Wednesday, April 7, 2021

The Milk Jug Experiment

 My last post ended with this graph:

Before I tell you what I did to generate it, let's first look at the "why". Here's a short section from a recent windsurfing session that illustrates the problem:
I compared three different GPS units in this test. At the highlighted section, the three GPS units disagree from each other by roughly 0.5 knots for a few points in a row. Areas like this are easy to find in many GPS traces - but what causes them? The GPS units can be much more accurate that this, as stationary tests show, where typical errors are about 10-fold lower. 

One potential cause of the larger errors are short-term changes in the satellite signals received by the GPS; specifically, changes in the signal and/or noise for one or more satellites, and in which satellites the GPS uses to compute speed and position. So the experiment was to induce controlled changes, without moving the GPS, and see what effect they had on the observed error (the speed, since the GPS was stationary) and the error estimates given by the GPS.

To disturb the signal, I took a one gallon milk jug and filled it with water. I then moved it around the GPS and on top of the GPS, keeping a distance of at least a few centimeters, for about 30 seconds. I did that twice, with 30 second control periods where I kept the jug away before, in between, and after. The periods where I moved the jug around the GPS are highlighted in the first graph.

The speeds that were reported by the GPS because of the distorted satellite signal were around 0.5 knots multiple times, and near 0.8-0.9 knots a few times.  I was a little surprised to see such "speeds" just because the signal was partially blocked - after all, the GPS should still have been able to get a perfectly clean signal from most of the 20-25 satellites it was tracking. But apparently, having just a few signals removed or distorted is sufficient to cause errors in the range that we can often see in windsurf tracks.

Now I don't usually windsurf closely to people waving milk jugs, but it's just an example of a sudden change in the satellite signal cause by external factors. During windsurfing, that could be something as simple as changing the position of the arm that the GPS is on, or the body position, so that a different part of the sky is blocked by the arm or the body. The more things move around, the more likely that is to happen - and if you don't have the luxury of windsurfing at a flat spot like Lake George or La Franqui, chop might just do the moving for you.

In a similar experiment, I looked at what happened when moving the GPS up and down rapidly, and the results looked similar, even when looking at directional speeds (velN and velE). But my lovely wife pointed out that I could not really be sure that I was moving the GPS straight up and down, without side-to-side movement, so this experiment would need to be repeated in a more controlled fashion. For now, I'll just say that it is possible that rapid up-down movements of the GPS, which are typical when sailing in small to medium chop, might also have an effect on GPS accuracy.

One interesting question arises when comparing the results from different GPS units that are very close together, and technically identical (or very similar). They both should "see" the same distorted signal, so why would the not compute exactly the same result?

Rather than answering this question directly, or speculating about why this may not be the case, I'll show a graph from a stationary experiment where I had two identical units positioned very close to each other, with a clear view of the sky:
One interesting result shown in the table above is in the "sats" column, which shows how many satellites each unit tracked. Since the units were very close to each other and technically identical, it is reasonable to expect that they would use exactly the same satellites. But at the start of the highlighted region, one GPS used 17 satellites, while the other used 20 - that's a pretty substantial difference! Here is a graph of the satellites tracked by the two Motion units over a time of about 1000 seconds (starting a few minutes into the test, after both units showed the same number of satellites):
For large parts of the tests, the two GPS units differed in how many satellites they used - which means that blockage or distortions of one or a few satellites could affect the two units differently, thus possibly leading to comparatively large discrepancies in the results.

How does all this relate to speedsurfing, you might ask? The blocking experiment is an example of non-random noise in the data. If you look back at the first graph, you may notice that the error estimates increase much less than the speeds. This means that there are multiple regions where the error estimates are significantly lower than the observed errors for more than 10 points in a row - here is an example:

Statistically, we would expect to see some points where the speed is higher than the error estimates, but we should practically never see this many points where the error is larger than the error estimate in a row - if the error is random. But if the error is not random, then we can not use Gaussian error propagation, which makes collecting at high data rates entirely pointless.


Sunday, April 4, 2021

GPS Noise and Data Rates

I'm noise sensitive, so perhaps it is quite fitting that I spent some time looking at "noise" in GPS units, and the relation between noise and data rates. Between the relatively cold water around Cape Cod and Cape Cod's dubious distinction to be the hot spot for the Brazilian P.1 variant of the COVID virus, on-the-water testing will have to wait a while. So the tests I report here are all stationary tests.

