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APRS MIC-Encoder

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The Mic-Encoder (Mic-E™) installs between your radio mic and radio and allows your GPS unit to transmit APRS AX.25 frames at designated intervals without needing a TNC! Very useful in many settings.

Bob Bruninga, WB4APR, uses the APRS Mic encoder while discussing its usage. The recorded audio was the results of recording the audio from a scanner listening to Bob talk on his 2-meter radio with GPS and APRS mic encoder installed. This is an excellent example of the combination of current narrow-band amateur technology with newer digital technology to enhance tracking options.

The unit is a semi-kit. This means that the board has been partially assembled, with interface wires, box, and other items still required to be configured and assembled for proper use.

Picture of assembled TAPR APRS Mic-Encoder outside the case. Picture of the unit with a Garmin GPS-20 mounted on top. Both units have the Battery backup installed, which does not come with the

Back panel of the Mic-E. The front panel (shown at top of page) measures 4-15/16 x 1-5/16. The case depth is 4-11/16 inches. The overall depth including knobs and connectors is 5-1/2 inches.



As mobile GPS/APRS continues to grow, we can eliminate the need for every mobile to have a TNC, digital radio, and second antenna by simply integrating the position report into a very brief tone burst at the end of a voice transmission over any two-way radio. The APRS Mic-E is this solution. With the Mic-E, no additional hardware is required in the vehicle, other than a GPS unit. The system not only reports position and vehicle type, but also one of 7 canned messages and 4 analog telemetry values.

Your APRS Mic-E Encoder Unit will provide you a quick, easy means of tracking your mobile operations when properly interfaced with your existing voice radio. Before operating your Mic-E on the air, you should inquire about APRS operations in your immediate area. Check 145.79 MHz (U.S.), 144.39 Mhz (Canada), and some areas may also use 145.01 MHz. By contacting APRS users in your local area you may find particular voice repeaters that already support Mic-E operations with a cross-band link to the local APRS network.

In order to make the APRS LOCATOR SYSTEM practical, the Mic-ENCODER has been designed with several design concepts:

  • Interfaces to UNMODIFIED radios via the MIC connector
  • Uses standard AX.25 for compatibility with existing TNC╝s
  • Compresses position report into about 0.3 seconds
  • Uses low enough in power to be powered from the MIC jack
  • Accepts the readily available NMEA output from GPS receivers
  • Optionally provides 4 analog channels for telemetry

The result is a 1200 baud position report compressed to 32 bytes including beginning and ending FLAGS. This equates to about 320ms, including CALLSIGN, DIGIPEATER PATHS, and a minimum message capability. Plus, the packets are still receivable on ANY AX.25 TNC.

The Mic-E (Mic-Encoder) installs between your radio mic and radio and allows your GPS unit to transmit APRS AX.25 frames at designated intervals without needing a TNC! Very useful in many settings.

The heart of the Mic-E is a PIC chip processor that is now sold commercially by Clements Engineering. In 1996, Gwyn Reedy, W1BEL, President of PacComm approached TAPR about handling the amateur version in a semi-kit solution. A semi-kit means that the board has already been built, but interface wires, box, and other items will still need to be configured and assembled for proper use by the owner of the kit.

The TAPR APRS Mic-E is the result of the collaborative process begun in October 1996 and finished in May of 1997.

Project History

Motorola Oncore VP Without Differential Corrections
The display below shows the Oncore VP collected non-differential GPS positions for approximatelly 9.5 hours.  SA Watch calculated the 95th percentile as 58 meters. April 23rd, 1997

  • Far right, MIC-E production without cables or GPS.

  • Right, MIC-E production board with cables attached and GPS-20 mounted.

    Click here to see the proposed front face plate on the production version.

  • Both units are outside their cases and don't have the 8-pin round MIC connector attached (open space in upper left corner).

  • Holes are provided on the board to mount Garmin, Motorola, and Trimble GPS units.

  • The Mic-E is easily interfaced to any radio with a round 8-pin Mic jack or the newer RJ-45 jack. All of the 8 pins from these front panel connectors pass straight through the Mic-E, out the back and to the radio unchanged, so there are no restrictions on any microphone/radio combination that use these two common connectors. To configure the Mic-E to the particular arrangement for your radio, isolate the PTT signal using the jumper header (seen in upper left of board) and wire the personality dip header (upper left of the board) that connects the Mic-E to the necessary signals on the microphone connection.

