Please see below message from CEO Marc Bailey (25 July 2018)
Following talks with the CAA yesterday at the Business Aviation Flight Operations Liaison Group (BAFOLG) we discussed the issue of GPS jamming. Something surfaced which you may wish your CAM to consider. Whilst examples of GPS jamming will obviously remove that navigation aid, all operators will have operational procedures in place to address this loss of GPS. However, you may have a dormant failure which could impact your GPWS. There have been examples of GPWS position input being taken from GPS, rather than actual position from other sources. Clearly this failure mode would place your GPWS warnings under question, but you might not see any indication as such of this performance reduction.
It might be worth processing this issue through your SAG.