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Discussion Starter #1
Does it automatically re-calculates the route guidance if you miss the turn?
 

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hey guys how much did yiu score for the streetpilot III.
How is it comparable to the acura nav?

By the way BRANDNEW4LESS.COM has the streetpilot III for $789 no shipping.

have some flip down screens too
 

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Acura's nav system is only an 8 satellite channel unit, whereas the StreetPilot III has the newer (and much more accurate) 12- channel system. Typically, the estimated horizontal accuracy on my StreetPilot III is 17 ft.
 

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Would not let the number of channels lead you to think "more accurate".

The Acura uses inertial nav and speed pulse as well as GPS, so it is tough to directly compare...but is probably "as accurate". The main error with GPS in today's systems (Garmin or Acura/Alpine) is the GPS Sats themselves.......we do not get the full military accuracy...
 

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Inertial gyros can only let the nav system know when the vehicle is turning (or changing velocity), but they can’t provide information regarding initial position (i.e. latitude and longitude numbers); that information is provided by the satellites. The StreetPilot III tells you which way it’s pointing (its heading) when it is pivoted (in my hand) about a point. Hence, it has the same capability as the Acura nav system with respect to heading information plus it is a 12-channel satellite system, which indeed is far more accurate than Acura’s 8-channel system. No doubt, this, along with possible erroneous map data, accounts for reports about Acura’s nav unit incorrectly showing the MDX driving along a road parallel with the one it’s actually on.

Below, I've provided a link comparing 8-channel accuracy with that for 12-channel. 8-channel systems have an average horizontal accuracy of about 27.5 meters versus an average accuracy of about 11 meters for 12-channel systems. That’s an increase in accuracy by a factor of 2.5 in favor of the 12-channel system.

By the way, the military no longer degrades the satellite signals since it has stopped S.A. (selectivity availability) for a few years now. Nav systems with WAAS (Wide Area Augmentation System), which are available to the public, have a horizontal accuracy better than 3 meters!

Typically, I find that the StreetPilot III is accurate (real-time estimate) to about 17 ft., but it has been as good as 12 ft. and as bad as 24 ft. depending on how many satellites it is receiving data from. Does your Acura nav unit give you a real-time estimate of its accuracy mcclendons?

Vicpai is very informed about nav systems, for his perspective check this out: http://www.acuramdx.org/forums/showthread.php?threadid=4731&pagenumber=2



Reference for (12-channel vs. 8-channel GPS nav units): http://www.garmin.com/support/faqs/8.html

Reference on WAAS: http://www.garmin.com/support/faqs/24.html
 

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There's no real-time estimate of positional accuracy in the X's Nav, but you can get a sense of the scatter by making multiple passes down a road thats not in the database - the scatter in the breadcrumbs gives a rough estimate of accuracy. The error seems to be 10-15 m, not bad, but enough to occasionally confuse a frontage road with a highway.

Note that the Nav's algorithms use a snap function to put the car icon on the nearest road, usually a reasonable assumption.

Ocean, where'd you get your info on the number of sats that the Acura Nav uses?
 

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Does the Acura Nav system really use gyros? Or does it use solid state accelerometers?

The salesman that took us on a test drive made the mistake of taking us on a brand new section of interstate. The nav system showed us driving across an empty field. It was pretty fun. Needless to say, I didn't order the Nav.

Does the Acura system have voice prompting like the Garmin?
 

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Hey WC, did you go to Beurkle? They did the same thing to me! Stretch of 694. Think they'd have figured that out by now! :rolleyes:

Yes there's a voice prompt, most of us have gotten to like her.

The lit calls it a gyro...
 

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ghost, wckrause...

Ocean, where'd you get your info on the number of sats that the Acura Nav uses?


Originally, I got the information stating that Acura's nav unit is only 8-channel from Vicpai's post (his research has proved to be reliable in the past), the link to which I provided in my last post. I also researched Alpine's nav unit since it is my understanding that the Acura nav unit is based on it. On Alpine's web page regarding their nav unit, they state that it is indeed only 8-channel and has gyro sensors. Perhaps the "gyro sensors" are only electronic as you suggest wckrause - this is very likely. The link is: http://www.alpine1.com/html/D2_n_1_n_n.html
Then, find their nav unit in the list of products.

