Text 13950, 99 rader
Skriven 2011-02-05 11:38:38 av Roy Witt (1:387/22)
  Kommentar till text 13928 av Robert Bashe (2:2448/44)
Ärende: The Smolensk crash on April 10, 2010
============================================
05 Feb 11 08:20, Robert Bashe wrote to Roy Witt:

 RB> Roy Witt wrote to Robert Bashe on Friday February 04 2011 at 09:57:

 RW>>>> I have a Garmin hand-held GPS that tells me at what height above
 RW>>>> sea-level I'm at.

 RB>>> Actually, all you need for that is the air pressure.

 RW>> This GPS is an electronic device, not a weather station.

 RB> Note what you wrote: "sea level", and what I wrote: "air pressure".
 RB> Sea level doesn't say a thing about your altitude above _ground_
 RB> level,

Like I said, sea level is a reference used to measure the height of ground
level. If you don't have a reference, there's no way to know what your
actual elevation is. Even your aircraft altimeter uses nominal pressure
at sea level as a reference.

 RB> but it's easy enough to determine your altitude abvove sea
 RB> level by measuring the air pressure.

Air pressure varies as you increase or decrease altitude. i.e. you have to
reset an aircraft barometric altimeter while on the ground.

Not as reliable as a radar altimeter for measuring altitude.

 RB> This is _also_ done in airplanes, but is not something that is
 RB> reliable enough to chance low-level flights in bad weather using it
 RB> as an altitude guide.

And yet it is done everyday.

 RB>>> But the big problem in flying is how far you are from ground level
 RB>>> - and depending on the terrain, "ground" level can vary tens and
 RB>>> even hundreds of feet from one minute to the next. That's why air
 RB>>> pressure altimiters have to be used with much caution - they often
 RB>>> give readings that differ from the actual height above ground.

 RW>> I don't know how it's done, but I do know that it works very well.
 RW>> The river down the hill is at 578ft above sea level...

 RB> You're still talking about sea level, and the point is what the
 RB> altitude above the GROUND level at any given moment is.

Ground level is determined by using sea level as a reference. Even your
typical aircraft barometric altimeter uses sea level as a reference.

 RB> Nothing else is important for an aircraft. Abnd I still dispute that
 RB> the altitude above GROUND level can be reliably determined by GPS,

That would depend on where the sats are at a given moment. Sats being on
the far horizons could make your altitude measurement as much as 1000
meters off. OTH, there are military sats up there that are radar enhanced
to help military aircraft and ground equipment find their way. If the
military is using such equipment in aircraft, then it must be accurate
enough to navigate by.

Google - Satellite  Altimetry, sometime.

 RB> air pressure or for that matter anything except an actual radar
 RB> bouncing system - and even then you have the problem of trees.

Trees don't completely block an RF signal in the radar frequency range.
Even if it did, unless you're flying over jungle where the ground is
invisible to the eye, it's still not invisible to radar.

 RB> This kind of thing - determining altitudes above GROUND level - isn't
 RB> anywhere near as easy as some people apparently think.

I suspect that when flying by the seat of your pants, altitude isn't an
easy thing to determine. That's probably why the altimeter was invented.

 RW>> ... my house is 610ft above sea level and the downtown 'circle' is
 RW>> at 633ft, while the hospital, which sits on a small hill above my
 RW>> place is at 653ft. Which is pretty close to what the actual
 RW>> elevation is.

 RB> All real fine as long as you're standing still and know the precise
 RB> heights of ground features above sea level.

It is a 'hiking' GPS unit, afterall.

 RB> But for an aircraft moving at several hundred mph over possibly rough
 RB> and uncharted terrain pretty worthless.

Not exactly worthless. Before GPS, there was the ground based system used
to navigate, mostly by land or water craft. I made an portable antenna
that a pilot could use to receive that ground based navigation signal to
fly to home base, newly purchased used aircraft.


                R\%/itt

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