Vesper Marine xb-8000 anchor alarm problem

[SoonerSailor]

Originally Posted by s/v Jedi

But we already do, it's called differential GPS

Not exactly. The WAAS and older DGPS broadcast corrections that recreational GPS receivers use to fine tune the pseudorange from the C/A code, but the survey grade receivers take it a step further and record carrier information that allows post-processing to figure out how many 1.5 GHz wavelengths they are from the GPS satellites, and at what phase angle of the carrier at that epoch. That takes much more accurate/precise ephemeris, clock, ionosphere, and troposphere corrections than what is available from WAAS, SBAS, and DGPS broadcasts.

If you will note in the table that WAAS by itself helps but doesn't get you down to sub-meter accuracy. That requires a different level of correction and signal analysis.

[Paul L]

Originally Posted by Seafarer7

Most of those high accuracy Trimble receivers use WAAS - ie: real differential GPS system with corrections streamed from a satellite (= Satellite-Based Augmentation System aka SBAS). They also track GLONASS (ie Russia’s GPS), doubling the number of tracked satellites

Trimble sells their own portable reference stations to obtain very high position accuracy used in surveying.
WAAS is use is available on most marine GPS systems.

[mlydon]

I haven’t had my 8000 too long, but Vesper says (and in my brief experience), the unit will calculate where the anchor actually IS, based on your swing pattern, AFTER you’ve dropped and swung about a while.
I haven’t had to worry about setting the alarm AS I drop, but then again, I always had a good breeze to stretch out the rode...

I slapped it in last fall coming back from my new boat purchase and used it for a week, and was very pleased with its performance, although I was NOT happy with using the anchor watch app - you have to leave the app up and running.

I’m just completing a proper install including the buzzer and switch, looking forward to next season with the whole thing properly rigged.

Matt

[Alan Mighty]

Originally Posted by SoonerSailor

GPS accuracy continues to improve somewhat as older satellites are replaced with newer ones. The link below gives results obtained by the US Forest Service when they tested a number of GPS capable devices. The table shows a radius in meters from a known position that the receiver will report its position within 95% of the time.

USFS Tested Horizontal Accuracies

The performance of the Xiaomi Mi 8 is very respectable! That's a US$400 smartphone delivering 1 - 2 metre accuracy.

[s/v Jedi]

Originally Posted by SoonerSailor

Not exactly. The WAAS and older DGPS broadcast corrections that recreational GPS receivers use to fine tune the pseudorange from the C/A code, but the survey grade receivers take it a step further and record carrier information that allows post-processing to figure out how many 1.5 GHz wavelengths they are from the GPS satellites, and at what phase angle of the carrier at that epoch. That takes much more accurate/precise ephemeris, clock, ionosphere, and troposphere corrections than what is available from WAAS, SBAS, and DGPS broadcasts.

If you will note in the table that WAAS by itself helps but doesn't get you down to sub-meter accuracy. That requires a different level of correction and signal analysis.

I was commenting on your previous post, where you only mentioned ground stations for survey grade receivers and that no ground stations are used for boats. Boats don't need sub-meter accuracy imho.

[Tillsbury]

Originally Posted by mlydon

I haven’t had my 8000 too long, but Vesper says (and in my brief experience), the unit will calculate where the anchor actually IS, based on your swing pattern, AFTER you’ve dropped and swung about a while.
I haven’t had to worry about setting the alarm AS I drop, but then again, I always had a good breeze to stretch out the rode...

I slapped it in last fall coming back from my new boat purchase and used it for a week, and was very pleased with its performance, although I was NOT happy with using the anchor watch app - you have to leave the app up and running.

I’m just completing a proper install including the buzzer and switch, looking forward to next season with the whole thing properly rigged.

Matt

You absolutely need the buzzer and switch. That way it doesn’t matter what happens to your phone or the app — the anchor alarm stays set.

Re: accuracy, this is a complete red herring. Anchor alarms don’t care about precise location, only relative location. In the same way I don’t care how accurately my clock knows what time it is — I can measure a number of minutes elapsed time with it even if it’s wildly out.

