. The MLS architecture provides inherent redundancy so that duplication of system equipment with critical switchover circuitry is not required. The MLS distributes all system operations within each of many transmitter modules (100) with the exception of the signal carrying frequency source, so that critical system failure will not occur if one component becomes non-operational.

Description

HIGH RELIABILITY FAIL-SOFT MICROWAVE LANDING SYSTEM HAVING HIGH CONTINUITY OF SERVICE AND INTEGRITY

BACKGROUND OF THE INVENTION

Field of the Invention The present invention relates to electronic scanning antennas, particularly to microwave landing systems which use multiple antennas, and more particularly to fail-soft microwave landing systems which use multiple antennas to radiate electronic signals over a wide area.

Description of the Prior Art

As shown in Fig. 1, a microwave landing system (MLS) disseminates microwave signals (generally designated by reference numeral 2) in order to guide aircraft 4 during final approach and landing. MLS uses ground-originated radio transmissions 2 that provide precise positional information to airborne aircraft 4 referenced to a single runway 6. Aircraft can approach an airport and make initial alignment with the chosen landing runway 6 by using navigation systems whose primary purpose is to provide en route guidance. However, in order to complete a landing, the aircraft should be precisely aligned with the runway heading and have a predetermined rate of descent. When there is adequate visibility, a pilot may be able to complete a landing without the use of instrument guidance. But, since today's larger and faster flying aircraft begin the landing phase of flight as far as 10 miles from the runway and at altitudes well above 2000 feet, total visual guidance is often impossible and landing systems, such as MLS, are required so that the aircraft 4 can have a proper landing approach in a wide range of weather conditions.

The technical standards specified by the Federal Aviation Administration (FAA) and the International Civil Aviation Organization (ICOA) define MLS signals and the

9UBSTTTU7ΕSHEET(RULE26) operational reliability requirements for the various per¬ formance levels of MLS (CAT I, CAT II, CAT III). The most reliable level defined by the standards is known as Category III (CAT III) . CAT III includes automatic landings in periods of very limited-to-no visibility. To ensure safety, the probability that an airplane will be involved in an accident during such a landing must be less than one in 10⁶.

There are two main factors relating to the design of the MLS ground equipment which are derived from this global safety requirement. The first, called Integrity, correlates to the situation wherein several system components fail in some sequence and alter the navigation landing signal that is received by the aircraft 4. In this situation, the signal that is received by the aircraft may be deceptive. However, the landing signal may not be obviously incorrect to the pilot. As a result, the pilot may attempt to land the plane in accordance with the incorrect information. The second component of the safety factor, called Continuity of Service (COS) , corresponds to the reliability of individual components and the likelihood of a total system shutdown as a result of a failure of one component which would leave the pilot attempting to land the aircraft without the benefit of a landing system signal. Of the two above-identified safety factors, Continuity of Service is the most limiting factor in the overall reliability and availability of the MLS because a single component failure can cause a signal outage.

Figure 2 shows a conventional microwave landing system 10 which generally includes primary and secondary signal generators 12 and 14 and phased array 16 having antenna elements 18. The conventional MLS also includes non-scanning, broad beam out of coverage indication (OCI) antenna elements 20, a data antenna element 22 and primary and secondary beam steering and control units 24 and 26. Specifically, the MLS includes primary signal generator 12 having a frequency source 28, DPSK signal modulation generator 30 and amplifier 32, and a secondary signal generator 14 also having a frequency source 28,

DPSK signal modulator generator 30 and amplifier 32. The signal generators are coupled to an RF switch 34 having an associated control circuit. The signal generators 12,14 produce carrier signals which are ultimately emitted by the antenna elements. Separate and independent circuits and sensors, herein called the monitor, monitors all aspects of the system behavior. As long as the primary signal generator 12 appears to be providing a proper signal, the primary signal generator 12 will be coupled by switch 34 to the remainder of the MLS circuit. However, when the monitor detects improper signals being produced by the primary signal generator 12, the RF switch 34 disengages its connection to the primary signal generator 12 and connects the secondary signal generator 14 to the remainder of the MLS circuit.

Coupled to the RF switch 34 is a second RF switch 36 which alternately connects the signal generators 12,14 to either: a third switching element 38 for alternately provid-ing the carrier signal produced by the signal generators 12,14 to at least one of the OCI antenna elements 20; the phased array 16 for providing the carrier signal produced by the signal generators 12,14 to antenna elements 18; or the data antenna 22. Coupled to the phased array 16 through logic switch circuit 40 are primary and secondary station control (SC) and beam steering units (BSU) 24,26. The logic switch circuit 40 operates in a fashion similar to that of RF switch 34. Specifically, the independent monitor monitors the signals provided by the primary control and BSU 24 and in the event that the primary control and BSU 24 is not properly operating, the monitor causes switch 40 to

9UBSTTTIΠΕSHEET{RULE26) couple the secondary control and BSU 26 to the phased array 16 in order to control the signal emitted by the radiating elements 18. While the disclosed MLS shown in Figure 2 is fail-soft to a very high degree, the conventional MLS of Figure 2 utilizes many multiple switches (34, 36, 38, 40) to create redundancy of the majority of system components by including secondary (standby) components so that the MLS can switch over to the secondary equipment in the event that the primary equipment fails. The switches are an integral portion of the design which provides the fail-soft feature to the conventional system. As used hereinafter, fail-soft refers to a system wherein despite the failure of a variety of components, the system will continue proper operation.

The fundamental limitation of the conventional fail- soft MLS is that if one of the switches (34, 36, 38) of the system becomes inoperable, the MLS could completely shut down and have no transmitting capability in spite of the redundancy. In addition, the actual redundancy of the system components causes the conventional fail-soft MLS to require more maintenance and is costly to produce. Moreover, the conventional fail-soft MLS is limited in that it cannot provide out-of-coverage indication (OCI) signals and a data signal, without employing a single thread multiple port RF switch such as RF switch 36. This design is not preferred because the MLS is subject to several single point failures which could shut-down the transmitting capability of the system.

