Order Tray | Contact Us | Home | SIG Lists

[aprssig] APRS RFID apps? (spec rev2)

Bob Bruninga bruninga at usna.edu
Thu Feb 11 17:52:25 UTC 2010

Here is a rev2 of the spec based on some good feedback from Wes:

> 1] Counting dupes will be troublesome....  
> Invariably, someone else will send 
> an RFID from another reader at the 
> same time and they'll get clobbered. 

No, the dupes are counted by the RFID reader PIC while the card is within he 6" of the coil.  A read takes  It makes the decision asa to whether the foot is moving, stepped on, or standing.  Then it sends only ONE packet that is flagged as to which of the 3 conditions is set.  However, there will be transmit redundancy.

Regarding the IN/OUT problem:

> 2].. sell them two cards.... 
> one for for LEFT, one for RIGHT shoe.... 
> one for coming, one for going....

Neat idea.  But Im now leaning back towards your original comment that:

> 3] if I'm going OUT, the [hotspot] puts me 
> in...the EOC, but 60 seconds later, I 
> drive away and my car "steals" the icon 
> out of the [EOC and on the road].  
> Same for Dayton.  As I walk between 
> buildings I'll end up moving my icon in 
> a few seconds anyway.

I agree.  That is the most likely scenario.  Probably the only place we need an OUT is by the large back door to the arena where most people go out. 

> 4] I... assign [my RFID card] the callsign ai4px-9.  
> Later, my kid wants to use it, so I... reassign 
> it to her callsign... [its a big problem to make
> sure the entire system at evrey location in the 
> world gets updated.. to the new association].

Easy.  Don't do that.  Once an RFID is associated with a callsign, the owner should never assume that he can change it if he wants to guarantee it will get recognized everwyhere.  I'll add this caution to the spec.

RFID SPEC rev(2) 

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, or locally at clubs to identify hams at the clubhouse or EOC.  

APRS HotSpots:  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 FR and IS network showing that person at that location. 

APRS RFID READER:  A $25 RFID reader is the only new hardware required. See http://www.sparkfun.com/commerce/product_info.php?products_id=8709 

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.  Two bits report the three cases if the foot was moving, or stepped, or standing.  The initial "RF" in the TOCALL 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 "RA". 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 or decrementing .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.  This works with all clients.  Just zoom in close enough and the table will appear. 

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. It is imperative to associate one and only one callsign permanently with each card because that association will be impossbible to reliably change later thorought the entire APRS system.  For multiple shoes, buy multiple $2 RFID cards.  Write the callsign on the RFID card. 

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... and should never be changed.

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, and repeats as long as the card is within the few inches of the coil. 

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

Special code in a "tracker" type pic processor watches these reports (up to 64 per second) and then generates 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). As noted above, the format of that packet is:


And the four bits of the SSID (I) of the HotSpot "SPOTID" contains two bits that indicate if the foot was moving, stepped on the tag, or is standing still on the tag.  These three states can be used by the user to signal such things as IN, or OUT, or AT on some tags.  If the foot is moving (in range for .1 second), the tag will be read on the order of 6 times.  If the foot is stepped, it will be read on the order of 60 times.  If the foot is standing for over 2 seconds, it will be read on the order of 120 times or more.  These times are used to set the state of the SSID.  The four SSID bits are designated as ABCD. 

AB = 00 this RFID reader does not discriminate foot movements
AB = 01 Foot was moving (under 10 hits)
AB = 10 Foot was there more than 10 but less than 40
AB = 11 Foot was standing more than 120 hits

CD are available for other TBD applications

In most cases, an OUT is not needed if people generally use the same callsign-SSID for their shoes as they have on their car.  Once the car reports, the foot RFID posit is replaced in the APRS system.

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 

More information about the aprssig mailing list