Communications: directive radio wave systems and devices (e.g. – Aircraft collision avoidance system – With transponder
Reexamination Certificate
2001-04-13
2003-07-01
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Aircraft collision avoidance system
With transponder
C342S042000, C342S043000, C342S051000
Reexamination Certificate
active
06587069
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to transmitters used in aircraft transponders and in particular to transmitters used in connection with a Traffic Alert and Collision Avoidance System, or TCAS.
Pilots primarily rely on “see and avoid” techniques to prevent mid air collisions between aircraft. Various avionics systems also aid the pilot and air traffic control in maintaining aircraft separation. These systems, generally known as surveillance systems, may include: transponders and/or collision avoidance systems that exchange aircraft data between aircraft and/or between aircraft and air traffic control.
Transponders are transmitters/receivers located aboard aircraft. The transponder transmits a 1090 MHz encoded message containing information about the aircraft in response to interrogation signals received from a ground based radar or other TCAS equipped aircraft. This radar, known as a “secondary surveillance” radar, or simply secondary radar, transmits an interrogation signal at 1030 MHz. The interrogation signal contains three pulses. The time interval between the first and third pulses determine what type of information is requested: eight microseconds for identification; and twenty one microseconds for altitude. If the aircraft transponder is a Mode A transponder, the transponder can reply only to the identification request. If the aircraft transponder is a Mode C transponder, the transponder can reply to both the identification and altitude requests.
The Mode A and Mode C systems are unable to relay additional information or messages between the ground based secondary radar and the interrogated aircraft, other than the identification and altitude information. The Mode Select, or Mode S transponder provides additional capabilities over those available in Mode A and Mode C. Standards for Mode S communications are contained in the Radio Technical Commission for Aeronautics (RTCA) document, “Minimum Operational Performance Standards for Air Traffic Control Radar Beacon System/Mode Select (ARTCBS/Mode S) Airborne Equipment,” RTCA/D0-181A, issued January 1992 and incorporated herein by reference.
In operation, a unique 24 bit address code, or identity tag, is assigned to each aircraft. Once per second, the Mode S transponder spontaneously and pseudo-randomly transmits, or “squitters,” an unsolicited broadcast, including the specific address code unique to the aircraft carrying the transponder, via first one and then the other of its two omnidirectional antennas. Whenever the Mode S transponder is not broadcasting, it is monitoring, or “listening”, for transmissions, including interrogation signals.
The unique 24-bit address code, or identity tag, assigned to each aircraft is the primary difference between the Mode S and Mode A/Mode C transponders. Interrogations are directed only to the particular aircraft using this unique address code and the replies are unambiguously identified. The unique address coded into each interrogation and reply also permits inclusion of data link messages to and/or from a particular aircraft.
The Traffic Alert and Collision Avoidance System (TCAS) is an airborne system that interacts with the Mode S and Mode A/C transponder system to alert the pilot of potential collision threats. The TCAS transmits interrogations which are received and replied to by other aircraft and used to determine the locations of other aircraft relative to own aircraft position. The TCAS system is described in RTCA document D0-185 entitled: “Minimum Operational Performance Standards for Air Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment”, issued Sep. 23, 1983, consolidated Sep. 6, 1990 and in DO-185A, “Minimum Operational Performance Standards for Air Traffic Alert and Collision Avoidance System II (TCAS II) Airborne Equipment”, dated December 1997 both of which are incorporated herein by reference.
FIG. 1
illustrates one known embodiment of the TCAS system having 4-element interferometer antennae
2
A and
2
B coupled to a radio frequency receiver
3
of a TCAS processor
4
. Receiver
3
is in turn coupled to a signal processor
5
operating known traffic alert and collision avoidance software. A radio frequency transmitter
6
is coupled to signal processor
5
for transmitting Mode S and Mode C interrogation signals. An associated control panel
7
for operating the TCAS system and a display
8
for displaying the TCAS information are each coupled to signal processor
5
of TCAS processor
4
as described in U.S. Pat. No. 4,855,748 and in co-pending U.S. application Ser. No. 09/369,752 entitled: “Multifunction Aircraft Transponder,” filed Aug. 6, 1999, each of which is incorporated herein by reference.
Transmitter
6
can consist of either a vacuum tube cavity oscillator or, more commonly, a solid state amplifier driven by an oscillator. The oscillator provides a radio frequency wave that is modulated by modulator
19
. To satisfy reception requirements, the maximum output power of the transmitter is 1000 W. In typical designs, the TCAS must deliver nominally 250 W measured at the rear of the unit.
FIG. 2
shows a typical Mode S transmitter and modulator in which an oscillator
20
drives an amplifier
21
composed of two zero bias Class C transistors
22
and
24
. The difference between the output of amplifier
21
and the power delivered to the antenna accounts for subsequent losses at the transmitting antenna and intervening circuit components and ensures that sufficient output power is available at the transmitting antenna.
A ceramic resonator
30
preserves the frequency stability of the oscillator output. The output is additionally filtered to reduce or remove sideband harmonics which may be significantly amplified by amplifier
21
and subsequently received by other aircraft and confused with the true signal. The input signal from oscillator
20
is split into two signals separated by a phase angle of 90 degrees. Each output of power splitter
32
feeds one of transistors
22
and
24
. The amplifier output is then supplied to a second power splitter
34
before being again filtered and supplied to one of antennas
2
A and
2
B.
One issue with use of the TCAS and secondary radar systems is that every Mode C/Mode A transponder that receives an interrogation signal replies. If the replies are received at the same time, or if portions of the reply signal overlap, the reply will be garbled. In addition, if an aircraft is interrogated by more than one ground facility or aircraft, a facility or aircraft other than the interrogator may receive the reply pulse. This unsolicited response is called FRUIT.
To minimize garble and FRUIT, the TCAS/transponder transmitter broadcasts the interrogation pulse using a technique called “whisper/shout.” The “whisper/shout” technique uses a variable level attenuator to transmit the interrogation pulse at varying power levels. The sequence of variable power interrogations reduces the amount of garble that the receiving TCAS must process and reduces the amount of FRUIT added to the secondary radar system.
FIG. 3
contains a block diagram of the TCAS transmitter device showing amplifier
21
and a whisper/shout attenuator
40
. After the signal to be transmitted is amplified by amplifier
21
it is then sent to attenuator
40
and phase shifters
42
a
-
42
c
before being broadcast from antennas
2
A and
2
B. Because attenuator
40
and phase shifters
42
a
-
42
c
occur downstream of amplifier
21
, these components must be sized to handle the up to approximately 600 W of power output by amplifier
21
. In addition, much of the output power generated by amplifier
21
is then wasted by attenuator
40
when in the whisper mode. In addition, because the Class C amplifier contained within amplifier
21
does not amplify linearly, the amplifier will introduce some amplification of the radio frequency pulse harmonics. This unwanted amplification is called spectral regrowth and an output filter is required to minimize its generation. The Class C amplifier can also only be turned full
Brandao Ruy C.
Brandao Ruy L.
Jones James B.
Nguyen Lien
Pham Dang
Andrea Brian K
Honeywell International , Inc.
Honeywell International Inc.
Nguyen Lien
Tarcza Thomas H.
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