Understanding mode S technology

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DefenseElectronics
Understanding mode S technology
Stemming from several mid-air collisions in the mid-1980s, mode S has been an integral part of airborne transportation today. Although air traf.c is the safest mode of transportation, more in-.ight information is needed due to the increase in traf.c. Enhanced surveillance and ADS-B address this need.
By Wes Stamper
W
hat is mode S? How does it work and why is it needed? What was wrong with the old mode A and C? What is Flight ID, UF and DF, squitter and elementary surveillance? This article will touch on many aspects of mode S technologies of today and tomorrow. The article will also discuss .ight ID, elemen-tary surveillance and UF and DF; explain the details of basic mode S surveillance, how it works and discuss the best practices to verify and test an installation. This discussion will also introduce the new enhanced surveillance, DF17 extended squitter and ADS-B; explain the difference between each of these new technologies and what is needed to verify and test installations.

What is mode S?
Mode S or mode ¡°select,¡± is a new way to interrogate an airframe by using a distinct address, such as an aircraft address, that only a particular airframe will respond. Many years ago, mode A and C were developed for airframe identi.cation and altitude reporting. This was and still is an important component of air traf.c control and air space manage-ment. As more and more airframes were available to the private and commercial .ying community, this basic form of surveillance was overwhelming the capacity of the air traf.c control radar beacon system (ATCRBS). Given the technology behind the mode A and C interrogation and reply, there were also problems with false reply uncorrelated in time (FRUIT), seeing replies from another inter-rogation, and garbling one reply interfering with another. The problem is analogous to attempting to listen to several conversations at the same time. As such, the capacity of the ATCRBS was being taxed to its limit. 
ATCRBS also uses the ¡°sliding window¡± technique to determine the azimuth position of the aircraft. This requires many interrogations and replies, which reduce the target-handling capacity of the ATC secondary surveillance radar (SSR). The mode S system uses a monopulse SSR, which has an electrically narrowed beamwidth, typically 2.5 ªÙ. Apart from better azimuth accuracy the monopulse technique reduces the number of interroga-tions required to track a target, as it theo-retically requires only one reply to obtain the azimuth and range of the airframe.

 

Where did mode S originate?
The mode S concept was mostly a develop-ment of MIT Lincoln Lab with coordinated efforts from the Federal Aviation Adminis-tration (FAA), Aircraft Owners and Pilots Association (AOPA) and the transponder manufacturing community. Mode S technol-ogy was .rst developed in the mid-1970s, but was not widely deployed until the early 1980s. The idea was to develop a way of using the same SSR that was being used in mode A and C, but to make it addressable, more accurate and reliable, and operate with greater capacity.
Primary surveillance radar (PSR) is still used to ¡°paint¡± the airframe with a radar pulse and place a target on the plan position indicator (PPI), which is the display of the ATC. However, the combined use of PSR and SSR allows for better surveillance without major upgrades to the existing PSR/SSR site. This provides an addressable means of gath-ering the same mode A and C information as well as basic information about the airframe. This new mode S technology is similar to the new digital cellular phones.
Similar to mode A and C, years ago there was the analog cellular phone, which allowed basic communications with minimal features. The current digital cellular phone has the same basic communication but affords more site capacity, better reliability and more capabili-ties such as text messaging, Internet access and global positioning system (GPS) location information. The same holds true for mode S surveillance. The mode S-equipped airframe can now report identity, intent, capability and location.

How does the interrogation and reply actually work?
The basicATCRBS system relies on pulsed RF as its means of communications. These pulses are 0.8 ¦Ìs wide and vary from 8.0 ¦Ìs

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to 21 ¦Ìs spacing for mode A and C respec-tively. The SLS (P2) pulse is also transmitted omnidirectionally and is used to suppress any replies to side lobe interrogations. The mode S SSR will interrogate using a 1030 MHz carrier with differential phase shift keying (DPSK) modulation. DPSK allows the interrogation frequency to have much more ef.ciency in sending information without interfering with mode A and C interrogations. DPSK also allows for up to 4 MBps of data. As the 1030 MHz, DPSK signal sends out the interrogation, the airframe will receive it, verify the request and integrity of the signal, and reply using a 1090 MHz carrier with pulse positioning modulation (PPM) transmission.
SSR is central to the mode S system. Mode S interrogations are generated at a rate of 50 times per second or 50 Hz pulse repetition frequency (PRF) and approximately 230 Hz for mode A/C interrogations. The reply will happen at the same PRF although mode S SSR has the ability to tell the mode S transponder not to reply to every mode S interrogation it

