# Patent

## Title
Systems and methods for providing ADS-B mode control through data overlay

## Abstract

Embodiments of the present invention disclose systems and methods for providing enhanced features using an ATC Overlay data link. Further, there are provided systems and methods for ADS-B Mode Control that enable a transponder to selectively transmit data on a desired link, such as the ADS-B or ATC Overlay link, based on the control inputs such as current active ADS-B applications, thus reducing RF interference while maximizing the amount of pertinent data being transmitted. In various embodiments, ADS-B Mode Control also offers a mechanism to include pilot-entered data onto the ADS-B or ATC Overlay link, thereby producing flexibility for future ADS-B In applications.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims full benefit of and priority to U.S. Provisional Patent Application 62/030,916 filed Jul. 30, 2014 titled, “Systems and Methods for Providing ADS-B Mode Control” and claims priority to and is a continuation-in-part of U.S. patent application Ser. No. 12/105,248, filed Apr. 17, 2008 titled “Systems and Methods of Providing an ATC Overlay Data Link”; and claims priority to and is a continuation-in-part of U.S. patent application Ser. No. 14/331,089, filed Jul. 14, 2014 titled “Interval Management Using Data Overlay”; the disclosures of which are fully incorporated herein by reference for all purposes.


DESCRIPTION OF THE INVENTION
1. Field of the Invention
The present invention relates to systems and methods for aircraft communication through use of encoded and modulated digital information, and more particularly, to systems and methods for providing a mode control function for ADS-B-based communications.
2. Background of the Invention
Travel by aircraft is generally a safe and efficient way for travelers to reach remote destinations. Over the years, as the popularity of air travel has dramatically increased, the need for techniques for safely managing the flow of aircraft has also risen. To address air traffic safety issues, aircraft have been equipped with avionics equipment such as transponders that assist air traffic controllers in identifying, tracking, and managing aircraft in flight.
Through radio frequency transmissions, transponders provide air traffic controllers and other suitably equipped aircraft with information such as aircraft identification, altitude, and other aircraft-specific data. Ready access to such information allows controllers and pilots to utilize airspace in a safer and more efficient manner. As the density of air traffic grows, it is understandable that there is a growing need for more information to be relayed between aircraft and ground stations on a near-real-time basis.
Currently, FAA Air Traffic Control and most other ATC controlling authorities around the world use standard modulation schemes to ensure interoperability of their radio frequency signals with other aircraft and systems. For example, the Minimum Operational Performance Standards for Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne equipment, promulgated by RCTA as RTCA/DO-181C (and incorporated by reference herein in its entirety) defines pulse position modulation on 1090 MHz for Mode S transponder and older transponder (ATCRBS transponders) replies to 1030 MHz ground station and TCAS interrogations. By using standard protocols aircraft state information as well as other data can be relayed aircraft to ground, ground to aircraft, or in some instances aircraft to aircraft.
The volume of information that must be transmitted from aircraft continues to increase as more advanced avionics and traffic control systems become available. Likewise, the need to transmit diverse information of all kinds also drives the desire to utilize aircraft systems to send data. However, because of the large number of required transponder replies in heavily travelled areas (such as in the vicinity of an airport, where hundreds of replies per second are generated), there are worldwide limits on the number of transponder broadcast transmissions permitted each second from each aircraft. For example, the limit for Automatic Dependent Surveillance Broadcast (ADS-B) is currently set to 6.2 transmissions per second to prevent the additional ADS-B interference from potentially all the aircraft near a major airport creating a situation where the ATC ground station becomes unable to receive surveillance replies from aircraft in the terminal area being controlled by ATC.
ADS-B squitter data content has already been defined for the most part by industry committees such as SC186, and there is little remaining room for future growth. In fact, systems currently envisioned and being developed by avionics systems designers will likely need to transmit more data than can be sent within the 6.2 squitters per second limit. The ability to employ more data in avionic systems is now and will continue to be needed.
As usage for Automatic Dependent Surveillance-Broadcast (ADS-B) grows, the need for additional bandwidth and data will become more important. ADS-B Out, as it is currently defined in DO-260B (which is hereby incorporated herein by reference in its entirety), allows for 6.2 Extended Squitter messages per second. The currently-defined squitter messages provide Aircraft State and Intent data for use by ground stations and other aircraft. With the advent of ATC Overlay (alternatively referred to as “ATC Data Overlay” or as “Phase Enhancement” by Aviation Communication & Surveillance Systems (ACSS), the inventors thereof), more data can be transmitted over the 1090 MHz link, however the decision of what data to send is still open for discussion.
The existing Mode S transponder reply data format (also known as squitters when they are sent unsolicited by an interrogation) is implemented with a pulse position modulation technique, where the position of a pulse determines whether a bit is a one or a zero. Referring to the transmission reply data format and timing diagram 200 in FIG. 2, the first four pulses 203 within the 8 microsecond preamble time 210 are called preamble pulses and are used to determine that the pulse position data that follows is for a Mode S reply (or squitter). ADS-B squitters use the long Mode S reply format and thus contain 112 bits in the data block 220 per squitter. In other applications, 56 bits may be transmitted.
Data is transmitted through digital data encoded in the Data Block 220. A bit interval 202 comprises two sub-intervals defining the logical state of a bit symbol. When a pulse is in the “1” sub-interval position (FIG. 3, 301) of a bit interval 202, that bit value is a 1 and when a pulse is in the “0” sub-interval position (FIG. 3, 302) of a bit interval 202, that bit value is a 0. Only one pulse either in a “0” or a “1” position is permitted for each bit interval or bit symbol period (such as bit interval 202) of the entire message shown 200.
Referring to FIG. 3, an expanded view of bit interval 202 is shown. A carrier wave in the form of a pulse waveform 310 is being transmitted in the “1” position 301, and no pulse is transmitted 315 during the “0” sub interval, and therefore, this bit interval represents the value of logical “1.” Note that the sinusoidal waveform 310 provided in the drawing is for illustration purposes only, and as a standard frequency for ADS-B replies is currently 1090 MHz+/−1 MHz, approximately 545 cycles of the waveform 310 would normally occur during the 0.5 microsecond sub-interval 301. The phase of the waveform 310 is also unimportant for existing transponder reply standards. What is needed is methods and systems to make more efficient use of transponder reply messages to increase data throughput and provide for additional communication links.
SUMMARY OF THE INVENTION
By way of definition, uses of the terms “ATC Data Overlay” and “phase enhancement” are considered interchangeable terms as described herein. Further, the term “ownship” and “own aircraft” refer to an aircraft in which systems and methods of the present invention operate from the perspective of crewmember that is piloting such aircraft, as distinguished from other aircraft that may be external to ownship or own aircraft.
ADS-B Mode Control may provide a mechanism for a transponder to selectively transmit data on a desired link, such as the ADS-B or ATC Overlay link, based on the current active ADS-B applications, thus reducing RF interference while maximizing the amount of pertinent data being transmitted. ADS-B Mode Control also may offer a mechanism to include pilot-entered data onto the ADS-B or ATC Overlay link, offering flexibility for future ADS-B In applications.
Future ADS-B In applications will require additional data be transmitted over a data link, such as ADS-B as defined in DO-260B or ATC Overlay. This additional data may only be relevant while an application is active (e.g., a closely spaced parallel approach, an In-Trail Procedure (ITP), Flight Deck Interval Management (FIM), Surface Indicating & Alerting (SURF IA)). There are provided in various embodiments methods to dynamically select which data may be sent on an ADS-B or ATC Overlay link, which allows for the minimum amount of RF interference, while providing the maximum amount of information through a limited bandwidth.
