AIS..how accurate is yours???

[donradcliffe]

Originally Posted by Jim Cate

In answer to several above posts;


The SOG/COG info we broadcast should be identical to that shown on the instrument screen, and that seems accurate compared to other instrumentation. this is not a fool proof test, but the best that I can do.

Jim

When I first got my transponder (West Marine), I hooked up ProAis underway and could see my broadcast SOG/COG. I was shocked to find that it wandered far more than my regular GPS readout (which averages the data to smooth the readout). In a seaway, the transmitted COG jumped around about 5-7 degrees on every transmission, which means that to a ships AIS my CPA is jumping around every 30 seconds). I went after WM, who asked SRT, and we were told that "international regulations prohibit averaging".

It sounded like lazy ass BS to me--if you only transmit SOG/COG every 30 seconds, a 10-15 second average is going to give a more accurate CPA.

That was 10 years ago, and I'd be interested if they ever fixed that problem. Jeff from Vesper should know.

[Jim Cate]

Originally Posted by donradcliffe

When I first got my transponder (West Marine), I hooked up ProAis underway and could see my broadcast SOG/COG. I was shocked to find that it wandered far more than my regular GPS readout (which averages the data to smooth the readout). In a seaway, the transmitted COG jumped around about 5-7 degrees on every transmission, which means that to a ships AIS my CPA is jumping around every 30 seconds). I went after WM, who asked SRT, and we were told that "international regulations prohibit averaging".

It sounded like lazy ass BS to me--if you only transmit SOG/COG every 30 seconds, a 10-15 second average is going to give a more accurate CPA.

That was 10 years ago, and I'd be interested if they ever fixed that problem. Jeff from Vesper should know.

good point, Don. My Vesper does provide some smoothing of speed and course info as displayed. Dunno about what is broadcast... Jeff, are you listening??

Jim

[barnakiel]

Originally Posted by DeepFrz

Barny, how can you tell the accuracy of your phone GPS? What Samsung phone do you have? My Samsung can use an enhanced location but it uses cell towers to enhance the accuracy of the GPS. It very nicely shows me where I am but how can you tell if its within 6 ft.?

I looked up the Apps, they are:

- GPS Test (1.3.2),
- AndroiTS GPS test (1.46),

Both show accuracy and the second one tells you how many US / RUS / CHN / (???) Japanese? satellites there are in view.

I have switched off what they call a-gps but cannot see any degradation. I do not know how the a-gps thing works.

I believe there may be many more Apps around now as I downloaded the Apps maybe 3 years ago or thereabout.

Samsung phone model GT-B5330. Android OS.

Cheers,
b.

[jeffrobbins]

Originally Posted by Jim Cate

good point, Don. My Vesper does provide some smoothing of speed and course info as displayed. Dunno about what is broadcast... Jeff, are you listening??

Yup, I'm here...

Our transponder's GPS receiver supports SBAS systems such as WAAS, EGNOS, MSAS. If you switch this on in places without SBAS then it might decrease the accuracy a little but shouldn't cause the wandering people are reporting with other GPS units. You can give that a try and see if it improves things when SBAS isn't available but I don't expect there to be large differences.

There are two things to consider... static accuracy and dynamic. Static is what you are testing when you are stationary and isn't necessarily indicative of what happens when moving.

FYI: We implement several optimizations. First is SBAS which you can switch on and off. We also have a smoothing algorithm which affects both static and dynamic characteristics by taking advantage of expected operating conditions such as sea-level and low velocity and acceleration associated with vessels. For that reason, you must switch our smoothing algorithm off if you are testing our units in the Colorado mountains or when installed on a boat that is faster than 40 knots.

Testing our GPS with the anchor watch feature isn't quite the same as when operating though. When you switch it into anchor watch mode, it changes the optimization and error correction algorithm to be more sensitive for example to prevent it from thinking a very slow drift is a wandering error situation.

Finally, on top of this stuff we implement an averaging algorithm which you can also switch on/off (this is newly released). This is used for displaying your COG/SOG and also it's internal CPA/TCPA calcs to smooth out SOG/COG jiggle. However, it does not affect the GPS data the transponder transmits (which is the output of the algorithms above).

Hope that helps explain our approach. Other manufacturers will do things differently I'm sure.

[jeffrobbins]

Originally Posted by donradcliffe

When I first got my transponder (West Marine), I hooked up ProAis underway and could see my broadcast SOG/COG. I was shocked to find that it wandered far more than my regular GPS readout (which averages the data to smooth the readout). In a seaway, the transmitted COG jumped around about 5-7 degrees on every transmission, which means that to a ships AIS my CPA is jumping around every 30 seconds). I went after WM, who asked SRT, and we were told that "international regulations prohibit averaging".

It sounded like lazy ass BS to me--if you only transmit SOG/COG every 30 seconds, a 10-15 second average is going to give a more accurate CPA.

That was 10 years ago, and I'd be interested if they ever fixed that problem. Jeff from Vesper should know.

I've also observed that behavior in SRT-based transponders but since we don't use any of SRT's stuff I've never investigated.

