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Author Topic:   Choosing a backup GPS/chartplotter
Frank O posted 10-22-2012 08:41 PM ET (US)   Profile for Frank O   Send Email to Frank O  
On my 1995 Outrage 21 I have a six-year-old Lowrance LCX-25C which serves as the head unit for GPS/chartplotter, sonar and radar functions. Unfortunately the extended extreme heat toward the end of summer here in Southern California seems to have taken a toll. The sonar seems to work okay, but the GPS has recently become what I'd call "glitchy" -- it doesn't load waypoints correctly, loses tracks, etc. I've spent quite a bit of time on the phone with Lowrance tech support and have tried system resets, firmware updates, etc. My gut feeling is that there's a hardware issue. The LCX-25C model is discontinued, so unless I want to buy a used unit on eBay (not particularly -- seems like a gamble), I'll need to replace it eventually. The current Lowrance models don't work with my radar unit, so I'm basically looking at having to replace the whole system.

Since the sonar seems to work okay for now, I'd like to buy a backup GPS/chartplotter to use while I see if I can get another year or so of use out of it before spending the big bucks. The most important things to me in the GPS are:

-- Must be able to transfer waypoints to and from my PC easily (via memory card, or possibly USB cable).

-- I'd like the unit to have a screen at least somewhat larger than a handheld (I have a Garmin eTrex Vista handheld, but it would not be convenient to use while navigating to, say, a shipwreck).

-- I don't want to spend a bundle (I'm setting ~$500 as my cutoff), and don't want to cut into the boat's console.

-- All else being equal, I lean toward the Garmin product line. I've used both Garmin and Lowrance units with PC-based software, and find the Garmins make for an easier workflow. In addition, it sounds from the Garmin techs as though I stand a pretty good chance of being able to download custom-built vectorized bathymetric maps that I create in Global Mapper to Garmins (I've done it with my eTrex Vista), whereas I've never found a way to do this with a Lowrance.

Given all of the above, I'm leaning toward a Garmin 521 or 421 at the moment. I gave brief consideration to a Garmin Nuvi 500 or 550, which supposedly have a marine mode, but these seem to basically be street map GPS units designed for cars, and there doesn't appear to be any way to transfer waypoints. With the 521 or 421, I'd be looking at fabbing up a base to attach the unit's yoke mount to, which in turn would sit on suction cups on the top of the console (I want to keep it removable).

Before I press the button on the above, are there any other solutions that anyone thinks I might consider, given my criteria?

20dauntless posted 10-22-2012 09:46 PM ET (US)     Profile for 20dauntless  Send Email to 20dauntless     
Do you have an iPad? If you already have one (preferably the 3G/4G model so you have a GPS chip built need for a data plan), pick up a waterproof case, RAM mount, 12V charger, and a map app (Navionics, iNavX, etc), and you'll have a great backup GPS with a big screen and wonderful user interface.
Frank O posted 10-22-2012 09:56 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Thanks for mentioning that. It's an idea I've considered, since I do have an iPad 2, but I see a couple of drawbacks. One is that the iPad screen can be really hard to read in sunlight. The other is that I've run a few tests with the iPad, and the accuracy of its GPS doesn't come out as well as what's delivered by the better dedicated units. If I were headed to a general waypoint it might not make a difference, but I put a lot of stock into being able to hit a specific spot on, say, a shipwreck when we drop the anchor.

jimh posted 10-23-2012 01:09 PM ET (US)     Profile for jimh  Send Email to jimh     
My understanding of the Apple iPad2 is that models that are WiFi-only have no GPS receiver in them. They deduce location using other methods. Models with a GPS receiver have a wireless cellular 3G modem. This article has some information on the iPad2 and GPS receiver capability:

I believe you can buy a GPS receiver that will work with an Apple iPad2 that does not have a GPS receiver internally. You could use a Bluetooth GPS receiver with an iPad, but there appears to be some qualifications necessary for it to work. See

Once you have a real GPS receiver connected to you iPad, there is no reason to think the position solution provided by that GPS receiver would be inferior to the position solution calculated by a specialized marine chart plotter with a GPS receiver inside. If you get any sort of decent GPS receiver using current technology, your position solution should be of comparable accuracy. All of these GPS receivers use the civilian L1 carrier. If they can receive four satellites, and if the satellite position geometry is good, you will get a position solution that should be as accurate as possible with GPS. If you have a WAAS precision-fix augmentation receiver, you will get a more accurate fix, but you can probably find an external GPS that will work with your iPad that also has WAAS capability.