The FAA and International Civil Aviation

Organization require that microwave landing systems emit signals 2 having a specified signal format, as shown in Figure 1. The MLS signal format requires multiple antenna functions to provide the signals over a wide area. The format also provides for signal transmission outside the normal flight pattern coverage, e.g., out-of-coverage indication signals (OCI) . The OCI signals alert the pilot of an approaching aircraft that the airplane is not in a proper position to receive landing signals. This is an important feature because it may occur that landing information reflects off objects such as buildings and hills proximately located to the airport. This deflection of signals could deceive the pilot into believing he is on a proper landing approach pattern. Inherent growth capabilities of the MLS, such as 360° continuous data coverage, also require additional antennas in many practical applications. Since MLS requires multiple antennas, an antenna switch (RF switch 36) is conventionally used to connect a transmitter sequentially in time to each antenna port. Additional switches are required for the redundant components such as the secondary signal generator 14 and secondary SC and BSU 26. These backup devices have traditionally been included to provide signal continuity in the event of a component failure and therefore improve the COS of the system. However, as previously mentioned, due to the inherent configuration of the MLS, the RF and logic switching components are a significant limiting factor in improving the signal continuity for high reliability CAT III MLS applications.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide an MLS employing distributed forms of: RF amplification, RF DPSK modulation, sector signal generation and antenna switching, beam steering, time division multiplexing, inter-station and intra-station synchronization and control electronics in order to enhance the fail-soft characteristics and overall reliability of the system. It is another object of the present invention to provide MLS architecture wherein all RF functions including power amplification, phase shifting, power control, antenna switching and DPSK signal modulation generation are distributed.

It is another object of the present invention to provide MLS architecture which has an inherent redundancy so that backup components which are switched into operation at critical times by a single thread, multiple port switch is not required.

It is another object of the present invention to provide MLS architecture wherein all control functions including beam steering, signal format timing, intra- station and inter-station synchronization, DPSK signal modulation generation and signal switching are distributed.

It is another object of the present invention to increase both the integrity of the system and continuity of service of the MLS architecture so that the probability of an accident occurring during the landing of an aircraft can be significantly reduced.

It is a further object of the present invention to provide a fail-soft MLS system which overcomes the inherent disadvantages of known fail-soft MLS systems.

In accordance with one form of the present invention, a microwave landing system (MLS) includes an active phased array having a first and second plurality of antenna elements. The first plurality of antenna elements corres-ponds to an array of primary antenna elements while the second plurality of antenna elements corresponds to an array of auxiliary antenna elements. Coupled to the active phased array is a means for supplying RF signals to the active phased array which includes a primary frequency source, a secondary frequency source and a switch for alternately coupling the frequency sources to the active phased array.

The active phased array includes a power divider coupled to the switch, and a plurality of transmitter modules having an input port and two output ports. Each of the plurality of transmitter modules is coupled to one of the plurality of output ports of the power divider. Coupled to a first output of the plurality of transmitter modules is the first plurality of antenna elements and coupled to the second output of each of the plurality of transmitter modules are power combiners each having a plurality of inputs and one output. Coupled to the output of the power combiners is the second plurality of antenna elements.

Each transmitter module includes an amplifier coupled to the output of the power divider means. Coupled to the amplifier is a switching means with a phase shifter disposed therebetween. The switching means has one input and two outputs in order to selectively couple the phase shifter to one of the two output ports of the transmitter module. The transmitter module also includes a second amplifier coupled to one of the outputs of the switching means.

The MLS of the present invention is controlled by a plurality of individual control elements contained within a control circuit located within each transmitter module. Each control circuit is coupled to a memory means and includes the individual control functions of that particular transmitter module of the MLS including beam steering, RF power level control for on/off and phased array antenna amplitude distribution taper, antenna selection, signal format timing, DPSK signal generation

SUBSrmjTISHEET RULE26) and synchronization of operation. In this way, main control circuits are not required to control the operation of the MLS. This eliminates the need for backup equipment with switching capability and it substantially eliminates the probability of single point failures within the system.

These and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a pictorial representation of a typical microwave landing system (MLS) for reference background.

Figure 2 is a functional block diagram of a conventional fail-soft microwave landing system (MLS) .

Figure 3A is a simplified block diagram of one version of the high reliability fail-soft microwave landing system (MLS) of the present invention.

Figure 3B is a block diagram of a high reliability fail-soft microwave landing system (MLS) of the present invention having high continuity of service and high integrity of the system.

Figure 3C is a block diagram of another embodiment of a fail-soft MLS architecture wherein fewer transmitter modules are used in conjunction with a more conventional phased array, such as may already exist. Figure 3D is a block diagram of a modification of the embodiment of the present invention shown in Figure 3C.

Figure 4 is a functional diagram of one of the plurality of individual transmitter modules constructed in accordance with the present invention.

Figure 5A is a functional diagram showing each of the plurality of individual transmitter modules used in the present invention connected to a common fail-soft bus in order to coordinate synchronization of operation of each transmitter module.

Figure 5B is a block diagram of the synchronization control circuit used in the present invention contained within each transmitter module.

Figure 6 is a block diagram of a possible ASIC

(Application Specific Integrated Circuit) control circuit used in the present invention contained with each transmitter module and coupled to an E²PROM.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to Figure 3A, a simplified form of the high reliability fail-soft microwave landing system (MLS) of the present invention is shown. The simplified form of the invention includes a primary frequency source 19 and a secondary frequency source 21. Both the primary and secondary frequency sources 19,21 are coupled to an RF switch 41. The RF switch 41 is coupled to an active phased array 80 which includes a primary antenna array 120 and a secondary antenna array 140. The primary antenna array 120 is envisioned to be used for providing the MLS scanning beam signals to approaching aircraft, and the secondary antenna array 140 is used to generate the MLS OCI signals and data signals to aircraft around the airport.

Referring now to Figure 3B of the drawings, one form of the high reliability fail-soft microwave landing system (MLS) having high continuity of service (COS) and high integrity of the present invention is shown. The high reliability fail-soft MLS is designed in order to eliminate many single port, multiple throw RF and logic control switches used in the conventional MLS so that the reliability of the system will be enhanced. The design of the high reliability fail-soft MLS inherently provides redundancy of the microwave and logic control functions so that standby backup equipment, which would become operative in the event a primary component fails, is not required. The invention distributes all system operations (except for the signal carrier frequency source) within each redundant transmitter module so that even if one system operation fails within one or more transmitter modules, the MLS will continue to operate. Therefore, the design of the present invention makes the MLS inherently fail-soft.

