29 Jun 2007 09:36:50
Nick Cox
GPS Speeds

I've been using a Foretrex 201 for about a year now to track my speeds
and I'm finding that it is extremely unreliable when it comes to
calculating speeds on a track. After my experience last year of losing
the unit off my wrist into Lake Ontario I am now wearing a PFD that
has a mesh pouch on the front. While the unit seems to have no
problems in recording the track, the speeds are absolutely wonky. For
instance last Wed. I was out in winds gusting to 20kts and supposedly
achieved a max. speed of 456 km/h! This is not the only time I've got
spurious speed data from this unit. I've heard that the NAVi gps units
are somewhat more reliable in reporting speed data. If anyone have any
experience with these units, I'd be grateful for your input.

Cheers,

Nick Cox



29 Jun 2007 11:48:11
wind.sh@dow
Re: GPS Speeds

Andreas Macke (I think) mentioned here recently about GPS max speeds
being affected in the midst of a crash (yard sale). Is there a
possibility you went down hard and that's when your max. speed went
skyward?

On Fri, 29 Jun 2007 09:36:50 -0700, Nick Cox <ncoxn225@rogers.com >
wrote:

>I've been using a Foretrex 201 for about a year now to track my speeds
>and I'm finding that it is extremely unreliable when it comes to
>calculating speeds on a track. After my experience last year of losing
>the unit off my wrist into Lake Ontario I am now wearing a PFD that
>has a mesh pouch on the front. While the unit seems to have no
>problems in recording the track, the speeds are absolutely wonky. For
>instance last Wed. I was out in winds gusting to 20kts and supposedly
>achieved a max. speed of 456 km/h! This is not the only time I've got
>spurious speed data from this unit. I've heard that the NAVi gps units
>are somewhat more reliable in reporting speed data. If anyone have any
>experience with these units, I'd be grateful for your input.
>
>Cheers,
>
>Nick Cox



29 Jun 2007 16:53:15
Steve Elliott
Re: GPS Speeds

The way a GPS calculates speed is by taking the distance and dividing it by
the time. This is done between trackpoints. Most GPS's allow you to set the
frequency of taking trackpoints. The more trackpoints, the more accurate
your track is, and the more memory is required to store them. In looking
for accurate speeds, the speed per trackpoint is not very reliable, but
taken over say 5 or 10 trackpoints, it's more reliable. If you capture a
trackpoint every second then you 10 trackpoints is 10 seconds.

With all that in mind, the accuracy of the speed is related indirectly to
the resolution. That is, if you are at 16 foot resolution, the trackpoints
are very accurate. If you're at 140 foot rsolution, they are not. If the
GPS thinks you have travelled 140 feet more than you actually have in a 1
second period, your speeds are off the chart. My experience with using a
GPS on the water is that accuracy goes away quickly when the unit is
submerged. So when analysing a track, I look for a trend. If I see a
stretch of readings that say 27 mph, 22 mph, 24 mph, 28 mph, 20 mph, I
figure I'm going around 24 mph.

Steve

Nick Cox <ncoxn225@rogers.com > wrote in news:1183135010.537313.177860
@n60g2000hse.googlegroups.com:

> I've been using a Foretrex 201 for about a year now to track my speeds
> and I'm finding that it is extremely unreliable when it comes to
> calculating speeds on a track. After my experience last year of losing
> the unit off my wrist into Lake Ontario I am now wearing a PFD that
> has a mesh pouch on the front. While the unit seems to have no
> problems in recording the track, the speeds are absolutely wonky. For
> instance last Wed. I was out in winds gusting to 20kts and supposedly
> achieved a max. speed of 456 km/h! This is not the only time I've got
> spurious speed data from this unit. I've heard that the NAVi gps units
> are somewhat more reliable in reporting speed data. If anyone have any
> experience with these units, I'd be grateful for your input.
>
> Cheers,
>
> Nick Cox
>
>



29 Jun 2007 13:37:18
Jrobb
Re: GPS Speeds

The GPS needs a clear view of the sky or you can get funky
spikes...like you found here. The mesh pouch on your PFD locates the
unit to only recieve roughly 50% of it's tracking window since your
upper body blocks any sattelites that may be low on the horizon.
Therein lies one issue. Submerging does result in the unit thinking
it just lost track and keeps counting on the present vector. When it
regains signal, it will "zoom " back and you get your accelerated
speed.

The Fortrex suffers from a "grid effect" part in parcel to it's
resolution. This is why at 1sec intervals it shows a more zigzaggy
track. Set to 2sec intervals, this smooths out the zig's and you get
a "better" but not superb result. Worn on the upper arm or wrist is
best for the unit.

