Pretty spectacular photos of it trying to loop!

http://www.sailrocket.com/live/?q=blog/3

It would seem more design work is needed

I've been watching Sailrockets progress for about 18 months now. I must say I'm really happy they have been able to do the speeds they have just done as it has been a labour of love for the owner of the boat and especially so cause he's an Aussie. If you haven't seen their website go check it out. It always makes for a good read and gives a special insight into what goes on in record attempts etc
Just glad that the boat is fairly unscathed from the loop and there is even more potential for it to go faster. For the record, their trimble timing recording showed sustained peaks of above 50 knots. This thing is quick....when it's not looping that is

'tearing an a-hole in the record books' apparently

Love the helmet.......should be orange with a couple of fluro stripes on it

Notice in the movie that they are using a GT-11 or 31 for instant feedback on the runs. (Can't tell which from the video).

Roo said... ...stuff...

This post is rather disingenuous as you just compared apples and trees... I'm not sure why you compared post-processed data from a propriatary processing engine to real-time data from a different propriatary engine -> thats just wrong on so many levels...

Until every GPS satellite is capable of transmitting on the 3 three frequencies available AND when GPS's use all 3 frequencies, the uncertainty in measurment will always be a problem. This applies equally to trackpoint (time-sync) vs. doppler. Interestingly, you correctly point out RF measurment as a key issue - however, until Trimble (and everyone else) release their propriatary algorithms so that they can be publicly reviewed, their system can rightly be criticised as there is no transparency in the quality of the data.

In any case, trackpoint data is no more accurate or inaccurate than doppler, given the BOTH can be post processed - and neither will be accurate until we can measure ionspheric delay, water absorption, etc. in real-time for the signal path actually transversed for each satellite. (If we lived in the Star Trek Universe, that would be different....)

Its a correct statement that doppler should be viewed with skeptisism - as it should for all data sources. However, that shouldn't cause us to completely ignore it as a significantly better data-source, than any other from similar capability devices.

[. The reason I compared the data from the Trimble with the GT-11/31 is because, for the first time, there is a scientific measurement of the error rather than an unfounded hypothesis based on quasi scientific theories that some have tried to prove with statistical analysis rather than measurement.

Not to mention the WSSRC accepts the Trimble unit and not the Navi.

Hi Ya Roo,i just tried the gps experience and i could,nt get your results. The navi varied from .007 to .01 sitting still with no mobile.Didn,t change when the mobile was near,when ringing etc. Can some one else try this experiement. Yes I accept that these are not good enough for wwssrc but, its the best where,ve got to play with at the moment and its great fun.

Hi Roo. Do you actually HAVE the data from both the Trimble unit (post processing) and the Navi unit the sailrocket pilot wore?
If you do. Share it with us to prove your point. If you don't it is pure speculation.
There are any number of explanations for the so called discrepancy that has been described.

boring, too technical for me

boogie, when Paul took off the rotational component would have made a difference between the GPSs, but looking at the video, the fastest part appeared to be Paul doing the pendulum swing... so would expect his arm to have been going faster than anything forward. There was also a significant difference on the previous run if I recall correctly. That could be put down to aliasing. Or a problem with the navi handling accelleration/decelleration. Lets hope he had the antenna tied down properly :)

Andrew and Craig, you might want to try it when moving to be sure that there is no zero speed filter being activated. Who knows whats in that SIRF black box in the latest firmware...

Clearly there is an issue with the algorithm when it comes to accelleration/decelleration thanks to Daffy's alpha detective work... Lets hope this can be resolved in firmware.

It's an interesting design. The video shows that at speed there's very little weight on the lee hull. The angle of the wing/sail has been set up so the pressure of the lee hull on the water is minimal. There are stays under tension to the upper tip of the wing. At speed the lower beam is probably mainly under tension - could almost be replaced by a piece of string. The force balances are very kite-like. I'd call it a kite boat.

The hydrodynamic force is a generated a bit more efficiently than a kite surfer, not needing the levered body weight in the system. The hydrodynamic force seems to be generated by a fin canted to leeward ie. the airfoil is operating in almost parallel opposition to the hydrofoil - very efficient. Righting moments and gravity don't play a big deal in the physics of the thing.