One big advantage about stationary tests is that we know exactly what speed the GPS should report: 0.00. Everything else is an error. As Tom Chalko did with Locosys GPS units many years ago, we can use this to learn about the error characteristics of GPS chips. There's one little caveat about the directionality of speed in speedsurfing, and the non-directionality of measured speed when the actual speed is 0, but we can ignore this for now.

For this series of tests, I am using a Sparkfun Neo M9N GPS chip with an active ceramic antenna (25 x 25 x 2 mm from Geekstory on Amazon). I'm logging either via USB to the a Surface computer, or with the Openlog Artemis I described in my last post. Compared to the M9 chip with the onboard chip antenna I used before, the current combo gets a lot better reception.

Let's start with an utterly boring test, where the GPS had a perfect, unobstructed view of the sky on a sunny day (the GPS was on top of the roof rack on our high roof Nissan NV van):

At the beginning, there's a little bit of speed when I switched the GPS on, and moved it to the top of the van. I had used the GPS just a few minutes earlier, so this was a "hot start" where the GPS very quickly found more than 20 satellites. After that, it recorded a speed close to zero (the graph on top), with an error estimate around 0.3 knots (the lower graph).

Let's switch to a more interesting example. The next test was done inside, over a period of almost 2 hours. The GPS was positioned right next to a wide glass door, so it had a clear view in one direction. Here's the graph:

At three different times, the GPS recorded speeds of more than 1 knot, even though it was not moving at all. With a typical estimated accuracy of about +/- 0.5 knots for each point, that number is actually a bit lower than expected. But what raises some red flags is that each point with a speed above one knot is closely surrounded by several other points that are also much higher than average. This is reflected in the top speeds averaged over 2 and 10 seconds: 0.479 knots and 0.289 knots. In fact, all top 5 speeds over 10 seconds are near or above 0.2 knots.

Let's look at one more test run - this one done at 8 Hz overnight, for about 10 hours:
The GPS was at exactly the same spot for this test. The overall results is similar, with a bunch of spikes between 0.5 and 0.9 knots. The top results for 2 second and 10 second averages are a bit lower, but we still see a couple of 10 second "speeds" of 0.2 knots.

Now wait a minute - I just said that the recording at the 3-fold lower data rate that covered a 5-fold longer observation period had lower observed errors. That is exactly the opposite of what we would expect! At first glance, the errors appear random, or at least mostly random. For random errors, statistic tells us that if we measure more often, the measured error will go down. Going from 25 samples per second down to 8 samples per second should reduce the observed error by about 77% - but that's not what we see!

The explanation for what we see is that the error is not entirely random - in fact, it has a substantial non-random element. That's quite obvious in the 25 Hz graph, where we can see a wave pattern in the error estimates, and clusterings of high speed points when the "error wave" is higher.

To have a closer look at the (non-)randomness of the observed errors, I copied the data from GPS Speedreader into a spreadsheet program, and then shuffled the observed speeds randomly. Next, I looked at all 2 second periods in the shuffled data, and compared the results to the results from the original data set. With completely random data, shuffling should not have an effect - we'd see the same top speeds. But if the original reported speeds (= errors) were not randomly distributed, we should see a difference. Here's a screen shot that shows the results for a test recorded at 18 Hz:


The average 2 second speed is 0.062 knots for the original and the 4 different shuffle tests, as expected. But in the shuffled sets, the maximum observed 2-second speed was between 0.093 and 0.097 knots - more than 3-times lower than in the original data set. Essentially, this proves that the error in the data set was not randomly distributed.

For comparison, here is the same analysis for a test recorded at 5 Hz:

For the 5 Hz data, we also observe a difference, but it is much smaller: the original data had a 2-second maximum that was about 50% higher than the shuffled data sets.

In my next post, I'll look into what's behind the non-randomness of the error. However, I'll leave you with a little puzzle and show the results of one of the tests I did:

Can you figure out what I did here? Here are a few hints: it involved a gallon plastic jug filled with water, and the numbers 2, 3, 5, and 30. Good luck, Sherlock!