Various embodiments of “ADS-B In” systems of the present invention are integrated with a TCAS computer (although they may be integrated into any other desired system), which in some instance are further coupled to a Mode S Transponder. Such integrated systems and connections to the transponder can be taken advantage of to provide information to the transponder indicating which ADS-B applications are currently active on-board an ownship aircraft, along with specifics about any other external aircraft. Through embodiments of the present invention, the transponder is provided with knowledge of the surveillance environment and can tailor the data it may transmit on the ADS-B or ATC Overlay link accordingly. In addition, this communication is expandable to include pilot-entered data (through user interfaces such as a Cockpit Display of Traffic Information (CDTI)), which could then be transmitted over ADS-B or an ATC Overlay link (e.g., a desired Flight Level (FL) during an ITP maneuver).
One present method includes a TCAS computer to communicating data in the form of an Application Information File (AIF), which in prior art systems is often transmitted by the TCAS to the CDTI. As mentioned below, the AIF format is a standard data block, defined in ARINC 735B (hereby incorporated by reference in its entirety for all purposes), which provides information surrounding the active ADS-B In applications. The AIF is transmitted through internal circuitry such as a TX 429 bus, and thus the AIF provides a source of control inputs to allow proper selection of signals to be modulated and overlaid for transmission on the transponder. Thus, by analyzing certain control inputs and data inputs, the best use of the available bandwidth is made as signals are selected for modulation on the primary ATC channel and the overlay modulation applied to that channel. Two approaches illustrate embodiments of the ADS-B Mode Control methods:
Closely Spaced Parallel Approach.
During a closely spaced parallel approach, the GPS antenna offset is useful to provide finer granularity and accuracy for aircraft position. In prior systems, GPS antenna offset is currently only transmitted while on the ground in the surface squitters. However, when the transponder determines that the closely spaced parallel approach application is active (e.g., via the AIF file), it may include the GPS antenna offset in the set of data transmitted on a desired link, such as the ADS-B or ATC Overlay link. Additional granularity could also be added to the GPS antenna offset in this mode if needed during the application.
In-Trail Procedure.
During an ITP maneuver, various embodiments of the present invention allow for ownship (e.g., own aircraft) to transmit additional information to either or both an aircraft in front of and an aircraft behind ownship, and such additional information may include, for example, desired flight level (FL). If embodiments of the present invention determine that the ITP application is active, the desired FL could be obtained from, for example, the CDTI & TCAS computer and then the data may be encoded and transmitted a desired link, such as the ADS-B or ATC Overlay link.
Embodiments of the present invention disclose systems and methods for providing enhanced features using an ATC Overlay data link. Further, there are provided systems and methods for ADS-B Mode Control that enable a transponder to selectively transmit data on a desired link, such as the ADS-B or ATC Overlay link, based on the control inputs such as current active ADS-B applications, thus reducing RF interference while maximizing the amount of pertinent data being transmitted. In various embodiments, ADS-B Mode Control also offers a mechanism to include pilot-entered data onto the ADS-B or ATC Overlay link, thereby producing flexibility for future ADS-B In applications.
In one embodiment of the present invention, a method is provided that includes: receiving a plurality of data inputs for transmission through a provided ADS-B transponder; receiving one or more control inputs; analyzing the control inputs and the data inputs to determine a transmission priority for the plurality of data inputs; selecting a first input of the plurality of data inputs based upon the transmission priority and encoding the selected first input to produce a modulated ATC signal; selecting a second input of the plurality of data inputs based upon the transmission priority and encoding the second input as an overlay message; and modulating the modulated ATC signal with the overlay message using an overlay modulation protocol. The overlay-modulated ATC signal may be transmitted through any conventional means, such as from at least one of a TCAS transponder; an ADS-B transponder; an ATC radar; and a ground station. Any desired modulation may be used for the overlay modulation, but in one preferred embodiment, the overlay modulation protocol is phase shift keying modulation.
The data inputs generally comprise information that is intended to be relayed to other aircraft or to ground stations, and examples include a radar altitude; a pressure altitude; an ownship heading; an ownship airspeed; a vertical climb rate; a vertical descent rate; a GPS location of ownship; a GPS antenna offset; a location of a second aircraft other than ownship; an ADS-B reply message; a desired flight level; an aircraft sensor value; a message for transmission to another aircraft; a message for transmission to a ground station; a transponder reply signal; a squitter signal; a 1030 MHz interrogation signal; a 1030 MHz TCAS signal; at least one of a signal within a voice band or a data band operating within an ATC HF, VHF, and UHF band; a VDL Mode 4 ADS-B modulated signal; a DME signal; a SATCOM signal; a signal originating from any avionics equipment that radiates a modulated RF signal; a baseband signal for transmitting data; and a pilot input, and combinations of any of those inputs.
Control inputs may originate from any number of types of data. For example, but not by way of limitation, control inputs may originate from an Application Input File (AIF). In many implementations, the Application Information File (AIF), is transmitted by the TCAS unit to a Cockpit Display of Traffic Information (CDTI) unit. The format of the AIF is a data block, defined in ARINC 735B (the disclosure of which is hereby incorporated by reference in its entirety for all purposes), and this data block includes a multitude of information surrounding the active ADS-B In applications. By transmitting this AIF over the existing standard TX 429 bus, the transponder can use the active ADS-B In application knowledge to provide appropriate data on ADS-B or an ATC Overlay link at the appropriate times. In various embodiments, additional labels could be defined for transmission over the TX bus to carry additional data that may not otherwise be defined in the AIF. Additional control inputs may include a list of ADS-B applications currently active on ownship; location information regarding a second aircraft; velocity information regarding the second aircraft; bearing information regarding the second aircraft; an input from a TX 429 bus; a pilot input; and an input received from a Cockpit Display of Traffic Information (CDTI) unit.
Any applications that are relevant to selection of data sources may be considered as inputs to the selection logic of embodiments of the present invention. For example, the applications identified in the control inputs may include: a closely spaced parallel approach, an In-Trail Procedure (ITP) application; a Flight Deck Interval Management (FIM) application; and Surface Indicating & Alerting (SURF IA) application.
Analysis of the control inputs to determine a transmission priority may be undertaken by any number of analysis techniques, such as comparing control inputs and/or data inputs to values in a table; comparing thresholds of control inputs to predetermined threshold values stored in the Data Selection & Link Assignment Logic function; analyzing control inputs and data inputs to determine a priority order most likely to minimize danger to ownship or an external aircraft (such as indications of imminent danger); analyzing applications to identify a high priority data request such as a request to change a flight level in an ITP scenario and raking such data request as top priority; ranking data streams based on desired timing of transmission; or any other method of priority scheme determination.
Additionally, in various embodiments, analyzing the control inputs and the data inputs to determine a transmission priority for the plurality of data inputs may further include determining that an a closely spaced parallel approach application is active; and selecting either a first input of the plurality of data inputs or a second input of the data inputs further comprises selecting a GPS antenna offset data value.
In another embodiment, analyzing the control inputs and the data inputs to determine a transmission priority for the plurality of data inputs further comprises determining that an In-Trail Procedure (ITP) application is active; and selecting a first input of the plurality of data inputs or a second input of the plurality of data inputs further comprises selecting a desired flight level input.
According to certain embodiments, a method can provide interval management by encoding an overlay message onto a provided modulated air traffic control (ATC) signal. The method can include employing a selected overlay modulation protocol. The method can also include modulating the provided modulated ATC signal with the overlay message using the selected overlay modulation protocol. The overlay message can be configured to provide interval management with respect to a flight path of a target aircraft. The ATC signal can be independently demodulatable from the overlay message, and in various embodiment, the ATC signal modulated with the overlay message can be recognizable as an ATC signal by legacy ATC equipment.