I've had mine in use for 1year now and about 60-70 sessions later with
no pack it's still fine. I am cautious to not let it submerge unless
I have to. No bad wipeouts and I only get at most lately max
accelerations in the range of 2-2.5m/sec...considered acceptable
amongst many speed sailors around the globe and a few who design some
of the software.

What software do you use to analyze?

J



29 Jun 2007 20:54:31
Steve Elliott
Re: GPS Speeds

I use OziExplorer. First, you need a map. It can be any bitmap image,
but then you need to calibrate it. That involves placing three reference
points on the image and entering the exact Longitude and Lattitude. Once
you do that, the image is saved as a map, and when you download tracks,
it shows it on the map. Then you can look at the trackpoints and see the
speed.

Here's an example on IWindsurf:

http://forums.iwindsurf.com/viewtopic.php?t=12365&highlight=

Steve

Jrobb <robbinskuma@gmail.com > wrote in news:1183149438.828940.320880
@n60g2000hse.googlegroups.com:

> The GPS needs a clear view of the sky or you can get funky
> spikes...like you found here. The mesh pouch on your PFD locates the
> unit to only recieve roughly 50% of it's tracking window since your
> upper body blocks any sattelites that may be low on the horizon.
> Therein lies one issue. Submerging does result in the unit thinking
> it just lost track and keeps counting on the present vector. When it
> regains signal, it will "zoom " back and you get your accelerated
> speed.
>
> The Fortrex suffers from a "grid effect" part in parcel to it's
> resolution. This is why at 1sec intervals it shows a more zigzaggy
> track. Set to 2sec intervals, this smooths out the zig's and you get
> a "better" but not superb result. Worn on the upper arm or wrist is
> best for the unit.
>
> I've had mine in use for 1year now and about 60-70 sessions later with
> no pack it's still fine. I am cautious to not let it submerge unless
> I have to. No bad wipeouts and I only get at most lately max
> accelerations in the range of 2-2.5m/sec...considered acceptable
> amongst many speed sailors around the globe and a few who design some
> of the software.
>
> What software do you use to analyze?
>
> J
>
>



29 Jun 2007 14:45:35
Nick Cox
Re: GPS Speeds

On Jun 29, 4:54 pm, Steve Elliott <jse6...@sbcglobal.net > wrote:
> I use OziExplorer. First, you need a map. It can be any bitmap image,
> but then you need to calibrate it. That involves placing three reference
> points on the image and entering the exact Longitude and Lattitude. Once
> you do that, the image is saved as a map, and when you download tracks,
> it shows it on the map. Then you can look at the trackpoints and see the
> speed.
>
> Here's an example on IWindsurf:
>
> http://forums.iwindsurf.com/viewtopic.php?t=12365&highlight=
>
> Steve
>

Thanks for all of the replies. I use Topofusion to analyse my tracks
and when I took a closer look at this track, the spike in the speed
occurred about 3 min. before I got hammered by a gust (so the spike
wasn't the result of a wipeout). What I think happened is that it
briefly lost contact with the satellite, and then when it reconnected
- that's when the spike occurred. With keeping the GPS in the pouch
I've never had a major problem with it losing the signal - but I guess
that waterstarting would create difficulties acquiring a signal. I'm
still interested to know if anyone has any experience with the NAVi
gps units.

Cheers,

Nick Cox



29 Jun 2007 19:18:08
Florian Feuser
Re: GPS Speeds

> The way a GPS calculates speed is by taking the distance and dividing it by
> the time.

Is there any other way?

;-)

I just got a GPS logger (no live readout, but I am not a speedster
anyway) and I am wondering what the polling rate of a GPS must be in
order to get acceptably accurate results. Are there required specs
posted anywhere - didnt see any on www.gps-speedsurfing.com


