Method of determining radio frequency link reliability in an aircraft ...

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METHOD OF DETERMINING RADIO
FREQUENCY LINK RELIABILITY IN AN
AIRCRAFT TRACKING SYSTEM

BACKGROUND OF THE INVENTION

The present invention relates generally to avionics for aircraft and more specifically to airborne collision avoidance systems and transponders.

Airborne collision avoidance systems provide protection from collisions with other aircraft. Conventional systems operate independently of ground-based air traffic control. The development of an effective airborne collision avoidance system (CAS) has been the goal of the aviation community for many years. In the late 1980's a system specification for airborne collision avoidance was developed with the cooperation of the airlines, the aviation industry, and the FAA. Systems compliant with this specification became known as the-Traffic Alert and Collision Avoidance System II (herein TCAS). Such systems were mandated by Congress to be installed on most commercial aircraft flying in U.S. airspace by the early 1990's. A chronology of the development of airborne collision avoidance systems may be found in "INTRODUCTION TO TCAS II", printed by the Federal Aviation Administration (FAA) of the U.S. Department of Transportation, March 1990, which is incorporated herein by reference.

Conventional TCAS equipment in a host aircraft listens for radio frequency (RF) transmissions from air traffic control transponders on aircraft in its vicinity. Some of the transmissions received by the host aircraft are unsolicited periodic transmissions (squitter) from other aircraft equipped with Mode S transponders; some are replies to interrogation requests made by the host aircraft (surveillance replies); and some are replies to interrogation requests made by other interrogators (aircraft or ground stations) in the vicinity, known as Frequency Replies Unsynchronized In Time (FRUIT). By computer analysis of these transmissions, the airborne TCAS equipment on the host aircraft determines which aircraft are "intruders," in that they may represent a potential collision threat. The TCAS equipment tracks the intruders, and if necessary, provides advisories about intruders to the flight crew to assure separation. Computer analysis of these transmissions can also be employed to passively track intruder aircraft that are too distant to track actively, and therefore not of immediate interest to the collision avoidance function.

TCAS equipment on the host aircraft may either passively track the squitter and FRUIT, or actively track (interrogate) the range of intruder aircraft to elicit a surveillance reply. Whether to commence active tracking of an intruder aircraft depends in part upon information about radio frequency communication reliability. Poor communication may be due to many factors, such as range to the intruder aircraft, RF interference, or shielded antenna paths, and is often associated with low RF signal strength.

In communication technologies (e.g., wired, wireless, radio, or optical) a message is understood by a receiver when it is received in accordance with a signaling protocol intended by the transmitter. Such a protocol may include for example a modulation scheme, a band of radio frequencies, and/or a frequency hopping technique. If the message is received, the transmitter and receiver are generally considered to be using (or sharing) a communication channel, herein also called a link. The link subsumes the signaling protocol and may also include higher level protocols gen

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erally associated with computer communications such as acknowledgements and network functions such as sharing of data and processing resources at the transmitter, the receiver, or both. The link may be transitory, persist for an exchange

5 of messages, or.be presumed to be dedicated to particular transmitter(s) and receiver(s). Generally, a link is considered to no longer exist when one or more messages are not received properly; or when there is an indication that one of the transmitter and the receiver may not rely further on the

10 existence of the link for its original purpose.

A conventional TCAS determines whether an RF link is robust enough to attempt acquisition of an intruder aircraft for active tracking, using a link reliability scoring algorithm as follows. Upon detecting an intruder via.receipt of an

15 unsolicited DF=11 squitter, or a DF=17 Automatic Dependent Surveillance-Broadcast (ADS-B) squitter, a link reliability score is initialized for that intruder. Each time that a subsequent squitter or FRUIT is received from the intruder aircraft, the score is incremented by a predetermined, fixed

20 value, which is based upon any previously unsuccessful acquisition attempts of the intruder aircraft. When the link reliability score reaches a predetermined threshold, acquisition of the intruder aircraft is attempted to acquire range information. The number of interrogations made during the

25 acquisition attempt depends upon the number of previously unsuccessful acquisition attempts.