Sunday, March 21, 2021

Olapap: Why We Need Ceramic Antennas

 This is a post about the Olapap GPS, a "plug and play" GPS logger put together with the Openlog Artemis (OLA), u-blox GPS chips, and a couple of cables. I'm sharing some windsurf test results that I have obtained with the "SparkFun GPS Breakout - NEO-M9N, Chip Antenna" board. This board has a powerful GPS chip (the M9), but a GPS antenna that is quite weak - similar to antennas used in phones.

I compared the "Olapa M9 chip" GPS to three other GPS devices that have been approved for use in the GPS Team Challenge:

  1. A Motion GPS (the original one with a screen), worn on my upper right arm.
  2. An Openlog prototype that uses the Beitian BN880 chip, worn on top of my helmet in a GoPro housing.
  3. Another Openlog Prototype that uses the (discontinued) Beitian BN280 chip, worn in a waterproof armband on my left arm

The Olapap GPS was in the same armband as the BN280 GPS, slightly above it. Both of these prototypes were in separate ziplock bags, in case the armband developed a leak. 

Here's a screenshot from a GPS Speedreader comparison of the top speeds for the 4 units:

If you look at the top row that shows the fastest speed over 2 seconds, you see that the three approved GPS devices all show 31.3 knots; they differ only in the second digit after the period. But the Olapap M9 chip GPS (in the last set of columns) shows a top speed of only 30.487 knots - almost a knot slower than the others! Furthermore, the M9 chip GPS also reports the highest error estimates in the "+/-" column: 0.85 knots, compared to 0.139 knots to 0.239 knots for the other units.

Let's have a look at the speed graph here:
The curves for the three approved devices (in blue, red, and green) are all close together, with just small point-to-point variations that look random. But curve for the M9 chip GPS (in magenta) looks quite different, sometimes being lower and at other times being higher than the approved GPS units.

When we look at the data points for this section, we can identify a likely cause of the problem:


The columns with the rectangles show the number of satellites that the GPS units used: about 18 for the Motion, 24 for the GPS on top of the helmet, 20 for the BN280, and only 12 for the M9 chip GPS (which was very close to the BN280, in the same armband).

The differences between the first 3 GPS units are primarily due to the GPS position: the unit on the head had the clearest view of the sky, and was able to use 4-6 more satellites than the units worn on the arm. The lower number of satellites for the M9 chip unit is due to the smaller, weaker antenna. I did a number of tests where I looked at the signal-to-noise ratios for the satellites in the Ucenter software, and the values for the M9 chip antenna were always significantly worse than for other GPS units. No real surprise here - it is common knowledge that larger ceramic antennas provide superior GPS reception.

The end result of this test is that the NEO-M9 board with the chip antenna does not provide data that are of adequate quality for speedsurfing. Since there is no easy way to replace the antenna, I returned the chip to Amazon. I have started looking at the Sparkfun SAM-M8 chip that has an older (M8) GPS chip, but a larger and more sensitive antenna, and will probably add other M9 chips with external antennas in the future. The initial M8 results look promising, but more test sessions on the water are needed. I'll post results here when I get them.


Saturday, March 20, 2021

The Olapap GPS


 This is a geeky post about making a GPS. I think it might be of interest to about five windsurfers in the world. If you're not one of them, I suggest you find something else to read. If you are not curious about what the picture shows, you are definitely not one of them!

Why make another GPS?

That's a good question. The short answer is that it is very hard to get a good GPS for speedsurfing competition - specifically, for the GPS Team Challenge (GPSTC). There are currently two GPS devices you can buy that are approved for the GPSTC. The first one is a watch: the Locosys GW60. Unfortunately, it has a few annoying habits, like wrist bands that just fall apart without any warning; buttons that you should NEVER press when the watch or your fingers are wet; and batteries that die if you don't use the watch for a while, and that require advanced soldering skills of you want to replace them. 

The second one is the "Motion Mini" logger. It's a fantastic little device, but very hard to get. Some people have waited many months to get their order, which is hand-assembled by one guy who can't keep up with the demand. He's also loosing money on his quest to provide a great GPS, so we don't know how long he'll be able to keep making the Mini Motions.

The question about alternatives comes up on a very regular basis on the Australian speedsurfing forum. Unfortunately, many GPS watches out there work well enough most of the time, but don't provide any accuracy estimates that allow software to automatically identify artifacts. Within the context of a competition where about 500 results determine the ranking every month, that's not good enough.