--
florian - NY22

http://www.kasail.com/windsurfing/team/florianfeuser.html


30 Jun 2007 05:03:24
Jrobb
Re: GPS Speeds

On Jun 29, 2:45 pm, Nick Cox <ncoxn...@rogers.com > wrote:
> On Jun 29, 4:54 pm, Steve Elliott <jse6...@sbcglobal.net> wrote:
>
> > I use OziExplorer. First, you need a map. It can be any bitmap image,
> > but then you need to calibrate it. That involves placing three reference
> > points on the image and entering the exact Longitude and Lattitude. Once
> > you do that, the image is saved as a map, and when you download tracks,
> > it shows it on the map. Then you can look at the trackpoints and see the
> > speed.
>
> > Here's an example on IWindsurf:
>
> >http://forums.iwindsurf.com/viewtopic.php?t=12365&highlight=
>
> > Steve
>
> Thanks for all of the replies. I use Topofusion to analyse my tracks
> and when I took a closer look at this track, the spike in the speed
> occurred about 3 min. before I got hammered by a gust (so the spike
> wasn't the result of a wipeout). What I think happened is that it
> briefly lost contact with the satellite, and then when it reconnected
> - that's when the spike occurred. With keeping the GPS in the pouch
> I've never had a major problem with it losing the signal - but I guess
> that waterstarting would create difficulties acquiring a signal. I'm
> still interested to know if anyone has any experience with the NAVi
> gps units.
>
> Cheers,
>
> Nick Cox

I tested out a Navi (GT-11) but didn't like the lack of ability to
navigate menues while on the water. My Fortrex was easier for that.
Other than that it's a great unit and much more precise. For
instance...the Fortrex's max speed readout is equivalent to roughly
3-5second speeds. So if it reads 32kts your max 2sec could be 34kts
and 10sec speed of 31.5...or thereabouts. I've noticed mine is
reading more close to actual speeds as I've been getting better at
making dry jibes so less spikes...almost none really. The Navi's
display of max speed is for all purposes almost dead on with your
actual max speed. So what you see is what you get...or equivalent to
your 2sec speed. The Navi also downloads NMEA data ( satellite
tracking info and doppler speed data along with trackpoints) to an SD
card. 512K card is good for about a month of sailing without
erasing. Upload of sessions is lightining quick. For more info on it
go to www.seabreeze.com.au and read up on the windsurfing-gpsand speed
sailing forum. Have a look at Roo's setup guide. One thing to note,
Cosmicharlie has made his presence known there and they are a bit
suspicious of newcommers. Keep this in mind when reading responses to
your posts at first. A calm head will ease skeptics.

If you're in the US contact Craig Berg at the Midwest Speedquest
site. Great guy to deal with and he's the North American importer of
the Navi's.

I use one of three programs GPSAR, GPSResults, and Real Speed. The
first is free, GPSR is free for trial 30days then limited access (only
first 1000 points analyzed), and Real Speed is now going to be for a
charge. Roughly around 30-40 USD for the pay-to play ones.

Nick, regarding your spike, it may not be obvious that is has lost the
signal and regained it as it relocks on to it eventually. The spike
is evidence of that happening. One thing to look at too if you can is
your elevation at the time of the spike. Fortrex has an altimiter too
(though it doesn't claim it on list of specs because it's not a
separate display page) if you have an elevation spike excessively low
or high that's evidence too of lost signal.


J



30 Jun 2007 05:06:42
Jrobb
Re: GPS Speeds

On Jun 29, 4:18 pm, Florian Feuser <flor...@SPAMTRAPfunnygarbage.com >
wrote:
> > The way a GPS calculates speed is by taking the distance and dividing it by
> > the time.
>
> Is there any other way?
>
> ;-)
>
> I just got a GPS logger (no live readout, but I am not a speedster
> anyway) and I am wondering what the polling rate of a GPS must be in
> order to get acceptably accurate results. Are there required specs
> posted anywhere - didnt see any onwww.gps-speedsurfing.com
>
> --
> florian - NY22
>
> http://www.kasail.com/windsurfing/team/florianfeuser.html

two ways I know of is trackpoinc calcs...distance over time.
and Doppler...difference in satellite signal reception similar to
audible doppler effect of a siren on cop car...or horn of a passing
car. Doppler is the more accurate one since the trackpoint resloution
is ok on most units but not difinitive.

J



30 Jun 2007 04:04:57
cosmicharlie
Re: GPS Speeds

On Jun 29, 7:18 pm, Florian Feuser <flor...@SPAMTRAPfunnygarbage.com >
wrote:
> > The way a GPS calculates speed is by taking the distance and dividing it by
> > the time.
>
> Is there any other way?
>
> ;-)
The speedometer on an automobile counts the revolutions on your wheel
and determines speed in another way.



30 Jun 2007 08:03:49
Florian Feuser
Re: GPS Speeds

> The speedometer on an automobile counts the revolutions on your wheel
> and determines speed in another way.

..and since the speedometer "knows" the circumference of your tires, it
can calculate speed by distance/time.

Good point about the doppler though, jrobb...


