Contents Glossary.................... ................ Introduction . .............................. .............................. What Is a Transponder ? ................ Why Use a Transponder ? ............... The ATC Beacon Radar System ......... Airborne Equipment .................. Transponder (Receiver-Transmitter) .............. Control Unit ........................ Digitizer............. ................ Antenna., ........................... ........................... Ground facilities ........ . ......... .... The ATC/Mode S System ............... Mode S Transponder ................. Ground Facilities ...................... Typical Operation ....................... Principles of Transponder Operation ..... How a Transponder Identifies an Aircraft .................................. .................................. How a Transponder Works ............. How ATCRBS/Mode S Works .......... Mode S Message Content ............ Mode S Interrogations................ ATCRBS/Mode S Al¡-Cal¡ ............. ATCRBS Only All-Call ................ Uplink Messages ...................... Mode S Replies ....................... Troubleshooting ........................ ... Questions and Answers ................... Appendix A (Mode S Uplink Field Fie ld Descriptions)........................... .... Appendix B (Mode S Reply Format Field Descriptions)..................... . ..... . ... Appendix C (TCAS Coordination Words)...............
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List of Illustrations Figure 1 2 3 4 5
Page Airborne ATC Beacon Radar Equipment ............................ ATC Radarscope Display ...... . ....... ATC Radar Beacon System ........... Pulse Spacing for Interrogation Signals ............... . ................. Propagation Pattern for 3-Pulse
viii 3 4 5
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Interrogation Signal................... Primary and Secondary Surveillance Radar System, Block Diagram .................... Transponder Block Diagram .......... Interrogation Signal Validity .......... Transponder Reply Pulses ............ ATCRBS/Mode 5 System Diagram ... Mode S Interrogations ............... Pulse Patterns for PAM Interrogations......................... Mode S Interrogation DPSK Signal .. Mode S Interrogation Formats ....... Transponder Replies.................. Transponder Interrogations and Replies ................................ ................................ Mode S Reply Formats ............... Side-Lobe Interrogations ............. Propagation of Interrogation Signal (2-Pulse System).... ................... Interrogation Signal Validity (2-Pulse System)....................... Sync Phase Reversal Location ........ Whisper-Shout Transmitter Sequence............................. Mode C Reply Pulses................. Sample Gilham Coded Altitudes ..... Vertical Sense/Encoded Sense Bit Agreement............................ TCAS Resolution Advisories Lock Request ............................... ............................... RR Field Code ........................ ........................ Tactical Message Subfield Codes ..... Flight Status Field Code Descriptions........................... Resolution Advisory Subfield Bit Definitions ............................ Resolution Advisory Complement Bit Definitions ........................ Extended Capability Codes in MB ................................. Reply Information Code Description ........................... TCAS Coordination Words ...........
Glossary ARINC ATC
Aeronautical Radio, Inc. Acronym for air traffic control.
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Interrogation Signal................... Primary and Secondary Surveillance Radar System, Block Diagram .................... Transponder Block Diagram .......... Interrogation Signal Validity .......... Transponder Reply Pulses ............ ATCRBS/Mode 5 System Diagram ... Mode S Interrogations ............... Pulse Patterns for PAM Interrogations......................... Mode S Interrogation DPSK Signal .. Mode S Interrogation Formats ....... Transponder Replies.................. Transponder Interrogations and Replies ................................ ................................ Mode S Reply Formats ............... Side-Lobe Interrogations ............. Propagation of Interrogation Signal (2-Pulse System).... ................... Interrogation Signal Validity (2-Pulse System)....................... Sync Phase Reversal Location ........ Whisper-Shout Transmitter Sequence............................. Mode C Reply Pulses................. Sample Gilham Coded Altitudes ..... Vertical Sense/Encoded Sense Bit Agreement............................ TCAS Resolution Advisories Lock Request ............................... ............................... RR Field Code ........................ ........................ Tactical Message Subfield Codes ..... Flight Status Field Code Descriptions........................... Resolution Advisory Subfield Bit Definitions ............................ Resolution Advisory Complement Bit Definitions ........................ Extended Capability Codes in MB ................................. Reply Information Code Description ........................... TCAS Coordination Words ...........
Glossary ARINC ATC
Aeronautical Radio, Inc. Acronym for air traffic control.
