Dead Toys and Lots of Noise

This is a two-part post. If you're a geek, keep reading to the end. If you just want an update about how things were recently, by all means stop after the first part.

Last Friday had looked so promising in the forecast: wind all day, with mid-20s in the afternoon! Sunny and warm! But it started badly enough, by giving me the finger - a blue finger:

I am still thinking I should sue Chinook for failing to put a warning label on their booms that reads "Remove fingers before closing". Fortunately, it's been too windy to find a lawyer. There's no fun to be had in court rooms!

Briefly, here are the other things that happened:

• My GoPro broke. Dead. Kaputt. 
• When the chop and crowds became unpleasant in front, I sailed to Lewis Bay on my Skate 110/Idol 5.6 combo, and loved the flat water I found. Came back to switch to slalom gear and drag Chris along. Had an absolutely crazy, overpowered, out-of-control ride through nasty chop to make it back to Egg Island - the wind had picked up to averages above 30. Chris promptly declared Lewis Bay as "absolutely unsailable" and sailed back, leaving me alone. Considering that I had barely made it there, I had doubts that I'd make it back, so I stayed. At least there were no crowds. But there was the security guard now posted at Egg Island that makes sure nobody but birds set foot there. 
• My Android phone that I use to announce my speeds through GPSLogit got wet and broke. This was the first time I had not double-bagged it, and also the first time I did not close the waterproof bag properly. It's terminally dead. Drying did not revive it. 

So, here's a pop quiz: what do I call a day of sailing like that? Think about it for a minute. I'll even post a picture to create some distance from the answer.

The answer is (of course): A great day of sailing!

In the morning, I had a few hours of freestyle on what I count as a small sail. I even made one or two wet Upwind 360s - the "Andy Brandt" variety, where you leave the sail forward when you carve, and backwind before loosing all speed. Still had to waterstart out of it, but getting closer.

In the afternoon, I was eventually joined at Egg Island by my lovely wife, who was nicely powered on 3.7. That made me feel a bit better about thinking that my 5.8 was a tad big, especially since the wind had dropped by then. I watched here hack away at the Flaka, with a few good tries. She now has tried Flakas on sails from 3.7 to 5.6 m .. impressive. I still think she'll get her first Flaka before the Cape Cod ABK camp in September.

I did not try anything, but practicing slalom sailing was fun, and I later got some perfectly flat spots for full-speed jibes - so cool! I ended the day with a final session on my 3S 96 with my Idol 5.0 in front, bumping and jumping. 125 kilometers of fun! Who would complain about a blue thumb and a couple of broken toys?

But I wanted to replace the GPSLogit phone, and picked up a Samsung Galaxy J1 for $30 at the local Best Buy. First order of business was checking how well it would work in comparison to the "standard" GPS units.

Warning: here begins the geeky part of the post. Continue reading at your own risk!

So I taped the phone and two Locosys GPS units to the dash board of our Nissan NV 2500 high roof van, and took the babies for a ride. The GPSLogit speech announcement worked as expected, and the speeds always agreed well with the speedometer. I even stopped in the middle to switch the GW-52 from 5 Hz recording to 1 Hz recording, just to see what happened.

Let's start by comparing the phone data with the GT-31 data. Here's a part of the doppler speed graph (click on the image for a large view):

GT31 (blue) vs. Samsung Galaxy J1 with GPSLogit (red)

The first impression was "pretty good", but a few small spikes are visible in the phone data. Here's an enlarged section:

The red phone data are a bit noisier, and have a few spikes that may overstate the speed by a knot or two. Indeed, the speed results in the GPS Team Challenge categories showed that the phone speeds were a bit higher than the GT-31 and GW-52 speeds (up to about 1/2 knot) - something that was seen before for other phones. But while the accuracy is not good enough for posting to the GPS Team Challenge, it's definitely good enough to hear how fast you are going while you're sailing. Great!

When I compared speed results from the GT-31 and the GW-52, the differences were minimal, as expected. However, there was a very noticeable difference in the 5 Hz data from the GW-52, compared the the 1 Hz data from the GT-31:

1 Hz GT-31 data (blue) compared to 5 Hz GW-52 data (red)

The GW-52 were collected every 0.2 seconds, so we have five times as many data points as for the GT-31 data which were collected every full second. The GW-52 data have a lot more spikes - little ups and downs. That had been reported by others before for windsurfing data, and the question arose whether these spikes are real differences in speed, or noise that was caused by the higher measurement rates. We'll get back to that question in a minute, but let's look at a comparison when the GW-52 unit is set to 1 Hz:

1 Hz GPS data from GT-31 (blue) and GW-52 (red) 

The data look very similar. 

So, let's have a closer look at the 5 Hz data. There are valid arguments that can be made why collecting data at a higher frequency is better. If (a) the errors in the speed measurement are purely random, and (b) the error per data point is the same at 1 Hz and at 5 Hz, then measuring at 5 Hz would give about 2.2-fold higher accuracy. But this is only true if both of these conditions are met.