I have written about making a simple little GPS logger with a u-blox GPS chip, an Openlog datalogger, and a couple of other bits and pieces in the past. While this thing was easy enough to build, and is at least as accurate as "approved" devices, it requires some soldering - which means most windsurfers won't even consider making one. But what if we can make a good GPS buy just buying parts and putting them together with a cable or two? That seems to be possible now!

Olapap GPS? Really?

Well, I had to give the thing a name, right? After many hours of searching copyright databases (just kidding!), I based the name on a critical component and the basic idea behind it. "Ola" stands for "OpenLog Artemis", the data logger part of the device. "PAP" stands for "Plug And Play" - no soldering! 

Parts needed

The Olapap GPS logger requires exactly three parts and one cable.

1. The Openlog Artemis from Sparkfun ($50). This is a nifty little data logger, nicknamed "OLA",  that can write to a micro SD card. It's similar to the Openlog I have used in my previous prototypes, but with two major difference: a much more powerful processor, and a "Quiic" connector to hook up a GPS chip. The Artemis processor on the chip is much faster than the one used by the old Openlog, and has several hundred times more memory. No more trying to optimize every single byte of memory and every CPU cycle! Sparkfun show the device on backorder right now, but you can probably find one on Amazon or your favorite electronics supplier.

2. A u-blox GPS chip with a Quiic connector from Sparkfun ($40 - $70). The u-blox chips are the only generally available GPS chips that (a) use Doppler for speed calculations, and (b) provide estimates of the speed accuracy based on the quality of the GPS signals received. We need one with the Quiic connector so that we can hook it up to the OLA, and Sparkfun offers a bunch of different types. So far, I've used two of them:

- The SAM-M8Q board ($40). This is an older (8th) generation GPS chip with a ceramic antenna that seems adequate. It is what I am currently using.

- The NEO-M9N board with a chip antenna ($70). This is a newer (9th) generation GPS chip that offers a higher data rate and use of the Chinese BeiDou satellite system, two functions that could lead to more accurate speed data. However, the chip antenna on this board is very small and inferior to the ceramic antennas that are typically used. In my tests, the reception quality of the antenna was so poor that the speed data were inaccurate. If you want to use an M9 chip, I strongly suggest that you get a NEO-M9 board with an antenna connector instead - either the Sparkfun board with the U.FL connector (which is a bit tricky to use!) or the Sparkfun board with the larger SMA connector

3. A battery. I am using a 1 Ah Lithium Ion battery from Sparkfun, but a smaller or larger battery should also work. If you order a battery from a different supplier, make sure that is has the correct polarity - some have the cables connected the other way around, which will kill your electronics!

4. A Quiic cable to connect the OLA and the GPS chip. I got a set of cables from Amazon.

5. A micro SD card (32 GB or less). Make sure to get a class 10 or better card!

6. A USB type C cable.

Getting started

Connect the pieces together (check the hookup guide at the Sparkfun site), press the reset button on the OLA - and voila, you start logging GPS data! But we're quite there yet, since the data will be logged as text files, with some of the essential data points missing. We need binary data (".ubx" files)!

Fortunately, the Sparkfun tutorial provides a link to a firmware specifically for GPS logging. Updating the firmware is pretty easy with the Sparkfun firmware update application. If you are using an M9 logger, you're all set to create .ubx files with the required "NAV-PVT" sentences. You just need to edit a settings file on the micro SD card that the OLA creates the first time it starts after installing the GNSS firmware to increase the logging rate to 5 Hz (or higher).

If you're using an M8 chip, though, you are a bit out of luck. The GNSS firmware only works with the newer M9 chips, not with the SAM-M8 chip I listed above. But the firmware is open source, so modifying it is straightforward. Well, at least if you know how to program in C/C++ and know your way around Arduinos. I've done both in the past, but rarely and reluctantly, so it took me quite a few hours to get things to work properly. The current version of my modified GNSS firmware is not pretty, but at least is works with the SAM-M8 chip. 

The next steps will be some more on-the-water testing of the prototype to see if the accuracy is adequate, and perhaps some testing with an M9-based GPS chip and an external antenna. If the results are positive, I'll clean up the modifications I made a bit and post the firmware so that anyone else interested can download and use it. 

Saturday, February 27, 2021

How Not To Get Beaten By A Girl

Short answer: pick the right gear! For those of you with more patience, let me give you the long story.  

