I don't know how you can prove that ANY gps measures speed via doppler just by looking at the output files. For gpx as an example, all it says is SPEED and gives a number. How it got that number is an internal algorithm, hopefully by doppler. It is prettly hard to reverse engineer the firmware code to ascertain whether it is doppler or not.
The guy who told me that the original Garmin Legend was doppler was a retired Garmin software engineer.
In any case, we all benefit from the speed accuracy of doppler compared to that of ds/dt. Because modern electronics has evolved to the point of being able to resolve the speed of light to sub-centimeter levels, there is no reason not to use doppler. Can't prove it, but I think they all do.
I run my GW-60 at 5 Hz, so the the 2-second speed is a running average of the last 10 points, and the 10-second speed is a running average of the last 50 points. Pardon me, I'm still gonna call this "smoothing." This, and SPD GENIE, are, by far, the best features of the GW-60 over pretty much all the others.
Steady on Daffy, Those of us who are not data analysts or GPS engineers would consider a moving average to be a type of smoothing.
As does Wikipedia en.wikipedia.org/wiki/Smoothing
True, I just sayin'. If you start your 10 second run at 35 knots, peak at 40 and finish at 35 again. The average may be 36,5, That suggests the run was smooth at 36.5knts. But that was not actually the case. Smoothing?
We all benefit from this discussion. Thanks.
All my handheld Garmin hiking units, including the aged Legend, output gpx files, with data taken at typically one point per second. Nice thing about gpx and nmea formats is that you can read them with a text editor.
If we had to depend on ds/dt for speeds, with s being ?10 meters most of the time, our speeds would be noisy all over the place. At our on-water windsurfing speeds, the standard deviations would be bigger than the moving means. Talk about noise. Since, however, our speeds are hugely less noisy than that, there must be a much better way to get speeds. Doppler is the only better one that I know of.
Since my old Legend, and later Legend HC, a couple different HCx units, the Oregons, the Montanas, and my Timex all report speeds that plot out with noise not too different from that of my GW-60, their speed points, even without moving averages, are definitely not ds/dt. Doppler has to be their method.
Again, just by looking at a gpx file, and looking at the number, I don't know how you can conclude whether it is doppler-derived or ds/dt-derived or somehow-otherwise-derived. You have to look at firmware code.
Have a look at the speed graph example I posted above. There are two lines. One is Doppler speed and one is speed derived from positional data. See that they are both about the same 'smoothness'. With 1Hz data, many GPS's already apply a 'smoothing' filter to to the positional derived speed, but even if they don't, there is often not a big difference in their appearance if there is excellent reception conditions and the atmospheric conditions are not changing rapidly. On the other hand, sometimes there are sudden changes.
yes you have to look closely at sailquick's graph to see there are 2 separate tracks overlaid, one doppler the other trackpoints.
It's a bit easier to see on my alpha graph above that there are 2 tracks. If there's doppler and trackpoints in a file it's very easy to tell. But I'm not sure about doing that with a text editor. May be later if I have time I'll try it with some ancient GPX files I know include doppler tracks.
Here's a Real Speed view of a GPX file from back in 2008, almost certainly from a GT11
This clearly shows the difference between doppler and trackpoints, (doppler green, trackpoints black). And how a doppler file is "smoother" than trackpoints, but doesn't have "smoothing".
Below is the same period in the GPX file, that this speed graph came from, via Real Speed.
It only has 1 speed reading with a max of 16.03m/s, (31.16kts)
Realspeed must be calculating the tackpoint speed from the positional data, as the below table doesn't show the huge spike above.
The speed in the below table must be doppler.
So the highest point in the file is 31.16kts, the highest doppler from the speed graph is the same.
Interestingly there is a gap of 5s here, I guess the old GT11s had problems!
The spike in the trackpoints is at 06:24:32, that's about 32kts, the GPX table's speed at that point is, 22.45. definitely doppler.
BUMMER, I can't post the raw GPX file, this smart page converts the HTML, so the first number appears to be "ele" no idea what that is.
Then there's date and time, speed in m/s and HDoP and lastly the number of sats.
