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Greg Guimond
10-08-2010, 08:32 AM
Is there a way to recalibrate my Garmin 76 handheld. I have compared it to three vehicles and my Livorsi and it seems to be off. Can GPS even be "wrong" ??

RickR
10-08-2010, 09:39 AM
Time got off on mine and would not accuire. That would cause an error.
I don't remember how I reset it :(

BUIZILLA
10-08-2010, 09:46 AM
look on your signal screen and see how many sat's it's locked-on to.. I think mine has a *possibility* of six, but needs at least 4 to correlate any accuracy..

oledawg
10-08-2010, 10:33 AM
Odds are that all three vehicles are actually off and the Garmin isn't. Most car speedo's are about 3-5 mph slower at 60 than than the speedo shows. In fact this is legislated on European cars. :bonk:

Greg Guimond
10-08-2010, 10:36 AM
I have the 76CSx model if that changes anything. My Livorsi also had a 2mph higher reading?

Greg Guimond
10-08-2010, 10:45 AM
Buiz, I have 10 sats in total on the bottom of the screen and it looks like 6 are active.

VetteLT193
10-08-2010, 10:54 AM
stupid question, but is the Garmin set to Nautical Miles per hour? that would make it seem nuts Vs. all the other speeds.

Greg Guimond
10-08-2010, 11:08 AM
Nope - regular ol MPH.......just seems odd that the Livorsi GPS would be +2mph

zelatore
10-08-2010, 11:19 AM
The guys are 100% right about vehicles being over optimistic on the stock speedos. I've never seen a car, bike, or boat that was dead-nuts accurate.

As for the Livorsi GPS speedo being 2 mph off, I wonder if that isn't a simple error in the digital to analog conversion. Could be as simple as the needle being slightly off on it's drive spindle.

The old Garmin you have is one of their best units. It's highly regarded among hikers and other users to the point that people pitched their newer Garmins (like the Oregon and Colorado) and went back to the old 76 even though it didn't have as many bells and whistles. And yes, Garmin knows this and is working on a replacement for their current hand-helds.

The best test would be to compare it with another GPS. If you've got a buddy with a hand held, take them both for a spin and see what they say.

Greg Guimond
10-08-2010, 11:29 AM
Good feedback Zelatore. Until I can figure it out I just use the lesser GPS speed as the "top" speed.

VetteLT193
10-08-2010, 02:16 PM
I will add that the whole concept of an analog GPS speedo is pretty retarded. :bonk: Also, I while back I had tracked down the supplier of the GPS units and they are to be quite blunt, cheap.

Mine hasn't worked all season. I still have to pry the puck off the dash and replace it, or just hook the gauge up to my Northstar.

gcarter
10-08-2010, 03:49 PM
We have several hand helds and there might be a little difference between them at times, not at others.
My Expedition w/new tires will register high speeds on the speedo. At 40 MPH, it's neutral, at 60 it's going 62, and at 75 it's going about 80.
When the tires are on the wear indicators, it's dead on.
Of course w/the new tires, the truck's trip computer is off.

Greg Guimond
10-09-2010, 11:36 PM
I called up Garmin and was told that it is perfectly normal to have a GPS speedo that off by 2-3MPH +/-. We can send people into orbit, trade trillions of funds around the world, but can't make a GPS that reads spot on. Oh well ..........:frown:

gcarter
10-10-2010, 07:18 AM
I called up Garmin and was told that it is perfectly normal to have a GPS speedo that off by 2-3MPH +/-. We can send people into orbit, trade trillions of funds around the world, but can't make a GPS that reads spot on. Oh well ..........:frown:

Greg, with this in mind, a good liquid filled Bourdon tube type speedo is just as accurate.
Sometimes you're better off just going simple.

OTOH, maybe all these wonderful, high, best effort speeds are all off and the boat's owners would have been better off spending their $500.00 elsewhere.
But I don't think my opinion will change anyone's minds.

Greg Guimond
10-10-2010, 07:59 AM
Good point George. My suspicion is that none of the GPS vendor's actually build the senders. That might explain why they are all off. After all, what can you expect for $237 for the Garmin 76C. I went through the factory re-set routine, and will see today if the Garmin lines up identically with the Livorsi unit. This might be the last good weekend for boating up this way before I have to turn to the parka for my testing LOL

zelatore
10-11-2010, 11:36 AM
Good point George. My suspicion is that none of the GPS vendor's actually build the senders. That might explain why they are all off. After all, what can you expect for $237 for the Garmin 76C. I went through the factory re-set routine, and will see today if the Garmin lines up identically with the Livorsi unit. This might be the last good weekend for boating up this way before I have to turn to the parka for my testing LOL

I'm not so sure about that. I think some of the big guys actually do build their own stuff, while the smaller guys just build the software/interface.