The high reliability fail-soft MLS includes redundant signal generators illustrated by first signal frequency source 19 and second signal frequency source 21, which is independent of the first signal frequency source 19. The first and second signal frequency sources 19,21 preferably provide an RF signal of approximately 5 to 10 milliwatts. The first signal frequency source 19 provides its signal to switching means 41 via an electrical conduit 31 such as a cable, waveguide or the like. Likewise, the second signal frequency source 21 provides its signal to switching means 41 via electrical conduit 50. Switching means 41 is a single-pole, double-throw (SPDT) RF switch. The SPDT switch 41 has input ports 55 and 60 with a single output port 65. The position of the switch is controlled by control logic circuit 66 which may be internal or external to the switch 41. The control logic circuit 66 is coupled to the first and second signal frequency sources 19, 21 as well as the SPDT switch 41. The control logic circuit 66 monitors the signals provided by the first signal frequency source 19 and second signal frequency source 21 in order to determine whether a failure of a respective signal frequency source has occurred. The single output of the switch 41 provides an RF signal from either the first signal frequency source 19 or the second signal frequency source 21 via electrical conduit 70 to active phased array 80. During normal operation of the MLS, the first signal frequency source 19 is selected and provides an RF signal to the switch 41 via conduit 31. The SPDT switch 41 would be in a first position such that input 55 is electrically coupled to output 65 in order to connect conduits 31 and 70. However, upon detection of a failure in the first signal frequency source 19, the control logic circuit 66 would cause the SPDT switch 41 to move to a second position where output 65 is electrically coupled to input 60 so that conduit 50 is connected to conduit 70. In this manner, the second signal frequency source 21 is selected to provide the necessary RF signal to the active phased array 80.

The active phased array 80 includes an N-way power divider 90. Electrical conduit 70 connects the output port 65 of the SPDT switch 41 to the input port of the N- way power divider 90 of the active phased array 80. The N-way power divider 90 receives the input signal provided on conduit 70, and equally divides and distributes the signal N ways to various power divider outputs 91. Each of the power divider outputs 91 are coupled to one of the

SUBSflTUTE SHEET(RULE 26) plurality of transmitter modules 100. Therefore, each of the plurality of individual transmitter modules 100 is provided with an equal portion of the RF signal provided to the N-way power divider 90.

In the preferred embodiment, the number of transmitter modules depends on the size of the array. For a 1 degree beamwidth scanning to +60° from the normal direction, approximately 120 transmitter modules 100 are preferably included in the active phased array 80. Each of the plurality of transmitter modules 100 has one input port 105 and two output ports. A first output port 110 of each of the plurality of transmitter modules 100 is coupled to a corresponding element of the primary antenna array 120 to power the primary antenna elements for providing a scanning MLS beam. A second output port 115 of each of the plurality of transmitter modules 100 is coupled to a corresponding input of one of a plurality of M-way power combiners 130 for powering antenna elements of secondary antenna array 140. Preferably, at least twenty transmitter modules 100 are coupled to each M-way power combiner 130 for output to one of the antennas of secondary antenna array 140. The twenty transmitter modules 100 that are connected to each power combiner 130 may be randomly selected. However for simplicity, Figure 3B shows transmitter modules 1-20 being connected to the first power combiner 130 and the next twenty transmitter modules (numbers 21-40) being connected to the second power combiner 130 and so on so that there are six power combiners 130, each being coupled to twenty transmitter modules 100. In the preferred form of the invention, each transmitter module 100 is designed so that by properly phase shifting and switching the signal within each transmitter module 100, the output signal can be transmitted by one of the antenna elements of either primary antenna array 120 or secondary antenna array 140. Preferably, in order to provide signals to the secondary antenna array 140, a plurality of smaller power combiners 130 are coupled to a portion of the total number of transmitter modules 100 in lieu of one large power combiner with an output for each antenna element of the secondary array 140 coupled to each of the plurality of transmitter modules 100. However, in the alternative, one large power combiner may also be utilized.

As shown in Figure 4, each of the plurality of transmitter modules 100 includes a preamplifier 145 coupled to a corresponding power divider output 91. The output of the preamplifier 145 is coupled to a phase shifting means 150. The phase shifter 150 is utilized to alter the phase of the signal that is received by the transmitter module 100 in order to control the scanning of the radiated beam and the combining of power in the power combiners 130. Coupled to the output of the phase shifter 150 is switch 170. The switch 170 is a SPDT RF switch having a single input port 171, and two output ports 172, 173 respectively corresponding to outputs 115 and 110 of transmitter module 100. Each transmitter module 100 preferably includes a power amplifier 160 coupled between switch output 172 and transmitter module output 115. The position of switching means 170 is controlled by an ASIC (Application Specific Integrated Circuit) control circuit 180 contained within each transmitter module 100. Coupled to the ASIC is an E²PROM (Electrically Erasable Programmable Read Only Memory) 185 for storing control data to be used by the control circuit 180. The control circuit 180 also coordinates the operations performed by the transmitter modules 100 including DPSK signal modulation generation and OCI signal generation in order to provide a proper output signal format 2 in accordance with FAA and ICAO specifications (Figure 1) . The control circuit 180 can also be coupled to amplifiers 145,160 and phase shifter 150 in order to control their operation.

