[Milsurplus] AN/AMT-3

[Michael Bittner]

Dave, Very nice summary of how the various radiosondes work.  Thanks for 
posting.  Al Klase's web site confirms that a Hair Hygrometer was used in 
the AMT-3.

Mike, W6MAB


----- Original Message ----- 
From: "David I. Emery" <die at dieconsulting.com>
To: "Michael Bittner" <mmab at cox.net>
Cc: <milsurplus at mailman.qth.net>
Sent: Tuesday, September 25, 2018 23:41
Subject: Re: [Milsurplus] AN/AMT-3


> 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."
> 


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