iFly GPS Forum

I have noticed a definate delay in target position as opposed to actual position.

Today, while returning from a SoCal mountain resort airport and at 8500 MSL receiving 6 towers, the Android phone showed a targer approching off my left wing and it indicated as 1,000 feet below me.

The aircraft was directly under me and the display indicated that it was approaching my 9 o'clock position.

When the display indicated that it was under me it was actually 1/4 mile on my 3 o'clock. I calculated at least a 10 second delay. I expect SOME delay when you figure ground radar to tower to aircraft and any delays in the IFLY presentation, but with jet speeds the target can be on you very quickly. I would think that airlines have ADS-B on 1090, but since we're not on that frequency there would be a delay since it has to go to a tower and then to my aircraft. Is this correct?

So I have mede it a point to look well ahead of where the target is on the display.

So this may explain why there was such a delay between the displayed position of the airliner under me and the actual position. It has to do with the rebroadcast necessary because we are on a different frequency than the airlines.

Automatic Dependent Surveillance–Rebroadcast (ADS-R): ADS-R takes position information received on the ground from UAT-equipped aircraft and rebroadcasts it on the 1090 MHz frequency. Likewise, ADS-R rebroadcasts 1090 MHz data to UAT users. In concert with TIS-B, ADS-R provides all ADS-B In-equipped aircraft with a comprehensive airspace and airport surface traffic picture. ADS-R delivers traffic data within a 15-nm radius 5,000 feet above or below relative to the receiving aircraft’s position.

Edited to correct: According to the FAA's Technical Documentation, the ADS-B System is allowed a latency of 6 seconds for towers with Terminal Volumes and 12.1 seconds for towers with En Route volumes. That's for TIS-B traffic. (http://adsbforgeneralaviation.com/wp-content/uploads/2011/12/SBS-Description-Doc_SRT_47_rev01_20111024.pdf). You were talking -Rebroadcast and quoted the spec for that. In either event, that's the latency to get the traffic data they have to you. I haven't found the spec on the latency they allow themselves to gather and tailor that data. (I see one second as a best case mentioned for reporting, but it's allowed to gracefully degrade.) As far as latency from the EFB, I expect it is nil. (Update: Well, since the iFly display updates once a second, one second is the best case low latency.)

My personal experience (so far) is that, here at my home airport, 1090 traffic is spot on. (Makes sense, since no towers are involved in getting the data to me when I'm listening on 1090 with a dual Stratux. So you might want to go dual band to get rid of ADS-R latency.) I don't have much of a feel for 978 traffic but I am beginning to notice that the traffic I see is always ahead of what's shown. So your thought to look ahead of the target is a good one. Traffic data is history. It is not the present.

(And, so, from an ergonomic point of view, I would make the back of the traffic arrow be the location of the target. That way, the length of the arrow to the tip would more correctly tell you where to look. (Turning traffic excepted.) But since the traffic arrow is a spec from TCAS, I doubt that anyone will change the way things are now.)