23
Once
the
altimeter
has
reached
its
“Arming
Altitude”
(ie.
rocket
is
launched),
it
will
write
a
1
second
(20
sample)
pre
‐
launch
ring
buffer
to
flash
memory
to
capture
its
pre
‐
launch
pressure
data.
Temperature
Data
and
Battery
Voltage
Data
is
recorded
to
flash
memory
at
1
Hz
rates.
These
are
instantaneous
1
Hz
snapshots.
The
exception
to
this
snapshot
sampling
method
is
applied
to
the
Battery
Voltage
during
the
Drogue
and
Main
events.
The
RRC3
performs
a
20
Hz
“low
voltage”
sample
during
the
duration
of
these
events
and
records
this
low
voltage
value.
Because
the
sampling
is
asynchronous
to
the
20Hz
baro
data
and
event
overlay,
there
may
be
some
offset
in
timing
when
observing
the
low
voltage
value
in
relation
to
a
given
event.
Pressure
Altitude
Conversion
The
RRC3
and
the
mDACS
software
both
employ
the
NOAA
“Pressure
Altitude”
calculation
method
to
convert
air
pressure
to
an
equivalent
altitude.
The
formula
is
as
follows:
The
web
reference
can
be
found
here:
http://www.srh.noaa.gov/images/epz/wxcalc/pressureAltitude.pdf
mDACS
Geopotential
Altitude
Conversion
If
you
have
accurate
temperature
for
your
ambient
launch
conditions
and
for
your
apogee
temperatures
and
you
know
accurate
ambient
launch
humidity
conditions,
you
can
apply
the
same
Geopotential
Altitude
calculus
used
by
NOAA
radiosondes.
The
temperature
data
is
interpolated
linearly
from
launch
to
apogee
and
back
down,
and
the
humidity
is
applied
as
a
constant
throughout
all
the
calculations.
The
Geopotential
references
can
be
in
the
Federal
Meteorological
Handbook
No.
3,
Appendix
D.
The
web
reference
can
be
found
here:
http://www.ofcm.gov/fmh3/pdf/12
‐
app
‐
d.pdf
mDACS
Flight
Simulator
Pressure
Conversion
When
the
Flight
Simulator
runs,
it
feeds
a
20
Hz
stream
of
pressure
data
to
the
RRC3
in
lieu
of
using
the
onboard
pressure
sensor.
The
simulation
altitude
to
pressure
conversion
uses
the
NOAA
Station
Pressure
calculation
using
a
standard
29.92
in
Hg
sea
‐
level
reference
(
Pa)
as
shown
in
the
following
formula:
