You are not going to get a gps jam just as your birds or ice or whatever it is disables your air data systems. Not going to happen.
Its a cinch
TAS INCREASES BY 50 kt per 10000. So if the following are at a safe speed at MSL you have all you need is this knowledge
MSL
250:kts IAS AND GS
FL 100. 250 kts IAS= 300 GS
FL200 250 IAS = 350:GS
etc etc
At FL350 250:kts IAS = 450 kts.
If you fly these GS in still Air you are safe. OK - then +/- the wind which is elementary maths.
You could fly a whole flight from takeoff to landing using this info and always have safe margins.