Let's look at the data points (I used GPS Action Replay's "Trackpoint Table" to create the screen shots):

GW52 "raw" 5 Hz data from a small subregion

For comparison, here at GT-31 data from the same region:

GT-31 data (1 Hz)

A picture says more than 100 words, so here is the doppler speed graph for this region:

1 Hz GT-31data (blue) and 5 Hz GW-52 data (red)

When we seen spikes like in the red curve about in windsurf data, we may believe that they are real: we constantly hit chop, gusts, and lulls, and it seems quite possible that our speed changes several times within a second. But the data above are not from windsurfing, but from driving a big, heavy van.  The Nissan NV2500 High Roof van is quite a beast. It weighs in at about 3 tons (6250 lb), and we've loaded it up with interior racks, 6 boards, 15 sails, masts, booms, etc. Does it change speeds several times a second like the GW-52 data indicate? Allow me to cite from The Princess Bride:

"Inconceivable!"

We can get a bit more formal in analyzing this. We can start by looking at the acceleration - the change in speed from data point to data point. The acceleration is shown in the tables above as "Linear m/s2". In the GT-31 data, the values change little from data point to data point; but in the GW-52 data, the acceleration jumps from 0.4 to 2.9 in 200 milliseconds! That's almost no acceleration to 1/3 g. I'm sorry, but I don't drive that crazy!

The change in acceleration is actually a pretty good indicator of noise in the data. It's pretty easy to calculate, and if you're mathematically inclined, you can call it the second derivative (the acceleration being the first derivative of the speed). Here's a graph of the acceleration and noise for GW-52 data:

For comparison, here is what this looks like for the GT-31 data:

That's a lot less noise in the G-31 data! The 1 Hz data do reflect reality better: I accelerated somewhat evenly over about 15 seconds, kept the speed more or less constant for about 1/2 minute, and then slowed down constantly.

This is some rather clear evidence that collecting GPS data at higher frequencies does indeed introduce addition measurement error, which negates any potential advantages of the additional data points at least partially. More data is just that - more data. It's not automatically better data. Nor does a lower error number that some software spits out necessarily mean that the data are indeed more accurate - if basic underlying assumptions are invalid, then the error numbers will also be invalid.
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Some of my readers may wonder: "Who are you that you think you can analyze this, and go against what everyone else says?". Well, that is a valid question. I admit that windsurfing is my primary addiction, but I have some scientific background. I got a M.S. in Biophysical Chemistry, a Ph.D. in Experimental Sciences, and have worked in scientific data analysis and software development for Bioinformatics for last 3 decades. I certainly enjoy developing algorithms for large-scale (and small-scale) data analysis... especially when it's not windy. 

Kalmus in NE

The picture above shows Kalmus on July 3rd. At times, there were close to 30 windsurfers on the water at the same time. Over the course of the day, more than 50 windsurfers sailed. Almost like old times.

Yesterday had good wind, too, but nobody sailed. Yes, it was Friday, instead of the middle of a long weekend. Yes, it was cloudy instead of sunny, and air temperatures where in the mid-60s instead of the mid-70s. But the bigger difference was the wind direction: north to northeast instead of the usual southwest. That's straight offshore at Kalmus. Even the iWindsurf meter readings looked gusty:

Lulls below 10, gusts in the mid-20s? That sounds like the Gorge, not like Kalmus!

But we know that wind meters can be deceitful. No way it would really be that bad! But gusts near 25 - there's a reason to go windsurfing! So we went.

The beach was almost empty - two or three vacationers braved the "cold", and no life guard was to be seen when we checked out the wind. Not many white caps to be seen, either, but my wind meter showed averages of 17, and gusts above 20 - good enough. By the time we had rigged, the life guards had shown up, and informed us that we absolutely could not launch on the Lewis Bay side. It did not matter that there were about 800 feet of completely empty beach next to the swimming area - "windsurfers must launch in front".  So we did, and sailed over to Egg Island. Here are my GPS tracks:

You may notice that I was planing the entire time. The lulls in Lewis Bay were nowhere near as bad as near the wind meter. The wind was a bit up and down, from just powered to slightly overpowered on my 6.5 - better than I had expected, and plenty of fun. The chop was small, mostly orderly, without any of the voodoo chop Kalmus is famous for.  The tide was high, so the inner sandbar was submerged completely, allowing runs that were the same length as typical runs in front of Kalmus beach. I played around with a few freestyle tricks, only to discover that the smaller (5.6) freestyle sails I have been able to use recently spoiled me a bit too much to enjoy wrestling the 6.5. Nor did it help that the chop was just big enough to keep the speed down a bit, and that gusts seemed to hit me every time I wanted to oversheet the sail. I kept thinking that slalom gear would have been the better choice for the conditions ... next time!

This weekend will see cloudy and colder weather with a chance of rain, but not much wind. But the forecast currently predicts a couple of nice southwest days for the middle of the week - see you on the water!

