Airborne_Again wrote:
You can also compensate using two different GPS signal frequencies, which is what the US military does.
Dual frequency is now also available for civilians, on GPS Navstar and on Galileo, although not necessarily all devices support it. My OnePlus 7 Pro pocket computer does, but my Samsung Galaxy Tab S6 Lite (Wi-Fi) does not.
Peter can you clarify what you mean by “the sats participate in the 3D solution”? The GPS sats are the same, whether we are talking about WAAS or EGNOS – both are different SBAS systems to apply corrections to GPS signals. I guess EGNOS can also be used on Galileo. Are you talking about the comms sats used to transmit the corrections?
With all due respect, I don’t think we should rely on one person’s experimentation to say SBAS does not improve position accuracy. Perhaps there are cases where non-augmented GPS is as accurate as GPS+SBAS and you have observed that, but it can’t be relied on to be generally true,
Separate point – I’m struggling with terminology here. People refer to “+V approaches”. However as I understand it there is no such thing. +V is a Garmin annunciation which means advisory glideslope, so in the case of LNAV+V it is an LNAV approach right? I get it that there is still a discussion to be had about the accuracy of the advisory information provided, but it is still just advisory.
Peter wrote:
WAAS should be better because the sats participate in the 3D solution, but here we have EGNOS which doesn’t.Experimentally, empirically, I see no vertical improvement with EGNOS satellite reception – as posted above.
That is not the major effect of SBAS improvement. The measurements in the reports I referenced are once per second for each location, so almost 8 million samples over the quarter for each location.
It looks like UK is doing some testing of their own SBAS. https://www.uksbas.org/
Uptimist wrote:
in the case of LNAV+V it is an LNAV approach right?
Legally officially it is. But you have a glideslope indication on your instruments, so in practice you can follow it, and officially legally cross-check your altitudes against the distance remaining to runway threshold every nmi or so, in the same way you would do on an LNAV approach, and lo and behold, while you are on the “advisory” glideslope, the altitudes all crosscheck magically. Just like you can fly an ADF approach following the more precise GPS “route”, and just crosscheck every 15s or 30s or so that yes, the ADF needle is still at the right place.
In practice you fly it like an LNAV/VNAV or an LPV, but legally officially you are flying an LNAV.
so in the case of LNAV+V it is an LNAV approach right?
Yes the minima is LNAV MDA, if you are flying LNAV on Dive-Drive or CDFA profile you could be above or under +V advisory guidance
The main advantage of LNAV+V with SBAS is that an autopilot can fly your advisory glide-slope in coupled mode all the way, legally, you should disconnect at 800ft agl (or whatever figure in AFMS), in practice it should works to MDA or even to touchdown: you would love how GPS-W with decent AP can fly “safe unofficial 3D” when operating under 2D approaches like LNAV, NDB, LOC or VOR…while you spend useful time monitoring the approach profile using raw data on second CDI/HSI and DME
So sorry, just being a bit pedantic, I understand the way the boxes provide guidance when annunciating +V, but we should not call it a “+V approach” or “LNAV+V approach”.
Peter can you clarify what you mean by “the sats participate in the 3D solution”?
They emit the GPS signals, in addition to the corrections.
For a GPS system, you don’t need a constellation of sats continually orbiting. You can also do it (over a limited area) with a number of geostationary ones.
So sorry, just being a bit pedantic, I understand the way the boxes provide guidance when annunciating +V, but we should not call it a “+V approach” or “LNAV+V approach”.
Feel free to be pedantic but it is extremely useful 
It is a glideslope in the same way as an LPV glideslope. Just in a different place vertically.
Uptimist wrote:
So sorry, just being a bit pedantic, I understand the way the boxes provide guidance when annunciating +V, but we should not call it a “+V approach” or “LNAV+V approach”.
The pedant in me can’t resist. The approach is one that conforms to the RNP APCH PBN Navigation Specification and is titled either RNAV in the US or RNP in the rest of the world. The minima line is LNAV or LP and is a 2D procedure. The annunciation is LNAV+V or LP+V where the +V signifies an advisory glide path is optional vertical guidance generated by the GPS manufacturer and may be used for the descent from the FAF to the MDA to aid the pilot in making a stabilized descent to the MDA. It is not charted on the approach procedure, nor is it part of the procedure design.
I get LNAV+V in raw NDB approach flown on magentas, what I should call that?