April, 1997

  • Production testing underway.

    Goal is still to try to have units available by Dayton. An announcement will be made on APRSSIG and TAPR-BB as soon as price and availability are determined.

    The picture at right is Ron Parsons, W5RKN, with one of his three dogs. Two production MIC-Encoders can be seen in the lower right corner on the table during testing. Picture to far right is prototype DCD conversion for MIC-E TNC repeater usage.

    The daughter board attached to the top of the TAPR State Machine DCD Upgrade unit reduces the false DCD hits on voice systems to zero. TAPR will be providing this daughter board this summer for those that have TNC's that can use the TAPR DCD and want to mute the repeater audio when MIC-Es are being used on them.

March, 1997

  • After beta testing, engineering changes are being made. Several new options and changes are being made based on the beta testing phase. Documentation still needs to be completed.

    The picture at right is Bob Bruninga, WB4APR, and Mark Humphrey, KE3XY, talking about the MIC-E and APRS during Greg's, WD5IVD, recent visit to Bob's office at the Naval Academy.

February, 1997

  • Beta testing is about finished. Several small project and docs still to be comepleted. Based on beta-testing a new design is being worked on to correct several issues discovered during the beta testing.

December, 1996

  • Beta Units shipped, testing begun

November, 1996

  • Picture on the right is the pre-design concept.

  • Beta Testers Selected
    TAPR has selected a small group of people to beta-test (or really use) the upcoming APRS MIC-E adapter.

    At this time, we have collected the necessary information to continue with the Beta-Test. TAPR would like to thank you for taking the time to come and take a look at the page. This looks to be an exciting project and one that will enhance many different capabilities.

    The purpose of beta-testing is to help generate and correct the kit documentation, ensure hardware problems are nonexistent in more variations not possible during the alpha-test, and that the software is functioning adequately in a wider range of applications. In addition, these beta-testers will become the core expert group to help others when the units are made available to everyone.

Left - Preliminary Front and Back panels (11/2/96)
Right - Front and Back Panels of beta units (1/1/97)
Note:Click here to see the proposed front face plate on the production version.


The Tucson Amateur Packet Radio (TAPR) APRS® Mic-Encoder (Mic-E) Kit was made possible by the pioneering efforts of

  • Bob Bruninga, WB4APR,
  • Will Clements, N3XLM,
  • Gwyn Reedy, W1BEL,
  • Greg Jones, WD5IVD (TAPR Project Manager)
  • and the active participation of a number of Amateur Radio operators and organizations.
      Alan K. Unangst, WC7R, Prescott, AZ Mike Parker, KT7D, Tucson, AZ
      Don Hilman, VE7FBI, Victoria, B.C. Steven Boyle, KD6WXD , Mountain View, CA
      Cliff Buttschardt, K7RR, Morro Bay, CA Stan Horzepa, WA1LOU, Wolcott, CT
      William Covey, W1GTT, East Lyme, CT Sam Guccione, K3BY, Camden, DE
      Neil Lauritsen, KA3DBK, Clearwater, FL Rich Garcia, N2CZF, Jupiter, FL
      Steve Dimse, K4HG, Summerland Key, FL Ralph Fowler, N4NEQ, Kennesaw, GA
      David Chesser, KA9NHL, Dekalb, IL Henry Van Bogaert, N9WMM, Midlothian, IL
      Larry Keeran , K9ORP, Hudson, IL Don Pfister, KA0JLF, Overland Park, KS
      Tom Kinahan , N1CPE, Westboro, MA Kevin Kelly, N6QAB , Lusby, MD
      Will Clement, N3XLM, Arnold, MD Bob Bruninga, WB4APR, Glen Burnie, MD
      Jonathan R Brockmeier , KB8SWB, Holland, MI Tony Drumm, AA0SM, Rochester, MN
      Rich Josephson, WA0UNS, St. Cloud, MN Jim Duncan, KU0G, Kansas City, MO
      Dale Lam, WA0NKE, Kansas City, MO Wilbur Goll, W0DEL, Kansas City, MO
      Bob Derderian, N2IPH, Bridgeton, NJ Mark Sproul, KB2ICI, North Brunswick, NJ
      Bill Healy , N8KHN, Incline Village, NV Alan Crosswell, N2YGK, Briarcliff Manor, NY
      George V. Chapek, N2AIG, Scotia, NY Michael E. Young, WB8CXO, Munroe Falls, Ohio
      Steve Caesar, AH7E, West Carrollton, Ohio Mark Humphrey, KE3XY, Chester Springs , PA
      Gould Smith, WA4SXM, Knoxville, TN Mark Endicott, WB0NOO, Nashville, TN
      Ron Parsons, W5RKN, Austin, TX Bob Winingham, KC5EJK, Dallas, TX
      Harry Burford , KA0TTY, Coppell, TX Greg Jones, WD5IVD, Austin, TX
      Mike Heskett, WB5QLD, Hurst, TX Frank McJunkins, K7RSD, Seattle, WA
TAPR wishes to acknowledge their contribution to this effort.