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ATCRBS
Abbreviation for air traffic control radar beacon system. A secondary surveillance radar using ground based interrogators and airborne transponders capable of operation on Modes A and C. BCD Binary Coded Decimal BITE Built-In-Test Equipment DDT Downlink Data Transfer DME Acronym for distance measuring equipment. A system that provides distance information from a ground station to an aircra ft. Downlink A signal propagated from the transponder. DPSK Differential Phase Shift Keying ELM Extended length Message 4096 Code The octal base, four-digit code used used between between framing pulses of a reply to identify the aircraft or for general use and emergency codes. Framing Pulse A pulse pulse that is used to mark the beginning or end of the coded reply pulses. GCA Acronym for ground-controlled approach. A system that uses a ground-based controller to control the approach approach of an aircraft by transmitting instructions to the pilot. Gray Code Special binary code used to transmit altitude data between between framing framing pulses of a transponder reply. A cyclic code having only one digit change at a time. Used in Mode C to transmit aircraft barometric altitude. Also known as Gilham code. Ident The action of the transponder transmitting transmitt ing an extra pulse along with its identification code (at the request of a controller). IF (if) Abbreviation for intermediate frequency. A frequency that a signal is shifted to as an intermediate step in transmission or reception of a signal. L-Band A radio-frequency band from 390 to 1550 MHz. Mode A The pulse format for an identification code interrogation. Mode B An optional mode for transponder interrogation. Mode C The pulse format for an altitude information information interrogation. Mode D An unassigned, optional transponder mode. MODE S Abbreviation for Mode Select. MTI Abbreviation for moving moving target indicator. This type of radar display will show only moving targets. The motion is measured re lative to the radar antenna. Nautical Mile Equivalent to 6076.1 feet or approximately 1.15 statute miles. Abbreviated nmi. PAM Pulse Amplitude Modulation PAR Abbreviation for precision approach radar. An X band radar which scans a limited area and is part of the ground-controlled approach system. PPI Abbreviation for planned position indicator. A type of radar display which shows aircraft positions and airways chart on the same display. PPM Pulse Position Modulation PSR Abbreviation for primary surveillance radar. The part of the ATC system that determines the range and azimuth of an aircraft in a controlled airspace. PWM Pulse-Width Modulator Radar Acronym for radio detecting and ranging. A system that measures distance and bearing to an object. Radar Mile The time interval interval (approximately (approximately 12.359 microseconds) required for for radio waves to travel 1 nautical naut ical mile and return (total of 2 nmi).
RF (rf) SLS (sls)
SPIP Squawk Squitter SSR Stage 1 Service Stage 2 Service Stage 3 Service Suppressor Pulse
TCAS Whisper Shout
Abbreviation for radio frequency. A general term for the range of frequencies above 150 kHz to the infrared region (10" Hz). Abbreviation for side-lobe suppression. A system that prevents a transponder from replying to the sidelobe interrogations of the SSR. Replying to side-lobe interrogations would supply false replies to the ATC ground station and obscure the aircraft location. Another designation for ident pulse. Reply to interrogation signal. The transmission of a specified reply format at a minimum rate without the need to be interrogated. (Filler pulses transmitted between interrogations) Acronym for secondary surveillance radar. A radar type system that requires a transponder to transmit a rep ly signal. A radar traffic service that provides the pilot with traffic information and limited vectoring, on a workload-permitted basis. Same as stage 1 service except the sequencing of aircraft into the traffic pattern is added to the services. Stage 3 radar traffic service provides radar vectoring, sequencing of aircraft into the traffic pattern, and separation between participating VFR aircraft and all IFR aircraft operating within the controlled airspace. A pulse used to disable L-band avionics during the transmitting period of another piece of L-band airborne equipment. It prevents the other avionics aboard the aircraft from being damaged or interfered with by the transmission and any noise associated with that transmission. Threat Alert and Collision Avoidance System. A sequence of ATCRBS interrogations and suppressions of varying power levels transmitted by TCAS equipment to reduce severity of synchronous interference and multipath.
CONTROL UNIT
L-BAND ANTENNA
Airborne ATC Beacon Radar Equipment figure 1
Introduction The airborne transponder is an important part of the air traffic control system being used today. The safety of passengers, aircraft, and crew depends on the ability of air traffic controllers to accurately identify and locate aircraft within the controlled airspace. This instruction guide presents the theory and principles of transponder operation and answers common questions about transponder operation. What Is a Transponder?
A transponder is the airborne receiver-transmitter (rt) portion of the ATC Beacon Radar System that sends an identifying coded signal, in response to a transmitted interrogation from a ground-based radar station, in order to locate and identify the aircraft.
Why Use a Transponder?
Air traffic controllers use the coded identification replies of transponders to differentiate between the targets (aircraft) displayed on their radar screens. Being able to identify the aircraft aids the controller in maintaining aircraft separation, collision avoidance, and distinguishing types of aircraft. The ATC/Mode S transponder is also capable of supplying identification and altitude information to a TCAS equipped aircraft via a data link. The ATC Beacon Radar System
The ATC Beacon Radar System consists of airborne and ground-based equipment. The airborne equipment consists of a transponder (receiver-transmitter), a control unit, an antenna, and a digitizer. The airborne equipment is illustrated in figure 1. The ground based equipment is comprised of a primary radar system and a secondary surveillance radar (SSR) system. The primary radar system consists of an antenna, a receiver-transmitter (rt), and an indicator. The SSR system consists of an antenna, a receiver-transmitter, and the necessary interface and control equipment for the ground station. Airborne equipment
Transponder (Receiver-Transmitter) The receiver portion of the transponder contains the necessary circuitry to receive, demodulate, amplify, and decode the interrogation signal. The transmitter section of the transponder is comprised of the circuits necessary to encode, modulate, amplify, and transmit the coded reply signal. The transponder also contains the circuitry required for checking the validity of the received interrogation signal and monitoring the integrity of the tr ansponder. Control Unit The control unit contains the circuits and controls necessary to select the identifying code. It also contains the controls necessary for selecting an altitude source, initiating a self-test condition, and selecting the transponder reply mode. Indicators on the front panel of the control will also display a system fault and the code selected. Digitizer The digitizer is a simple converter that converts an analog signal, representing barometric altitude, to a digital format. The digitized barometric altitude can then be encoded and shipped as part of the reply signal. The digitizer circuit could also be part of the rt in some ATC Beacon Radar Systems. The digitizer output may be in the form of a digital bus, a discrete digital output (such as a Gilham code), or a synchro output (26-V ac, 400 Hz, XYZ.)