The Acura nav unit has voice prompting.
 

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Yes it was a Beurkle salesman taking us up on that new road. The guy was brand new and looked to be about 17. He didn't know anything about the MDX much less the Nav system.

"Gyro sensors" doesn't sound like a mechanical gyro to me. There's no way they're using laser gyros (it would cost more than the car). Must be a combo of dead reckoning and GPS as you described.

Still waiting for delivery!
 

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Ocean,

With GPS, it takes 4 channels (SATs) to get a fix. An extra one or two will get you a better fix, but with each SAt the amount of "better" gets smaller. Some 8 channels (and some 12s) do not even look at all channels at once---6 or so gets a very good fix. 12 would not get a substantially better fix than 8 if data to them were perfect.

When I said the main error is "SATs", I did not mean the "SA" for the military that you mention. I was refering to the signal propagation effects from the SATs, timing errors in the SATs, etc. The militay does have an extra capability not avialble to Joe Public, and that is how that can get some more error out----but once again this is differnt than SA.

My SP III does not give me a direction, until I start moving. That direction does come from GPS, vs gyros. In the Acura, you have the gyros (would guess solid state accels but?????) to give turning, plus a speed pulse to give movement. The Acura will keep tracking in a parking garage (with some error), the SP III loses lock.

The importance of 12 channels is not accuracy, but is how quick a system can acquire once turned on and how well it does when a SAT drops off horizon and it has to find a new one. "More is better" here, up to 12 (the number of SATs one could expects to see at Max). Some comanies will preach the "12 is more accurate" to sell a product, but missing the boat. Attached is a link for info:

http://celia.mehaffey.com/dale/why12.htm
 

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mcclendons...

12 would not get a substantially better fix than 8 if data to them were perfect.


Since it only requires 4 satellites to get a 3-D position fix, if the data were perfect, that's all you would need - just data from 4 satellites and no more with nav unit capacity to track 1 or 2 more to replace any satellite dropping below the horizon. The problem is that the data are never perfect, and that's why a 12-channel nav unit is more accurate than an 8-channel unit.


The militay does have an extra capability not avialble to Joe Public, and that is how that can get some more error out----but once again this is differnt than SA.


The public has access to WAAS nav technology, which has been measured to provide an accuracy of 2-3 meters. What navigational technology does the military have that is better than that? Or would you have to kill me if you told me?:D ;)

From the source that you referred to:
Aren't 12 channel parallel units more accurate?”
“Not necessarily. The accuracy specifications for both parallel and multiplexing units are often the same. In practice a unit that can calculate an over determined solution is likely to produce a slightly more accurate position. However, Selective Availability reduces the accuracy below the threshold of any differences between the units in this regard.”


“Not necessarily” only because of Selective Availability – which now is turned off! When the StreetPilot III is receiving data from 11 satellites, estimated accuracy is typically 12 ft. When only 7 or 8 satellites are tracked, accuracy drops significantly. This empirical fact (observed while watching the StreetPilot III in operation) is consistent with the information contained in the reference that you provided. Mathematically, it only takes 4 satellites to provide a 3-D position fix. The problem is that data from each satellite has some error associated with it. In an “over determined solution” the data from the additional satellites tends to reduce the error introduced from the first 4 satellites for a more accurate position fix. However, as more and more satellites are added you do get diminishing returns with respect to accuracy.
 

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SA Turned Off?

If SA degration has been turned off and the ACURA nav cost $2K, why is it that we can't do better than positional accuracy of 100 feet?

BTW, I thought SA was reactivated after September 11, 2001.

Are there any performance advantages to the Acura Nav versus a handhold units beyond the integration/theft/aesthetics/functionality considerations?
 

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mdxpecting...

S.A. was not turned back on after Sept. 11. This afternoon in Las Vegas, the estimated accuracy for my StreetPilot III was 12 ft. (the unit itself calculates this) while 11 satellites downloaded data into it. No way would this be possible with S.A. active.

It is my understanding that Acura's nav unit has many more "points of interest" (POI) than does the StreetPilotIII. So its data base is superior in that regard.
 

