U.S. patent number RE32,368 [Application Number 06/620,798] was granted by the patent office on 1987-03-10 for collision avoidance system for aircraft.
This patent grant is currently assigned to Toyo Tsushinki K.K.. Invention is credited to Chuhei Funatsu, Toshikiyo Hirata.
United States Patent |
RE32,368 |
Funatsu , et al. |
March 10, 1987 |
Collision avoidance system for aircraft
Abstract
A collision avoidance system for aircraft in which one aircraft
is equipped with an interrogation station having a secondary
surveillance radar. .[.Coarse.]. .Iadd.A .Iaddend.distance
measurement is effected either by passive or active distance
measurement or by both of them. If the detected distance lies
within a certain limit, the output power and/or period of the
interrogation signal of the secondary surveillance radar of the
subject aircraft is altered .[.so as to effect fine distance
measurement.].. This system can be applied without increasing
interference against the existing secondary surveillance radar
system by keeping the output power and period of interrogation
signal in a minimum required extent. By the same reason the system
can keep the interference at a small extent between proximate
aircraft, each mounting this collision avoidance system.
Inventors: |
Funatsu; Chuhei (Yokahama,
JP), Hirata; Toshikiyo (Samukawa, JP) |
Assignee: |
Toyo Tsushinki K.K. (Samukawa,
JP)
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Family
ID: |
14253567 |
Appl.
No.: |
06/620,798 |
Filed: |
June 14, 1984 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
714335 |
Aug 13, 1976 |
04107674 |
Aug 15, 1978 |
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Foreign Application Priority Data
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Aug 15, 1975 [JP] |
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50/99674 |
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Current U.S.
Class: |
342/32; 342/45;
342/455; 342/30; 342/37; 342/42 |
Current CPC
Class: |
G01S
13/933 (20200101); G01S 13/781 (20130101) |
Current International
Class: |
G01S
13/93 (20060101); G01S 13/78 (20060101); G01S
13/00 (20060101); G01S 009/56 () |
Field of
Search: |
;343/6.5R,6.5LC,455,6.8R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Buczinski; Stephen C.
Attorney, Agent or Firm: Wenderoth, Lind & Ponack
Claims
What is claimed is: .[.1. A collision avoidance system for aircraft
each equipped with an ATC transponder comprising;
an interrogation station mounted on one aircraft, having secondary
surveillance radar function emitting an interrogation signal to be
responded to by the ATC transponder of a second aircraft.
coarse detection means on said one aircraft for detecting the
existence of the second aircraft in proximity to said one aircraft,
and
output control means for altering at least one of output power and
transmission period of the interrogation signal so as to effect
fine detection of said second aircraft when said coarse detection
means delivers a signal representing information concerning said
second aircraft which exceeds a certain value..]. .[.2. The
collision avoidance system for aircraft as claimed in claim 1,
wherein said coarse detection means deduces the distance between
the aircraft by detecting the time difference between reception of
a second interrogation signal sent from a ground secondary
surveillance radar at said one aircraft and a response signal sent
from the ATC transponder mounted on said second aircraft in
response to said second interrogation signal..]. .[.3. The
collision avoidance system for aircraft as claimed in claim 1,
wherein said coarse detection means deduces distance between the
aircraft by detecting the time difference between transmission of
an interrogation signal sent from the interrogation station mounted
on said one aircraft and a response signal sent from the ATC
transponder mounted on said second aircraft in response to said
interrogation signal..]. .[.4. The collision avoidance system for
aircraft as claimed in claim 1, wherein said coarse detection means
deduces the distance between the aircraft by detecting the
receiving level of a response signal sent from the ATC transponder
mounted on said second aircraft responding to an interrogation
signal sent from a ground interrogation station..]. .[.5. The
collision avoidance system for aircraft as claimed in claim 1,
wherein said coarse detection means deduces the altitude difference
information between the aircraft by detecting altitude information
included in the response signal delivered
from the ATC transponder of the second aircraft..]. .[.6. The
collision avoidance system for aircraft as claimed in claim 1,
wherein said coarse detection means deduces the variation of
altitude difference between the aircraft by consecutively deducing
