iFly GPS Forum

Agree, having relative altitude would be great. However, Walter brought up a good point, there is an issue in the ADS-B traffic until 2020 as ADS-B out traffic uses Pressure Altitudes. However transponder traffic being broadcast on the TIS-B is pressure altitude, and there can be a difference of 5-10% difference between pressure vs GPS altitude.

You could potentially be at FL6 to FL10 and have a aircraft showing it is 500 ft above you and climbing, when in reality it is 100 below you and climbing into you.

I think it would be safest to show Altitude, Climb/Descent, and Speed. It's pretty simple to look at traffic altitude and look at your altitude (on GPS) and reference how far it is above or below you. Then the climb or descent would tell what they are doing, the speed would be nice to tell relative closure or distance. IE, if it is 3 miles away but moving at 250kts, look fast! Just some thoughts we had. Keep the feedback coming.

I'd have to confirm with Walter, but I think we can determine if it is GPS or Pressure altitude, depending on the source. However, getting pressure altitude into the iFly or a portable device is a challenge.

I think the traffic info is very useful. Anything to assist you in where to look to get a visual is very useful. You just have to know what numbers you are looking at. Keep in mind it is also designed to be a reference system that is a perk for GA aircraft. Come 2020, when everyone is ADS-B out compliant, then you should be looking at no issue as that is the one of the big benefits of ADS-B, more accurate GPS position data.

Think of the same scenario with your altimeter (pressure attitude) and your GPS. You'd want to report your Altimeter reading (pressure altitude) to ATC, not GPS as they will be different. Same for staying out of airspace, you would really want to make sure to stay above or below based on pressure altitude, as that is what ATC references. It makes you think, huh. Have a great weekend. Great dialog.

Now it might be possible to pull pressure from the ADS-B info and correct for pressure altitude, provided you have accurate in air ADS-B data. I'll bounce that around the office.

Tim,

They see both and it is my understanding, they correct it. I've got a questions in to find out a little more on how ATC corrects and how the ADS-R (Rebroadcasts of 1090ES/Mode S) conveys altitude. I should know more shortly.

All true ADS-B out (978 or 1090) will report GPS Altitude, however a 1090ES via Mode S is likely reporting Pressure Altitude from the Transponder. How they are handleing them for the re-broadcast would be better known by one of the ADS-B manufactures. That is what I've asked. Stay Tuned.