I'll have to ask; Michele had a couple of GPS companies as clients, including Garmin and Trimble, who makes some hyper-acurate stuff for surveying and the like but doesn't have much in the consumre market.

I'm just upset she can't get free demo gear from them. :wink:

Greg Guimond
10-11-2010, 12:04 PM
Interesting, it would be revealing to get some "inside" data from the GPS folks on who builds what for whom.

gcarter
10-11-2010, 12:19 PM
A friend, who's a software engineer, told me the GPS cards in cell phones cost roughly $9.00 each.
What would you expect for that?
Something else to consider is that a GPS receiver used only for speed determination is a very simple device compared to what other devices are capable of, so I wouldnt exect great accuracy.

VetteLT193
10-11-2010, 01:30 PM
A friend, who's a software engineer, told me the GPS cards in cell phones cost roughly $9.00 each.
What would you expect for that?
Something else to consider is that a GPS receiver used only for speed determination is a very simple device compared to what other devices are capable of, so I wouldnt exect great accuracy.

If memory serves correct the Livorsi unit is a generic GPS unit (i.e., not just speed) and it costs about 15 bucks. so ditto to your accuracy comment.

Greg Guimond
10-11-2010, 08:10 PM
10-11-10 ...............:screwy:

HIGH LIFE
10-12-2010, 06:53 PM
Greg, Looks like your Livorisi needs to traded in for a 100 GPS !!! "HIGH LIFE"

Greg Guimond
10-13-2010, 02:12 PM
Right now I just want to "pin" the needle on the Livorsi I have :eek:

VetteLT193
10-13-2010, 03:21 PM
where is the needle right after you power up but are still stopped? is it dead on zero?

dsparis
10-13-2010, 03:34 PM
GPS Speed:
How accurate is it? How fast can I go? How HIGH can I go? GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed.
HOW ACCURATE IS THE SPEED READING?

From the NAVSTAR GPS User Equipment Introduction document Section 3.7:
GPS receivers typically calculate velocity by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent, (multipath, obstructed sky view from the dash of a car, mountains, city canyons, bad DOP) but 0.2 m/sec per axis (95%) is achievable for PPS and SPS velocity accuracy is the same as PPS when SA is off.
Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on.

gcarter
10-13-2010, 05:49 PM
This is interesting.
I was driving this morning on I-75, the road was dead flat, speed was 75 indicated, and on cruise control.
It was fascinating watching the Garmin indicated speed moving from 74 to 78 repeatedly, but not in any kind of repeatable cycle or repeatable numbers.
It must have been variations in the unit's reception because I would swear the trucks speed wasn't varying .5 MPH.

zelatore
10-14-2010, 11:36 AM
I've seen cheap GPSs like my iPhone jump around and take a long time to become accurate, but I just figured that was because they used the cheapest possible chipset in them as it was more of an added on gimmick than a true function of the device. Most of my dedicated units have been pretty steady and repeatable.

I spoke to Michele about her old clients and she thinks Garmin buys the actual GPS chip then designs their own receiver around it. Trimble, who makes professional/survey equipment might make their own GPS chips (ASIC) and definitely designs their own receivers.

I would expect most of the GPS companies we're familiar with do it like Garmin. It just can't be worth it for the average company to own a silicon fab. Smaller companies might buy both the chipset and receiver from a 3rd party. Trimble is one of a handful of companies I can think of that might need such tight control of it's chips they would design their own, but even then I wouldn't be surprised if they outsource the actual silicon fab.

Fluffy Foo-Foo
10-14-2010, 12:15 PM
FWIW I tried to track down the manufacturer of Livorsi/Gaffrig gauges and found these guys. http://www.sanav.com
Tried to buy a receiver/antenna direct but they sent me to Livorsi!

There are a lot of sites that sell receivers and antennas and the basic stuff is VERY cheap money wise.

zelatore
10-14-2010, 04:37 PM
I'm pretty sure my Livorsi speedo will accapt standard NMEA 0183 inputs from any GPS - so if you've got a small chartplotter or something like that you could just wire the speedo to it and forget about the Livorsi GPS all together.

Greg Guimond
10-14-2010, 06:15 PM
So now that we have a few posts under our collective belts here are my observations.

1. Everyone who runs a GPS (anolog or digital) on this board is posting up speeds that are probably not accurate + - 2-3MPH.

