I happened to drive around in my van to test a new $30 phone for use with GPSLogit, and ended up coming to some insights about the spikes in 5 Hz GW-52 data. Bottom line is that there is a lot of additional noise when recording at 5 Hz when compared to 1 Hz data from GT-31s. Not all of the speed variations we see at the 200 ms level is real - maybe very little of it is. So the actual improvement in accuracy that is achieved by recording at 5 Hz rather than 1 Hz is definitely smaller than the theoretically possible factor of 2.2. I attached an image that illustrates the measurement noise in 5 Hz data.
The full report is at boardsurfr.blogspot.com/2016/07/dead-toys-and-lots-of-noise.html (scroll down a bit to get to the GW-52 stuff).
Precisely why I asked locosys to display 1hz average max speed instead of 5hz instantaneous max speed. I'm not interested in a noise spike, that gives you a very false impression of what the 2s average will be when you crunch the numbers.
Looks like this isn't going to happen.
What interest me is the nature of the noise.
At one stage I was testing 2 GW52s with them side by side the two signals showed very similar noise, in phase with each other. When I had one unit on my head and one on my upper arm, the signals looked similar, but the phase between them varied, about 80% was 180deg out of phase.
It's not 'noise', it's DATA.
The speed errors are random and therefore more data leads to far better accuracy over many points.
We are not interested in 0.2 knots speeds. (we are looking at the Forest, not the individual trees)
We do not give much serious credibility to the accuracy of even 2 second speeds.
We are interested in longer periods of 10 seconds and more. In all situations, more data is far better.
The 1Hz data from a GT-31 is logically derived from an average of multiple readings over that second. We just don't know how many and the method used. Is it any wonder that agrees closely with 1 second averaged data from the GW-52, which is an average of the 5Hz data.
It is interesting to note that speed graph traces from the 10Hz Trimble survey grade GPS (cm accuracy) from Maquarrie Innovation and Sailrocket show the same sort of sawtooth appearance. It is, for all intents and purposes, irrelevant.
As I have said many times before. The one calculation top speed figures given by ANY GPS are highly unreliable. (high error margin)
Even the 2 second figure calculated in software has a high error margin. Getting too puffed up about your 2 second figures from a 1Hz GPS is kidding yourself a bit. That is where the big benefits come from higher Hz.
On the matter of higher Hz I will agree to disagree since I have not seen any actual evidence to the contrary and plenty of evidence in support.
The level of noise in the sampling rate can be directly attributed to the quality of the antenna. The GW52 has a very cheap one and it's reflected in the noise created. The GT31's is a better patch type but the true level of noise is masked by filters and averaging over 1 second. Fitting a higher quality antenna is the key to improved accuracy, here's the result from a helix one fitted to a 18hz U-Blox 8 gps proto. The m/s figure is much smoother. N.B. the 16 sec. time differential is caused by the leap year correction in some gps units.
Yes, GW-52 antenna costs $0.25/0.50
The helix one is $15
Diference between cost optimized and result driven design...
Yes, it is quite possible that the antenna is a limiting factor that increases noise disproportionally at higher rates. But there are other possible factors, too; for example, electromagnetic interference can create problems when going from 1 Hz to 5 Hz (as seen with when trying to adopt the Flysight).
The important thing is to take a close look at the data to see if the underlying assumptions are valid. I did a few test drives with a ublox-7 based GPS dongle and the GW-52 today. Interestingly, the 1 Hz GW-52 data give a higher estimated accuracy than the 5 Hz data:
That's because the error estimates for the individual data points are about 3-5-fold higher in the 5 Hz data. At least in this example, the 1 Hz data are more accurate than the 5 Hz data for 10 second runs. From what I have seen so far, I would not be surprised if that is always the case. BTW, the 10 second segment I selected was from the same stretch of road, and two drives that were about 20 minutes apart.
