A couple of days ago on Christchurch Estuary. GW60 set to 5Hz and both Motions set to 10Hz. I uploaded all three sets of data to KA72. The following three photos are of KA72's representation of the tracks. ..It shows my deliberate attempt at a hairpin shaped Alpha, that includes my 2sec high (29+knots) on the inbound leg - It didn't amount to much because the wind dropped in the gybe. ..But I'm showing it here because of the apparent and very significant track error founded on GPS alone (first photo). Several other differences are noticeable as well.




All three devices were on my helmet:

Good example of what positional errors look like. Typical errors are in the range of +- 3 meters for GPS only, and can go down to +- 1.5 meters when using 3 GNS systems, like the Motion. In general, neighboring points are all shifted in the same direction, which your tracks nicely illustrate.
For alphas, that means if the ends are exactly 50 meters apart, there's an almost 50% chance that the measured distance is larger than 50 meters, and that the alpha therefore does not count. But with a properly worn accurate GPS like the Motion, alphas with a 48 m distance between the ends are very likely to fall within the 50 m range, even if there is a positional error of 1.5 meters.
On the water, knowing the difference between 48 and 50 meters would require properly spaced markers, and sailing quite close to the markers. Most actual alphas have legs that get together much closer, like 30 or 40 meters. The extra 10 or 20 meters then often get added to the faster leg, so that the end points are 50 m apart.

Hi Boardsurfr, I notice your reference to three systems. Do you have the latest Motion (with screen), are you using Galileo as well?

I think that the later Motion Firmware versions use 3 GNSS @ 10Hz. (I can't remember offhand in what version this changed or if my memory is even correct - Julien will be able to confirm this) The number of satellites used is limited by the firmware to Max 18 with the strongest signals.

Edit: I just checked one of my Motions (LCD) that is loaded with fw3132 (test fw). In earlier fw, the note "(2 x GNSS)" appeared beside the setting for 10Hz. In this fw is does not have that note, which strengthens my memory that later fw's are 3 x GNSS @10Hz.

Other well built Ublox M8 based loggers will get up to around 30 satellites on 3 x GNSS, but @10 hz, this can, and often does cause missed points, which is why the Motion Strategy. I have a prototype M9 based logger that handles 3xGNSS 25+ sats @10Hz without missing points, (and apparently even @20Hz as far as I have seen from limited testing). It will be interesting to see if the 'about to be released' Ublox M10 based Motions can handle a greater number of satellites on 3 GNSS, but it appears that the gains in using more than 18-20 sats may be quite small anyhow. (diminishing returns with increasing numbers).

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Phromsky said..


All three devices were on my helmet:

That is an excellent test setup by the way. Well done!

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Phromsky said..
Hi Boardsurfr, I notice your reference to three systems. Do you have the latest Motion (with screen), are you using Galileo as well?

The only Motions I have are the original ones (and they are almost dead, both screens and USB connector). They usually get around 25 satellite, which means they are using Galileo.

From all the data I have looked at, the difference in accuracy between 5 hz and 10 hz is very small, while the difference between 2 and 3 GNSS systems can be quite noticeable. So recording at 5 hz with 3 GNSS systems typically gives better actual accuracy than using only 2 GNSS systems at 10 hz. Unfortunately, just looking at the +/- estimates given by analysis software typically gives the opposite impression. But if you look at the actual correlation between observed errors over time, in particular when reception is not ideal, you'll see that a lot of the apparent "accuracy gain" seen when you go to 10 hz is a mathematical artifact.

I tend to shy away from mathematics and especially large volumes of numbers, so I've really only looked at the situation in the short time frame - As in 5Hz and 10Hz two second highs. As such I'd tended to think that 10Hz at least provided greater insight into the plausibility of the 2 second high.

Recently I'd been thinking that maybe 10Hz could provide for a new competitive element. And one that might level the playing field a bit for lightweights and women. ..Comprising a score founded in the ratio of a conventional scoring Alpha to its low speed point in the gybe. With the low speed point measured in a 1 second time frame - thereby employing the same amount of data as a 5Hz two second high.

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sailquik said..
I think that the later Motion Firmware versions use 3 GNSS @ 10Hz. (I can't remember offhand in what version this changed or if my memory is even correct - Julien will be able to confirm this) The number of satellites used is limited by the firmware to Max 18 with the strongest signals.

Edit: I just checked one of my Motions (LCD) that is loaded with fw3132 (test fw). In earlier fw, the note "(2 x GNSS)" appeared beside the setting for 10Hz. In this fw is does not have that note, which strengthens my memory that later fw's are 3 x GNSS @10Hz.

Other well built Ublox M8 based loggers will get up to around 30 satellites on 3 x GNSS, but @10 hz, this can, and often does cause missed points, which is why the Motion Strategy. I have a prototype M9 based logger that handles 3xGNSS 25+ sats @10Hz without missing points, (and apparently even @20Hz as far as I have seen from limited testing). It will be interesting to see if the 'about to be released' Ublox M10 based Motions can handle a greater number of satellites on 3 GNSS, but it appears that the gains in using more than 18-20 sats may be quite small anyhow. (diminishing returns with increasing numbers).

Hi Sailquik, prior to seeing your post above I hadn't anticipated that the constellation reception status of the Mini Motion could be altered with firmware. Meanwhile both of my Motion's came to me with 3031, which Julien has confirmed (consistent with your indication) limited 10Hz logging to GPS+GLONASS. ..In fact because I hadn't realised that the system reception status can be altered, when 3049 came out I only uploaded it to one of my Motion's, and so the other one remains in 3031 status to date.

The implication seems to be that my three photo's as seen above, actually show three different system reception statuses - in order as follows:
GPS
GPS+GLONASS+GALILEO
GPS+GLONASS