altimeter setting

[TXCAP4228]

It was either three or four and I couldnt remember. A Squared, you may correct me if I am wrong but if you have enough satelites for RAIM (I know its a different issue but I think its 6, or 5 with a barro-assist which obviously doesn't apply here) then you have the same vertical precision as lateral precision. IE, less than the error that is allowable for an IFR approved altimeter.

That's not bad. In any case its a heck of a lot better than using weather radar.

Am I completely off base here?

 

[A Squared]

Texcap,

Having more satellites will improve the accuracy (not precision) of the soulution, including the vertical componnent. I don't know how much.

RAIM isn't so much to increase the accuracy, but to verify the accuracy. It does it with redundant measurements.

If you have 3 satelites, you can get a non-redundent position fix if you supply the altitude.

If you have 4 satellites, you can get a non-redundant 3-d position, or if you supply the altitude, you get a 2-d fix with one redundant measurement.

If you have 5 satellites, you can get a 3-d position with one level of redundency, or if you supply the altitude, you get a 2-D fix with 2 redundant measurements.

If you have 6 satellites, you can get a 3-d fix with 2 redundant measurements.

Now what the redundency does is allow error checking. If you have no redundency, and you have a bad measuremnt, your position will bad, but you can't tell. If you have one redundent measurement, and one measurement is bad, you can tell that one is bad, but you can't tell *which* one. If you have 2 redundent measurements, and one measurement is bad, then you can tell which measurement is bad and drop it out of the soulution.

That conceptually is what RAIM does, how exactly that is programmed, I couldn't tell you, I'm sure it is buried in a TSO someplace.

I *think* (but I'm not sure) that if you're using baro aiding, the altitude displayed on your GPS screen is just your barometric altitude from the encoder.

Anyway, back to the question at hand, all of the things suggested have some use, If I was in the situation, I'd use a non-baro-aided GPS altitude as a reality check, I'd use the Wx radar to make sure I wasn't flying toward terrain at or above my altitude, I'd fly an ILS and check the altitude against DME readouts and crossing the outer marker. All of these things are a tool, none is the answer by itself.