Note: Descriptions are shown in the official language in which they were submitted.
DOCKET R4387.01 1 3 2 5 2 6 1
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1 AUTOMATIC, REAL-TIME FAULT MONITOR VERIFYING
2 NETWORK IN A MICR0WAVE LANDING SYSTEM
3 BACKGROUND OF THE INVENTION
4 1. Field of the Invention
The present field of the invention relates
6 to a process and network in which an executive monitor
7 is connected within a microwave landing system ("MLS")
8 to evaluate whether or not an internally-generated out-
9 of-tolerance signal activates an alarm system. If the
alarm system is activated, then the proper fault moni-
11 toring function of the executive monitor is verified.
12 2. Description of the Prior Art
13 An instrument landing system ("ILS") has
14 served as the prior art approach and landing aid for
aircraft for many years. The ILS, however, has a number
16 of basic limitations, such as being site crittcal and
17 expensive to install, being sensitive to extraneous
18 reflections, having a limited number of channels,
19 lacking the flexibilTty required for aircraft
operations, and producing erroneous informat7on in rough
21 terrain and mountainous reglons. As a result of these
22 limitations, an MLS has been proposed as a standard ILS
23 replacement for world-wtde implementation since it can
24 reduce or eliminate these basic limitations.
The MLS consists of various antenna stations
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1 adJacent to a runway which transmit wave energy informa-
2 tion to approaching aircraft enabling said atrcraft to
3 calculate the following data to safely land on an air-
4 port runway: azimuth from an AZ station, elevation from
an EL station, range from a precision distance measuring
6 equipment (DME/P) station, and back azimuth from a BAZ
7 station. The AZ station provides an aircraft with head-
8 ing or approach guidance to runways or helo pads at an
9 airport. The EL station provides for a wide selection
of glide slope angles needed by a pilot to land his
11 plane safely on a runway. The DME/P station provides
12 for continuous range information needed by a pilot to
13 ascertain the distance between his aircraft and the
14 airport runway on which he is landing. The BAZ station
is similar to the AZ station and is intended to supply
16 guidance to a pilot for missed approaches to and depar-
17 tures from an airport.
18 More specifically, the AZ station includes an
19 antenna wh7ch generates a narrow, vertical, fan-shaped
beam which sweeps to and fro across the area to be
21 covered by the AZ station. Before the start of a scan a
22 test pulse is transmitted, then the "to" scan starts.
23 At the end of the scan, there Is a pause before the
24 "fro" scan starts. A second test pulse marks the end of
25 the scanning cycle. The atrcraft receives a "to" pulse
26 and a "fro" pulse. The time difference between pulses
27 is then measured by the aircraft and gives the angular
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1 location of the aircraft relative to the AZ station.
2 The EL station also includes an antenna which generates
3 a narrow horizontal fan-shaped beam which sweeps up and
4 down through the area to be covered at the airport. The
time difference between receipt of the up and down
6 pulses is used by the aircraft to determine the eleva-
7 tion angle of the aircraft relative to the EL station
8 and thus its displacement from the glide path angle
9 selected by the pilot to land his aircraft on a runway.
The elevation scan cycle requires much less time than
11 the azimuth scan cycle. The elevation scan cycle is
12 normally repeated 39 times per second as compared with
13 13 times per second for the azimuth cycle. The BAZ
14 station includes an antenna which generates a narrow,
fan-shaped, vertical beam which sweeps to and fro
16 horizontally through the area to be covered at the
17 airport. The same angular measurement principle used
18 for determlning the approach AZ angle Ts used for
19 determining the BAZ angle. The DME/P station includes
an antenna which transmits wave energy travelling at a
21 known rate. By calculatlng the tTme the wave energy
22 travels from the antenna to the aircraft and knowtng the
23 rate at which the wave energy travels, the distance or
24 range between alrcraft and the airport station can be
calculated. The above information Is calculated by the
26 approaching alrcraft as a dTrect result of the
27 meantngful Information transmitted by the antenna
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l stations included within a given MLS at an airport.
