Flying had become somewhat routine – hundreds of hours in the logbook, an instrument rating, and I had now gone up in varying weather conditions as a cautious and careful pilot without incident. I had recently purchased one of the new Stratus ADS-B In receivers, and appreciated the subset of traffic that appeared on my iPad mini.
It was an unremarkable flight so far, but suddenly the large letters “TRAFFIC” plastered across my screen with corresponding alert. Three hundred feet below and slightly behind was an airplane, approaching fast. I had overflown an airport, but hadn’t seen anybody. I banked left and right in my low wing craft, looking for the guy, who must be right below me, now 200 feet. On a collision course. He was going to rise up into my gear or clip my wingtip. This was an extremely dangerous situation. Craning my neck in all directions, still negative contact.
The other airplane was ever so close, but still below, now slightly off to the right. Evasive action was now mandatory. I looked to my left and above: clear. I did a chandelle, pulling a few G’s. I was VFR, without flight following this morning, and this should nicely take me out of harm’s way.
Except it did not – by awful luck the other pilot had also turned left and was still on my tail! Perhaps he had done the same maneuvering when he saw me. I looked around, still no dice. Well, about this time I figured it out, my stress level dropped significantly, and the experienced reader has probably determined what was happening, given the series of improbable events. In order to test my theory, I made a gentle circle toward back on course, and sure enough, this pesky other pilot followed me back on heading, and then suddenly disappeared altogether.
I was dogfighting with myself. Fortunately, other than a few cups of wasted avgas and a ounce of knowledge gained, there was no harm done.
Traffic Information Service (TIS) obtains radar-derived location data of aircraft transponders; this location data is uploaded to a receiver for display as an aid in situational awareness and avoiding potential traffic conflicts. TIS-A used the Mode S data block alone and was restricted to terminal radar control areas, while the newer TIS-B integrates ADS-B location data in addition to radar data for subsequent re-broadcast from a series of ground stations.
The TIS-B uplinks for display are performed both using the 1090 MHz frequency (often called extended squitter) or the 978 MHz frequency (called UAT, for Universal Access Transceiver); in areas of reception for TIS-B this gives the receiver traffic, even without the newest ADS-B Out equipment. This is integrated into the more complete system as part of the full NextGen ensemble – in essence the ADS-B ground stations will uplink both traffic information (ADS-B and radar airplane locations) and FIS-B (flight information service, primarily weather updates).
As ADS-B rebroadcast is upon request and only includes traffic that is relevant to the airplane receiving the broadcast, only airplanes with ADS-B Out equipment will receive a complete traffic picture. Airplanes with ADS-B In equipment, however, may “piggyback” and receive the airplane locations and display them on their screen when in the vicinity of an airplane that has requested ADS-B data.
Thus, my own airplane, with only the Mode S transponder (not ADS-B Out), was being tracked by ground radar, and I received the location of myself via the TIS-B upload, as I was near to a major airport providing this (radar-based) service and another airplane that had triggered the broadcast upload. The application running on my iPad, ForeFlight, did its best to determine what my own track was, but displayed the slightly laggy radar data on my screen as another ship. My non-baro-aided ADS-B In only “knows” of the GPS location, so if this differs enough from the TIS information, the unknown craft will be displayed.
In talking with ForeFlight support, their programmers note that ADS-B/TIS-B In will attempt to eliminate the ownship based on the current GPS location, however, this is imperfect and sometimes the uploaded data will show. With ADS-B Out on the aircraft, ForeFlight will eliminate ownship because it knows your airplane’s exact location and N-number from the Out transmission.
Since my initial surprise, I have had numerous appearances of ownship alerts using ForeFlight, all in TIS-B areas which generally exist around busier Class B airports. A hallmark is the slight lag of the upload, so the ship appears slightly behind and does whatever maneuvering you have been doing; and altitude is invariably within 300 feet or so.
Awareness of these events will prevent pilots from being surprised by the sudden appearance of phantoms, and help the ADS-B user to distinguish the real threats from the sudden appearance of ghost planes from self-tracking. In this situation the primary danger that could result is from excessive or inappropriate maneuvering as part of a startle or panicked reaction, which could be even more dangerous than a real threat.
Until ADS-B Out is required, shadows of ourselves, our own ghost airplanes, will remain on occasion.