Antenna The antenna is an L-band, monopole blade-type antenna. The antenna is usually mounted in an area of the aircraft that will not be shielded from interrogation. This prevents the aircraft's identification from disappearing from the controller's radar screen. Ground Facilities
The ground facilities consist of a primary radar system and the secondary surveillance radar system. The primary radar system works like other radar systems. A narrow rf type beam, transmitt ed through a rotat ing antenna, is reflected by any targets in its pat h and returned to the antenna. By calculating the elapsed time between transmission' and reception of the rf beam, the distance to target is determined. The angle of the antenna is also noted so that the bearing of the target can be determined. This information is displayed on a 2-dimensional radar screen. Refer to figure 2. AIRWAYS CHART SUPERIMPOSED ON DISPLAY
'SKIN PAINT' ECHO (NONTRANSPONDER EOUIPPED OR TRANSPONDER SET TO DIFFERENT CODE.)
TRANSPONDER SET TO PROPER CODE
TRANSPONDER TRANSMITTING IDENT PULSE
ATC Radarscope Display figure 2
The problem with this type of display is that it does not separate aircraft by altitude. To get this type of information to the air traffic controller, the secondary surveillance radar (SSR) system was developed. The SSR system uses an antenna that is mounted directly to the
primary radar antenna and pointed in the same direction or synchronized with the same rotation as the primary radar.
The secondary surveillance radar (SSR) system interrogates the aircraft about its identity and altitude by transmitting two sets of pulses. The first set of pulses is called Mode A and the
second set is called Mode C. The Mode A pulses are spaced 8 microseconds apart and interrogate the transponder about the identity of the aircraft. The Mode C pulses are 21 microseconds apart and interrogate the transponder about the altitude of the aircraft. The pulses in both modes are identical except for the spacing of the pulses. There are two other modes (Mode B and Mode D) which are optional modes for transmitting the identification and altitude information. Refer to figure 4. The ground station assumes that the reply signal it receives after an interrogation is in response to that interrogation. The interrogation set of pulses consists of three pulses. The rotating directional antenna radiates two pulses, designated P1 and P3. The P1 and P3 pulses are spaced according to the SSR mode of operation. A pulse designated P2 is radiated by an omnidirectional antenna 2 µS after the P1 pulse from the directional antenna. The P2 pulse is a reference pulse for side-lobe suppression (sls) with an amplitude the same as that of the maximum side-lobe pulse from the directional antenna.
All pulses are 0.8 µs wide. Figure 5 illustrates the propagation pattern from the SSR. The transmitted interrogations from the ground station are at a frequency of 1030 MHz. The received reply signals are at a frequency of 1090 MHz.
P1 AND P3 PULSES
DIRECTIONAL ANTENNA SIDE-LOBES
OMNIDIRECTIONAL ANTENNA PATTERN
Propagation Pattern for 3-Pulse Interrogation Signal Figure 5
The received signal from the airborne transponder is electronically encoded so it can be displayed on a controller's radar screen. The type of radar screen used is called a planned position indicator (PPI). The images presented on a PPI remain on the screen till the next sweep of the screen. In this way (he controller does not have to remember where the aircraft is between sweeps. The replies processed by the SSR system will produce either a single or double slash for the target. The number of slashes depends on what the controller has selected. The ground controller selects which identification codes will be displayed on the PPI. If the controller is not interested in a particular aircraft, the identification code of that reply signal will be ignored. In some cases the controller may be able to display only those aircraft returning a transponder signal, instead of all primary radar targets. The code is selected by flight personnel and, in case of emergencies, there are codes that can identify the aircraft as being in an emergency situation. An emergency identification code (7700 or 7777) causes the slashes to appear wider and brighter in addition to initiating an aura¡ warning to the controller. Code 7600 is selected when the aircraft's vhf transceiver is not operational.
The ATC/Mode S System The ATC/Mode S transponder equipped aircraft and ground station enhance the operation of the ATCRBS system by adding a data link capability, discrete interrogation capability, and performance improvements. In addition, the ATC/Mode S transponder is still capable of operating with an ATCRBS only ground station. The airborne equipment remains the same as the airborne ATCRBS equipment except the ATC transponder is replaced with an ATC/Mode S transponder.
Mode-S Transponder
The Mode-S transponder provides the transponder and the communication capabilities (data link) required for TCAS. The Mode-S transponder consists of the transponder receivertransmitter, two omnidirectional L-band antennas, a control panel, and an altitude encoder. Ground Facilities
The ground facilities consist of a primary radar system and the secondary surveillance radar system. The primary radar system works the same as other radar systems. The ATC/Mode 5 secondary surveillance radar system interrogates the aircraft, in a fashion similar to the ATC ground station, except that when it interrogates an ATC/Mode S transponder equipped aircraft, that transponder will reply with a Mode S reply. The interrogator antenna uses monopulse processing to determine the azimuth bearing of an aircraft from a single reply, instead of a series of replies, as required by ATCRBS. Another difference between the ATCRBS ground facility and the Mode S ground facility, is the data link feature, that allows the interrogating stations to exchange position and identification information of aircraft. Refer to the section "How ATCRBS/Mode S Works" for more details. Typical Operation 1. 2. 3. 4. 5.