The technical specification for the recommended Bluetooth GPS receiver, the DUAL UNIVERSAL GPS, are quite impressive:

The advantage of this device is you could use it in many other situations with your iPad2, and not be limited to just using it on the boat.

jharrell posted 10-23-2012 04:29 PM ET (US)     Profile for jharrell    
Keep in mind the screen is not the only reason a iPhone or iPad not work on your average non-pilot house boat. Overheating is also an issue, put these devices in a waterproof case mounted to the top of your console in full sun and you will receive an overheat warning in short order, similar to this one: png

I big part of what you pay for in your average chart plotter is the sunlight viewable screen and the physical design to withstand the open marine environment, a big part of that being temperature extremes.

David Pendleton posted 10-23-2012 06:47 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I have the Dual GPS and it performs well. Here is a screenshot from my ipad of their GPS Status app. screenshot.png

David Pendleton posted 10-23-2012 06:50 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I should also note that the GPS was in indoors on my living room coffee table in front of a window. The ipad (and I) were in another room 20' away. Note the diminished battery charge also. I'm very impressed with this thing and I will absolutely use in on my boat next season. I already use it in the truck.
Frank O posted 10-23-2012 09:19 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Thanks for all the information, very enlightening. I have to say that with the precision of the Dual GPS, the iPad is tempting.

I took my iPad 2 (3G + Wifi) outdoors today and looked at a mapping app in full sunlight with the screen brightness turned up all the way. With sunglasses on, it was acceptable, though not great. Since my sunglasses are polarized, when the iPad is held in portrait orientation the screen appears totally dark. This is not a problem, though, in landscape orientation. I can see that the combination of screen brightness and GPS use would cause the iPad to burn through its battery really quickly, so I'm thinking that on the boat I'd need to keep the iPad plugged in to a charger running off the boat batteries.

Although my boat is a center console, it has a T-top, so I'm thinking temperature may not be that much of a factor. It would be in full sun primarily very early in the morning. In six years of boating at all hours in a variety of conditions, the top area of my console has never gotten wet, so I'm also thinking I might be able to get away without a waterproof case.

One question I'm still pondering is what kind of mount I'd put an iPad on. I was wondering if I could get away with a ball-and-socket mount on a suction cup -- these are designed for car windshields, and I'd hope it would grip to the gelcoat on my console. Then again, we do experience an occasional hard bounce when taking waves, and a mount like the following would involve only minimal drilling:

Probably my most serious reservation at this point is that I've yet to see an iPad GPS map app that works for me on all fronts. I have MotionX GPS HD on my iPad which is pretty fair, with an included marine chart, but it doesn't zoom in close enough for my purposes anchoring on a waypoint. I'd also like to be able to import vector maps I create from depth contour data in Global Mapper, but the MotionX support team tells me it does not accept any form of custom map. The app iNavX looks interesting, but it also seems as though it would get expensive -- $50 for the app itself, a similar amount for the Navionics chart, and then, if I understand it correctly, $120 a year subscription fee to be able to continue using the Navionics chart. So I'm still looking for an app that works in all ways.

One other issue is that my partially failing Lowrance chart plotter is no longer sending NMEA 0183 position data to my radio and the OptiMax engine's SmartCraft system. This is not that big of a deal on the radio, but I was really liking seeing the SmartCraft data based on boat speed calculated from GPS. I looked at a multiplexer like the iMux ([ur][/url]), but this appears at first glance to be designed to work in the other direction -- pulling data via NMEA 0183 from various systems that is displayed on the iPad.

So -- it's tempting, but I'm still thinking it through.

David Pendleton posted 10-23-2012 09:50 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
One of the first apps I installed to my (wife's) ipad was EarthNC. I'm going to look at the other chart applications at some point, but EarthNC is a bargain and looks great.

You can also use their online viewer to see what the charts look like.

Frank O posted 10-23-2012 10:11 PM ET (US)     Profile for Frank O  Send Email to Frank O     
David, EarthNC's Marine Charts looks pretty nice, but before I spend $20 on another nav app, can you tell me:

-- If you zoom in all the way, what kind of area does the screen cover?

-- Can you transfer waypoints and tracks easily between the iPad app and a PC?

David Pendleton posted 10-23-2012 10:23 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
-- If you zoom in all the way, what kind of area does the screen cover?

I don't understand the question.

-- Can you transfer waypoints and tracks easily between the iPad app and a PC?

EarthNC does support GPX files. This is described here:

Frank O posted 10-23-2012 10:36 PM ET (US)     Profile for Frank O  Send Email to Frank O     
What I mean by my first question is: Is there a scale marked on the charts to indicate the size of the area covered by the screen if you zoom in all the way?