In certain embodiments, a method can create a data link for interval management. The method can include encoding a first data stream into an avionics signal using a first modulation protocol to create a modulated avionics signal. The method can also include modulating the modulated avionics signal with a second data stream using a second modulation protocol to create an overlay-modulated signal. The second data stream can be configured to provide interval management with respect to a flight path of a target aircraft. The method can also include transmitting the overlay-modulated signal through a transponder. The method can further include receiving the overlay-modulated signal in a receiver. The method can additionally include extracting the second data stream from the overlay-modulated signal by using a second demodulation protocol. The method can also include extracting the first data stream from the overlay-modulated signal by using a first demodulation protocol.
A method, in certain embodiments, can create a data link for interval management. The method can include encoding a first data stream into an avionics signal using a first modulation protocol to create a modulated avionics signal. The method can also include modulating the modulated avionics signal with a second data stream using a second modulation protocol to create an overlay-modulated signal, wherein the second data stream is configured to provide interval management with respect to a flight path of a target aircraft. The method can further include transmitting the overlay-modulated signal through a transponder to enable control an aircraft with respect to the target aircraft.
A method of interval management, according to certain embodiments, can include obtaining a spacing goal for an aircraft relative to a target aircraft. The method can also include determining clearance instructions for the aircraft, wherein the speed guidance is based on the spacing goal. The method can further include transmitting the clearance instructions in a computer-readable format to the aircraft, wherein the instructions are provided by an overlay-modulated signal of a provided modulated air traffic control (ATC) signal, and wherein the instructions are configured to enable control of the aircraft to achieve the spacing goal.
The overlay data may comprise any information desired to be transmitted, including but not limited to additional or supplementary ATC information. Existing transponder and ground station frequencies may be utilized for the transmission, and the signals modulated with the overlay data may be received by any receiver, including but not limited to receivers in aircraft and ground stations. The modulation protocols or types utilized in both in the primary ATC signal and the overlaid signal are selectable from any suitable modulation schemes, but those of skill in the art appreciate that modulation protocols utilized in various embodiments of the invention may be selected to be a non-varying modulation, including, but not limited to fixed modulation protocols. As used herein, the term “overlay modulation” includes modulating a signal that has previously been modulated, including chases where a single or a plurality of modulations were previously applied to the signal.
Embodiments provide a method for encoding an overlaid message onto a provided modulated ATC signal, the method comprising selecting an overlay modulation protocol; and modulating the provided modulated ATC signal with an overlay message using the selected overlay modulation protocol. The provided modulated ATC signal may be modulated with any protocol such as a pulse position modulation protocol, and the overlay modulation protocol may be any protocol such as phase shift keying modulation. When phase shift keying modulation is utilized, phase states may be assigned using Gray Code to further reduce bit error. In one embodiment, modulating the provided modulated ATC signal with an overlay message using the selected overlay modulation protocol further comprises applying the overlay modulation to one or more modulated message bit intervals within a data block in the modulated ATC signal. In various embodiments where the overlay modulation comprises a PSK modulation protocol, a phase transition in a carrier signal in a bit interval is modulated into the carrier, and such phase transition may occur at any location in the carrier signal in the bit interval. In one embodiment, such phase transition may occur in the carrier signal proximate to a mid-point of the bit interval, or proximate an initial point of the bit interval. A plurality of phase transitions may also be modulated into the carrier signal within any particular bit interval. The location of a phase transition within a bit interval may be selected to optimize spectral performance, noise performance, or any other criterion.
The ATC modulation protocol and the overlay modulation protocol may comprise any single or combination digital modulation scheme, including, but not by way of limitation any of the following modulation protocols: binary phase shift keying modulation (BPSK); quadrature phase shift keying modulation (QPSK); 8-phase shift keying modulation (8-PSK); differential phase-shift keying (DPSK); DNPSK modulation, where N is an even integer and a multiple of 2; frequency shift keying (FSK); amplitude shift keying (ASK); quadrature amplitude modulation (QAM); orthogonal frequency-division multiplexing (OFDM); minimum-shift keying (MSK); asymmetric phase-shift keying, (APSK); pulse position modulation (PPM); amplitude modulation (AM); frequency modulation (FM); or any other modulation form using amplitude, phase, or frequency signal characteristics and methods of use; and combinations of any of those protocols. Single or multiple modulation operations may be performed on the ATC signal to encode additional overlay information. The modulated ATC signal may be a standard ATC format as described above, or may comprise any number of signal types alone or in combination, such as a transponder reply signal; a squitter signal; an ADS-B reply signal; a 1030 MHz interrogation signal; a 1030 MHz TCAS signal; a General Aviation UAT transceiver signal on a UHF frequency band; at least one of a signal within a voice band or a data band operating within an ATC HF, VHF, and UHF band, a VDL Mode 4 ADS-B modulated signal; DME; SATCOM; or any other avionics equipment that radiates a modulated RF signal; a baseband signal for transmitting data; and combinations of those signal types.
The overlay data may be encoded, encrypted, supplemented, interleaved, or otherwise processed before or after overlay modulation occurs. Such processing may be intended to reduce bit error rates by providing supplemental check bits, parity bits, CRC bits, Reed-Solomon bit error detection and correction coding or data, or other information to conduct error checking or error correction coding. Interleaving may be used to spread overlay data or data symbols across multiple bit intervals within a transponder reply or squitter message, or across multiple reply or squitter messages in order to reduce the effect of burst noise on the bit error rate of the transmitted message.
In another embodiment, a method for decoding an overlaid message from an overlay modulated signal is provided. The method includes demodulating the overlay modulated signal with a first modulation protocol to produce a first message; demodulating the overlay modulated signal with a second modulation protocol to produce an overlay message; wherein the first message and the overlay message may be independently demodulated from the overlay modulated signal.
The first or second modulation protocol may comprise any single or combination digital modulation scheme, including, but not by way of limitation any of the following modulation protocols: binary phase shift keying modulation (BPSK); quadrature phase shift keying modulation (QPSK); 8-phase shift keying modulation (8-PSK); differential phase-shift keying (DPSK); DNPSK modulation, where N is an even integer and a multiple of 2; frequency shift keying (FSK); amplitude shift keying (ASK); quadrature amplitude modulation (QAM); orthogonal frequency-division multiplexing (OFDM); minimum-shift keying (MSK); asymmetric phase-shift keying, (APSK); pulse position modulation (PPM); amplitude modulation (AM); frequency modulation (FM); or any other modulation form using amplitude, phase, or frequency signal characteristics and methods of use; and combinations of any of those protocols.
Embodiments of the present invention provide that the received overlay data may be decoded, decrypted, supplemented, de-interleaved, or otherwise processed after receipt. Such processing may be intended to reduce bit error rates by providing supplemental check bits, parity bits, CRC bits, Reed-Solomon bit error detection and correction, or other information to conduct error checking or error correction coding. De-interleaving may be used to extract overlay data from multiple bit intervals within a transponder reply or squitter message, or across multiple reply or squitter messages in order to reduce the effect of burst noise on the bit error rate of the transmitted message. Further, additional demodulations may occur to extract additional overlaid data from the received signal. These additional demodulations may use any modulation protocol as defined herein.