Problems are associated with this method of determining when to commence active tracking of an intruder aircraft. Specifically, this method frequently results in premature

30 attempts to acquire a tracking of the intruder aircraft's range, and if range is acquired, an accurate tracking of range to the intruder aircraft may be difficult to maintain. That is, the link used by the host aircraft may not be robust enough for the host aircraft to acquire the track, and if the track is acquired,

35 the link may not be robust enough to maintain continuous accurate information about the track. For instance, receipt of as few as three unsolicited replies within a 12-second interval may generally trigger an interrogation-intensive acquisition attempt. If the intruder is distant or the intruder's

40 antenna is shielded, significant power may be transmitted by the host aircraft trying to acquire track information of an intruder aircraft. In accordance with the TCAS specification, RF transmissions may be subject to a maximum transmitted power during a prescribed time period (e.g., a budget or

45 allocation). Subsequent tracking may consume a significant percentage of the power resources or power allocation available to the aircraft tracking system and may preclude timely acquisition and active tracking of other intruder aircraft using perhaps more reliable RF links.

An improved method of determining when to commence active tracking of an intruder aircraft is desirable. Without such a method, power may be wasted, important links neglected, critical advisories omitted, and hazardous flying

55 conditions develop that may lead to a loss of life and destruction of property.

BRIEF SUMMARY OF THE INVENTION

The following summary of the invention is provided to go facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention may only be gained by taking the entire specification, claims, drawing, and abstract as a 65 whole.

The present invention includes a method for receiving a message or reply from a sender; for determining the signal strength of the message or reply; for determining the time interval that has elapsed since a prior signal from the sender was received; and for determining whether to attempt acquisition of the range of (distance to) the sender in accordance with the signal strength and the time interval measurements. 5

The present invention includes a system for use in an aircraft, comprising at least one input for accepting antenna signals, and a memory comprising indicia of instructions for performing the method described above.

Premature attempts at such an acquisition are avoided 10 when the determination of whether to attempt an acquisition of a sender's range is based upon signal strength and upon the time spanning between received squitter. In addition, time, processing resources, and power may be better utilized as a consequence of avoiding maintenance of an unreliable 15 RF link with the sender. Inaccurate communication may also be avoided with consequential improved system performance and reliability.

By employing signal strength and time interval measure- 2Q ment to determine whether to attempt an acquisition of the sender's range, the system has more resources for transmissions with other senders that are transmitting messages or replies with stronger signal strengths or that are transmitting messages or replies that are more frequently received. 25

By employing signal strength and time interval measurements to determine whether to attempt an acquisition of the sender's range, available power is not wasted and power allocations are more effectively utilized.

BRIEF DESCRIPTION OF THE DRAWINGS 30

Embodiments of the present invention will now be further described with reference to the drawing, wherein like designations denote like elements, and:

FIG. 1 is a diagram of an aircraft system in accordance 35 with various aspects of the present invention;

FIG. 2 is a diagram of signal paths received by a host aircraft in accordance with various aspects of the present invention;

FIG. 3 is a diagram of a processor of the aircraft system 40 of FIG. 1;

FIG. 4 is a first flow chart of the methodology for the radio frequency (RF) reliability scoring in accordance with an embodiment of the present invention; and

FIG. 5 is a second flow chart of the methodology for the radio frequency (RF) reliability scoring in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

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An aircraft system according to various aspects of the present invention operates in a host aircraft to decide when to actively attempt acquisition of the range (for the purpose of tracking) of other aircraft in the vicinity (also known as 55 intruder aircraft). This determination is made with reference to characteristics of squitter received from Mode S transponders in other aircraft.

The present invention determines which intruder aircraft to attempt to actively acquire range information for initiating 60 active tracking of the aircraft or validating range derived from ADS-B squitters. An. aircraft equipped with a system of the present invention is capable of actively tracking the range of other aircraft reliably within approximately a 20 mile radius of itself. 65

The determination of which intruder aircraft to attempt to actively track also depends in part upon radio frequency

(RF) link reliability. Poor RF link reliability may be due to many factors, such as range to the intruder aircraft, RF interference, or shielded antenna paths, and is often associated with low signal strength.