When the switching means 170 couples input 171 with output 173, the RF signal originally generated by one of the signal frequency sources 19 or 21, is provided from phase shifting means 150 and from output 110 of the transmitter module 100 to primary antenna array 120. Amplifier 145 functions to amplify the signal provided by power divider 90 to approximately 0.2 watts, so that when all of the signals generated by the plurality of transmitter modules 100 are provided to primary antenna array 120, the power density of the scanning beam required by FAA and ICAO is achieved. When the switching means 170 couples input 171 with output 172, the RF signal is provided from phase shifter 150 to amplifier 160 and from output 115 to a corresponding one of the plurality of power combiners 130 wherein the RF signal is combined with other RF signals provided by other transmitter modules 100. Thereafter, the combined signal is provided by the corresponding power combiner 130 to a corresponding antenna of secondary antenna array 140. Power combiner 130 is a standard combiner network, which recombines the signals provided at the input according to a predetermined coupling arrangement and provides the combined signals at the output of the array. The signal produced by each power combiner 130 is ultimately provided to different inputs of the secondary antenna array 140. Amplifier 160 functions to amplify the signal provided by power divider 90 to approximately 1.0 watt, so that when all of the signals are combined by power combiners 130, the power densities of the data and OCI signals required by FAA and ICAO are achieved. The active phased array 80 preferably utilizes Gallium Arsinide monolithic microwave integrated circuit (MMIC) phase shifters, switches and amplifiers in each transmitting module 100 instead of the usual PIN diode phase shifters and switches. The Gallium Arsinide MMIC amplifiers amplify the RF signal, which is preferably a low level signal at C-band at a power of 0.1 watt for output 110 and 1.0 watt for output 115. The array taper for low side lobes is developed by electronic control of the MMIC amplifier or by an electronically controlled attenuator (not shown) connected in series with each phase shifter of the active phased array 80. An alternative embodiment of providing the array distribution amplitude taper is by utilizing a series of couplers or fixed attenuators (not shown) in the RF distribution circuit between the outputs of the MMIC devices and the elements of the primary antenna array 120.

An MLS generally has two modes of operation. In one mode, a narrow TO-FRO fan beam is scanned through the primary antenna array 120 in order to provide aircraft with scanning beam azimuth and elevation information. In the second mode of operation, the secondary antenna array 140 radiates signals which provide essential and auxiliary data (landing information) and including out- of-coverage indication (OCI) signals to aircraft within the range of the MLS. Primary antenna array 120 includes a plurality of antenna elements which, when supplied with wave energy signals, provide a narrow beam of radiated energy. The beam is electronically scanned TO and FRO by varying the phase of the input signals provided to the antenna elements. This is accomplished by altering the phase shift of the signal at outputs 110 and provided to each transmitter module 100 by employing the control circuit 180 within each transmitter module 100 to individually manipulate the phase shifters 150. During the second mode of operation, the switch 170 is set to couple input 171 to output 115 and the phase shifter 150 within each transmitter module 100 is manipulated so that the phase of the signals provided to the power combiner 130 will result in maximum power being transferred to the particular antenna which is connected to that power combiner 130.

The operation of the internal components, such as the phase shifter 150, amplifiers 145, 160 and switch 170 of each transmitter module 100 is regulated by control circuit 180. Preferably, the control circuit is an Application Specific Integrated Circuit (ASIC) and in order to provide fail-soft operation of the MLS, the control circuit 180 is located within each individual transmitter 100 module for redundancy. In the preferred embodiment, as shown in Fig. 6, the control circuit 180 includes an arithmetic and logic unit (ALU) 300 which, though a data bus 305, is coupled to a beam steering logic circuit 310 for manipulation of the phase shifting means 150. The ALU 300 is also coupled to an RF power level control circuit 315 for manipulation of the amplifiers 145 and 160, a function monitor control circuit 320, sequencer circuit 325 for sequencing of the output signal provided by the antenna elements to FAA and ICAO multiplex transmission cycle specifications (Figure 1) , DPSK signal generator circuit 330 for providing DPSK signal generation for the output signal provided by the data antenna of the secondary array, clock and synchronization generation circuit 335 which provides synchronization control of the corresponding transmitter module functions, auto calibration/auto stabilization circuit 340 and a remote monitoring subsystem (RMS) interface circuit 345. Each of the individual circuits are known. However, the structure and concept of distributing and synchronizing each of the circuits within each transmitter modules 100 of an active phased array 80 of an MLS are unique.

It is an important aspect of the present invention to provide synchronization of operation between each of the control circuits contained within each transmitter module 100 of each active phased array 90 and to establish the time synchronization among the several active arrays that comprise a complete MLS (azimuth, elevation, back azimuth) . The synchronization of the control circuits 180 within each transmitter module 100 permits the MLS to operate as though there was one master control circuit even though control signals are independently generated within each transmitter module 100. As a result, the operation of the MLS does not rely on a single external control circuit. The synchronization of the control circuits 180 also ensures that each transmitter module 100 that is currently operating will simultaneously provide output signals to generate the required MLS signal format. The purpose of the synchronization of the control circuits 180 is to allow all transmitter modules 100 to operate in unison in each active array 80 and to sequence the transmission from each active array according to the FAA and ICAO time division multiplex cycle (Figure 1) . This permits each transmitter module function to be individually distributed or located within each transmitter module which provides fail-soft characteristics to the MLS. Therefore, a control circuit 180 within transmitter module 100 may falter causing the corresponding transmitter module 100 to shut down or loose synchronization with the plurality of modules. However, the non-operation or asynchronous operation of a limited number of transmitter modules 100 will not cause a total system failure. As previously stated, this fail-soft design is essential in achieving an extremely high Continuity Of Service (COS) and Integrity.

Referring now to Figure 5A, each of the plurality of transmitter modules 100 is shown connected to a common synchronization bus 200 via electrical conduits 205,206 and isolation resistors 240 (Figure 5B) and to frequency sources 19,21 via RF frequency source control bus 200A. Figure 5B illustrates a block diagram of the clock and synchronization circuit 335 contained within each transmitter module 100. The synchronization circuit 335 includes a counter 210 and a synchronization bus interface circuit 215. The counter, such as Part No.74161 manufactured by National Semiconductor of Santa Clara, California, includes a clock input 211, a reset input 212, hold input 213, and an overflow (OF) output 214. The OF output 214 of the counter 210 is preferably coupled to the hold input 213 and a 10MHz clock signal is provided to the counter through the clock input 211.