Nina and I have been speedsurfing 3 out of the last 4 days, and she's beaten my top speed in 2 of the 3 sessions. Not by a small margin, either - she typically was about 2 knots faster. Nor was this the first time - she did the same thing in a couple of speed sessions in January. Sure, she is a better windsurfer than I am - but I am taller and heavier, two things that should help in speedsurfing.

The last 3 days made it clear that gear choice is a big factor on who's faster. Three days ago, Nina was on a 6.3 m sail and 89 l slalom board; I was on a 7.0 m sail and 99 l board. Her 2 second top speed was 31.6 knots, pretty decent for the conditions - and 2 knots faster than my top speed. She got her top speed at a deep downwind angle - 130 degrees of the wind. My top speed was at a lower angle, about 110 degrees off the wind. The 20 degree difference explains the speed difference: deeper is faster. But since the apparent wind goes down at deeper angles, you need enough sail to go deep. Her 6.3 was big enough, my 7.0 was too small. I weigh almost one third more than Nina, so my sail size should be about 1/3rd larger, too - not just 10 percent larger.

Yesterday, I was able to turn the tables (for once). We rigged when the wind was barely touching 20 mph. Nina very much prefers the 6.3 to the 7.0, so I suggested she should a bigger board - the Falcon 99 instead of the Falcon 89. I went with the Falcon 112 and a 7.8 m sail. The wind picked up to around 24 mph just as we started sailing, so I ended up nicely powered. Nina also had plenty of power, but had a hard time controlling the board with the 23 cm delta fin that I often use with the board. This time around, I ended up with 31.6 knots, while she could not get above 30. The difference in "feel" was similar: my setup was nicely balanced and easy to sail, which she was fighting for control. 

The first instinct was to simply blame the board - we both had been on the Falcon 99 when we were slower. But as tempting as this explanation is, it's likely to be wrong. I had used the same board and fin a few weeks earlier, and set a spot and board PB with 33.09 knots. Furthermore, I had used the Falcon 112, a board which I never liked much, and which I had never been able to push past 30 knots. I had started to think of the board as "slow", at least for me - but clearly, I was wrong.

The wind forecast for today was a couple of knots higher than for the last few days. When we left home, the sky was still cloudy, but we hoped that the sun would burn away the clouds, as it does most days. Our optimism explain why we both opted for the same sails, but smaller boards, today: Nina for the Isonic Speed W54, and I for the Falcon 99. Unfortunately, the clouds stayed, and the wind never picked up, staying around 20 mph and leaving both of us a bit underpowered. We launched from the JFK Causeway today, despite the many dead fish from last week's cold on shore: 

On the water, though, there were fewer dead fish than on previous days. I guess they all washed up onshore...

Nina got out a bit earlier, and caught a nice gust in her very first speed run that propelled her to 31.7 knots - almost 3 knots faster than I was today. Overall, she also beat my speeds in 4 of the 6 GPS categories today, even jibing the little speed board better than I jibed my "go to" board. My setup today felt unbalanced, and I was fighting the entire time to get upwind. I did a few short speed runs, but had to pay for it every time afterwards fighting back upwind. It seems I made the mistake of going down one board size, but 2 fins sizes, and the 31 cm weed fin was too small for the conditions. Never mind that Nina was on a 19 cm fin...

The cool thing about all this is that the "match racing" allows us to learn about the "right" gear for the conditions. Most of our windsurfing has been freeriding and freestyle sailing, where you often pick the smallest sail possible to get planing. A 5.6 m freestyle sail in 18 knots wind (or less), and a 5.0 in 21 mph, would be plenty of power. On slalom gear, though, the 7.8 is barely large enough to get going in 18 knots, and just right right in 21. Knowing that you need a bigger sail is one thing - but there's always a tendency to pick a smaller-than-ideal size. The other complication is that everything needs to fit together - board and especially fin need to match both sail size and conditions, and going too big can be just as bad as going too small. But with two of us on the water, chances are higher that at least one of us gets it at least half-way right, and shows what is possible under the conditions. Now I only have to convince myself to always go two sail sizes larger than Nina, no matter how windy it looks...


Thursday, February 18, 2021

Freezing In Texas

"Everything is bigger in Texas!"  I learned that five years ago, when we started to spend part of the winter in Corpus Christi. But who would have thunk that applies to winter thingies, like power outages? Not me!