-35 2008-01-26T06:23:56.815Z 13.05 1.4 6
-35 2008-01-26T06:23:57.815Z 12.53 1.4 6
-35 2008-01-26T06:23:58.815Z 12.12 1.4 6
-35 2008-01-26T06:23:59.815Z 12.37 1.4 6
-35 2008-01-26T06:24:00.815Z 12.93 1.4 6
-35 2008-01-26T06:24:01.815Z 13.02 1.4 6
-35 2008-01-26T06:24:02.815Z 13.57 1.4 6
15 2008-01-26T06:24:07.814Z 16.03 1.4 6
17 2008-01-26T06:24:08.814Z 15.58 1.4 6
20 2008-01-26T06:24:09.814Z 15.12 1.4 6
24 2008-01-26T06:24:10.814Z 15.05 1.4 6
27 2008-01-26T06:24:11.814Z 14.74 1.4 6
30 2008-01-26T06:24:12.814Z 14.61 1.4 6
34 2008-01-26T06:24:13.814Z 14.30 1.4 6
37 2008-01-26T06:24:14.814Z 14.17 1.4 6
41 2008-01-26T06:24:15.814Z 13.81 1.4 6
44 2008-01-26T06:24:16.814Z 13.37 1.4 6
48 2008-01-26T06:24:17.814Z 12.96 1.4 6
52 2008-01-26T06:24:18.814Z 12.41 1.4 6
55 2008-01-26T06:24:19.814Z 12.22 1.4 6
60 2008-01-26T06:24:20.814Z 11.67 1.4 6
64 2008-01-26T06:24:21.814Z 11.67 1.4 6
68 2008-01-26T06:24:22.814Z 11.53 1.4 6
71 2008-01-26T06:24:23.814Z 11.52 1.4 6
75 2008-01-26T06:24:24.814Z 11.46 1.4 6
79 2008-01-26T06:24:25.813Z 11.62 1.4 6
82 2008-01-26T06:24:26.813Z 11.46 1.4 6
86 2008-01-26T06:24:27.813Z 11.33 1.4 6
90 2008-01-26T06:24:28.813Z 11.26 1.4 6
93 2008-01-26T06:24:29.813Z 11.07 1.4 6
97 2008-01-26T06:24:30.813Z 11.06 1.4 6
101 2008-01-26T06:24:31.813Z 11.27 1.4 6
105 2008-01-26T06:24:32.813Z 11.55 1.4 6
-42 2008-01-26T06:24:46.350Z 12.05 2.0 5
-40 2008-01-26T06:24:47.349Z 11.94 1.4 6
-38 2008-01-26T06:24:48.349Z 11.89 1.4 6
-37 2008-01-26T06:24:49.349Z 11.72 1.4 6
-37 2008-01-26T06:24:50.349Z 11.26 1.4 6
-36 2008-01-26T06:24:51.349Z 10.99 1.4 6
-36 2008-01-26T06:24:52.349Z 11.17 1.4 6
-36 2008-01-26T06:24:53.349Z 11.15 1.4 6
-36 2008-01-26T06:24:54.349Z 11.02 1.4 6
-36 2008-01-26T06:24:55.349Z 11.38 1.4 6
-36 2008-01-26T06:24:56.349Z 11.53 1.4 6
-36 2008-01-26T06:24:57.349Z 11.70 1.4 6
-36 2008-01-26T06:24:58.349Z 11.62 1.4 6
-36 2008-01-26T06:24:59.349Z 11.99 1.4 6
-36 2008-01-26T06:25:00.349Z 12.16 1.4 6
-36 2008-01-26T06:25:01.349Z 12.23 1.4 6
-36 2008-01-26T06:25:02.349Z 12.08 1.4 6
-36 2008-01-26T06:25:03.349Z 11.96 1.4 6
-36 2008-01-26T06:25:04.349Z 11.68 1.4 6
-36 2008-01-26T06:25:05.348Z 11.43 1.4 6
-36 2008-01-26T06:25:06.348Z 11.27 1.4 6
-36 2008-01-26T06:25:07.348Z 11.27 1.4 6
-36 2008-01-26T06:25:08.348Z 11.01 1.4 6
-36 2008-01-26T06:25:09.348Z 10.72 1.4 6
-36 2008-01-26T06:25:10.348Z 10.58 1.4 6
-36 2008-01-26T06:25:11.348Z 10.81 1.4 6
-36 2008-01-26T06:25:12.348Z 10.95 1.4 6
-36 2008-01-26T06:25:13.348Z 10.84 1.4 6
-36 2008-01-26T06:25:14.348Z 10.42 1.4 6
-36 2008-01-26T06:25:15.348Z 10.45 2.4 5
-36 2008-01-26T06:25:16.348Z 10.57 1.4 6
-36 2008-01-26T06:25:17.348Z 10.50 1.4 6
-36 2008-01-26T06:25:18.348Z 10.37 1.4 6
-35 2008-01-26T06:25:19.348Z 10.11 1.4 6
-35 2008-01-26T06:25:20.348Z 10.23 1.4 6
-35 2008-01-26T06:25:21.348Z 10.43 1.4 6
-35 2008-01-26T06:25:22.347Z 10.42 1.4 6
-35 2008-01-26T06:25:23.347Z 10.65 1.4 6
-35 2008-01-26T06:25:24.347Z 10.65 1.4 6
-35 2008-01-26T06:25:25.347Z 10.94 1.4 6
-35 2008-01-26T06:25:26.347Z 11.15 1.4 6
"For well over a decade, most GNSS receivers use a combination of position deltas and Doppler shift to determine speed."
The issue I had seen with a "non-doppler" GPS before was that spikes in the positional speeds "bled through" into the speeds, albeit much reduced. I just looked at a whole bunch of GT-31 and GW-60 files, and did not find a single example of that happening. Here is a typical comparison of the positional speeds (top) and doppler speeds (bottom) from a GW-60:
In general, the positional speeds are very similar, and that includes the noise. The obvious big difference is a spike after a crash - probably when the GPS lost reception briefly, and then re-acquired satellites and come up with a brand new position. That spike is completely absent in the doppler data, as would be expected for pure doppler-based speed.