A definition of squitter is a reply format transmission without being interrogated. These “unsolicited replies” or squitters are used to provide TCAS 2-equipped airframes with the discrete address of the squittering airframe, to enable the TCAS 2 system to acquire and track the airframe using mode S formats UF/DF0 and UF/DF16.

Squitter has its origins in distance-measuring equipment (DME) transmissions. The DME ground station would broadcast unsolicited replies or squitters. When the airborne DME interrogator was in range, the squitter would be seen and the DME interrogator would then transmit a range interrogation and receive range replies from the DME ground station. This served to limit unnecessary transmissions over the air and optimized DME ground station-handling capability. TCAS 2 systems use mode S squitters in a similar fashion; the TCAS just listens for the DF11 squitters, which contain the sending aircraft's discrete address, thereby reducing the need to interrogate over the air. The discrete address, once obtained, is placed on the TCAS 2 processor's roll call of addresses for ongoing tracking. Mode S technology has two types of squitter, a short (56 bit) DF11 acquisition squitter and the extended (112 bit) DF17 squitter.

The functional components of mode S are uplink format (UF) and downlink format (DF). UF is a specific interrogation originating from SSR or another airframe asking specific, addressable information about that airframe. DF is the reply from the airframe in regard to the UF interrogation. UF 0, 4, 5, 11 and 16 make up basic surveillance. Basic surveillance messages are comprised of the airframe address, parity bits and a 56-bit data word known as “short” interrogations and replies.

UF0 is a short air-to-air surveillance for TCAS/ACAS (Figure 1). The DF0 reply will include the mode C altitude as well as the mode S address. To test DF0, its altitude reply is compared to the mode C altitude and the mode S address for verification. Also encoded in the DF0 reply is the vertical status (VS) bit, the reply information (RI) field. The VS bit will indicate a 1 if the airframe is on the ground and a 0 if it is on the air. The RI field is a four-bit word containing the airframe's true speed capability and type of reply to the interrogating airframes.

UF4 is a short, ground station request for altitude similar to the UF0 request, but initialized by the ground station (Figure 2). Testing the DF4 reply is verified against the mode C altitude and the mode S address for validity. Also encoded in the DF4 reply is the flight status (FS) field, downlink request (DR) field, utility message (UM) and the altitude code (AC) field. The FS field is a three-bit word reflecting eight different conditions of airborne, alert and special position indicator (SPI) status of the airframe. The DR field is a five-bit word that will contain the request to downlink certain airframe information. The UM field is a six-bit word that contains transponder communication status information. The AC field is a 13-bit word that contains the altitude of the airframe with special encoding 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 identification and is compared to the mode A 4096 code for validity. Also encoded in the DF5 reply is the flight status (FS) field, downlink request (DR) field, utility message (UM) and the identification (ID) field. The FS field is a three-bit word reflecting eight different conditions of airborne, alert and special position indicator (SPI) status of the airframe. The DR field is a five-bit word that will contain the request to downlink certain airframe information. The UM field is a six-bit word that contains transponder communication status information. The ID field is a 24-bit word that contains the mode A identification 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) field, parity/interrogator identifier (PI) field, the interrogator identifier (II) and surveillance identifier (SI). The CA field is a three-bit word that contains the communication capabilities of the transponder. The PI field is a 24-bit word that will report the interrogator identification code with a parity overlay. The II field is a four-bit word, from 0 to 15, containing the identification code of the interrogator. The SI field 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 verification. Also encoded in the DF16 reply is the vertical status (VS) bit, the reply information (RI) field and the altitude code (AC) field. 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 field is a four-bit word containing the airframe's true speed capability and type of reply to the interrogating airframes. The AC field 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) field. This field 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 contains a 56-bit message field for transferring downlinked aircraft parameters (DAPS). To properly test the DF20 reply, the UF20 must contain a reply request (RR) of 17, a designator identifier (DI) of 7 and a reply request subfield (RRS) of 0. The working component of the DF20 reply is the comm B message field (MB). Contained in the MB field is aircraft address (AA), downlink request (DR), flight 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 field for transferring downlinked aircraft parameters (DAPS). To properly test the DF21 reply, the UF21 must contain a reply request (RR) of 17, a designator identifier (DI) of 7 and a reply request subfield (RRS) of 0. The working component of the DF21 reply is the comm B message field or MB. Contained in the MB field is aircraft address (AA), downlink request (DR), flight status (FS) and identification code (ID). 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 UF21 interrogation.

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