--
florian - NY22

http://www.kasail.com/windsurfing/team/florianfeuser.html


30 Jun 2007 12:40:39
Re: GPS Speeds

On Jun 30, 8:03 am, Florian Feuser <flor...@SPAMTRAPfunnygarbage.com >
wrote:
> > The speedometer on an automobile counts the revolutions on your wheel
> > and determines speed in another way.
>
> ..and since the speedometer "knows" the circumference of your tires, it
> can calculate speed by distance/time.
>
> Good point about the doppler though, jrobb...
>
> --
> florian - NY22
>
> http://www.kasail.com/windsurfing/team/florianfeuser.html

"....and since the speedometer "knows" the circumference of your
tires, it can calculate speed by distance/time."

Just right. The older mechanical speedometers, with the cable
connection, are calibrated in revolutions (of the input cable) per
mile. This takes into account the differential ratio and the tire
size.

The newer electronic speedometers are calibrated in "pulses per
mile" (the pulses come from a sensor at the differential or from an
encoder in the wheel). On some cars, you can actually see a small
calibration number on the face of the speedo. One of my cars, for
example, says "k10095" in tiny letters on the speedo dial. This means
means that the speedo electronics is calibrated to register 60 mph
when the pulse rate is 10065 pulses per minute (60 mph equals 1 mile
per minute). The encoder creates 10065 pulses through 1.000 mile of
travel. Tire size is also critical in this calculation, as you said.




30 Jun 2007 06:20:05
Hanginon
Re: GPS Speeds

Before we get too far in this thread, a small correction.

A GPS does it's calculations using change in position (not distance)
over time. This way you get distance, velocity, and heading.



30 Jun 2007 06:54:29
Nick Cox
Re: GPS Speeds

On Jun 30, 1:03 am, Jrobb <robbinsk...@gmail.com > wrote:
> On Jun 29, 2:45 pm, Nick Cox <ncoxn...@rogers.com> wrote:
>
>
>
> > On Jun 29, 4:54 pm, Steve Elliott <jse6...@sbcglobal.net> wrote:
>
> > > I use OziExplorer. First, you need a map. It can be any bitmap image,
> > > but then you need to calibrate it. That involves placing three reference
> > > points on the image and entering the exact Longitude and Lattitude. Once
> > > you do that, the image is saved as a map, and when you download tracks,
> > > it shows it on the map. Then you can look at the trackpoints and see the
> > > speed.
>
> > > Here's an example on IWindsurf:
>
> > >http://forums.iwindsurf.com/viewtopic.php?t=12365&highlight=
>
> > > Steve
>
> > Thanks for all of the replies. I use Topofusion to analyse my tracks
> > and when I took a closer look at this track, the spike in the speed
> > occurred about 3 min. before I got hammered by a gust (so the spike
> > wasn't the result of a wipeout). What I think happened is that it
> > briefly lost contact with the satellite, and then when it reconnected
> > - that's when the spike occurred. With keeping the GPS in the pouch
> > I've never had a major problem with it losing the signal - but I guess
> > that waterstarting would create difficulties acquiring a signal. I'm
> > still interested to know if anyone has any experience with the NAVi
> > gps units.
>
> > Cheers,
>
> > Nick Cox
>
> I tested out a Navi (GT-11) but didn't like the lack of ability to
> navigate menues while on the water. My Fortrex was easier for that.
> Other than that it's a great unit and much more precise. For
> instance...the Fortrex's max speed readout is equivalent to roughly
> 3-5second speeds. So if it reads 32kts your max 2sec could be 34kts
> and 10sec speed of 31.5...or thereabouts. I've noticed mine is
> reading more close to actual speeds as I've been getting better at
> making dry jibes so less spikes...almost none really. The Navi's
> display of max speed is for all purposes almost dead on with your
> actual max speed. So what you see is what you get...or equivalent to
> your 2sec speed. The Navi also downloads NMEA data ( satellite
> tracking info and doppler speed data along with trackpoints) to an SD
> card. 512K card is good for about a month of sailing without
> erasing. Upload of sessions is lightining quick. For more info on it
> go towww.seabreeze.com.auand read up on the windsurfing-gpsand speed
> sailing forum. Have a look at Roo's setup guide. One thing to note,
> Cosmicharlie has made his presence known there and they are a bit
> suspicious of newcommers. Keep this in mind when reading responses to
> your posts at first. A calm head will ease skeptics.
>
> If you're in the US contact Craig Berg at the Midwest Speedquest
> site. Great guy to deal with and he's the North American importer of
> the Navi's.
>
> I use one of three programs GPSAR, GPSResults, and Real Speed. The
> first is free, GPSR is free for trial 30days then limited access (only
> first 1000 points analyzed), and Real Speed is now going to be for a
> charge. Roughly around 30-40 USD for the pay-to play ones.
>
> Nick, regarding your spike, it may not be obvious that is has lost the
> signal and regained it as it relocks on to it eventually. The spike
> is evidence of that happening. One thing to look at too if you can is
> your elevation at the time of the spike. Fortrex has an altimiter too
> (though it doesn't claim it on list of specs because it's not a
> separate display page) if you have an elevation spike excessively low
> or high that's evidence too of lost signal.
>
> J