The pilot selects an identification code or is instructed to select a certain identification code from the air traffic controller. The SSR system transmits a coded interrogat ion signal (at 1030 MHz) as the primar y radar syst em detects the aircraft. The interrogation signal is received, detected, and decoded by the airborne transponder. The transponder then encodes and transmits a set of reply signals (depending upon mode and code selected). The reply signal is then received, decoded, and displayed at the ATC ground station.
Principles of Transponder Operation How a Transponder Identifies an Aircraft
A transponder identifies an aircraft by producing a unique coded reply in response to a group of transmitted pulses (interrogation) from a ground station. The transmitted pulses from the ground station are radiated from a directional SSR antenna mounted with the primar y radar antenna. The transponder is interrogated ever y time the radar scans the airspace it occupies. The transponder receives, detects, and decodes these pulses. Depending on the spacing of the pulses, a set of pulses is initiated encoding the selected identification code and/or the barometric altitude of the aircraft. These reply pulses are then transmitted back to the SSR antenna, where they are decoded and applied to the PPI as part of the video signal. The plane is then located by range, identifying code, and altitude (if the aircraft is equipped to reply to a Mode C interrogation).
How a Transponder Works
Refer to figure 7. The transponder receives the interrogation pulses at a frequency of 1030 MHz. The signals are then passed through a duplexer, a device that switches the antenna from receive to transmit at the proper time. From the duplexer, the received signal is sent to a preselector that amplifies and filters the 1030-MHz signal. The filtered 1030-MHz signal is mixed to produce an electrically manageable signal that is amplified and applied to the detector. The detector checks the validity of the pulses and separates them from the carrier frequency. To prevent an invalid interrogation by the side lobes of the SSR, a sidelobe suppression (sis) system is incorporated in the transponder. Refer to figure 8. The detector checks for the pulse amplitude and spacing before it is applied to the decoder as a valid interrogation. The valid interrogation signal generates an internal suppression pulse which brackets the transmission pulses. If the interrogation pulses are deter, mined to be invalid, the decoder circuits are disabled and the transponder will not reply. The pulses are then shipped to a pulse decoder. The pulse decoder measures the amount of time between pulses and determines whether the pulses are Mode A, Mode B, or Mode D. The decoder then applies a trigger signal to the encoder and the mode selector. The valid interrogation signal also generates a suppression pulse to be applied to other L-band equipment when the transponder is transmitting. Suppressor pulses protect the receiver portions of the L-band equipment aboard the aircra ft by temporarily preventing the equipment from receiving during the brief transmission period of the transponder.
The mode selector chooses what group of reply pulses will be sent. If a Mode A is selected, only the identification will be transmitted. If a Mode C is selected, the identification and the altitude information are both encoded. The pulses shown in figure 9 are for an identification code of 7777. The "ident" pulse, or SPIP, is transmitted when the ident button is pressed on the control. This causes the slashes (representing the aircraft) to enlarge, so it can easily be distinguished from the other returns. This is the maximum number of pulses that would be transmitted.
After encoding the identification code and the altitude information, the reply pulses are sent to the modulator to be added to the carrier signal. A feedback signal from the modulator, called the automatic overload control voltage, is sent back to the encoder to
prevent the trans mitter from being tr igger ed by more tha n 2000 interrogat ion signals/second. The carrier signal is multiplied up to the transmit frequency of 1090 MHz and applied to a power amplifier before transmission. The duplexer switches the antenna from the receiving mode to the transmit mode and the reply signal is sent back to the SSR system. The reply signal is sent two microseconds after the initial interrogation pulse is received. The ground radar system then decodes the received pulses and displays the information on the controller's radar screen. The transponder also contains monitoring circuits that check for unit reliability and protect against over-int errogation. The transponder will respond to only 2000 interrogations/second. Most radar stations interrogate at 400 interrogations/second. If there are more interrogations, the transponder limits its sensitivity to respond to the strongest 2000 interrogations.
How ATCRBS/Mode S Works The Mode S transponder-equipped aircraft and ground station enhance the operation of ATCRBS by adding a data link feature and a discrete interrogation capability, in addition to performance improvements in determining the aircraft location. The Mode S transponder data link capabilities include bidirectional air-to-air information exchange, ground-to-air data uplink (Comm A), air-to-ground data downlink (Comm B) and multisite message protocol. The Mode S transponder may also function as part of an airborne separation assurance (ASA) system when interfaced with a Traffic Alert and Collision Avoidance System (TCAS). Refer to figure 10.
The ATCRBS/Mode S system operates in a similar fashion as ATCRBS. As a transponder equipped aircraft enters the airspace, it receives the ATCRBS/Mode 5 all-call interrogation, which can be identified by both ATCRBS and Mode S transponders. ATCRBS transponders reply as described in previous paragraphs, while the Mode S transponder replies with a Mode S format that includes the discrete 24-bit Mode 5 address. The interrogator uses monopulse processing to determine the azimuth bearing of an aircraft from a single reply, instead of a series of replies as required by ATCRBS. The monopulse antenna generates two separate patterns: a single or sum pattern and dual lobe (difference) pattern. The two patterns simultaneously receive each transponder reply. The ratio of the energy received by the sum pattern to the energy received by the difference pattern determines t he bearing of the aircraft from the antenna boreline.
