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.Wednesday, April 7, 2021
The Milk Jug Experiment
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.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.
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:
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:
- A Motion GPS (the original one with a screen), worn on my upper right arm.
- An Openlog prototype that uses the Beitian BN880 chip, worn on top of my helmet in a GoPro housing.
- 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.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.