I was just crusing today and happen to find out some interesting stuff.

Ok, awhile ago, there was a thread about finding out your actual moving speed in the little climate control dash. But it sucked as it could only display speed in km/h and it was by 10 digits... but now, i found out one that displays in .1 significant digit!

So this is how you do this-

1) go into nav diagnosis screen. (hold map, menu, setup buttons altogether)
2) go into GPS information
3) hold menu button for 5 seconds
4) in the middle, you shoule be able to see your speed and much more info

I can't say it is the EXACT speed that you're getting but I think it's pretty accurate. From what I think, the speed is measured by GPS system.

I hope this isn't a repost!

 

I don't have the factory Nav, but from my experience, the GPS calculated speed is usually about 2 seconds delayed, so that's something to keep in mind if you're going to use that information...

 

what does this tell me that the speedometer doesn't ?

 

I'm not sure I understand the point of looking at your speed in the NAV screen. Didn't your TSX come installed with a speedometer?

 

ditto on the other two posts... very confused why you'd go through all that trouble to find out what the speedometer tells you.

 

GPS works by taking points. So the system is measuring the time you traveled between two points, and giving you the speed.

So was it similar to the speed on the speedometer, or was it only in the km/h?

 

Digital vs. Analog

 

I have to say that this the most rediculous thread ever posted. Why would anyone want to lose the everything the nav screen displays (Audio/AC/Nav) just to get a DIGITAL readout of your speed?

 

If you change tire sizes, your speedo could be off, but the nav speed display will still be very accurate.

 

That screen also shows your compass direction in degrees as well. That is something I wish was available on the default map screen. I'm sure all the pilots and sailors will agree with me that it should have the option such that when I'm heading due east, my compass should say "090" instead of "E."

But yes, the speed on that screen is computed from satellite triangulation rather than wheel rotation.

 

Are you sure that's how the TSX does it?

Many navigation systems calculate speed based on the Vehichle Speed Sensor aka VSS wire from the ECU. This sends out a certain number of pulses per second based on your vehicle speed.

 

I'm not sure that's how Alpine does it, but that is typically how GPS calculates speed.

 

no, Map + Menu + Cancel buttons

 

Not really a dumb post at all IF the TSX nav uses the sats to determine speed. Speedometers read low on purpose and this lets you know exactly whet the speed is, nice to know if you travel a lot on the highway so you can run the real speed (say 78 in a 70 zone) not just the speed your speedo says. I have used stand alone nav systems in all my cars at least once just to determine the real speed. My cars are between 4-6 miles slow at an indicated 70.

 

Um, hate to burst everyone's bubble, but the GPS calculated speeds are usually inaccurate for many reason's, I'll give you just two:

1) GPS speed calculations assumes you are travelling in/on a level plane, unless the road is very flat and straight, this will affect the calculations, sometimes a lot.

2) The navigation system uses the best 4 satellites out of all the ones visable (typically 4-8 visable at any one time), as it switches between satellites (due to loss or incomplete navigation messages from the various satellites), the accuracy of the calculation changes 'cause different satellites provide better signals than others, this can cause big fluctuations (the ERD's on satellites can vary by several meters).

GPS calculated speeds are very handy/accurate on bodies of water, so the Navy loves to use it (usually better than other ways), or when you don't have some better way. Still, it's fun to play with

 

thanks for clearing it up

 

^^^ what he said


maybe this is really useless but it doesn't hurt to know

 

Now actually, this brings up an interesting point. I dont think that the speedo will ever be 100% accurate for the reason being when i measure my mileage from "trip computer A or B" and then i flip to the trip computer. The #'s NEVER have matched up before. By the end of the tank, it always ends up being like .5 miles off. I know it might be the tire tread or something minor like that but it's been doing that since day 1 and since i've had it a year. Wierd...

 

1.) No, that is wrong. It triangulates your position, so it will be just as accurate on a 20% grade as it is on the flat. Otherwise it would give different readings at sea level than at higher elevations on the flat.

2.) You only need two satellites for triangulation. Four satellites gives you six different pairs from which to triangulate. Those six different calculations will be plenty accurate.

The only thing you'll notice is about a half second to full second lag on the change of speed.

 

1) you are wrong and right at the same time, it WILL triangulate your position and be just as accurate at 20% grade as dead flat. HOWEVER, if you are driving up a hill and go 1 mile you may have only covered .80 miles and gone up some, think pythagorean theorem, a^2 + b^2 = c^2. so you're wrong, the gps will think you have travelled .8 miles when in reality you covered a mile cuz you were going uphill.

