TELEMTRY.txt 6.5         APRS TELEMETRY SYSTEM
                   Using the Micro.Interface.Module (MIM)

NOTE: THIS INFORMATION IS PRELIMINARY, AND ALTHOUGH A PROTOTYPE IS WORKING
FINE, HE AND I ARE STILL FINE TUNING THE PROTOCOLS AND FINAL DESIGN SPEC.
IF YOU ARE INTERESTED LET US KNOW.

      Carl Wick, N3MIM, has developed a simple, yet powerful Micro-Interface-
Module that can be used as a single chip APRS telemetry system.  Although the
intent of his original design was to make a very simple, light-weight, throw-
away module for experimental balloon flights, his design has now evolved into
a very useful APRS packet tool.  Using a single chip microprocessor, he has
implemented a complete TNC (transmitter only) on a chip.  This chip has four
analog inputs, five digital bit inputs, a receive audio input (for propoer
CSMA operation) and outputs PTT and AX.25 audio using digital synthesis.  The
only external components besides the sensors themselves, are a crystal and a
transmitter.  A 0.2 cu in. 500 mw xmtr is available too!  The pin-out follows:

                    _____________
        Analog 3 --O|           |O-- Analog 2
        Analog 4 --O|   M.I.M   |O-- Analog 1
       Rcv input --O|           |O-- Xtal
           reset --O|   AX.25   |O-- Xtal
           Gnd   --O|           |O-- 5 volts
     Input bit 1 --O| Telemetry |O-- AX.25 out
     input bit 2 --O|           |O-- AX.25 out
     input bit 3 --O|   Chip    |O-- PTT
     input bit 4 --O|___________|O-- Input bit 5


APRS TELEMETRY RECEIVING SYSTEM:   Any telemetry system requires a table of
equations and labels to make the values useable at the receiver site.  The
problem with most experimental telemetry systems, is that the receiver system
must be tailored for every new application.  In the case of a one-time
balloon launch, (or any experiment assembled to meet a schedule) the
equations are not usually available until just hours or minutes before
launch.  This means that it is next to impossible to distribute the equations
and parameter definitions to a large number of tracking stations and to be
able to have receiving telemetry software ready to go.

      For this reason, APRS has been designed to serve as a general purpose
telemetry tracking system for the M.I.M.  Using APRS it is possible to
transmit the telemetry equations, parameter definitions and channel units IN
REAL TIME!  Once any APRS station receives these parameter transmissions, it
is then ready to receive and to display the real-time telemetry values in the
proper engineering units.  The TELEMETRY page is displayed using the alt-T
command.  Hitting this command causes APRS to scan the READ MAIL screen
looking for the telemetry equations, and then to scan the ALL_BEACONS pages
looking for TELEMETRY values.  Up to 16 samples are displayed per page, for
a total of 75 samples.  The TELEMETRY samples are saved in the normal LOG
files.  A sketch of the APRS telemetry display is shown below:


APRS TELEMETRY FOR XYZ BALLOON LAUNCH

SER TIME Battery  AirTemp  BTemp  Pres   Altude Camera Chute  Sun    10m  ATV
NUM      volts    deg.F    deg.F  Mbars  K feet BIT    BIT    BIT    BIT  BIT
--- ---- -------- -------- ------ ------ ------ ------ ------ ------ ---- ----
101 1215  12.8     86       85    999      0    ...    ...    ...    ...  ...
102 1216  12.8     86       85    999     1000  ...    ...    ...    ...  ...
103 1217  12.6     87       87    998     2000  ...    ...    ...    ...  ...
104 1218  12.4     84       80    980     4000  clik   ...    on     on   high
105 1219  12.3     80       76    900     8000  ...    ...    ...    on   high
106 1220  12.1     75       70    850     16000 ...    ...    on     on   ...
107 1221  12.0     70       65    800     32000 clik   ...    ...    ...  ...
108 1222  12.0     65       60    730     64000 ...    ...    on     ...  high


    Notice that the M.I.M module transmits a value for each of its four
analog channels and each of its five digital bits once every sample time.  On
receipt, the fourth analog channel is displayed along with a fifth relative
channel which has its own separate equation for the 4th value, such as for
Pressure and Altitude in a balloon experiment.  The sample periodicity can be
set from any value from 1 second to 16 minutes depending on the application.
Each sample includes a unique serial number.  In addition, not only can the
parameter name, units and equations be specified for each of the analog
channels, but the word to be associated with either the 0 or 1 value of each
digital bit can also be specified.

     To configure all APRS stations to properly decode the telemetry from the
M.I.M module, the net control station (or any other designated station in the
APRS network) needs to transmit the proper parameter definition packets.
These packets are transmitted as APRS messages TO the CALLSIGN of the M.I.M
module.  If the M.I.M module is using the callsign of N3MIM, then the
parameter definition station would send the following four messages:

To N3MIM:PARM.Battery,BTemp,AirTemp,Pres,Altude,Camra,Chute,Sun,10m,ATV
To N3MIM:UNIT.Volts,deg.F,deg.F,Mbar,Kfeet,Clik,OPEN!,on,on,high
To N3MIM:EQNS.0,2.6,0,0,.53,-32,3,4.39,49,-32,3,18,1,2,3
To N3MIM:BITS.10110,PROJECT TITLE...