Figure 2. Ground station request.
receives. Once the SSR has received the reply it will decode the mode (A, C or S) and de-modulate the information within each mode. aircraft identi.cation (DF4) and basic airframe squitter and the extended (112 bit) DF17
There are three interrogation types in a information (DF11). The SSR is the main squitter. mode S SSR system: component of the interrogation and reply. The
1.
ATCRBS all call: This interrogation interrogation happens about 50 Hz PRF. The UF and DF consists of P1, P3 and a 0.8 ¦Ìs P4 pulse. P2 reply will happen at the same PRF. Once the The functional components of mode S SLS is transmitted as normal. All ATCRBS SSR has received the reply it will decode the are uplink format (UF) and downlink format transponders reply with the 4096 identi.cation mode (A, C or S) and demodulate the informa-(DF). UF is a speci.c interrogation originating code for mode A interrogations and altitude tion within each mode. from SSR or another airframe asking speci.c, data for mode C. Mode S transponders do not addressable information about that airframe. reply on this interrogation. What is squitter? DF is the reply from the airframe in regard to

2.
ATCRBS/mode S all call: This interro-A de.nition of squitter is a reply format the UF interrogation. UF 0, 4, 5, 11 and 16 gation is identical to the former except P4 is transmission without being interrogated. make up basic surveillance. Basic surveil-

1.6 ¦Ìs long. ATCRBS transponders reply These ¡°unsolicited replies¡± or squitters are lance messages are comprised of the airframe with the 4096 code or altitude data as per the used to provide TCAS 2-equipped airframes address, parity bits and a 56-bit data word ATCRBS all call. Mode S transponders re-with the discrete address of the squittering known as ¡°short¡± interrogations and replies.ply with a special code, which contains the airframe, to enable the TCAS 2 system to UF0 is a short air-to-air surveillance for identity and the aircraft¡¯s discrete address.  acquire and track the airframe using mode S TCAS/ACAS (Figure 1). The DF0 reply will
3. Mode S discrete interrogation: This formats UF/DF0 and UF/DF16. include the mode C altitude as well as the interrogation is directed at a speci.c mode Squitter has its origins in distance-mea-mode S address. To test DF0, its altitude reply S transponder-equipped aircraft. The inter-suring equipment (DME) transmissions. is compared to the mode C altitude and the rogation consists of P1, P2 and P6. P2 is The DME ground station would broadcast modeSaddressforveri.cation.Alsoencoded transmitted via the directional antenna and unsolicited replies or squitters. When the in the DF0 reply is the vertical status (VS) hence is the same amplitude as P1 and P3. This airborne DME interrogator was in range, bit, the reply information (RI) .eld. The VS effectivelysuppressesATCRBStransponders the squitter would be seen and the DME bit will indicate a 1 if the airframe is on the from replying. P6 is actually a DPSK data interrogator would then transmit a range ground and a 0 if it is on the air. The RI .eld block that contains either a 56-bit or 112-bit interrogation and receive range replies from is a four-bit word containing the airframe¡¯s message. The DPSK modulation produces a the DME ground station. This served to true speed capability and type of reply to the spread-spectrum signal, which has immunity limit unnecessary transmissions over the interrogating airframes.to interference. air and optimized DME ground station-UF4 is a short, ground station request for
When the transponder receives a valid handling capability. TCAS 2 systems use altitude similar to the UF0 request, but initial-mode S discrete interrogation, it will return mode S squitters in a similar fashion; the ized by the ground station (Figure 2). Testing a reply 128 ¦Ìs after reception. The reply is TCAS just listens for the DF11 squitters, the DF4 reply is veri.ed against the mode C transmitted on 1090 MHz and uses a 56-bit which contain the sending aircraft¡¯s discrete altitude and the mode S address for validity. or 112-bit PPM transmission. address, thereby reducing the need to Also encoded in the DF4 reply is the .ight
Each mode S interrogation will have a interrogate over the air. The discrete address, status (FS) .eld, downlink request (DR) .eld, 24-bit address unique to the aircraft as well once obtained, is placed on the TCAS 2 utility message (UM) and the altitude code as a 24-bit parity check for validation. In processor¡¯s roll call of addresses for ongoing (AC) .eld. The FS .eld is a three-bit word basic mode S surveillance, the informa-tracking. Mode S technology has two types of re.ecting eight different conditions of air-tion is limited to altitude reporting (DF0), squitter, a short (56 bit) DF11 acquisition borne, alert and special position indicator (SPI) status of the airframe. The DR .eld is a .ve-bit word that will contain the request to downlink certain airframe information. The UM .eld is a six-bit word that contains tran-sponder communication status information. The AC .eld is a 13-bit word that contains the altitude of the airframe with special encod-ing for feet or metric units and if the altitude resolution is 25 feet or 100 feet.

