Monday, March 19, 2018

Texas Wrap

After three days of driving, we're back from Texas, and were once again greeted by snow on the ground. I'm not sure what the bigger shock was, though - the temperature differences or the "sticker shock" in the supermarket, where exactly the same food costs 1/3 to 1/2 more. Maybe I can understand that for fresh fruit from Mexico - but for frozen food? Well, at least the selection of vegetarian pizza is better here. Texas is not the best place for vegetarians!

The weather this year was weird, with a lot of cloudy and rainy days. I still got to sail 31 days, about the same as last year, but that included 4 light wind days, and 4 days on South Padre Island. My biggest sail (the 7.8) also was the most used sail, and only 6 of the 31 days were on sails smaller than 7.0 (2 x 5.6, 4 x 6.3).  It's not quite as bad as it might seem, though - since I only sailed slalom gear, I was on the 7.0 race sail in 23 mph wind averages, where I would use a 5.0 on freestyle gear. The two days on the 5.6 had wind averages of 27 and 32 mph.

We got only one "real" speed session this year, and not a single day with northwesterly wind that would have been perfect at the South Bird Island Slicks. On the upside, we sailed a couple of spots for the first time that are great for long distance speed: the North Flats at South Padre Island, and Grassy Point in Corpus Christi. At low water levels, both spots are great for nautical mile and one hour runs, so it's no surprise that 4 of my top 5 sessions for both of these disciplines are from this year. Good practice for the OBX long distance race next month! And maybe all that "big sail" practice will be useful at the US Nationals a week later, where I'll be sailing a 9.0 in the Kona One category. It's cool to see that more than 50 windsurfers are already registered for the US Nationals, including racers from the US, Canada, Denmark, Great Britain, Belgium, FranceBrazil, Poland, Italy, and Germany. The list includes multiple national champions, so the racing should be hot! See you there or in Avon in a few weeks!

Monday, March 12, 2018

The Difference Between Good and Bad

No, this is not a philosophy post. It's about GPS reception. Surprised?

When I looked at GPS test results from yesterday and today, one thing puzzled me. Look at the data and take a guess:
Why was the phone GPS so inaccurate on 3/11, with large errors in every GPSTC discipline, but quite accurate on 3/12? I'll give you some clues, so you can see how early you can guess the answer.

Clue #1: A cold front pulled in yesterday afternoon. Temperatures dropped from the 80s into high 60s yesterday evening, and to 50s today.

Clue #2: That made me wear a 4 mm wetsuit today, while I was sailing in a lycra top yesterday.

The ardent reader of my blog may have a suspicion on what was going on by now (ha!).

Clue #3: The arm band I use for my phone tends to slip on the lycra, but much less so on the wetsuit. Yesterday, I stopped several times just to re-orient the phone so that it was pointing to the sky; it often was on the side of my arm.

Clue #4: Compare the track points from one run where the phone reported about 1 knot less speed than the GW-60 shown here:
to a run where both devices gave almost the same speed:
Clue #5: Check the "Sats" column for the phone data (the numbers on the right side) for the picture above.

So: yesterday, the armband with the phone slipped often, so that the phone was on the side of my arm, instead of pointing up. The runs that deviate most from the GW-60 data had reception from fewer GPS satellites, and changes in the number of satellites during the run. It seems that having the phone facing sideways on the arm during speed runs screws things up. That's not really very surprising - perhaps the more surprising thing is that the USB dongle gave accurate results yesterday (it was in the same bag, lying on the screen of the phone).

Can we verify that turning the phone sideways and blocking one side screws up GPS reception? Sure! I used GPSLogit, which has a graph that shows satellite reception. Here's a screen shot showing the reception with the phone facing up:
The phone used GPS 13 satellites, and had a decent horizontal accuracy of 4.6 meters. Just turning the phone onto its side did not change much, but then putting a hand close to the top of the phone on the back side did:
Now, the phone had good signal (green bars and dots) from only 7 satellites, and the estimated positional accuracy was 19.7 meters - four times worse!

