Ibra wrote:
You don’t need SBAS signal to fly LNAV+V
So was it that modern receivers decided to provide LNAV+V without any SBAS corrections as a design choice? Or did they improve the calculation of the (advisory) glide path in order to no longer need SBAS to get acceptable performance?
TSO146 box run different set of code, for instance LNAV lateral and “advisory” +V vertical guidance are angular on short final and are very precise…the FDE and RAIM algorithms are more involved as they use more satellites, however, you need SBAS signal or BARO signal to perform the “official” glide path integrity in LVNAV/VNAV (or SBAS signal for LPV)
In TSO125 box, the LNAV is lateral 0.3nm and there is no angular or vertical guidance, this was done to be backward compatible with DME+INS systems for RPN APCH which are pure 2D
That reads very much like the generic definition of DGPS (just using one of the possible means for sending the DGPS correction: a satellite
Yes; one can transmit correction data just locally over a radio link; you often see that in survey work. I believe it is also used in navaid calibration where the calibrator aircraft flies within some tens of cm of the glideslope etc.
How would the legality and practicality of using that equipment be any better than the use of an LNAV+V certified installation (on approach down to some 200ft in both cases) ? (Honest Q)
I don’t think there is any difference in safety, but there is a slight difference in the height of the glideslope; an LPV glideslope is probably just below the +V one. Is it really? I think it must be. The +V one is set up to start at the FAF and then miss all SDFs.
Also flying a +V down to 200ft is busting the nonprecision IAP DH which applies to +V, but you didn’t ask that Q because you obviously know that 
So was it that modern receivers decided to provide LNAV+V without any SBAS corrections as a design choice? Or did they improve the calculation of the (advisory) glide path in order to no longer need SBAS to get acceptable performance?
No idea, but it is surely better to have this feature than not have it. SBAS doesn’t give you more accuracy; it just gives you more data for verifying integrity. The US (WAAS) GPS system is better than the European EGNOS because WAAS satellites participate in a position solution, whereas EGNOS ones merely transmit integrity data; see around here. Much harder to spoof GPS if you have SBAS. Various threads – example.
Peter wrote:
SBAS doesn’t give you more accuracy; it just gives you more data for verifying integrity.
I’m pretty sure it gives you both. It gives you correction factors to apply to the satellite signals. See https://en.wikipedia.org/wiki/European_Geostationary_Navigation_Overlay_Service#Services
Open Service (OS): It improves positioning accuracy by correcting error sources affecting GNSS signals (…). The corrections transmitted by EGNOS help mitigate the ranging error sources related to satellite clocks, satellite position and ionospheric effects.
I am sure that’s the intention but it is not supported experimentally, except maybe marginally. By the time you are receiving say 10 satellites, I on’t think SBAS makes any difference and that’s what I am seeing in my tests.
You get 0ft error for GPS true altitude at LFPN with SBAS
I have not tested without SBAS
So GTN “visual approaches” are to LNAV+ V accuracy, not LPV?
My GTN VA puts me on the centreline every time…far more accurately than the ILS( in vmc)
TSO C145/146 went through several iterations. The performance specification is based on RTCA DO 229. The most current revision of the TSO is 146E and the DO is 229E. The first WAAS GPS was certified using TSO C146A including the GNS 480/CNX 80 and the GNS 430W/530W. The GTN series used TSO C146C, the Avidyne used TSO C146D, and the latest Garmin GPS 175 uses TSO C146E. DO 229C had the following note regarding advisory vertical guidance, but all the GPS units until the Avidyne only provided +V advisory vertical guidance when in an SBAS SSV. Avidyne added +V outside the SBAS SSV and Garmin eventually updated the software in the GTN series and later to do the same. The section 2.2.4 referenced in the note describes the path implementation for LNAV/VNAV. Since the +V is just one of many possible paths to follow down to the MDA. it really doesn’t affect the safety of obstacle avoidance, since obstacles are avoided if you stay above the MDA and any step down fixes in the final segment.
Note: LNAV equipment may provide advisory vertical guidance. Advisory vertical
guidance is defined as supplemental guidance where the barometric altimeter
remains the pilot’s primary altitude reference. This advisory guidance should
use the vertical path and deviations defined in Section 2.2.4. This advisory
guidance may be provided even when SBAS corrections or integrity information
is not available. This advisory guidance cannot be used to descend below the
LNAV MDA or step-down altitudes.
Peter wrote:
SBAS doesn’t give you more accuracy;
Yes, it does. The very purpose of SBAS is to give more accuracy. Reliability is taken care of by RAIM. It is true that with SBAS you don’t need RAIM, but that’s a bonus.
Technically, SBAS gives more accuracy by enabling compensation for ionospheric delays which are variable and thus an unavoidable error source with unaugmented GPS. (You can also compensate using two different GPS signal frequencies, which is what the US military does.)
You can read these two most recent quarterly performance reports for basic GPS and WAAS corrected. Both are good but WAAS is much better.
https://www.nstb.tc.faa.gov/reports/2022_Q3_FAA_SPS_PAN_Report_119_v1.0.pdf
https://www.nstb.tc.faa.gov/reports/2022_Q2_FAA_WAAS_PAN_Report_81_v2.0.pdf