The address and the location of ¡he Mode S aircraft is entered into a roll-call file. On the next scan, the Mode S aircraft is discretely addressed. The discrete interrogations of a Mode S aircraft contain a command field that may desensitize the Mode S transponder to further Mode S al¡-cal¡ interrogations. This is called Mode S lockout. ATCRBS interrogations (from ATCRBS interrogators) are not affected by this lockout. Mode S transponders reply to the interrogations of an ATCRBS interrogator under al¡ circumstances. As a Mode S aircraft flies into the airspace served by another Mode S interrogator, the first Mode S interrogator may send position information and the aircraft's discrete address to the second interrogator via ground lines. Thus, the need to remove the lockout may be eliminated, and the second interrogator may schedule discrete roll-call interrogations for the aircraft. Because of the discrete addressing feature of Mode S, the interrogators (SSRs) may work at a lower rate (or handle more aircraft). In regions where Mode S interrogators are not connected via ground link, the protocol for the transponder is for it to be in lockout state for only those interrogators that have the aircraft on the roll call. So, if the aircraft enters an airspace served by a different Mode S interrogator, the new interrogator may acquire the aircraft via the reply to an all-call interrogation. The Mode S only all cal¡ is used by the interrogators if Mode S targets are to be acquired without interrogating ATCRBS targets (that may be present). The destination address of this interrogation is all logic level ones. All Mode S transponders will reply to this interrogation with their discrete address. In turn, an ATCRBS only all call is used by the interrogator if ATCRBS targets are to be acquired without elicting replies from the Mode S targets. Aircraft are tracked by the interrogator throughout its assigned airspace. A Mode S aircraft reports in its replies either its altitude or its ATCRBS 4096 code depending on the type of discrete interrogation received. During each scan, interrogations of ATCRBS aircraft are made in both Mode A and Mode C. The Mode S interrogation is transmitted using binary differential phase-shift keying (DPSK). The modulation of the downlink transmission from the transponder is pulse position modulation (PPM). Each Mode S interrogation contains a 24-bit discrete address that allows a very large number of aircraft to operate in air traffic control environment without the occurrence of a r edundant address. When an interrogator does not receive a valid reply as it scans through a Mode S aircraft's location, the interrogator can reinterrogate a limited number of times. Normally, interrogators interrogate at low power ('whisper') and reinterrogate at high power ('shout') if the low power interrogation fails. Referring to figure 11, the Mode 5 transponder receives an interrogation from the ground. The transponder will initiate a reply to an ATCRBS ground station, until the presence of the P4 pulse is detected. Upon detection of the P4 pulse, the ATCRBS reply is terminated. If the width of the P4 pulse is 0.8 µs seconds (ATCRBS Mode A or Mode C only al¡ -cal¡), no reply is transmitted. If the width of the pulse is 1.6 µseconds (ATCRBS/Mode S all-call), a Mode S reply is generated 128 µseconds after the leading edge of the P4 pulse. The Mode S reply is the same reply generated in response to the Mode 5 al¡-cal¡ interrogation. The standard replies are discussed in later paragraphs.
Side lobe suppression for Mode S is accomplished by overlaying a pulse (P5) on the P6 pulse to prevent the transponder from detecting the sync phase reversal (SPR) position of the P6 pulse. Pulse P5 will be overlaid on al¡ Mode S only al¡-cal¡ interro gations. If the SPR position is masked by P5, the SPR position will not be detected by the transponder in the time interval expected, and the transponder will not reply. The Mode S transponder is capable of improving air-to-air surveillance and communications, when it is equipped with two antennas. Such systems are termed diversity systems. One of the antennas is mounted on the top of the aircraft, and the other antenna is mounted on the bottom of the aircraft. The diversity Mode S transponder will have automatic selection of antenna, based on the relative strength of detected interrogation signals (if the signals are received simultaneously). If the detected interrogation signals are not received simultaneously (within 0.125 microseconds of each other), the transponder selects the interrogation signal that arrived the
earliest. The selected antenna is used to receive the remainder of the interrogation, and if required, transmit the Mode S or ATCRBS reply. Mode S Message Content
The minimum data link transponder supports all surveillance functions, in addition to bidirectional air-to-air data exchange, ground-to-air data uplink (Comm A), air-to-ground data downlink (Comm B), and multisite message protocol. In addition, the transponder is capable of receiving extended length messages (ELMs) from the ground. ELMs are received in the Comm C format. ELM transmittals to the ground use the Comm D format. (Note: The TPR720 ATC/Mode S Transponder is capable of, but is not certified for Comm D replies.) All discrete Mode S interrogations and replies (except the all-call reply) contain the 24 bit discrete address of the Mode S transponder upon which 24 error detection parity check bits are overlaid. In the all-call reply, the 24 parity check bits are overlaid on the Mode S interrogators address and the transponders' discrete address is included in the text of the reply. The primary function of Mode S is surveillance. To accomplish this function, the Mode S transponder uses the 56-bit transmissions (in each direction). In the 56-bit transmissions, the aircraft reports its altitude or