APRS Mic Encoder Message Formats

Contributed by Alan Crosswell, N2YGK.

This is DRAFT documentation of equipment under beta test and may not be correct or may be subject to change. Revised 4/25/97 for Rev. 1 changes.

Overview

This chapter describes the on-air protocol used by the APRS Mic Encoder. 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 which is available at http://www.tapr.org/tapr/html/ax25.html.

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) 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;
Bearing in degrees;
Velocity in MPH;
Three binary message bits (8 values)
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
Flag	Address	Control	PID	Info	FCS	Flag
01111110	112/560 Bits	00000011	11110000	72...N Bits	16 Bits	01111110
	(see Fig. 2)	UI	no level 3	(see Fig. 3)

Destination Address

Fig. 2 -- Destination Address Encoding
Callsign: Latitude & msg, etc. bits						SSID
Ar1DDDD0	Br1DDDD0	Cr1MMMM0	Nr1MMMM0	Lr1HHHH0	Wr1HHHH0	CrrSSID0

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.

[SSID] AX.25 SSID, interpreted as :

0	Use conventional digipeater path, if any.	8	North path
1	WIDE-1 omnidirectional flooding	9	South path
2	WIDE-2	10	East path
3	WIDE-3	11	West path
4	WIDE-4	12	North path + WIDE
5	WIDE-5	13	South path + WIDE
6	WIDE-6	14	East path + WIDE
7	WIDE-7	15	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
flag	Longitude			Speed & Course SPD & CSE			symbol	table
F	D+28	M+28	H+28	SP+28	DC+28	SE+28	$	T

The information field is encoded as follows:

[F] Mic-E flag, one of:

60h GPS data is valid.

27h GPS data is old.

1Ch GPS data is valid (Rev. 0 beta units only)

1Dh GPS data is old (Rev. 0 beta units only)
[D + 28] Binary degrees of longitude. To decode:
1. subtract 28
2. subtract 80 if 180 <= D <= 189
3. or, subtract 190 if 190 <= D <= 199
4. if the [L] bit is set, add 100 to get final value of longitude.
[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.

The speed and course bytes are packed binary-coded decimal. After first subtracting 28 from the byte, each 4-bit nibble contains a single digit of the course or speed.

[SP + 28] First part of speed in MPH. 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 MPH, 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:

2Fh Use primary APRS symbol table.

5Ch 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)
flag	Telemetry
F	11	22	33	44	55

The flag is one of:

60h	2 printable hex telemetry values follow (channels 1 and 3).
27h	5 printable hex telemetry values follow.
1Dh	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

60h	GPS data is valid.
27h	GPS data is old.
1Ch	GPS data is valid (Rev. 0 beta units only)
1Dh	GPS data is old (Rev. 0 beta units only)

2Fh	Use primary APRS symbol table.
5Ch	Use alternate APRS symbol table.