David Pendleton posted 10-23-2012 10:48 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Yes, there is a scale in the app (but not the web viewer) that constantly adjusts as you zoom in and out. It appears at the bottom left of the display.
Frank O posted 10-23-2012 11:05 PM ET (US)     Profile for Frank O  Send Email to Frank O     
And what does that scale show as the size covered by the screen when it is zoomed-in completely?

By way of comparison, my MotionX GPS HD app has controls on each side of the screen, so the map area is roughly square.

The scale indicates that 7/8" on the screen is 0.04 mile, or about 211 feet.

The map area on the screen is about 8.5" square. Thus, when zoomed in all the way, the map covers about 2,050 by 2,050 feet, or approximately 0.4 x 0.4 mile.

For my navigation purposes, I need an app that allows me to zoom in much closer than this.

Frank O posted 10-23-2012 11:13 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Sorry, I mis-measured. The map area covers about 5.6" square on the screen. So the area represented is about 1,355 by 1,355 feet, or about a quarter mile by a quarter mile. Still, I need to be able to zoom in much closer.
David Pendleton posted 10-23-2012 11:14 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I couldn't find any good videos on YT for EarthNC. Give me a bit and I'll put one together for you to demonstrate what it looks like.
Frank O posted 10-23-2012 11:21 PM ET (US)     Profile for Frank O  Send Email to Frank O     
I can see from the web-based viewer generally what the maps look like, and they're nice. But the specific area covered on the screen when it is zoomed-in fully is a really important point to me. If you don't think you can determine what it is, I can probably ask their tech support.

Chuck Tribolet posted 10-24-2012 11:55 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
BTW,there's at least one plugin GPS for the iPad. It comes
from a company called Bad Elf. I have one. However, it is
almost as expensive as the difference between the WiFi-only
iPad and the one that has cell service (and therefore GPS).

Without a GPS, the WiFi-only deduces its position by the
WiFi it finds. It's not GPS accuracy, but generally gets you
in the right block.


ericflys posted 10-24-2012 02:08 PM ET (US)     Profile for ericflys  Send Email to ericflys     
I use both EarthNC and Navionics, both of which don't have subscription fees. Of the two I prefer the Navionics app.

If you purchased either the IPad with Retina display or the new IPad Mini, you shouldn't need an external antenna for a very precise and quick position fix. This is because these unit along with the IPhone 4s and IPhone 5, have a chip set the picks up GPS and Glonass. Glonass is the Russian GPS equivalent. So essentially you only need to see a much smaller portion of the sky to get a position fix.

I am holding off upgrading my marine gps until later next year, when I believe new units will be released from most of the major manufactures that can receive GPS, Glonass, and Galileo. This should result in much faster initial position fixes, much faster refresh rates, and would still perform with a partially obstructed view of the sky.

ericflys posted 10-24-2012 02:10 PM ET (US)     Profile for ericflys  Send Email to ericflys     

for some info on the IPad GPS antennas.

Frank O posted 10-24-2012 08:51 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Thanks for the information.

I did see the Bad Elf GPS receiver on Amazon. Apparently it plugs into the iPad's 30-pin port, but it has a power pass-through so you can simultaneously keep the iPad charged. Still, the Bluetooth capability of the Dual XGPS150A sounded interesting, so I ended up ordering one of those.

I got some additional data on the scale provided by various GPS apps at maximum zoom-in. EarthNC got back to me by email; they said that, when zoomed-in fully on their chart, one inch on the iPad screen represents about 100 feet.

In the MotionX app that I have, I discovered that the scale depends on the map selected. MotionX allows the user to choose between various topo, street and satellite maps, as well as a NOAA marine chart. When using the marine chart, at maximum zoom-in one inch on the iPad screen represents about 240 feet. Not good! However, when using Google imagery, one inch on the screen represents 60 feet. This would check the box for my precision-anchoring exercises. In MotionX it's easy to toggle between the selected maps, so I could run using the marine chart most of the time, and switch to the Google map when trying to nail a spot. The only wrinkle is that Google doesn't allow its map data to be downloaded and stored on the local unit. Perhaps this wouldn't be an issue, as much of my boating is within cell phone range of shore. Then again, if I can switch to the Google map for the greater zoom factor, it doesn't really matter to me whether or not I see map data -- the important thing is watching the distance and position relative to the waypoint I'm trying to hit.

I also sent an email to Navionics to ask them about the scale of their maps at maximum zoom, but haven't heard back from them.

Interesting about the newer Apple models and Glonass. My smartphone is an iPhone 4S, and I have to say I'm not impressed by the accuracy of the location services at least while in the Google Maps app (I'm on iOS 5.1.1). Today when I started the app, the blue dot for my position jumped around a bit, then settled down at a point about 30 feet from my true position (as indicated by aerial imagery).

