Android Start Line Apps Anyone?

Originally posted by RTFM So what part of my Differential GPS explainiation do not understand? In did not say anthing about  precise GPS - that's RTK which I assume you also no nothing about...

I was using precise GPS in its general sense, not in a proprietary sense.
I wouldn't expect anyone who can't spell 'know' to appreciate the difference though.
However, let's put all that aside, this is not the place for this discussion.
Would you agree that the OP is wasting his time with a simple GPS ap to tell if a dinghy is over the line?

Originally posted by RS400atC Originally posted by RTFM >>To get relative postions with any great accuracy, you have to make sure the receivers are prioritising >>the same satellites. That is how Differential GPs gets much better accuracy. Not quite correct. If I may explain...   Differential GPS involves the cooperation of two receivers, one that's stationary and another that's roving around making position measurements. The stationary receiver is the key. It ties all the satellite measurements into a solid local reference. Remember that GPS receivers use timing signals from at least four satellites to establish a position. Each of those timing signals is going to have some error or delay depending on what sort of perils have befallen it on its trip down to us. Since each of the timing signals that go into a position calculation has some error, that calculation is going to be a compounding of those errors. Luckily the sheer scale of the GPS system comes to our rescue. The satellites are so far out in space that the little distances we travel here on earth are insignificant. So if two receivers are fairly close to each other, say within a few hundred kilometers, the signals that reach both of them will have traveled through virtually the same slice of atmosphere, and so will have virtually the same errors. Differential GPS can eliminate all errors that are common to both the reference receiver and the roving receiver. These include everything except multipath errors (because they occur right around the receiver such as masts and sails etc) and any receiver errors (because they're unique to the receiver). That's the idea behind differential GPS: We have one receiver measure the timing errors and then provide correction information to the other receivers that are roving around. That way virtually all errors can be eliminated from the system, even the pesky Selective Availability error that the DoD can put on if they so wish... Hope that helps... Yes, that helps me see that you don't really know very much about precise GPS. I suggest you set up a mock start line and see what happens. Any time spent on Android apps could be more profitably be spent I think.

>>To get relative postions with any great accuracy, you have to make sure the receivers are prioritising

Edited by RTFM - 25 Feb 13 at 10:28pm

not sure if they are reading the GPS direct or just the possition.  One of them (I forget which) seemed to be doing lots of very fancy stuff with pitch and yawl etc.

If I was writing the basic GPS algorithm I'd tell it to work out where it should be based on speed and direction travel.  That may cause some issues as I almost always tack near the line!

RTFM wasn't suggesting you'd be 10m behind (or over) the line. But that it would be able to take you back to the same spot on the same day.  The way you configure the start line is to sail to the committee boat and stop with the committee boat about 1m to your starboard side. Hit the RC button and set its possition from the GPS.  Then sail to the pin and repeat.  So it doesn't matter if the global possition is 10m out provide it stays 10m out for the rest of the day.  (Although it'd make me happier if I can save some of the fixed marks for eternity but they will also move a little in wind etc.)
 

How accurate is GPS you may ask?
Turn on any GPS receiver and it will give you an approximate horizontal position of 10-15 metres. It does depend on how much positional averaging the manufactures GPS unit does, and if you moving or not. Accuracy is also affected by a number of other factors, including the satellite positions, noise in the radio signal, atmospheric conditions, and barriers to the GPS signals, such as sails and masts.
However having a receiver that is accurate to approximately 10 metres is not an issue with the race sailing application, as the 10m accuracy stated above is the absolute position, and not a relative position accuracy. All of the sailing boats within the same race area will have the same GPS errors in there calculated horizontal positions. (The largest error is mainly attributed to the ionosphere & troposphere delays as mentioned above). This can be seen by taking two GPS receivers on a single boat (with the same GPS logging configuration) and the ‘base line’ distances between them will remain reasonably constant.