The synchronization bus interface circuit 215 includes a receiver 220 having its output coupled to the reset input 212 of the counter 210. In the preferred embodiment, the receiver 220 is a comparator, such as Part No. LT485 manufactured by Linear Technology Corp. of Milpitas, California, having a first input 221 coupled to the synchronization line 200 and a second input 222 coupled to a voltage source (not shown) so that the receiver 220 can compare the voltage on the synchronization bus with the value set by the voltage source. Preferably, the second input 222 of the receiver 220 is coupled such that a first resistor 225 is interposed between the second input 222 and the voltage source. In addition, a second resistor 230 is interposed between the second input 222 of the receiver and ground. Thus, resistors 225 and 230 form a voltage divider circuit. In a preferred embodiment, the voltage source is set and the resistors 225,230 are selected such that the reference voltage at the second input of the receiver 220 is preferably approximately 0.75 volts.

The synchronization bus interface circuit 215 also includes a driver 235, such as Part No. LT485 manufactured by Linear Technology Corp., for amplifying the output signal provided on the OF output of counter 210. The input of driver 235 is coupled to the OF output 214 of the counter 210 while the output of the driver 235 is coupled to the synchronization bus 200. Preferably, interposed between the output of the driver 235, receiver 220 and the synchronization bus 200 are current limiting resistors 240.

As shown in Figure 5A and as previously described, each of the plurality of transmitter modules 100 is coupled to the synchronization bus 200. The synchronization bus 200 functions as a voltage divider wherein if N synchronization bus interface outputs are connected via corresponding conduits 206 to the synchronization bus and if the OF output 214 of counter 210 transitions to a HIGH output, the driver 235 will increase the voltage on the synchronization bus by 1/N volts. In the preferred embodiment, the comparison voltage of each receiver 220 is preset so that when the voltage on the synchronization bus 200 reaches 75% of the maximum voltage, which is approximately 0.75 volts (i.e., when 75% of the OF outputs 214 of counter 210 are set HIGH) , the output of all receivers 220 will transition to a logic HIGH state causing the counter 210 in each transmitter module to reset and start counting from zero. As a result, each counter within the transmitter modules 100 will be in synchronization and begin counting together. The voltage level of the synchronization bus 200 is monitored by the synchronization circuit 335 of each control circuit 180 in each transmitter module 100 in order to ensure synchronization of operation of all transmitter modules 100.

The operation of the synchronization circuit 335 for ensuring that all control circuits 180 and corresponding transmitter modules 100 operate in unison preferably occurs as follows. When the MLS is first turned on, there may occur the situation wherein counter 210 in each control circuit 180 of the transmitter module 100 will be counting and in a different and random state from counters in other transmitter modules 100. Therefore, initially each counter 210 within the synchronization circuit 335 is reset. Additionally, the comparison voltage at input 222 of receiver 220 is set to 0.75 volts. Thereafter, the 10MHz clock input is generated within frequency source 19 or 21 and is provided to the clock input 211 of each counter 210. The maximum count of each counter 210 will be uniformly set so that after a predetermined time period, the OF output 214 of each counter 210 will transition to a logic HIGH state. Since the HOLD input 213 of the counter 210 is coupled to the OF output 214, when the OF output 214 provides a logic HIGH output, the counter 210 will stop counting. The HIGH OF output is provided to driver 235 which adds 1/N volts to the synchronization bus 200. This operation occurs for each synchronization circuit 335 within each control circuit 180. The receiver input 221 monitors the voltage on the synchronization bus 200 to determine if the synchronization bus voltage is .75 volts or greater.

When at least 75 percent of the synchronization circuits 335 within each control circuit 180 provides 1/N volts to the synchronization bus, the receiver 220 will detect this voltage and provide a logic HIGH output to the RESET input of the counter. When this occurs, the counter 210 is cleared and the 10MHz clock input which is provided to the counter 210 causes the counter to begin a new count.

Coupled to the RESET input of the counter 210 of the clock and synchronization circuit 335 is an electrical conduit 305 of the control circuit 180. As a result, when a new counter cycle is begun by providing a logic HIGH to the RESET input of the counter, a master synchronization signal is provided on the conduit 305 so that all control functions regulated by the control circuit 180 can also be reset to start a new cycle. The predetermined time period of counter operation is preferably selected to coincide with the operation of the beam steering logic circuit 310, the signal sequencing circuit 325 and the DPSK signal generator circuit 330 of each control circuit 180. Therefore, when the beam steering logic circuit 310, signal sequencing circuit 325 and DPSK signal generator circuit 330 complete their cycle of operation, they are simultaneously reset by the clock and synchronization circuit 335 within each control circuit 180 and, because all synchronization circuits 335 are coupled via synchronization bus 200, all control circuits 180 are simultaneously reset in order to insure uniform transmitter module operation.

As shown in Figure 5A is the data bus 200A. This bus controls the on/off action of the frequency source

19,21 from the plurality of ASIC control circuits 180 in a fail- soft manner similar to the above description.

The reference voltage of the receiver 220 at input 222 of the synchronization circuit 335 is chosen such that if a component of the synchronization circuit 335 fails wherein either: a) the synchronization circuit driver 235 is shorted to ground or the voltage source; b) the synchronization circuit 335 provides a 1/N voltage to the synchronization bus 200 at an erroneous time; or c) the synchronization circuit 335 never provides a 1/N voltage to the synchronization bus 200, a failure of up to at least 25% of the synchronization circuits 335 will have no effect on the operation of the MLS. For example, if the output of the driver 235 of one synchronization circuit 335 is shorted, the driver output will constantly add 1/N volts to the synchronization bus 200. However, since each synchronization circuit 335 is designed to require at least 75% of all synchronization circuits within the transmitter modules to be in the same state before a proper system transition is recognized and a system reset is performed, the shorting of one driver 235 will have almost no effect on the operation of the MLS. The combination of resistor isolation along with the voltage division of the synchronization bus 200 allows the MLS to exhibit the desired fail-soft properties even during a failure of the clock and synchronization circuit 335 itself.

As a result of linking the synchronization circuits 335 in the above-described manner, all functions performed by the control circuit 180 and the operation of all transmitter modules 100 will be in a similar corresponding state (in synchronization) as if there was one master controlling source providing a master control signal to influence the operation of all transmitter modules 100. To synchronize the various other active arrays within the complete MLS time division multiplex cycle, it is common practice to establish the azimuth array as the timing master. This azimuth (AZ) equipment establishes and communicates an inter-station synchronization signal to the other equipments (EL and the back azimuth are not shown in Figure 1 for simplicity) . In the preferred embodiment of this invention, the inter-station synchronization signal is created in the azimuth active array using distributed means in a manner previously described for synchronizing the control circuits of each active array. This precise timing information is encoded in a signal and sent by cabling from the azimuth equipment to the other MLS equipment where it is distributed to each sync and clock generator circuit in every trans-mitter module via a fail-safe bus. These circuits account for the cabling time delay using existing methods and they establish the overall timing of the transmissions from that MLS equipment in accordance with the same principles of generating distributed synchronization described above.