It started harmless enough. The forecast predicted a couple of days with temperatures below freezing, and down into the 20s at night (that's -6 C, for my friends used to metric numbers). No big deal, we thought - we've seen freezing or near-freezing temperatures almost every year we were down here. Usually, temperatures would get back up into the 60s a couple of days later.

So when our power went out on Sunday evening, we did not worry. So what if we never experienced power outages while living in Germany? We've been in the US long enough to get used to them. Usually, they just last a few minutes, or at most a few hours, even if a snow and ice storm with near-hurricane force winds hits Cape Cod.

We were in for a lesson about what "small government" means. "Small government" is considered a great thing in Texas; even the legislature meets only every other year. For electricity, this means that Texas never entered any agreements to share electricity with neighboring states, since doing so would have put its power grid under Federal jurisdiction. 

Together with millions of Texans, we learned the consequences of this decision, and the "In business we trust! Regulation is bad!" philosophy, over the next few days. We called out landlord about the outage after half an hour, and heard back from them that the utility company thought that fixing it might take until midnight. So we went to bed early, hoping to have power again when we woke up. Hah!

We woke up to near-freezing temperatures in the apartment. This time, it took the landlord more than an hour to get a hold of anyone at the electricity company, and the news were not good. During the night, power companies all over Texas had started "rotating outages". Supposedly, they cut power to some customers for 15-60 minutes at a time. The reality, however, was very different. Some customers had power the entire time - we could see the outside lights burning all the time at the buildings across from ours. Others had "planned" outages that lasted many hours. But for many, the power did not come back on for days.

After the first night, our landlords tried to help by allowing us to use the fireplace, which had been off limits until then as a "fire hazard". Being good Germans, we of course had followed that rule, and thus not bothered to get any firewood. When we tried to drive to a grocery store to get some, we were in for another surprise. Not that the little stores on North Padre Island were closed - that was to be expected, with power outages now common. The surprise was that the police had closed the JFK Causeway - the only connection to the main land (and open grocery stores). Apparently, there was some ice on the bridge, which made driving way too dangerous. This made me realize how wicked spoiled we are in New England, where somehow the streets remain useable even in snow and ice. 

Fortunately, the landlord had allowed me to use some wood he had lying around in the garage - 2x4s under the work bench (but not the wood next to it that was cut to size for sealing up the place during hurricane warnings!). He even had a manual saw hanging on the wall, which gave me an excellent warmup and workout - let's just say its best years were long behind it.

The fireplace was in the back corner of the apartment. Using it helped me understand the difference in heating capacity between a wood burning stove, like we have at home, and a fireplace installed in a climate where typical winter temperatures are in the high 60s. The wood stove in our Cape Cod home can actually heat up the entire place; the little corner fire place looks very nice. But if you sit just a foot or two away from it, just out of range of the sparks that the construction wood will generate, you can stay warm. Yes, we definitely were lucky.

At some time, our neighbor managed to organize an emergency generator that he ran near-constantly from then on. Since houses are about 6 feet apart here, and the generator was on his back patio and did not have any noise suppression, we absolutely knew when his generator was running. It did not quite sound like a plane trying to take off, but it clearly seemed to be aiming in this direction. My admiration for my lovely wife grew a bit when I saw she could sleep through that noise. I got some sleep, too, at least until my ears hurt so bad from the ear plugs that I had to take them out.

On day 2 of the Big Freeze, temperatures during the day rose above freezing, and the police opened the bridge to the mainland again. Afraid that they might spoil the islanders too much, and that there might be some ice hiding on the road at other places, they decided to keep the main highway into Corpus Christi closed, though. We needed to get to the hardware store which was on the other side of Flour Bluff - usually a 10 minute trip on the highway. But with the 6-lane highway closed, the only other option to get there was to use a little 2-lane bridge a few miles further south. No problem - that was the way we often took after windsurfing to go to Lazy Beach Brewing! So what if we had to share the small roads with thousands of other cars, which extended to 10-minute trip to more than an hour - at least we go there! And we were able to score some burnable construction wood, at prices that were only twice as high as on Cape Cod.  And sawing that wood into small pieces was a lot easier than before thanks to a brand new saw. 

We had big hopes to get power again on day 3, because the forecast had predicted 50 degrees (10 C) and sunshine. But once again, the weather remained much colder than predicted, and the sun never made it through the clouds. We saw a few work trucks at the local power substation, and some business and intersections on North Padre Island got their power back, but our house never did.