For the ublox-7 GPS data, the result is quite different. Here, the error estimate is about 2x lower for the 5 Hz data than for the 1 Hz data (+- 0.3-0.5 for 1 Hz, 0.12-0.25 for 5 Hz). That's what would be expected if (a) measuring at the higher rate does not increase the measurement error, and (b) the 1 Hz data are sampled, not averaged (or filtered).
Note that in both cases, we rely on the error estimates given by the instruments. They are using different chips from different manufacturers, so it is very likely that the numbers are computed differently. One chip might use one standard deviation, the other one two or three. We're not comparing apples and oranges, but we may be comparing apples to sets of three apples.
But if we ignore that for the time being, it's cool to see that the sub-$20 USB dongle got similar accuracy as the GW-52. Now I just need to find a cheap phone with USB host mode support...
Error estimate for 10 sec.
18hz U-Blox was 0.091 GT31 0.259 Run 1
18hz U-Blox was 0.078 GT31 0.533 Run 2
18hz U-Blox was 0.137 GT31 0.414 Run 3
18hz U-Blox was 0.081 GT31 0.268 Run 4
The U-Blox always has higher individual error estimates than the Sirf GPS chipsets. I think the reason is the U-Blox is a 3d estimate versus a 2d estimate for Sirf.
Very nice numbers, Roo. Roughly 4-fold better than the GT-31, in line with what would be expected from bumping the data rate up. I'd guess that a lot of the improvements between the Sirf3 and the ublox8 are necessary to go to an 18-fold higher data rate without adding measurement noise.
Pretty well covered in the U-Blox data sheets. VACC is the velocity accuracy estimate, velocity being calculated in 3 dimensions, x, y and z. The Sirf calulates SDOP, speed dilution of precision, speed SOG calculated from a scalar measurement.
I think the error rate is due to GW-52 design, I checked files from the following units
- 1hz GT-31
- 10hz Ublox PAM-7, std Ublox antenna
- 18hz Ublox Max-M8q, helix antenna
All three devices have comparable +/- values when you check the values on the same runs.
GT-31/PAM-7 checked 2 tracks in NL : 0.2xx against 0.3xx
GT-31/MAX-8q check 2 tracks on La Franqui : 0.2xx against 0.3xx
Changing value from average to gaussian didn't change the AVG outcome from the 4 tracks.
Used GPS Results 6.147 Windows version.
I don't own a GW-52, can't compare the devices...
My guess is that the watch design was build for the small (chip)antenna and that the only added the big antenna without changing the device design.
I just checked a couple tracks, but there is always a movement in 3 dimensions.
Lowest value in run 1 was 830.9 meter and highest 831.5 m (0.6 meter) 8 sats
Lowest value in run 2 was 830.8 meter and highest 831.9 m (0.9 meter) 9 sats
Lowest value in run 3 was 832.0 meter and highest 833.1 m (1.1 meter) 8 sats
Lowest value in run 4 was 829.5 meter and highest 831.6 m (2.1 meter) 8 sats
Lowest value in run 5 was 829.1 meter and highest 830.0 m (0.9 meter) 8 sats
Especially run 4 has a very high value, others values are the highest or the lowest when you look at the accuracy and speed +/- values.
I also checked the run in the overview and it is a run where I almost took of with my big slalom gear, that run had a lot of vertical movement anyway...
Calculations in 3D should be done when we use a new device.
Every time we use a GPS we are moving in 3D. We are moving in space relative to the satellites that we use to triangulate our position, to them we are not on a flat surface but moving in 3 directions. To dismiss this introduces an error into our calculations. The Sirf SDOP is speed dilution of precision which is based on speed over ground not 3D velocity that the U-Blox uses.
Back from holiday and really bad wifi :)Here is a screenshot from GPSAR
Red GT-31
Yellow Gyro (Ublox-M8q@18hz, data comes from UBX and was saved to SBP via GPS Results)
Speed is around 27kts average, nothing spectacular
The highlighted part is 10sec.
Tracks can be made available on request :)
Two things immediately obvious.
1. The Doppler data is far better at high Hz (but we already knew that).
2. The time offset.