In order to be able to transmit coherent RF signals from the active array 80 all transmitter modules 100 must be fed from the common RF frequency source 19 or 21. Since the MLS is a time division multiplexed (TDM) system, the RF source 19 or 21 must be respectively turned on and off during periods of transmission and non- transmission. To distribute this control within each transmitter module 100 in a fail-soft manner, in the preferred embodiment all RF power level control circuits 315 drive a bus 200A similar to the synchronization bus 200 used for the synchronization and clock generator control circuit 335. A receiver in the RF power level control circuit 315 may be implemented to determine when at least 75% of the RF power level control circuits 315 are in the same state. When this is determined, the RF power level control circuits 315 within each control circuit 180 are simultaneously set to a different corresponding power level. Using this technique, the MLS of the present invention has the capability to manipulate the operation of a non-distributed component of the system such as frequency source 19 and 21 from within each transmitter module 100 in a way which has the same properties as the fail-soft synchronization and clock generator circuit 335. In an alternative embodiment and as part of the transmitter module function, it may be desirable to extinguish the RF signal from an improperly functioning transmitter module 100. In the case of the aforementioned distributed control where there is not a single central control circuit for the functions of the transmitter modules 100, it is difficult to determine when a particular trans-mitter module has failed. In order to determine when a transmitter module 100 has failed, each control circuit 180 with the transmitter module 100 contains a built-in-test (BIT) circuit 313 which compares signals generated by a corresponding transmitter module 100 to the signals generated by the remainder of the transmitter modules 100 called an ensemble signal. The ensemble signal is generated using a similar technique as that used for the synchronization circuit. Specifically, as each transmitter module 100 transmits a signal corresponding to a particular control function of the control circuit 180 (e.g. DPSK signal generator circuit 330, beam steering logic circuit 310) , an encoded message is provided on a fail-soft "bus" similar to that described above. Since all the transmitter modules 100 are operating in unison, all transmitter modules generate the encoded message simultaneously. A receiver in each individual control function circuit of the control circuit 180 transitions to a particular logic state if at least 75% of the modules are in one logic state. Logic circuits in each control circuit 180 of the transmitter module 100 compares the receiver output, which represents the ensemble signal provided by the other transmitter modules with the state of that particular transmitter module 100. If the transmitter module 100 is not following the signals generated by other transmitter modules for a predetermined period of time, the BIT logic turns off the improperly functioning transmitter module and communicates this status to the remote monitor subsystem circuit 345.

As previously described, conventional MLS have relied upon duplication of system operation, including the BSU (beam steering unit) , wherein the timing and sequencing commands for scanning the beam are sent over a single control line to the plurality of phase shifters in order to provide a fail-soft system. However, this approach is not preferred for a CAT III MLS because the outputs of the BSU's are provided to the phase shifters on a single control bus via a single SPDT logic switching arrangement which creates the opportunity for single point failures. Since the control bus must have a fairly large number of conduits for rapid updates of the plurality of phase shifters, single point failures can occur at a rate which significantly limits the COS of the MLS.

In order to overcome this problem of single point failure within the BSU function, the present invention is designed so that the beam steering control logic 310 and each control function of the control circuit 180 is distributed across the array of transmitter modules 100. With regard to the beam steering units, this results in a fail-soft system because a number of individual phase shifters 150 or beam steering logic circuits 310 can fail without significantly affecting system operation. This technique substantially eliminates single point failures by completely eliminating the need for a single control line or bus which carries all beam steering commands to control the phase shifters. In the preferred embodiment, beam steering consists of presetting each phase shifter 150 within the transmitter module 100 to a known initial state. The preferred embodiment of the phase shifter is a digital device with four or five bits (smallest bit being 22.5° or 11.25°) . Then, the beam steering is conducted at predetermined points in the MLS format cycle, such as when the synchronization and clock generator circuit 335 provides a synchronization signal on conduit 305. Thereafter, each phase shifter 150 is stepped at an optimum scanning rate for the duration of the scan. In addition, a technique known as phase cycling control is used to average out any phase shifter errors. This technique modifies the initial state of all phase shifters 150 at the start of each scan in order to assure proper scanning for the MLS signal. It is preferred that in order to achieve the benefits of distributed beam steering, the necessary logic controls that can interpret the beam steering control signal provided by the beam steering logic circuit 310 should be incorporated within each transmitter module 100.

The steering control used in the beam steering logic circuit 310 can be based on any one of a number of sequence controllers in use today such as is available from Altera Corp. of San Jose, California. A major difference between the beam steering control of the present invention and that of the prior art is that the beam steering data of the present invention is manipulated so as to allow each transmitter module 100 to contain only phase shifting data relevant to that particular phase shifter. This significantly reduces the amount of information required to be stored within each transmitter module (preferably less than 1000 bytes) . If this technique were not employed, each transmitter module 100 would have to contain information for all phase shifters 150, which would make the technique significantly more expensive and physically large.

