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A notam issued Sept. 1 announced that, beginning Sept. 2, both ADS-B surveillance and TCAS may be unreliable in Virginia, North Carolina, South Carolina, Georgia, and Florida, as well as in airspace extending approximately 200 nautical miles off shore. The situation is expected to last through Oct. 1 as a result of military exercises in the area.
But similar military exercises in the past have caused no interference with civilian ADS-B or TCAS, and AOPA is asking the FAA to explain both why the notam was issued so late and what has changed to raise these new concerns.
"We are working to get answers for our members," said Rune Duke, AOPA director of air traffic and airspace. "This notam has caused considerable alarm and much confusion, while giving pilots little time to prepare. The long duration, ambiguous language, and short notice of this notam are all cause for serious concern. We have spoken with representatives of the FAA and the Department of Defense and will continue to pursue this until we get the answers pilots need."
The wording of the notam has led many general aviation pilots to believe that ADS-B-based traffic information might not be available to them. But the notam does not specify any interference with 978 MHz ADS-B systems, which are most commonly used by light GA aircraft. As a result pilots should continue to have access to TIS-B and FIS-B services. The greatest impact will be on aircraft using 1090 MHz ADS-B systems or TCAS, which are primarily used by larger, faster aircraft operating in the flight levels. Air traffic controllers will help ensure separation between military and civilian traffic during the exercises, and no delays or reductions in ATC services are anticipated.
...we read that:
"Pilots are advised that the traffic alert and TCAS may fail to establish tracks on nearby aircraft and may fail to receive traffic alerts (TA) or resolution advisories (RA). Operators should be aware that tracks may first appear within close proximity to their aircraft, and may immediately go into TA/RA status.
Pilots are advised to maintain an increased visual awareness in this area. If operators believe that an aircraft should have triggered an alert, the incident should be reported to air traffic control as soon as possible."
https://www.nbaa.org/ops/airspace/regional/20150901-tcas-adsb-unreliable-in-southeast-united-states-beginning-september-2.php
In other words, planes may appear to be there, but are not; and planes that SHOULD be there, may not be!