Originally Posted by ShinyShoe
I was under the possibly mistaken impression that 406MHz between the 70's of latitude had geostationary cover so you'd expect almost instant transmission. 60 second delay inbuilt in set to avoid false alarms. Then time for a satellite possition lock then transmit was my understanding.
I thought the LEO sats were not geostationary and were able to position the transmitter to within a couple of miles or so but had a delay as they needed to move to do the positioning.
That sounds to me like it took 30mins to get a sat fix. Thats very slow.
I think youre getting satellites and GPS mixed up a bit, First is the initial distress alert, which would have gone out almost instantly, but when falmouth fax or GD92 the info over to milford haven, part of it wil have read 'unresolved' which is relating to position.
Geostationary sats don't move relative to the earth, so there is no doppler effect, and therefore no means of telling how far away you are from the sat. (your position)
The PLB has a GPS built in, which can presumably populate a 'position' field when sending the 406 distress alert, it's possibe that it took he PLB a while to attain a GPS fix (alarm type noise), I know some can be awkward if they don't have a clear view of the sky, if it was in a lifejacket, or pocket, or maybe they are just slow?( I know older GPS units take a long time to get a 'fix' when turned on after a long time off, presumably because they need to find the satallites again)
OR the alarm type noise described was a LEO sat passing over and allowing a position to be deterimed using doppler.
either way, in the grand scheme of things, and considering what these were originally designed for (off shore alert where help might be hundreds of miles away) I don't consider 30 minutes a ridiculous amount of time.