chflyer wrote:
No, not in this context (approach phase up to MAP). It is relevant wrt missed approaches where the first leg is a climb on heading to altitude. Without baro-aiding, the pilot needs to manually sequence. With baro-aiding, the GPS will automatically sequence to the next leg once altitude is reached.
Baro-Aiding is RAIM terminology. You are discussing having a corrected Baro Altitude input from an AHRS/PFD system or air data unit that provides a pressure altitude with a baro-pot input to determine the local altimeter setting.
The important difference is that the glideslope in LNAV/VNAV is a part of the designed procedure and is certified, while the +V is, as you say, merely an uncertified calculation done by your navigator to help you from FAF to MAP. You should not rely on it, you are on a 2D approach and should be using RoD checked against check altitudes every mile.
Secondly, and in a very few cases crucially, the LNAV/VNAV glidepath is surveyed to the MAP (ie the threshold), whereas +V can (and in one or two cases does) fly you into the ground between DA and the threshold.
Peter wrote:
+V is implemented by Avidyne also so it must be Jepp who did the database coding for it
We have discussed this before. It is not correct. Both Garmin and Avidyne have (I assume independently) written a simple algorithm to produce a slope from FAF to MAP. I will try to find the last time I told you that, standby…
And again here.
Peter wrote:
This is what the pre-W IFR certified GPSs have. It is slightly bizzare because as soon as there are 4 or more sats received, the baro data is disregarded, IIRC. So it looks like a lot of effort (wiring up the 11 gray code wires to the GPS, etc) for almost no gain because some 99.9% of the time you are getting way more than 4 satellites. I have never understood the reasoning behind this feature.
Peter,
It also applies to WAAS GPS when operating outside of an SBAS service volume. Without Baro Aiding, there would be entirely too many situations where a satisfactory RAIM calculation could not be performed, causing delays or forcing the cancellation of flights. It is not just how many satellites are in view, it involves their geometry. Satellites closely bunched together don’t provide good geometry and the position error can easily exceed the RAIM criteria for NPA. Crude as it is, the pressure altitude with 100 foot precision and not corrected to MSL, is way better than the 0.3 NM criteria (556 meters).
OK Timothy I will try to remember for the 4th time 
However this
Secondly, and in a very few cases crucially, the LNAV/VNAV glidepath is surveyed to the MAP (ie the threshold), whereas +V can (and in one or two cases does) fly you into the ground between DA and the threshold.
should be moot since these are all non precision approaches, so below the DA all bets are off as per protocol, anyway. Only ILS and LPV are assured of the GS being obstacle-free all the way to the tarmac (ILS has to be for CAT3 to work, obviously).
Thanks again NCY. When I get my 650+750 boxes, about a year from now, I will know because I will be drawing up the wiring from the KEA130A
Peter,
LNAV/VNAV is an APV type of approach and uses a DA. The lateral path is the same as LNAV and based on GPS (WAAS not required). The vertical path is based on a baro-altitude input (static source) corrected for the local barometric pressure and a GS is constructed via the FMS computer using the GPS position and the altitude at that position.. Baro-VNAV equipment capable of flying these approaches is found on most air carrier, turbojet, and turbo prop aircraft. Recently the capability has been added for some G1000 installations, which brings this capability to piston class aircraft. This type of procedure can be flown without WAAS and predates the general availability of WAAS in the US. Most air carrier aircraft are not WAAS equipped as are many business aircraft. Since the GS is based on altimetry, the vertical path is subject to the errors introduced by temperature. This is why there are temperature limitations on these procedures when flown with baro-VNAV systems that don’t use temperature compensation. Some Baro-VNAV systems include temperature sensor input and compensation. The vertical path is not a straight line in space, as MSL follows the curvature of the earth. WAAS GPS’es incapable of generating a glidepath based on a Baro-Altitude input (GNS/W or GTN) can fly these procedures if located in a SBAS service volume with sufficient integrity of the vertical position. Since a WAAS calculated path is a straight line in space, accommodations on the vertical CDI FSD were made to adapt to the curved path of the baro-VNAV. This was accomplished by limiting the vertical CDI to be a fixed amount beyond a distance from the threshold where the angular CDI FSD reaches +/- 150 meters. With this adaption, the FAA approved the use of WAAS vertical guidance to be used on an approach with LNAV/VNAV minimums.
Many years ago, there where hundreds of RNAV approaches with both LNAV and LNAV/VNAV lines of minima. Today it is really difficult to find an RNAV procedure that has LNAV and LNAV/VNAV lines of minima that does not also have an LPV line of minima. So in the US, it is largely academic that a WAAS GPS can be used to fly to the LNAV/VNAV minimums. Outside the US, this is not academic if the country permits the WAAS GPS to fly these procedures.
Note, TERPS uses a separate specification to determine the DA for LNAV/VNAV approaches. Unlike LPV, where there is a single sloped vertical obstacle surface that must be clear of obstacles, the LNAV/VNAV uses two obstacle surfaces, one is sloped and the other is level. Close in to the runway, the sloped obstacle surface transitions to a level obstacle surface. This difference in vertical obstacle criteria can sometimes lead to situations where the LNAV/VNAV has a lower DA that the LPV DA because of how obstacle penetrations in the visual segment affect the location of the DA. The lateral obstacle surface for the LNAV/VNAV is the same as that used for LNAV. The LPV uses an angular width the same as is used on an ILS, so laterally the LPV is superior as it is not affected by some obstacles that would be an issue for the LNAV/VNAV.
So there are different specifications in TERPS for all of these RNAV approach types:
LPV, LP, LNAV/VNAV., and LNAV.
Many thanks NCYankee for the explanation. So LNAV/VNAV is BARO-VNAV. Interesting too that WAAS is not required.
I am amazed that the difference between the MSL curvature and the “straight line in space” is relevant to obstacle clearance, over the typical FAF-MAP distance…
WAAS is certified to emulate the LNAV/VNAV glidepath, so, although it was developed for BaroVNAV, BaroVNAV is not required.
There is a choice. Most CAT uses BaroVNAV and much of it does not have WAAS, most GA uses WAAS and does not have BaroVNAV.
There are aircraft that have both and many that have neither.
