The mobile APRS W2 digi left town this morning

APRS is characterised by a lack of standards or guidance, but mostly by a lack of compliance with those standards and a lack of understanding and common sense.

VK2UWP is travelling around the countryside and from the look of it, setting up a WIDE2 digipeater in the local caravan parks without regard to its dysfunction. Continue reading The mobile APRS W2 digi left town this morning

TNC-X – DCD behaviour

I have been investigating ‘lost APRS packets’, a result of many root causes.

One is a perception that when I drive in the local area, I often do not hear my own digi ‘echo’ yet I find that it did receive a packet and submit it to APRS-IS. This problem is much more apparent using a TNC-X that with any other KISS TNC in its place.

A possible explanation is that it did echo but I did not hear it due to collision with another digi.

So that led me to explore whether TNC-X fails to quickly detect the other digi already transmitting. Continue reading TNC-X – DCD behaviour

Latency in APRS networks

APRS-IS depends very much on a timestamp created at the time that a record from an  iGate is received.

Very few stations transmit a timestamp in the packet header even though accurate time is usually available when the GPS provides valid position data.

So in a network where packets are carried through many digipeaters (a result of the end user supplying an inappropriate PATH), and many of these digipeaters having several iGates listening, there is a need to weed out duplicates.

The mechanism to discard duplicates falls down when the duplicate submission is delayed significantly, and some network infrastructure is notorious for latency.

Continue reading Latency in APRS networks

Should you use MIC-E compression in your APRS tracker

At first, the MIC-E compressed packets might seem to have advantage in reducing channel utilisation and improving the probability of successful packet transmission… but it comes at a cost.

The linux iGate daemon aprsd 2.2.5-15 has had a problem with corruption of MIC-E packets for more than ten years resulting in occasional gross errors in position reports processed by those nodes. Continue reading Should you use MIC-E compression in your APRS tracker

MFJ-1270B TNC review

MFJ1270BFront

I have been doing some research on APRS recently, and Brian, VK2AH, kindly donated a MFJ-1270B.

MFJ-1270B Over the years I have heard so many reports that the MFJ-1270B does not accommodate twist (an amplitude / frequency slope usually the result of lack of pre-emphasis, usually the result of audio overdrive of the transmitter, and usual for hams). More detailed discussion is at The extent to which APRS works is often an accident.

There is no shortage of bad signals to test the tolerance of the TNC, with two local overdriven digis, and travellers through the area. Continue reading MFJ-1270B TNC review