No Spot messages for over 36 hours.01/10/2010 at 12:52 am | Posted in Uncategorized | 6 Comments
It has now been over 36 hours since the last message from the Spot tracker. So we really have no idea where Pinta is. Based on previous speed measurements she *probably* couldn’t have made it to land by now but its not totally impossible. Ever since we launched the reliability of the Spot hasn’t been as good as we’d hoped for. Between the time we launched and now 32.5% of all messages that we expected have failed to deliver. If you discount the last 36 hours then this rate drops to around 27%. Things have got a lot worse in the last few days though, between the 17th and 26th only 6 messages failed to arrive. On the 27th we lost 2 messages, on the 28th we lost 3 and on the 29th another 2.
There are lots of potential causes for this, here’s a list of just a few that we’ve thought of:
- Pinta has sunk or been run over by a ship. Although there’s very little shipping in that area and most of it sails closer to land than Pinta’s last known position.
- The keel has broken off and Pinta has capsized.
- The Spot has fallen off and sunk or is floating (attached to some foam) but no longer attached to the boat. If this happened gradually it might account for the deteriorating message reliability in the last few days.
- Water has entered the Spot box and damaged the electronics. The timings of the messages we have received suggest that the PIC which controls the Spot hasn’t been rebooting and it was very well sealed.
- The Spot has run out of battery. This is unlikely unless we miscalculated or overlooked something. We’ve run several long term (up to a month) tests on the Spot and found that it lasts about a month on a pair of AA lithium batteries when transmitting every hour. The Spot on Pinta has 6 batteries and only transmits every 6 hours.
- The Spot (globalstar) satellite is having trouble. This has happened before and wouldn’t be a total surprise although its now been going on a long time. If anyone knows of a webpage where you can find out the status of the Globalstar satellites we’d be interested in seeing it.
- The higher the waves, the harder it is to transmit. We’ve had problems before when buildings, bits of car, trees, mountains etc have obscured the Spot’s sky view and this has caused messages to fail. It is possible that waves can cause the same problem. The problem is threefold. Firstly the rocking motion might mean any attempt to send data to the satellite is interrupted. The wave period has typically been between 6 and 10 seconds, which should be a lot longer than it takes to send a message. Second if transmission occurs when the boat is in the trough of a wave (especially when wavelengths are very short) it effectively has a wall of water on either side of it which will block transmissions. Finally the boat is likely to be heeled on one side a lot of the time which will reduce the amount of sky seen by the Spot’s antennas.
- Spot don’t like us automatically pressing the buttons on their unit and have decided to cut us off. If they have, they’ve not told us!
To see if there was any correlation between wave height and Spot reliability the graph below shows median daily wave heights (in metres) from the closest functioning data buoy to Pinta. This was the M3 data buoy (located near Mizen Head) until September 18th and the K4 data buoy (about 150nm WNW of Westport, 54.550 N 12.367 W) after this date. Unfortunately for much of the time the M1 databuoy to the west of Galway would have been more appropriate but it is not currently functional. The graph also shows the number of Spot messages received each day. This should almost never be more than 4, although as the interval between messages is actually slightly less than 6 hours there have been two days when we expected to receive 5 messages in one day but never actually achieved this. This analysis is probably too simplistic as it doesn’t consider the heading of the boat in relation to the waves and wind which will have a big influence on the angle of heel and the rocking motion of the boat. However, there does appear to be a (inverse) correlation. If we look at the 15th of September the waves increase to 4 metres and the Spot goes from getting 4 to 2 messages. As the waves die down during the 16th and 17th the Spot increases back to 4 messages a day and maintains this until the 22nd when the waves again increase and the number of messages drops. On the 25th the trend breaks as both the waves and number of Spot messages drops, but the Spot recovers again on the 26th while the waves are still relatively small. Since the 27th the waves have again increased in height and the Spot has again decreased in reliability. This still needs to be tested properly with a correlation coefficient but it looks like there might at least be something to the “big waves stop the spot from working” hypothesis. On reflection it is quite impressive looking at the wave graph that a 3 metre dinghy intended for sailing around very calm waters has survived for 17 days in waves almost never less than 2 metres high and sometimes over 4 metres (and with a period range of only 6-10 seconds).