altitude difference information..]. .[.7. The collision avoidance
system for aircraft as claimed in claim 1, wherein the output
control means for altering at least one of output power and
transmission period of the interrogation signal sent from the
interrogation station functions in response to detected variation
of time difference between transmission of the interrogation signal
and reception of the response signal..]. .[.8. The collision
avoidance system for aircraft as claimed in claim 1, wherein the
output control means functions in response to detection of the
repetition frequency of the response signals, i.e. number of
responses during a certain duration, sent from the ATC transponder
on the second aircraft..]. .[.9. The collision avoidance system for
aircraft as claimed in claim 1, wherein said output control means
functions in response to detection of repetition frequency of
interrogation signals sent from ground secondary surveillance
radar, i.e. number of second interrogation signals during a certain
duration..]. .[.10. Collision avoidance system as claimed in claim
5, wherein the response signal of the other aircraft is sent in
response to an interrogation signal sent from the interrogation
station mounted on said one aircraft..]. .[.11. Collision avoidance
system as claimed in claim 6, wherein the response signal of the
other aircraft is sent in response to an interrogation signal sent
from the interrogation station mounted on
said one aircraft..]. .Iadd.12. A collision avoidance system for
aircraft which are each equipped with an ATC transponder
comprising:
an interrogation station located on a first aircraft and having a
secondary surveillance radar function, said interrogation station
emitting an interrogation signal which is to be responded to by an
ATC transponder of a second aircraft;
a detection means located on said first aircraft for detecting the
existence of said second aircraft in proximity to said first
aircraft;
an output control means operatively connected to said detection
means for altering at least one of an output power and a
transmission period of said interrogation signal emitted by said
interrogation station when said detection means detects information
concerning said second aircraft which exceeds a predetermined
value. .Iaddend. .Iadd.13. A collision avoidance system as recited
in claim 12, wherein said detection means deduces distance between
said first and second aircraft by detecting a time difference
between reception of a second interrogation signal by said first
aircraft which has been sent from a ground secondary surveillance
radar and a response signal sent from said ATC transponder located
on said second aircraft, said response signal being generated in
response to said second interrogation signal. .Iaddend. .Iadd.14. A
collision avoidance system as recited in claim 12, wherein said
detection means deduces distance between said first and second
aircraft by detecting a time difference between transmission of
said interrogation signal sent from said interrogation station
located on said first aircraft and a response signal sent from said
ATC transponder located on said second aircraft, said response
signal being generated in response to said interrogation
signal. .Iaddend. .Iadd.15. A collision avoidance system as recited
in claim 12, wherein said detection means deduces distance between
said first and second aircraft by detecting a received level of a
response signal generated by said ATC transponder located on said
second aircraft when said response signal is responding to an
interrogation signal sent from a ground interrogation station.
.Iaddend. .Iadd.16. A collision avoidance system as recited in
claim 12, wherein said detection means deduces altitude difference
information between said first and second aircraft by detecting
altitude information included in a response signal generated by
said ATC transponder of said second aircraft. .Iaddend. .Iadd.17. A
collision avoidance system as recited in claim 12, wherein said
detection means deduces variations of altitude difference between
said first and second aircraft by consecutively deducing altitude
difference information. .Iaddend. .Iadd.18. A collision avoidance
system as recited in claim 12, wherein said output control means
functions in response to a detected variation of time difference
between transmission of said interrogation
signal and a reception of said response signal. .Iaddend. .Iadd.19.
A collision avoidance system as recited in claim 12, wherein said
output control means functions in response to a detection of the
number of response signals during a predetermined time period
generated by said ATC transponder located on said second aircraft.