When the overlay modulation is phase shift keying-type (PSK) modulation, recovery of encoded data phases may be complicated by fluctuations or drift in the phase of the carrier frequency. Normally, with ATCRBS or ADS-B type replies, the phase of the carrier does not matter as long as the pulse position is correctly modulated. To obtain an accurate data phase and reduce bit errors of the PSK, embodiments of the present invention compensate for the phase drift whether synchronous or differential PSK techniques are utilized. In one embodiment, the second modulation protocol comprises phase shift keying; and a phase error is subtracted from a signal phase to produce a data phase. The phase error may be computed by identifying one or more preamble pulses within the overlay modulated signal; identifying one or more data pulses within the overlay modulated signal; determining one or more phases of a carrier frequency within at least one of the one or more preamble pulses and the one or more data pulses; and comparing the one or more phases of the carrier frequency to a predetermined frequency to compute a phase error. In the case of a differential PSK modulation, embodiments of the present invention provide that the second modulation protocol comprises differential phase shift keying; and a first phase of a first carrier wave pulse within a first bit interval is computed by comparing the first phase of the first carrier wave pulse within the first bit interval to a second phase of a second carrier wave pulse within a second bit interval.
In another embodiment, a method of the present invention provides for transmission and receipt of overlay data through an ATC-type response or squitter signal. Embodiments provide an overlay data link through a provided ATC signal modulated with a first modulation protocol, through a method comprising: modulating the provided modulated ATC signal with an overlay message using a second modulation protocol to produce an overlay modulated signal; transmitting the overlay modulated signal by an ATC transponder; receiving the overlay modulated signal at a receiver such as with a TCAS receiver; and extracting an overlaid message from an overlay modulated signal, wherein a first message is obtained by demodulating the overlay modulated signal with the first modulation protocol; and the overlay message is retrieved from the overlay modulated signal by demodulating the overlay modulated signal with the second modulation protocol. As described previously, the first and overlay modulation protocols may comprise any type of digital modulation schemes, such as one embodiment where the first modulation protocol is pulse position modulation and the overlay modulation protocol is phase shift keying modulation. Alternatively, the first or second modulation protocol may comprise any single or combination digital modulation scheme, including, but not by way of limitation any of the following modulation protocols: binary phase shift keying modulation (BPSK); quadrature phase shift keying modulation (QPSK); 8-phase shift keying modulation (8-PSK); differential phase-shift keying (DPSK); DNPSK modulation, where N is an even integer and a multiple of 2; frequency shift keying (FSK); amplitude shift keying (ASK); quadrature amplitude modulation (QAM); orthogonal frequency-division multiplexing (OFDM); minimum-shift keying (MSK); asymmetric phase-shift keying, (APSK); pulse position modulation (PPM); amplitude modulation (AM); frequency modulation (FM); or any other modulation form using amplitude, phase, or frequency signal characteristics and methods of use, and combinations of any of those protocols.
There is also provided an ATC overlay data link system. One embodiment comprises a first modulator, the first modulator configured to modulate ATC data into a first modulated signal through a first modulation protocol; a second modulator coupled to the first modulator, the second modulator configured to modulate an overlay message into the first modulated signal using a second modulation protocol to produce an overlay modulated signal; a transponder comprising a transmitter and an antenna wherein the transponder is coupled to the second modulator and the transponder is configured to transmit the overlay modulated signal; and a TCAS receiver coupled to an antenna, a first demodulator and a second demodulator, wherein the first demodulator is configured to extract the ATC data by demodulating the overlay modulated signal with the first modulation protocol; and the second demodulator is configured to extract the overlaid message from an overlay modulated signal.
As described previously, the first and overlay modulation protocols may comprise any type of digital modulation schemes, such as one embodiment where the first modulation protocol is pulse position modulation and the overlay modulation protocol is phase shift keying modulation. Alternatively, the first or second modulation protocol may comprise any single or combination digital modulation scheme, including, but not by way of limitation any of the following modulation protocols: binary phase shift keying modulation (BPSK); quadrature phase shift keying modulation (QPSK); 8-phase shift keying modulation (8-PSK); differential phase-shift keying (DPSK); DNPSK modulation, where N is an even integer and a multiple of 2; frequency shift keying (FSK); amplitude shift keying (ASK); quadrature amplitude modulation (QAM); orthogonal frequency-division multiplexing (OFDM); minimum-shift keying (MSK); asymmetric phase-shift keying, (APSK); pulse position modulation (PPM); amplitude modulation (AM); frequency modulation (FM); or any other modulation form using amplitude, phase, or frequency signal characteristics and methods of use; and combinations of any of those protocols.
Embodiments of the present invention provide that the system may encode/decode, encrypt/decrypt, supplement, interleave/de-interleave, or otherwise process data for overlay and data extraction. Such processing may be intended to reduce bit error rates by providing supplemental check bits, parity bits, CRC bits, Reed-Solomon Bit Error Detection and Correction, or other information to conduct error checking or error correction coding. Interleaving/De-interleaving may be used to extract overlay data from multiple bit intervals within a transponder reply or squitter message, or across multiple reply or squitter messages in order to reduce the effect of burst noise on the bit error rate of the transmitted message. Further, additional demodulations may occur to extract additional overlaid data from the received signal. These additional demodulations may use any modulation protocol as defined herein.
There is further provided an overlay data link embodiment. The overlay data link comprises a first modulator, the first modulator configured to modulate a first data stream data into a first modulated signal through a first modulation protocol; a second modulator coupled to the first modulator, the second modulator configured to modulate an overlay message into the first modulated signal using a second modulation protocol to produce an overlay modulated signal; a transmitter, wherein the transmitters is coupled to the second modulator and the transmitter is configured to transmit the overlay modulated signal; and a receiver, a first demodulator and a second demodulator, wherein: the first demodulator is configured to extract first data stream by demodulating the overlay modulated signal with the first modulation protocol; and the second demodulator is configured to extract the overlaid message from an overlay modulated signal. In one embodiment of the overlay data link, the first modulation protocol is pulse position modulation and the overlay modulation protocol is phase shift keying modulation. The first and second modulation protocols may comprise any appropriate amplitude, phase, and frequency modulation, including, but not limited to binary phase shift keying modulation (BPSK); quadrature phase shift keying modulation (QPSK); 8-phase shift keying modulation (8-PSK); differential phase-shift keying (DPSK); DNPSK modulation, where N is an even integer and a multiple of 2; frequency shift keying (FSK); amplitude shift keying (ASK); quadrature amplitude modulation (QAM); orthogonal frequency-division multiplexing (OFDM); minimum-shift keying (MSK); asymmetric phase-shift keying, (APSK); pulse position modulation (PPM); amplitude modulation (AM); frequency modulation (FM); and combinations thereof.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a block diagram of an embodiment of a system of the present invention.
FIG. 2 illustrates a standard prior art transponder reply data format and timing diagram.
FIG. 3 depicts an expanded view of an exemplary bit interval of the reply data format, with a logical data value “1” pulse encoded.
FIG. 4 shows one bit interval of modulated ATC signal with overlay data, where the carrier has been phase shifted ninety degrees, the phase shift occurring at or near the beginning of the bit interval.
FIG. 5 shows one bit interval of modulated ATC signal with overlay data, where the carrier has been phase one hundred eighty degrees, the phase shift occurring approximately at the midpoint of a carrier within a sub-bit interval.
FIG. 6 shows one bit interval of modulated ATC signal with overlay data, where the carrier has been phase shifted ninety degrees, and frequency drift has affected the carrier.
FIG. 7 illustrates one and one half bit intervals, illustrating a merged carrier signal from a bit value of 0 from a previous bit interval, and a bit value of 1 from a following bit interval with midpoint one hundred eighty degree phase shifts.
FIG. 8 shows an exemplary modulator of the present invention.
FIG. 9 illustrates an exemplary demodulator of the present invention.
FIG. 10 shows a constellation diagram and corresponding Gray Code symbol assignments for an 8-PSK modulation scheme of the present invention.
FIG. 11 illustrates a block diagram showing one exemplary system configuration of the present invention.
FIG. 12 shows a functional block schematic representation of aspects of the present invention.
FIG. 13 illustrates a flowchart for one embodiment of a method of the present invention.