At this point, we were getting a bit tired of the cold, and the idea of a warm shower seemed like something heavenly. So we looked at hotels in the area. We had done so haphazardly before, but this time, we were serious! But every hotel that we called to verify that the rooms shown on the internet were still available either had no power, or no water. The water was a new thing: in many areas in Texas, the water supply suffered within a couple of days of the power going down. Sometimes, the water plants had power outages, and no backup generators. At other places, many people without power opened the faucets  to prevent pipes from freezing and bursting; and often, pipes did burst, but the water was not cut off for various reasons. All that reduced water levels to dangerously low levels, and reduced the water pressure to a slow trickle - if the water was not shut off completely.

On North Padre Island, we were lucky, because the water was never shut off all the way. For a day or so, the pressure dropped so low that toilet tanks had to be refilled by hand, and filling a gallon mug with water took a few minutes - but we still had water. Around that time, I also realized how lucky I was that my Raynaud's disease is just a very mild form, so that my fingers only hurt briefly after washing my hands, until they were warmed up again at the fireplace.

But today, on the fourth day without power or hot water, we had reached the point were a 3-hour drive to South Padre Island seemed too tempting. Temperatures were a bit warmer down here, and hotels still had power, running water, and free rooms. On the drive from North to South Padre Island, we saw hundreds of wind turbines working just fine in the strong wind; interestingly, we also saw at least 50 wind turbines that were not running, right next to others that ran. That's at least 100 MW of unused capacity - for no apparent reason, since temperatures were in the mid-40s, and wind speeds were just about ideal.

Some Republican politicians, including the governor, have jumped on the opportunity to blame renewable energy sources for the power problems. That strategy will probably work well with their voters: deflect the blame to something they had. However, their statements have been proven to largely be a lie (check this article in Newsweek, or this article from NPR). If you dig just a bit deeper, you can see that the problems arise directly from the free, largely unregulated energy market in Texas. Less than 20% of the electricity in Texas is generated by electric utilities; about 80%  is produced by "independent power producers" and similar commercial sources. Those independent providers have no responsibility to the consumer at all - instead, their only objective is maximizing profits. They have increasingly chosen wind turbines simple because this is the most profitable energy source in Texas, where wind and space are plenty. This happened even though Texas does not provide any state subsidies for renewable energy - the state only subsidizes fossil energy sources (oil and gas). But in the quest to maximize profits without responsibility to customers, the energy providers run an extremely lean ship, with a "grid reserve margin" of only 7.4%. When they installed wind turbines, they chose not to install cold weather packages, which would increase costs, but allow wind turbines to work perfectly fine in temperatures down to at least -4 F.

The current power crisis in Texas (which was preceded by power outages last September) is a logical consequence of all this - profit maximization with minimal regulation. At least one Texas gas company has been very happy with the recent problems, saying that it has "hit the jackpot" since it was able to sell gas at highly inflated prices during the crisis. 

Well, enough of that. We are now in South Padre Island. It's very windy, but we did not even bring our wetsuits - air temperatures in the 40s and water temps that are probably in the 30s just did not seem attractive after 4 days of freezing. At least we are warm now!

Saturday, February 13, 2021

Fun With Polar Plots: Fin - Freeride Foil - Race Foil

Have a look at this GPS track from one of my windsurfing sessions last summer:


This is how almost everyone windsurfs at Kalmus (and many other spots): back and force, back and force, and then some more back and force. If we plot the speeds against the angle to the wind in a "polar plot", here's what we get:


The filled are shows the maximum speed at a given angle to the wind; the inner blue line shows the average speed (ignoring speeds below 5 knots). The plot includes speeds while turning, which confuses the picture a bit. Here's what it looks like if we ignore all turns, and look only at speed when going in a straight line:


In GPS Speedreader, we can also look at how often we surfed at any given angle to the wind by selecting "Frequency" in the pulldown menu at the bottom:


This tells a pretty boring story: almost always on a beam reach in one direction, and pinching upwind just a little bit when going the other direction. The highest upwind angle is just around 30 degrees (60 degrees relative to the wind).