In the preferred embodiment, the plurality of transmitter modules perform the dual functions of generating the scanning beam signal and the sector signals. The scanning of an MLS beam occurs as follows. First, the beam steering control logic circuit 310 of each transmitter module 100 provides the starting phase states and phase-step durations to its corresponding phase shifter 150. The duration counter of the phase shifter controls the time duration of each individual phase step provided by the phase shifter 150. It should be noted that the starting phase state and phase-step durations are unique to each phase shifter 150 and are predetermined at the time of the design. The durations are represented as a count of the BSU clock pulses and stored in an E²PR0M contained within each phase shifter. At the start-of-scan, the duration counter begins decrementing from an initial count toward zero. When the duration counter reaches zero, the phase shifter is incremented (or decremented) , the RF phase shifter 150 is incremented (or decremented) by the smallest step value (22.5° or 11.25°) and the next signal duration is loaded into the phase shifter 150 by the beam steering logic circuit 310. This process is repeated until the end-of-scan. At the start of the next scan, the phase cycle counter is incremented, and a new starting phase is provided to the phase shifter 150 by the beam steering logic circuit 310 and so forth. The phase cycle counter is utilized to continually modify the initial phase state of all phase shifters 150 in the same way, thereby having the effect of averaging any phase shifter error over one phase cycling period without changing the initial pointing direction of the scanned MLS beam. Since the phase cycle counter has a repetition rate which is many

SUBSTfTUπSHEET(RULE26) scans in duration, a separate synchronization signal may be used to synchronize this portion of the beam steering control. This synchronization signal ensures that the phase cycle counters of all phase shifters 150 are in the same state.

A sector signal (DPSK or OCI) is generated in a similar way but with some differences. During this part of the MLS time division multiplex cycle, no scanning beam operation takes place. The control circuits 180 select the group of modules associated with a specific power combiner 130 which provides the RF output signal to a specific antenna element of secondary array 140. All other transmitter modules associated with the other power combiners are turned off by their respective control circuits. The phase state of each phase shifter in the active group is provided by the beam steering logic. The phase states are not stepped as in scanning beam generation; instead they remain static during the generation of the sector signal. The main purpose is to have in-phase addition of each transmitter module's signal at the power combiner output port. Phase cycling is preferred on each subsequent sector signal generation to average out phase shifter inaccuracies. During the activation of one group, it is preferred to set the phase states of each transmitter module in the non-activated groups such as to provide a cancellation of any leakage signal at the output port of their respective power combiners. If the active group provides the DPSK signal, the 180 degree bits of their respective phase shifters are toggled simultaneously according to a pattern stored in each transmitter module's memory and provided to each phase shifter by the control circuit. This process transmits essential and non-essential MLS data and avoids the separate DPSK modulator typically associated with the frequency source of the prior art. The duration of each sector signal is determined by the control circuits in a manner similar to but simpler than for beam scanning. The control circuits sequence the groups according to the time division multiplex cycle based on information stored in each module's memory.

In some cases an MLS equipment design already exists and is only capable of meeting the Integrity and COS requirements for Cat I, which is much less stringent than for Cat III. To upgrade such a design to Cat III capability, another embodiment of the invention can be used and is shown in Figure 3C using the same principles as those described herein for the preferred embodiment. Here, the scanning beam antenna 120 uses a plurality of phase shifters (typically PIN diode devices) 500 and an RF power divider 90 as may already exist in conventional systems except the beam steering (BSU) is accomplished by the same distributed beam steering means (i.e., control circuit 180 and memory 185 in each transmitter module) as described herein for the preferred embodiment. A second but smaller transmitting array 140 comprises a second RF power divider 90A and a plurality of active transmitter modules 100A, similar to those in the preferred embodiment but with one input and one output. Each transmitter module contains RF amplifiers 145 and 160 and phase shifting means 150. The MLS of this embodiment is controlled by the plurality of individual control elements contained within a control circuit 180 located within each transmitter module in much the same way as the preferred embodiment except the beam steering circuit 310 and function for the scanning beam antenna are not included, and RF switch 170 is omitted so that the output signal from the amplifier 160 is passed to the output 115 of the transmitter module 100A (Figure 4) . Each control circuit 180 is coupled to a memory means 185 and includes the individual control functions of that particular transmitter module 100A including RF power control on/off sector, antenna selection, signal format timing, DPSK signal generation and synchronization of operation. The RF outputs from these active transmitter modules 100A are combined in an RF power combiner 130A. Proper phase relationships created by the phase shifting capability of each active transmitter module 100A causes the RF energy to be selectively switched to any of the output ports of the power combiner 130A which are connected to the plurality of antenna elements comprising the OCI, data and scanning beam antennas. More specifically, a plurality of outputs of power combiner 130A are each coupled to a respective antenna element of OCI and data array 140 while one output of power combiner 130A is coupled to the input of power divider 90 which distributes power to the individual elements of the scanning beam antenna 120, as shown in Figure 3C. Power combiner 130A is a standard combiner network such as a Blass, Butler or Lopez network.

As shown in Figure 3D, a further refinement of the architecture shown in Figure 3C uses transmitter modules 100B similar in structure to the module 100 of the preferred embodiment but without the beam steering circuit 310 and function, each module 100B having two switched outputs. One switched output 110 from each transmitter module is connected to a dedicated power combiner 130B with one output connected to the input of the power divider 90 for the scanning beam antenna 120. The other switched output 115 from each transmitter module is combined in the manner already described with respect to the embodiment shown in Figure 3C. This arrangement provides potentially greater isolation between the MLS scanning beam and any of the OCI and data antennas because of the use of RF switch 170 in each transmitter module 100B. It can be seen from the above description that the high reliability fail-soft MLS of the present invention provides inherent redundancy of system operations and components including the synchronization of the system operations and components so that backup, standby components which switch operation at critical times are not required.

Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.

Claims

WHAT IS CLAIMED IS;

1. A microwave landing antenna system for radiating RF signals in selected regions of space in a desired radiation pattern comprising: a) an active phased array circuit having a first plurality of antenna elements and a second plurality of antenna elements; and b) a first frequency generating means coupled to the active phased array circuit, the first frequency generating means supplying a plurality of first RF signals to the active phased array circuit.

2. A microwave landing antenna system for radiating RF signals in selected regions of space in a desired radiation pattern as defined by claim 1 further comprising: first switching means being interposed between and electrically coupling the active phased array circuit to the first frequency generating means, the first switching means selectively coupling the first frequency generating means to the active phased array circuit.

3. A microwave landing antenna system for radiating RF signals in selected regions of space in a desired radiation pattern as defined by claim 2 further comprising: a second frequency generating means for supplying a plurality of second RF signals, the second frequency generating means being coupled to the first switching means wherein the first switching means selectively couples the second frequency generating means to the active phased array circuit.

