NCYankee wrote:
GPS altitude and MSL based on QNH are quite different. The error varies with temperature and height above the airport and can be quite substantial, certainly on the order of hundreds of feet and in extreme cases more than a 1000 feet.
Yes, but on the ground the error should be zero; and for DA/MDA purposes, the GPS altitude is safe. While in warmer weather it will be below the altitude indicated by a barometric altimeter, it will still be the same height above ground so absolutely safe.
However, in practice that does not work because the published airport elevation is NOT a GPS altitude, and there is no standard GPS altitude to start with.
Ibra wrote:
I have never seen more than 100ft between GPS ALT and AD ELV on ref point
The reason you see any error more than a few metres (with WAAS) at all is that other than latitude an longitude, the MSL datum is the elevation consistent with all in other elevations in the country. These were all established with triangulation well before GPS was even feasible, sometimes more than two a century.
The same used to be the case for latitude and longitude (for example the UK meridian and the WGS84 meridian are 100m apart and UK airfields used the UK meridian), but aviation has standardised on WGS84 so by now airports publish their WGS84 coordinates.
But there is no GPS standard altitude for aviation. There are three “revisions” of the geoid model (EGM96, EGM2008. and EGM2020) which could all give different elevation figures for the same point on earth, and because all of them are approximations, they will not match what is currently charted.
GPS altitude is geometric and the error is unaffected by temperature or pressure. GPS altitude error does not change very much with altitude and is more suitable for obstacle and terrain avoidance than a barometric altimeter. The issue is with the barometric altimeter which can have substantial error the further above and away from the point where QFE is determined. Although the vertical path provided by LPV is very accurate with very little vertical path deviation, the DA is determined by the barometric altimeter indication corrected by QFE. In spite of the error at higher altitudes, temperatures, inversions, pressure variations, the barometric QFE altitude error is small at the MDA/DA if the correct QFE setting is applied. Using a GPS altitude to set a pseudo QFE, especially in a warmer than ISA environment can introduce a barometric altimeter indication error that can easily exceed the total obstacle clearance that is provided for with the DA. A colder than ISA temperature would most likely increase the altitude above the true QFE altitude. I did a rough calculation at 3000 feet above the airport at ISA +/- 15 C would introduce 167 foot error in the Barometric indication. To this, one would need to add the GPS error.
A side note, GPS altitude is not normally displayed on an LPV approach and is not approved for determining the DA.
Cobalt wrote:
However, in practice that does not work because the published airport elevation is NOT a GPS altitude, and there is no standard GPS altitude to start with.
The GPS altitude used for the purposes of the LPV is based on WGS84 and is called a geometric altitude as it is based on the spheroid surface, not the real surface. The altitude of the airport reference is in terms of a geometric altitude included as the “LTP/FTP Ellipsoidal Height” element in the FAS block. For that matter, the geometric ellipsoidal height is what is specified in the database for obstacles and terrain. So internal to the GPS, everything is in terms of the the geometric ellipsoidal height. This is also the form of altitude that is broadcast by ADS-B Out.
I guess near AD datum with SBAS correction the difference, or it’s variation is expected to be tiny (or at least near some static value of WGS error from model surface & real surface error)
But yes we are far from certification of GPS Altitude for the purpose of ATC terrain clearance or traffic separation in terminal airspace with QNH baro, or to fly in RVSM flight levels on STD baro 
maybe when we have current weather balloon soundings with GPS ALT & QNH ALT & STD ALT on a given day
Ibra wrote:
maybe when we have current weather balloon soundings with GPS ALT & QNH ALT & STD ALT on a given day
I see you are joking, but QNH is a local setting based on a surface location. In effect it accounts for the temperature and pressure at the local site and elevation, so a remote site in a balloon would not be of any use.
Just joking (no way one can get a serious “local QNH in every 3D space”, they have to decide between Euler & Lagrange formulation first and for data that would be scalars, vectors or tensors…I can’t imagine how much bureaucracy would come up out of that )
Just bumped into this thread searching for something about Strausberg.
I see there are currently 2 RNP approaches with LPV and their own EGNOS channels.
Does nobody know what happened in 2021 when those approaches were removed and then later (don’t know when) restored?
The important part about the IFR procedures at EDAY is to obtain the PPR from Bremen ACC (See AD 2.20, §2.1).
If you don’t do it ahead of time, Approach will definitely complain about it on the radio, and they may reject your request. Last fall we were two (unrelated) flights arriving IFR from the north into Strausberg, neither had done it. I was the second flight, the weather was more than fair so I did not fight it, apologized for the mistake and we finished VFR.
The person at the airport office where you pay the bills remind you of this before you leave. With so many people missing that point, they have printed the phone number on small papers (paper ad style) and distribute them to all the owners of an IFR flight plan. I could get my departure PPR over the phone literally 5’ before stepping into the plane.
Strausberg is only an AFIS airport with no controlled airspace, they can get you your clearance on the ground, but they cannot do more coordination like a TWR would. Whether this was the reason for the suspension in 2021, I don’t know.
Other than that, both SID and STAR work.
Does nobody know what happened in 2021 when those approaches were removed and then later (don’t know when) restored?
I remember legal disputes regarding certification of QNH measurement equipment. Then using a QNH from nearby airport, then adjusting minima for possible error etc. etc. German precision at its best…
The important part about the IFR procedures at EDAY is to obtain the PPR from Bremen ACC (See AD 2.20, §2.1).
If you don’t do it ahead of time, Approach will definitely complain about it on the radio, and they may reject your request.
And the bad thing is, Bremen ACC does accept it only on the day of flight, not ahead of it. And only two aircraft the hour are accepted. I tried it once and the guy on the phone, who was really polite and didn‘t like the rule as well, suggested a call late in the night before flight or very, very early in the morning to be sure to get the slot.
I decided finally to go VFR.
