Article: 2273 of rec.aviation.owning Path: newshost.ncd.com!ncd.com!olivea!spool.mu.edu!howland.reston.ans.net!europa.eng.gtefsd.com!emory!news-feed-2.peachnet.edu!news-feed-1.peachnet.edu!news.duke.edu!eff!news.kei.com!yeshua.marcam.com!zip.eecs.umich.edu!newsxfer.itd.umich.edu!jobone!lynx.unm.edu!carina.unm.edu!news-user From: ldonovan@unm.edu (leslie donovan) Newsgroups: rec.aviation.owning Subject: Terra/AD Date: 28 Apr 1994 10:59:53 -0600 Organization: University of New Mexico, Albuquerque Lines: 91 Distribution: na Message-ID: <2poq29$gui@carina.unm.edu> NNTP-Posting-Host: carina.unm.edu There seemms to be some confusion about this Terra/FAA story on other newsgroups. Since the topic has come up here, I thought I'd post the following to set the record straight. Here is the straight scoop from Terra Corporation President, Richard W. Donovan-- who happens to be my father. Note particularly paragraph 3 below. You read this and decide for yourself. It's technical, but careful readers will get the picture about the real story here. Terra is a small company; the FAA is a big organization. Terra is trying very hard to handle this problem as best they can. The last paragraph gives a phone number to call if you have any questions-- they mean it. They really will talk to you personally about this. They're as unhappy about this as all the consumers are. Oh-- and if any of you are also on Compu-Serve, Terra would really appreciate it if you would cross-post this to the aviation group there. We hear misunderstanding is happening on that network too, but we don't have access to that one. What follows was written by my father, Richard Donovan, president of Terra. Regarding the TRT 250 Series transponders manufactured under TSO C74c since 1985-- It has been discovered that this transponder and Federal Aviation Administration's (FAA) new Mode S Traffic Control System and the Traffic Alert Collision Avoidance System (TCAS) on airliners are incompatible. These systems were heretofore thought to be compatible with all ATCRBS (Mode A/C) transponders such as the TRT 250 Series. However, interrogations by these systems have been found to momentarily disrupt the TRT 250 transponder's microelectronic timing circuitry so that aircraft equipped with these transponders are rendered invisible to Mode S ground stations and TCAS equipped aircraft. The proposed FAA Airworthiness Directive (AD) remedies this unsafe condition by requiring the modification or replacement of the affected units in a time frame not to exceed six months from the date of the AD. As a result of testing the first Mode S ground sensor in the Baltimore area, it was found that an ATCRBS transponder Model TRT 250 manufactured by Terra Corporation did not reply to new ATCRBS ONLY ATCRBS/MODE S ALL CALL interrogations by the Mode S ground sensor. These interrogations, developed to support both Mode S and TCAS II, are not included in the ATCRBS specifications, adopted in 1970. The Terra transponder was thus not designed to reply to them. The specifications are just now being changed to require that all ATCRBS transponders reply to these new Mode S interrogations. The Massachusetts Institute of Technology (MIT) Lincoln Laboratory tested 18 ATCRBS transponders during the development of the new interrogation formats. All of those transponders were shown to be compatible with the new Mode S signal formats (reference, Lincoln Laboratory Report No. ATC 25, dated 11/25/73) and thus the ATCRBS specifications were not changed. A subsequent report (reference Lincoln Laboratory Report No. ATC 42, Rev. D, dated 8/29/86) confirmed that ATCRBS transponders were unaffected by the Mode S environment; however, Terra's TRT 250 transponder was not tested by MIT. The TSO C74c approval for Terra Model TRT 250 was granted in 1984 under the original specifications that did not require testing for compatibility with Mode S and TCAS. The recently discovered incompatibility with Mode S and TCAS has been traced to the new ATCRBS/Mode S signal formats, which include additional information that is added at the end of a standard ATCRBS-Mode A/C interrogation. Mode S transponders are designed to receive and process this information as part of the new digital communication data link between Air Traffic Control Centers and aircraft. For compatibility, ATCRBS Mode A/C transponders manufactured under TSO-C74c must now reply to the ATCRBS ONLY or ATCRBS/MODE S ALL CALL interrogations, but ignore (i.e. not recognize or act upon) the additional interrogation data. Unlike older transponders, the Terra transponder model TRT 250 Series employs advanced solid state microelectronic circuitry that processes the additional interrogation data as an invalid interrogation and consequently suppresses any response. The FAA has developed an interim fix at the Mode S ground sensor that permits the Terra transponder to function normally. In order to bring Mode S ground sensors on line around the country in support of the new digital communication data link implementation, this temporary alteration to the planned process must be removed as soon as possible following the modification or replacement of all Terra TRT 250 Series transponders. More than 5,000 of these transponders are in the field at this time. Contact Dick Donovan (505) 884 2662 if you have further questions or require additional facts regarding this matter. from Leslie Donovan University of New Mexico, Albuquerque Technical-Vocational Institute email: ldonovan@carina.unm.edu