UF5 is a short, ground station request for the airframe identity. The DF5 reply is the airframe¡¯s identi.cation and is compared to the mode A 4096 code for validity. Also encoded in the DF5 reply is the .ight status (FS) field, downlink request (DR) field, utility message (UM) and the identi.cation (ID) .eld. The FS .eld is a three-bit word reflecting eight different conditions of airborne, alert and special position indicator (SPI) status of the airframe. The DR .eld is a .ve-bit word that will contain the request to downlink certain airframe information. The UM .eld is a six-bit word that contains tran-sponder communication status information. The ID .eld is a 24-bit word that contains the mode A identi.cation of the airframe.
UF11 will request the airframe¡¯s mode S address. The DF11, or all-call reply, will reply with the airframe address (squitter address) as well as the capability (CA) .eld, parity/inter-rogator identi.er (PI) .eld, the interrogator identi.er (II) and surveillance identi.er (SI). The CA .eld is a three-bit word that contains the communication capabilities of the tran-sponder. The PI .eld is a 24-bit word that will report the interrogator identi.cation code with a parity overlay. The II .eld is a four-bit word, from 0 to 15, containing the identi.ca-tion code of the interrogator. The SI .eld is a six-bit word, from 0 to 63, used to identify the types of surveillance.
UF16 is a long, air-to-air surveillance for ACAS and is the long form of a UF0. Where the DF0 is 56 bits long, the DF16 is 112 bits long. The DF16 reply will have in it the mode C altitude, as well as the mode S address. Testing DF0, the altitude reply is compared to the mode C altitude and the mode S address for veri.cation. Also encoded in the DF16 reply is the vertical status (VS) bit, the reply information (RI) .eld and the altitude code (AC) .eld. The VS bit will indicate a 1 if the airframe is on the ground and a 0 if it is in the air. The RI .eld is a four-bit word containing the airframe¡¯s true speed capability and type of reply to the interrogating airframes. TheAC .eld is the mode S altitude that is compared to the mode C reply for validity. Also in the DF16 is the message comm V (MV) .eld. This .eld contains information used in air-to-air exchanges (coordination reply message).
UF20 is the long form of a UF4. Where the DF4 is 56 bits long, the DF20 is 112 bits long. The DF20 reply is also known as comm A altitude request. The DF20 reply also con-tains a 56-bit message .eld for transferring downlinked aircraft parameters (DAPS). To properly test the DF20 reply, the UF20 must contain a reply request (RR) of 17, a desig-nator identi.er (DI) of 7 and a reply request sub.eld (RRS) of 0. The working component of the DF20 reply is the comm B message .eld (MB). Contained in the MB .eld is aircraft address (AA), downlink request (DR), .ight status (FS) and altitude code (AC). These parameters are compared to the DF11 reply for validity. A transponder that does not have an active subsystem that will accept comm A data will not reply to a UF20 interrogation.
UF21 is the long from of a UF5. Where the DF5 is 56 bits long, the DF21 is 112 bits long. The DF21 reply is also known as comm A identity request. The DF21 reply also contains a 56-bit message .eld for transferring downlinked aircraft parameters (DAPS). To properly test the DF21 reply, the UF21 must contain a reply request (RR) of 17, a designator identi-.er (DI) of 7 and a reply request sub.eld (RRS) of 0. The working component of the DF21 reply is the comm B message .eld or MB. Contained in the MB .eld is aircraft address (AA), downlink request (DR), .ight status (FS) and identi.cation code (ID). These parameters are compared totheDF11replyforvalidity.Atransponder that does not have an active subsystem that will accept comm A data will not reply to
a UF21 interrogation.

