Primary and backup GPS

sneuman · #1 (**permalink**) 12-11-2004

I am currently fitting out my 28ft sloop for a Hong Kong to Vietnam (then on to Thailand) trip. The first leg will take about 8 days across the South China Sea - south of Hainan island on to Nha Trang in Vietnam south. I have a hand-held Magellan GPS but want a not-too-expensive primary. I''ve been advised by a delivery skipper to get a fixed-mount set, but no good explanation as to why. I''m trying to keep as many systems off the boat''s main 12v - although I have alternator and solar recharge capability, I just feel better carrying a stash of AA cells. Also, is it adviseable to get two different brands (say a Magellan and a Garmin) so that any weakness in one would be covered by the other? Any thoughts?

WHOOSH · #2 (**permalink**) 12-11-2004

S:

I think we heard from you a while back, at the planning phase for your trip down to Thailand via Vietnam. Congratulations on getting close to departure! I surely do hope you post some observations on this run, somewhere accessible by the web, and us to them here at a later date.

The basic GPS needs aboard a cruising boat are so generic-vanilla in nature that it''s easy to say it doesn''t matter what you buy...but were I you, I''d have some specific preferences.

First, I think there''s two reasons why you might be hearing recommendations for fixed-mount units: 1) they have larger screens and so are more easily used, whether flight planning at the nav station or when glancing down to the display from the cockpit; larger also equates to bigger buttons, sometimes a help in an active seaway or when you''re tired; 2) fixed-mount units often have an external antenna (since the assumption is they live belowdecks) and this can be a bit more effective with low SNR sats; this may be due to a better (more sensitive) antenna, that it has a built-in pre-amp and/or due to the portable GPS antenna''s connection, which can become a bit corroded over time. So...my vote would be for ''fixed'' for all these reasons.

Second, I''d play a bit with the Magellan and determine how I feel about the user interface: is it simple (for the basic tasks I know I''ll need) and - for me - fairly intuitive? If the answer is yes, then my first preference would be to shop for a fixed mount Magellan unit that has a similar user interface. Common user interfaces make life easier when you''re half seasick and/or tired. (And don''t forget, you may end up putting the portable at work in the cockpit; how nice to have the same user interface both above decks and below). If you''re handheld unit is old and the new Magellan user interfaces are simply ''different'', I''d fall back into a general shopping mode, comparing interfaces on the brand choices available to me along with price, form factor, etc.

(BTW I found Magellan to be a much harder product to become acquainted with than Garmin; just a thing about their user interface vs. my brain. I have a fixed unit plus two portables of 3 different generations, but all the user interfaces are all but identical and I appreciate this a lot).

Re: your reservations about driving a GPS off your house bank, these units draw barely more than the natural voltage loss inherent in wet cell batteries, so they aren''t going to make any meaningful difference in the longevity of the house bank before recharging it. If you''re worried about having an independent power source for the fixed mount GPS, I think you have two ''fallbacks''. First, your GPS will work off an almost dead house bank, all the way down to 10V, I believe...so ''dead'' isn''t necessarily dead. Second, you can buy a small/cheap alkaline-type battery holder at an electronics shop that holds the correct # of portable batteries to provide 12V power, and keep that in reserve as a ''last case'' before falling back onto your portable unit. Radio Shack in the States carries these things for a few dollars and simple wire nuts would temporarily attach the leads from your fixed mount unit to the battery holder. Thus, you''d end up with 3 power alternatives and 2 units for redundancy. You''re set!

Jack

sneuman · #3 (**permalink**) 12-11-2004

Whoosh. Thanks for your excellent advice and encouragement. I''ve been looking forward to this trip for a long time, but the preparations (and money!) have been difficult, especially as I am having to fly back and forth between Bangkok and Hong Kong to do the work.

Your points about interfaces, etc. are well taken. I agree with you about Magellan''s - I find it difficult, but have nothing to compare it with, as I''ve never used a Garmin.

camaraderie · #4 (**permalink**) 12-12-2004

I''ll ditto woosh''s notes on the Garmin being easier to use. We''ve been out for 3 years now using a Garmin portable (GPS76) as our primary GPS with another one in a waterproof box down below (with lots of extra AA batteries). We have the Garmin "holder" and the power data cable wired into the 12V system and feeding our NEMA data needs and have had no problems. I am uncomfortable with units that need external antennas and cannot be run without 12V battery power. Lost ALL of our antennas and electronics in a recent escapade, but the GPS76 was still just fine and I''ve lost battery power due to an alternator failure before and the battery power option got us to port safely. I see no reason that you need a fixed mount unless you simply want the large display but I''d still have a handheld as a backup at the very least.Have fun!

starcresttoo · #5 (**permalink**) 12-12-2004

what are you going to do when that theoretical time comes that all the satellites are turned off?

hamiam · #6 (**permalink**) 12-13-2004

I have had several Garmin units; the 12, 48, and, currently the MAP76. All handheld and all excellent in design and function. All waterproof to the standard but you may want to place the backup in a waterproof bag all the same.

starcresttoo · #7 (**permalink**) 12-14-2004

there are already devices being sold by certain governments made specifically to disrupt gps guided smart bombs.these may have an affect on all other gps units too.

sneuman · #8 (**permalink**) 12-15-2004

if they''re dropping smart bombs anywhere near me, i''ve probably got bigger problems than losing my GPS!

flicker · #9 (**permalink**) 12-15-2004

Some time ago, as I poorly recall, another contributer to this site said that his GPS was off by a mile when trying to reenter NY harbor. He had to navigate by visual alone, but fortunately was familiar with the area. Later, he heard POTUS was in town and believed that the Secret Service screwed up the GPS service locally to protect him. If this is true, it won''t take an act of war to turn off GPS, just a bureaucratic decision.

Chas.

ct · #10 (**permalink**) 12-15-2004

We have a GPSmap 182C as our primary unit with a Garmin 120 backing the 182C up. I concur that Garmin units are very user friendly, simple to use and so far very dependable.

As far as the GPS being a mile off in NY, I find that a little hard to swallow. The risk to all commercial air traffic, civilian aviation as well as the maritime interests would surely result in at least prominate nation-wide if not worldwide news coverage. With all the dependence on accurate GPS data to prevent catastrophies, it would have been the scandal of the century.

Nope...can''t buy it.