What 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 flight ID, elementary 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.

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 identification and altitude reporting. This was and still is an important component of air traffic control and air space management. As more and more airframes were available to the private and commercial flying community, this basic form of surveillance was overwhelming the capacity of the air traffic 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 interrogation, 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 °. Apart from better azimuth accuracy the monopulse technique reduces the number of interrogations required to track a target, as it theoretically requires only one reply to obtain the azimuth and range of the airframe.

The mode S concept was mostly a development of MIT Lincoln Lab with coordinated efforts from the Federal Aviation Administration (FAA), Aircraft Owners and Pilots Association (AOPA) and the transponder manufacturing community. Mode S technology was first 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 gathering 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 capabilities 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.

The basic ATCRBS system relies on pulsed RF as its means of communications. These pulses are 0.8 µs wide and vary from 8.0 µs to 21 µs spacing for mode A and C respectively. 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 efficiency 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 receives. Once the SSR has received the reply it will decode the mode (A, C or S) and demodulate the information within each mode.

There are three interrogation types in a mode S SSR system:

1. ATCRBS all call: This interrogation consists of P1, P3 and a 0.8 µs P4 pulse. P2 SLS is transmitted as normal. All ATCRBS transponders reply with the 4096 identification code for mode A interrogations and altitude data for mode C. Mode S transponders do not reply on this interrogation.

2. ATCRBS/mode S all call: This interrogation is identical to the former except P4 is 1.6 µs long. ATCRBS transponders reply with the 4096 code or altitude data as per the ATCRBS all call. Mode S transponders reply with a special code, which contains the identity and the aircraft's discrete address.

3. Mode S discrete interrogation: This interrogation is directed at a specific mode S transponder-equipped aircraft. The interrogation consists of P1, P2 and P6. P2 is transmitted via the directional antenna and hence is the same amplitude as P1 and P3. This effectively suppresses ATCRBS transponders from replying. P6 is actually a DPSK data block that contains either a 56-bit or 112-bit message. The DPSK modulation produces a spread-spectrum signal, which has immunity to interference.

When the transponder receives a valid mode S discrete interrogation, it will return a reply 128 µs after reception. The reply is transmitted on 1090 MHz and uses a 56-bit or 112-bit PPM transmission.

Each mode S interrogation will have a 24-bit address unique to the aircraft as well as a 24-bit parity check for validation. In basic mode S surveillance, the information is limited to altitude reporting (DF0), aircraft identification (DF4) and basic airframe information (DF11). The SSR is the main component of the interrogation and reply. The interrogation happens about 50 Hz PRF. The reply will happen at the same PRF. Once the SSR has received the reply it will decode the mode (A, C or S) and demodulate the information within each mode.

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