Just picked up my RX-8 on Saturday and I've been having a problem with my NAV:

-It seems to be slightly off, by maybe 60-80 feet or so. When stopped at the light of an intersection, the map shows me further back than I really am, even when I'm stopped directly AT the light.

-When passing major street intersections, I'll pass the main street, then the Nav shows me crossing the intersection several seconds after I've already crossed it. It doesn't seem to be a delay, it just seems to be slightly off.

-Sometimes, when instructed to turn, I end up missing it because I'm fooled by the position on the map.

I'm not sure what to do. Is this normal? The map's not WAY off, just enough to confuse me on occasion. Any way to recalibrate this thing?

 

Check whether the GPS symbol comes up on the screen. The GPS signal will initialize the inertial gyros.

What I suggest, is let the car it at a staitionary location for a few minutes (5 at least) with an unobstructed view of the sky. Once the inertial gyros are initialized, it should not matter whether you have the sattelites tracked or not (if it is not too long without GPS). The inertial is your primary NAV system with GPS being your secondary.

 

It's not a true inertial navigation system like you'd find on aircraft so the idea of letting the gyros get initialized isn't completely accurate. True innertial navigation systems use a combination of gryos and accelerometers to detect the direction and velocity of motion from a known point.

Ideally you want the navigation system to track multiple sattelites and that by default is going to be the most accurate method of navigation. If you're not seeing the GPS indication on the right side of the navigation display you're having signal reception problems with the sattelite receiver. This is primarily going to be caused by two things.

First you could be in an area where sattelite coverage isn't going to be all that great. This would include inbetween large buildings, under dense tree coverage, and the obvious, in a tunnel.

The second possibility is that there's someting obstructing the antenna's view of the sky. This could be some object left on the rear window deck between your speakers (that's where the antenna is, under the plastic above the pass through) is obstructing the signal. Or you've recently had your windows tinted and the film used to tint them has properties that reduce the amount of RF energy that can pass through the glass. This is what I experienced with my car about this time last year.

The secondary navigation portion of the navigation system works in conjunction with the GPS and other sensors in the car. What happens is when the system loses it's primary source of navigation data (GPS Signal) it switches to a dead reckoning mode. There's a gyro in the DVD unit in the center console that tells the computer when the car turns left or right. The linear distance travelled is provided by sensors in the drive train almost exactly like your odometer/speedometer. The navigation computer looks at your known position, your recent heading changes, and the distance travelled and it estimates your car's position based on the map data it knows about. Basically it looks at where you have been and are going and compares it to the known road structure around you. When the GPS isn't working at it's normal level you rely more on the dead reckoning features of the system. I found that I was constantly having to do position updates manually because of the roadway changes in my area due to construction.

Additionally when the GPS is operating properly the accuracy of the distance travelled "coefficient" is updated to compensate for tire pressure changes, tire wear, and even changes in tire size if you replace your wheels/tires with non-standard sizes.

As far as "calibration" goes there's three things you can do. As azharo points out, find a place with a nice clear view of the sky and let the car sit there for a few minutes. This will allow the GPS to get an accurate fix on your location and essentially sets a starting point if dead reckoning is used.

You can manually set your position using the setup menu. Essentially you place your car at a known and well defined point on the map and then you tell the computer that's where your car is and what direction it's facing. If you're having GPS reception problems this is likely your only solution. If you have devitations from the known roads because of construction or simple map database errors you'll likely have to do this frequently.

This last "calibration" is pretty much automatic and requires the GPS to be operating normally. The computer monitors your groundspeed and distance travelled as reported by the GPS and compares it with the sensors in the car that report how far you've travelled. By looking at the two values together it can recompute the coefficent used in the dead reckoning mode. You'll likely need to do this only if the tire sizes significantly change. The description of this calibration can be found in the back of the owner's manual where the operation of the navigation system is described.

Good luck with your navigation system problems and if you need more help you've come to the right place.

 

Pavehawk,

You are right. But in the RX8 or any other factory-installed NAV system, the primary sensors are not the satellites. They use the internal accelerometers and wheel counters (crude but effective). Your primary system is the 'inertial' (for lack of a better termI.

The accyracy of GPS is controlled by the DoD. But in tiimes of war tpday, they use theatre jamming rather than degrade the whole system world-wide. Technically, they have removed the selective availability as the civilian industry (especially airlines) are getting more heavily dependent on the GPS system.

 

So, is my Nav the only one acting this way? I read someone else on this forum complain theirs was about 100 feet off as well...

Also, does adjusting the "vehicle position" in the setup menu actually work? I feel like I've tried that, only to have it revert back to the same margin of error later...

 

mine's not 100 feet off, but 10 feet or so. it's always behind at an intersection. it only started after i had my nav upgraded to the latest version. maybe they're all like that.