WA8LMF Mirror of WB4APR Website - 21 July 2008 APRS at Field Day

Global APRS messaging for Groups!
Such as JOTA, SCR, Skywarn, SATERN, AMSAT, or even FD!

Bob Bruninga, WB4APR

Now APRS has global group messaging capabilty!

APRS has always been a two-way tactical real-time communications system which can support group messaging in the local area on RF. But once messages are injected into the APRS Internet system (APRS-IS), they only return to RF when addressed to individual callsigns. But now with the CQSRVR, not only can group messaging be supported worldwide, it can also support global CQ's! too.

GLOBAL CQ's: . In the Fall of 2007, CQSRVR written by Pete, AE5PL, can be used to support any kind of global messaging groups such as SCOUTS or SCR (School Club Roundup), or, JOTA or SKYWARN, SATERN or even Field Day! . Using this capability, even if there are no other local groups on APRS in local RF range, you can still make two-way contact with other stations throughout the world.

BACKGROUND ON APRS MESSAGING: . Normally, APRS uses the local RF network on the national APRS channel (144.39 in North America) for communications. . But all of these local APRS packets are also injected by local I-Gates into the global real-time APRS-Internet system as well. . Everything goes into the internet... (where there is plenty of bandwidth) and generally, nothing comes back out because the 1200 baud channel cannot normally support it. . However, the one exception is messages. . Since APRS messages are one-to-one, passing them back to RF from the internet is not a problem. . This is why APRS users can send and receive messges anywhere on the planet as long as the recepient's callsign is known. . The network will assure a packet addressed to a certain callsign will be sent back to RF by the closest IGate that hears him anywhere in the world.


On the TH-D7 APRS HT, Just press the MSG button, select INPUT on the MSG Menu, enter CQSRVR as the address, and make the first text of the message be CQ FD CQ FD (or whatever your event is) followed by your location. . The example screens below are not customized for this web page, but show a message to EMAIL that contains the text A3XYZ@AMSAT.ORG OK in OceanCity with HT & whip!. . This actually shows how to send an email but that is a different topic

If your message is digipeated via the local digipeater, then you will see MY MESSAGE flashed on the screen indicating success. . Once you have sent your message to the CQ SERVER, you will then receive similar messages from any other identical GROUP stations that do the same. . Once you get one of their messages, then you should send them a message to their callsign to complete the exchange. . And that is only a hint of what these HT's can do (see event data entry example).

Having to know the recepients callsign in advance is why APRS has not been all that useful for SCOUTS, SCR or Field Day in the past, where random contacts are the normal mode of operations.

Of course, sending a message in APRS is simple. You can either user APRS software running on a laptop or PC connected to a packet station on the North American APRS frequency of 144.39 MHz, or you can simply key in a message on the D& and D700 APRS radios as shown below:

USING CQSRVR TO FIND OTHER SCOUTS, SCR or DEMO GROUPS: The purpose of CQSRVR is to permit stations to send a special CQ GROUP and to receive any similar CQ's from that group. . This way, each SCOUTS station can send out a CQ to all other SCOUTS stations and then those other stations can see your callsign and know you are on the air. . Then they can send you a normal APRS message for the two-way contact. . Once two stations are exchanging message packets, then the local IGate will also send out a complementary position report for that station as well. . This allows your map to begin to fill with those other stations no matter where they are.

CQSRVR DETAILS: . Operating with CQSRVR is simple. . There is only one command that you send to CQSRVR as an ordinary APRS message. . The same GROUP message not only logs you on to CQSRVR for GROUP but also is the CQ GROUP message that goes back out to all other logged on users. . The message is sent to the APRS address of CQSRVR. . Then the message body contains the two keywords CQ and GROUP. . Here are some examples:

CQ SCOUTS CQ SCOUTS from WB4APR in Annapolis Maryland (all after the first CQ SCOUTS is free form).
CQ SCR CQ SCR from WB4APR at the Naval Academy
CQ SATERN CQ SATERN from WB4APR giving a demo to students

Notice the limitation that a SPACE character must separate the GROUP name from the rest of the message. . No other delimiters are allowed. . And also, since this message will arrive with the first part truncated off, then you have to repeat the CQ twice so that the recepient will get the second half of it and know that is came as a CQ GROUP message. It is very important that the message contain that second GROUP name, so that recepients know that it was a group message, and not just a random individual message. The recepient cannot tell, unless the CQ GROUP CQ GROUP is included twice in the original message.

GROUPNAMES: . The following are some regularly used GROUP names: FD, JOTA, SCR, SKYWARN, SATERN, AMSAT, etc. . You may only send one of these messages no more often than once every 30 minutes to indicate your continued presence on CQSRVR. After 24 hours, you will be dropped from the CQSRVR list unless you send another CQ message. . CQ FD CQ FD...: . The only problem we anticipate with this process is CQ FD during the national Field Day event if hundreds and hundreds of groups may try to use it during the 24 hours of Field Day. At those levels, we may need to add some kind of automatic throtteling mechanism that will prevent overload of the RF network. The worst case scenario is where there are multiple FD sites in the same high density RF area which is served by too many IGates. There is no need for all of those sites to get the same copies of each CQ. . I think this can be solved by a bin counter that looks at the lat/lon of each outgoing message (to the nearest degree) and keeps the throughput into each bin below some threshold (maybe 2 packets per minute randomized)...

Using the APRS messaging system and CQSRVR, your APRS station can be as small as just the TH-D7 APRS handi-talkie shown here. . It can send and receive up to 10 messages. . To keep from over-filling the walkie-talkies message buffer, you will need to keep killing messages after you read them to make room for more.


TROOP AND STATION SCORING: . Another frequent application at scout camporee events is the collection of scores and transmission of those scores by HAM radio. The TH-D7 HT is ideal for data entry at special events, and the D700 is ideal for convenient data display at the point of use. The top photo above shows a D700 APRS radio control head attached to a clipboard for taking data from reporting stations... We use this system at scouting events where troops move from station to station while competing in skills competitions such as the Klondike Derby shown to the left.

From each station at every time schedule, a troop number and score is reported by HAM radio back to headquarters. In the past this was all done by voice reports from up to 20 ham radio operators with HT's, all on one voice net.

This year, we fielded 5 Kenwood TH-D7 HTs and the one D700 so that some operators could directly enter their information on the keypad and have the data automatically transmitted as an APRS message to the Headquarters D700 where the information was available to the HQ staff at their convenience, and not in conflict with the demands for the voice net.

For more info, please go to the APRSevent web page for more detail.

APRS POSITION TRACKING SUPPORT: Of course, APRS can be used to a significant advantage for keeping track of moving stations and objects at the event. Everyone with a D7, D700 or othere APRS GPS device can also see the usual APRS displays and position information for knowledge about where where other operators are (although 121 miles shown is not typical of a local event!): Or if the other HT station is moving, another operator can check his ALTITUDE, COURSE and SPEED page on that station:

APRS has lots of uses at any special event. . APRS operators simply bring along their D7 or D700 equipment and it is ready to go. . Besides we need to practice all the time to keep ous skills up for when we might need APRS, tracking, and communications for real...

Bob Bruninga, WB4APR


WA8LMF Mirror of WB4APR Website - 21 July 2008