Now I remember, ( also discovered you can add a bit to a post later on using the edit button) the forum came up with something a while ago, Seahorse I think it was, where the idea was to get righting moments out of the system by using opposing hydro and aero foils. Someone else AUSXXX? pointed out that this was an old concept and it had a name. Different implementations of the concept look very different because you can put the driver pod just about anywhere.

This one looks like a goer, but if the hydofoil cavitates the whole thing will leap out of the water. What's the latest on cavitation theories?

Maybe not exactly a kite boat, depends on your definition of a kite. How about " A wing or a sail that flies in the air under the control of strings?" If the lower beam is in tension it might as well be string as far as the physics is concerned.

slowboat said...



Maybe they've inadvertently switched it on. Has been noticed with some GPSs in the past. Besides Locosys dont know whats really happening in the SIRF engine AFAIK.

Why would you think that Chris? Have you been talking to them?

slowboat said...


Biggest concern I have with it is resolving the inconsistencies we are seeing with large accelleration/decelleration. Until thats done the alpha scores will be pretty noisy.

I agree. The issues we are seeing with the GT-31 are really all on the trackpoints side though. This seems to be as a result of the 'predictive smoothing' where they use Doppler values to weight the trackpoint position. We have test firmware that turns this 'smoothing' off which makes the trackpoint and Doppler graphs pretty much mirror image of each other. We need a bit more testing and evaluation of this. It seems much more likely that any acceleration errors will be mostly or all in the trackpoints and that they are not an issue with the Doppler within the limitations of anti-ailiasing mechanisms.

Here is an explanation from Manfred Fu chs on the method of testing and proving speed accuracy using the geostationary method employed so far:

"There are currently 28 operational GPS-satellites orbiting the Earth at a height of 20180 km on 6 different orbital planes. Their orbits are inclined at 55? to the equator, ensuring that at least 4 satellites are in radio communication with any point on the planet. Each satellite orbits the Earth in approximately 12 hours and has four atomic clocks on board.

This means that the satellites travel at a speed of about 13900km/h=7500knots. The Earth rotates once in 24 hours so at the equator the speed is about 1700km/h=900knots. So we are talking about a highly dynamic system. The speed of the GPS unit is determined by the frequency shifts between the different satellites in view (Doppler-effect). These shifts depend on the satellite positions (and thus on the time) and on the latitude (rotation of the Earth). When we are talking about a stationary GPS this unit is of course in no way stationary (only relative to the surface of the Earth, we should better call this quasi-stationary).

When we measure the speed of a quasi-stationary units it moves in reality with really high speeds relative to the satellites. After considering the Earth's rotation we get the relative speed to the rotating Earth. So if we measure at different latitudes or at different times we cover all possible frequency shifts that are much larger than the small additional frequency shifts produced by our rather slow 50knots additional speed relative to the rotating Earth. So measuring 0knots for a quasi-stationary unit over a longer period of time or at different latitudes is a proof of accuracy for all speeds covered up to the bandwidth of the RF-receiver which must be much higher than 50knots in order to determine the large relative speeds to the satellites.

A quasi-stationary unit thus gives us easy access to all constant speeds we are interested in without the need to take care of aliasing effects which are due to inadequate bandwidths or inadequate sampling rates of the units and show up when measuring non-constant speeds with non-synchronized units. The GT-11 for example has a bandwidth of about 2Hz which would required a sampling rate of at least 4Hz (Nyquist theorem) to be able to interpolate between the sampled points and thus integrate of longer periods like 10seconds or 500m. If the bandwidth of the unit is too high the measured speeds represent instantaneous speeds only and the interpolation between them is not possible. If the bandwidth is adequate, the sampled speed over time curve is completely determined and interpolation and integration is possible with high accuracy."


Specific data on the relative accuracy of the three main types of GPS used in recreational speed sailing are on Manfred's website here:

http://www.gps-speed.com/gpsaccuracy.html

Note that the GT-31 gives accuracy to 0.05 knots or better with 99.5% confidence and better than 0.07 knots with 99.9% confidence. (20 second runs over 20 knots)

Compare this with Garmin trackpoint devices where the same 99.9% confidence level is at about 1.5 knots!!!!

Note also that the data shows a very significant improvement in the GT-31 Doppler over the GT-11 Doppler.

If we were seeing the sort of errors that Kean claims are possible we would not get figures anywhere near this good.


Edit: Manfred tells me the number of satellites was 28 in 2002, today there may be 31 sats.