.Iaddend. .Iadd.20. A collision avoidance system as recited in
claim 12, wherein said output control means functions in response
to a detection of the number of interrogation signals during a
predetermined time period generated by a
ground secondary surveillance radar. .Iaddend. .Iadd.21. A
collision avoidance system as recited in claim 16, wherein said
response signal of said second aircraft is generated in response to
said interrogation signal generated by said interrogation station
located on said first aircraft. .Iaddend. .Iadd.22. A collision
avoidance system as recited in claim 17, wherein said response
signal of said second aircraft is generated in response to said
interrogation signal generated by said interrogation station
located on said first aircraft. .Iaddend.
Description
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a distance measurement system for
collision avoidance of aircraft. The system is to prevent aircraft
collision by suitably providing, for instance, a threat signal
depending on a result of measurement of mutual distance between
proximate or approaching aircraft in flight should danger of
collision occur.
(2) Description of the Prior Art
According to recent development of air traffic, the danger of
collision between aircraft has been increased very much because
more and more large numbers of various aircraft take the air of
same area and same altitude within the same time zone. Therefore it
is a very important to establish and maintain safe navigation of
aircraft. There are provided ground radar units mainly for the air
traffic control purposes. But it would cost an enormous amount to
establish a safe navigation control system using only such ground
radar. In addition a number of technical difficulties must, yet be
solved in realizing such a system. Furthermore, such a control
system is not effective outside the range of the ground radar.
The conventional distance measurement system including that using a
secondary surveillance radar of a subject aircraft may be
classified into two major systems, i.e., a passive system and an
active system.
A passive distance measurement system is a system based on a
principle of measurement of incoming information only. In this
system, a radar beam from a ground station functioning as a
secondary surveillance radar and air traffic control (ATC)
transponder outputs of the subject aircraft and of other nearly,
i.e. threatening aircraft delivered in response to the ground
secondary surveillance radar are utilized for identifying the
distance between the two aircraft.
In more detail this sytem is mainly based on the measurement of a
time difference between an ATC transponder output of the subject
aircraft delivered in response to a ground secondary surveillance
radar beam of a ground station and an ATC transponder output of the
other nearby aircraft delivered in response to the same ground
radar beam.
The passive system is further based on the following facts.
(1) The locus of the points at which the above time difference
becomes constant takes the form of ellipsoid having the subject
transponder antenna and the ground antenna as the focuses.
(2) ATC transponders of the subject and the other aircraft can only
receive the ground radar beam when the two aircraft are covered by
an effective radiation pattern from the ground radar beam.
(3) The transmitting wave strength of an ATC transponder is a
certain value decided by regulation so that the reception wave
strength of the ATC transponder output of other aircraft can be
used to represent a function of distance between the two
aircraft.
This passive distance measurement system has certain
disadvantages.
In the system of deducing the distance by obtaining the above time
difference, there will be no time difference when the second
aircraft is located on a line connecting the subject aircraft and
the ground antenna or comes very close to said line. In this case
the time difference becomes same as the time required for the
response to the transmission of the ATC transponder from the second
aircraft irrespective of the distance between the second aircraft
to the subject aircraft.
To assume the distance between aircraft, by the level of received
wave strength set forth above (3) also has a certain danger.
Because the electric field strength of the ATC transponder of the
second aircraft may not have a constant value and it may vary
depending on the course and attitude of flight of the second
aircraft mainly by a reason of non-uniformity of the transmission
pattern of the antenna of the ATC transponder.
An active distance measurement system overcomes most of the
abovementioned disadvantages of the passive system. In an active
distance measurement system, the ground station and its
transmission radar beam pattern are not utilized, but the collision
avoidance system on the subject aircraft is given the facility of
transmitting an interrogation signal acting as a kind of secondary
surveillance radar. By measuring the time difference between the
transmission of the interrogation signal and reception of a
response signal delivered from the ATC transponder of the second
aircraft responding thereto by the collision avoidance system of
the subject aircraft, the distance between the two aircraft can be
obtained. This active distance measurement system affords a
substantially high accuracy in the distance measurement compared
with the passive distance measurement system.
It is preferred to arrange the transmission of the secondary
surveillance radar, in the active distance measurement system
equipped on an aircraft, during an interval of scanning of the
ground radar beam, which has a very sharp directivity. By the above
arrangement, the influence of the transmitted signals of the
subject and other aircraft to the ground station can be minimized.
