GPS mis-placed Triple-T

[yoda]

Hi Everyone,

Weird thing happened coming home from work today.... The nav system suddenly showed my position about 75 feet from where I actually was! It even labeled me on the wrong street!

It self-corrected after about 2 minutes, but that was weird. It WAS pretty cloudy out - does that affect how the GPS works?

Has anyone else had that happen?


Yoda

[bigdaddy]

<div class='quotetop'>QUOTE(yoda\";p=\"98833)</div>

It WAS pretty cloudy out - does that affect how the GPS works?

Has anyone else had that happen?
[/b]

I don't have Prius (yet), but have used handheld GPS units for years (the reason being www.geocaching.com) and cloud and tree cover, tree lines, tunnels, urban canyons, etc effect GPS accuracy, 75 feet off on a very cloudy day would not surprise me.

I think I read someplace about the Prius / Lexus nav sensing when things were awry with the GPS signals and it used car speed/direction to compensate, which might have happened in your case.

[jfh3]

You probably lost site of the satellites and the NAV's dead reckoning system took over to "guestimate" where you were and then self-corrected when it could see the sats again.

[Blue]

The GPS receiver in the Prius (or any car for that matter) likely has a horizontal accuracy of about 15 meters. It would be very costly to equip a vehicle with a more accurate (submeter) receiver.

A GPS receiver determines location by measuring the signal travel time from the satellite to the receiver, assuming that the signal travels at the speed of light. Atmospheric and multipath errors (mentioned above) can increase the travel time. Multipath errors, for example, occur when the signal is reflected off of another surface, such as a building or a body of water.

Another term you might hear is Position Dilution of Precision (PDOP), which refers to satellite geometry. Essentially, the PDOP increases when satellites are closer together, resulting in an increased potential for error. If the satellites in view are farther apart, PDOP usually goes down (with a decreased potential for error).

Errors can also be introduced with the Ephemeris data, as well as what is called clock drift. Ephemeris data is transmitted from the satellite to the receiver, and contains information about satellite position. If this information is in error, then the receiver will think the satellite is in the wrong location. Clock drift occurs when the atomic clock on the satellite is "off". For example, a one nanosecond discrepancy can translate into a one foot error on the ground.

Any of these errors could account for the 75 foot (or more) offset that you experienced. Since the database doesn't know any better, it just snaps you to the closest street.

[prius04]

Last fall the NAV told me that I was about 1/2 mile to the left. But 1/2 mile to the left was out over the bay. As the icon approached land, the icon kept jumping to nearby streets. When the street on the display ended, the icon seemed to stop and then jump to another street.

This happened for about 60 seconds and then bingo, the icon jumped to where we actually were.

Another time, the icon seemed to be about 100 feet off.

This is now twice in 12000 miles. The first time was during a thunderstorm, the second time was a normal sunny day.

Other than those two times, I've been very impressed with the accuracy of the NAV.

I love this car.

[Bill Merchant]

[font=Comic Sans MS:d0d2745cee]Yoda, when I start out in the morning it frequently takes several minutes for the nav to sense the GPS signals. Take a look at the map display and if you don't see the letters "GPS" under the scale icon, you are using dead reckoning.

Dead reckoning works best when the inertial sensors can provide input to the nav system, to match turns to the roads in the database. It's a pretty straight shot south out of my place, and sometimes the display shows me skidding down the road sideways, until I make a few turns or GPS is acquired.

As the manual states, the nav is not a substitute for your own good sense. Don't try to drive in the bay with Prius04... [/font:d0d2745cee]

[yoda]

Hey Thanks for all the input everyone!

I wasn't actually too concerned about the system itself. Being a compu-geek myself, I know that systems have problems at times.

But I appreciate all the responses. I love being educated about how things work, and now I know a LITTLE more about Triple-T's Nav system.

The fact that it self-corrected pretty quick DID lead me to believe that (for whatever reason) the system had simply lost the sat for a short time. Probably the heavy cloud cover at the time.

Anyway - I still say the Nav system ROCKS and I just love it.

Thanks again for the resonses. Like I said - I DO love learning about the systems.

Yoda

[yoda]

P.S. A special thanks to both Blue and Bill Merchant - Your posts where enlightening as to how it works....

8)

[DaveinOlyWA]

very curious to hear from people who have the Prius Nav. what does the Nav do with streets that are new or have been realigned?

does it still snap to the street as defined on the map or does it ignore the street data it has and give you the real position?

i ask because i use Route 66 software with a portable GPS receiver and when i went on a road that had been straightened, it showed me on my real path even when there was no road on the map.

your statement about "snapping to the nearest road" makes me wonder how the system would react to new roads that are not listed in the mapping software loaded on the DVD

[Bill Merchant]

[font=Comic Sans MS:d29bcf7a57]Hi Dave. The nav system does try to find a road if there's one nearby in the database, but if it can't, it just shows you maneuvering in space. Compass mode always shows your GPS longitude and latitude.

A new freeway exit was constructed on the way to the dealer I bought the Silver Cloud from. If I follow nav directions it will lead me astray and then back to the dealer, but it's not direct. Some people near me don't have guidance to their homes, since they are new. So nav gets you close, but you are on your own if reality doesn't match the database. [/font:d29bcf7a57]