Radio Nav Systems RQ - Mode C XPDR???

[FRQ Charlie Bravo]

Reading the AIP for a bit of study got me talking to a very knowledgeable LAME (E, I & R) friend.

He mentioned something about ATC directing a Go Around (Missed Approach) if, whilst making a GNSS Approach and not yet "visual", they lost the aircraft's Mode C readout. My mate said it was due to some earlier GPS receivers which took barometric aiding readings from the encoder attached to the altimeter (a now discontinued practice).

I cannot find any reference in the AIP (Book) which specifies that Mode C is required for a specific approach. (Sure it's required for flight in Radar CTA generally but I'm referring to a failure in flight.)

Any body out there know what he was referring to and able to back it up with a Reference?

FRQ CB

[AerocatS2A]

That doesn't make sense. A GNSS approach is non-precision and as such it doesn't have a glideslope presentation (yes I know that some GPS/FMS units with VNAV will display a pseudo glideslope, but it is a guide only and isn't part of the approach.) I don't see how anything other than the normal IFR altimeter requirement would apply.

[Bevan666]

Whilst I know nothing of a requirement for ATC directing a go-around, I do about the GPS side of things.

A TSO-C129 GPS (KLN89, KLN90, GNS430 etc) does use mode C input from the transponder. It uses that as a pseudo satellite input (one which is located at the centre of the earth). It uses that to help exclude bad GPS signals, which is a requirement for RAIM (receiver autonomous integrity monitoring).

Without mode-C input to the GPS, RAIM will not be available, and your GPS will not go 'active' during an approach.

This may change with TSO-145 kit (such as the GNS430W)

Bevan..

[Oktas8]

Barometric aiding is certainly available on TSO'd GPS receivers, but it is not a requirement. Sufficient satellites in a sufficiently helpful distribution in the sky will also provide RAIM. RAIM is required for a GPS approach, but the rules don't state, and the pilot won't know, how the GPS receiver achieves the required integrity and accuracy for RAIM to be available.

I doubt ATC would know or care whether I'm using barometric aiding on any given approach. The whole point of RAIM is that it is Receiver Autonomous - that is, it requires no monitoring or intervention from outside the aircraft.

If an aircraft ceased squawking Mode C on the approach, ATC might tell it to go around for separation reasons - I wouldn't know - but not for GPS reasons I think.

Cheers, O8

[Jack Ranga]

There is NO WAY an ATC would direct a go around from a GNSS approach if the mode C was lost. It's up to the pilot to assess visual and either land (with a clearance) or make missed approach.

ATC use mode C for separation not to decide whether a pilot is visual or not.

[Masif Eego]

might be best if he sticks to swinging on spanners....:)