The applicants had disclosed abovementioned arrangement in Japanese
Patent Application Publication No. 29,358/73. There is also a
possibility that both the subject and the threatening aircraft are
navigating in a zone where no ground radar exists. In such a case,
the secondary surveillance radar on the aircraft may be arranged to
transmit the interrogation signal at a certain time interval
irrespective of the existence of other approaching aircraft.
In the active distance measurement system, since a highly accurate
distance measurement is possible, the system may be modified to
have a function of measuring the variation of the abovementioned
time difference. In this case mutual speed of the two aircrafts can
be obtained without much difficulty and this would contribute in
avoiding collision.
Thus the active distance measurement is a much improved system
compared with the passive distance measurement system, however,
this system has still disadvantages substantially mentioned
below.
(1) Fruit noise of an overall radar system may be increased by the
interrogation of the secondary surveillance radar which is mounted
on the aircraft and by the response signal thereto.
(2) During the interrogation and response between the subject
aircraft and the other or threatening aircraft aircraft, the ATC
transponder on the second aircraft becomes insensitive for the
further interrogation and therefore the function of the secondary
surveillance radar is spoiled.
In order to avoid aforementioned disadvantages of the active
distance measurement system either of the following two steps must
be taken;
(a) decrease the number of interrogation signals from the subject
aircraft.
(b) suppress radiation power of the interrogation signal.
SUMMARY OF THE INVENTION
The present invention has its primary object the provision of a
system mitigating the aformentioned disadvantages of both the
passive and the active systems.
Another object of the invention is to establish a distance
measurement system for collision avoidance of the aircrafts by
supplying, for instance, an alarm or threat signal, without
applying any particular requirement such as, for instance, mounting
additional equipment on the threatening aircraft but just using the
secondary surveillance radar function already in use on an aircraft
and the ATC transponder with which aircraft are usually
equipped.
The present invention is a system in which both features of the
passive and the active distance measurement systems are combined.
In accordance with the system of the present invention the
aforementioned disadvantages of the existing two systems are
avoided by automatically altering either the interrogation period
or the transmitting power of the interrogation wave of the subject
aircraft depending on an identified distance between it and the
threatening aircraft obtained by either of the passive or active
system or a combination thereof.
The present invention utilizes the existing ATC transponder for
collision avoidance, however, the system of the invention is so
arranged as not to give any interference to the existing ground
secondary surveillance radar systems.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram for explaining the principle of the
passive and active distance measurement systems using a secondary
surveillance radar system,
FIG. 2 is a diagram for explaining the principle of the passive
distance measurement system,
FIG. 3 is a block diagram of an inventive apparatus equipped on the
subject aircraft for collision avoidance, and
FIG. 4 is a more detailed block diagram of the apparatus shown in
FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In order to first give a clear understanding of the present
invention, existing passive and active distance measurement systems
will be explained by referring to the drawings.
In FIG. 1, a ground interrogation station 1 having the function of
a secondary radar system for air traffic control purposes is
coupled to an antenna 2 for radiating an interrogation radar beam.
The radar beam is transmitted with a sufficient directional
characteristic and is arranged to scan a certain given area.
By the transmission of the radar beam from the directional antenna
2, a zone 3 is defined at a certain instance having its wave
intensity exceeding a certain level, for instance, higher than a
respondable level of the ATC transponder.
The transmitted radar beam from the antenna 2 is received by an ATC
transponder 5 mounted on the subject aircraft through an antenna 4.
The ATC transponder 5 transmits a response signal after a certain
short time delay from the reception of the interrogation signal.
The response signal is transmitted through the antenna 4.
In this case the aircraft is also provided with a collision
avoidance system 6. In FIG. 1, it is assumed that there is another
aircraft in the neighborhood of the first or subject aircraft and
this second aircraft is also provided with an ATC transponder 8 and
its antenna 7.
