The concept of DF17 extended squitter is similar to elementary and enhanced surveillance with one exception: DF17 is a squitter and does not need an interrogation. Therefore, the DF17 will report its information regardless of any ground station or airframe asking. DF17 extended squitter is supported by the FAA and will make an important part of automatic dependent surveillance — broadcast (ADS-B). DF17 extended squitter includes airborne position (BDS 0,5), surface position (BDS 0,6), extended squitter status (BDS 0,7), identity and category (BDS 0,8) as well as airborne velocity (BDS 0,9) reporting. Airborne position (BDS 0,5) includes the longitude and latitude of the aircraft, the barometric altitude, the GNSS (GPS derived) height and surveillance status. Surface position is similar to airborne position with the longitude and latitude of the aircraft, and the movement and heading of the aircraft. The extended squitter status report will reflect the surface squitter rate, altitude type and extended squitter status. BDS 0,8 or identity and category will report the ADS-B emitter category type ranging from “no reporting” to surface vehicle to space vehicle. The last of these, and possibly the most intense, is airborne velocity. This will report the east/west and north/south velocities, heading, vertical rate and altitude source, the difference between the barometric and GPS altitude, the IFR capability and airspeed.

As previously discussed, DF17 is the integral and working portion of automatic dependent surveillance broadcast (ADS-B) (Figure 3). Breaking down the meaning of the terms: automatic — there is no interrogation needed to start the data or squitter coming from the transponder; dependent — as it relies on onboard navigation and broadcast equipment to provide information to other ADS-B users; and surveillance — it is a means of automatic surveillance and traffic coordination. Some of the benefits of ADS-B technology are better use of airspace, improved aircraft-on-ground surveillance and better safety for traffic avoidance and conflict management.

Will the air transport industry of tomorrow truly be a “free flight” community, without the need for ATC or ground surveillance? This is another discussion entirely. However, mode S will continue to evolve to meet the needs of an ever-expanding airborne community. The notion of small private aircraft and large air transports sharing the same airspace without the ever-watchful eye of ATC may be a far-fetched idea for the near term, but may become a reality in the future. Only 50 or 60 years ago, pilots coined the original phrase of IFR or “I Follow Road” for guidance and surveillance. All the while the pilots are relying on radio contact and visual acquisition for collision avoidance. Today, we rely upon high-tech electronics to autonomously navigate our airspace. What will the next 50 years reveal? For now, an understanding of the mode S of today and the new technologies of tomorrow is needed to provide for a safe flying environment.

Wes Stamper is a sales support engineer for Avionics Products at Aeroflex. He has been with Aeroflex (formerly IFR Systems, 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@aeroflex.com.

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