Mode S alone is not ADS-B out. You need a mode S xponder with the ES extended squitter add on.
With no ground station signal you can get air to air on a 978 receiver or a 1090 receiver or a dual band receiver but you will only see the planes that are on the same band that you have a receiver for.
A mode-S xponder can receive TIS-A traffic.
For areas where you have no ground station reception, a dual band receiver will get all ADS-B out equipped planes. You just would not see the composite TIS-B based traffic info.
With no ground station signal you can get air to air on a 978 receiver or a 1090 receiver or a dual band receiver but you will only see the planes that are on the same band that you have a receiver for.
A mode-S xponder can receive TIS-A traffic.
For areas where you have no ground station reception, a dual band receiver will get all ADS-B out equipped planes. You just would not see the composite TIS-B based traffic info.