You guys are so helpful... I just noticed recently that when I'm watching my Nav map, sometimes my car will jump to a different street, one that's right next to the one I'm on. Sometimes, if I'm driving on a freeway, and I drive past a ramp, my car's position will jump over to the ramp, when I'm actually still on the freeway. Does this happen to anyone else?

The only thing I can think of that might cause this is that I just put in a 10" sub and it is in a box directly underneath the Nav unit in the trunk. Will the magnet from the sub adversely affect the Nav?

Another thing that happens with my Nav is that when bass hits on the sub, the screen gets a few small lines through it (interference or electrical noise?). During the install we originally were using 12 gauge wire for the battery lead to the amp and the interference was really bad. After replacing the 12 gauge with 8 gauge wire it improved quite a bit, but I'm still getting a little interference. Anyone know a way to eliminate this?

Thanks again.

 

i get that jumping every so often (no sub in trunk). once it happened when there was a service station to the center of the freeways, and i went straight but the nav thought i went to the service station way, but eventually jumped back.

 

I’ve had this happen a few times, but nothing that was a major nuisance.
The “calibration” of your NAV may be off by a bit.

I’ve had the BASS pumping so LOUD that my V1 was bouncing of the windshield glass and my NAV has never skipped a beat.

I think it has an amazing shock absorbing system in the drive case.

Shawn S

 

It happens all the time. We are being positioned by satelites. Although it looks as like the roads are far apart to you, just think if you were the satelites. How far is it then? These things are not perfect. At least in seconds it corrects itself!

 

Every once in a while I will get that "noise" pattern in the NAVI screen. Generally when I'm shutting down. I am not sure what it is, but intermittents are a pain to fix.

The NAVI jump around seems to happen to me about the time I say, "Gee, the NAVI has been working perfectly".

The GPS system (without a special ground correction station) is only accurate to 50-150 feet. If you are driving along, the map is trying to make decisions on where you are. This same logic is what helps you out when your off-road, then get back on the road and it moves the car's icon back on the correct street.

I could put you to sleep with all of the issues going on, but as long as it works MOST of the time, you are ok! The jumping to parallel roads is documented in the manual (at least so I though).

 

yeah, this happened to me twice on a trip to myrtle beach, sc. i missed two ramps onto the highway because of it. i guess if its accurate most of the time, it should be ok like ericl said. i've had the car since feb. and its never happened before. we'll see what happens.

 

The Navi uses a combo of GPS and inertial navitgation to determine position. The GPS is only acurate to (gulp) +/- 10 meters for commercial/civilian uses.

The military also runs occasional test on the satelites which WILL screw you up - get used to it. All in all, the system is still the best available.

 

My Navi does it also...it is a common problem. Part of the problem is that the System we have in our cars uses an 8 Channel Receiver instead of the Better 12 Channel ones like in the Boat and Hand Held models.

Also the system caculates your postition from the satilites ... now depending on the time it takes the siginal to return to your Receiver (this is affected by weather or Ionosphere delays) it could have you in a different postion all together. That is where the software comes into play.....it assumes that your still one the same road that you were before. If the siginal is just a little late, the software tries to correct that and determine the spot where it thinks you are (This is where the system jumps from road to road). Also when this happens check the Upper Left hand corner of the Navi screen and see if the Icon is Green or Yellow .....Yellow means that the system has you on a 2D Fix....this is alot less accurate that when the system has a Green Icon. The Green means that the system has you on a 3D and is caculating your alitutide along with your current position. This is very accurate(could be as close as 3 meters, depending).

There is probally no fix for this it is just the best the system can do with the equipment that we have. I hope that this helps and does not confuse anyone.

 

The "inertial navigation" is actualy just the wheels sensors for forward motion, an additional sensor for steering position, and finally a yaw sensor (inside the NAVI) for angle (this is the only inertial reference). The wheels are the only source of forward motion info (according to the Helms). The GPS updates the position it has inferred from the wheel movement and compares it to the GPS lat/lon info to do on the fly corrections.

An interesting note -- someone suggested that it didn't matter if the bars were yellow (you would just loose the altitude -- this is true). However, if you are going up and down hills, the true length of travel is NOT correctly calculated without the altitude (Z-axis) info -- this can allow for errors!!!!


Also, the amount that the signal is off (wihtout differential GPS):


Standard Positioning Service (SPS)

Civil users worldwide use the SPS without charge or restrictions. Most receivers are capable of receiving and using the SPS signal. The SPS accuracy is intentionally degraded by the DOD by the use of Selective Availability.

SPS Predictable Accuracy
100 meter horizontal accuracy
156 meter vertical accuracy


http://www.colorado.edu/geography/gc...gps/gps_f.html

Depending on the positions of the sats in the sky, additional errors can result -- see the link above for any and all info you could want...

(I lost the site where this guy keeps a day-to-day log of commercial band accuracy -- it is shown on a web page as a 2-d scatter graph.)

 

Here is some more info that I thought you might like to see:
Sources of GPS signal errors


Factors that can degrade the GPS signal and thus affect accuracy include the following:

Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.

Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.

Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.

Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.

Satellite geometry/shading — This refers to the relative position of the satellites at any given time. Ideal satellite geometry exits when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.
Intentional degradation of the satellite signal —

Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

Garmin Home Page

 

As a former Air Force officer who controlled GPS satellites (up until January), I thought I'd chime in here. Fast-CL is right...selective availability was turned off in May of 2000, so even though guaranteed accuracy of the civilian code (SPS) is only 100m, the actual accuracy will be much better than that. Besides the ionospheric delays and other causes of interference mentioned earlier, the position of the satellites makes a huge difference in accuracy. In laymans terms, satallites farther apart give greater accuracy then several right above you. So even though you may have 7 satellites in view (our receivers don't tell you how many they see), your position reading could actually be less accurate than if you had 4 widely spaced satellites. These things fly at an altitude of 10,998 NM, (as a point of reference, the shuttle flies at ~200 NM) so to accurately triangulate at that range, you need to have satellites spaced appropriately.

 

You guys are correct -- the scatter plot I was looking at was a pre May 2000 plot...

Here is the transistion on May 2000, with the accuracy doing a dramatic improvement:



Picture of sats in non-optimal positions:


Ideal/better positions:


Stuff a lot of us have to deal with (like me):


[ 07-11-2001: Message edited by: EricL ]