2 The MLS is capable of operating on any one of 200
3 channels in the microwave frequency band. The present
4 microwave frequencies in use are between 5043 and 5090.7
megahertz. The AZ, BAZ and EL stations all transmit on
6 the microwave frequency. The DME/P station transmits a
7 paired frequency in L-Band. The MLS signal format has
8 the potential to transmit signals from the above-
9 mentioned various stations in any desired order to
approaching aircraft. A preamble or data word is trans-
11 mitted by each station to approaching aircraft prior to
12 the main wave energy being transmitted in order to
13 inform the approaching atrcraft of which function (AZ,
14 EL, BAZ, DME/P) will be transmitted next. As soon as
the aircraft decodes the message it waits for the wave
16 energy to be received in order to perform the desired
17 calculation. Then, the aircraft awaits the next
18 preamble to ascertain the identity of the next trans-
19 mitted function. As can readily been seen, the MLS has
numerous advantages over the ILS.
21 All MLS installations transmlt the following
22 basic data to approaching aircraft: facility (landing
23 runway) identlflcation; azimuth threshold distance,
24 coverage and off-set (distance from AZ antenna to fixed
spot on center line of runway); beam wTdths (AZ, EL);
26 DME/P dlstance, off-set and channel (distance between
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1 station and runwayj; and elevation height, off-set and
2 distance from threshold. Most of this information is
3 needed by the equipment aboard the approaching aircraft
4 to make the necessary computations for an approach to
the airport. Any malfunction in the MLS equipment will
6 cause the approaching aircraft to make faulty calcula-
7 tions and rely upon erroneous data. For this reason, it
8 is absolutely essential to continuously maintain the MLS
9 system and to verify that the MLS system stations are
transmitting accurate information.
11 The MLS was the first system designed to utilize
12 a maintenance program. The advantages of such a mainte-
13 nance program include a reduction in the time spent in
14 travel maintaining the system and a reduction in the
maintenance and record-keeping for the system, which in
16 turn allows more effectlve use of a smaller number of
17 maTntenance personnel operating from a smaller number of
18 maintenance bases. Overall, such a maintenance program
19 is economtcal, reliable, and efficient.
Each MLS station is supported by an executTve
21 monitor and a maTntenance field monltor, both of which
22 are tools implementing the maintenance program. The
23 executive monltor samples the information being trans-
24 mltted by each antenna station to approaching aircraft.
In other words, the executive monitor evaluates the same
26 7nformation that the antenna is transm7tting to
27 approaching aircraft to ensure that the tnformation
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l being sent to the aircraft is reliable. For example,
2 the executive monitor checks the accuracy of the angle
3 code throughout ~he antenna coverage and thus can detect
4 when a given sample of the angle code is beyond a pre-
determined limit. Examples of such transmitted param-
6 eters which are checked by the executive monitor to
7 ascertain whether or not they are out-of-tolerance are
8 (a) scanning beam mean angle error, (b) function
9 preamble, (c) effective radiated po~er (whether it be
for the function preamble, the EL and BAZ scanning
11 beams, clearance pulses, or an out of coverage
12 indicator), (d) timing error in signal format, (e)
13 synchronization error in time division multiplexing, (f)
14 digital phase shift keying ("DPSK") data transmission,
(9) interstation synchronization, (h) array integrity
16 parameters (such as dynamtc sidelodes, channel failures,
17 frequency channels, etc.), and (i) clearance angle. If
18 any of these transmitted parameters a-J are out-of-
19 tolerance, then the executive monitor automatically
initiates an alarm, the stat10n is shut down, and the
21 approach7ng aircraft does not recelve information from
22 that station. The AZ, EL, BAZ, and DME/P antenna sta-
23 tions each contaln an executTve monitor. If the EL or
24 BAZ station is shut down, the other statlons are still
operable and transmit information to the approachlng
26 airçraft. If the AZ station is shut down, however, all
27 stations are disabled and do not transm7t tnformation to
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l approaching aircraft. Since the executive monitor
2 initiates an alarm and shuts down a station or the
3 system when an out-of-tolerance parameter Ts detected
4 over a predetermined period of time it is necessary to
verify that ~he fault monitoring function of the execu-
6 tive monitor is operating properly and will provide such
7 an alarm when such an out-of-tolerance parameter is
8 detected over that predetermined period of time. Other-
9 wise an approaching aircraft may be erroneously relying
on MLS supPlied information which should have generated
11 an alarm and shut down the station or the MLS without
12 transmitting information to approaching aircraft.
13 SUMMARY OF THE INVENTION
14 An obJect of the invention is automatic real-time
verification of the fault monitoring operation of an
16 executive monitor so that an alarm is generated and
17 declared valid when an internally generated out-of-
18 tolerance or erroneous signal is detected by the
19 monitor.