Referring to FIG. 1, there is shown a block diagram of an aircraft system 100 of the present invention. System 100 includes a computer system 12, a transponder 16, a directional antenna 10, an omnidirectional antenna 11, an aural annunciator 13, a resolution advisory (RA) display 15, a traffic advisory (TA) display 14, a control panel 17, and a transponder antenna 19, all suitably coupled together.

Transponder 16 transmits electromagnetic signals via transponder antenna 19. Information conveyed by or derived from the electromagnetic signals includes range, altitude and bearing of the aircraft transmitting the signal. Transponder 16 may be of the type known as a Mode S transponder that additionally conveys on the electromagnetic signal the identifier of the aircraft transmitting the signal. Interrogations may be directed to a particular aircraft, using this aircraft identifier. That is, particular aircraft generally respond to interrogations directed to them and ignore interrogations not directed to them. AMode-S transponder-equipped aircraft is also capable of transmitting a periodic transmission (squitter) to alert others of the aircraft's presence or convey position or other aircraft information.

Many other configurations of an aircraft system 100 are possible; for instance, it is possible for the TA and RA displays to be combined into one display (not shown). Further, it is possible to replace omni-directional antenna 11 with a directional antenna 10. Aircraft system 100 in one implementation complies with the Minimum Operation Performance Standard (MOPS) for Traffic Alert and Collision Avoidance System (TCAS), manual document number DO-185 or DO-185A published by RTCA, Inc., which is incorporated herein by reference.

Referring now to FIG. 2, an example of an in-flight situation 200 is illustrated, in which aircraft system 100 on host aircraft 230 performs a method in accordance with the present invention. From time to time signals 266, 226, and 216 from nearby aircraft 260,220, and 210, respectively, are received by aircraft system 100 installed in aircraft 230. Aircraft system 100 receives squitter and/or FRUIT 266, 226, and 216, which are sent from aircraft within range 265, 225 and 215, respectively. Signals 216, 226, and 266 are shown transmitted by any type of antenna(s). Because signal strength generally decreases with distance, signal 266 is stronger than signal 226, and signal 226 is stronger than signal 216.

Host aircraft 230 may attempt to acquire an RF link with (i.e. actively track) all three aircraft; however, the characteristics of the squitter received by aircraft system 100 is employed by the present invention to determine with which aircraft an RF link is attempted. The decision depends in part upon the RF link reliability. RF link reliability may be predicted, in part, from the signal strength of the squitter received from an aircraft. Signal strength may suitably be an amplitude or power level measured at a particular time, or during a duration of measurement, or an integral, or average of such measurements.

RF link reliability may also be predicted, in part, from time interval measurements of the squitter received from the aircraft. Mode S transponders emit squitter at a regular periodic frequency (for instance, every second), but the squitter may not be received by the host aircraft when the intruder aircraft is beyond a 20 mile range or the signal path is obstructed. Thus, if an intruder aircraft flies in and then out 5

of the 20 mile range of the host aircraft, the intruder aircraft's squitter will only be regularly received while the intruder emits squitter within the 20 mile range. Once the intruder is out of the 20 mile range, squitter will be received irregularly or not at all by the host aircraft. Since squitter is emitted at a periodic frequency, a time interval can be measured to indicate when the intruder is no longer within range for reliably maintaining a track.

An RF link is not attempted with an intruder aircraft until aircraft system 100 predicts that such a link will be reliable. Aircraft system 100 monitors the squitter until indicia representing the squitter time interval measurements and signal strengths accumulate to a given threshold. Once the given threshold is reached, an RF link is predicted to be reliable and aircraft system 100 attempts to acquire the range of the intruder aircraft for active tracking.

Thus power is conserved that might otherwise be spent acquiring an RF link to an aircraft when the associated RF link reliability scores are low; because, an RF link is attempted only when an associated RF link reliability score reaches a threshold.

Referring now to FIG. 3, a computer system 12 of the aircraft system 100 of FIG. 1 is illustrated. Computer system 12 suitably comprises, e.g., a processor 310, a main memory

320, a memory controller 330, and a direct access storage device (DASD) 370, all of which communicate via a system bus 360. In one implementation, computer system 12 performs interrogation and provides resolution advisories (RA's) and traffic advisories (TA's) in any conventional manner (e.g., is compatible with RTCAMinimum Operation Performance Standard (MOPS) for Traffic Alert and Collision Avoidance System (TCAS II)).