The PARM format specifies the name of each of the ten parameters.  The UNITs
format specifies what units are to be displayed, and for the digital bits,
show what label is associated with the digital condition.  The parameters and
units for the first two can be up to 8 characters, the next 6 can be 6
characters, and final 2 can be four characters each.   The EQNS format has
three coeficients for each of the four analog channels, plus the fifth relative
channel that uses a different equation related to the channel 4 value.  The
BITS format specifies either a 1 or a 0 for each of the five digital channels
to indicate which state is associated with the indicated label.  This permits
the payload designer to use 1's or 0's as convenient with his circuity without
being forced to always use 0 for OFF and 1 to mean ON.  A title can also be
included in the BITS definition which will be used by APRS to title the
TELEMETRY page.  The three values for each of the analog channels are the
coeficients of a quadratic equation:

  Final value = A*X^2 + B*X + C  Where X is the M.I.M transmitted value

FORMAL SPECIFICATION:  The specific format for the TITLE, PARM, UNIT, and EQNS
message packets are shown below.  They are entered as messages to the address
of the MIM module:

  PARM.P1,P2,P3,P4,P5,B1,B2,B3,B4,B5  Where Pn and Bn are the parameter names

  UNIT,U1,U2,U3,U4,U5,L1,L2,L3,L4,L5  Where Un are the units for analog ports
                                      and Ln are the labels for the bits

  EQNS,A1,B1,C1,A2,B2,C2,A3,B3,C3,A4,B4,C4,A5,B5,C5       Where the An,Bn,Cn
                                      are the coeficients for each of the four
                                      analog channels, plus the 5th relative
                                      channel.

  BITS.XXXXX,Title-up-to-23-chars     The x's specify the state of the bits
                                      that match the BIT Labels.

  T#sss,111,222,333,444,xxxxx         This is the on-air format for the UI data
                                      frame, where sss is the serial number
                                      followed by the four 3 digit analog
                                      values and the five binary values.
                              

APPLICATIONS:

   1)  OBVIOUS Balloon payloads using only party balloons, not needing the big
       WX balloons and all the paraphanalia.

   2)  TRAFFIC monitoring MILE posts!  This is a neat idea!  Given that HAMS
       will be commuting with APRS moving Map displays, why not build a match
       box sized traffic SPEED detector (solar powered MIM module) that can be
       stuck on the side of a highway pole ?  Via a $1.29 crystal MIC from
       radio shack, use DSP to figure out the speed of the traffic based on
       audio analysis!  Beacon this SPEED once every two minutes at about 10
       mW.  The beacon will, of coure, include the LOCATION of the device.
       What the APRS commuter sees on his MAP is these MILE posts ahead of him
       showing traffic speeds!  He can then decide on alternate routing!

            We have plenty of room in the MIM to add this DSP (maybe), IS
       THERE ANYONE OUT THERE THAT IS INTO DSP THAT CAN DETERMINE THE
       ALGORITHM TO DETERMINE SPEED FROM THE AUDIO OF TRAFFIC??????????
       (or the amplitude fluctuations of a photo cell?) Even cheap X band
       doppler motion detectors are possible, since they only need to turn
       on briefly to get a speed measurement.  This thing has to be VERY small
       and low power to be able to be SOLAR powered and able to be COVERTLY
       installed with out a lot of STATE HIGHWAY bureaucracy.

LOW POWER TELEMETRY TRANSMITTERS:  To complement this less than ONE-CUBIC
inch MIM telemetry system, Agrelo Engineering in NY makes a 1.5 x 0.5 x 0.25
inch 2 meter transmitter for $99.  It outputs 500 mW at 6 volts 140 ma and
120 mW at 3 volts 50 ma.  See more cheap transmitters in the GPS.TXT file.


ORDERING YOUR M.I.M. SYSTEM:   The packets from a M.I.M. chip are sent as
AX.25 UI frames only.  There is no provision for any connected protocols
which would make the device overly complex.  Therefore you must specify the
UNPROTO string for the device.  The default periodicity is burned in to the
single value specified, but there is an initialization process that can be
used to modify this default at power-up by grounding a combination of the
four analog lines.  Using this process, you can select 1/4th, 1/2 or 2 or 4
times the default rate.  In the table below, the defaults for all
options are shown.  Please be very accurate in specifying the following items
when ordering: (DO NOT ORDER YET!   THIS IS PRELIMINARY!)

   Callsign and SSID:   (required!) __________________________________

   TO Address:          (APRTLM)    __________________________________

   Digipeater VIA path: (WIDE)      __________________________________

   Default periodicity: (64 sec)    __________________________________
                        (other defaults supported 8,16,32,64,128,256,512

   Since the PERIODICITY is frequently different for different applications,
   there is a mechanism for changing it in the field.  The four ANALOG
   inputs are sensed during the power up initialization process and if they
   are at +5 volts, they are interpreted in the following manner:

   A1 - divide period by 4    } A1 and A2 => divide period by 8
   A2 - divide period by 2
   A3 - Multiply period by 2
   A4 - Multiply period by 4  } A3 and A4 => Multiply period by 8

   The analog inputs are used for this initialization process, since their
   attached circuits are usually high impedance and can be temporarily
   pulled high without problems.  This means, for example, that A1 and A2
   can be diode ORed to a single push-button to give a start up period of
   8 seconds for a nominal 64 second device.  Or a resistor/capacitor
   combination on one of these pins can be used to automatically select
   a periodicity at power up.
  

  