DESCRIPTION OF THE EMBODIMENTS
Reference will now be made in detail to the present exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.
Embodiments of the present invention provide methods for overlaying additional data on top of existing ATC data without affecting existing ATC transponder/TCAS/Ground Station/Multilateration or other existing equipment. Embodiments of the present invention may use any form of modulation overlaid on top of existing ATC data, and depending on implementation, the overlaid data may be undetectable by equipment that processes legacy ATC signals. This permits the use of existing equipment within the ATC environment, and the use of new equipment that can be either backwards compatible and/or independent of ATC existing data formats permitting more data throughput efficiency using the same ATC bandwidth and frequency assignments of 1030 and 1090 Mhz. Embodiments of the present invention utilize a transponder reply data format and timing diagram similar to that shown in FIG. 2. However, as discussed in more depth below, embodiments of the present invention provide additional modulation within each pulse interval 202 that may not be detected by existing ATC equipment or methods, but can provide additional data bits within the same reply.
Turning to FIG. 1, a block diagram 100 illustrates an embodiment of a system of the present invention. A primary ATC data stream 103 is input to an encoder and modulator 105 that produces a modulated signal (such as by pulse position modulation) that complies with existing standards such as an ADS-B squitter or reply transmissions (see, e.g., FIG. 2). Data 104 that is desired to be transmitted through an overlay data link is encoded 110, if desired, with information such as parity bits, CRC (cyclic redundancy check) codes, encryption keys, or any other information or encodings that are desired to be applied to the overlay data. The encoded overlay data is then further 115 modulated into the modulated ATC stream 115 in a manner that provides a reply/squitter signal that is compatible with existing hardware yet contains information in excess of that defined by current transponder standards. One exemplary method to achieve such overlay modulation compatibility is to initially modulate the ATC signal with a pulse position modulation that is acceptable by currently deployed hardware, and then apply the overlay modulation with a technique such as phase shift keying “PSK.” PSK is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (e.g., the carrier wave), as opposed to selectively applying a pulse to bit symbol times as is utilized in pulse position modulation. Since varying the phase of the carrier signal within each defined time frame in an ATC-encoded reply signal does not impact the ability of legacy hardware to decode the original ATC data message, the PSK modulation is nondestructive and may be independently demodulated. Put another way, the twice-modulated signal 117 carries the ATC data 103 that is modulated and directly decodable by conventional PPM techniques, in addition to additional data 104 that has been overlaid on the modulated signal in a non-destructive manner, such as by PSK modulation.
The modulated signal 117 may then be transmitted 120 and received by a receiver 125 that is configured to receive ATC transponder reply transmissions (such as a ground station or another transponder in a suitably equipped aircraft). In various embodiments, the transmitter 120 and the receiver 125 may comprise any electronic equipment capable of sending and/or receiving RF signals, including, but not limited to ATC radars, TCAS transponders, ADS-B transponders, and ground stations of any type. In one embodiment, ADS-B ground stations can receive and transmit ATC Overlay messages on top of various outputs such as Traffic Information Service Broadcasts (TIS-B, Automatic Dependent Surveillance Re Broadcasts (ADS-R).
The received signal is then demodulated 135 by PPM demodulation 135 to recover the original primary ATC data 103 after any necessary decoding. The received signal is also demodulated 130 to obtain the overlay data 104, which may be presented in several channels after any necessary decoding. Therefore, the twice-modulated signal 117 may be demodulated with multiple techniques independently and each data stream (103, 104) respectively independently recovered.
Any form of modulation may be overlaid on top of any ATC-modulated signals, whether such modulation currently exists or is created at a future date. In alternate embodiments, additional data may be overlaid by modulation on top of an overlaid modulation as well, and this modulation is not limited to modulation that is undetectable by existing equipment. Further, the modulation is not limited to 1090 MHz squitters and replies, but can also be used on 1030 MHz (currently interrogation by radars and TCAS), on the General Aviation UAT transceiver UHF frequency band, ATC HF, VHF and UHF voice and data bands, on the VDL Mode 4 ADS-B modulation used for ADS-B in limited locales within Europe, DME; SATCOM; or any other avionics equipment that radiates a modulated RF signal and at baseband for ATC terminal or other networks currently sending data over landline or microwave links. Thus any form of ATC modulation that is used can have an overlaid modulation applied to it at any frequency. This can also include, for example, overlaid modulation on a modulation applied to Air Transport airborne weather radar for communication links. Therefore, embodiments of the present invention provide that any overlay modulation type (whether interfering in some manner or not) may be overlaid on or added to or encrypted with any modulation type for whatever purpose desired, such as, but not limited to ATC applications, any commercial data relay purposes, low probability of intercept applications, low probability of detection techniques, low probability of spoofing purposes, and encryption.
Embodiments of the present invention may employ phase shift keying (PSK) modulation within each of the 112 bit pulses in the data block 220. Turning to FIG. 4, an expanded view of one exemplary bit interval 202 of the data block 220 is shown, with an ATC data value of logical “1” (carrier pulse 400 is present in “1” sub-interval position, 301) and the phase shifted carrier signal 400 is overlaid with phase modulation by phase shift 410 of ninety degrees. The phase shift in this embodiment occurred at or near the beginning 403 of the logical “1” sub-interval position 301. The original unshifted carrier signal 310 is shown for comparison. Depending on the particular PSK implementation, this phase shift may correspond to one bit or several bits of overlay data. Note that the sinusoidal waveform 310 provided in the drawing is for illustration purposes only, and as a standard frequency for ADS-B replies is currently 1090 MHz+/−1 MHz, approximately 545 cycles of the waveform 310 would normally occur during the 0.5 microsecond sub-interval 301. As the applicable ATC standards do not care about the phase of the carrier wave, the presence of an acceptable frequency and amplitude waveform in the appropriate bit sub-interval position 301 is all that is needed to provide the logical “1” originally encoded.
Phase information can then be sent with a differential phase between each pulse representative of a plurality of states used to represent a plurality of bits. Each phase difference is detected by examining the phase of a previous pulse to the phase of the next pulse to eliminate the effects of relative aircraft motion. For the first phase reference pulse relative to Bit 1 case, any of the preamble pulses 203 may be used, and from then forward each previous bit pulse phase can be used as the reference for the next bit phase.
Other PSK methods can be used to send data such as a synchronous transmission of phase, where comparison to a reference that is synchronized in phase to the incoming reply signal modulation is used to determine a phase value for each pulse. The preamble pulses of the Mode S reply message can be used to synchronize a reference oscillator source to the incoming message RF signal using, for instance, a phase-locked-loop oscillator or a Costas loop that can be locked to the incoming signal phase and frequency, which is then used as a phase reference for all the data bits.