4. A microwave landing antenna system for radiating RF signals in selected regions of space in a desired radiation pattern as defined by claim 1 wherein the active phased array circuit comprises: a) power divider means coupled to the first switching means, the power divider means having an input port and a plurality of output ports, b) a plurality of transmitter modules, each of the plurality of transmitter modules being coupled to one of the plurality of output ports of the power divider means, each of the plurality of transmitter modules having at least first and second output ports, each of the plurality of transmitter modules alternately providing a first signal through the first output port and a second signal through the second output port, and c) a plurality of power combiner means, each second output port of the plurality of transmitter modules being coupled to one of the plurality of power combiner means, each of the plurality of power combiner means having an output port, each of the plurality of power combiner means combining the second signals provided by corresponding transmitter modules which are coupled to the power combiner means, wherein each of the first plurality of antenna elements are respectively coupled to the first output port of a corresponding one of the plurality transmitter modules, and wherein each of the second plurality of antenna elements are respectively coupled to a corresponding output port of one of the plurality of power combiner means.

5. A microwave landing antenna system for radiating RF signals in selected regions of space in a desired radiation pattern as defined by claim 4 wherein the second plurality of antenna elements provides an out of coverage indication (OCI) signal.

6. A microwave landing antenna system for radiating RF signals in selected regions of space in a desired radiation pattern as defined by claim 4 wherein each of the plurality of transmitter modules comprises: a) first amplification means having an input port and an output port, the input port of the first amplification means being coupled the power divider means; b) phase shifting means having an input port and an output port, the input port of the phase shifting means being electrically coupled to the output port of the first amplification means; c) second switching means electrically coupled to the phase shifting means, the second switching means having an input port and at least two output ports, each of the at least two output ports of the second switching means corresponding to the first and second output ports of a respective one of the transmitter modules, the second switching means selectively coupling the phase shifting means to one of the first and second outputs of the transmitter module; and d) second amplification means coupled to one of the at least two outputs of the second switching means.

7. A microwave landing antenna system for radiating RF signals in selected regions of space in a desired radiation pattern as defined by claim 6 wherein each of the plurality of transmitter modules further comprises a control circuit coupled to and providing control signals to the first and second amplification means, the phase shifting means and the second switching means, the control signals regulating the operation of the first and second amplification means, the phase shifting means and the second switching means.

SUBSmUTESHEET(RULE26)

8. A microwave landing antenna system for radiating RF signals in selected regions of space in a desired radiation pattern as defined by claim 7 wherein each of the plurality of transmitter modules further comprises a memory means coupled to the control circuit, the memory means storing control functions and operations to be used by the control circuit.

9. A microwave landing antenna system for radiating RF signals in selected regions of space in a desired radiation pattern as defined by claim 7 wherein each control circuit within the plurality of transmitter modules is coupled to a bus for synchronization of operation of each of the plurality of transmitter modules.

10. A microwave landing antenna system for radiating RF signals in selected regions of space and in a desired radiation pattern as defined by claim 7 wherein each control circuit within the plurality of transmitter modules further includes a synchronization control circuit comprising: a) counter means having a reset input port and a clock input port, the counter means also having a hold input port and an output port, the output port of the counter means being coupled to the hold input port; and b) bus interface means having at least two input ports and at least two output ports, a first input port and a second output port of the bus interface means being coupled to the bus, a second input port and a first output of the bus interface means being coupled to the counter means.

11. A microwave landing antenna system for radiating RF signals in selected regions of space in a desired radiation pattern as defined by claim 10 wherein the first output port of the bus interface means is coupled to the reset input port of the counter means and the output port of the counter means is coupled to the second input port of the bus interface means.

12. A microwave landing antenna system for radiating RF signals in selected regions of space in a desired radiation pattern as defined by claim 10 wherein the bus interface means comprises: a) comparator means having a first input port and a second input port, the first input port being coupled to a voltage source in order to set a predetermined threshold value, the second input port being coupled to the bus, the first input port of the comparator means corresponding to the first input of the bus interface means, the comparator means having an output port corresponding to the first output of the bus interface means; and b) amplification means having a first input port and a first output port, the first input port of the amplification means being coupled to the output port of the counter means, the first input port of the amplification means corresponding to the second input port of the bus interface means, the first output port of the amplification means being coupled to the bus, the first output port of the amplification means corresponding to the second output port of the bus interface means.

13. A microwave landing antenna system for radiating RF signals in selected regions of space in a desired radiation pattern as defined by claim 12 wherein first resistor means is interposed between the voltage source and the first input port of the comparator means, second resistor means is interposed between the first input port of the comparator means and a ground potential reference, and wherein third resistor means is interposed between the first output port of the amplification means and the bus.

14. A fully distributed transmitter antenna system for radiating RF signals in selected regions of space in a desired radiation pattern comprising: a) a first frequency generating means for supplying a plurality of first RF signals, and b) an active phased array circuit having; power divider means electrically coupled to the first frequency generating means, the power divider means having an input port and plurality of output ports, a plurality of transmitter modules, each of the plurality of transmitter modules being coupled to one of the plurality of output ports of the power divider means, each of the plurality of transmitter modules having at least first and second output ports, each of the plurality of transmitter modules providing a first signal through the first output port and a second signal through the second output port, a first plurality of antenna elements, each of the first plurality of antenna elements being respectively coupled to the first output port of a corresponding one of the plurality transmitter modules, a plurality of power combiner means, each second output port of the plurality of transmitter modules being coupled to one of the plurality of power combiner means, each of the plurality of power combiner means having an output port, each of the plurality of power combiner means combining the second signals provided by corresponding transmitter modules which are coupled to the combiner means, and a second plurality of antenna elements respectively coupled to each output port of the plurality of power combiner means.

15. A fully distributed transmitter antenna system for radiating RF signals in selected regions of space in a desired radiation pattern as defined by Claim 12 further comprising: first switching means interposed between the active phased array circuit and the first frequency generating means, the first switching means selectively coupling the first frequency generating means to the active phased array circuit.

16. A fully distributed transmitter system for radiating RF signals in selected regions of space in a desired radiation pattern as defined by claim 13 further comprising: a second frequency generating means for supplying a plurality of second RF signals, the second frequency generating means being coupled to the first switching means wherein the first switching means selectively couples the second frequency generating means to the active phased array circuit.