GPS Aqua pack location

Hi NotWal, I got the Walkie Talkie Pac, which I think might be the same width
When I got my Navi, it was all that was available in the deep north, so I grabed it and hit the water.
It is a fairly tight fit, but no real problem, just slide it in. To get her out, I blow into the end to unstick it, then push her out.
Playing with the toggle after it's in is a pain, but hasn't caused any grief once set. I do plan to get a nylon washer though, see if that helps.
Hope this helps, I know it's not what you asked

Thanks for that Roo, I'm a paranoid old b*stard, so I'll keep an eye on it.

I've seen another Navie get wet and die (replaced under warranty), so keep them dry, no matter what the specs say

quote:

---

Originally posted by sick_em_rex

actually Mathew, the unit is totally unobstructed by any part of my body. I can easily look down at the unit as I'm sailing to see the screen. It is effectively at waist height and due to the natural leaning back stance of windsurfing the unit is always face up to the sky and pulling in all 4 satellites 100% of the time. I have never had any loss of signal and rarely had a spike when wiping out so I think I'm on a good thing. But you're welcome to your point of view

---

In most modern GPS devices, they can lock up to 12 satellites at a time; for most locations on the water you probably get 6 or more at any given time (although occasionally the orbits can coincide to get less than this). In theory it is possible find your location using only 3 satellites, however reality kicks in and requires that you should never trust data that is only locked onto 4 satellites, as the error rate is far too high to be useful -> you really need to lock onto at least 5. You may not "loose signal" with less than 5, but your positional error and doppler-speed error increase dramatically. This is why both RealSpeed and GPSResults will mark a track-point as invalid if there are less than 5 satellites locked.

I would be surprised if your body didn't obstruct in some way as the GPS units will lock onto a satellite which is quite close to the horizon. Given the GPS is at your waist/stomach, your upper body will be blocking some part of the field of view.

My understanding is that the Garmin units will "lock" onto any satellite it can see BUT will only use 4 for calculating the speed. If you look at the NMEA data put out by the Foretrex it gives all the satelittle data of all the satellites it can see. Unfortunately, the strings are so long with all this unused data that the Garmin can only update every 2 seconds as it is restricted to 4800 baud in NMEA out mode. Still if you hook your Foretrex up to a PDA you can improve it's accuracy by 10 times. Not very practical though and cheaper to get a Navi/GT11.

Anyway , back to the location thing. It is possible to get flattering results from the Garmin unit when worn on the body. The body will block some signals. If you lean back in a gust you may pick up a different satellite which the garmin unit can get more accurate data from (depends on their position above the horizon)and so it may change one of the satellites used for the calcs hence giving you a jump in location. Unfortunately the Garmin seems to have a track smoothing filter so it will not be apparant that there was a jump (ie. a spike) as the unit averages the speed over several seconds. I used to wear my gps on my chest but got some smooth tracks suggesting 40 when I knew I was doing no more than 35. Others have noticed this also. I am not sure if the Navi/GT11 has something similar but I don't think so.

Since using the upper forearm I never loose a signal and never have spikes or flattering peaks.(unless fully earned)

Yo

Some info...

Sail blocking: at the frequency(s) used by the GPS system 1.2/1.5 GHz, the sail is essentially transparent (not quite but close enough), so sail blocking isn't so important. Similar effect for mast/boom.

Garmin: depending on the Garmin used as to what it does, but yes Foretrex does what yoyo said; also Garmin are quite secretive about the algorithms in use, so most of we know is from testing/speculation. But importantly for the Foretrex data-logging, it has what has become known as the "grid effect". Since the data storage capacity is quite low, Garmin has traded storage space vs. precision. The effect is that we loose the last decimal place or two, thus loosing accuracy.

12 satellite lock: I should clarify what I wrote... The term *lock* is a bit of a misnomer - most modern units will *track* 12 satellites using multiple receivers or multiplexing a single receiver (ie: if the US put more satellites up so that 13 come into view, the unit would track up to 12 of them), then *use* a number of them for its calculations. I have read that some Garmins use 6, but I cant remember what the Foretrex used specifically. The Navi/GT-11 units use as many satellites as meets a minimum criteria (what that criteria is some of us are still trying to find out... **).

Smoothing: One effect that yoyo mentioned was the filtering/smoothing effect. If you take a Foretrex, get a good lock and go for a walk for about 20m then cover the unit with a tin-can, turn around that walk back, then uncover the unit; do this a few times. Now download the data, you will see that there are "spikes" in your path where you managed to walk *extremely* fast... The unit is applying a dead-reckoning algorithm to the data-logging; which for our speed-sailing purposes is generally agreed to be a bad thing.

Wrist/shoulder mounting: some people have suggested mounting the GPS on a helmet so that it gives the best possible sky-view. But I'm not convinced of that (yet), as I have tracks where I have locked onto 11 satellites (wrist mounted). Yoyo suggested adding some velcro to the wetsuit -> this is a great idea; maybe the wetsuit manufacturers would do this as another selling point (watch mounting, etc). Mounting it anywhere is fine, provided it can track/use a sufficient number of satellites, ie: that could mean the spreader bar is fine.

** Just to let everyone know if you don't already, there are a group working in the background, trying to figure out how to increase the accuracy of GPS measurement of our speeds. You may have heard some mention of "GPS-Doppler" or "Doppler speed". The manufacturers of the GT-11 have accepted some suggestions and now log NMEA and binary data to the SD cards. We/they are also looking at the next generation devices to allow for logging of absolute error. The idea of these suggestions is that we are aiming for precision on the order of cm/s or mm/s -> that way, when someone breaks 40/50 knots while wearing their GPS, they know that it is a "world class" result.

whew, hope that explains a bit more...

quote:

---

Originally posted by sick_em_rex

It certainly seems like when my Foretrex bites the dust, as it will, then I'll be buying a Navi.

---

I use both... I much prefer to use the Foretrex while on the water -> it is easier to use, the screen is bigger, you can show multiple readouts (I use speed, max-speed, time, distance). I primarily use the GT11 only for accuracy and data-logging.

And for me since I am lucky enough to live close to Sandy Point, I like having a backup unit just in case the batteries go flat, or one of them dies.

quote:

---

Originally posted by Roo

I developed the accuracy level parameters for the NAVi last year in an attempt to increase the reliability and accuracy of the speed readings we could access through the logged NMEA data. The requirement for 5 satellites was based on the accepted levels in the surveying industry using high end dual frequency receivers. Macquarie Innovation use the same parameters for their record attempts. The other part of the equation was a HDOP less than 4, this is an error level that is saved in the NMEA data. Using this information the data could be verified and the level of accuracy of the speed readings confirmed.

---

These values are great for most sailors as they are based on real-world experience. Those two metrics indicate when the positional error is increasing beyond a reasonable limit --> 5 and 4 are good for windsurfing. It is also good when mates compare their runs as they can compare high-quality data.

==> You did good work with this and pretty much got the ball rolling with the requirement to increase GPS accuracy to a high standard.

For the more anal sailors, these may be not enough. The main problem with using those values only, which wont concern 99.9% of sailors, is that it assumes that that the on-board GPS algorithms are openly available, so that a third-party can very the integrity of the algorithm and to verify if the implementation is correct, ie: only logging those values doesn't allow the third-party to do their own calculation. It means that instead of logging the actual error, we are logging the apparent effect of the error (but as I said, this only matters to the top 0.1% of sailors).