The interrogation radar beam having the directional characteristic
shown by 3 is also received by the ATC transponder 8 of the second
aircraft which delivers a response signal after a certain time
through the antenna 7. The antenna 7 has generally a
non-directional characteristic so that the response signal is
received by the collision avoidance system 6 through its own
antenna 11 to which system 6 the response signal delivered by the
first ATC transponder 5 is also supplied.
The locus of a point at which the time difference identified by the
system 6 between the one response signal delivered from the first
ATC transponder 5 and other response signal delivered from the
other ATC transponder 8 becomes constant is given by an ellipsoid 9
as shown in FIG. 2 having two focuses one being that of the first
antenna 4 and the other being that of the ground antenna 2. By
using the time difference an approximate distance between the two
aircrafts can be obtained. In this case it is assumed that the
antenna 4 and the system 6 are located in very close proximity.
The instance wherein both the ATC transponders 5 and 8 can receive
the same ground interrogation wave is when the two aircraft are
located in the same radiation pattern 3 of the ground radar
beam.
By measuring the field intensity of the response signal of the ATC
transponder 8, which must deliver a certain constant level, an
approximate distance between the two aircraft can be deducted.
Namely in the passive distance measurement system, either the time
difference between the first ATC transponder response signal and
the second ATC transponder response signal or the field intensity
of the received second ATC transponder response signal is used to
identify the distance between the two aircraft.
In an active distance measurement system, the ground station 1, the
antenna 2 and the radiation pattern 3 are not used but the
collision avoidance system 6 on the subject aircraft is given a
function of transmitting interrogation signal of the secondary
surveillance radar system. The time difference between the
transmission and the reception of the response signal from the ATC
transponder 8 on the second aircraft is used for measuring the
mutual distance of the two aircraft.
A basic principle of the system of the present invention will now
be explained by taking up some possible embodiments.
EMBODIMENT 1
In one embodiment of the present invention, an interrogation wave
of 30 Watt peak to peak (P--P) output level is transmitted from the
collision avoidance system 6 mounted on the first aircraft. This
interrogation signal will provide an electric field strength that
any ATC transponder of an aircraft located within 5 NM (nautical
mile) can respond thereto.
The interrogation period is adjusted to be 3 seconds interrogation.
The abovementioned output power and the period are the minimum
requirement for avoiding collision for aircraft having speed less
than 1 Mach in view of probability. By selecting the above values,
the increase in disturbance ratio to the existing secondary radar
system by fruit noise or the like is less than 2%. Furthermore, the
increase of the insensitivity ratio of the existing ATC transponder
can be kept less than 4.5%. Therefore, it can be said that the
system of the present invention does not affect either the function
of the existing secondary radar system or that of the ATC
transponder.
By using passive distance measurement, if the collision avoidance
system 6 detects the second aircraft within a range of 10 NM as for
instance by the reception of a response signal from the second
aircraft exceeding -60 dBm, the system 6 automatically increases
the transmitting output of the interrogation signal to a level of
300 watts and thus expands the surveillance range to 10 NM. Also
the interrogation period is lengthened to 12 seconds.
The abovementioned output power and the period are the sufficient
values to avoid collision of aircraft having speed less than 1 Mach
in view of probability. Furthermore, the extent of the influence to
the existing secondary surveillance radar and to the existing ATC
transponder is very minor and is the same degree as mentioned
above.
The increase of the output power of the interrogation signal and
the elongation of the interrogation period are continued until the
distance between the aircrafts reaches 13 NM by passive of active
distance measurement or combination thereof. At this distance of 13
NM there is no substantial danger of collision.
For the period when the distance obtained by active measurement is
5.5 NM to 3.5 NM, the interrogation period is automatically altered
from 12 seconds to 3 seconds linearly and consecutively. By using
active distance measurement by its one interrogation, the next
interrogation period is thus decided automatically. The system 6 is
so modified to include such function. During this period the
passive distance measurement is continued to establish the backing
up of the active distance measurement.
At a range of less than 4.5 NM by active distance measurement, the
output power of the interrogation wave is decreased to 30 Watt.
This output power is returned to 300 Watt, when the distance
becomes over 5 NM and the period is lengthened to 12 seconds.