 

Sorry, wrong again. If you'll look on those diag menus, you'll notice an altitude reading. Why would you think the engineers would have missed something so obvious?

 

So the GPS track positions in 3D? That's pretty cool. Makes me wish I had nav.

 

Your map display doesn't show topo, but yes, the system does calculate your altitude based on it's triangulation.

 

Very cool...

 

It would seem to me that you need 3 satellites for a triangulation. 2 satellites would only accurately describe a circle around the earth on which you could be sitting.

 

i didnt really think of that...i stand corrected

 

No. Two satellites and one receiver constitute a triangle. All you need is the target (your car) and two external points of reference (two satellites) to get a triangulation calculation. Having three satellites gives you three of these calculations - one for each grouping of two. (If you have satellite A, B, C, and car X, you could have ABX, ACX, BCX.) Having four satellites gives you six calculations, etc. The accuracy of the calculation with 4+ satellites is very accurate, because you are getting multiple confirmations of the results.

 

Bear with me here. Let's say you've got sat A and sat B in geosynchronous orbit over California and New Jersey on the same latitude (35 degrees). If you are in Oklahoma at 32 degrees latitude, sat A says you are X distance away and sat B says you are Y distance away on your receiver. If this is the only info your receiver gets, it doesn't know if you are in Oklahoma at 32 degrees or in Kansas at 38 degrees. Unless it gets info from a third satellite.

 

wow some serious debate going on here

I think answer to that is, nav can't accurately calculate speed when at slope. See it for yourself, when I was going up/down hill, the speed was significantly less than speedometer; while on flat, speed seemed to be about equal.

 

No. It's not just distance, it is vector. Distance could be in any direction; vector is a distance in a specific direction. Look on the diag screen under nav info. You'll see the actual positions of the satellites.

 

I haven't noticed that to be the case. If that is true, then it is a limitation (or more accurately a programming logic error) of software rather than a limitation of the GPS technology.

What I have noticed is that there is a 1-1.5 second update lag, as it will often show me travelling at ~15MPH a second or so after I stop before it drops to zero.

 

Unfortunately, I don't own a TSX yet (dammit). If the sats do work on vectors, you're absolutely right. In fact only 1 bird would be necessary, but that would be grossly inaccurate. But we're not talking about triangulation anymore. Sounds like expensive technology, maybe $2000 for the navi is a bargain

 

There's way too much misinformation about GPS in this thread.

If you don't want to really know how GPS works, don't bother replying or reading this topic. However, if you're genuinely interested in how GPS systems really calculate your position on the earth's surface, please click here.

Click here for even more advanced info.

Note that OEM NAV systems typically combine GPS signals with a gyroscope and the VSS to estimate position in the temporary absence of GPS signals.

 

Actually, I'm not wrong. 3 satellites *will* give you a fixed location, but no altitude, you need four satellites for that. So simply put, you need 3 satelites for a 2 dimensional fix, 4 satelites for 3 dimensions. To calculate speed correctly, you need to measure a change in location in *3* dimensions, continuously, preferably using the same 4 satellites (which, hopefully, are 4 of the better performers).

I'm curious, you speak with authority, your expertise on GPS comes from...?

 

This is mostly correct. However, you do not need more than 3 sats to fix a location in 3 dimensions, assuming that the sats are not all in the same plane. This also assumes that the navi software is aware of altitude changes on the ground. Otherwise, it would presuppose that you were floating on air.

I don't speak with TSX navi authority (yet), but I do speak with college physics knowledge.

 

I stand corrected. I spoke only from the perspective of mathematical requirements. Apparently the GPS implementation uses other methods as well, which require more than 2 EPRs for a fix.

 

hmm.. never mind my comment. I checked out again today and it seemed to be equal either it's flat or sloped. I think the lag caused me confusion when I was going down hill since the speed was gaining..

 

The error in speed accuracy when traveling on a grade as opposed to flat is statistically insignificant. That is, any error will be many times smaller than the error caused by GPS position calculation accuracy.

Even on moderate slopes (up to 8%) you go up 8 feet for every 100 feet traveled. The actual distance traveled every 100 feet is 100.08 feet. That's negigible.

 

how do u display speed in the climate control dash????

 

If you mean the NAV screen, read and follow the instructions in the first post of this thread.