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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #1  
Old 09-17-2001
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GPS Anchor Alarms

When I bought my sailboat a few years ago, the previous owner left the old Garmin 75 GPS for me. I''ve used it ever since, reluctant to upgrade because of all the waypoints and routes that I''ve loaded in. Maybe next year...

One of the features I turned off and forgot about was the anchor drag alarm. Back when GPS positions were intentionally distorted with "selective availability," turning on your anchor alarm almost guaranteed that you would be awakened in the middle of the night for no good reason.

Now, since SA was turned off by the Clinton administration, anchor alarms work, even with my old, obsolete unit. In my boat, I have a GPS mount at the Nav table, so I can hear the alarm easily from my berth.

If your unit, like mine, requires you to set the alarm distance in decimal fractions of miles, remember that a nautical mile is 6000 feet, .1 nautical miles is 600 feet, and .01 nautical miles is 60 feet.

I like to set the alarm for .02 NM, or 120 feet. That''s usually about how much rode I have out, and it will go off if the boat swings near 90 degrees. I definitely want to get out of bed for any 90-degree swing.
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Old 09-24-2001
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GPS Anchor Alarms

Noticed you have a Garmin GPS 75. The boat I am looking at to buy has the same unit. Can you tell me what your thoughts are on the unit? Can you compare it to newer models? Should I purchases a new (WAAS)GPS day one? Overall opinion would be helpful. Thanks much!

Andy
andy@wepi.com
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Old 09-25-2001
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GPS Anchor Alarms

I think that the extra accuracy claimed by the newer units is not that significant to boaters. If selective availability (induced position inaccuracy) was still in effect, a WAAS or differential GPS would have tremendous advantages. Now, I suspect it''s mostly marketing. They didn''t know that SA was going to be turned off when they developed these things.

That said, get a new GPS, one with a charplotting capability. Almost all new GPSs have some sort of charting built in. Having your position imposed on a chart, no matter how small, is a powerful thing.

Also, the expense of any GPS is dwarfed by your investment in learning to use it, and the time you spend entering your own routes and waypoints. That means that whatever you buy, you''ll someday be reluctant -- like me -- to give it up.

So, get the latest and greatest, but not necessarily the most expensive. Learn to use it really well.
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Old 09-25-2001
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GPS Anchor Alarms

EA,

There is a fair amount of accuracy/repeatability difference between GPS with selective availability off and WAAS. Have a look at this URL an I think you will agree.

http://www.raymarine.com/recreational/latest/index.html

The graph was origianlly built as part of the (former parent company Raytheon) work with the FAA on WAAS.

The Garmin line of GPS receivers is/has gone exclusively to WAAS.

Wayne
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Old 09-25-2001
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GPS Anchor Alarms

The accuracy figures that most people use (including Garmin) are:

1. GPS with SA: 100 meters
2. GPS w/o SA: 15 meters
3. Differential GPS: 3-5 meters
4. WAAS: 2-3 meters

While 12 a meter error (a boatlength) could certainly make a difference navigating a narrow channel in very poor visibility, it is unlikely that the graphic display on a WAAS GPS would even render that level of detail.

So, for me, it''s not really an accuracy issue. I''ll buy a new GPS unit for the chartplotter and for the built-in navaid database.
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Old 09-26-2001
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GPS Anchor Alarms

1) The resolution of the display has nothing to do with the accuracy or repeatability of the geolocation calculations. The delta in successive calculations alarms the anchor warning not the pixel location on a chartplotter.

2) the quoted accuracys are CEPs. The boat''s position could be calculated in the LR portion of the CEP at time 1 then at time 2 (possibly minutes later) calculated in the UL portion of the CEP. Both calculations within the 95% probability - but disbursed by as much as the diameter of the CEP not the radius from "ground truth". In this case 15 meters is now nearly 30 - enough to keep you awake at night listening to anchor warning alarms from the unit regardless of manufacture... unless of course you have a very big boat.
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Old 09-26-2001
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GPS Anchor Alarms

I agree that the GPS display resolution has nothing to do with the accuracy of an anchor drag alarm.

I thought we had digressed to discussing the value of WAAS accuracy for everyday navigation tasks. For general pilotage, I maintain that a marginal improvement in accuracy isn''t useful unless the granularity of the display will represent it.

In practice, my obsolete non-WAAS, non-DGPS anchor alarm doesn''t fire at night when I set it to .02 nautical miles, (120 feet).

It does go off every time we drive the boat away from the drop point after we pull up the hook--I always forget to turn it off--and it goes off when the boat swings, so I know it works.

If the alarm did go off spuriously, I''d just set it to 180 feet.
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Old 10-01-2001
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GPS Anchor Alarms

If you decide to upgrade to a newer Garmin, you can probably transfer your waypoint and route data to the new unit using the Garmin to Garmin transfer feature.
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