[SoonerSailor]

Accuracy is not a red herring here. True that you don’t need to know the lat/long of your dropped anchor, but your GPS position, even if immobile with respect to earth, will wander quite a bit over time. Usually the worst result of this is going to be a false anchor drag alarm, but it behooves you to know the limits of GPS precision when you are counting on it to keep you off the rocks.

[a64pilot]

Back in the day we had what was called selective availability, which was a “dithering” of the time signal of the GPS which kept it intentionally from being very accurate, you could see errors up to 100 meters.
Bill Clinton turned off SA, reason was with it on, the Russian system was going to be more accurate and there would be a mass exodus to the Russian system, with SA turned off, the US system was more accurate.
Back in the day a ground based transmitter could also transmit a differential signal locally and that could enable GPS to be accurate enough to steer a farm tractor or enable an aircraft precision approach.
Farms that used GPS to steer their tractors saw an increase in productivity just based on the rows being more precisely spaced, which meant many more rows per field. A human just can’t steer as precisely as GPS can, it’s that accurate.
Just about every Agricultural aircraft uses GPS for precision application, many start and stop the application based on GPS automatically, and vary application rates on ground speed, and even “prescription map” that is based on satellite photos they know which parts of the field need more chemical and which less, and the GPS based system turns up application rates and decreases them based on GPS position.
Then came WAAS, which has pretty much made differential or DGPS no longer required.
There are ways that a GPS positional data can be extremely accurate, for instance I had an Apollo Precedus aircraft GPS way back when that had a survey mode, you placed it in one spot and it began taking measurements and averaging them, if you have enough measurements and average them, you can average put the error and get an astonishingly accurate position.
If your curious Garmin got into the aircraft GPS business by buying Apollo. Garmin has a long history of buying into businesses.
Oddly enough Garmin bought Apollo from UPS, the freight people.
The DOD can and I’m sure has turned selective ability back on, and can adjust SA to degrade accuracy as much as they want, a concern is/was to prevent someone from making a poor man’s cruise missile out of an old Cessna.
Civilian GPS’s use the “C” code transmitted from the satellite, Military use the “P” code, which requires a secure “fill” to receive, the P code frequency is much more powerful to penetrate foliage etc, and way more accurate.

All civilian GPS’s have a max speed at which they will operate. I had to prove once on exporting aircraft that the Garmin 430 installed could not be used as a navigation device for a missile. Garmin provided the data that made Customs happy. Cause I thought sure it could in fact be used, but due to its max operating speed, it can’t.

ITAR or International Traffic in Arms Regulation is what I ran into.

Bottom line, GPS with SA deselected, especially if your receiving WAAS signals is way more than accurate enough for an anchor alarm.
Aircraft for example use GPS to shoot instrument approaches, which would not be allowed if they weren’t very accurate, and they do this without DGPS now.
Phones however can often be led astray by them triangulating cell tower signals.
Before I got my Vesper I used to use my IPhone and Drag Queen, it would often alarm in the middle of the night, but you would wake up and it would have already corrected itself. You could look at he position that would stay good, then just for a very short time it would go way off, then return

[SoonerSailor]

a64pilot - take a look at the table linked to earlier in the thread to get the best idea about what kind of precision you can expect from GPS receivers, with and without WAAS. You are right on about occasional wide excursions of GPS receiver reported positions. Recreational GPS receivers - which only pick up the L1 frequency and use only the C/A code, even with WAAS, are not good enough for navigating farm tractors. Trimble, John Deere, and others sell base/rover combinations that are, and I think tractor navigation can also be done with receivers that have a cell link to subscription based proprietary reference station networks to markedly improve accuracy and precision - but I don't have personal experience with that.

Aircraft GPS receivers have, or will have, access to a relatively new signal (L5) from newer GPS satellites that is in a protected aviation frequency band (normal civilian L1 and L2 frequencies are in bands shared with other devices/applications). That in combination with the L1 and/or L2 frequencies will provide a greater degree of dependability and accuracy for safety of life applications. I think aircraft GPS ILS approaches still require the use of GBAS, a local type of short range DGPS in the VHF band.