[Jim Cate]

Thanks, Jeff. That clears up several points for me!

Jim

[Snowpetrel]

Originally Posted by barnakiel

Accuracy of the Garmin I read from the Garmin interface. The unit is Garmin72.

Accuraccy of the phone I read from any nav App that has a position accuracy field.

I do not use cell towers on this phone as I have removed the card and use this unit as our mini-tablet / gps dongle only. Tested in the bush (off cell range) by far the most accurate GPS we have at this point but I know newer ones are even better.

I can look up the name of the app if you want. Ours is a 'cheapo' 100USD Samsung smartphone and the system is an older Android.

b.

I wouldn't put too much faith in that accuracy feild, its usually just HDOP rejigged through a simple calculation such as HDOPxUERE to convert it to a figure in meters or feet. Better to watch DOP, satellite numbers and strengths, and make sure RAIM is on and set up correctly.

[StuM]

Originally Posted by DeepFrz

Barny, how can you tell the accuracy of your phone GPS? What Samsung phone do you have? My Samsung can use an enhanced location but it uses cell towers to enhance the accuracy of the GPS. It very nicely shows me where I am but how can you tell if its within 6 ft.?

The display of many GPS applications give minutes to 5 decimal places - which is about 3/4 of an inch

Many applications show an accuracy figure as well. I'm currently tied up in a marina and GPS Test Plus on my Samsung phone currently says "Accuracy 10ft". According to that app, my Latitude is slowly drifting between 9° 27.955xx' and 9°27.956xx. That 3rd significant digit means that it is varying over about 6 feet. Google Maps with satellite imagery on and zoomed in, my "location dot" is currently right over where I am sitting in the saloon.

That's good enough for me without SBAS

[StuM]

Originally Posted by barnakiel

I have switched off what they call a-gps but cannot see any degradation. I do not know how the a-gps thing works.

I believe there may be many more Apps around now as I downloaded the Apps maybe 3 years ago or thereabout.

Samsung phone model GT-B5330. Android OS.

Cheers,
b.

A-GPS (Assisted GPS) uses cell phone information to speed up initial position determination (Time To First Fix). It does not enhance the accuracy of GPS location.

Here's the official Samsung stance on it:
Differences between A-GPS and GPS

(I set my phone and tablet Locating Method to "GPS only" when using it as a navigation device away from civilisation)

[barnakiel]

Originally Posted by Snowpetrel

I wouldn't put too much faith in that accuracy feild, its usually just HDOP rejigged through a simple calculation such as HDOPxUERE to convert it to a figure in meters or feet. Better to watch DOP, satellite numbers and strengths, and make sure RAIM is on and set up correctly.

Yes. This is most likely as you say.

I actually know a better method (long term parallel logging and then drawing a regression line and comparing the distributions of the variables). Still, with every testing method there are some limitations and you can bias (or get biased) the results depending on what one wants to prove (this is a very common practice in online tests, not something I recommend).

So, given the same calculation method on my Garmin units and on my Samsung phone, I think I can pretty safely compare garmins with samsungs.

b.

[barnakiel]

BTW

I have done some googling today and I found that Thrane LT-300 offers 72 channels, gps, glonass and beidou, and all augmentation constelations, and better than 2 meter accuracy. I like it. Best non-phone gnss I found online today (other than geodesy oriented ones).

And ... I am not buying one. I will settle on the 66 channel two system augmented gps/glonass that will come with the new Standard Horizon VHF/AIS transceiver. I have an AIS/VHF from SH now and we love it.

The Thrane gps is 400 freedom tokens and the SH AIS B/VHF combo is expected at 800 units, so in our gadgets starved boat the AIS B / VHF combo makes so much more sense, even if I can get slightly less accurate gps with it. It will also talk to an IPX7 wireless remote so I can give up on tablet in the cockpit.

Big sweet hug to everyone. Very good reading in this thread!

Off to bed now,
b.

[DeepFrz]

I just turned on my phone and selected Maps, which was left zoomed in, and watched with amusement as the little blue dot scooted around the screen. It has me sitting in the middle of the lawn, about 60 ft. from my living room. Of course, I have a concrete floor above my head and a roof above the apartment above me. So I don't think that 60 ft or so is to bad at all. When I am outside it is much more accurate.

[barnakiel]

Yes. They all get affected, newer ones less, old ones very much.

There is big military money for grabs if you build one that works inside tall buildings, bunkers, etc. I thought about it and my feeling was without small gnss satellites set throughout the structure this cannot be done - unless we improve the acceleration sensor to some crazy sensitivity levels (comparable to the best tectonics equipment - which, unfortunately, weighs tonnes.

;-(
b.

[DeepFrz]

This is where I live. About 60 ft. to the right of the blue dot. Well, it looks a little different now.

https://www.google.ca/maps/place//@4...!4d-97.1166069

[TheThunderbird]

A bit out of topic, but.... :-)

Do any cruisers use CLASS A AIS !?
IS it really necessary to have a transreceiver? I think that its usefulness rests with the TX section 80% of cases, the 20% being on the RX side.

Do you agree?