Uplink extended length messaging (UELM )
This is known as comm C capability of a transponder. A transponder must be a level 3 to accept a UELM message. UELM oper-ates over UF24, which allows for long data messages to be sent from the ground station to the airframe. UF24 works similar to any other interrogation with one exception. The UELM message is capable of up to 16-bit to 80-bit message segments for this extended message. The working component of UELM is the comm C message (MC) .eld. This MC .eld works in conjunction with the number of C (NC) or segments to allow for the extended message.

Downlink extended length messaging (DELM )
This is known as comm D capability of a transponder. A transponder must be a level 4 to transmit a DELM message. DELM operates over DF24, which allows for long data messages to be sent from the airframe to the ground station. DF24 works similar to other interrogations with one exception. The DELM message is capable of up to 16-bit to 80-bit message segments for this extended message. The working component of DELM is the comm D message (MD) .eld. This MD .eld works in conjunction with the number of D (ND) segments to allow for the extended message.

What is .ight ID?
Flight ID has its origins in the International Civil Aeronautics Organization (ICAO). The

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.ight ID is an eight-character identi.cation report (BDS 1,7), aircraft identi.cation (BDS airborne velocity. This will report the east/west that is entered by the pilot usually via a .ight 2,0), and ACAS active resolution advisor and north /south velocities, heading, vertical deck CDU. The .ight ID may contain the report (BDS 3,0). A EuroControl mandate rate and altitude source, the difference between company-assigned number for that particular stated that elementary surveillance installa-the barometric and GPS altitude, the IFR .ight. If the company-assigned number is tions started March 31, 2003 with a transitional capability and airspeed. not available or not used, the .ight ID then period up to March 31, 2005. becomes the aircraft tail number. The .ight ID What is ADS-B? supplements the unique 24-bit aircraft discrete What is enhanced surveillance? As previously discussed, DF17 is the address and is used by ATC for monitoring Enhanced surveillance, as in elementary integral and working portion of automatic purposes. surveillance, is as easy as 4, 5, 6¡ªthat is BDS dependent surveillance broadcast (ADS-B)
4, 5 and 6. Take the concept of elementary (Figure 3). Breaking down the meaning of the What level is the transponder surveillance and add new aircraft intent report-terms: automatic¡ªthere is no interrogation being tested? ing .elds. These .elds are selected vertical needed to start the data or squitter coming from
The level of a transponder is an important intent (BDS 4,0), track and turn report (BDS the transponder; dependent¡ªas it relies on point when testing an installation. The level 5,0) and heading and speed report (BDS 6,0). onboard navigation and broadcast equipment reports the capabilities of the transponder. The selected vertical intent report (BDS 4,0) is to provide information to other ADS-B users; A level 1 transponder is one with basic used to indicate such items as the barometric and surveillance¡ªit is a means of automatic surveillance capabilities. This is UF0, UF4, altitude of the airframe, the selected altitude surveillance and traf.c coordination. Some of UF5 and UF11 reporting. A level 1 tran-of the mode control panel or .ight control the bene.ts of ADS-B technology are better sponder will have no provision for datalink unit (MCP/FCU) and the .ight management use of airspace, improved aircraft-on-ground capabilities or extended length messaging. A system (FMS). The target altitude of the surveillance and better safety for traf.c avoid-level 2 transponder will have all the features aircraft also reports what altitude the aircraft ance and con.ict management. of a level 1 with capabilities for UF16, UF20
and UF21 or comm A/B protocol. Most tran-The level of a transponder is an important
sponders installed today are level 2 capable.
The level 3 transponder adds uplink extended point when testing an installation.