According to the abovementioned embodiment 1, the first aircraft is
protected by active measurement by its secondary surveillance radar
having output of 30 Watt and interrogation period of 3 seconds. In
the present system, the active distance measurement is
automatically adjusted in its interrogation frequency and power
output by using passive distance measurement in conjunction with
the ground surveillance radar and the occurrence of fruit noise and
the system disturbance are suppressed to very minor level.
It is evident that the embodiment 1 is altered to change either one
of the output power or the interrogation period which still has a
good collision avoidance facility. It may be imagined easily that
the alteration of output power and the period can be effected by
either one of the passive or active distance measurement.
Altitude Information
Altitude information is included in a response signal of an ATC
transponder for responding to a particular kind of interrogation
signal so that it is easy to detect an altitude difference between
aircrafts by using passive or active distance measurement.
Accordingly, it is possible to derive the altitude difference
signal by arranging the interrogation signal of the secondary radar
equipment on the first aircraft to contain such particular
requirement. Furthermore, by arranging the behavior of alteration
of the transmission power and the period by referring also the
detected altitude difference, it is possible further to suppress
the aforementioned disturbance effect against existing secondary
surveillance radar system, accompanied with the introduction of
this system, without decreasing the collision avoidance effect.
The required devices or circuits to be added to the system 6 for
realizing this function are minor ones.
Approaching Speed
Particularly in the active distance measurement system, the
approaching speed between first aircraft and other aircraft can be
obtained with substantial accuracy by effecting the distance
measurement consecutively.
It is obvious that approximate approaching speed can be obtained by
the passive distance measurement.
The system disturbance can be effectively decreased further, by
introducing controls for the interrogation output power and the
interrogation period by using the information of the approaching
speed in addition to the distance and altitude information.
EMBODIMENT 2
The distance deduction by using the passive and/or active distance
measurement requires a complicated calculation process. A
calculation to obtain three dimensional approaching speed including
the altitude factor becomes more complicated. If the apparatus is
to have added a function to control transmission output power and
period of the interrogation signal level and of indicating alarm to
a suitable value, then the apparatus must be a bulky one for
instance to include a microprocessor.
FIG. 3 shows a block diagram of one embodiment of the collision
avoidance system to be mounted on an aircraft. In this embodiment,
an antenna ANT is commonly used by an ATC transponder T
corresponding to the ATC transponder 5 of FIG. 1 and the other
portions of the diagram which correspond to the collision avoidance
system 6 shown in FIG. 1.
The signal received by the antenna ANT is fed through a switcher SW
and a divider DIV both to the ATC transponder T and to a passive
sensor PS. The ATC transponder T responds to the incoming signal
and transmits a response signal through the same route in reverse
direction. The output of the ATC transponder T and the output of
the passive sensor PS are supplied to a central processor unit CPU.
The central processor unit CPU has further connection to an
interrogation transmitter TX, a response receiver RX and to the
switcher SW. The result of passive distance measurement, active
distance measurement, altitude information, approaching speed
information and other information are processed under certain
mutual relationships by the central processor unit CPU. According
to the result of the above process the output power and period of
the interrogation signal, kind of threat signals and others are
decided displayed and used as further instructions. The device
designated by SC is a directional coupler such as a circulator.
The program to be stored in the central processor unit CPU should
be one to effectively decrease the fruit noise and system
disturbance at a possible minimum extent under consideration of any
encountering conditions. Generally a most suitable program is
decided by effecting a simulation test.
EMBODIMENT 3
FIG. 4 shows a block diagram of another embodiment of the system
according to the present invention to be mounted on the
aircraft.
