The pressure altitudes that have been archived in the general RAF C-130
data set use a standard altitude reference pressure and temperature of
1013.246 mb, 288.15K. This often leads to pressure altitudes during flight
that are inaccurate, particularly during low level legs over the ocean.
The research radar altimeter was not operatioal during ACE-1 so pressure
altitude is the only measure of altitude that we have.
We have now reprocessed Flights rf3-rf30 and corrected the pressure altitude.
The pressure altitude is being calculated by
PALTC = (Tr/gamma)[1-(P/Pr)^X],
where Tr and Pr are sea level values of temperature and pressure, gamma
is the standard lapse rate , and X is R*gamma/g,
where R is the universal gas constant for dry air, and g is the acceleration
of gravity.
Rather than use a single reference temperature and pressure, we use the
temperature and pressure at takeoff and landing as two reference points
(Anchorage, Honolulu, Xmas Island, Fiji, Christchurch and Hobarth were all
nearly at sea level +- 5 m). We then found all flight legs that were flown
at 100, 300 or 500 ft and determined the altimeter setting (surface pressure)
at these points using the transform
PSURF = P*exp(g/R(HGM/Tbar)
where P is the pressure at the flight altitude, g and R are as defined
above, HGM is the altimeter reading from the pilots radar altimeter, and Tbar
is the average temperature between surface and flight altitude. The surface
temperature was assumed to be the ocean temperature determined by the
Heinmann thermometer and flight level temperature was taken from the radome
right Rosemount probe. The pilots had a working radar altimeter so we know
the low level flight legs fairly precisely, at least within +-5 m.
We interpolate linearly between all the surface pressures to get estimates
between the reference points. These are then used in the corrected calculation
of pressure altitude given above.
A major caution: Whereas we think that these new pressure altitudes are
much more accurate than the original values, there still will be some
uncertainties that result from the assumption of a standard lapse rate.
The values close to the surface will be most accurate and the uncertainty
will increase with increasing altitude.
The lidar data are presently being processed to determine aircraft altitude
when the lidar is pointing downward. These will be the most accurate
height data, once processed, and will be used to ascertain the accuracy
of the corrected pressure altitudes. In the meantime, we are reluctant
to place an uncertainty estimate on the corrected altitude values. We are
probably safe in saying that within the boundary layer, the corrected
altitudes are acccurate to within +- 25m.