Hi J,

Thanks for your input. When I took another look at the track the
elevation of the speed spike comes out at about 285 feet, but what is
interesting is that there are lower speeds (correctly identified) for
which it has determined higher elevations (i.e. 340 ft+). I don't
really know what that means and I must admit I've always found the
elevation anomalies interesting since I always thought that the waves
I encounter aren't THAT large! The NAVi unit looks like an interesting
unit and I think that I'll recommend to Toronto Windsurfing Club that
they get one so that it can be compared to the Foretrex.

Cheers,

Nick



30 Jun 2007 14:31:15
Jrobb
Re: GPS Speeds

On Jun 30, 6:20 am, Hanginon <fjmast...@yahoo.com > wrote:
> Before we get too far in this thread, a small correction.
>
> A GPS does it's calculations using change in position (not distance)
> over time. This way you get distance, velocity, and heading.

moving to a different position within a calculated amount of time is
the same as distance over time.

If I move to a position 1mile away to the East from where I currently
am within 1hour and have been doing so for 3hours, I've gone 3miles on
heading of 90, and am traveling at 3miles per hour. If the unit
didn't know what the difference between Lat and Long coordinates it
wouldn't be able to measure distance or speed but only heading. If it
didn't measure all this on a backdrop of the passage of time, you'd
only get heading, and distance...

J



30 Jun 2007 15:51:09
Steve Elliott
Re: GPS Speeds

GPS units have two ways to determine elevation. The first is altimeter
based and the second is based on satellite triangulation. The latter is
fare less accurate. If your GPS uses an altimeter, it has the ability to
measure air pressure. Before you use it, you need to set the elevation to a
known value to take into account atmospheric pressure variations due to
weather. I suspect your unit uses triangulation and that's why you see such
discrepancies among readings. (Unless the wind that we sail in affects an
altimeter moment to moment. Not sure about that.)

Steve
> Hi J,
>
> Thanks for your input. When I took another look at the track the
> elevation of the speed spike comes out at about 285 feet, but what is
> interesting is that there are lower speeds (correctly identified) for
> which it has determined higher elevations (i.e. 340 ft+). I don't
> really know what that means and I must admit I've always found the
> elevation anomalies interesting since I always thought that the waves
> I encounter aren't THAT large! The NAVi unit looks like an interesting
> unit and I think that I'll recommend to Toronto Windsurfing Club that
> they get one so that it can be compared to the Foretrex.
>
> Cheers,
>
> Nick
>
>



30 Jun 2007 15:56:37
Steve Elliott
Re: GPS Speeds

Good point Hang, and me with a degree in Physics. Since we're splitting
hairs tho, notice in my original post:

"The way a GPS calculates speed is by taking the distance and dividing it
by the time."

which is technically correct, as Speed is defined as distance over time.
Velocity is change in position over time.

Steve

Hanginon <fjmasters@yahoo.com > wrote in news:1183209605.411198.55990
@n2g2000hse.googlegroups.com:

> Before we get too far in this thread, a small correction.
>
> A GPS does it's calculations using change in position (not distance)
> over time. This way you get distance, velocity, and heading.
>



06 Jul 2007 14:41:17
jeff feehan
Re: GPS Speeds

Florian Feuser wrote:
>> The way a GPS calculates speed is by taking the distance and dividing
>> it by the time.
>
> Is there any other way?
>


yes - there is, and your GPS uses it!


most GPS receivers calculate speed by the doppler effect, not by distance/time.

unusual speeds are supposedly due to multipath errors.

jeff


> ;-)
>
> I just got a GPS logger (no live readout, but I am not a speedster
> anyway) and I am wondering what the polling rate of a GPS must be in
> order to get acceptably accurate results. Are there required specs
> posted anywhere - didnt see any on www.gps-speedsurfing.com
>
>


06 Jul 2007 15:21:43
jeff feehan
Re: GPS Speeds



i think that GPS receivers with newer chips - like the SIRF III chip
that garmin is putting in many new models - will have fewer erratic
speeds reported.

the foretrex 201 doesn't have the new chip.

the forerunner 205 does, and it's pretty small - it's a wristwatch - but
it only has 3000 trackpoints.

otherwise, the smallest model with the SIRF III is the new legend HCX,
and a similar Vista model.

jeff