Why could I see all the airplanes on my iPad? I have TIS radar capability on my Garmin 750 and they didn't show up on the 750 as TIS traffic. We were about 50 miles from any airport with TIS radar and below 2000' AGL plus the 750 indicated the TIS was unavailable so I don't think that is the answer.
Also, can anyone explain the "hockey puck" of 30 nm radius and a few thousand feet deep?
If you are flying in an aircraft equipped with ADS-B In, you may receive traffic information from the following sources:
A. Directly (air to air) from an aircraft equipped with ADS-B Out.
B. ADS-B information from an aircraft equipped with ADS-B Out
relayed through ATC and an ADS-B ground station.
C. Mode C/S radar information relayed through ATC
and an ADS-B ground station.
You can receive B & C only if you are within range of an ADS-B ground station, AND
you are flying within an active "hockey puck." The "hockey puck" is a cylinder of
airspace 30 miles diameter and 7000' altitude that is activated wherever an
ADS-B Out equipped aircraft flies. If your aircraft is ADS-B Out equipped, you
are always activating your own "hockey puck." Otherwise, you are dependent on
the kindness of strangers.
I have heard it said that it is possible to receive radar information (C above)
reflecting an aircraft not equipped with a transponder, but I have never seen it.
I was under the impression that only ADS-B Out equipped aircraft would show as traffic on a display fed from an ADS-B receiver.
No, ADS-B also has other sources of traffic info. In addition to Ron's link, see https://www.faa.gov/nextgen/equipadsb/capabilities/ins_outs/ and AIM 4-5-6 - 4-5-10.
We were about 50 miles from any airport with TIS radar and below 2000' AGL plus the 750 indicated the TIS was unavailable so I don't think that is the answer.
Typical range is 55nm (per AIM).