APRS® Mic-Encoder Message Formats

Contributed by Alan Crosswell, N2YGK. (June 1997)
Updated by Ron Parsons, W5RKN (December 1998)


This section describes the on-air protocol used by the APRS Mic-Encoder (Mic-E™). You don't need to read it unless you are writing software to decode Mic-Encoder packets, for example. Reference is made to the AX.25 standard, AX.25 Amateur Packet-Radio Link-Layer Protocol Version 2.0, October 1984.

The APRS Mic-Encoder compresses the APRS position report and message bits into the destination address and information fields of a standard AX.25 UI (unproto) frame. Although the destination address appears to be quite unconventional, it is a valid AX.25 address, consisting only of printable (shifted one bit left) 7-bit ASCII values. The result of this compression is a frame 232 bits long (plus any sync characters sent for TXdelay) which contains:

  • Transmitting station's callsign and secondary station ID (SSID);
  • Latitude & Longitude in degrees, minutes, and hundredths of minutes. Position ambiguity is allowed. Spaces may be used instead of digits in a manually-entered position. Thus you can enter a position indicating approximately where you are and it will not be misinterpreted by the viewer.
    So instead of 3859.11N/07629.11W (spaces are illustrated by a ^)
    enter 3859.1^N/07629.1^W.... and the icon becomes a 600' circle below 4 mi. or
    enter 3859.^^N/07629.^^W.... and the icon becomes a 1 mi. circle below 16 mi. or
    enter 385^.^^N/0762^.^^W.... and the icon becomes a 10mi. circle below 64 mi. or
    enter 38^^.^^N/076^^.^^W.... and the icon becomes a 60 mi. circle below 512 mi.
  • Bearing in degrees;
  • Velocity in knots;
  • Three binary message bits (7 standard message values plus 7 custom values). Custom messages have no pre-defined definitions.
  • A display symbol code (e.g. car, truck, RV, etc.); and
  • A specially encoded digipeater path (only understood by "APRS SSID digipeaters").

To this basic message, the following additions are sometimes made:

  • [+40..88 bits] Two or five 8-bit analog telemetry values (analog-to-digital values ranging from 0..255).
  • [+N bits] Arbitrary beacon text.
  • [+56..336 bits] Conventional (non-SSID) digipeater list ("VIA" path).

Figure 1 shows the basic AX.25 UI frame. Figures 2 and 3 describe the special encoding.

Fig. 1 -- Mic-E UI frame









112/560 Bits



72...N Bits

16 Bits


(see Fig. 2)


no level 3

(see Fig. 3)

Destination Address

Fig. 2 -- Destination Address Encoding

Callsign: Latitude & msg, etc. bits









Since the MIC-E is transmitting unconnected (broadcast) AX.25 frames, the destination address does not have to be a callsign-SSID as is required for connected-mode (two-way station-to-station) communications. Conventional APRS transmissions set the destination address to a generic broadcast callsign such as APRS. The MIC-E compressed transmission instead makes good use of these seven bytes by embedding the latitude and various flag and message bits as follows:

  • Six binary decimal latitude 4-bit nibbles giving degrees, minutes, & hundredths of minutes (DDDD DDDD MMMM MMMM HHHH HHHH).
  • [ABC] Three message bits, complemented (e.g. message 5 is 010).
  • [N] Northern/Southern latitude bit (N = 1, S = 0).
  • [W] Eastern/Western longitude bit (W = 1, E = 0).
  • [L] 100's of longitude degrees (L = 1 means add 100 degrees to longitude in the Info field.)
  • [C] Command/Response flag (see AX.25 specification).
  • [r] reserved for future use (currently 0).

To allow for position ambiguity where spaces are used in the lesser-significant digits of the lat/lon data and to allow for seven custom messages in addition to the seven standard messages, the following table shows the ASCII value (after the destination byte is right-shifted one bit) for various values of the latitude digit and the message/N/W/L bits.