Computer system 12 may be implemented by any computer technologies (e.g., circuits, firmware, operating system, and software) suitably adapted for performing methods for acquiring one or more links based on accumulating indicia of squitter characteristics and.comparing them to a threshold.

Processor 310 performs computation and control functions of computer system 12, and comprises a suitable central processing unit (CPU). Processor 310 may comprise.a single integrated circuit, such as a microprocessor, or may comprise any suitable number of integrated circuit devices and/or circuit boards working in cooperation to accomplish the functions of a processor. Processor 310 suitably executes computer applications stored in main memory 320.

Direct access storage device (DASD) 370 allows processor 310 to store and retrieve information from auxiliary storage devices, such one or more disks (e.g., magnetic disk, optical disk, or disk array).

Memory controller 330 moves requested information among main memory 320, DASD 370, and to and from processor 310. Memory controller 330 may be implemented as a separate entity. Portions of the function provided by memory controller 330 may, in various embodiments, be implemented in circuits packaged with processor 310, main memory 320, and/or DASD 370.

Main memory 320 and/or DASD 370 provide storage and read/write access to data and instructions used by processor 310. For example, main memory 320 and/or DASD 370 may include: an operating system 329, a threshold link validity value 324, a monitor state 326, a minimum value 330, a maximum value 332, a measure link reliability application

321, an active tracking application 323, a time interval field 325, an acquisition state 328, a tracking state 331, a squitter

record 380, an aircraft identifier field 382, a time stamp 384, an intruder record 350, an assigned state field 351, a previous acquisition attempt field 355, an aircraft identifier field 354, a link reliability score field 356, a time of last received squitter field 358, a base increment value 357, a decrement value 359, a signal strength value field 386, and an intruder database 372. For convenience of reference, such database records and software applications are briefly described in Table 1.

TABLE 1

Database,
Record, Field

or Application Description

Once an intruder is detected by receipt of squitter, measure link reliability application 321 creates an intruder record 350, stores the value of aircraft identifier 382 in aircraft identifier 354, stores a value of monitor state 326 in assigned state 351, and initializes the link reliability score 356. Link reliability score 356 is incremented by increment value 357 each time that an additional squitter or FRUIT is identified to that intruder. If link reliability score 356 exceeds a predetermined threshold link validity value 324 and assigned state 351 equals monitor state 326, measure link reliability application 321 stores acquisition state 328 in assigned state 351. If active tracking application 323 indicates that the intruder's assigned state 351 should be reset to monitor state 326, measure link reliability application 321 reinitializes link reliability score 356 and increments previous acquisition attempt value 355. For every surveillance interval in which no squitter or FRUIT is received, link reliability score 356 is decremented. When link reliability score 356 falls below a minimum value 330, and assigned state 351 equals monitor state 326, measure link reliability application 321 deletes the intruder record 350.

Intruder database 372 stores an intruder record 350 for each detected intruder. Intruder database 372 stores an intruder record 350 associated with each detected intruder. Any number of intruder records 350 may be stored in main memory 320.

Aircraft identifier 382 includes the aircraft identifier information that is encoded in squitter or FRUIT received from a Mode S transponder. When measure link reliability application 321 creates an intruder record 350, the value of aircraft identifier 382 is stored in aircraft identifier 354. To determine if aircraft identifier 382 belongs to an intruder for which an intruder record 350 already exists, aircraft identifier 382 is matched against aircraft identifier 354.

The previous acquisition attempts field 355 is initialized to zero when an intruder record 380 is created, and incremented each time that active tracking application 323 indicates that the intruder's assigned state 351 should be set to monitor state 326.

Link reliability score 356 is initialized when an intruder record 350 is created. Link reliability score 356 is reinitialized when active tracking application 323 indicates that the intruder's assigned state 351 should be set to monitor state 326. Link reliability score 356 is incremented by increment value 357 each time that subsequent squitter or FRUIT is received for the intruder associated with intruder record 350, constraining the resulting link reliability to maximum value 332. Link reliability score 356 is decremented by decrement value 359 each time that an predetermined time interval passes without