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.