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APRS Telemetry System

Using the Micro Interface Module (MIM)

The MIM module is a complete telemetry TNC transmitter on a chip. It has a serial data port, 5 analog and 8 digital telemetry inputs. It outputs PTT and transmit audio AX.25 tones. The MIM was developed by Carl Wick, N3MIM, as a very simple, light-weight, throw-away module for experimental balloons. He has teamed up with Will Clement and refined the chip into a very useful APRS packet tool. The only external components besides the sensors themselves, are a transmitter and optional GPS card and battery. A 0.3 cu in. 800 mw xmtr is available from Agrelo Engineering too!

INPUTS                               OUTPUTS
        Analog 1 --O|           |O-- PTT to XMTR
        Analog 2 --O|           |O-- Audio to XMTR
        Analog 3 --O|           |O
        Analog 4 --O|   M.I.M   |O
        Analog 5 --O|           |O
     Rcv Audio   --O|   AX.25   |O   (receiver audio is optional for)
     NMEA Serial --O|           |O   (colision avoidance CSMA)
     Input bit 1 --O| Telemetry |O
     input bit 2 --O|           |O 
     input bit 3 --O|   Chip    |O 
     input bit 4 --O|           |O
     input bit 5 --O|           |O
     input bit 6 --O|           |O
     input bit 7 --O|           |O
     input bit 8 --O|           |O


APRS TELEMETRY FORMAT: The on-air packet telemetry format is as follows:


  T#sss,111,222,333,444,555,xxxxxxxx  where sss is the serial number
                                      followed by the five 3 digit analog
                                      values and the eight binary values.

Each telemetry value is actually a decimal number between 000 and 255. The user can adjust his sensors to meaningful values, or the telemetry equations can be modified on receipt. Usually just a a simple 10k and 2.4k resistor divider is used on channel 1 to give you battery voltage in tenths. So a reading of 138 would mean 13.8 volts. It is easy to make APRS display any other quadratic telemetry equation too. See following sections.

EXTENDED LIFE OPTION: For extended operation (up to a year or more), the MIM can be configured to go to sleep between reports. A single set of AA Alkaline batteries could power the MIM and 1 watt transmitter for a YEAR at one report every 30 minutes. This extend timer mode TBD.

Additionally, the power to the GPS can be programmed separately to allow time for obtaining a current fix. Assuming a worst case of the GPS needing 15 minutes per fix for a full sky search, this could result in a system that could "check its position" once a day, report its position, telemetry and status every 30 minutes using a 1 watt XMTR for a YEAR on a single set of alkaline D cells.

APRS TELEMETRY RECEIVING SYSTEM: To make the APRS Telemetry page generic and able to show real engineering values and units, APRS can receive on-air packets to define the Telemetry labels, units, and equations. This means that APRS does not need to be progammed for each different application. These paramaters may be uploaded to all APRS stations live via four one- line BULLETINS. The first one defines the telemetry labels, the second defines the units, the third defines the telemetry equations, and th forth defines the project name and digital bit definitions. 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. The last 16 values are displayed. The TELEMETRY samples are saved in the normal LOG files. A sketch of the APRS telemetry display is shown below:



SER TIME Battery  AirTemp BTemp Pres  Altud Camra Par Sun 10m ATV 5th 6th etc 
NUM      volts    deg.F   deg.F Mbars K ft  BIT   BIT 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  hi  ... ...
105 1219  12.3     80      76    900   8000  ...  ... ... on  hi  ... ...
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  ... hi  ... ...


Notice that the M.I.M module transmits a value for each of its five analog channels and each of its eight digital bits once every sample time. The sample periodicity can be set from any value from 1 second to hours 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 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


The PARM format specifies the name of each of the 13 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 EQNS format has three coeficients for each of the five analog channels. 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,etc    Where Pn and Bn are the parameter names

UNIT,U1,U2,U3,U4,U5,L1,L2,L3,etc 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,etc Where the An,Bn,Cn are the coeficients for each of the five analog channels, BITS.XXXXXXXX,Title-up-to-23-chars The x's specify the state of the bits that match the BIT Labels.

T#sss,111,222,333,444,555,xxxxxxxx This is the on-air format for the UI packet, where sss is the serial number followed by the five 3 digit analog values and the eight binary values.

PARAMETER NAMES: Due to the 80 character screen width in DOS, each parameter has a fixed NAME/UNITS length. The lengths for the 5 analog channels are 7, 6, 5, 5 and 4 characters. The lengths for the 8 digital bits are 5, 4, 3, 3, 3, 2, 2, and 2 characters respectively. So you may need to decide early on what channel to use for what purpose based on the number of characters available in the display...

DEFAULTS TO APRS Mic-ENCODER: Since the predominant application of the MIM module is in the APRS Mic-Encoder, the default telemetry parameters and units for the Mic-E are normally displayed. These will go away if any on-air parameters or equations are received...



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


  • Tracking wildlife or packages


  • 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. A new 800 mw model is now out! See more cheap transmitters in the GPS.HTM file.



Clement Engineering
PO Box 1086
Severna Park
MD 21146

Phone 410 268-6736
FAX: 410 268-4612