The iPad Mini looks like it could be a nice size of display to use on a center console. I might experiment with my iPad 2 for the next few weeks, and think about an iPad Mini if I want to stay with an Apple device. I'll be interested to see what the marine GPS manufacturers come out with next year.

Mr T posted 10-24-2012 10:51 PM ET (US)     Profile for Mr T  Send Email to Mr T     
For what it is worth I'm not sure if you have an internal GPA antenna or the external one, but if you have an external antenna, there have a number of reported issues with GPS fix due to low voltage at the antenna. I'd suggest that if you do have an external antenna, to check the connections and voltages going to it, as it can cause a lot of the issues you have related here.
jimh posted 10-25-2012 09:26 AM ET (US)     Profile for jimh  Send Email to jimh     
There is a general notion that an electronic device will be subject to malfunction if it is operated with a supply voltage that is out of tolerance with its specifications, so you can say that about a GPS receiver, too. If you don't supply it with the proper power, it may not work properly.

There is a general expectation that the position solution obtained by a GPS receiver should improve in accuracy as the number of satellites in view increases, and by combining the GPS and GLONASS systems, a modern GPS receiver should have more satellites in view than one that only sees the GPS constellation. Whether or not this improvement is obtained in practice in the implementation of the receiver in the newer Apple iOS devices is something I cannot affirm or deny. A receiver with both GLONASS and GPS ought to be able to see more satellites and ought to be able to deduce a more accurate position solution.

Once the error of the position solution is in the range of 30-feet, I think you need to begin to worry about the accuracy of the position being used as the standard. You'd need to go to some monument or marked location whose position was precisely determined in order to know if the error being seen were due to the GPS receiver incorrectly deducing the position or if the error were in the map cartography. A variance of 30-feet is not much of an error.

ericflys posted 10-25-2012 11:52 AM ET (US)     Profile for ericflys  Send Email to ericflys     
All major manufacturers agree on the benefits of the new antennas that can receive four different types of navigation satellites. Currently Garmin offers this option for their marine GPS:

Some other manufactures already have this capability in their recievers.

I that every new receiver released from here on out would have the new antennas and software. I will be upgrading my GPS this year, but am waiting to see new units from Standard Horizon and Garmin with this capability. They are rumored to be released in the May-June timeframe.

My point is, if you are in the market for a marine GPS, and can hold out a bit, it's worth it in my opinion. Electronics are always going to get better with time, and if you always wait for the next greatest thing, you will never buy one, but I think this upgrade is worth waiting for.

jimh posted 10-25-2012 09:40 PM ET (US)     Profile for jimh  Send Email to jimh     
Eric--What are the four navigational satellites you make reference to?

I am aware of the GPS and GLONASS constellations. What are the other two?

ericflys posted 10-25-2012 10:38 PM ET (US)     Profile for ericflys  Send Email to ericflys     
See the Garmin link in my above thread. One is Galileo(European Union) and the other is QZSS. I believe there is a fifth system coming on line in the future as well.
jimh posted 10-26-2012 12:30 PM ET (US)     Profile for jimh  Send Email to jimh     
Eric, thank you for explaining what you were referring to when you said there were "four different types of navigation satellites" that all manufacturers agreed upon as being beneficial. I disagree with your characterization.

I don't think there are other operational systems besides GPS and GLONASS at this time.

Wikipedia says GALILEO won't be fully operational until perhaps 2019 at the earliest.


The Quasi-Zenith Satellite System or QZSS system is not a global system. It only works in the region of Japan.


I don't see that being able to use GALILEO or QZSS would be a benefit at present to any GNSS users in North America. At this time for users in North America a GNSS receiver that gets GPS and GLONASS is about as good as it gets from what I can tell. Of course, I am including WAAS in GPS.

The only other very useful addition would be a DGPS receiver to get the United States Coast Guard differential GPS correction signal. There are very few devices available now at a low cost that can get DGPS. Most GNSS receivers offer only WAAS. DGPS would be helpful for people in regions were the service is provided, which would be mainly coastal areas and the Great Lakes.

ericflys posted 10-26-2012 01:20 PM ET (US)     Profile for ericflys  Send Email to ericflys     
Systems like Galileo don't have to be fully operational for them to provide some benefit to a receivers navigation solution.

When shopping for a new GPS, I would prefer to have one that can take advantage of current and future positioning systems. I think by the end of 2013, this will be the standard and incorporated into most position receivers.

