Thwice now I've had ADS-B IN OFFLINE caution messages on my Skyview. In between I've had flights where everything worked perfectly. TIS traffic has been visible when the ADS-B wasn't working. I haven't been able to locate a source for adsb ground station status (NOTAMs?) to help determine if it's my problem or theirs. I'm in Spokane, WA and have been in the local area both times I got the failure message.