So like the already existing VOR/DME network?
> The idea is to use these alternative signals to calculate a vehicle’s position and velocity.
All commercial planes have an IMU/ADIRU in them. Most are doubly if not triply redundant.
> There is no question GPS is still the gold standard for navigation
Not for flight. There are plenty of areas in Europe where it is compromised to the point of being useless.
> Scientists refer to it as “signals of opportunity”
Way back in the day pilots would use FM radio stations in the same way. If you were really lost out somewhere over a dark patch.
> If these signals are clean enough for navigation,
The bigger problem you have is while these are "fixed location" services our database of the locations isn't exactly awesome or well maintained in the face of maintenance or physical changes.
"Signals of opportunity" for flight is a bizarre regime to even consider.