As previously mentioned, the benefits are faster initial position fixes, much faster refresh rates, and the ability to operate with a much more obscured view of the sky. There is no downside other than the limited current availability of devices.

jimh posted 10-26-2012 01:47 PM ET (US)     Profile for jimh  Send Email to jimh     
I think GALILEO when fully deployed will have 32 satellites. I think they currently have two in operation. Generally we would say that in a 32-satellite system there should be at least four in view at any time. Scaling this down to two satellites, I think we would find that a satellite would be in view only one-quarter of the time. So at present having a GNSS receiver that gets GALILEO would provide you with a chance to get one more satellite in view about one-quarter of the time. This is in contrast to GLONASS, which I think has four satellites in view all the time. So in making a comparison we could say that GLONASS is 16-times more likely to improve the accuracy of your position fix than GALILEO can at the present.

Whether or not one ought to invest in hardware for systems that are not operational at the moment or are in the very early phases of their operational status is a reasonable question. I bought a GPS and chart plotter instrument that was one of the first to support WAAS. Several years later there was a change in the WAAS configuration and that caused by device to become incompatible. Now when I bought the device WAAS was fully deployed and working, but because I bought at such an early phase, I ended up missing out on long term compatibility. I use this experience as a guide to buying GNSS receivers. I don't think there is a guarantee that you get a GNSS receiver today that is going to work with GALILEO as it ends up being operated seven years into the future.

Heck, it we were to look seven years into the future we might be buying devices that would work with future improvements planned and underway now for GPS. The USA plans to add new features to GPS and in seven years they might be operational. Would I buy a receiver now that will work with them seven years in the future? No, I will wait. That's my basis for suggesting that buying GALILEO or QZSS support in a present day GNSS receiver is not mandatory.

I have been surprised and have mentioned before that the current offerings in marine GNSS receivers seem to be way behind the times, as none of them seem to be particularly capable--and especially for the prices charged.

I do want to thank you, Eric, for indirectly pointing me to the Garmin GPS-19 series of GNSS receivers. They look like the best-in-class of the current marine-grade GNSS receivers, and their cost (around $220) is not out of line with other lesser devices (such as the Lowrance LGC-4000). The specification for the Garmin GPS-19 look much better than competitors' devices.

ericflys posted 10-26-2012 05:21 PM ET (US)     Profile for ericflys  Send Email to ericflys     
The good news is the devices that support GPS and Glonass in marine navigation world, also support those other technologies that, as pointed out, are of little use to us right now. So we don't really have to factor them in, the real decision for someone in the market for a unit currently is, do I buy a GPS only device(I'm talking about devices with internal antennas), or wait a few months are get one that will recieve a wider range of navigation signals.
jimh posted 10-26-2012 05:45 PM ET (US)     Profile for jimh  Send Email to jimh     
For me the notion of waiting several months to buy a new GNSS is simple; I won't be boating for at least six months, and it is easy to wait. Modern electronics have age cycles like dog years. Every six months to a year there is almost an entirely new product cycle introduced.
Frank O posted 10-26-2012 07:55 PM ET (US)     Profile for Frank O  Send Email to Frank O     
I heard back from Navionics. At maximum zoom-in, the resolution of their Marine&Lakes: USA HD app for the iPad is a bit over 300 feet per inch on the screen. So it offers the least zoom range of the apps I've looked at so far.

In the spirit of full disclosure, I should admit that, when anchoring on a particular obscured target such as a shipwreck, I use GPS to get as close as possible, then focus on sonar to find its signature. Still, being able to zoom in as close as possible with GPS is a good thing.

In other news, I just took delivery on a Dual XGPS150A GPS receiver module for the iPad. I thought I'd take it, the iPad 2, an iPhone 4S and a Garmin handheld to a geodetic monument near my office on my lunch hour to run a few tests.

jimh posted 10-26-2012 11:31 PM ET (US)     Profile for jimh  Send Email to jimh     
Frank--I look forward to the results of your test at the geodetic monument with the various GPS receivers. Please let us know the outcome.
Chuck Tribolet posted 10-27-2012 02:00 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Frank: The position on that monument may well be datum NAD27,
not the NAD83/WGS84 almost universally used by GPS receivers.
The two datums are about 300 feet apart up here in NorCal,
not at all surprising given that NAD27 started out by making
a best guess the location of someplace in the midwest relative
to Greenwich and the equator, and then surveying to California
with transits and tapes. 300' is damn good when you think
about it.

It would be better to compare the iStuff to a known good WAAS
GPS receiver. I'll try to remember to compare my BadElf GPS+iPAD3 to my Garmin 162 this weekend.

