Just posted on GPSSS
All (all right many of) your GPS questions answered!
www.mikeg.me.uk/guides/GPS_Devices_-_Speed_Sailing_-_Feature_Comparison.pdf

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lao shi said...

Just posted on GPSSS
All (all right many of) your GPS questions answered!
www.mikeg.me.uk/guides/GPS_Devices_-_Speed_Sailing_-_Feature_Comparison.pdf

Nice info.
But Garmin Fortrex 101 and 201 are 7 years old and have been discontinued. Garmin has Fortrex 301 and 401
with SiRF STAR III chip.

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AUS4 said...

lao shi said...

Just posted on GPSSS
All (all right many of) your GPS questions answered!
www.mikeg.me.uk/guides/GPS_Devices_-_Speed_Sailing_-_Feature_Comparison.pdf

Nice info.
But Garmin Fortrex 101 and 201 are 7 years old and have been discontinued. Garmin has Fortrex 301 and 401
with SiRF STAR III chip.

And do they offer any advantage to a speed sailor interested in accuracy over the older Garmins? Unfortunately not.

Hi guys,

Thought I would sign up to this board as I've ended up here several times when searching Google. :)

I have added a heap of additional Garmin devices to my guide so it now covers all of the models from the common ranges. Hopefully it is still relatively easy for people to digest!

I don't have any hands-on experience of the eTrex H range (high sensitivity models) so I have had to rely on the manuals for that information. The same is true for the Foretrex 301 and 401 so any further information could be helpful. I've also listed some modern Garmin devices which should be avoided and why.

Looking forward to the day when I can play with a GT-32 or whatever Locosys choose to call it. ;)

Mike

Mike,

That's a top guide. If you don't mind, I'll linkto it from ka72.com at some point. It will be very useful.

Just some minor corrections.

GT-11, I think, should be considered "Discontinued." They are not that easy to come by.

Also, the reference to "Move Long" on page 18, you say that you should answer "Yes" if the device has move spatially (more than 100kms) or has been switched off for a long time (several days.) I don't believe it has anything to do with time. Happy to be corrected on that point, but I was pretty sure it just referred to the distance.

Your assertion (also on page 18) that SBN files can be manually joined together using the copy /b command from the DOS prompt is interesting. I'd like to know if anyone has tried to do that and then upload a file to ka72.com? There is a header in the file that might interfere with it working as expected, but I don't know. That is a really great tip if it works.

Cheers,

Dylan.

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Dylan72 said...
Also, the reference to "Move Long" on page 18, you say that you should answer "Yes" if the device has move spatially (more than 100kms) or has been switched off for a long time (several days.) I don't believe it has anything to do with time. Happy to be corrected on that point, but I was pretty sure it just referred to the distance.

Then I'll correct you.

GPS's receive some telemetry from the satellites (in addition to timestamps), that being ephemeris data - aka satellite positions. Since atmosphere and other factors cause differing drag effect on the satellites, the USA calculates and uploads the expected trajectory for a few weeks, which the satellites then downlink to the GPS's. This is why the first time a GPS is used, it needs to sit outside for 30mins or so... ie: to allow time for the GPS to scan the sky so that it can get the ephemeris. So... if your GPS isn't used for a while, then the ephemeris will be stale.

For a similar reason, the GPS will use the ephemeris to calculate the expected azimuth/elevation of the satellites -> when you move too far away from last use, the satellites wont be where the GPS expects them to be.

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Your assertion (also on page 18) that SBN files can be manually joined together using the copy /b command from the DOS prompt is interesting. I'd like to know if anyone has tried to do that and then upload a file to ka72.com? There is a header in the file that might interfere with it working as expected, but I don't know. That is a really great tip if it works.

There was a technical discussion some time ago (when the file format was first proposed) that it should be possible to join files together... but I am unsure whether any of the programs have incorporated it as a feature.

Mathew,

Thanks for the note. I've scanned through all my links and other info about the GT-11 and can't see any reference to the "Travel Long" having anything to do with time.

The GT-11 manual states:
"When the unit has not been used for a long time, or it's taken to a far distance from where you initialized it, you will need to re-initialize the unit before use."

which stops short of saying it explicitly. I always just assumed that the unit fell back to a full re-init if it was switched on after a long time. After all, it has an internal clock, so it should know whether its data is out of date or not without having to ask.

I honestly don't know one way or the other, and it was only a minor point, regarding an obsolete GPS anyway.

Hi Dylan,

Thanks for the feedback. Feel free to add a link on ka72.com.

I've got a few minor tweaks to make so I will take your comments on board and address them in the next issue. Quickly though...

Discontinued: Agreed. Just an oversight.

Move long: I don't think that it is stated explicitly but I am pretty sure that "move long" is related to stale ephemeris or azimuth data and answering "yes" forces a cold start. As you say, it shouldn't really have to ask and it's the only GPS that feels the need to ask the user!

Joining SBN: As you know, the SBN format is made up of packets that have start / end sequences, packet length and payload length. Although joining files results in the header (username, serial, firmware, etc) being replicated in the middle it should simply be ignored by the software (unless the programmer was a bit lazy, lol). GpsarPro, GPSResults and RealSpeed all load joined SBN files without problem but I haven't tried any other software. It's a useful trick on the odd occasions where it is needed.

Feel free to point out any other sections that need clarifying in the next issue.

I've made a few minor refinements to the guide (issue 2.2) and uploaded to the original URL.

As always, please let me know if anything needs further work.