Communications: directive radio wave systems and devices (e.g. – Aircraft collision avoidance system – With transponder
Reexamination Certificate
2002-12-18
2004-07-27
Pihulic, Daniel (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Aircraft collision avoidance system
With transponder
C342S037000
Reexamination Certificate
active
06768445
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to avionics electronics. More specifically, the present invention relates to methods and devices for qualifying Mode-S signals and for controlling reply by transponders to Mode-S interrogations.
A variety of transponders exist today for use with the Air Traffic Control Radar Beacon System (ATCRBS) and that support various communications protocols, such as Mode-C, Mode-A and Mode-S communication's protocols. ATCRBS ground dish that transmits a beam having directional characteristics to transmit and receive information to and from aircraft within the relevant air space. The radar dish transmits over a common frequency to all of the aircraft. Each ATCRBS ground station also includes an omnidirectional antenna co-located with the directional radar dish. The omnidirectional antenna transmits, over a control frequency, among other things, side lobe suppression (SLS) signals which, as explained below in more detail, are synchronized to, and used in combination with, transmissions over the common frequency from the directional radar dish. The SLS signals are utilized to prevent aircraft outside of the beam from replying to transmissions generated by the directional radar dish. The aircraft transponders compare certain pulses transmitted from the radar dish over the common frequency with certain pulses transmitted from the omnidirectional antenna over the control frequency. The aircraft transponder determines whether to reply to received signals depending upon the relation between the compared pulses.
In general, an ATCRBS ground station sends approximately 250 to 450 Mode-S interrogations per second per radar frequency. In a ten second period, the radar dish will maintain a specific aircraft within its radar beam for no more than approximately 100 milliseconds which enables approximately 25 to 45 replies to be received by the ATCRBS ground station from each aircraft during each sweep of the radar dish.
To partially address the clutter of the air communication space created by excessive and unsolicited replies, the Mode-Select (Mode-S or discrete beacon address system, DBAS) was developed which permits active transmission of information to and from the aircraft. Mode-S transmissions have greatly reduced the transmission interference or garble previously experienced. In a Mode-S system, the ground station transmitter/receiver interrogates aircraft discretely based on specific 24 bit address assigned to each aircraft. The ground station transmits a Mode-S signal to each aircraft from which a reply is sought. The Mode-S protocol was developed to operate within the existing Mode-A or Mode-C environment.
The ground station produces a tag for each aircraft in its surveillance area through the use of two different methods in order to individually address each aircraft. In one method, a Mode-S SQUITTER is transmitted by the aircraft transponder pseudo randomly with a unique identification code for the aircraft embedded in the transmission. In the other method, a Mode-S ALL CALL signal is transmitted by the ground station. When the ground station transmits an ALL CALL signal, the Mode-S signal includes an interrogation command intended to elicit a reply from the transponders of every aircraft that receives the interrogation command. Each transponder that receives the ALL CALL signal replies by transmitting its unique 24 bit address.
The protocol for the Mode-S signals includes an identifying preamble containing two pulses, namely a P
1
pulse and a P
2
pulse, separated by a predetermined time interval. The P
1
and P
2
pulses are transmitted in accordance with a particular pulse width, modulation technique, and frequency. When transmitting a Mode-S interrogation, the ground station after transmitting the P
1
and P
2
preamble pulses, transmits a differential phase shift keyed (DPSK) data segment of predefined length, such as 56 or 112 bits or chips. The DPSK data segment contains, among other things, the interrogation command. The DPSK data segment includes 24 parity bits to provide a cyclic redundancy check (CRC). The DPSK data segment and CRC bits are embedded within a P
6
pulse. The P
6
pulse also contains a synchronization phase reversal (SPR) signal that precedes the first data bit/chip by a predetermined time set forth in the protocol.
The aircraft and ground station operate asynchronously with respect to one another since the aircraft transponder is driven by its own internal clock that operates independent of the clock used to drive the ground station transmitter/receiver. This is why, the aircraft transponder first synchronizes incoming received signals with the clock of the aircraft transponder before being able to read the DPSK data segment contained within the P
6
pulse of the Mode-S signals. Signals received at the aircraft transponder represent a collection of signals transmitted from different sources, for different purposes and in varied formats. The aircraft transponder searches the collective incoming signals for various identifiers, such as Mode-A, Mode-C and Mode-S indicators. A Mode-S signal is identified by its preamble and more particularly by the pulse width and interval between P
1
and P
2
pulses. When the transponder detects a valid Mode-S preamble, the transponder next searches for the P
6
pulse containing the DPSK data segment. To demodulate the DPSK data segment, the transponder must first be synchronized with the phase of the received Mode-S signal. The transponder achieves synchronization by first identifying the SPR signal contained within the P
6
pulse. At the ground station, the P
6
pulse is formatted such that the DPSK data segment is transmitted by a predefined time interval after the SPR signal.
At the aircraft, the transponder continuously monitors received signals and, upon receipt of valid P
1
and P
2
pulses, begins searching the received signal for a P
6
data segment and once located, begins searching for the SPR signal. The transponder must detect the SPR signal within an allotted time window following the leading edge of the P
6
pulse. Once the aircraft transponder identifies an incoming Mode-S preamble and locates the subsequent corresponding P
6
signal and the SPR signal, the transponder is able to become synchronized with the DPSK data segment. If the SPR signal is not received within the allotted time window, the transponder determines that the received signal is not a Mode-S signal.
However, existing transponders have met with certain limitations. As noted above, when a ground station transmits a Mode-S ALL CALL interrogation, it is desirable for an aircraft to reply only when the aircraft is within the radar dish beam. It is preferable that aircraft outside of the radar dish beam not reply as such communications unduly garble the transmission airspace and are not properly receivable by the radar dish. In an attempt to limit aircraft replies only to aircraft within the radar dish beam, a protocol has been defined that must be satisfied by received signals at the aircraft transponder before replying. At the ground station, the radar dish transmits the P
1
, P
2
and P
6
pulses over the common frequency for a Mode S interrogation. The omnidirectional antenna also transmits a P
5
pulse over the control frequency (as explained below in more detail). The P
5
pulse is transmitted in all directions uniformly by the omnidirectional antenna. Thus, the strength or amplitude of P
5
pulse received by a particular aircraft is independent of the angular relation between the omnidirectional antenna and the aircraft. In contrast, the P
1
, P
2
and P
6
pulses transmitted by the radar dish are directional and thus, signal strength is stronger within the beam formed by the directional radar dish. Hence, the strength or amplitude of P
1
and P
6
pulses received by an aircraft is dependent upon whether the radar dish beam is directed at the aircraft or not and where the aircraft is located within the beam (e.g., the center or edge).
While the P
6
signal is strongest withi
Garmin International, Inc.
Pihulic Daniel
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