Coming This Summer: Speed Limit

[fatlazyless]

Haven't got anything intelligent to say. Think maybe I saw the Skater that's for sale yesterday, white w/ red trim. Didn't have any 'for sale ' signs. While the big vee hull go-fasts roar around, the Skater makes a buzzing sound......buzz...buzzz....buzzz....like a big chainsaw.

Hey, today's www.citizen.com says the speed limit zones' starting time have been pushed back till Aug 1 due a legislative process issue, or something. The screws of politics maybe cannot get a grip on this one.

[WeirsBeachBoater]

The 42' was out this morning between the snake and sleepers, and I couldn't help but notice they were leveling the radar gun at me. Hope they recorded my 25 mph speed!

[Silver Duck]

Actually, it would be fairly straight forward to calibrate the Raymarines:
1. Turn a few handheld GPSs into "transfer standards" by comparing their readings (on land) to radar guns which have been shown to be accurate and whose readings are acceptable in court.
2. Put those GPSs aboard boats and make repetitive test runs past the Raymarines at a range of speeds, while documenting both the GPS readings and the Raymarine readings. (I'd do this using multiple GPS-carrying boats travelling in multiple directions concurrrently.)
3. If the GPS readings agree with the Raymarine readings, you've calibrated the Raymarines and the data they produce should be acceptable in court.

Silver Duck

[Dave R]

Originally Posted by Silver Duck

Actually, it would be fairly straight forward to calibrate the Raymarines:
1. Turn a few handheld GPSs into "transfer standards" by comparing their readings (on land) to radar guns which have been shown to be accurate and whose readings are acceptable in court.
2. Put those GPSs aboard boats and make repetitive test runs past the Raymarines at a range of speeds, while documenting both the GPS readings and the Raymarine readings. (I'd do this using multiple GPS-carrying boats travelling in multiple directions concurrrently.)
3. If the GPS readings agree with the Raymarine readings, you've calibrated the Raymarines and the data they produce should be acceptable in court.

Silver Duck

You know and I know this would work fine and make reasonably accurate measurements, but a lawyer would have a field day with this method. This method basically has a radar set calibrated by a GPS device which is calibrated by a hand held radar unit which is calibrated by a standard in some lab. Too many links in that chain and too many opportunities for error to prosecute someone unless they were grossly over the speed limit, in which case testimony alone would probably suffice.

[GWC...]

Originally Posted by Dave R

Well, if someone were to challenge a ticket, they'd have to ask how the radar is calibrated etc. That would end that coiurt session in a hurry.

Next time on the Lake, why not visit one of the MPs doing the speed checks and ask how they are calibrated...

http://www.citizen.com/apps/pbcs.dll...321/-1/CITIZEN

[Gavia immer]

Originally Posted by Silver Duck

3. If the GPS readings agree with the Raymarine readings, you've calibrated the Raymarines and the data they produce should be acceptable in court.

Silver Duck

That's Judge Silver Duck, presiding?

[Phantom]

Reported on WCVB Television (Chan 5) in the Boston area last nite was a small piece about the Speed Limits on Winni -- Although they were not detailed in their reporting as to where the speed limits were, they reported that MP was out currently taking "test data" and that the Speed Limit would go into effect August 1st !!!!!!

Did they misreport the start date???? I thought it went into effect June 1 !

[Mee-n-Mac]

Originally Posted by Silver Duck

Actually, it would be fairly straight forward to calibrate the Raymarines:
1. Turn a few handheld GPSs into "transfer standards" by comparing their readings (on land) to radar guns which have been shown to be accurate and whose readings are acceptable in court.
2. Put those GPSs aboard boats and make repetitive test runs past the Raymarines at a range of speeds, while documenting both the GPS readings and the Raymarine readings. (I'd do this using multiple GPS-carrying boats travelling in multiple directions concurrrently.)
3. If the GPS readings agree with the Raymarine readings, you've calibrated the Raymarines and the data they produce should be acceptable in court.

Silver Duck

It would have to be a range of speed and ranges as the radars have to determine distance travelled from at least 2 measurements of distance (and then time). To determine distance travelled it'll need to calculate the geometry and that gives you 2* main sources of error, the range error and the angle error. I don't know what Raymarine units the MP has but quickie look at the Raymarine site listed these as 70' or 1.5% of the range setting and 1 degree, respectively, for these. You could calbrate them with GPS but the next question asked would be how long this calibration is good for. Can you go for a year between cals ? A week ? A day ? The best bet would be to have Raymarine specify the accuracy and cal period and live with that. For all I know the units the MP have may be good enough.


*Of course this assumes the MP boat doesn't move in any way. Which of course it will for any "long" observation time. So the MP's would then have to have a GPS and compass reading fed into the calculations. Certainly to prove these factors weren't a factor, they'll need to record them to show they didn't change appreciably during the observation time.

[Silver Duck]

Mee & Mac

Point taken about the range of distances! Actually, if I were doing this at work, I'd probably set up an exploratory test with a 10 distance point by 10 speed point matrix to see what the unit was capable of, then narrrow at least the speed range down to something like 40 to 65 mph for the actual operational qualification testing. You're also correct about establishing a calibration interval; that would need to be done experimentally, too.

I do think that, in the long run, qualifying the Raymarines (if possible) would be a more practical alternative than hand held radar guns that need to be pointed at a specific boat. That way, the MP could monitor everything in the area simultaneously.

I, too, suspect that the Raymarine setups are good enough to do the job off the shelf, but for use in court that would need to be proven. I also suspect that the MP is not going to be interested in 46 mph vs 45, but will be looking for more like 50 or so as a minimum.

Gavia Immer

It would convince me if I were the Judge, but then, I set up testing programs for process equipment for a living! Actually, I would hope that it would be necessary only to convince one court-appointed expert rather than each judge; I seriously doubt that most judges have much understanding of how radar works but they accept radar gun data anyway!.

Silver Duck