Why do ADS-B outages occur without simultaneous RAIM outages?
RAIM (Receiver Autonomous Integrity Monitoring) is a function within an avaition GPS receiver. It uses redundant measurements for a consistency check to determine the intergrity of the position measurement. 

Since RAIM requires redundant measurements, it is only available when there are sufficient measurement sources available. In practice, this mainly boils down to the number of functioning GPS satellites that can be seen from the receiver, and the geometry of the satellites positions relative to the receiver. 

For normal RAIM prediction, the RNP integrity limit of 1 or 2 NM is used. This means that the RAIM function needs to be able to verify that the GPS receiver will not produce a lateral position error greater than 1 (or 2) NM with a probability exceeding 10-7 per hour. 

The  online predition tool simulates the GPS constellation and calculates whether a GPS RAIM system would be able to reach that integrity limit with the (healthy) satellites in view of the receiver at the time of flight. 

For ADS-B, the position transmitted needs to have a certain level of quality in terms of accuracy and integrity. The accuracy of ADS-B position (encoded as NACp) need to be 0.05 NM or below, and the integrity (encoded as NIC) needs to be 0.2NM or below. 

To determine the integrity of the ADS-B position, the GPS receiver again uses RAIM. To qualify for ADS-B, not only needs RAIM to be available, it thus needs to determine the integrity at 0.2 NM or less, and at the same time an accuracy of 0.05 NM needs to be achievable. 

Since the ADS-B Integrity requirements are much more stringent (0.2 NM vs 1 or 2 NM), the prediction of integrity for ADS-B shows more frequent outages. 

Note that the prediction tool only provides a prediction. The actual values during flight may be different. 



References: 


RAIM prediction requirements FAA AC 20-138D section 5-2.3pdf
ADS-B requirements: 14 CFR 91.227 c) 1) i) &amp; iii)