Monday, February 22, 2010

2-way APRS Communications Vs. Rx Only iGates

I operate a iGate at my house, and a Digi/iGate on the local mountain top. I reciently got a TinyTrak4 with a display unit, and was attempting to send an email via APRS, and I wanted to share what I learned, what I did, and the eventual results.

2 Rx Only iGates in town, and a Wide1-1 Digi on the hill that also gates. I found several options for sending an email, but I wanted to make sure that 2 way communication via this method was possible. If I am out camping in one of my favorite areas, there is no repeaters to get into, but I might be able to get my Digi. The ability to communicate out of the area is important, as it is near Death Valley and no cell phone coverage.

Here are the instructions for sending email via APRS that I found, I used the 1st one because of the 2 way option, but the 2nd one is a little easier to remember how to use without a cheat sheet.

http://www.aprsca.net/email.html
http://www.aprs-is.net/Email.aspx

When sending messages, I noticed that I wouldn't always get the Acknowledge back. I checked my paths and the local iGates were gating my traffic to the APRS-IS, but because my iGate is a RX-Only, and the other one has the Transmitter disabled, I figured the return traffic was never being sent to RF.

I started researching filters used for iGates, and I found that my gate was not sending any filter commands to the servers it was connecting to, except range. So when the packet would return to my iGate, it would just be discarded. Even if it was handed to the TNC, I am using a scanner as a reciever, so there is no transmitter. I added in the following to the filter string, "t/p". The reason was positions usually only travel to the APRS-IS cloud. When you apply a filter, you are basically telling the APRS-IS server what you want from it or can handle. Now when I send a message, and my iGate picks me up, the Acknowledge is sent by the full iGate/Digi on the hill.

Ref: http://aprs-is.net/javAPRSFilter.aspx

So the big thing that I learned here, is RX-Only iGates must have the filters set properly as to not inhibit 2-way communication via APRS. Now if I can convince the owner of the other iGate in town to add that filter line to his login string....
Ed
KI6PSP