I just today got a cool new speedometer for my bus. It is a GPS speedometer that replaces a regular speedometer. The speedometer head is digital and has an odometer included. I ordered this from www.nordskogperformance.com back before I started spending money like crazy with my new radiator core and such. This is a new product for them and they have limited production runs right now.
No more worries about inaccurate speedometers because of tire size or any similiar issues. This should be close to 100% accurate. There are several marine GPS speedometers out there, but none of them have odometers as boats don't need them.
I'm not sure if I will get this installed before I leave Tuesday or not. The big issue is my dash is 24 volt and I have to install my 24 to 12 volt converter before I can install this. (I could not find a switched 12 volt circuit in my main electrical panel.) My bus does have a Vanner to run lights and such, but it is just easier to install a $10 12 to 24 volt converter under the dash.
Brian: Did you ever post the solution to your fan belt problem?
Quote from: Stan on September 21, 2007, 06:09:45 AM
Brian: Did you ever post the solution to your fan belt problem?
No, I did not post anything about the belts.
I talked to two local belt places and they both said a standard B97 belt would not work as they only have a 25 HP rating. The one guy said it wasn't the fan that was an issue rather the 350HP on the crankshaft. I talked to Jim Shepard from the board here and he said a pair of standard B97 belts would be just fine. He recommended against going to a B98 belt as he said the tensioner probably could not take up the slack.
I ordered a pair of B97 belts from Motion Industries and got them installed Wednesday. We had to remove the fan assembly to get the belts installed. My friend who was helping thought hitting the starter quick would get the belts on, but I had accidently left the ignition on overnight and killed the batteries.
I ordered another set of belts after the first ones fit so I have a spare set for the road.
So this is an actual speedometer that goes in the dash that gets it's info from GPS? That's pretty cool. I have a portable GPS unit and also my laptop with a GPS receiver and Microsoft Streets & Trips and I have found that all of my car speedometers are incorrect, even the new cars! It would be convenient to have the true speed right there in front of you and not have to look away to a GPS screen.
It would be very accurate.
Not knocking Your choice, but at $499.00?
I could replace all my gauges and senders for that.
Cliff
Quote from: Songman on September 21, 2007, 06:54:04 AM
So this is an actual speedometer that goes in the dash that gets it's info from GPS? That's pretty cool. I have a portable GPS unit and also my laptop with a GPS receiver and Microsoft Streets & Trips and I have found that all of my car speedometers are incorrect, even the new cars! It would be convenient to have the true speed right there in front of you and not have to look away to a GPS screen.
You are correct. This unit replaces the speedometer head and connects to a little black box that is a GPS receiver.
It was only $350 as the product I got is not on Nordskog's website yet. My current speedometer is wildly inaccurate in part because it uses the old fashioned four studs on the brake drum to sense speed. The dealer I got the bus from put in several used speedo heads before they got one that even worked. I was quoted $250 for a new speedo head and sensor and I figured that for the extra $100 I could get something that is nearly 100% accurate and needs no calibration.
I know it isn't cheap, but I thoght it was cool so I ordered it. This was before I spent a ton on a new radiator core and such.
Brian,
That is a cool product....
Just a cautionary comment. 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 will be curious how it works for you going down the road.
A bus item that is a whole lot more fun than radiator stuff!
Skip
Quote from: belfert on September 21, 2007, 06:32:07 AM
I talked to Jim Shepard from the board here and he said a pair of standard B97 belts would be just fine. He recommended against going to a B98 belt as he said the tensioner probably could not take up the slack.
If Jim Shepherd said to use the B97, that's what I'd do. I'd wager he knows more about belts than any local salesman.
I got to wondering about how the GPS speedo handles the initial power up. I've had some experience with some GPS units that take a few moments to lock on to the satellites. Let's say you stop and park your bus briefly and shut everything off. A few minutes later you hop back in, and being already aired up, you are ready to roll as soon as you start the engine. Is the GPS quick enough to hang with you? What if it takes 30 seconds to lock on the sats? Would the odometer not register the first hundred yards of movement?
Or, let's say the GPS stores it's last position in non-volatile memory when it is shut down. Maybe the odometer adds on the calculated distance from the shutdown location to the next location it gets a satellite lock. If so, what happens if you load the bus on a ferry? At the end of the crossing, when you fire the bus back up, with the ferry trip get added to your odometer?
Yeah, I know. My mind works in strange ways.
I didn't think about satellite linkup delays. Still no worse than the handheld GPS I have been using, but the handheld is left on when fueling and such.
The odometer might not be 100% accurate, but the speedo should be and that is what I care about the most. I have a hubodometer matched to my tire RPMs.
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. I just found this out.
Craig, yes I did use the B97 belt after talking to Jim.
I've used GPS and Loran in airplanes since day one and I found that there are many situations in which neither one worked well or sometimes not at all.
Between tall buildings, in mountain passes, inside anything including tunnels and sometimes just for no reason at all.
I can't figure out how the odometer ever catches up for distances lost between these failures since there is no way it knows what route you took. GPS essentially works in straight lines so from a failure point to the next lock on will not be accurate.
The battery is just there to maintain the user added stuff like waypoints and the last position so it can power up faster.
It is a puzzle to me why the wheel stud systems didn't work since this is a very simple system. Sounds as if the heads were the problem since there isn't much to the sender. I had planned on installing one of these to replace my cable driven one which is awful and is probably not to far from breaking the cable since it is obviously kinking or catching somewhere in that 40'+ run.
QuoteMy current speedometer is wildly inaccurate in part because it uses the old fashioned four studs on the brake drum to sense speed
After reading literally thousands of posts from several different boards and for many years, this is the first time I have ever heard mention of this problem.
DML uses this system and I never had any problem of any kind for more than 15 years and 150,000 miles.
Richard
The speedometer on our MC-8 is the original OEM with the 4 bolts in the front hub. When we bought the bus, the speedometer was intermittant (when it worked it was accurate, but would frequently quit and sometimes start again). After contacting Precision Speedometer in Phoenix and getting instructions for testing the system, we found we had a bad sensor on the wheel. We replaced the sensor and had to do a couple minor adjustments of the air gap between the sensor and the bolts to get an accurate reading. Once the gap was set, it is very accurate (compared to our GPS) and dependable. No reading until about 5-10 MPH 30MPH at GPS 30 MPH, 46 MPH at GPS 45 MPH, and 63 MPH at GPS 60 MPH. This is as close or closer than our Grand Cherokee or Ford F350 dually.
However, that GPS Speedometer has got the others all beat when it come to COOL!! Jack
4 or 5 years back I decide that I didn't realy like my dash so started looking around at other options. Found Dakota Digital in Sioux Falls, SD at a car show. Told them what I had and they showed me how to do it. Now have the cleanist dash I have seen in a bus. All digital with the ability to set flash limits, either high or low, on all readouts as well as adjust speedo down to the last hair for speed. One instrutment about 1" high and 18" long has Oil press, water temp, rpm, volts, fuel, speed and odometer anlog with high head light and turn indicaters. Another smaller gage has airpressure.
Just my way -- your milage may vary
JimH
JimH,
A picture would be nice.
Skip
Here's the link I found.
http://www.dakotadigital.com/
Checking it out now. Sounds really cool.
Paul
Quote from: belfert on September 21, 2007, 12:25:16 PM
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
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
Quote from: maria-n-skip on September 22, 2007, 08:43:07 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
http://OurOdyssey.BlogSpot.com
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
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".
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.
steve
Quote from: maria-n-skip on September 22, 2007, 09:55:01 AM
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
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
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.
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.
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.
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
4104
Quote from: jeepme on September 23, 2007, 09:31:01 PM
... 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
Subject seems to be drifting from speedometer to GPS, so here is my GPS relative post. We run Delorme and MS Streets and Trips simultaneoulsy on our laptop (split screen S&T on left half of screen and Delorme on right half each usng their proprietary antenna). THe S&T altitude seems to always be inaccurate and considerably different readings on different occasions at the same location. Our drive way has registered as anywhere from -23' to 114' (our actual elevation is 18'). Speeds seem to both be very accurate (checked with portable radar speed displays used on the highways). Anyone have any ideas why the altimeter function does not work properly? Jack
As far as GPS altitude goes, one friend of mine said that the government purposely doesn't allow accurate altitude readings on civilian GPS. This is supposedly to prevent use as a guidance system for weapons. I've never researched this and have no idea if true.
Sean, yes, the GPS speedometer I bought has an external antenna.
Quote from: JackConrad on September 24, 2007, 05:33:17 AM
... each usng their proprietary antenna). THe S&T altitude seems to always be inaccurate ... Speeds seem to both be very accurate ... Anyone have any ideas why the altimeter function does not work properly? Jack
Yes...
First off, understand that the little box you connect to the computer is not just an antenna, it's an entire GPS receiver. Your computer itself can't possibly do the sophisticated signal processing required to receive GPS signals. What the little box sends to the computer is a position report (latitude, longitude, altitude) at frequent intervals.
GPS receivers can have as few as four "channels" and as many as 12 (or more for military and high-precision units). Unlike channels on your TV, or on a Ham radio or a police scanner, where you can receive only one channel at a time, the number of channels on a GPS receiver represents the number of different signals the unit can be receiving simultaneously.
GPS receivers locate themselves at a unique point in three-dimensional space using simple triangulation. At any given point in time, the satellites are at known positions, broadcasting a continuous stream of clock pulses. When the receiver sees a clock pulse, knowing the speed of radio energy, it can determine how far away that satellite is. As always with triangulation, knowing your distance from one known point places you anywhere on a sphere equidistant from that point. Two known points puts you anywhere on a circle defined by the intersection of two spheres. And three known points will place you, usually, at either of two points along that circle, where it intersects with the third sphere.
This is why, in order to get a unique fix, you need to be receiving four satellites simultaneously. The fourth satellite allows you to determine which of those two points is your actual position. However, there is a "trick" that receivers use to establish a fix with only three satellites: They use the sphere defined by the surface of the earth at a given altitude, and assume you must be somewhere on that sphere. The receiver uses the last known altitude for this trick, and that works nearly perfectly for boats, and almost as well for land vehicles and commercial aircraft. Things that change altitude rapidly, like fighter jets, space craft, and missiles, can't use this shortcut.
So the ability of any GPS to give you an accurate, continuously updated fix depends on its ability to always have a continuous signal stream from four satellites. You'd think that could be done with only four channels, but it can't, for several reasons. First, GPS satellites are constantly "rising" and "setting" above and below the horizon. So even if you had a perfectly clear view to the horizon on all sides from your antenna, when any of the four satellites sets, you will need to search for, pick up, and synchronize with a different satellite that is still above the horizon. This takes time, during which you will have lost part of your fix.
Secondly, we seldom have a good view to the horizon all the way around. So, while there are usually about 12 satellites above the horizon (half the constellation), any given satellite may fade in and out as we pass in front of buildings, under trees, past mountains, etc.. This is particularly an issue if your receiver's antenna is sitting on your dashboard instead of on the roof -- the signal will go through glass, but not anything else like the coach's roof. So your vehicle itself will generally block your view of several of those satellites.
For this reason, good receivers have 12 channels -- enough to at least try to acquire every bird above the horizon. And, since the chipsets are now cheaply mass-produced, it's hard to imagine anyone making a unit without 12 channels nowadays. However, many older receivers have only six, which was a common cost-cutting measure.
So, long buildup, but the reason for the difference in performance between your two receivers is either (1) the more accurate unit has a better antenna, with more "gain" and thus able to receive more marginal signals than the other receiver or (2) the less accurate receiver has fewer channels, and thus is having to switch birds constantly to maintain a fix. And whenever any receiver drops below four input signals for any period of time, it will switch to a two-dimensional navigation mode as I described above, where it attempts to place you accurately in latitude an longitude (and give you correct speed-over-ground), and punts on the altitude. When this happens, you will see inaccurate and sometimes rapidly changing altitude displays.
-Sean
http://OurOdyssey.BlogSpot.com
Quote from: belfert on September 24, 2007, 07:33:05 AM
... the government purposely doesn't allow accurate altitude readings on civilian GPS. This is supposedly to prevent use as a guidance system for weapons. ...
Well, sort of. The system does belong to the military, and it is designed so that the accuracy of the system can be lowered on command. In the event of missiles inbound to the US, the system can be de-tuned or even shut down in a matter of minutes.
There is a name for this: it is called "Selective Availability". And ten years ago, Selective Availability was in constant use, and civilian receivers could not get a fix better than about 30 meters or so (about 100 feet). Military receivers are unaffected, because, while they use the same satellites, they use a different and highly encrypted time signal unavailable to civilian receivers. As more and more of the world grew dependent on the civilian GPS system, including commercial aviation and shipping, pressure mounted to turn off Selective Availability, and I don't remember the date anymore (although I'm sure it can be Googled), but it was in fact turned off sometime during the Clinton administration.
So the rubric about civilian receivers not being accurate for military reasons is now out of date. Today you can get a fix accurate to about ten meters with the GPS satellites alone, and even better than that, down to 2-3 meters, if you apply corrections to it which are now constantly broadcast from a pair of geosynchronous satellites operated by the FAA, known as WAAS (Wide Area Augmentation System).
In any case, even if Selective Availability was turned back on, the system has no way to control "altitude" versus any other part of the fix -- see my previous post. Your receiver is doing simple triangulation to determine its position in three dimensions. It's just that receivers built for navigation prioritize determining latitude and longitude over determining altitude, and so, when not enough data exists for a completely accurate 3-D fix, the altitude number is the one most likely to be wrong.
-Sean
http://OurOdyssey.BlogSpot.com
Thanks Sean,
Since I can't change my altitude in the bus, is not really that important, just informational. And come to think of it, the Delorme (the accurate elevation) GPS did say something about being WAAS compatible. Jack
Quote from: gus on September 21, 2007, 07:31:19 PM
I've used GPS and Loran in airplanes since day one and I found that there are many situations in which neither one worked well or sometimes not at all.
Between tall buildings, in mountain passes, inside anything including tunnels and sometimes just for no reason at all.
How did the airplane in the tunnel part work? I just gotta know???? ::) ::)
QuoteI 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?
Handheld Magellan, no external antenna. Tracks 12 satellites so adding an ext antenna would probably, according to your explanation of how they work, make signal loss an issue only in tunnels.
Jason Whitaker
4104
Quote from: JackConrad on September 24, 2007, 05:33:17 AM
THe S&T altitude seems to always be inaccurate and considerably different readings on different occasions at the same location. Our drive way has registered as anywhere from -23' to 114' (our actual elevation is 18'). Speeds seem to both be very accurate (checked with portable radar speed displays used on the highways). Anyone have any ideas why the altimeter function does not work properly? Jack
According to my calculations You are at 18 ft ASL, Your antenna is located on top of your MCI, You have an 8 inch roof raise, So you should technically be reading 30 feet ASL. ;D HTH
I've ben watching this thread with some amusement for som etime now - all for a speedometer ;D
Sean's description is the closest I've seen to the reality of GPS to date. Although, there was a missed point... Specifically, around the problem of tunnels and parking garages and "urban canyons"...
GPS receiver companies have been trying to get auto-makers GPS recivers that were suitable for in-dash navigation in less-than-deal singal quality situations. Enter dead-reckoning ("DR", via accelerometers). Once the GPS receiver gets its fix on your trajectory, the DR sub-system begins assisting the actual locations by smoothing errors, and keeping track of changes in your course and speed when the birds are out of sight.
Accelerometers need occational updates to correct for minute errors in the tracking, and GPS receivers need short-term help keeping the heading info coming... Happy marriage ;).
I've been in a few vehicles with the new DR+GPS hardware like this (http://www.trimble.com/lassendrgps.shtml) and they track very well even in tunnels, multi-story parking garages, and sky-scraper lined cities (like say San Francisco).
Any one can make a speedometer out of an off-the-shelf GPS with a serial output. The standard "VTG" NMEA 0813 word (which can be set to be continually output from most any GPS receiver) gives Ground Speed and "Track Made Good". This is easily converted to a simple digital display (what Nordskog has done) for MPH readings. If the GPS digital dash speedo can keep giving you speed reading when you're in a tunnel, they used a $250 embedded GPS receiver (but if it loses the signal and goes to zero - you probably over-paid for that speedo - the trimble embedded DR+GPS development kit was quoted to me at $650, which included 3 receivers :o).
Cheers!
-Tim
Quote from: Tim Strommen on September 24, 2007, 10:13:54 PM
... Although, there was a missed point... Specifically, around the problem of tunnels and parking garages and "urban canyons"...
Actually, I did say "Without another input source, such as wheel sensors, it's just not possible for a system based solely on GPS to know [how fast you're going all the time]"
Quote
GPS receiver companies have been trying to get auto-makers GPS recivers that were suitable for in-dash navigation in less-than-deal singal quality situations. Enter dead-reckoning ("DR", via accelerometers). ...
You say this like DR is a new discovery. Come to think of it, several GPS manufacturers seem to think this too. However, I can assure you that dead-reckoning in-dash navigation systems actually pre-date GPS by some time.
Back in the early 80's, I dealt with a company known as Carlin & Collins. (Or maybe Karlin was spelled with a "K" -- it's been a long time.) They had invested hundreds of engineer-years and millions of lines of code, as well as a good deal of hardware development, in exactly such a system, which worked pretty well, that they were trying to persuade the major automakers to adopt. No accelerometers involved, back then -- the system used a magnetic compass, and sensors in wheels on both sides of the car. The wheel sensors gave the distance/speed, but the differential between the two sides also gave turn information, which was checked against the compass.
In order for the whole thing to work, however, the system needed to know what the roads looked like, in order to match your movements against them (and correct your position to match). Thus the company was extremely focussed on making hyper-accurate, machine-readable maps for every major city in the US (the initial target market). The system looked and worked great, and was very impressive, and I had the sense of being in a James Bond movie (where the "technology" debuted in an Aston Martin, you may remember). Somewhere in the middle of my dealings with them, they changed their name to what you may know them as today: Navigation Technologies (NAVTEQ), to better reflect what they were producing.
Of course, in the middle of all this, the release of civilian capability for the GPS came down the pike, wiping out virtually all of their investment in the DR measurement portion of the system nearly overnight. (Ironically, in typical Silicon Valley style, Trimble Navigation opened up shop nearly literally across the street in Sunnyvale.) Even with GPS, though, which in the early days of civilian receivers was hardly more than a digital display of position, speed, and heading, the demand for the machine-readable maps was greater than ever. So NAVTEQ ditched their entire navigation system project, and re-focussed strictly on making the maps. They are now one of the premier electronic cartography companies in the world.
It's taken twenty years for the major consumer GPS players to get back around to installing dead-reckoning inputs to their navigational displays. Cheap accelerometers and other technology that avoids the problems of installing those pesky sensors out in dirty places like the wheels is one of the reasons -- it's finally becoming cost-effective to add some of the terrestrial-based information back into the system.
And now you know, as Paul Harvey would say, the rest of the story.
-Sean
http://OurOdyssey.BlogSpot.com
According to my calculations You are at 18 ft ASL, Your antenna is located on top of your MCI, You have an 8 inch roof raise, So you should technically be reading 30 feet ASL. HTH
Capn Ron,
Actually that test was done with the both antennas laying on the drive way. And you are correct, with the WAAS enabled Delorme, raising the antenna to the top of the bus did give us a different reading. Jack
The full story......
http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf (http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf)
FWIW
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Quote from: Sean on September 24, 2007, 11:01:36 PM
..You say this like DR is a new discovery. Come to think of it, several GPS manufacturers seem to think this too. However, I can assure you that dead-reckoning in-dash navigation systems actually pre-date GPS by some time...
Quote from: Tim Strommen on September 24, 2007, 10:13:54 PM
GPS receiver companies have been trying to get auto-makers GPS recivers that were suitable for in-dash navigation in less-than-ideal singal quality situations. Enter dead-reckoning ("DR", via accelerometers). ...
The "revolution" is the micro-machined, failry accurate, cost-effective accelerometers that has recently become avilable, as well as years of experimentation in the proper use of such devices. As you mentioned about Navtec, the very mechanically intricate and, hard to calibrate (requiring military/survey-grade mapping and a huge database) is not cost competative in comparison to a few $15 electronic parts out of harms way - and is subject to accuracy based on the reliability of a large group of individual components. Auto makers are deliberatly trying to get away from this type of complex system, to improve product quality and reduce warranty liability.
Dead-Reckoning is simply knowing one's speed and direction over time and comparing it to a good map. This "technology" has been in use for more than a century or more - infact it could be said that the first satellite navigation and dead-reckoning was done well before GPS electronic satellites were put in orbit. One could (for the sake of argument) use a callendar, sextant, star chart, map, hourglass, and compass - along with your boat's speed in knots charted over time to figure out where you are (frequent updates on your position "plot" via sextant improve your positional accuracy like a GPS reading does - Navy guys will recognize this as the classic Quarter Master's job). I had practice doing this while helping a friend's dad move his sailboat up the pacific coast a few summer's ago.
With GPS systems that have map "datums" (thanks to companies like Navtec), 12-channel Receivers looking at as many "stars" of a known position (through the use of GPS "almanacs") as they can, high-accuracy real-time clocks (updated by GPS time), and now accelerometers and digital compasses built-in, the dead-reckoning process is now mostly digital, self-contained, and automatic - and really accurate.
Quote from: Sean on September 24, 2007, 11:01:36 PM
...Actually, I did say "Without another input source, such as wheel sensors, it's just not possible for a system based solely on GPS to know [how fast you're going all the time]"...
Sorry I missed that... You are indeed correct. Not trying to clarify for you necisarily, but rather for others ;).
Quote from: Sean on September 24, 2007, 11:01:36 PM
...It's taken twenty years for the major consumer GPS players to get back around to installing dead-reckoning inputs to their navigational displays. Cheap accelerometers and other technology that avoids the problems of installing those pesky sensors out in dirty places like the wheels is one of the reasons -- it's finally becoming cost-effective to add some of the terrestrial-based information back into the system...
This was the point I probably didn't make very well, but you definately filled in the blanks :).
Cheers!
-Tim
My GPS speedometer works great since I plugged it back in. I couldn't get it to work initially and left it unplugged. I called the company and they said it needed to to be plugged in for 20 minutes the first time before it would acquire lock.
The only issue is losing lock twice due to the antenna turning sideways. I need to make a mount for the antenna.