[Milsurplus] AN/AMT-3

[David I. Emery]

On Sun, Sep 23, 2018 at 10:37:43PM -0700, Michael Bittner wrote:
> The altitude part is easy, a bellows (aneroid barometer), that expands or 
> contracts with changing pressure, moves a lever arm across the different 
> grooves on the disk.  Note that the un-raised 3/4 ths of the disk allows 
> the lever arm to freely disengage from one groove and move to another.
> 
> But darned if I know how temperature and humidity were done.  In other 
> radiosondes, separate resistive elements that change resistance with 
> temperature and humidity are switched into a blocking oscillator to change 
> its frequency and then modulate the transmitter.  Similarly, the altitude 
> lever arm functions as the arm of a potentiometer.  But that doesn't seem 
> to be the case with this AMT-3.
> 
> Mike, W6MAB

	I feel silly to try to comment from distant recollections of
around 50+ years, but when I was in HS (prep school) in the mid 60s I
had access to a couple of the class of dropsondes that contained a xtal
controlled 1 tube transmitter and a rotating record and sent HF CW.  
These came from NOS government military surplus in boxes made available
to schools... but I cannot for sure say they were AMT-3s specifically
though the description sounds right for what I had.

	IIRC the ones I played with had three swing arms at different
angles around the record for the pressure, temperature and humidity 
reports... and a hair hygrometer for the humidity and some kind of
coiled bimetallic spring like device for the temperature (similar to
what is in many big dial analog thermometers today) along with the
aneroid barometer device to register pressure.
	
	IIRC there was some mechanism (cams on the side of the record
closing a contact or similar) to switch between the arms as the source
of the transmitter keying.

	I do remember each of the things came with its own individual
calibration chart in the box for translating the characters into
meteorological values.   Seemed to have been prepared by some kind of
automated testing setup of the era... and I vaguely remember one needed
to use a couple of the values transmitted  - together on the chart - to
get the correct actual numbers for the weather report (particularly
humidity).

	As for radiosondes of the era - the ones I played with back then
had the arm moved by the aneroid barometer cell move across a  etched
circuit on top of a black plastic substrate (Bakelite ?)  with a comb of
closely spaced contacts separated by insulation.  Some of the etched
contacts activated a small relay that switched between the temperature
sensor (a thermistor on a holder outside the case) and the humidity
sensor (a hygroscopic gel on a plastic plate containing some ionic
substance that would conduct better when wet and thus have lower 
resistance).  This was indeed used to control a blocking oscillator that
modulated the transmitter (403 MHz or 1680 MHz).

	Another set of contacts interdigitated with the first switched
in a ground across the sensors to provide a periodic reference
calibration such that you had T H T H T H T H R T H T H  or similar.

	These were calibrated (again by some automated setup
individually) with the specific pressures at which the switches between
T H and R  occurred and IIRC the patterns were such that it was possible
to figure out where you were in the sequences (and thus the more
significant digits of pressure) from the pattern of switches.

	On the ground the frequency of the modulating tone was recorded
on a strip chart which was read manually and used with a calibration
chart for the temp and humidity sensors to get the actual temperature
and humidity at a particular pressure as determined from when the last
and next switch from T to H to R happened.

	Altitude was determined from time from launch based on the
amazingly predictable rate at which a standard latex weather balloon
inflated to a certain size with hydrogen or helium and with weight of
payload set for a certain measured upwards buoyancy rises upward.

	This predictable rate of accent allowed determination of
pressure at various altitudes from when the switches between T, H and R
happened in elapsed time since launch.

	Sometimes a radar reflector was attached in addition to the
parachute and radiosonde, which allowed cross checking this by radar
tracking of the balloon.

	Winds aloft were determined mostly by radio theodolite... eg
DFing the rising transmitter, in later systems with an autotrack 
driving a mount for a 6 foot or so 1680 MHz dish equipped with a
mechanical digital angle printout mechanism driven by synchros on the
dish that printed the el and az angles and time on a roll of paper
every few seconds.

	These angles and the altitude determined from time and pressure
allowed determining the drift of the balloon with winds aloft and thus
measurement of the wind speed and direction.

	Of course modern stuff does all of this with solid state sensors
for pressure and temperature (not sure about what exact current
technology is used for the humidity sensor) and a GPS receiver and does
not require any kind of actual tracking, optical or DFIng or radar.  
Downlink is digital messages in the 400 MHz area... which can be
received with  a more or less usual omni UHF antenna and suitable
receiver.  (There are some 3rd party programs available for demodulating
these messages and decoding  them and displaying the weather info
therein).

	And with GPS information available all the wind speed and
direction  and pressure versus altitude information can be derived quite
accurately from 3 space GPS position tracking over time and obviously be
processed and transmitted in real time by PC programs... no human labor
involved  or fussiness about correctly inflating the balloon and
correcting for various sources of error in that and other measurements.

-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."