Can you trust your GPS?

[Ivan Cloherty]

2. The principle….Is based on two well known optical laws, one…the angle of incidence is equal to the angle of reflection and two…
If a ray of light suffers two successive reflections by two plane mirrors in the same plane, then the angle between the first Ray of light and the second
Ray of light is equal to twice the angle subtended between the inclination of the mirrors.
.

Well remembered John

I always thought the angle of the dangle was influenced by the inclination of the female

[j.sabourn]

#21 If the female is on the visible horizon Ivan you can always use the corrections to get her on the sensible horizon and then she will have no choice but to choose the Rational horizon and subject herself to your carnal desires. With a bit of luck that is and may you finish up in the position you hope to be. Cheers JS

[happy daze john in oz]

Then you look in the mirror and think, well maybe I should have a shave before I approach her.
No luck with stubble there!!!!

[j.sabourn]

Well John if she is just touching the visible horizon and you yourself are swimming for your life , just pause a moment in your wishful swimming the breast stroke and work out how far away she is , say your eyeballs are a foot above sea level then the square root of 1 is 1 multiply that by 1.15 so she is 1.15 nautical miles away from you. Don’t suppose she will notice your stubble from that distance. You might even be lucky and she might be Venus but there again Venus rhymes with another word and maybe to your misfortune a fellow shipmate out of UCL whose real name is Bill but one of the other variety . Cheers JS

[David Varley]

I was kindly allowed to join RIN after submiting an idea of detection of GPS spoofing which, I claim, would work on any vessel with a proper inertial or optical north seeking gyro (as well, I suppose as my electronic nav qualifications from AMEC).

In addition to what ever else has been clagged on to the GPS add a GPS compass and a 'comparitor' (very simple same idea as off course alarm) to compare GPS heading with the proper heading reference. When the two diverge then staff will be alerted to investigate cause. It maybe that spoofers would be able to alter the apparent speed of the vessel (I suppose one could also include the vessel's log in the comparison) but that would be all. It would not tell staff which device was inaccurate but one must leave something to the human in charge.

If one is really cheapskate (and I think it is now within the rules) and your ship has already, and only, a GPS heading reference, you're on your own.