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I spent the day yesterday GPSing a number of field plot locations with a WAAS enabled 12 channel Garmin GPSmap76. A couple observations:

While the system could pick up 12 Sats, it generally would only pick up 6-8, and mostly overhead. These were under pretty much open-sky conditions, some peripheral forest canopy which might obscure low horizon satellites (i.e. the ones that would give the best triangulations). Also, the satellites constantly wink on and off, with corresponding changes in accuracy.

The best accuracy I was getting was 4-6 m, but generally it was 8-10, and sometimes as bad as 12 or more. I felt pretty stupid wading 3-4 m into a wetland to get a good fix on a location that has an inherent error of 30 ft! :rolleyes:

There is a lot of variation in accuracy that most folks aren't aware of - the key thing is that you have sufficient accuracy to achieve your purposes. Chances are with a handheld GPS, you'll be able to find your X if you get within 10 meters of it! With the X, the system can tolerate a fair amount of error and still keep you on the right road. If you're doing surveyor work, you'll need to pop for the additional post-processing steps to do the differential corrections, etc.

BTW, just because the military turned off SA doesn't mean they don't have other tricks up their sleeve. There is that meter based fiber optic grid they've laid under the countr,,,aaaahhhh...
 

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The main advantage an integrated unit has is that when GPS is lost (3 or less SATs), it keeps tracking using the gyros/speed pulse. That way, when you get GPS back in 20 seconds (or whatever) you probably have not missed a turn.

The the SP III, in downtown DC for example, loses locks and just "sits in place". By the time it catches up......... I do love my SP III, but the Acura unit is superior (and should be).

The military capability I mentioned uses P(Y) code. Most civilian units use only C/A code, a few can take P code, but P(Y) takes special equipment. Using C/A and P code, a unit can compensate largely for atmospheric errors ( as they are 2 differnt Freqs.) greatly improving accuracy. Agree that WAAS makes a huge differnce, but many units do not use WAAS, and WAAS is not everywhere yet. I do not know if SPIII uses P code or not. I do not have WAAS on mine (or Acura for that matter)
 

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ghost...

WAAS is most suitable in aviation or in marine applications since units so equipped need an unobstructed view of the southern sky to achieve the highest level of accuracy. Since you are in the northern part of the country, the WAAS satellites are low in the southern sky and are easy to obstruct - hence you could only achieve high accuracy some of the time. By the way, WAAS covers all of the continguous 48 states, unlike differential GPS.

Regarding the StreetPilot III, when driving through the tunnel that runs under McCarran airport here in Las Vegas (must be about 1/2 a mile long), I do lose contact with the satellites (the voice prompt tells you this as well as display), but quickly reacquired contact (5-6 seconds) once out of the tunnel to make my turn onto I-215 west only about 1/3 mile beyond the tunnel with the aid of this nav unit (route display and voice prompt). When driving through the tunnel, the StreetPilot did lose heading and track (the tunnel is curved in an arc), but continued to advance the map as though it was assuming that I was maintaining velocity. Since the route (showing me where to turn) continued to be displayed, losing satellite contact didn't matter anyway since I still had the information that I needed to stay on route even if the turn had been immediately beyond the tunnel. While driving up and down the strip (Las Vegas Blvd.), I always have had contact with enough satellites to provide an accuracy of at least 23 ft or better, although the unit can read more satellites with an unobstructed horizon. Over the last month or so, I've tested this nav unit by driving about 3 thousand miles in and around Las Vegas. The only real problem that I've had occasionally is with inaccurate map data due to reconstruction in this rapidly growing city.

About one week ago, I saw the Nav Tec guys driving around town in one of their specially equipped cars apparently mapping the streets.
 

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mcclendons...

I hope that you're right about the military having nav accuracy even greater than what WAAS can provide. That way, they can target a rocket on bin Laden, not on his left cheek, not on his right cheek, but right up his ***. :D
 

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Discussion Starter #20
oceanMDX said:
When driving through the tunnel, the StreetPilot did lose heading and track (the tunnel is curved in an arc), but continued to advance the map as though it was assuming that I was maintaining my velocity.
Yes, I read a review that basically describes this feature: 20-30 seconds of "dead reckoning" for the SPIII. This should compensate to some extent for the lack of gyros.
http://www.gpsinformation.net/spiii/sp3review.htm This is a very good review with lots of technical info on the SPIII for those people like me thinking of buying it.
 
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