JimH: I'd be upset with the vendor of that GPS that some WAAS
change obsoleted. My Garmin 162 dates to 2000, when WAAS was
just an idea at Stanford. Garmin did a firmware update to
it when WAAS started testing, and it's worked fine ever since.


Chuck Tribolet posted 10-27-2012 02:07 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
DUH. I just realized I had a known good WAAS GPS receiver
here, the little Garmin eTrex I use for a bike computer.
The etrex reads out in degrees and decimal minutes, the app for the
BadElf in decimal degrees, so a little conversion will be

BadElf: N37.096310 W121.635956
which converts to:
BadElf: N37 05.7786 W121 38.157360
eTrex: N37 05.780 W121 38.157

Close enough for me.


Chuck Tribolet posted 10-27-2012 02:17 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
JimH: I had a differential GPS receiver hooked up to the
Garmin 162 before WAAS went operational. WAAS is better.
IIRC, WAAS is about 3x better than dGPS. I gave the dGPS
receive away to a friend who was doing trail mapping in the
hills. At the time the WAAS birds were very low on the
horizon here on the west coast, but the low frequency dGPS
signals got down into the canyons. I personally don't
understand why we still maintain the dGPS network (other
than interdepartmental politics between the USCG and FAA).


Chuck Tribolet posted 10-27-2012 02:22 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
QZSS is an augmentation system similar to WAAS, not a
navigation system like GPS/GLONASS/Galileo. Like the WAAS
birds, it does also function as a GPS satellite.


jimh posted 10-27-2012 08:37 AM ET (US)     Profile for jimh  Send Email to jimh     
Chuck--The change in the PRN's used on WAAS sunk my older GPS receiver. And, yes, QZSS is an augmentation system, but it still only works in Japan or near Japan. It may be great for boaters in Japan, but it wouldn't help me find a more precise position on Lake Huron. The satellites wouldn't be visible and the data would not be useful.

As for the USCG and their DGPS, there was or perhaps still is a plan to deploy more reference stations, spreading out across the whole continent. I don't know if that is still in the budget.

There is a DGPS reference station in Detroit, right on the bank of the Detroit River. And another in Cheybogan, again right on the bank of the Cheboygan River. I have seen both of them and the installations are modest. There is a little transmitter building and a guyed tower. The tower height is only about 100-feet or less. Excluding the land--you only need about a acre--you could probably build the tower and the transmitter building for about $75,000; the government will probably spend twice that. The transmitter is low power, probably something like 100 to 200-watts, and probably cost $100,000. So each DGPS station could be on-line for $250,000. Considering that one GPS satellite costs about $800-million to build, and probably will consume another $200-million (or more) to launch into orbit, a DGPS station is a bargain. If one GPS satellite in orbit is $1-billion, then for the same money we could build 4,000 DGPS stations.

What DGPS needs to revive interest is a low-cost consumer-grade receiver. If there were one available and the cost were $200 or less, I'd get one for the boat.

As for the accuracy of a fix augmented with DGPS compared to WAAS, I would think DGPS would be more accurate. The correction is for a smaller area. Each DGPS station only covers a limited region and provides real-time correction for that region.

Chuck Tribolet posted 10-27-2012 11:48 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
WAAS breaks things down into regions too.

The GPS birds are A) necessary to navigation, B) necessary to
our defense plan, so are going to happen anyway.

Why would anybody pay $200 when WAAS takes nothing but
firmware in the GPS receiver, and is about as good. If you
want more accuracy, you need survey grade dGPS (centimeter
accuracy, I hear).


jimh posted 10-27-2012 05:56 PM ET (US)     Profile for jimh  Send Email to jimh     
If you could get more accuracy, more availability, and more redundancy for $200 I think you could attract a lot of users. If the DGPS were built into a [modern marine GNSS receiver], it could probably cost much less.

You must explain to me an other readers how WAAS is regionalized. I think the ground segment of WAAS--used by airplanes--is regionalized, but the space segment--used by marine GPS receivers use--is not. Let me hear from you on that.

Chuck Tribolet posted 10-27-2012 06:49 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The ground segment of WAAS, which is what counts, is
regionalized. WAAS has about 40 ground stations stations.
That's enough to make it exceed it's specified accuracy
of 25' by a bunch. Wikipedia says WAAS actual horizontal
accuracy is typically 1M, which is less than the distance
my Danforth anchor will sail when I drop in 50' feet of water.

It doesn't matter at all that the space segment of WAAS isn't
regionalized to any great extent (some of the WAAS birds dont
really cover everywhere).

More groud stations wouldn't hurt, but I just don't see the
need for terrestrial transmission.

