Lowrance Mark 4 GPS speed accuracy

[walt]

I have a Lowrance Mark 4 that I use in a 15 foot "experiment" boat that likes to plane. Other than the problem Ill talk about below, I like this instrument as I can set up the display how I want and it doesnt use much power (about 2.2 watts as shown). In the picture, I have from top to bottom Speed, Peak Speed, water temp + battery voltage, depth. Overall just about perfect for this electric trolling motor powered sailboat - since I even can see the battery voltage.

However, I am having trouble with the GPS peak speed reading. In the video http://www.youtube.com/watch?v=DjZmwFZdaas&feature=youtu.be
I had the instrument mounted under the tiller and when I got done sailing, the display said I had a peak speed of over 15 mph - which is clearly an error (I was actually seeing peak speeds in the 8 to 8.5 mph range). I moved the instrument out from under the tiller (wood will disturb the RF signal) and next time had a peak speed of 11 mph. I moved the instrument again today to make sure it has a completely clear view of the sky and the boat sitting in my driveway still had one peak speed reading of about 10 mph. Just in case you missed it, the boat was in the driveway - not moving.

So I’m wondering if this is just how this brand works (which would be very disappointing to me) or do I have a defective unit?

FYI, the depth finder is sometimes a little intermittent also. Maybe I just got a bad unit. I believe some of the folks here have either this unit or the similar Elite 4. Does your peak speed reading seem accurate?

 

[Roadking Larry]

I've never seen any "way off" speed readings on mine. I've never set up the peak speed display, probably because I didn't now it would do that, might have to read the manual one of these days.
My transducer is mounted inside the hull with toilet bowl wax and the only time I've seen depth readings that were out of expected range was when backing with the outboard, other than that it is consistent and pretty well matches charted depths for my lake. Which just happens to be the "demo" lake at least on my unit.

 

[walt]

You can get to the trip stuff by (I belieive) settings - system - trip calculator. You have to go into this screen to turn on the trip calculator and also reset it. I also have the peak speed displayed.

FYI, I never see the peak speed errror happening since Im never watching the display 100% of the time. Its likely a very transient reading that goes away quick but it gets captured.. I have seen false GPS peak readings in the past while ice boating and it was usually caused by having the GPS in the coat pocket and your arms moving quickly between the GPS and the sat signal. But the unit I have now gets these peak errors just sitting still with a direct view of the sky..

I also have the Elite 4 and will try it also but its 850 miles away at the moment, I wont get to it until early August.

Im guessing something is screwed up my unit. A few hours ago, I had reset the trip calculator, left it on in the boat and just got another peak speed reading of 9.5 mph... The attached picture is from a sailing session last week. Everything looks about right but no way did I go over 11 mph.

 

[walt]

Follow up.. I went to the retail store (Bass Pro) and they are going to replace the unit. Good news is that I havent heard of any others having this issue so it hopefully is just my particular unit that has some problem. This type of problem is hard to find at the manufacture since it apears to work fine most of the time.

Ill make sure to do a final follow up when I get the replacement and test it out (might take up to a week)- hopefully with good news as this particular instrument can be configured to do exactly what I want.

 

[Roadking Larry]

I made a trip to the marina this morning and played with the Mark-4 a little. I pulled up the trip stuff and there was data there that showed a max speed of 9.5 kts. While possible, according to the previous owners report of 10+ I'm not convinced I've hit that speed.
As soon as my back gets to where I can sail again I'll see what I get out of it.
Even with the odd random error I'm not too worried about it. Any GPS unit is still subject to errors and I'm not putting my life on the line with it so I can live with it.

 

[walt]

Tonight I reset the trip calculator and at about 2.5 hours achieved a new peak driveway speed of 29.2 mph. Pretty good speed! (considering the boat was not moving). When I look at the instrument, it is very well behaved showing a constant 0 mph and showed 0 mph peak speed for over two hours, then the big error. Maybe a cosmic ray went through the processor or something. Im going to remove the transponder and wiring in the morning and get a new unit.

Any chance you got the 9.5 knts while loading the boat - ie, driving in the parking lot. Ive never come anywhere near that speed either sailing or motoring with my 26S.

 

[Roadking Larry]

I didn't install the unit till the boat was already in the water. I don't for a minute think I've sailed it at 9.5 kts, but I could see a brief transient while manhandling it at the dock but even that sounds unlikely.
You're readings sound like a problem though 29kts in the driveway is kind of out there.

