While Daffy was here, I've been picking his brains about SDoP, and although I might not have it all down pat, I thought it a good idea to post here, as Roo has stated, not many of us has a clew what it's all about.
So here goes, any corrections welcomed, thanks to sausage for proof reading.

Some GPS chips, (sirf and ublox) output the calculated accuracy of their doppler speed data and this accuracy depends on; the number of satellites, their relative positions, and their signal strength and quality. This is called the Depletion of Speed Precision or SDoP value and the lower the number the greater the accuracy.

The windows version of GPSResults makes good use of it in its display options, and by ticking SDOP you can instantly see the speed accuracy (the SDoP envelope) on the speed graph.
As an example, below are the GPSResults for the same gybe from a GT31 (on my arm) at the top and a GW52 (on my head) at the bottom. The sdop wide envelope clearly shows the GT31 data to have less accuracy and also calculates this to be the best alpha for the session, whereas the GW52 has a smaller SDoP envelope and greater accuracy and calculates this alpha as the 2nd best for the session. The sdop values are also included in the table firstly as the +/- value, 0.333kt for the GT31 and 0.168 for the GW52.
I guess the mSDoP is the worst SDoP value, 1.749knots for the GT31 and 0.467 knots for the GW52





Recently I've checked a few alphas this way, and it's become obvious that the GPS's worn on the head or shoulder produce the cleanest alpha data, the chest or back are consistently compromised, and the upper arm is sometimes good sometimes bad. It seems to depends on how the sky view changes when leaning into the gybe.

One of the unique features of VDOP (Velocity Dilution of Precision), SDOP is not the actual name but one made up for convenience, is that with the U-Blox chipset it is an unfiltered measurement. In other words it is based on the raw 3D (dimensional) data from the GPS.

GPS units process the data and create 2D output, sadly this dilutes the level of precision as any time a number is processed and rounded up/down it loses some accuracy. Manfred setup GPSResults to have the ability to use 3D data produced by the U-Blox for greater accuracy, it was part of the 10hz 3D GPS project I developed back in 2007.

Roo

Select to expand quote

Roo said..
One of the unique features of VDOP (Velocity Dilution of Precision), SDOP is not the actual name but one made up for convenience, is that with the U-Blox chipset it is an unfiltered measurement. In other words it is based on the raw 3D (dimensional) data from the GPS.

GPS units process the data and create 2D output, sadly this dilutes the level of precision as any time a number is processed and rounded up/down it loses some accuracy. Manfred setup GPSResults to have the ability to use 3D data produced by the U-Blox for greater accuracy, it was part of the 10hz 3D GPS project I developed back in 2007.

Roo

References?????

Let me know what you need Andrew.

Roo

Thanks Roo. The info you sent me was interesting. The GT-31 and GW-52 use the 3D Doppler and error data as well. But we suspect there is some 'smoothing' to 'averaging' going on in the data speed data acquisition. It appears in some tests that the data is being acquired at up to 10z but 'averaged' over a 1 second value. But with the Ublox chips having it at 10z, or more, a lot of the need for that sort of filtering is eliminated and accuracy confidence increases significantly with the raw data.