WAAS is plenty good enough for me to find the dive sites.
I've got other things to do with $200. A tape measure does
just fine to build a redwood fence. I don't need a digital
slide caliper.


jimh posted 10-28-2012 12:50 AM ET (US)     Profile for jimh  Send Email to jimh     
Chuck--your reply was ambiguous. I could not tell if you were agreeing with me. I said that I did not think the data from the WAAS signals-in-space segment--which is the data you get on your marine GPS receiver with WAAS augmentation--is particularly regionalized. I think the FAA ground segment stations that transmit to aircraft provide data about their local conditions, or regionalized data. But marine GPS receivers do not use that data.

So when you said "WAAS breaks things down into regions" I do not agree. The data sent from the signals-in-space source is not particularly regionalized, from what I can tell. It seems to be a general area correction signal or something like that which mainly helps correct for ionospheric changes.

Also, the ionospheric correction information does not have to be instantaneously real-time data. You can use data that is ten minutes old and still get a better fix. Some newer GNSS receivers will hang on to WAAS data for a while, even if they lose the WAAS source signal, and keep using it until they feel it is too old. This can be useful if the WAAS signal goes out of view for a short while.

The advantage of the USCG DGPS is it seems to be nicely localized since you can only get the signal from a particular DGPS station for about 100-miles or so.

I don't know how much it costs the FAA to lease the transponders they are using for their WAAS signals-in-space segment from the operators of the geo-stationary satellites that carry the data, but I'd bet it is not cheap. I bet it costs more than it would to build a few DGPS ground stations.

jimh posted 10-28-2012 09:57 AM ET (US)     Profile for jimh  Send Email to jimh     
Chuck--Your proposed analogy about fence building was not well chosen. A GPS system with high accuracy would be quite useful in locating the fence on your property. If you install the fence and it is on your neighbor's property, you will wish you had used a more accurate position locating system. You might appreciate a precision fix with DGPS in that case.

High-accuracy position fixing for vessels can be quite useful. Suppose you were trying to pilot the container ship COSCO BUSAN under the Oakland Bay Bridge in dense fog. Or, suppose you were trying to pilot the DELTA MARINER under the Eggner's Ferry Bridge on a dark night when the obstruction lights were extinguished. A high-precision fix might be very useful in those circumstances.

David Pendleton posted 10-28-2012 12:13 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
What kind of accuracy to you think you need? As long as my EPE is less than or close to the length of my boat, I'm fine with that.

Even if you had accuracy to within a few feet, no electronic (or paper) chart is that accurate.

jimh posted 10-28-2012 06:05 PM ET (US)     Profile for jimh  Send Email to jimh     
On a sunny day with fair weather, you don't need really need any electronic navigation on the Great Lakes. I spent the first 25-years navigating the North Channel without any electronic position solution. But, as long as you have electronic navigation gear on board, I don't think there is a good case for using less accuracy than is available. As long as more accuracy is available and the cost is nominal, why not use more accuracy?

With DGPS you should be getting position solutions with 1-meter accuracy as long as you are within about 75-miles of a DGPS station. That is quite nice accuracy.

Yes, the charts may be less accurate than that.

David Pendleton posted 10-28-2012 06:41 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
As long as more accuracy is available and the cost is nominal, why not use more accuracy?

Because it serves no purpose. The charts aren't accurate enough for it to matter, and if I'm physically a few meters away from where my GPS is reporting, it's no big deal.

Chuck Tribolet posted 10-28-2012 10:45 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
What David said.


jimh posted 10-28-2012 10:58 PM ET (US)     Profile for jimh  Send Email to jimh     
More accuracy can always serve a purpose. I gave you two good examples already where more position accuracy might have been useful:

--placing Chuck's fence on his side of the property line, and

--piloting in the fog

David Pendleton posted 10-28-2012 11:46 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
--piloting in the fog

That's what RADAR is for. If your chart is crap (and they all are) what good is super-accurate GPS?.

Jefecinco posted 10-29-2012 09:14 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
Paging through some old "Yachting" magazines yesterday I saw an article with an opening line that said, "With a fathometer and RDF offshore navigation is now easy."

Remember when we got along just fine with a sounder, compass and paper chart?


6992WHALER posted 10-29-2012 12:59 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
In 2011 we did a trip in Georgian Bay Canada.
I mistakenly thought that my chart plotter's electronic charts covered the area.

We had to relay on paper charts. It was very nice to be able to look at the GPS and see our coordinates so we could find our location very quickly on the charts. If the paper charts are the most accurate depiction of the region, I would like to have the most accurate GPS to help me figure out were I am on the paper chart.

