Thursday, July 11, 2013

How to Interpret the Tracker

June 11, 2013

Nani is making slow but steady progress. Alex posted a message on the tracker this morning saying they are finally sailing and making 5 knots into a 10 knot breeze. Sounds like things are a bit tilty on the boat!
Alan sent me a e-mail yesterday with some information on how to best interpret the tracker information, and it answered some of the questions we have had, so I'm sharing it here. 

From Alan:

I was looking at the Delorme tracker and thought that it might be informative to your readers to have a little more insight into what they are seeing.
The gaps in the data are usually due to the tracker running out of battery power.  It is powered by 2 AA batteries and they don’t last very long.  At the beginning of the trip, Alex was using lithium AA batteries that had longer life.  They are gone and he is using standard AA batteries.  You will also see that the spacing of the dots varies ( based on the the reporting interval as well as boat speed).  The Delorme defaults to reporting a position update every 10 minutes.  To save battery power, Alex has changed the interval to 2 hours. 
You will see on 7/10 at 0323 (3:23 AM), there is a position reported then there is a gap when the batteries died sometime there after.
At 0714, it looks like the batteries were replaced.  However, the reporting interval reset itself to the default 10 minutes.
At 0738, Alex reset the interval to 2 hours.  This has to be done manually using the Android tablet.
When reviewing the data reported, the position information is probably fairly accurate though the Delorme sends the position signal on a space available basis on a low earth orbit satellite network.  Sometimes it’s possible that the signal is not transmitted exactly at the time reported.  The speed and heading information reported can be somewhat suspect. Instantaneous values are not very accurate – a wave, a brief change in heading, a gust of wind, or just reporting by the GPS introduces several sources of error.  Calculating average speed and heading between two reported positions is more accurate.
You could try to read tea leaves inferring that low speeds and headings to the north or NNE might be sailing, followed by a significant course change and increase in speed that could suggest motoring but there is no way to know for certain and speculation serves little purpose.  It is a fantastic look at a trip unfolding.  Looks like they are making good progress.  Here’s to good sailing!

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