An interrogation signal sent from a ground station acting as a
secondary surveillance radar, such as shown in FIG. 1 by the
station 1, is received by an antenna ANT1 corresponding to antenna
4 of FIG. 1. The received signal is fed through an ATC transponder
T1 to a time comparator TC. On the other hand, a response signal
sent from an ATC transponder of the second aircraft corresponding
to the transponder 8 of FIG. 1 and received by an antenna ANT2,
which corresponds to the antenna 11 of FIG. 1, is fed to the time
comparator TC through a receiver RX1 and a decoder DE. The time
difference between the former and the latter signals can be derived
by the time comparator or time counter TC. If this time difference
is less than 120 .mu.S for instance, a signal is sent from the time
comparator TC to a timing control TCL. One output of the timing
control TCL is fed to a transmitter TX1 through a coder C, and
another output of the timing control TCL is fed to the same
transmitter TX1 through a power control PC. Under control of the
TCL the transmitter TX1 transmits an interrogation signal of for
instance 12 second period and of 300 W output power through the
antenna ANT2.
It the time difference between the two signals obtained by the time
comparator TC exceeds 120 .mu.S, then the interrogation signal is
changed to have 3 second period and 30 W output power.
The response signal of the ATC transponder of the second aircraft
corresponding to the transponder 8 of FIG. 1 received by the
antenna ANT2 and the receiver RX1 and decoded by the decoder DE is
on the one hand supplied to a level comparator LC. If the response
signal is over -60 dBm, a signal is sent from the level comparator
LC to the timing control TC and an interrogation signal same as
before and for instance having 12 second period and 300 W output is
transmitted through the antenna ANT2.
If the response signal level is less than -60 dBm, the
interrogation signal is changed to have 3 second period and 30 W
output power. An output of the ATC transponder T1 is further
supplied to an interrogation counter IC and counted the number of
the interrogation signals. If the number of interrogation signals
is less than 100 during a period of 12 second for instance, a
signal is derived from the interrogation counter IC to the timing
control TC and an interrogation signal having 12 second period and
300 W output power is transmitted through the antenna ANT2 same as
mentioned before.
Also in this case if the number of signals is more than 100 during
12 second period, the interrogation signal is changed to have 3
second period and 30 W output power.
The response signal of the ATC transponder of the second aircraft
responding to the interrogation signal sent from the antenna ANT2,
is received by a receiver RX1 and sent to a decoder DE and decoded
therein.
An output signal of the decoder DE is supplied to a range register
RR and the mutual distance from the other aircraft is obtained
therein.
In this case if the obtained mutual distance is less than 5 NM for
instance, the interrogation signal transmitted from the antenna
ANT2 is changed to have 3 second period and 30 W output power. This
is controlled by an output signal delivered from the range register
RR to the timing control TC which controls power control PC and the
coder C and eventually transmitter TX1.
Another output signal from the range register RR is sent to a range
tracker TR and the relative speed between the aircraft is obtained
thereat. Output signal of the range tracker RT is sent to a threat
evaluator TE. The danger of collision with other aircraft is
evaluated by the threat evaluator TE and instruction is given to an
indicator IN if a danger of collision is evaluated.
A further output signal of the decoder DE is fed to an altitude
register AR. By using the decoded signal and an output signal from
an altitude digitizer AD representing altitude of the own aircraft,
the relative altitude between other aircraft can be obtained.
By using the abovementioned relative altitude information, if the
relative altitude is less than .+-.3,400 ft for instance, the
variation rate of the relative altitude is obtained by using the
altitude tracker AT. By using the output signal thereof, the threat
evaluator TE evaluates the danger of collision between other
aircraft and sends an instruction signal to an indicator IN to send
an alarm or a threat signal if a signal is supplied already by the
range tracker RT.
If the relative altitude is more than .+-.3,400 ft for instance,
the threat evaluator TE does not send an output even though the
range tracker RT sent an output signal to the threat evaluator TE.
It is also possible that this altitude difference information is
supplied to the range tracker RT as shown by dotted line so that
the interrogation output power and period are not changed even when
the detected distance is small.
The system of the present invention will give a substantially
improved effect for collision avoidance and its influence for the
existing secondary radar system can be kept at a very minor
extent.
This system has also an advantage that the interference between the
collision avoidance systems mounted on several aircrafts can be
kept at a small extent by the same reason set forth above.
Accordingly, the invention may contribute for the safe navigation
for the more and more increasing air traffic.
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