ADS-B is the automated process by which appropriately equipped aircraft transmit technical data such as position, flight direction and altitude, but also information such as identification and flight number. The transmission is undirected (broadcast, the "B" in ADS-B) and unsolicited on the frequency 1090 MHz. The transmission is not encrypted. With an inexpensive SDR plus suitable software or a special receiver, reception is easily possible - provided you have a good antenna for ADS-B.
WiMo offers antennas for outdoor mounting and antenna kits with preamplifier for 1090 MHz and cable. Even in moderately good locations you can often achieve ranges of up to several hundred km to receive ADS-B transmissions.