A further object of the inventton is to provide
21 for the veriflcation of the fault monitoring operatlon
22 of an executive monitor in which a station control board
23 is connected on line with the executive monitor and
24 provides a signal thereto representative of erroneous or
out-of-tolerance data in order to initiate an alarm in
26 the executive monitor. Another obJect of the invention
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1 is to store the actual data obtained by the executive
2 monitor until proper fault monitoring verification
3 occurs.
4 Another obJect of the invention is to employ a
filter counting means to record a history of the out-of-
6 tolerance data received by the executive monitor so that
7 the filter counting means can be preconditioned during
8 the verification process to receive one additional out-
9 oftolerance sample to thereby generate an alarm within
the executive monitor.
11 Another object of the invention is to employ a
12 switch which will permit an internally generated out-of-
13 tolerance sample to be analyzed instead of the sampled
14 information generated by the antenna system.
For a better understanding of the present inven-
16 tion together with other and further obJects, reference
17 is made to the following description in conjunction with
18 the accompanying drawTngs.
19 BRIEF DESCRli'TlON OF THE DRAWINGS
Figure 1 is a schematic block diagram of a cir-
21 cuit for verlfying proper operatTon of the fault moni-
22 toring function of the executlve monTtor accordlng to
23 the Inventlon.
24 Flgure 2 is a flow-chart Illustratlng a sequence
of operatTons for verifylng proper operatlon of the
26 fault monitorlng functlon of the executive monitor.
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1 DETAILED DESCRIPTION OF THE INVENTION
2 Figure l illustrates the EL station 14 components
3 of an MLS system whlch are used to process one of many
4 MLS parameters transmitted to approaching aircraft and
to verify the proper functioning of a fault monitor
6 system within the MLS should an out-of-tolerance param-
7 eter be detected by the MLS system.
8 Since many parameters are verified in an MLS (see
9 parameters a-J listed above), the one chosen to best
illustrate the invention and the one shown in Figure 1
11 is the scanning beam mean angle error parameter.
12 Although the scanning beam mean angle error parameter is
13 illustrated, it is to be understood that any of the
14 other parameters verified by the landing system could
have been chosen to illustrate the invention, and in
16 this regard, the invention is not to be limited to the
17 illustrated parameter. The scanning beam mean angle
18 error from an EL sta~ion provides the difference between
19 the actual height of the approaching aircraft in angular
degrees and the height the approaching aircraft already
21 calculated. The EL station transmtts scanning beam mean
22 angle error Information to approaching a7rcraft by an
23 array antenna means 1. The antenna means 1 also samples
24 this transmisslon Internally wlthin the MLS system by
providing an output of hlgh frequency RF voltage signals
26 to an RF receiver detector means 2. The detector means
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l 2 in conventional fashion converts the voltage 5 i gnals
2 into a series of D-C video pulses and outputs them to an
3 executive monitor means 3 over a manifold video llne.
4 The executive monitor means 3 contains a local CPU 4,
local memory means 5, and a memory means 6 shared with a
6 control means 9, called a station control board.
7 The shared memory means 6 contains two filter
8 counters 7. Like the executive monitor means 3, the
9 control means 9 also contains a local CPU 11 and memory
means 10. CPUs 4 and 11 are thus able to read from or
11 write into the shared memory means 6. CPUs 4 and 11
12 perform comparisons and calculations. The two filter
13 counters 7 are used in the executive monitor means 3
14 (together wlth an angle decoder not shown) to detect the
time of occurrence of the rising and falling edges of
16 the video pulses and thus an out-of-tolerance scanning
17 beam angle. If a small scanning beam angle error is
18 detected, one of the two filter counters 7 lncrements.
19 If a large scanning beam angle error is detected the
other filter counter 7 increments. If the calculated
21 scanning beam angle is withln a first predetermlned
22 limit, then the filter counters 7 decrement, but never
23 below a value of zero. The approachlng aircraft detects
24 the same timing tnformatlon as the executlve monltor
means 3. The pulse edges are defined as the 3dB points
26 of each pulse and are detected when the video pulse
27 amplitude equals a first reference voltage corresponding
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1 to 0.707 times a second reference voltage which is
2 stored in the executive monitor s local memory means 5
3 in digital form. The comparison of the video pulses
4 with said second reference voltage eliminates the
counting of spurious false alarm signals and thus
the erroneous calculation of guidance information.