If you want to reproduce this, but don't have GPSLogit, there are plenty of free GPS test apps on the Google Play Store available that could be used instead. The phone's GPS antenna is typically located near one of the top corners of the phone. You'll have to wait a few seconds to see changes in the satellite reception (probably because that information is not updated as often as position and speed info).

So, if you want to get accurate speeds from your phone with GPSLogit or Windsport Tracker, make sure the phone is facing to the sky! Of course, that's the same for the GW-60.

Sunday, March 11, 2018

Dongles Beats Phone

Here's a graph that compares the accuracy of the GPS from an Android phone to the USB GPS dongle that I have been testing:
The bars show the (absolute) difference between the results of the GPS units to the results from the "Gold Standard" GW-60 GPS in the six GPS Team Challenge disciplines over 10 different windsurfing sessions. The unit is knots. Shorter bars are better.

The results show that the phone GPS (red bars) can give quite accurate results, but sometimes does not. In 5 of the 10 sessions, at least one number (usually for 2 second average speed) was off by 0.3 knots or more. In three sessions, the observed difference was larger than 0.5 knots, which is definitely unacceptable for competition (but still good enough for just recording your sessions, and getting an idea how fast you were).

In contrast, the USB dongle had a maximum error below 0.4 knots in all 10 sessions, with a maximum deviation of 0.2 knots in 9 out of 10 sessions. The one session with a higher error was the one I reported about previously, where the arm band with the phone and dongle had slipped for large parts of the session. In all other session, the accuracy was very good. For comparison, the observed differences when wearing two GW-60 watches (one on each hand) are often in the 0.1-0.2 knot range.

Compared to the phone GPS, the USB dongle has another advantage (in addition to the higher accuracy): the u-blox GPS chip in it can provide accuracy estimates, which allows the automatic detection and elimination of artifacts, for example those that can happen during crashes or when swimming.

Anyone interested more detail can look at the data in this PDF file. You may notice that not all sessions have entries for all 6 GPS disciplines. There are various reasons for this: some sessions are shorter than 1 hour, so GPSResults did not give 1 h results; other sessions are different length in the different files (for various reasons, including a little bug that stopped the dongle logging in earlier versions), so distance differences where meaningless; and so on.

If you're really interested and want to look at the raw GPS data yourself, you can download them from here. It's a 32 MB ZIP archive with about 35 files. Note that you may need to define the time range in GPSResults for some of the sessions where the different units recorded different length sessions if you want to get meaningful results. Also, a few of the GW-60 session contain data from multiple sessions, so make sure to select just the session you want to compare.

Friday, March 2, 2018

Another Naughty Spot

I recently wrote about how great Grassy Point in Corpus Christi is for nautical mile runs (naughties). Today, I finally got to try out a different naughty spot that I had wanted to sail for years: the shipping channel to the right of the JFK Memorial Causeway between Corpus Christi and North Padre Island. Check out the water depth map:
The channel is between the causeway and the big green spoil area in the lower right quadrant of the map. We cross the causeway every time we go shopping, and many times when the water level was low, parts of the spoil area have been above water; the boundary region next to the channel often was just inches deep. A perfect speed channel?

Today, the wind direction was just right: NE, a 90 degree angle to the channel (note that the map above is not in the usual "north on top" orientation). At around 19 knots, it was weak enough to let me use the Falcon 112 - when exploring, a little extra volume is always welcome! After a 5 minute drive from our condo, I was on the shore, ready to rig. Here are today's tracks:

The water level today was quite high. I got off at one spot that's shown in green on the depth map, and the water was above my hips. That meant the channel had some chop - perhaps a foot high, but quite orderly, allowing me to pick lanes. The channel is about 50-80 m wide, so it allows some mini "downwind" runs at about 15 degrees angle to the channel direction. One of those got me a top speed of 28.1 knots over 2 seconds. The best nautical mile run was 25.45 knots, pretty decent for square runs in 19 knot wind. My jibes today were quite poor, partly because I went for very tight turns which the Falcon 112 does not like much, so the hour came out at 20.06 knots. But with a bit more wind and a lower water level, this would be a great spot for 1 hour runs; with a slightly less square wind direction, it will be a great spot for naughties.