ATCRBS 4096 code, and the flight status (airborne, on ground, alert, special position identification (SPI), etc. The squitter transmission is an all-call reply which is transmitted by a transponder approximately once every second. The squitter signal is observed by aircraft equipped with airborne collision avoidance systems. Special surveillance interrogations from airborne collision avoidance systems are addressed to Mode S equipped aircraft based upon the address of the squitter signals. These interrogations are used for Mode S target tracking and collision threat assessment. The discrete addressing and digital encoding of Mode S transmissions permit their use as a digital data link. The interrogation and reply formats of ¡he Mode S system contain sufficient coding space to permit the transmission of data. These data transmissions may be used for air traffic control purposes, air-to-air data interchange for collision avoidance, or to provide flight advisory services such as weat her reports, or automated terminal information system (ATIS). Most Mode S data link transmissions will be handled as one 56-bit message included as part of a long 112-bit interrogation or reply. These transmissions include the message in addition to the surveillance data. Longer messages are transmitted using the extended length message (ELM) capability. The ELM is capable of transmitting up to sixteen 80 bit message segments, either ground-to-air or air-to-ground. The ELM can be acknowledged with a single reply or interrogation. ELMs uplinked need not be replied to individually, but can be acknowledged in a reply containing a summary of the received interrogations. ELMs do not contain surveillance data. Mode S Interrogations
The Mode S transponder supports al¡ surveillance functions, in addition to bidirectional airto-air data exchange, ground-to-air data uplink (Comm A), air-to-ground data downlink
(Comm B), uplinked extended length messages (Comm C), downlinked extended length messages (Comm D), and multisite message protocol. The Mode S PAM (pulse amplitude modulation) interrogations are shown in figure 92. The following six interrogations are exclusively PAM signals: ATCRBS Mode A, ATCRBS Mode C, ATCRBS Mode A/ Mode S all-call, ATCRBS Mode C/Mode S all-call, ATCRBS Mode A only all-call, and ATCRBS Mode C-only all-call.
NOTES: 1. 2.
3. 4.
P4 IS NOT PRESENT FOR ATCRBS MODE A AND ATCRBS MODE C INTERROGATIONS. P4 PULSE IS LONG (1.6 µS) FOR ATCRBS MODE A/MODES ALL CALL AND ATCRBS MODE C/MODE'S ALL CALL. P4 IS SHORT (0.8 µS) FOR ATCRBS MODE A-ONLY ALLCALL AND ATCRBS MODE C-ONLY ALL-CALL. CARRIER FREQUENCY FROM ATCRBS INTERROGATORS IS 1030 ±0.2 MHZ AND 1030 ±0.01 MH2 FROM MODE S INTERROGATORS. P2 PULSE AMPLITUDES WILL VARY FROM P1 AMPLITUDES. P3 AMPLITUDES ARE EQUAL TO P1 AMPLITUDES ±1 DB. P4 AMPLITUDES ARE EQUAL TO P3 AMPLITUDES ±1 DB.
Pulse Patterns for PAM Interrogations Figure 12
All Mode S interrogations are binary differential phase shift keying (DPSK) signals. Refer to figure 13. The P1-P2 pulse pair preceding P6 suppress replies from ATCR85 transponders to avoid synchronous garble (caused by the random triggering of ATCRBS transponders). A series of 'chips' contain the information within P6. The chips start 0.5 microseconds after the sync phase reversal (spr). A chip is an unmodulated interval of 0.25 microsecond duration, preceded by possible phase reversals. If preceded by a phase reversal, a chip will represent a logic level one. If preceded by no phase reversal, a chip represents a logic level¡ zero. There are either 56 or 112 chips within each P6. The last chip is followed by a 0.5-microsecond guard interval to prevent the trailing edge of the P6 from interfering with the demodulation process. P5 may be overlaid on P6 by the interrogator as a side lobe suppression (sls) signal in any Mode S interrogation, and spaced 0.4 microseconds from the sync phase reversal. Side
lobe suppression pulse P5 would overlay the location of the sync phase reversal of P6. P6 will be overlaid by P5 on all Mode S-only all-call interrogations.
ATCRBS/Mode S All-Call
The transponder will detect the interrogation as an ATCRBS/Mode S interrogation if the received interrogation contains a 1.6-microsecond pulse in the P4 position and the amplitude of P4 is above the amplitude of P3 less 1 dB. The transponder will detect the interrogation as an ATCRBS interrogation, if the amplitude of P4 is below t he amplitude of P3 by 6 dB. ATCRBS-Only All-Call
If the transponder receives a valid ATCRBS interrogation followed by an 0.8-microsecond pulse in the P4 position, the transponder will accept the interrogation if the P4 amplitude is below the P3 amplitude, minus 6 dB. The interrogation will not be accepted if the amplitude of P4 is above P3 minus 1 d8. Mode S transponders do not accept t he ATCRBS only all-call. Uplink Messages
The formats of uplink messages contain either 56 or 112 bits, with the last 24 bits being used for address or parity, and the remaining bits used for information. The bits are numbered in order of transmission, beginning with bit 1. The numerical values are encoded in groups of
bits called fields. The first bit transmitted is the most significant bit (MSB). The information encoded in fields consists of at least one bit. The decimal equivalent of the binary code within the field is used as the designator of the field function. Each Mode S transmission contains two essential fields: one describes the format, and the other (24 bits) carries parity information and contains either the address or the interrogator identity overlaid on the parity. The format descriptor is the field at the beginning of the transmission and the 24 bit field always occurs at the end of the transmission. Refer to figure 14 for the Mode S interrogation formats currently assigned. Refer to appendix A for a description of Mode S Uplink fields and subfields.