I also agree that radar is a great way to deal with fog (The best is to not go in it), but radar overlaid on a chart is better. When moving slowly in the fog, GPS chart plotters tend to not keep up with the radar, adding a heading sensor will help but the more accuracy and faster redraw the better.

ericflys posted 10-29-2012 10:19 PM ET (US)     Profile for ericflys  Send Email to ericflys     
A heading sensor should be unnecessary, for anything but standing still by the end of this coming year, with devices from all major manufactures. As redesigned models come out the refresh rates should be so fast, based on multiple navigation signals, that unless at an absolute standstill, the data presented should provide accurate heading as well as position.

The previous twenty posts get down in the weeds a bit. The simple reality for consumers looking to buy a GPS unit is this: Almost all units released or resigned in the next year will be able to receive navigation data from more sources than just GPS. I believe this is the biggest advance in chart plotter technology to happen for some time. Do you need it? Hard to quantify, but if you're in the market for one definitely worth getting one with these advancements, especially since they aren't that cheap and the wait shouldn't be that long. Whether or not in your area you can receive every type of signal the new chart plotters will be capable of receiving is a moot point. In every area, you will receive more than than you receive now with a receiver that can only receive GPS signals.

Most of us are happy with the performance we get from our current GPS receivers, but if you can have better, without any additional cost or effort on our parts why not? Depending on the geography of the areas one operates in (i.e. bay surrounded by high mountains) the performance may be much better, with less interruptions of service than you can get with a receiver that only receives GPS. Additionally, if you buy a unit with an internal antenna, you may be able to mount it in an enclosed cabin and not have an external antenna as is necessary on some boats.

jimh posted 10-30-2012 09:26 AM ET (US)     Profile for jimh  Send Email to jimh     
The trend that Eric has forecast, that marine chart plotters will soon move to having a GNSS receiver instead of a GPS-ony receiver and will be able to receive GLONASS, is very likely underway, as seen in the Garmin external receiver already on the market. As to the question of inclusion of GLONASS providing an immediate benefit, there could be some complications. I said earlier

A receiver with both GLONASS and GPS ought to be able to see more satellites and ought to be able to deduce a more accurate position solution.

That is the goal, but obtaining more accurate position solutions may not be guaranteed or obtained in actual practice. I found some grumbling from surveyors using GNSS receivers who noticed their position solutions were not particularly better when using the GNSS mode of a modern receiver. See

With modern global navigation systems the present accuracy is already rather amazing, and improving it further will require some careful application of technology and computation.

As for a low-cost back-up GPS receiver, if a USB interface can be used it will he hard to beat the GlobalSAT BU-353S4. See the linked page below for details: gs_en_product_cnt_id=76&img_id=563&product_cnt_folder=3

The BU-353S4 typically sells for under $50. It is using the SIRF IV chip, and the sensitivity specifications are fabulous, -163 dBm.

Frank O posted 11-25-2012 05:20 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Just an update on this topic. After spending some time working out how to mount my iPad on the Outrage's console, I discovered a previously unforeseen show-stopper -- a no vote from my spouse on using the tablet on the boat. As I'm sure you all know, permanent members of the Security Council have veto power.

So I went back to the idea of a low-end(-ish) Garmin chartplotter, and ended up with a GPSMAP 541 unit. On a positive note, it feeds lat-lon and speed information to my radio and the Mercury Optimax's SmartCraft system. I generated a nice map of depth contours from bathymetric data for our area, and was able to upload this to the Garmin as a background map. As for my existing Lowrance, although GPS is glitchy, the sonar function still works well enough. It's kind of nice to have a full-screen sonar display and a separate GPS display:

I'll probably think about a new end-to-end system for GPS, sonar and radar next summer or fall, but will keep the GPSMAP 541 (possibly mounting it in my T-top's upper locker) as a backup.

ericflys posted 11-25-2012 09:10 PM ET (US)     Profile for ericflys  Send Email to ericflys     
On a negative note, the 541 is being phased out by a much more capable device. Of course this could mean good deals to be had on a previous generation device, if it met ones needs...
Frank O posted 11-25-2012 09:49 PM ET (US)     Profile for Frank O  Send Email to Frank O     
I got a pretty good deal on the GPSMAP 541 (a couple of hundred dollars off list). This was one of those "I need this right now" situations, and I don't think the newer-generation models were available (at least I didn't see anything appropriate on the Garmin website in my price range). I'll definitely be looking for newer models when I shop for the more extensive instrumentation revamp next year.
jimh posted 11-25-2012 11:09 PM ET (US)     Profile for jimh  Send Email to jimh     
ASIDE to Frank--the console looks great. I love those multiple gauges you have. They look great.

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