7 As another safeguard at least two rising and two
8 falling edges (two pulses) are analyzed in real-time by
9 the CPU 4 in the executive monitor to determine the
scanning beam angle error. Thirty-one samples of these
11 paired video pulses must be fed to the two filter
12 counters over a 0.8 second time period in order for the
13 executive monitor to accurately and reliably determine
14 if an out of tolerance scanning beam mean angle error
exists. Such determination is made by comparing the
16 number of incremented counts in each filter counter with
17 a second predetermined limit for each stored in
18 memory. If the number of incremented counts exceeds the
19 second predetermined limit for either filter counter
then the filter counter output indicates that mean
21 erroneous guidance - the scanning beam mean angle error
22 is beyond acceptable limits`- exists for the prescribed
23 0.8 second time period; an alarm 8 is therefore
24 generated in the executive monitor and noted in the
memory means 6 shared by the monitor means 3 and the
26 control means (station control board) 9 when such
27 erroneous guidance is detected. Under these conditions
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1 an alarm signal is then fed by the station control board
2 9, to the transmitter of the array antenna means 1
3 shutting the system down. Of course, if erroneous
4 guidance is not indicated by eTther filter counter, then
the alarm system will not be activated and the system
6 will not be shut down. The approaching aircraft
7 realizes that a fault condition has been detected in a
8 particular station when it does not receive any
9 information from that station.
The real-time, automatic operation of the present
11 fault monitoring verificatton scheme will now be
12 explained with reference to Figure 2. Figure 2 shows
13 through logic the manner in which the proper operation
14 of the fault monitoring function performed by the
executive monitor is verified. Using the scanning beam
16 mean angle error parameter by way of example only,
17 verification is accomplished by having the contents of
18 the two filter counters 7 saved in the shared memory
19 means 6 in a location separate and apart from the filter
counters 7. Between antenna scans the two filter
21 counters 7 are then provided with a predetermined count
22 that will cause the generation of a real-time, automatic
23 alarm If one more sample outsTde scanning beam mean
24 angle error limits is recelved. Thls precondltioning of
the two fllter counters is accomplished in the memory
26 means 6 shared by the control means (station control
27 board) 9 and executive monitor means 3 shown in
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1 Figure l. During the next antenna scan, the control
2 means CPU ll sends a signal over the MV SEL-Gl and G2
3 line activating switch 12 which replaces the parametric
4 video output received by the executive monitor means 3
with one out-of-tolerance pulse pair sample generated
6 internally. The internally genera~ed out-of-tolerance
7 pulse pair sample is sent by CPU 11 over the MV pulse
8 generator line to the executive monitor which then
9 increments the filter counters 7. The internally
generated pulse pair represents a sample outside filter
11 counter limits and when summed with the predetermined
12 count in filter counters 7 should activate alarm 8.
13 If an alarm is generated as a result of the
14 introduction of the internally-generated out-of-
tolerance pulse pair to the filter counters 7, then the
16 test is determined to be valid, i.e. the fault monitor-
17 ing function of the executive monitor is working
18 properly. Under these conditions, the previously saved
19 contents of both filter counters are restored in order
for normal operation of the system to be resumed on the
21 next antenna scan. If elther filter counter does not
22 generate an alarm, then an executive monitor failure Is
23 declared in the statlon control board 9, and the actual,
24 real-time alarm ts activated. Under these circumstances
the station or system is automatically shut down by the
26 station control board 9 and In need of repaTr. This
27 7nforms maintenance personnel that the system is not
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l properly monitoring fault conditions when scanning beam
2 mean angle errors Tn excess of a second predetermined
3 limit are detected. The above real-time verification
4 process is automatic software controlled and conducted
every 15 minutes in an MLS. The MLS system also can
6 request the performance of a verification test from
7 human sources~
8 Specific structural details for the shared memory
9 means 6 and local memories 5 and 10 would be apparent to
one skilled in the art. Although the alarm 8 and
11 filter counters 7 are shown in Figure 1 to be contained
12 in the shared memory means 6 those functions could
13 be performed by separate hardware or in local memories
14 5 and 10 with assistance from processors 4 and 11. An
ari~hmetic logic unit could be used in combination
16 with the memories and processors to perform calcula-
17 ttons with intermediate and final results stored in
18 memories 5 6 or lO. The verTficatTon process
19 although described in reference to an MLS may be used
with any type of landing system. All of the above
21 changes could be made without departlng from the true
22 scope of the inventlon.
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