My GDL 82 UAT has "anonymous mode" available when squawking 1200.
"Want to fly with anonymity?
There are private pilots. And then there are extra-private pilots who want to keep their identities off the traffic screens of other ADS-B “In” equipped aircraft (containing business competitors, perhaps?). GDL 82 and its UAT technology can allow for this extra measure of identity security with “Anonymous mode.” Most conventional ADS-B devices are designed to transmit your aircraft’s ICAO number — a discrete “hex code” assigned by FAA as part of your aircraft’s certificate of registration. If you’re flying VFR, however, you can have GDL 82 set to mask your aircraft’s ID from being transmitted to other receivers whenever your transponder’s squawk code is set to 1200. ATC can still track you with ADS-B precision. But your GDL 82 simply won’t transmit your aircraft’s N-number to other pilots in the airspace."
Part 91 and the TSO for UATs allow anonymity. The others with Mode S/1090ES out are not allowed to.
(d) Minimum Broadcast Message Element Set for ADS-B Out. Each aircraft must broadcast the following information, as defined in TSO-C166b or TSO-C154c. The pilot must enter information for message elements listed in paragraphs (d)(7) through (d)(10) of this section during the appropriate phase of flight.
(1) The length and width of the aircraft;
(2) An indication of the aircraft's latitude and longitude;
(3) An indication of the aircraft's barometric pressure altitude;
(4) An indication of the aircraft's velocity;
(5) An indication if TCAS II or ACAS is installed and operating in a mode that can generate resolution advisory alerts;
(6) If an operable TCAS II or ACAS is installed, an indication if a resolution advisory is in effect;
(7) An indication of the Mode 3/A transponder code specified by ATC;
(8) An indication of the aircraft's call sign that is submitted on the flight plan, or the aircraft's registration number,
except when the pilot has not filed a flight plan, has not requested ATC services, and is using a TSO-C154c self-assigned temporary 24-bit address;
(9) An indication if the flightcrew has identified an emergency, radio communication failure, or unlawful interference;
(10) An indication of the aircraft's “IDENT” to ATC;
(11) An indication of the aircraft assigned ICAO 24-bit address, except when the pilot has not filed a flight plan, has not requested ATC services, and is using a TSO-C154c self-assigned temporary 24-bit address;
(12) An indication of the aircraft's emitter category;
(13) An indication of whether an ADS-B In capability is installed;
(14) An indication of the aircraft's geometric altitude;
(15) An indication of the Navigation Accuracy Category for Position (NACP);
(16) An indication of the Navigation Accuracy Category for Velocity (NACV);
(17) An indication of the Navigation Integrity Category (NIC);
(18) An indication of the System Design Assurance (SDA); and
(19) An indication of the Source Integrity Level (SIL).
