32 Knots, backwards...

[dickh]

There was I, sailing gently from Walton Backwaters to River Deben on Sunday morning, 3 knots on the log, when I glanced at the GPS repeater and noticed I was doing 24 Knots - funny I thought, then looked again and it gradually crept up to 32 knots... Then I noticed I was apparently sailing the reciprical course on the GPS.
Left it for a few minutes slightly bemused and decided to switch off the gps, switched on again and after a few minutes showed the correct course and speed. Good job I wasn't relying it, just using eyeball Mk1.
What happened?

dickh
I'd rather be sailing...

 

[ChrisJ]

Sounds as though it happened about the same time as:

"Aye Aye Sir. The GPS says we are nowhere near land"
"Look out of the window"
"Why bother? There aren't any islands anywhere near here".

"Bump"

 

[charles_reed]

3 knots sideways

The Garmin 35 I have feeding the chart-plotter sometimes gets overexcited and plots a course at about 50% of actual speed at 90 degrees to my actual course.
When first I reported this to Garmin they put it down to selective degradation, but it has carried on doing this, occasionally, ever since.
The attack only lasts about 4-5 minutes and turning the unit through 90 degrees always appears to fix it.
As I lack total trust in any single navigational input, these hysteric attacks came as no surprise to me especially in view of the orbital decay of the GPS satellites, which means that GPS positioning will become increasingly apocryphal.
Unfortunately the EU appears to have stalled on Galileo, Decca is no more, so we'll have to get used to the +/- 4 miles of Loran and relearn our EP skills.
I've got into the habit (unfortunately) of relying on the GPS time clock for noon sights.

 

[Idlerboat]

I wonder if the british warship that ran up on to wolf rocks Queensland Australia a couple of days ago had the same problem with his GPS !

 

[claymore]

Attention seeking behaviour.
They like to feel needed and its their way of gaining your attention and respect. Decca's used to do it somewhere off Kintyre in bad visibility - they'd suddenly start telling you that they were unreliable.
I feel that they are of the female gender which made me wonder why the decca was always called Desmond - perhaps it was a cross decca? (sorry)

regards
Claymore

 

[Guest]

Definately not wanting to be picky but it was at Lord Howe Island about 300 NM off the NSW coast. So I'm not sure they belong to Queensland but probably NSW. Anyway more importantly they are going to Court Marshall the poor guy and he wasn't even on the boat at the time. Apparently he was off visiting the natives at the local, and at the time the ship was under his offsiders control!!! The locals reckon the last boat that crashed into that particular rock was in 1836 slightly before GPS was invented. Since them it has been clearly marked on every chart and pilots log ever written for the area.

Hope his superannuation is topped up...

Woops!!

 

[Idlerboat]

Global Position Somtimes

 

[jfkal]

Dunno whether you were gently sailing and whether the sea was gentle as well.
A few thoughts.
Background: GPS is nothing but a precise timing signal received from various satellites with known position. Accuracy depends mainly on the number of satellites in "view" at any given time and the software in the GPS device.
GPS positioning is quite accurate 2D, height becomes already a bit of a problem.
Speed is a different matter especially with speed changes over a small time period.
(You may want to make a small test: Use a handheld GPS. Hold it with your hand outstretched {have your reading glasses on} and move your arm in a, as swift as you can, half circle, or if your arms are long enough swiftly back to your body. The speed can run up to 7 knots easily. Now your feet have not moved at all right?
Depending now on the position of the antenna (mast head not a good idea) and motion of the boat (pitching, rolling etc) the antenna might reach those speeds in short bursts indeed. Now it depends on the software of your device whether it is smart enough to dampen the display like it is done with the FLUXGATE compass data for your autopilot.
GOS is great if it is used for what it was intended with the right equipment )

 

[tome]

GPS receivers use a Kalman predictive filter to dampen speed and take out noise. More likely the receiver was tracking an unhealty SV, eg today SVN30 is unusable 0900 - 2100 UTC. GPS receivers should decode the health status, but I'm not sure they all do in practice.

Was this after the set had been out of use for a few days, and shortly after power-up? Could be it was using old almanac data.

 

[dickh]

tome, I had used the GPS the previous day and it appeared OK. I noticed it about ½ hr after switching on for the trip back. The trip was only about 9 miles.
After the winter layup, it would not send SOG and COG info to the cockpit repeater, and I found I had to reset this, (after posting this problem earlier in the year) it had turned itself off over the winter. Weather mine decodes satelite health status I'm not sure, it is fairly old.
Normally it is reliable - but as mentioned in my first post, it occasionally "loses Data", but then is OK after a few minutes.

dickh
I'd rather be sailing...

 

[tome]

Sounds like it may be developing a memory fault (like the rest of us) and had dropped the almanac it gathered the previous day? Some early sets rely on an internal battery to keep the memory alive, and it may be on it's way out. Try leaving it on for a couple of hours before you need to use it so that it gathers up-to-date almanac data.