Went to an interesting Royal Institute of Navigation lecture on SatNav for General Aviation some years ago. Got lost in Helmert Transformations and Molodenski Formula, which was ironic, but a very interesting chap called Huw Baumgartner from CAA/NATs summarised the CAA’s position on technical issues.
He made 3 navigational equipment system definitions.
A Sole Means Air Navigation System – approved for specific phases of operations. VOR/DME is an example.
A Stand Alone Air Navigation System – is not combined with other navigational sensors or systems. It gets no help from another source such as when an IRS is updated by multi DME - DME.
A Supplemental Air Navigation System – is an approved system that can be used in conjunction with a Sole Means System.
The GA community is looking for an approved Stand Alone – Sole Means – cleared for enroute and approaches- viz. GPS. The difficulty here is that the signal is weak and subject to in band interference and out of band interference. The former relates to problems at or near the propagated signals spectrum – satellite comms systems for example and in the latter case, transmissions on apparently remote frequencies that interfere with the signal. Harmonics on VHF frequencies such as the thirteenth harmonic of Gatwick Approach falls in the middle of GPS spread. Arc welders also have an interesting effect!
The CAA’s position is that GPS is a Supplemental System when approved.
Wilful interference is a key topic that is no doubt being addressed by security agencies. An ice cream van fitted with a jammer the size of a shoebox could, if sited on Hog’s Back in Surrey, could render all GNSS approaches inoperative, at all South East UK airports. These are the times we live in.
The conclusion was that CAA is supportive of satellite based navigation but it has to be conducted at an appropriate level of safety. Safety in a security context seems to be a moving and political target. The provision of a dedicated European satellite in the near future to provide a form of WAAS is eagerly anticipated.
I use a B-RNAV approved GPS for enroute navigation. I get at least one RAIM failure per flight on average. They don’t last for long and other non approved GPS installations in the A/C give no indications other than they are fat and happy – whilst the main GPS is red flagging the HSI and beeping away like a good ‘un.
Came away from the lecture committed to always fly a Sole Means Air Navigation System side by side with GPS.