NOTES: 1. 2. 3.
XX:M DESIGNATES A FIELO 'XX' CONTAINING M BITS. - N - DENOTES FREE CODING SPACE WITH N AVAILABLE BITS. FOR UPLINK FORMATS (UF), 0 THROUGH 23 THE FORMAT NUMBER CORRESPONDS TO THE BINARY CODE IN THE FIRST FIVE BITS OF THE INTERROGATION. FORMAT NUMBER 24 IS ARBITRARILY DEFINEO AS THE FORMAT BEGINNING '11' IN THE FIRST TWO BIT POSITIONS, WHILE THE FOLLOWING THREE BITS VARY WITH THE INTERROGATION CONTENT.
FIELD DESIGNATORS AP = ADDRESS/PARITY AO = ACOUISITION SPECIAL DI = DESIGNATOR, IDENTIFICATION II = INTERROGATOR IDENTIFICATION MA = MESSAGE, COMM A MC = MESSAGE, COMM U NC = NUMBER OF C-SEGMENT PC= PROTOCOL PR = PROBABILITY OF REPLY RC = REPLY CONTROL RL = REPLY LENGTH RR = REPLY REOUEST RR = REPLY REQUEST SD = SPECIAL DESIGNATOR
Mode 5 Interrogation Formats Figure 14
Mode S Replies The replies transmitted by a Mode S transponder consist of an ATCRBS reply in response to an ATCRBS interrogation, and a Mode S reply consisting of a preamble and data block. Refer to figure 15 for an example of the Mode S reply data block.
The reply data block is formed by pulse position modulation (PPM) encoding of the reply data. A pulse transmitted in the first half of the interval represents a logic level 1, while a pulse transmitted in the second half represents a logic level 0. The first preamble pulse occurs 128 (±0.25) microseconds after the sync phase reversal of the received P6 of a Mode S interrogation. The first preamble pulse will also occur 128 (±0.5) microseconds after the P4 pulse of an ATCRBS/Mode S all-call interrogation. Refer to figure 16 for a diagram illustrating the replies for ATCRBS and Mode S interrogations and the timing relationship with the corresponding interrogation.
The ATCRBS replies to an ATCRBS interrogation are identical to the replies transmitted by any non-Mode S transponder. There are two framing pulses, nominally spaced 20.3 microseconds apart. Within these framing pulses are thirteen information pulses (the X pulse is reserved for future use). These pulses are used to make up the 4096 identification code selected by the pilot. The code designation will consist of digits 0 through 7. The digits will be defined by the sum of the postscripts of the information pulse numbers. The first digit will be defined by pulse group A, the second digit will be defined by pulse group B, the third digit will be defined by pulse group C, and the fourth digit will be defined by pulse group D. As an example, let us assume the identification code is 3615. This identification code would consist of pulses A1 and A2, pulses B2 and B4, pulse C1, and pulses D1 and D4. An SPI (special position identification) pulse is transmitted (upon interrogator request) following the last framing pulse of a Mode A reply. The SPI pulse is activated by the IDENT switch (of the controller), and is transmitted for a period of 18 (±2) seconds. The Mode S replies lo a Mode S interrogation are in a much different format, so the reply can contain much more information than previous generations of transponders. The standard reply formats are shown in figure 17. Only the reply formats (downlink format) defined at the time of publication are shown. The subfields shown in figure 17 ar e defined in appendix B.
Troubleshooting The basic idea of troubleshooting avionics equipment is to isolate the trouble to a single component or circuit that can be replaced or repaired. Two methods are commonly used to accomplish this. One method is to start at the antenna or input and check components along the signal path until the faulty one is found. The other method is to troubleshoot the equipment in sections and work by halves. This is done by locating the trouble in one half of the unit or the other half, then dividing the troubled half in half to isolate the trouble even further. This second technique is especially useful for avionics equipment. The typical avionics unit can usually be divided into two or three sections: receiver, computation, and/or transmitter. The transmitter of a transponder can usually be checked by noting whether the reply lamp on the control lights. This indicates that the transmitter has transmitted a signal. To test the other two sections, most transponders have a self-test capability for checking the receiver and pulse decode/encode section. The interrogation rate might affect the transponder's performance. At higher interrogation rates, the transponder could start to reply randomly instead of to each interrogation. Transponders that are used in areas where there are a number of SSR systems should be checked at several interrogation rates. When complaints are received about transponder operation, the details of the problem should be noted. For example, if a transponder is not responding properly, does this happen on al¡ the codes or just a particular code? This information would pinpoint a problem to either the code selector or the control. Intermittent reception of replies by the ground station could be the result of improper pulse spacing or shape, which the ground station would reject. The antenna installation may also cause poor reception i n some directions but not others. The installation of the antenna should be checked to ensure that the antenna pattern is not interfered with. A problem that occurs in avionics equipment using high voltages, such as a transponder, is that at high altitudes it is easier for sparks to jump across terminals. After repairing a transponder, precautions should be taken to ensure that sharp points and other possible sparking points are eliminated. Postcoating of circuits after repair is another way of preventing sparking at higher altitudes. While these troubleshooting hints do not cover all possible problems, they do indicate information and troubleshooting clues to look for. More specific troubleshooting aids are discussed in the overhaul manual of each part icular piece of equipment.