For comparison, let's look at the tracks from a windfoil session:


The tracks are spread out a little more, which is easier to see in the polar plot:


The maximum upwind angles are about 15 degrees higher than with a fin. While speeds drop quickly when going upwind on the fin, the speed drops on the foil are much smaller. This was a session in about 11 to 13 knots of wind, gusting to 15 knots, with a 6.5 m sail, a Slingshot Infinity 84 foil, and a Fanatic Stingray 140. The i84 foil is known to not go upwind as well as the smaller Infinity 76. I was reasonably well powered, but not really fully powered; a bit more wind would have allowed a few more degrees against the wind. But even with this setup and in the lighter wind, going upwind was totally effortless - much easier than with a fin.

Freeride foils are known to be easy to use and can be tons of run, but they are slower than race foils, and  do not go upwind nearly as well. I don't have a race foil, but after reading on the Seabreeze forum about someone who had just gotten a race setup, I downloaded the track from ka72.com, and had a closer look. Here's the polar plot:


That's quite a different story! The speeds are comparable to my windsurf session, but both upwind and downwind angles are a lot better. Where I had a hard time pointing 30 degrees into the wind, the race foil reached almost 60 degrees, while barely slowing down relative to the beam reach speed. That enabled the foiler to take a nice excursion around some big sand banks at his spot:


At least for me, this would have been impossible to do with a fin - maybe on a longboard with a huge daggerboard, but even that would have required a lot more tacks. 

The "frequency" polar plot for the race foil also looks very different from the one with the fin:


The distribution is much wider than in the fin plot further up, especially on the left side (starboard) - meaning he did not go upwind or downwind a couple of times, but rather most of the time. More options, more fun! Maybe I need to get one of these race foils...


Tuesday, February 9, 2021

Foil Turns

The forecast predicted 10 mph. Fog was rolling in from the ocean. But the arctic air that has started chilling large parts of the US is predicted to make it down to Texas in a couple of days, with lows close to freezing even on the coast - so when the wind meter readings got up to 12 mph, we just had to go foiling at Bird Island.

My GPS tracks tell the story:


Speeds are knots, except for the wind speeds which are in miles per hour. We foiled from 3:35 pm to 5 pm. I used a 6.5 m sail with a Slingshot Infinity 84 and a Fanatic Stingray 140, Nina used her 5.4 m wing, Slingshot Infinity 99 foil, and custom wing board. We both used 71 cm (28 in) foil masts. 

The tracks show that I was foiling almost the entire time, which was nice. But the even nicer part was that I managed to keep speed in almost all of my jibes. Most of them included a touchdown at the end, but it usually just needed a couple of pumps to get back up flying. So easy! It seems to be twice as much fun if you don't have to start over after every turn. A couple of times, I even managed to switch the feet while still flying, and if there was a touchdown, it was very brief - maybe a second or two. On port, I did sail-first jibes, trying to remember everything Andy Brandt told me in a private a few weeks ago. The closer I staid to his advise, the better the jibes were - no surprise! But on starboard, it felt more natural to do step jibes. Before today, I'd typically loose most of my speed when flipping the sail, but today, controlling height and turn radius somehow worked much better. I also must have remembered Nina's advice, because I started pushing down on the boom when stepping, which made the foot change much less dramatic. To my surprise, I ended up foiling through one of the step jibes, which also ended up being the jibe with the highest minimum speed in the entire session. 

Most likely, the near-perfect conditions today made things easy, and explain most of the progress. The water was quite deep (0.5 ft above 0), but very flat thanks to the light wind. But the polar diagram shows that I still got upwind angles of almost 45 degrees, which is perhaps 5-10 degrees better than in many sessions. That's a definite hint that (a) the wind was reasonably strong, at least in gusts, and that (b) tips that Andy Brandt gave me in the private lesson made a big difference (besides improving my jibes, he also showed me how to go upwind better).  So there is some hope that, at least to some small extend, the improvements may be due to me finally improving my foil jibes.

Nina had a fantastic day winging. She's been foiling through jibes for months, but foiling through tacks had remained elusive - until today! She discovered that she had to move her arms a bit differently, and promptly foiled cleanly through a couple of tacks, without the board making any water contact. In about half a dozen more tacks, the board just touched the water very briefly. Naturally, she was super happy.

With a little luck, we'll get another session tomorrow, before temperatures start dropping all the way into the 30s over a few days. Going windsurfing or foiling in such temperatures here in Texas is just inconceivable