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sail0r
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Joined: Jan 13, 2008
Posts: 2
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 Posted: Sun Jan 13, 2008 3:45 am Post subject: hand held gps that operates at low speeds |
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| I need a waterproof handheld gps that can provide course and speed at less than 1 kt, down to .2kt. I bought a Lowrance h20c, and found out that it shuts itself off at 1 kt. Now it is a paperweight. They say that only DGPS will give accurate info at those speeds. Any ideas? |
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philpugh
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Joined: Dec 28, 2005
Posts: 2003
Location: Antrobus, Cheshire
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| Posted: Mon Jan 14, 2008 3:48 pm Post subject: Re: hand held gps that operates at low speeds |
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| sail0r wrote: | | I need a waterproof handheld gps that can provide course and speed at less than 1 kt, down to .2kt. I bought a Lowrance h20c, and found out that it shuts itself off at 1 kt. Now it is a paperweight. They say that only DGPS will give accurate info at those speeds. Any ideas? |
Not sure about the lowest speed displayed but have a look at the GARMIN range - there are some specific marine handhelds there - but I think that this only means that the base map is different. Problem is that at very low speeds then the positional inaccuracies become significant. Atmospheic conditions / satellite geometery etc can cause succsessive readings to be several metres different even though you aren't moving - so the reply you had before - use DGPS for better location precision is a good reply. Perhaps your current system manufaccturer chooses not to display info below 1kt as they know it is unreliable. Check the system set up - some devices incorporate an anti-wandering feature that will behave in a similar manner that you describe - this is designed for higher speed applications (eg in car) to remove the apparent wandering caused by anomolies when stationary.
From your name I am assuming this is a nautical application of GPS? At such low speeds what are you trying to achieve? If you have your GPS logging information then you will get a record allowing calculation of the average speed over the log interval. I wouldn't be relying on the GPS instantaeneous speed for dead-reckoning. Just use the position information for the ship's log entries and verify with any land fixes you can make. The unit should still show you course/bearing to next waypoint - even if "stationary" - it's just that if you are virtually static then the anomolies previously mentioned will cause the direction to next waypoint to keep moving alarmingly.
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Phil |
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sail0r
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Joined: Jan 13, 2008
Posts: 2
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| Posted: Tue Jan 15, 2008 2:47 pm Post subject: |
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You are correct, this is nautical. I am trying to find something that can be used on the final approach while docking a large tanker at an offshore platform, when you have left the ships GPS in the wheelhouse, and are now standing on the bridgewing. Especially at night, there is very little visual reference for speed and direction, since the actual movement of the vessel is no longer necessarily the ships heading, since tugs pushing, current, wind, etc are now much bigger factors with the ship is moving between 1 and 2 kts. I guess I will just have to go to a store, and take the different models outside for a slow walk. Thanks for the reply,
Ted |
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aj2052
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Joined: Jul 03, 2005
Posts: 1431
Location: Leics,UK
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| Posted: Wed Jan 16, 2008 2:51 am Post subject: |
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| I remember standing in the bridge of a P&O ferry in dry dock looking at their GPS Display on a big screen and it showed our position about a mile or so inland, if this is the standard when you got hundreds of passengers onboard then god help you,Admitedly it was about 8 or 9 yrs ago tho. |
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