An ADS-B "Out" Transmitter is used to broadcast a WAAS GPS-derived position to other users of the ADS-B system. These Transmitters use one of two frequencies.
The ADS-B "Out" Transmitters show on this page are 1090 MHz Extended Squitter (1090ES) Transmitters. They are permitted at any altitude and are required above 18,000 feet. While it seems redundant to have ADS-B "Out" transmission on two different frequencies, the 1090ES band was created for higher performance and more capable solutions.
If your aircraft flies above 18,000 feet, you need an ADS-B "Out" solution shown here.