Latitude Digit

Message/N/W/L Bit = 0

Message Bit/N/W/L = 1

Custom Msg. Bit = 1













































  • [SSID] AX.25 SSID, interpreted as :


Use conventional digipeater path, if any.


North path


WIDE-1 omnidirectional flooding


South path




East path




West path




North path + WIDE




South path + WIDE




East path + WIDE




West path + WIDE

Information Field

The Information field (text) is used to complete the position report that was begun in the destination address. The encoding used is different from the destination address since the content is not constrained to be printable, shifted 7-bit ASCII, as it is in the address. However, full 8-bit binary is not used: all values are offset by 28 and further operations (described below) are performed on some of the values to make the data close to if not completely printable ASCII. This enables one to use a TNC in MONitor mode to display the data without confusion over whether a carriage-return is part of the data or the end of line. Also, software that suffers from the limitations of only accepting 7-bit printable ASCII is accommodated.

NOTE: If the first character of the information field is not one of the four valid MIC-E flag characters (1Ch, 1Dh, 60h or 27h), then the frame is not a MIC-E compressed report and must be interpreted as a conventional AX.25 packet.

Fig. 3a -- Information Field Encoding: Mandatory fields

MIC-E mandatory Information Field data bytes



Speed & Course












The information field is encoded as follows:

  • [F] Mic-E flag, one of:


GPS data is valid.


GPS data is old.


GPS data is valid (Rev. 0 beta units only)


GPS data is old (Rev. 0 beta units only)

  • [D + 28] Binary degrees of longitude. To decode:
    1. subtract 28
    2. if the [L] bit is set, add 100 to get final value of longitude.
    3. subtract 80 if 180 <= D <= 189
    4. or, subtract 190 if 190 <= D <= 199
  • [M + 28] Binary minutes of longitude. To decode:
    1. subtract 28
    2. subtract 60 if M =>60
  • [H + 28] Binary hundredths of minutes of longitude. Note that 00..03 hundredths will be unprintable.
  • [SP + 28] First part of speed in knots. Subtract 28 and multiply by 10.
  • [DC + 28] Second part of speed and first part of course. Subtract 28 and divide by ten. Quotient is units of speed. Remainder is hundreds of degrees.
  • [SE + 28] Second part of course in degrees. Subtract 28, the add to remainder, above.

Finally, make these course and speed adjustments:

  • if speed => 800 knots, subtract 800
  • if course => 400, subtract 400.

The remaining fields are:

  • [$]Symbol to use (see APRSdos README\SYMBOLS.TXT)
  • [T] Primary/alternate symbol table flag (Rev. 1 and higher only), one of:


Use primary APRS symbol table.


Use alternate APRS symbol table.

The information field is optionally extended either with telemetry values or comments/btext. If the next byte is one of the flag characters (60h, 27h, 1Dh), then it contains telemetry.

Fig. 3b -- Information Field Encoding: Optional telemetry

MIC-E optional Information Field data

optional telemetry (follows mandatory data)









The flag is one of:


2 printable hex telemetry values follow (channels 1 and 3).


5 printable hex telemetry values follow.


5 binary telemetry values follow (Rev. 0 beta units only)

Each telemetry value is a 2 digit printable hex representation of a binary value ranging from 0..255. For example, 254 is represented as `FE'. (Except Rev. 0 beta units.)

For comments/btext, the remainder of the information field simply contains textual data as configured by the MIC-E user. You can put in a standard APRS-formatted comment (see APRSdos README\PROTOCOL.TXT) which will cause the APRS display software to override any position data the Mic-E has encoded. This is most useful if you are using a Mic-E without a GPS.

Fig. 3c -- Information Field Encoding: Optional comments

MIC-E optional Information Field data

optional comments/btext (follows mandatory data)

comments or btext

APRS® is registered to Bob Bruninga, WB4APR. Mic-Encoder™ is a trademark of Bob Bruninga, WB4APR. Parties interested in commercial development and commercial sales of the MIC-E both within and outside of the amateur community should contact Clement Engineering.

It is published here to document the TAPR project and to allow amateur experimentation. This ownership does not intend to restrict individual amateur designs and construction, but it does require that all commercial development and commercial sales both within and outside of the amateur community must license the trademarks from Bob Bruninga and Clement Engineering who welcome such applications.

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