[aprssig] APRS RFID apps? (spec rev1)

Bob Bruninga bruninga at usna.edu
Wed Feb 10 14:23:41 CST 2010

RFID SPEC rev(1)

> I'd really like to have my shoes have the same 
> callsign as my car (or other ssid)... when I 
> [drive away] it will "snatch" my icon away 
> from the EOC table and out on to the road.

Good idea.  I have revised the spec below so that one can assign any callsign-SSID to any of his cards, with a suggestion to make them all the same if they are just different shoes, or different for different applications.

I have also eliminated the -D SSID on the local RFID packet since it violates the use of the TOCALL field in APRS.

> How do you define the location of the reader?

That is exactly where it is.  But that position (which shows up on the map as a HotSpot reader) is not the same as the origin of the LATITUDE table of RFID callsign's that have passed over it.  That origin is offset during set-up to make the vertical list on the map be in a logical place.  For example, the HotSpot-IN tag would put its list INSIDE the HARA arena.  The OUT reader would make the list on the outside.

>  I'd sorta envisioned ... instead of just 
> incrementing .01 latitude, you could do 
> a spiral effect around the reader...

I did that with my first APRStt application back in 2001 and quickly learned that it is confusing to viewers that do not know they are randomsized positions.  Therefore for APRStt and RFID' I think it best to clearly show up on the map as a distinct vertical table located in a logical place on the map.

I have also nailed down the RFID symbol which is "RA" and had alredy been defined.  So here is an updated RFID SPEC:


The following is a possible design of an innexpensive ham radio APRS RFID system for application in large venues such as the Dayton Hamvention to keep track of walking operators.  Ten Inch round RFID yellow/red hot spots are placed in each doorway or special location.  APRS participants wear a $2 RFID card in their shoe.  When they cross an APRS hot spot, the hot-spot reader sends the RFID to a centralized on-site PC that associates that RFID to a callsign and then generates an APRS packet on the APRS network showing that person at that location. 

APRS RFID READER:  A $25 RFID reader is the only new hardware required. 
The 9600 baud serial output from this device is connected to a PC for one-door applications or, for large multi-door locations, to a typical APRS "tracker" which has special parsing code to convert the RFID number into a special AX.25 short-range packet (preferably on a separate frequency from he national APRS channel).  The RFID packet format is: 


Where the SSID of the hotspot reader (I) contains 4 bits of additional info such as direction, speed, etc. The initial "RF" will make this an ALTNET (non-APRS) packet, and the dddd:dddddd is the 10 byte HEX ASCII RFID code.  In addition, a SPOTID position packet is sent out once every 10 minutes direct so that the SPOTID itself shows up on the map.

The central processor receives the above RIFID from a given SPOTID and then matches the RFID to a callsign and generates the following standard APRS position packet for the local network (and/or APRS-IS): 

Packet:  !DDMM.mmNRDDDMM.mmWAdddddddddd free text 

Where CALL-SS is the callsign associated with that card. The APRS SYMBOL is "R and A". The RFID number is included in the first 10 bytes of the packet (which happens to show up well on most mobile and handheld APRS displays.  The Free text is associated with that hotspot.  Such as "Entering HARA", Leaving HARA, or ARRL Booth, or KENWOOD Booth etc. 

The position of someone passing over the RFID HotSpot is built by incrementing .01 Latitude from an origin on the map associated with that RFID HotSpot.  This way, everyone that crosses that hot spot will appear in a neat vertical table on the map. 

The users CALLSIGN and SSID is associated with each of his RFID tags.  There are two schools of thought depending on your own use of your tags.  Either make them all have your CALL-7 SSID to show you were walking, or make them the same as your car CALL-9 so that when you arrive or leave a place, your CALL-9 goes with you and is not left at the HotSpot.

CALLSIGN ASSOCIATION:  The initial RFID number to callsign association is made at the central processor or point-of-sale of the RFID card -OR- anywhere else that a keyboard or keypad is attached.  Or even send it in an APRS packet from any other radio.  It doesnt matter, since EVERY such association will generate the above APRS packet and from that , ALL systems locally or globally can collect these associations from the APRS-IS.  Also clubs with large numbers of card holders can submit RFID files to pre-load the APRS-IS system.  Or anyone with any APRS system can manually prepare the above packet and send it once from their HT or Mobile.  From then on, the association is made. 

RFID READER SERIAL DATA:  The RAW output from the RFID reader is at 9600 baud and can be read on any serial port.  The format is as follows: 

 Where S is STX (02h) 
 DDDDDDDDDD are 5 bytes in 10 ASCII HEX characters 
 KK is a HEX checksum 

Special code in a "tracker" type pic processor can read this and generate the local RFID Reader packet which is sent on a separate frequency to the central processor.  Or for one-door applications, direct into a local PC connected to the APRS system (RF or Internet). 

RFID Tag (125 KHz): The Credit card sized RFID's are under $2 depending on quantity.  They are available from: 


Anything I missed? 
Bob, WB4APR 

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