2. Many people on this board think that it is ok for $300 consumer GPS/NAV device to provide inferior performance based on components that are built by outside vendors and $50 or less of the $300 total cost

3. No one has a vendor that will provide accurate speeds only, at any cost under $1,000

Well, not where I thought it might end up but interesting none the less :frown:

gcarter
10-14-2010, 07:17 PM
So, back to my long held belief, that a good liquid filled Bourdon tube speedo is as accurate as the typical GPS we might have.
So, save your money for something that makes a difference.

OTOH, a hand held aircraft type GPS might be more accurate.
They, I suppose , can measure speed and position in three dimensions simultaniously.
I suspect they're more expensive also.

Greg Guimond
10-14-2010, 07:23 PM
Post a pic of this "bourdon" transom mounted thing. Worth a look, but I'm not a fan of extra drag on the keel plus it must cost at least $100 all in correct?

I have no idea what a commercial GPS would cost and if in fact it is handheld, but would also like to see a pic of one of those :yes:

gcarter
10-14-2010, 11:20 PM
Greg, a Bourdon tube is the part inside the gage that motivates the needle. You can Google it.
It consists of a U shaped tube, one end is attached by a hose to a pitot tube on the transom. The Bourdon tube, when pressure is applied, unwinds slightly and the needle is moved by a link and a couple of gears. When properly designed and built, like Livorsi does, they are extremely accurate.
The pitot tube will not add any noticeable drag until you exceed 100 MPH.

gcarter
10-15-2010, 12:06 AM
You can get a good used one for well under $100.00.
In fact, I have one, in red I'll offer. Monster size, red rim, white face, Gaffrig by Livorsi, very good condition.
Actually, I have two of them somehow. I'm planning on having Livorsi replace the rim w/a platinum rim on one of them.

VetteLT193
10-15-2010, 08:43 AM
to extend George's posts... The other thing about GPS is it is speed over ground Vs. speed in the water. A proper setup with both a GPS and standard speedo you should see both speeds nearly the same +- 1 or 2 MPH in calm water with no current.

As the current, wind, waves vary you'll see the two numbers vary. The difference in the numbers can help properly operate your boat because you'll better understand the water you are on. It also helps for top speed runs etc. because you won't get a false read running with the wind/current because the variation will appear in the difference in speedos.

I am still baffled by how much confidence is put into GPS speed numbers by most people when it is only one piece of the puzzle

gcarter
10-15-2010, 09:02 AM
to extend George's posts... The other thing about GPS is it is speed over ground Vs. speed in the water. A proper setup with both a GPS and standard speedo you should see both speeds nearly the same +- 1 or 2 MPH in calm water with no current.

As the current, wind, waves vary you'll see the two numbers vary. The difference in the numbers can help properly operate your boat because you'll better understand the water you are on. It also helps for top speed runs etc. because you won't get a false read running with the wind/current because the variation will appear in the difference in speedos.

I am still baffled by how much confidence is put into GPS speed numbers by most people when it is only one piece of the puzzle

Good point Bob.

tmh
10-15-2010, 09:08 AM
I've seen a lot of posts with individuals strugging over tenths of miles per hour for top speed and the accuracy level may be off by a few mile per hour? Somebody's world may have just been upended.

gcarter
10-15-2010, 09:09 AM
http://knowledgepublications.com/doe/images/DOE_Instrumentation_Bourdon_Tube.gif

Here's a Bourdin tube gauge.
Every mechanical pressure gauge used in industry works like this.
Even gauges that require +/- .05% accuracy work like this.
The speedos made by Livorsi are some of the most accurate available anywhere. They are glycerine filled to dampen the needle and remove any "jiggling".
The main design consideration is to have a range great enough so that the maximum speed is always somewhere close to the middle of the range.
I.E., if the maximum speed is 80 MPH, then a 100 MPH speedo is needed.
Two reasons for this;
The gauge is the most accurate in the middle of the range.
Also, the mechanism can be damaged (it can jam) if it is allowed to reach the upper end of the scale.

Greg Guimond
10-15-2010, 04:10 PM
Yes, great point Bob. George, thanks for the explanation. This is part of what makes this place a good forum, you can always learn something new. I am probably the only one that did not really understand all the intrinsic details surrounding consumer GPS shortcomings especially as they relate to Bourdon type units. To your point TMH, it looks like the 2010 Fastest Classic thread is now rendered obsolete, thanks for pointing that out.

Well, at least it was fun to watch some of the posts anyway :smash: