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Author Topic: Got a cool new speedometer - basically 100% accurate  (Read 4450 times)
maria-n-skip
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« Reply #15 on: September 22, 2007, 07:31:21 AM »


 JimH,

     A picture would be nice.

   Skip
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Dreamscape
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« Reply #16 on: September 22, 2007, 08:02:05 AM »

Here's the link I found.

http://www.dakotadigital.com/

Checking it out now. Sounds really cool.

Paul
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Sean
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« Reply #17 on: September 22, 2007, 08:17:46 AM »

The GPS receiver has a rechargable battery to keep in sync with the satellites when the ignition is off.  Of course, this doesn't help if the vehicle is under a roof or canopy.

And Gus wrote:

Quote
The battery is just there to maintain the user added stuff like waypoints and the last position so it can power up faster.

Actually, if the battery is there to power the receiver when the ignition is off, neither of these statements is entirely correct.   And I would guess that's what the battery does, because (1) this is a vehicle speedometer, which needs to be ready-to-go as soon as the vehicle starts, and (2) nowadays, we use NVM (non-volatile memory, usually flash) to store things like waypoints -- no battery required.

When a GPS receiver loses satellite coverage due, for example, to the vehicle being parked under a canopy, the  internal oscillator ("clock") continues to operate.  (Incidentally, the difference between a $100 receiver and a $5,000 receiver is the accuracy of the oscillator -- and I'm talking about just the receiver, not any mapping components.)  The receiver also has its own copy of the almanac, and so it can continually update the "expected" position of the birds.  So as long as the receiver itself is not moving, and the receiver remains powered up with a full almanac and running clock, as soon at the obstruction (canopy) is removed, the reacquisition of signals (and thus accurate speed and position information) will be nearly instantaneous (on the close order of hundreds of milliseconds -- not minutes or even seconds).

Since no oscillator is 100% accurate, and since the GPS constellation itself sends out correction messages, the apparent position of the receiver will "drift" over time until it no longer has a correct model of the where the birds are, and when this happens, the first bird it reacquires will appear to be out of position.  When this position difference exceeds the tolerance of the system, the position accuracy will drop (and some receivers will refuse to display any position at all) until enough birds have been reacquired to derive a new fix.  So if you are parked under a canopy for months on end, you may experience a delay or inaccuracy in speed and position reporting once you come out.

Going through tunnels presents a similar problem -- the birds will not appear in the "expected" positions to the unit, since you come out of the tunnel in a different place than you entered.  Most receivers try to extrapolate a range of expected positions based on last known speed, heading, and position, to speed up the reacquisition time.

Also, Skip wrote:

Quote
Our data collection crews this summer occasionally would run into not enough birds for the GPS unit to work. Each year it gets better and your chances of being in the wrong place at the wrong time is statistically slim.

I'm not sure what data Skip is collecting, or what accuracy is required, but for a vehicle speed and distance application, you are not going to experience this ever at any point on the surface of the earth unless you are in a narrow canyon or on the streets of a major city surrounded by high-rises.  At any given time from any point on the surface of the earth, as long as you have a view "mostly" to the horizon in all directions, there will be between six and twelve birds visible.  You need only four to establish an initial fix, although more birds will yield a more accurate position.  Also, to get the best accuracy requires a correction source, and most modern GPS receivers can use a geosynchronous satellite-based correction, known as WAAS.  (Incidentally, the FAA just shut down two of the WAAS satellites and replaced them with backup birds, and some older receivers lost their WAAS correction as a result -- go to your GPS manufacturer's web site to see how to update your receiver's WAAS system.  More recent receivers make the change automatically.)  It will take a little while to get the WAAS correction and apply it, but this will be of only limited value in a speed-and-distance receiver (vs., say, a mapping unit).

One you have the initial fix, though, maintaining correct speed and distance (over ground) requires only three birds, and can be done for short periods with only two.  Again, it is a matter of the receiver having already downloaded a full copy of the almanac, having a good fix, and having synchronized its clock to the birds.

The issue raised by several posters about odometer accuracy is a good one, though.  Without another input source, such as wheel sensors, it's just not possible for a system based solely on GPS to know whether you're driving or being carried (by a ferry, rail car, tow truck, etc.), or whether that tunnel you just went through was straight or curved.  Certain educated guesses can be made, like ignition on = driving, ignition off = being carried, and adding distance for tunnels by interpolating speed between signal loss and reacquisition and multiplying by the time, and that will get very, very close.  But probably not legal for the required odometer disclosure in many states.

FWIW.

-Sean
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maria-n-skip
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« Reply #18 on: September 22, 2007, 08:43:07 AM »


 Sean,

    collection digital image of road every 10 meters at posted road speed.

   trimble pathfinder pro... azm. 30deg, pdop >4.

 But then what do I know

Skip
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« Reply #19 on: September 22, 2007, 09:00:04 AM »

    collection digital image of road every 10 meters at posted road speed.

   trimble pathfinder pro... azm. 30deg, pdop >4.


Right -- so I would think you need a really good position fix for that.  Absolutely not necessary for a speedometer, which doesn't care where you are, just how fast you are going.  It's not important if the fix is incredibly accurate, as long as the fix at time T+1 is roughly the same degree off as the fix at time T.  Since accuracy improvement (as more birds are acquired, or WAAS kicks in) happens in a gradual way (WRT speeds in the realm of automobiles), the speedometer will still be incredibly accurate even while the fix is improving over time.

So I stand by my statement:  for a speedometer application, this is just not going to happen as long as you have a good view of the sky.

-Sean
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maria-n-skip
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« Reply #20 on: September 22, 2007, 09:55:01 AM »


 And I stand by mine
 Quote
Our data collection crews this summer occasionally would run into not enough birds for the GPS unit to work. Each year it gets better and your chances of being in the wrong place at the wrong time is statistically slim.

 ~ 874,666  images each collection cycle with over 3.5 Tera bytes total of GPS data.

 The DMI calculates the 10 meters with GPS as a back up. Each image has a Coordinate associated with it.
 Part of QA/QC makes sure each image has a coordinate and each year we have to heads up digitize a coordinates
 for the images where we didn't have enough birds for triangulation. worst case 2 miles this last season

  In our state only an LS can establish accuracy. So for resource stuff it is what it is.

 Skip


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belfert
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« Reply #21 on: September 22, 2007, 10:14:10 AM »

I think Sean is right on this thing working pretty much all the time except in a tunnel or on a covered bridge.  The $99 handheld GPS I have been using has never lost signal except in a tunnel.  I don't care if the miles on the odometer are off a little bit since I don't know the exact mileage of my coach anyhow.  I care more about accurate speed.

The manual says there is a battery that lasts a week between uses, but I don't see a battery on the board unless on the back.  There is a large capacitor on the board that may be the "battery".
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« Reply #22 on: September 22, 2007, 10:32:34 AM »

There are actually several GPS speedo's availible though they may not all have an odometer. Livorsi Marine and I think Gaffrig Marine make them. we have used nothing but GPS speedos in the boats for the last ten years or so. very accurate, has worked flawlessly from south argentina to canada and california to norway. granted we dont run the boats in tunnels or around the base of tall buildings but they have worked great for us and as a speedo only they require an antenna that is the size of a hockey puck.
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Sean
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« Reply #23 on: September 22, 2007, 01:54:37 PM »

And I stand by mine...


Skip, I don't think we are disagreeing.  I'm merely stating that your application is completely different from that of a speedo, and there will be no such lack of availability for a speedo application (short of a major failure of the GPS system, such as satellite failure, or a return to Selective Availability).

Also, when I wrote "I'm not sure what data Skip is collecting..."  what I meant, and what I was thinking, was "I'm not sure what data Skip is collecting, but it probably requires more accuracy than a speedo...".  I was most assuredly NOT thinking, and did not mean "I'm not sure what the h_ll kind of data Skip is collecting...".  So, please, don't get your hackles up.  It's all too easy to misinterpret someone's intended tone in a written medium such as a post.  No offense was intended, and I apologize if it came across that way to you or anyone else.

The statistics you've quoted regarding number of images collected and terabytes of data accumulated, as well as the Position Dilution of Precision figure you mentioned earlier, are all very interesting in their own right, and one could even say impressive.  But they are not at all relevant to whether or not one can build an accurate ground speedometer/odometer utilizing the Global Positioning System, or whether such a speedometer will suffer from LOS problems.

