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Car is doesn't know which way it is pointed
- Thread starter Skotty
- Start date
Yes, because a compass is a magnetic field detector, and putting one of those inside a steel cage (yes, the model 3 is mostly steel). that is full of wires carrying up to 1000A and driving in tunnels full of rebar next to other vehicles full of who knows what makes them worthless. Once you have a lock via GPS even once, the gyros are a much more accurate way to track heading.
It's not about the cost.
Source: I've designed electronic compasses for airplanes. You can see the distortion in them when the airplane taxis over rebar in the pavement or near a building or other aircraft. Calibrating them is a massive pain in the butt, and planes are not built out of steel. Then you get into the fun of "soft" and "hard" iron- some things actually bend the earth's magnetic field, but differently depending on the way the field enters them.
This is why the old compasses in mirrors only said N, NE, E, etc. But that's nowhere near accurate enough to navigate by. You need basically degree precision.
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Well yes I'm aware of those issues, but I'm skeptical that you can find north with JUST a gyro and GPS (in a car that is). And out of curiosity I just took my iPhone for a drive in my "steel cage" model 3 and it had no trouble tracking north using its built-in compass, though its certainly possible it was also using the gyro to compensate somewhat.
I'm confused about how you can find North using GPS and just gyros? GPS gives you position, but I'm not aware that the antennas on cars are clever enough to give you direction as well? To be sure, once you HAVE north the gyros can kick in, but until then???
GPS would be enough I think... Gps gives you position yes. Once you start moving, look at the various positions you receive multiple times per second. IF you do the delta between two positions you'll get a direction. With the coordinate system, you know which direction is North, normally Y axis positive. The problem with just GPS is that you don't know which way you are pointing until you start moving...
Like @GtiMart said, GPS absolutely can give you direction as long as you are in motion. It's super accurate direction too. Once you get a single update of direction, you can now use Gyros to integrate turns when you don't have GPS. You can even use GPS to calibrate out the gyro drift over time, which then makes the gyros even better when they're needed.
The fact you took your iPhone for a drive means it's highly likely it was also using GPS in the background. What app did you use to be sure it was only showing you raw magnetometer based direction ("compass") with no augmentation? If you do find a raw compass app, you don't need to go for a drive. Just move the phone around in the car, and you'll see how much it changes with location in the cabin.
There are actually ways for GPS to give direction without motion- it just requires two antennas with some physical separation. Now the direction is just the line between the difference in position of the two antennas. They'll often do this with big ships that need to know both what direction they are pointed as well as what direction they are going (which are not the same thing when you are floating on a fluid or flying in the air - heading vs track). Airplanes and boats are like cars that are drifting all the time.
The GPS was giving multiple updates because every 15 seconds or so the car on the nav would snap back onto the right location before it would start driving into the lake or into some building again. I just checked Google maps, and the last time this happened the trip was about 4.5 miles and 8 minutes by Google estimate before it finally corrected it's direction. This was driving in a suburban area, just housing and some trees, mostly 4 lane divided road so it's not like the tree cover was significant.
stopcrazypp
Well-Known Member
This is actually fairly trivial and worked like this for ages in standalone GPS units without built in compasses. Just travel along the road and the GPS compares the vector between the points you have traveled. That immediately gives the direction the car is travelling. It only requires a short distance of driving for this to work.
stopcrazypp
Well-Known Member
Are you saying only your direction is wrong or are you saying the location wrong also? Has this always been how it was in your car? If this is something that popped up recently and also puts the car at a wrong location, it may be related to this thread:
GPS issue since the past few weeks
Someone claims the cause is the cabin camera interfering with the GPS antenna. If cabin camera is disabled and the problem goes away, that should be an easy way to test.
The way GPS syncs the direction should not take minutes to accomplish if there is a GPS signal. It should literally take a few seconds of driving with GPS coverage for it to work as it really only needs 2 different points to determine your travel direction.
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When it would snap back, the location was correct, but the arrow was pointed in the wrong direction. As I continue to drive at, say, 35 MPH, as I would move 35 MPH one way, the arrow would move 35 MPH in another way (the way it was pointed, which is not the way I was actually pointed), so the location gets more and more off, until a short period of time goes by (which I estimated at 15 seconds or so), at which point it would snap back to the right location again, with arrow still pointed in the wrong direction. As noted, this lasted about 8 minutes over a course of about 4.5 miles last time it happened, and the prior time it was roughly the same.
stopcrazypp
Well-Known Member
So basically your location is wrong also, that seems very similar to the problem described in that thread. Interference with the GPS antenna certainly can cause something like this. If this only happens at a certain location however, it may also be poor GPS reception.
It makes sense for the arrow to flip in the opposite direction given the location is snapping back and forth (and there may be some buffering/hysteresis going on that prevents it from immediately flipping back, which generally helps deal with noise in the data)
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Hm...yes, at least a few of those people seem to have the same issue. It hasn't always been present. Seemed to start a number of weeks ago. One theory on that thread is that an update activated the cabin camera and that is causing interference of some sort. And that is why it is only a recent problem. Seems like a solid theory. My concern is that service center will just randomly replace stuff without actually fixing anything. The problem with that under warranty is that it is wasteful, and every time you take part of the car apart and put it back together, it adds wear. Clips break, trim gets damaged, parts get looser, etc. Counterproductive. I have a service appointment, and they are ordering a part, but I'm feeling rather dubious about it.
I took the car in to the service center, where I'm not sure anything happened (maybe diagnostics?). Then they made a mobile appointment for a week or two after that. On the mobile appointment they replaced the GPS antenna. Honestly, I haven't driven the car enough since that appointment to know if it's fixed or not. It was intermittent to begin with.
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