Invisible transponders

Invisible Transponders

In yet another aspect of transponders not appearing on the air traffic controller’s display (which is totally unrelated to “selective rejection”), I was personally affected by the “P4 problem” (aka “The Terra Problem”). That occurred when my aircraft’s transponder became invisible to our newly installed Mode S interrogator (see Letter2Talotta.pdf). That problem seemed to go away shortly thereafter, as the FAA installed a “temporary” fix. This archived message may help you to understand the beginning of that issue. In January 2009, I learned that all FAA “Mode S” interrogators still utilize a software fix (referred to as the “Terra Algorithm”) to deal with other transponders that exhibit “Terra like characteristics,” and that long-range radars have not even been interrogating in the Mode S format since June 2006. See “Turning off Mode-S: A Maintenance Alert was issued on 6/1/2006 instructing BI-6 sites to stop interrogating Mode-S. Instructions for doing this are included in the alert. Go to TechNet for a copy of the alert.”

Thomas G. Lusch

May 27, 2009

Update April 22, 2010: I found it extremely puzzling that Lincoln Labs would have created an interrogation pulse waveform in such a way that ATCRBS transponders could inadvertently become invisible. I did some research and learned that originally the technical specifications had the P4 pulse 1.5 microseconds after the P3 pulse. That 1.5 microsecond spacing certainly wouldn’t have caused the Side Lobe Suppression (SLS) to inadvertently activate on ATCRBS transponders, resulting in the possibility of them becoming invisible. However, a change from 1.5 to 2.0 microseconds spacing took place to fix an unanticipated problem. One can read in the Thursday, March 6, 1980 Federal Register, on page 14780, where the FAA proposed the change. It reads...

Changes in RF waveforms. Changes to the RF waveforms have been made in two areas. The P4 pulse used in the ATCRBS/DABS All-Call interrogation has been lengthened and relocated to prevent false All-Call decodes due to multipath on the interrogation link.”

Michael C. Stevens, in Chapter 9 “Multipath and Interference” of his book Secondary Surveillance Radar, discusses the early Mode S Trials. As best I understand this technical stuff, due to the effect of multipath (i.e. radio waves arriving at the airborne transponder from more than one path...like when they come directly from the radar and bounce off a building and arrive a tiny bit later), a classical ATCRBS interrogation could result in a the Mode S transponder thinking it was seeing a “Mode S All Call” interrogation, which in effect said “do not reply to this classical ATCRBS interrogation...but instead reply as a Mode S transponder.” That apparently generated a false response from the Mode S transponder, and formed a false target at an erroneous range.

Thus, it appears that we may have observed The Law Of Unintended Consequences, in that it appears that in an effort to fix an unanticipated problem (false All-Call decodes due to multipath), a later unanticipated problem (“The Terra Problem”) was created.