Automatic Dependent Surveillance Broadcast (ADS-B) is the relatively new method for an airplane to broadcast its ICAO ID, location, altitude, ground speed, and track to any 1090 mHz receiver. It uses the relatively new Mode S Protocol. Mode S transponders are compatible with Mode A and Mode C transponders, which are used to respond to a request for an "ident" and an altitude from an ATC (Air Traffic Control) controller. ADS-B uses the airplane's GPS information for its location, altitude, ground speed, and track.

ADS-B transmitters will be required in all airplanes flying in controlled airspace by 2020. Airplanes that only fly below 18,00 feet might get by with a 978 mHz transmitter to broadcast their ADS-B info.

An important shortcoming of this broadcasting is that each broadcast can interfer with another. So they're broadcast very frequently, and the receivers must disregard over-lapping broadcasts and only accept good ones. It's messy, but it's been shown to be the best way after years of testing.

There are several Uplink/Downlink Formats in the Mode S Protocol. Downlink Format 17 (DF17) is used for ADS-B broadcasts. It contains 14 bytes of information.

DF17 transmits an airplane's ICAO ID along with with either altitude and location, airplane callsign, or ground speed and track. It needs to be received at least 4 times to completely receive all of an airplane's useful information. The 4th broadcast is required because to know an airplane's location, 2 different location broadcasts need to be received (because the 34 bits (a little over 4 bytes) transmitted containing latitude and longitude data aren't enough to hold complete location information, so an elaborate method of compressing the full information was used, which uses 2 location reports).

If you buy a 1090 mHz receiver like the NooElec R820T2 or the RTL-SDR USB dongle with antenna ($20 or $25), and plug it into your computer's USB port, and then download the free SDR (Software Defined Radio) RTL1090, which receives data from the USB dongle and sends ADS-B data to a computer's TCP port 31001, THEN USAPhotoMaps can receive it and displays that data on its map.

The receiver chip in the NooElec and RTL-SDR is the RTL2832U, a high-performance DVB-T COFDM demodulator (see DVB-T in Wikipedia) by Realtek, and the tuner chip is the R820T2.