You can today put together a GPS system with sub-meter accuracy and put it on your boat, but it is relatively expensive, and for navigation it is moot since navigation charts don't carry that precision - at least now. It would make for a super accurate anchor alarm, though.

[ReneJK]

Originally Posted by jeffrobbins

Just heard from our tech support team and they received your email and replied with suggestions for diagnosing what's going on. They are looking forward to hearing back from you when you arrive.

.

Thanks Jeff !
I was able to shoot a video of the issue and am working with the techs following up on some suggestions

Will keep the ticket updated and let the support team know if things change or not , may take a few weeks (again) as its not quite easy and quick to reproduce the problem

[B25Matt]

Originally Posted by SoonerSailor

a64pilot - take a look at the table linked to earlier in the thread to get the best idea about what kind of precision you can expect from GPS receivers, with and without WAAS. You are right on about occasional wide excursions of GPS receiver reported positions. Recreational GPS receivers - which only pick up the L1 frequency and use only the C/A code, even with WAAS, are not good enough for navigating farm tractors. Trimble, John Deere, and others sell base/rover combinations that are, and I think tractor navigation can also be done with receivers that have a cell link to subscription based proprietary reference station networks to markedly improve accuracy and precision - but I don't have personal experience with that.

Aircraft GPS receivers have, or will have, access to a relatively new signal (L5) from newer GPS satellites that is in a protected aviation frequency band (normal civilian L1 and L2 frequencies are in bands shared with other devices/applications). That in combination with the L1 and/or L2 frequencies will provide a greater degree of dependability and accuracy for safety of life applications. I think aircraft GPS ILS approaches still require the use of GBAS, a local type of short range DGPS in the VHF band.

You can today put together a GPS system with sub-meter accuracy and put it on your boat, but it is relatively expensive, and for navigation it is moot since navigation charts don't carry that precision - at least now. It would make for a super accurate anchor alarm, though.

What most people fail to understand is that WAAS is not about getting a better position, although that is a small side effect (as stated above and supported in the tables). It's all about knowing the error bounds of your position, so that it can be used in aviation. If your error bounds are too high, you can't fly a GPS approach, the GPS will flag its position as bad.

[a64pilot]

And yet, my Vesper Vision seems to indicate over time a position that is accurate to a foot or so in its anchor alarm function. I’ll drop anchor and back down on it and watch, it will stabilize and stay within one foot over a couple of minutes, then I know for sure I’m not dragging.
I assume it may due this by averaging measurements?

I can be 80’ away from the anchor and set the alarm at 90’ and stay for days, and it won’t alarm.
How does it achieve this level of accuracy?

[SoonerSailor]

A GPS position over a given short period may or may not stay within a few feet. It is more likely to do so at night and when the ionosphere is settled. I have watched position plots from a fixed dual frequency (L1 & L2) receiver stay within a 1 meter radius for long periods of time then take a brief 4 meter flyer despite good satellite geometry. With the typical single frequency (L1) receiver these excursions will be larger. Averaging does help some, but it has to be on the order of tens of minutes and more to really start to tighten the accuracy of a fix.

[a64pilot]

If you look at this picture, you’ll not see any of those excursions, in fact the width of the band of the track is due to wind strength, high winds and the distance will stretch to 80’, low winds maybe 70’.
However the GPS plotted position stays within one or two feet, way closer than my understand of GPS accuracy. set the anchor drag to 10 feet more Than you have rode out and the alarm will never sound, but drag ten feet and it will.
I don’t think GPS position is within a constant couple of feet, so how does the Vesper do this?

[s/v Jedi]

I see the same accuracy as A64, not just with the Vesper but with all my current GPS sensors. I think some of these also use the Russian system but others don't and I see no obvious difference.

It's the same with other boats around me in the marina: they seem to be positioned wrong, but that is caused by not having a heading indication and/or not having entered the correct position of their GPS antenna.

The difference I see is that I sometimes touch the finger pier, other times not. Let's say 1-2 foot variation, of which at least 6" is my actual movement with wind and tide changes