OAT for RADAR on fighters?

Ed Rasimus wrote:
On 19 Jun 2006 07:50:02 -0700, "Don McIntyre" wrote:

While reading through Hans-Heiri Stapfer's "Walk Around MiG-21 Fishbed
Part 1" (Squadron-Signal), he makes mention of a temperature probe
mounted low on the nose on an MiG-21F-13. Why would a radar rangefinder
need to know the OAT?

Dunno about MiGs, but every high performance tactical aircraft I ever
dealt with had an OAT input to the Central Air Data Computer (or
whatever it was called on a particular system.) Air temp is a
component of density and that is a factor that impacts range of a
weapon.


AFAIK temperature determins the speed of sound all on its own. Air
density seems to have nothing to do with it unless you get to near
space. Air density of course would determin dymanic pressure.

Technically the speed of sound is the square root of the ratio of air
pressure over air density all multiplied by a constant known as adiatic
exponent of air. However as the ratio of pressure to density is a
constant at a specific temperature temperature is all that is needed.

Best look at it here
http://www.sengpielaudio.com/SpeedOfSoundPressure.pdf

The WW2 British blind bombing system Oboe made compensations for air
density variations so possibly modern radars use it as well, probably
not so much for ranging but for doppler or anti-jamming measures.

Since dum**** fighter drivers don't like numbers the presentation on
the radar screen would be little lines indicating min and max ranges.
When the radar gets a range, the computer whiz-bang calculates how far
the gizmo can get at the current density altitude.


Ed Rasimus
Fighter Pilot (USAF-Ret)
"When Thunder Rolled"
www.thunderchief.org
www.thundertales.blogspot.com

On 20 Jun 2006 07:27:56 -0700, "Eunometic"
wrote:


Ed Rasimus wrote:
On 19 Jun 2006 07:50:02 -0700, "Don McIntyre" wrote:

While reading through Hans-Heiri Stapfer's "Walk Around MiG-21 Fishbed
Part 1" (Squadron-Signal), he makes mention of a temperature probe
mounted low on the nose on an MiG-21F-13. Why would a radar rangefinder
need to know the OAT?

Dunno about MiGs, but every high performance tactical aircraft I ever
dealt with had an OAT input to the Central Air Data Computer (or
whatever it was called on a particular system.) Air temp is a
component of density and that is a factor that impacts range of a
weapon.


AFAIK temperature determins the speed of sound all on its own. Air
density seems to have nothing to do with it unless you get to near
space. Air density of course would determin dymanic pressure.

Technically the speed of sound is the square root of the ratio of air
pressure over air density all multiplied by a constant known as adiatic
exponent of air. However as the ratio of pressure to density is a
constant at a specific temperature temperature is all that is needed.

Best look at it here
http://www.sengpielaudio.com/SpeedOfSoundPressure.pdf

You are correct, but irrelevant to the discussion of the relationship
between OAT and radar ranging. Mach is temperature dependent and since
the atmosphere gets colder as you rise, the speed of sound will
change.

But, while high performance aircraft do have mach indicators, they
operate on the basis of airspeed--which leads to an entirely more
complex discussion as we start to grapple with indicated, calibrated,
equivalent and true airspeeds as well as ground speed along with the
trigonometric impact of dive/climb angle relating to bombing.

Highly recommend you don't fly landing approach on mach.

Ed Rasimus
Fighter Pilot (USAF-Ret)
"When Thunder Rolled"
www.thunderchief.org
www.thundertales.blogspot.com

Hey, Ed, M 0.1 is a maximum cruising speed for most old fuds on the
Green Stamp.
BTW I don't think that OAT probe has much to do with the radar. I'd
suspect it's for the air data computer to feed the Machmeter (for some
reason the 21 has a big one, probably so the pilot will fly the exact
Mach commanded by his controller) and the engine and anti-ice systems.
BTW when the 21 was first placed in commission Sov philosophy didn't
allow the interceptor pilot much freedom at all; he was essentially
just the interface between the GCI controller and the two Atolls. I
used to watch their training intercepts on GCI radar at one base i was
at.
Cheers - Walt BJ