Backtracking in APRS

Always interesting to look at glitches on APRS maps and investigate the cause. In this case, a local NMEA log provides evidence that the APRS track is faulty.

googleearth 14/11/2015 , 12:18:38 Google Earth

Above is an image of a track, red is from APRS and blue from a local NMEA capture.  Two ‘backtracks’ are evident on the red trace and not on the blue trace, they are artifacts of the design of APRS. Continue reading Backtracking in APRS

Mapping trips from APRS archives

I recently created a map from APRS archives of a recent trip by some friends over about eight weeks through central and north west Australia and back by the southern coast.

Google Earth googleearth 01/11/2015 , 08:43:23

Above is a graphic of the created map, but the ‘real’ map is not simply an image, but it is a kml file for Google Earth which you can view / zoom / scroll, for example in Google Maps by clicking on the map above.
Continue reading Mapping trips from APRS archives

Zig zags in APRS tracks

There are many causes of zig zag errors in APRS tracks, and they fall generally into two main categories:

  1. incorrect positions (ie the tracker was never there); and
  2. correct positions with incorrect timestamps.

The first is common and has a number of causes, but principally defects in software used in iGates, most of which is not maintained either by the iGate operator or original developer. APRS is pretty static, but most old software has significant defects.

The second category is again common, and mostly the result of the design of the APRS radio network and its vulnerability to network delays (some of which can be caused by defective equipment).

This article looks at a case in the second category where a vehicle appears to have done a U-turn on the highway, travelled back some distance then another U-turn and caught up their original track speed at the next posit. It is clearly an out of order packet. This article shows how to diagnose the cause from the raw packet log for the tracker.


Screenshot - 30_10_2015 , 08_36_44

Above is a map of the glitch that is not uncommon in APRS for one of several reasons. No, VK2HJ has not done U-turns on the highway, the zig zag track is incorrect. Continue reading Zig zags in APRS tracks

OpenLog for TinyTrak – drive test


A drive test of the OpenLog logger collecting raw NMEA data in parallel with the TinyTrak (VHF) was conducted. To maximise the performance of APRS, a fill-in digi / iGate was run at my home. The tracker used a 65W transmitter with quarter wave vertical in the centre of the car roof.

Google Earth googleearth 29/10/2015 , 08:19:01

Above is an overview of the APRS and OpenLog tracks. Click on the image for a scaleable / zoomable view in Google Maps. Continue reading OpenLog for TinyTrak – drive test

OpenLog for TinyTrak

There are a host of factors that contribute to data loss in APRS, to name just some:

  • non-standard / sub-standard / poorly configured digipeaters;
  • defect ridden iGates that lose, duplicate and corrupt packets;
  • poorly configured mobiles;
  • network congestion and interference;
  • unpredictable equipment failures;
  • basic geographical coverage of the network; and
  • dependence on the ionosphere for HF APRS.

This article describes an enhancement to the popular TinyTrak (and its clones) to also capture the GPS stream to an inexpensive local data logger.

The logger does not interfere with normal radio APRS, it coexists with it and creates a properly timestamped fine detail log of positions over a very long time, a log that can be post processed into a range of graphic / map and tabular reports.

Data logger

The datalogger used in an OpenLog. It is a simple logger that writes data to a micro SD card formatted FAT16/32 up to 32GB, costs about $A12 (inc post) for the logger and about A$10 (inc post) for a 16GB Class 10 micro SD card. (A slower card could be used, but they aren’t much cheaper.)


Above, the OpenLog data logger.
Continue reading OpenLog for TinyTrak

A cheap and cheerful data logger

I had need of a portable serial data logger for proof of concept of a supplementary data logger for an APRS tracker.

The requirement is to capture RS232-TTL data at 4800bps, 8N1 to a data file for later extraction. The logger needs to restart automatically and append new records to the existing file.

A spare Raspberry Pi2 was applied to the job as a headless data logger.



Above is the RPi2 with an inexpensive FTDI USB/RS232-TTL adapter. Only the ground and RD wires attach to the modified TinyTrak. Continue reading A cheap and cheerful data logger

Adjusting KISS TNC AFSK tx level using an improved isochronous test packet

Adjusting KISS TNC AFSK tx level using an isochronous test packet explained a technique to drive a KISS TNC with a specially constructed packet that contains an ISOCHRONOUS test packet, a packet that will produce equal high and low tone alternation in the transmitted AFSK signal. The improved packet should be repeated by most digipeaters, allowing observation of their modulation performance.


Above is the waveform recovered from a receiver without de-emphasis (a Motorola R2009D communications analyser in this case).
Continue reading Adjusting KISS TNC AFSK tx level using an improved isochronous test packet

On-air testing of APRS digipeater with a crafted suite of test records

It is one thing to read code, and perform traces of live traffic to test proper function of a digipeater. This is a more valid technique than injecting test records directly into the software as for instance, viscous digi depends on the neighbour digis. Continue reading On-air testing of APRS digipeater with a crafted suite of test records

APRS traffic study – VK2OMD-3 08/02/2015

APRS traffic study – VK2OMD-3 11/10/2014 reported a study of APRS traffic.

This article is an update after configuration changes at VK2RHR-1 to make it a ‘fill-in digi’, it repeats packets addressed to WIDE1 only.

The study of APRS traffic was conducted by analysing the log of VK2OMD-3, a VHF APRS ‘fill-in’ digi and iGate.

The capture period was during from 0700-1700 local time on Sunday 08/02/2015. Continue reading APRS traffic study – VK2OMD-3 08/02/2015