I couldn't get my GT11 to find any satellite signals this afternoon. Weird. It worked fine two days ago.
Today, it turned on OK, displaying the "Card Found" message.
After a period of time, displayed "Indoors? Yes/No". Responded "No" - was at edge of Lake Burley Griffin.
After a further period of time, displayed "Travel long? Yes/No". Responded "No" - I was at exact same place I last used it.
After about 5 minutes of no apparent progress, I turned it off, then on again. Same sequence of dialogues; this time I watched the screen showing satellite signal strength. Was void of any signals. 
I tried the GT11 at home tonight; took ages to start, but seems OK now. That was a relief.
I don't think it could be an undervolt problem; I charged it up after Wednesday's sail.
Has anyone else experienced a transient "no signal" situation? Is there any logical explanation as to why it occurred? Is it a sign of impending complete failure of the GT11.
Prior to this, the device has performed flawlessly. I can't complain; I have certainly had my money's worth from it. I am not sure that I can now trust it as much as I used to.
Put it in a warm spot, card door open, even place it in a bag with a handful of rice. Sometimes the moisture gets to the workings. Make sure the card tangs are nice and clean. Use an eraser to clean them up
was it sitting on something stationary, or were you wearing it?
The GT11 sometimes has trouble finding itself while it's moving about.
Appreciate your advice, Decrep and Mineral. 
I will use some sachets of silica gel to help dry it out. Will also give the card terminals a bit of a polish. Re "moving about vs. stationary" , it was sitting on a post, face up, with a clear sky view. Has started up OK on the post many times before.
For what it's worth, it started without problems at home this morning.
Thanks again.