Voices In My Head

Maybe I windsurf too much. Today was the 4th day in a row. Maybe that's nothing for Corpus Christi or the Gorge, but for Cape Cod, that's a lot. So today, I had a little voice in my head that told me how fast I was going. It only made me want to go faster! It worked, too - I finally broke the 60 km/h barrier (32.4 knots, 37.3 mph).

Meter readings today showed averages around 29 mph, with gusts up to 36 mph. There was no sand flying, though, making the readings appear a bit high. Nina wanted to go to Egg Island to work on Flakas using a 4.0 m sail, so I took my 90 l slalom board and 5.8 m speed sail and tried to go fast. The voice in my head? That was GPSLogit. Very useful. Just like Roo said.

Conditions were near-perfect for speed runs (and Flaka tries, says Nina). I heard "32" (knots) several times, which is faster than I had ever sailed before. 33 seemed so close.. but I later discovered that I switched from top speed runs to nautical mile runs just as the wind got strongest (and then dropped quickly). But my sail was probably too small for "real" speed, anyway: going back upwind was easy. That means I did not have much power in the downwind speed runs. Oh well. I'm happy with a new 2 second personal best. And it was fun! Yes, fun.

The way back was interesting. The wind had dropped, which seems to be the rule for Egg Island sessions. Without enough wind, the 90 l board becomes a submarine. Not a problem - if you don't want wet feet and knees, you should not windsurf. I opted for the easy way home, aiming for the Lewis Bay side beach at Kalmus. I even got a lucky gust that let me plane through the ferry lane. Submarine schlogging practice was fun, too. When I hit the wind shadow near the yacht club, I was almost on shore, and the swim was short. As warm as the water is, that was fun, too.

Yesterday?  Fun on a 6.5 in south wind - big, orderly swell, than flat. Fog to keep us cool. Fun.

2 days ago? Another Egg Island/Lewis Bay session, again on 6.5 (Nina on 5.0). Sailed 135 km, jibed 150 times. Fun!

3 days ago? SSE wind at West Dennis, high 20s, low tide. Fun waves outside, nice and flat inside. Nina on 4.0, I used a 5.0 Idol. Found that it worked well with the slalom board. Fun!

But today was the best day - lots of wind, lots of speed, lots of sun. And for the first time, my ankle did not hurt at all for the entire session. That means I have run out of excuses not to do freestyle. Bummer! Not.

Nina is following Brendon's advice and is hacking away at the Flaka, and nothing else. Sure, she'll throw in a Clew First Push Tack or a 360 every now and then, but those don't count because she can do them already. She's making slow but steady progress. Could happen any day now!

I'll leave you with a picture of today's GPS tracks:

Twisted

After many days of good forecasts that did not happen, the wind finally came yesterday. It had been windy all night long, and meter readings early in the morning were around 25. When we got to the beach, they were 23. When we got onto the water at around 10 am, they dropped to 20. Still enough to have fun, after a bit of re-rigging and switching boards.

Martin was smarter. He was at the beach at 8 am. When I started re-learning my Carve 360s, he showed me how it's done, carving through one in style, with the sail all the way down to the water. Thanks, Marty! I got a few good ones right after that. Seeing how it's done helps. A lot.

Then I saw Marty go for loop crashes. I asked him about it, and he said he was working on the sail ride part. So I figured I'd show him a Gecko Loop. That's a non-planing trick where you can practice pushing yourself up on the boom to ride the sail. I had just learned it at the ABK camp in Hatteras a couple of weeks ago.

Unfortunately, I had forgotten that (a) I had learned the move in about 10-12 mph wind, and (b) when the wind picked up to 15-18, my tries got very bad, so that I quickly stopped. Here are a couple of pictures from these later tries:

In this try, I at least got the nose into the water, although not quite enough. Otherwise, the board is too flat - it should be tilted to the leeward edge is in the water, and the windward edge up. My back foot came out of the strap.

In this try, the board is even flatter - I did not even get the nose to go down. Fortunately, both of my feet got ripped out of the straps. This is somewhat similar to many bad loop tries I have done and seen where the timing is off.

When I tried to show Marty the Gecko Loop yesterday, I combined the worst elements of the two pictures above. I kept the board way to flat as in the second picture; but I also had my front foot nice and tight in the foot strap, so that it did not come out. That twisted the ankle in ways it was not supposed to be twisted. Quite a few tendons on the inside screamed at me in protest.

I took a little break to see if they would calm down, and then went back out for a trial run. But every little bump on the water sent little pain signals from my ankle, so I had to call it a day. Too bad, since the 25 mph wind returned a few hours later, and it was sunny and warm. But at least we were early enough to get a seat at one of our favorite places on Main Street, and were pleasantly surprised that the new cook at Gringo's can make some decent veggie tacos. So overall, I'll call the day a win - several good things (wind, sun, friends on the water, and decent food) beat the one bad thing. Seems I was lucky, anyway, and did not break or tear anything: a day later, I can walk (mostly) without pain, so this seems to be just a little strain that should go away in a few days. Maybe even in time for more wind on Tuesday...