I suspect that if both units lose reception concurrently then it may be interference.
In many planes, with less than properly done RF cabling and old radios, doing a VHF transmission on the 11th or 13th subharmonic of 1575MHz (e.g. 121.15MHz) will wipe out GPS reception in seconds.
It is only when approving an IFR GPS installation for flying GPS approaches, or for BRNAV, that these tests are specified.
Anyway,
here is the perfect solution to all GPS dropouts - you sack the relevant official