Again, remember that the speedometer only needs to "see" two satellites at any given moment, and even then, it's sufficient to see pairs of satellites only every couple hundred milliseconds or so.  Periodically, it will need to see three satellites at once to check itself, but these checkpoints can be on the close order of minutes or even tens of minutes apart.  Even more rarely will it need to see a fourth bird, to recalibrate its altitude.  Only when it has been powered down and taken out of service will it need to go through the lengthy process of acquiring four birds, plus downloading the almanac, from scratch.

-Sean
http://OurOdyssey.BlogSpot.com
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« Reply #24 on: September 22, 2007, 02:15:56 PM »

Hi Jim, any chance of you taking a picture of your Glass Panel and posting it for those of us who only dream of a panel like that?

I've been a pilot for over 40 years and am so used to the 'steam guages' I'm more comfortable with them in a plane...but in the bus, where I don't have to depend on much more than separation and directional control it sure would be fun to build a panel such as you describe.

Thanks,

NCbob
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« Reply #25 on: September 23, 2007, 07:06:22 AM »

I've used a lot of GPS over the years, starting with a Garmin 45.  My experience & understanding of the technology matches Sean's advice - for speedo purposes the constellation will always be "available" except in extremely narrow passages, extended tunnels or heavy forest canopy. 

We used GPS for sub-meter vehicle guidance in our business and would occasionally experience loss of signal days but even on those days my Garmin e-trex would still give me a speedo reading.

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Songman
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« Reply #26 on: September 23, 2007, 07:14:11 AM »

My GPS usually takes less than 30 seconds to lock on to a satellite. My GPS is on a constant hot so I have to turn it on and off. There is a button on it that just turns the screen off while leaving the rest of the unit active. If I stop for dinner or something, I just turn the screen off and am ready to get back on the road at the touch of a button.
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« Reply #27 on: September 23, 2007, 12:04:18 PM »

Sean is correct, the first half of my statement;

 "The battery is just there to maintain the user added stuff like waypoints and the last position so it can power up faster."

is not correct.

I was going way back in my memory when one GPS I owned would lose position when I shut off the aircraft elect power if the GPS battery pack was not installed. It did not lose user waypoints or preferences. I usually left the pack off because it was so big.

That was the only one I ever owned that did that, don't remember which brand, but most if not all do not do that anymore.

I always found that two satellites would provide navigation but not altitude/elevation.

Since we now travel by bus rather than airplane I don't use my GPS except for local flying in my putt putt so they probably have made great advances in technology. Magellan was always my favorite brand. Never was impressed by Garmin plus they are overpriced.

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« Reply #28 on: September 23, 2007, 09:31:01 PM »

I've been using a gps for a speedo for the last 5000 or so miles and it works good. I turn it on as I get into the bus and by the time everyone's settled, the bus is on and I'm in my seat it's ready to go. Needs batteries every couple days of travel.

Besides Eisenhower tunnel it's lost the signal 2-3 times. Unfortunately each time has been when I was in the mountains and needed to know my speed.  There is no indication that it lost the signal, it just freezes on say 29 mph until it updates all the sudden to 35 and you realize that you're going way too fast for 2nd gear.

Which is why fixing my conventional speedo is on my list.

Jason Whitaker
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« Reply #29 on: September 24, 2007, 12:47:52 AM »

... Needs batteries every couple days of travel.

... it's lost the signal 2-3 times. Unfortunately each time has been when I was in the mountains and needed to know my speed.  There is no indication that it lost the signal, it just freezes on say 29 mph until it updates all the sudden to 35 ...


I gather from the battery part of your post that this is a portable unit.  Are you using an external antenna, or just the built-in?

With a built-in antenna on a portable unit, in a bus (or even in most cars), you are bound to lose signal from time to time as parts of the vehicle itself will get in the way.

It has gone without saying thus far, but the antenna needs to be on the roof, at the highest point, where no part of the vehicle can obstruct the signal.  I presume that the dash speedo we have been talking about comes with a roof antenna.

-Sean
http://OurOdyssey.BlogSpot.com

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