length messaging (UELM) to the level 2
transponder. The level 3 transponder is capable is intending to use. The track and turn report Conclusion
of receiving a UELM from the ground interro-(BDS 5,0) will indicate the roll angle, true Will the air transport industry of tomorrow
gation using comm C (UF24) format, but does track angle and rate, ground speed and true truly be a ¡°free .ight¡± community, without the
not need to reply directly. The UELM reply air speed of the aircraft. The heading and speed need for ATC or ground surveillance? This is
will include a summary of the interrogation. report will re.ect the indicated air speed and another discussion entirely. However, mode S
A level 4 transponder is the most advanced mach, the barometric altitude rate, the mag-will continue to evolve to meet the needs of
in its capabilities. This transponder adds netic heading and inertial vertical velocity. an ever-expanding airborne community. The
downlink extended length messaging (DELM) notion of small private aircraft and large air
to the level 3 transponder. The DELM is used What is DF17 extended squitter? transports sharing the same airspace without
similar to the UELM except the DELM is The concept of DF17 extended squitter is the ever-watchful eye of ATC may be a far-
air to ground transmissions. similar to elementary and enhanced surveil-fetched idea for the near term, but may become

lance with one exception: DF17 is a squitter a reality in the future. Only 50 or 60 years ago, Comm B and BDS and does not need an interrogation. Therefore, pilots coined the original phrase of IFR or
These parameters make up an important the DF17 will report its information regard-¡°I Follow Road¡± for guidance and surveillance. part of elementary and enhanced surveil-less of any ground station or airframe ask-All the while the pilots are relying on radio lance. Comm B is the integral part of DF20 ing. DF17 extended squitter is supported by contact and visual acquisition for collision and DF21 that has the message B (MB) .eld the FAA and will make an important part of avoidance. Today, we rely upon high-tech elec-within its reply. The MB message .eld may automatic dependent surveillance¡ªbroadcast tronics to autonomously navigate our airspace. contain downlink data requested from the tran-(ADS-B). DF17 extended squitter includes What will the next 50 years reveal? For now, sponders¡¯BDSregisters.TheseBDSregisters airborne position (BDS 0,5), surface position anunderstandingofthemodeSoftodayand or comm B designated sub.eld registers (BDS 0,6), extended squitter status (BDS 0,7), the new technologies of tomorrow is needed to contain information about the status, intent identity and category (BDS 0,8) as well as provide for a safe .ying environment.

and location of the airframe. airborne velocity (BDS 0,9) reporting. Airborne position (BDS 0,5) includes the What is elementary surveillance? longitude and latitude of the aircraft, the Elementary surveillance is as easy as 1, 2, barometric altitude, the GNSS (GPS derived) 3 or BDS 1, 2 and 3. This concept is driven height and surveillance status. Surface posi-largely in part by EuroControl, that takes basic tion is similar to airborne position with the surveillance (UF0, 4, 5, 11) a step further in longitude and latitude of the aircraft, and the adding the features of DF20 comm B com-movement and heading of the aircraft. The munications for datalink and identi.cation extended squitter status report will re.ect of the airframe. Elementary surveillance is the surface squitter rate, altitude type and made up of several new components. These extended squitter status. BDS 0,8 or identity are 25-foot resolution altitude decode, inter-and category will report the ADS-B emitter rogator identi.cation (II) and surveillance category type ranging from ¡°no reporting¡± identi.cation (SI) veri.cation, .ight status, to surface vehicle to space vehicle. The last data link capability report (BDS 1,0), GICB of these, and possibly the most intense, is

ABOUT THE AUTHOR

Wes Stamper is a sales support engineer for Avionics Products at Aero.ex. He has been with Aero.ex (formerly IFR Sys-tems, Wichita, Kan.) for 17 years. He has authored several papers on avionics test methodologies and other technologies, as well as given presentations, discussions and training sessions on the subject of avionics test. He can be reached at wes. stamper@aero.ex.com.