 

[Sumner]

29.2 is nothing. When we first used our Garmin Map 76 in Canada we got speeds over 200 mph a couple times. You could look at the track and for a couple seconds it would be 100 miles away or so. I talked to Garmin and they told me how to do a re-boot that set the GPS back to factory and that seemed to of taken care of it.

I'd still bet that momentary speeds of 10-15 mph might be possible just due to other errors ....

http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System

... I didn't try and understand all or most of that link to see what the error could be, but I did see where a moving vehicle with a GPS could have less of an error than a stationary one,

Sum

[FONT=Arial, sans-serif]============================[/FONT]

Our Endeavour 37

Our MacGregor 26-S Pages

Our Trips to Utah, Idaho, Canada, Florida

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[walt]

29.2 is nothing.

LOL.. I thought I had finally achieved something significant

 

[walt]

I looked at the article and couldnt make the connection to the 10 to 15 mph speed error or that a moving vehicle would have less of an absolute error unless the error is a percent of the actual speed. Ie, 1 mph error if your going 1 mph is a much larger pecent than if your doing 100 mph.

What I also didnt see was anything that mentioned the rate of change of the position error. This would seem to be the one that is important for the speed error.

I did see some stuff about the theory of relitively in there, I wonder if my $200 Lowrance has the mentioned theory or relitivey compensation?

This was kind of interesting, I remember back when this turned this off:

GPS included a (currently disabled) feature called Selective Availability (SA) that adds intentional, time varying errors of up to 100 meters (328 ft) to the publicly available navigation signals. This was intended to deny an enemy the use of civilian GPS receivers for precision weapon guidance.
SA errors are actually pseudorandom, generated by a cryptographic algorithm from a classified seed key available only to authorized users (the U.S. military, its allies and a few other users, mostly government) with a special military GPS receiver. Mere possession of the receiver is insufficient; it still needs the tightly controlled daily key.
Before it was turned off on May 2, 2000, typical SA errors were about 50 m (164 ft) horizontally and about 100 m (328 ft) vertically

It seems to me and I could be wrong that the instrument software should be able to throw out the bad readings. Ie, if you have a string of measurments at 5 to 7 knts and then get one at 100 knt, it probably isnt right. If it is poor software filtering, getting a new unit with the same software wouldnt fix my problem.. I looked all throught the settings to see if there was some soft of filtering options, there were not.

 

[Sumner]

....It seems to me and I could be wrong that the instrument software should be able to throw out the bad readings. Ie, if you have a string of measurments at 5 to 7 knts and then get one at 100 knt, it probably isnt right. ...

I'd think you are right on that one if it is taking a bunch of readings over a short period of time. Too much math in that article for me :cry:. Hopefully your new unit should solve the problem. Our reboot seemed to do the trick for ours.

One thing that I was amazed at was how many miles total we traveled swinging on anchor over 7+ weeks,

Sum

[FONT=Arial, sans-serif]============================[/FONT]

Our Endeavour 37

Our MacGregor 26-S Pages

Our Trips to Utah, Idaho, Canada, Florida

Mac-Venture Links

 

[walt]

I mentioned this already but back a long time ago (10 years), I had some Garmin handheld GPS with a peak speed reading and would use it for ice boating. If I put the instrument where it had a clear uninterupted view of the sky, it always seemed to have a reliable peak speed reading. But if I put it in a coat pocket where my arms would get between the GPS and the sky in a transient manner from controlling the sheet, I would sometimes get a peak speed reading that was way high - like 40%. I think its just the radio sometimes getting a false position reading and software that doesnt throw out bad readings but instead uses them to calculate speed.

I have doubts that the new unit will solve the peak speed reading unless it comes with a better newer version of software. I believe you can also go on line and download the latest software and load it into the unit with the micro SD card. Since I beleive there is something also wrong with the depth transducer, I didnt try the software download, maybe I should have..

 

[walt]

follow up.. I picked up a brand new Lowrance Elite 4 and installed it today and of course had to also take the boat out within hours. Absolutely wonderful day sailing and the Elite 4 overall worked well but I still got a false peak spead reading of 16.34 mph. You can see in the pictures that the instrument has about as clear view of sky as possible.. so I give up on the peak speed, I think the software for this needs a little work still. Still like the instrumement for all the other things it does - speed overal works well. Before I got the bad readiing, the peak speed was about 7.8 mph which I believe.