In legacy ATC systems, the frequency of the signal being transmitted are in the range of 1090 MHz plus or minus 1 MHz, and drift of the phase of the carrier signal is generally not of concern provided the signal envelopes within predetermined timing intervals is within established ranges, such as those set forth in RTCA standard DO-181C. To recover a phase shift encoded within the broadcast carrier signal, embodiments of the invention provide for obtaining the data phase by synchronous or asynchronous means. Turning to FIG. 6, original carrier 310 (shown only partially for clarity) would have undergone a frequency drift 610, producing a new drifted-phase carrier 612. Without knowledge of the amount of drift 610, the exact phase of the phase-shifted carrier 400 may be difficult to determine. In one embodiment, phase drift 610 is accounted for via phase synchronous means but with the frequency asynchronous, for instance by utilizing an integrator and accumulating and storing the phase drift as a phase error signal that can be subtracted out from the phase of the current signal. In another illustrative embodiment, after a predetermined amount of data pulses are received (for instance 7 pulses), a local oscillator tracks the received signal via a Costas loop, allowing frequency and phase synchronization. The local oscillator is then driven with an offset frequency to maintain synchronization. Through this approach, interfering signals with different frequencies can be distinguished through use of a matched filter, and through use of matched filters with synchronized frequency and phase, noise performance can be improved significantly, for instance by about 3 dB.
Aircraft motion and signal-to-noise-ratio, as mentioned above, may be taken into account when determining an acceptable bit error rate (BER). For instance, if two aircraft, one transmitting a reply and the one receiving the reply are traveling towards one another at a rate of 1200 knots, which is 2000 ft. per second, the relative velocities can significantly affect the phase error seen by the receiving aircraft. Since the wavelength in free space is about 1 foot at ATC 1030 MHz and 1090 MHz frequencies, a phase error of about 2000 ft./sec.×360 degrees phase×112 usec (112×10−6 seconds per Mode S reply message length)=80 degrees phase error. Since the 80 degrees of error is spread over the entire Mode S message and each phase can either be differentially compared from the previous to the next bit or synchronized out as described above, the phase error between each bit then is about 80 degrees/112 bits per microsecond=0.71 degrees phase error per bit. Thus, any encoding scheme that can tolerate a phase error of 0.71 degrees between each bit is realizable, but is also affected by the signal to noise ratio to accurately measure phase. For a reasonable signal to noise ratio that exceeds 10 db, a PSK encoding scheme that provides 360 states or 1 degree per state could be tolerated. Usually a power of two is used for binary encoding, so 256 (28) states could be used to provide 8 bits of data per PPM pulse.
To decrease the effects of noise on signal to noise ratio, embodiments of the present invention use a D8PSK (Differential 8-state Phase Shift Keying) modulation scheme. Referring to FIG. 10, each of 8 states is shown from 0 degrees through 360 degrees with each state separated by 45 degrees representing 3 bits. This provides 112 bits×3=336 bits per message additional to the 112 bits of the original Pulse Position Modulation (PPM) Mode S reply message. Thus, if an additional 3 bit message is sent for each of 6.2 squitters per second, a total of 336 bits per message×6.2 messages=2083 additional bits can be sent via the overlay modulation. Therefore, by modulating each pulse-bit whether in the zero or one position, with D8PSK modulation, a new data link is then created with 3 bits of data for every previous PPM bit. In one embodiment, the new data link would not be detected by existing ATC TCAS and transponder equipment unless it is equipped to detect the D8PSK modulation, ensuring backward compatibility with existing systems.
In various embodiments, states can be encoded to reduce the number of bit errors per symbol. Other schemes using additional parity bits to produce symbols that are completely orthogonal can be used to provide additional interference immunity.
In alternate embodiments, modulation schemes providing more bits per second such as D16PSK (4 bits per message bit) or D32PSK (5 bits per message bit) can be used depending on the amount of noise immunity versus data rate required. Secondary modulation schemes may also be used in conjunction with error correction and control schemes in order to minimize bit error rate and correspondingly increase signal to noise ratios in noisy environments. Also, bits can be encoded into states that only permit one bit change per adjacent state change (Grey Code). This can reduce bit errors to only one bit for changes between adjacent states and helps with noise and interference immunity, as discussed later.
Additional techniques can be used to minimize frequency spectral power effects or bandwidth required to accommodate the additional modulation. For instance, when two pulses are positioned next to one another, such as shown in FIG. 7, and in the case of a sudden change in phase 720 in the first pulse 705 to a different phase in the next pulse 710, an amplitude notch may occur between pulses affecting the bandwidth of the reply. However, if the phase is slowly varied between the middle of the first pulse to the middle of the second pulse, the phase variation occurs over a greater time period and the amplitude notch can be made to be very small, permitting compliance to existing bandwidth requirements for Mode S replies and having no effects upon existing equipment in the field. The phase information can then be read in the beginning of the pulse for the first bit interval (before any phase changes to the next pulse) and near the end of the second pulse for the second bit interval (after a phase change from the first to the second pulse is complete) when two pulses have merged into one, permitting the use of this MSK (Minimum Shift Keying) phase change technique. This approach also can be adapted for significant or multiple intra-sub-interval phase shifts, if embodiments provide for multiple phase shifts per sub-interval.
MSK type of modulation phase change across the pulse will further reduce any spectral effects. The worst case spectral effect is for a phase shift from 0 to 180 degrees, and could be accommodated and tested to ensure compliance with existing industry specifications for a Mode S reply. Spectral and amplitude specifications for Mode S reply emissions can currently be found in RTCA DO-181C.
Additional modulation types with varying degrees of BER, detection gain, and bit throughput can be used. These include, but are not limited to: general phase shift keying modulations; binary phase shift keying modulation (BPSK); quadrature phase shift keying modulation (QPSK); 8-phase shift keying modulation (8-PSK); differential phase-shift keying (DPSK); DNPSK modulation, where N is an even integer and a multiple of 2; frequency shift keying (FSK); amplitude shift keying (ASK); quadrature amplitude modulation (QAM); orthogonal frequency-division multiplexing (OFDM); minimum-shift keying (MSK); asymmetric phase-shift keying, (APSK); pulse position modulation (PPM); amplitude modulation (AM); frequency modulation (FM); or any other modulation form using amplitude, phase, or frequency signal characteristics and methods of use; and combinations of modulation techniques.
In principle, any modulation type can be used, but should be constrained to meet DO-181C amplitude and spectral requirements such that existing ATC equipment is not affected, but new equipment can transmit and detect the overlaying modulation. In addition, some form of bit error detection and correction can be used such as that presented in RTCA DO-181C, Mode S Minimum Operational Performance Specification, and RTCA DO-185A, TCAS Minimum Operational Performance Specification, as an example to improve message reception.
A more suitable means of bit error detection and correction for Phase Shift Keying modulation can, for example, include the use of additional bits to provide orthogonal symbol (cross correlation is zero for one bit errors) encoding with zero correlation between symbol sets (bit patterns between symbols are significantly different by several bit states). For example, the bit values can be represented, as shown below in Table 1 (for two bits to simplify the discussion). Note that this method may require that more bits be used within each symbol to represent the lesser number of bit values (or states), so that a more unique bit pattern is transmitted and received, and can be corrected in the presence of noise or interference with a higher degree of certainty.










TABLE 1







Bit Values
Symbol Data









00
0000



01
0101



10
1100



11
1001










Another embodiment of the present invention addresses limiting the effect of burst errors, such as for a single interfering pulse where the “BIT 1” pulse position of the Mode S reply has been overlaid by a stronger in amplitude interfering pulse. This situation could destroy an entire 3 bit symbol for the case of D8PSK (3 bits per symbol). However, if each D8PSK bit value is sent as only one bit of 3 separate messages, then only one bit of each message may be affected, making it possible for 2 of the messages to remain uncorrupted. For example, from Table 1, the last bit of the Symbol Data for Bit Value 10 could be corrupted into a 1, resulting in the sequence from Table 1 to be a 1101 instead of a 1100. Then, the 1101 is incorrect, since it is not a valid symbol and by knowing which bit has been corrupted from the interfering pulse position it is possible to determine that 1100 is the correct symbol for a one bit error, since none of the other symbols are of the form “11XX”.
Non-ideal communications channels, noise, atmospheric conditions, interference, or other phenomena may induce errors into the message being communicated, whether or not the transmitted message has been overlaid with additional encoded information. In various embodiments of the present invention, conventional error reduction or correction approaches may be applied to reduce bit error rate, such as utilization of a parity overlay or CRC check scheme as described in RTCA/DO-181C, Reed-Solomon bit error detection and correction, or other error conventional detection and correction schemes.
Embodiments of the present invention include the use of a Gray-Code for each phase state transmitted which is most useful for the case where an interfering pulse is near to the same amplitude as the data pulse and cannot be detected, use of a four bit message symbol where only one parity bit is used for every three message bits to detect and correct single bit errors (as opposed to traditional 8 bit symbols where half the bits are parity bits), interleaving of message bit symbols to limit the number of errors per message symbol due to sequential interfering pulses so that most errors can be corrected (after de-interleaving) by changing the state of a single bit, and using a parity overlay encoding scheme on top of the last 24 bits of the address of the intended recipient for point to point messaging or with an all ones address for broadcast messages as is done now for Automatic Dependent Broadcast messages (ADS-B) as described in RTCA DO-260A ADS-B MOPS in conjunction with the error detection and correction algorithm as described in RTCA DO-185A TCAS MOPS. Alternate embodiments provide methods for bit error detection and correction, and may be more efficient in terms of the number of phase message bits that can be corrected per each 112 bit PPM Mode S reply.
FIG. 11 shows a block diagram 1100 of an embodiment of the present invention. Three central components, a TCAS computer unit 1105, a selection logic and overlay encoding module 1110, and a mode S transponder 1120, are coupled together either directly or through buses 1150A and 1150B. In various embodiments, the TCAS computer unit 1105 may be physically remote from the mode S transponder 1120, and/or the selection logic block 1110; or in another embodiment all three blocks 1105, 1110, 1120 are integrated within one electronics assembly. Various inputs 1130 are provided to the system 1115 and may include a wide variety of inputs in the form of data to be transmitted, and control inputs to be analyzed to determine which of the data inputs should be modulated and transmitted by the transponder 1120. Among the inputs may be data directly received from a Cockpit Display of Traffic Information (CDTI) 1140, which may comprise pilot- or crew-entered information, in addition to information regarding active applications. Information provided to the system 1115 may include an AIF file that is relayed by a data bus as explained above, and which contains details of active applications that may require data to be transmitted.
FIG. 12 shows a further elaborated view of a system embodiment of the present invention, with aspects similar to that shown in FIG. 1. Data selection and link assignment logic 1110, accepts inputs from various items of data 1230, as well as control data items 1220. The data selection of link assignment logic analyzes the control data inputs 1220, as well as the data items 1230, to determine which of the data inputs 1230, should be selected as either a primary ATC data signal, or an overlaid data signal. Although two outputs are shown in FIG. 12, that is a primary ATC data for transmission signal 103, and a selected overlay data signal for transmission 104, embodiments of the present invention may utilize two or more overlaid signals to allow additional data to be transmitted by the transponder 120.
Any appropriate method may be utilized to perform prioritization of the data inputs 1230 that are to be selected for encoding and overlay. Analysis of the control inputs to determine a transmission priority may be undertaken by any number of analysis techniques, such as comparing control inputs and/or data inputs to values in a table; comparing thresholds of control inputs to predetermined threshold values stored in the Data Selection & Link Assignment Logic function; analyzing control inputs and data inputs to determine a priority order most likely to minimize danger to ownship or an external aircraft (such as indications of imminent danger); analyzing applications to identify a high priority data request such as a request to change a flight level in an ITP scenario and raking such data request as top priority; ranking data streams based on desired timing of transmission; or any other method of priority scheme determination. In the illustrated embodiment, two of the data inputs 1230 are selected for transmission, one of which is a primary ATC data input 103, and the other is a data input that will be applied as an overlay for data transmission 104. Similarly to FIG. 1, the two respective signals are respectively encoded/modulated 105, 110, and then the overlay data stream modulated 115 with the ATC data stream, to produce an ATC signal which is then transmitted 120 through hardware such as a Mode S transponder.
A primary ATC data stream 103 is input to an encoder and modulator 105 that produces a modulated signal (such as by pulse position modulation) that complies with existing standards such as an ADS-B squitter or reply transmissions (see, e.g., FIG. 2). Data 104 that is desired to be transmitted through an overlay data link is encoded 110, if desired, with information such as parity bits, CRC (cyclic redundancy check) codes, encryption keys, or any other information or encodings that are desired to be applied to the overlay data. The encoded overlay data is then further 115 modulated into the modulated ATC stream 115 in a manner that provides a reply/squitter signal that is compatible with existing hardware yet contains information in excess of that defined by current transponder standards. One exemplary method to achieve such overlay modulation compatibility is to initially modulate the ATC signal with a pulse position modulation that is acceptable by currently deployed hardware, and then apply the overlay modulation with a technique such as phase shift keying “PSK.” PSK is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (e.g., the carrier wave), as opposed to selectively applying a pulse to bit symbol times as is utilized in pulse position modulation. Since varying the phase of the carrier signal within each defined time frame in an ATC-encoded reply signal does not impact the ability of legacy hardware to decode the original ATC data message, the PSK modulation is nondestructive and may be independently demodulated. Put another way, the twice-modulated signal 117 carries the ATC data 103 that is modulated and directly decodable by conventional PPM techniques, in addition to additional data 104 that has been overlaid on the modulated signal in a non-destructive manner, such as by PSK modulation.
The modulated signal 117 may then be transmitted 120 and received by a receiver 125 that is configured to receive ATC transponder reply transmissions (such as a ground station or another transponder in a suitably equipped aircraft). In various embodiments, the transmitter 120 and the receiver 125 may comprise any electronic equipment capable of sending and/or receiving RF signals, including, but not limited to ATC radars, TCAS transponders, ADS-B transponders, and ground stations of any type. In one embodiment, ADS-B ground stations can receive and transmit ATC Overlay messages on top of various outputs such as Traffic Information Service Broadcasts (TIS-B, Automatic Dependent Surveillance Re Broadcasts (ADS-R).
FIG. 13 illustrates a flow chart 1300 of an embodiment of the present invention. Data inputs are received 1310 for transmission through an ADS-B transponder. Such data inputs may comprise a wide variety of information and may include, for example a radar altitude; a pressure altitude; an ownship heading; an ownship airspeed; a vertical climb rate; a vertical descent rate; a GPS location of ownship; a GPS antenna offset; a location of a second aircraft other than ownship; an ADS-B reply message; a desired flight level; an aircraft sensor value; a message for transmission to another aircraft; a message for transmission to a ground station; a transponder reply signal; a squitter signal; a 1030 MHz interrogation signal; a 1030 MHz TCAS signal; at least one of a signal within a voice band or a data band operating within an ATC HF, VHF, and UHF band; a VDL Mode 4 ADS-B modulated signal; a DME signal; a SATCOM signal; a signal originating from any avionics equipment that radiates a modulated RF signal; a baseband signal for transmitting data; and a pilot input, and combinations of any of those inputs. Also received by the system are control data inputs 1315, that are utilized to prioritize the data inputs from step 1310. Such prioritization results in determination of priority of data inputs 1320 and selection of an appropriate number, for example two of a plurality of data inputs, of data inputs to be formatted for transmission through a Mode S transponder. The selected data streams are assigned 1325 as primary and overlay data sources for respective transmission through the ATC data channel, and the overlay data channel. Encoding and modulation 1330 of the ATC data channel and overlay channel is then performed, and the overlay data is then modulated with the ATC data to form an overlaid modulated signal 1335. The overlay signal may then be transmitted 1340 hardware such as a Mode S transponder.
Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

## Claims

1. A method comprising:
receiving a plurality of data inputs for transmission through a provided ADS-B transponder;
receiving one or more control inputs;
analyzing the control inputs and the data inputs to determine a transmission priority for the plurality of data inputs;
selecting a first input of the plurality of data inputs based upon the transmission priority and encoding the selected first input to produce a modulated Air Traffic Control (ATC) signal;
selecting a second input of the plurality of data inputs based upon the transmission priority and encoding the second input as an overlay message; and
modulating the modulated ATC signal with the overlay message using an overlay modulation protocol.

2. The method as disclosed in claim 1, further comprising transmitting the overlay-modulated ATC signal from at least one of a TCAS transponder; an ADS-B transponder; a ATC radar; and a ground station.

3. The method as disclosed in claim 1, wherein the overlay modulation protocol is phase shift keying modulation.

4. The method as disclosed in claim 1, wherein the plurality of data inputs comprise two or more of: a radar altitude; a pressure altitude; an ownship heading; an ownship airspeed; a vertical climb rate; a vertical descent rate; a GPS location of ownship; a GPS antenna offset; a location of a second aircraft other than ownship; an ADS-B reply message; a desired flight level; an aircraft sensor value; a message for transmission to another aircraft; a message for transmission to a ground station; a transponder reply signal; a squitter signal; a 1030 MHz interrogation signal; a 1030 MHz TCAS signal; at least one of a signal within a voice band or a data band operating within an ATC HF, VHF, and UHF band; a VDL Mode 4 ADS-B modulated signal; a DME signal; a SATCOM signal; a signal originating from any avionics equipment that radiates a modulated RF signal; a baseband signal for transmitting data; and a pilot input, and combinations thereof.

5. The method as disclosed in claim 1, wherein the overlay modulation protocol is selected from the group consisting of:
binary phase shift keying modulation (BPSK);
quadrature phase shift keying modulation (QPSK);
8-phase shift keying modulation (8-PSK);
differential phase-shift keying (DPSK);
DNPSK modulation, where N is an even integer and a multiple of 2;
frequency shift keying (FSK);
amplitude shift keying (ASK);
quadrature amplitude modulation (QAM);
orthogonal frequency-division multiplexing (OFDM);
minimum-shift keying (MSK);
asymmetric phase-shift keying, (APSK);
pulse position modulation (PPM);
amplitude modulation (AM);
frequency modulation (FM); and
combinations thereof.

6. The method as disclosed in claim 1 wherein the one or more control inputs comprise at least one of: an Application Input File (AIF); a list of ADS-B applications currently active on ownship; location information regarding a second aircraft; velocity information regarding the second aircraft; bearing information regarding the second aircraft; an input from a TX 429 bus; a pilot input; and an input received from a Cockpit Display of Traffic Information (CDTI).

7. The method as disclosed in claim 6, wherein the applications comprise one or more of: a closely spaced parallel approach, an In-Trail Procedure (ITP); a Flight Deck Interval Management (FIM); and Surface Indicating & Alerting (SURF IA)).

8. The method as disclosed in claim 1 further comprising encrypting the overlay message.

9. The method as disclosed in claim 1 further comprising applying a second overlay modulation protocol to the overlay-modulated signal.

10. The method as disclosed in claim 9, wherein the second overlay modulation protocol is selected from the group consisting of:
binary phase shift keying modulation (BPSK);
quadrature phase shift keying modulation (QPSK);
8-phase shift keying modulation (8-PSK);
differential phase-shift keying (DPSK);
DNPSK modulation, where N is an even integer and a multiple of 2;
frequency shift keying (FSK);
amplitude shift keying (ASK);
quadrature amplitude modulation (QAM);
orthogonal frequency-division multiplexing (OFDM);
minimum-shift keying (MSK);
asymmetric phase-shift keying, (APSK);
pulse position modulation (PPM);
amplitude modulation (AM);
frequency modulation (FM); and
combinations thereof.

11. The method as disclosed in claim 1 wherein:
analyzing the control inputs and the data inputs to determine a transmission priority for the plurality of data inputs further comprises determining that a closely spaced parallel approach application is active; and
selecting a first input of the plurality of data inputs further comprises selecting a GPS antenna offset data value.

12. The method as disclosed in claim 1 wherein:
analyzing the control inputs and the data inputs to determine a transmission priority for the plurality of data inputs further comprises determining that a closely spaced parallel approach application is active; and
selecting a second input of the plurality of data inputs further comprises selecting a GPS antenna offset data value.

13. The method as disclosed in claim 1 wherein:
analyzing the control inputs and the data inputs to determine a transmission priority for the plurality of data inputs further comprises determining that an In-Trail Procedure (ITP) application is active; and
selecting a first input of the plurality of data inputs further comprises selecting a desired flight level input.

14. The method as disclosed in claim 1 wherein:
analyzing the control inputs and the data inputs to determine a transmission priority for the plurality of data inputs further comprises determining that an In-Trail Procedure (ITP) application is active; and
selecting a second input of the plurality of data inputs further comprises selecting a desired flight level input.