Questions and Answers This section is intended to answer commonly asked questions about transponder operation and transponder principles. Q. What is the operating range of a transponder? A. The operating range of the transponder depends upon line-of-sight distance between aircraft and station and the transmitter section of the transponder. The minimum power output is approximately 125 watts. In this case, the 'operating range is about 100 nautical miles. line-of-sight range is 100 nautical miles at 10 000 feet. Most transponders, though, operate as far out as 200 nautical miles (at an altitude of 30 000 feet). Q. What is a LO-SENS?
A. A LO-SENS is an operating mode of the transponder. In the LOSENS mode, the receiver section sensitivity is decreased so that only a strong signal will interrogate the transponder. The air traffic controller usually requests that the pilot switch his transponder to LO-SENS when the aircraft is close to a ground station and the transponder is responding to side-lobe interrogations. This is in addition to the sis circuits already incorporated in the transponder. Refer to figure 18. LO-SENS is also requested when the ground station radar may be overworked by the return of too many aircraft in high-traffic areas.
Q. What specifications must a transponder meet? A. Transponder specifications are governed by RTCA documents DO-138, DO-160, and ARINC specifications 572 and 730. These documents define the operating parameters and environment of an airborne transponder system. Q. What is the difference between primary and secondary radar? A. The difference between primary and secondary radar is the return signal. The return signal is reflected from the target for primary radar, while in secondary radar the incident signal triggers a response from a transponder.
Q. What is the 2-pulse interrogation system? A.
The 2-pulse interrogation system is another method of propagating the interrogation signals. Refer to figure 19. The pulse-pair interrogations from the SSR are a composite of both the rotating directional and fixed omnidirectional antenna radiations. There is no P2 pulse, as there is in the 3-pulse system. The first pulse, P1, is transmitted by the omnidirectional antenna. The last pulse, P3, is transmitted by the directional rotating antenna and is used in amplitude comparison with P1 to determine whether or not the interrogation is valid. Figure 20 illustrates a valid and invalid interrogation signal for a 2-pulse system.
Q. What is sync phase reversal? A. Sync phase reversal is the method used in transmitting messages via the transponder data (ink. It is a position in the Mode S interrogation word that synchronizes the decoding of the interrogation. Figure 21 shows the Mode S interrogation word and the sync phase reversal position.
Q. What is whisper-shout? A. Whisper-shout is a method lo control ATCRBS synchronous interference and aid the operation of TCAS in high traffic density airspace. Basically, whisper-shout refers to the power levels of each interrogation and the associated suppression pulse of each interrogation. Refer to figure 22.
The first pulse of an interrogation serves as the second pulse of suppression. Each aircraft within an airspace will have a different effective sensitivity level due to variations in receivers, cable losses, and antenna shielding. Using a system of increasing power in interrogations and accompanying suppression pulses, each aircraft within the airspace will most likely respond lo two interrogations before it is turned off by the higher power suppression pulse accompanying the next higher power interrogation. By this method, the synchronous interference is reduced and also the severity of multipath effects on the interrogation link.
Q. What is DABS? A. DAOS is an acronym for discrete address beacon system. It was a precursor to the TCAS system, but differed in that it depended upon a ground based interrogation system. It would reduce interference by discretely addressing particular aircraft when interrogating transponder equipped aircraft.
Q.
How is altitude encoded in replies?
A. Mode C replies from ATCRBS transponders encode the aircraft altitude in a gray code format that is also known as the Gilham code. The Gilham code is an 11-pulse format, that uses the presence of pulses lo indicate increments in the altitude. Refer to figure 23 for a diagram of the mode C transponder reply. The Gilham code is arranged so that the bits D2, D4, A1, and A2 define the 64000 to 8000 feet altitudes, and the A4, B1, B2, and B4 bits define the 4000 to 500 feet altitudes. The C bits along with the parity of the A, B, and D bits define ihe 100-foot increments. In order to display negative altitude values down to -1000 feet, the 0 value starts at - 1000 feet, instead of 0 feet. Refer to the table of figure 24 for examples of Gilham encoded altitudes.
Mode S replies send the altitude information in the AC fields of formats 0, 4, 16, and 20. The 13-bit AC field expresses the encoded altitude in the sequence of C1, A1, C2, A2, C4, A4, M, B1, D1, B2, D2, B4, and 134. The M bit allows for the possible future use of encoding altitude in metric units. Zero is transmitted in each of the 13 bits if the altitude information is not available.
Q. How does (he Mode S transponder coordinate with the TCAS equipment? A. The Mode S transponder functions as the communication link by which the TCAS equipped aircraft are able lo communicate with potential threat aircraft, and the link used by two TCAS equipped aircraft lo coordinate avoidance maneuvers.
