[Milsurplus] High & Mighty SCR-718

[D C *Mac* Macdonald]

That's the simple beauty of "pressure pattern" navigation.
The difference plus or minus of radio/absolute altitude
and pressure altitude (with 29.92 set in altimeter) tells
the navigator whether wind is from port or starboard.
The rate of change between one measurement and the
next can be used to calculate the speed of the apparent
wind.  With wind vector, compass heading, and air speed,
a DR position can be calculated.

When you set your altimeter at 29.92 inches of mercury
and fly the airplane at a specified altitude, the aircraft will
o up and down as actual local barometric pressure varies.
The absolute altitude is what the radio/radar altimeter
indicates.  You use the difference between the two for
your navigation calculations.

If one or two celestial LOPs (lines of position) can be
obtained and cross the DR plot, you have a valid fix.
No, it probably won't be as accurate as a three-star
fix you can get at night, but it  WILL  be good enough
to keep you pretty close to your desired course.

Believe me, it does work; not as well as inertial navigation
or GPS, but you will get close enough to find SFO or any
other coastal airport for which you are searching.

Best regards,


Mac, K2GKK/5
ex-Navigator and SAC EWO (sextant shooter)



----Original Message Follows----
From: C Whitaker <whitaker at pa.net>
Reply-To: whitaker at ieee.org
To: milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] High & Mighty SCR-718
Date: Sun, 31 Jul 2005 18:12:30 -0400

de WB2CPN             2005.07.31.2214

I'm missing something about this.  How can the
radio altimeter and the pressure altitude tell
anything except the barometric pressure, adjusted
for sea level, at the time?  If you fly 29.92 at
the indicated assigned altitude you're going to
see the absolute altitude, (the radio altimeter),
change if the barometric pressure changes because
the aircraft will climb or decend as the pilot
maintains the indicated altitude. If you had weather
charts that showed the current isobars you could get
some sense of heading by knowing if you wanted to be
flying across isobars, or parallel to one.

Then there's the way they measured pressure altitude
in "Strategic Air Command", I think, by measuring
the temperature of boiling water.

Then there's the Cat and the Duck that every pilot
knows about.  A cat and a duck are carried in their
cages, and when the pilot begans to wonder which way
is up and which way is down, he dumps the cat out
of it's cage and notes which way it falls.  For IFR
penetrations without instruments he releases the duck
and follows it down.

73  Clete        Sorry 'bout that, Mac.