Travel Time Estimate

[charliek]

Had new Z2 installed this past weekend, works great. However I have noticed that the travel time estimate when using route guidance is 30-40% long. Is this normal, or could something be hooked up wrong?

[inferno999]

On new installs, the speed isn't 100% accurate as it takes time to calibrate itself to your car. Having the VSS hooked up helps, but it's not necessary.

 

Also, you need to tell it how fast you drive on freeways and other roads. By default, it assumes you drive the speed limit. The setting is under Regional Settings.

Menu -> Settings -> Regional settings -> average speed.

 

For more, see page 97 in your manual.

[charliek]

On new installs, the speed isn't 100% accurate as it takes time to calibrate itself to your car. Having the VSS hooked up helps, but it's not necessary.

 

Also, you need to tell it how fast you drive on freeways and other roads. By default, it assumes you drive the speed limit. The setting is under Regional Settings.

Menu -> Settings -> Regional settings -> average speed.

 

For more, see page 97 in your manual.

 

Thanks! I set the speed averages the first day, so I'll just drive for a while and see if it gets more accurate over time. Thanks again!

[mikek]

when you are on a trip and you know about how long it takes, set your travel speeds until the time is about right.

[charliek]

when you are on a trip and you know about how long it takes, set your travel speeds until the time is about right.

 

Great minds think alike! I had just the same thought, I'll do that on the way home tonight. Thanks.....

[kafklatsch]

Hello All,

I have been using the GPS for travel to the tune of about 2,000 mi/wk. I have learned that I can use it as a guide with the following realizations.

1) I set my highway speed to 75 mph average(your avg speed setting may vary),

2) the secondary road average speed and the residential street average speed are not very accurate (how can they be - depending on time of day, traffic patterns and general population density which very greatly between Des Moine and say Manhattan - the estimates could be very close or very far off),

3) the accuracy goes up if you have a medium to long trip with lots of highway travel (and a good avg speed setting by the driver),

4) the accuracy goes down if you have a short to medium trip (1-3 hours) with secondary and residential routes.

Finally, the system keeps triangulating as you drive until it gets very close which is "not coincidentally" near the end of your trip. It is not possible for any system to be accurate with any consistency. I have found that adjusting my highway setting has given my best eta's. the other road type speed settings are so "hour to hour traffic dependent" that I dont even bother to adjust them. play with the settings for your driving conditions, it may help.

by the way, nothing within the Avic software will magically change this situation.

[rigpig]

I thought a GPS system was able to calculate speed ? Why should you have to enter an avg speed ?

When you display the Vehicle Dynamics Screen and use the speedo, where does that signal come from ? Is there a way to get the speed based on a GPS calculation to be displayed ?

 

I found this article, which I thought I would share :

GPS Speed:

How accurate is it? How fast can I go? How HIGH can I go?

 

GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed.

 

HOW ACCURATE IS THE SPEED READING?

 

 

From the NAVSTAR GPS User Equipment Introduction document Section 3.7:

 

GPS receivers typically calculate velocity by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent, (multipath, obstructed sky view from the dash of a car, mountains, city canyons, bad DOP) but 0.2 m/sec per axis (95%) is achievable for PPS and SPS velocity accuracy is the same as PPS when SA is off.

 

Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on.

 

HOW FAST CAN I GO AND HAVE MY GPS READOUT MY SPEED?

All Currently manufactured CONSUMER GPS receivers we know about will measure speed to 999 miles per hour.

 

Garmin (uniquely as far as we know) manufactured the G-38, G-40, G-45, and the G-II (not PLUS) units which had a 90 mph speed readout limit. These units are manufacturing discontinued and we know of no other manufacturer who made a unit which would not go to 999mph.

 

HOW HIGH CAN I GO AND HAVE MY GPS READOUT ALTITUDE?

Defense department regulations prohibit standard consumer GPS receivers from functioning above 60,000 feet and 999mph (simultaneously). Most GPS receivers seem to set hard limits at EITHER 999mph or 60,000 feet.

[ecrawford]

I hate this about the Z series units. My little garmin nuvi is much more accurate at deciding my arrival time past on my average travel speeds during the trip.

[kafklatsch]

another dead tread that left a little confusion.

 

the GPS actual speed is very accurate. But the GPS cannot tell the future, i.e. your estimated time of arrival.

 

ETA is a logical calculation based on average speeds over certain roads. Many variables change that estimate..... something as simple as a meal stop... to something more complex like "why everyone and their mother left the exact same time you did"... the navigation systems has some way of assigning these speeds based on the road traveled.

 

The average speed setting you select with the avic is based on your own driving habits.... you may drive slowly and average 55 on a 70mph posted hwy when traffic is factored in..... or you may average 80 on an overnite jaunt..... thankfully you can adjust it.

 

the navigation system only uses an average speed based on road type (interstate, secondary, rural residential, etc..)and figures in a certain amount of "congestion and other conditions" to come to an average speed over those roads and thus gives an "ETA". The initial estimate is not even a factor of actual GPS speed data.

 

it will give you the exact same time for driving a 200 mile trip if you leave in rush hour or if you leave at midnight. your estimated arrival time will only change as you put miles behind you. it will then keep recalculating using the same factors AND the amount of trip remaining. If you are hauling A** the estimated time drops. If you get stuck in a 4 car pile-up your estimated time will increase..... the GPS actual speed only plays a part in trip time estimates, and thats after you have put some miles behind you..

 

A simple example. If you are driving 100mph and you have 400 miles left on a rural hwy with a 55 mph Posted limit. the navigation system WILL NOT give you a 4 hr ETA (100MPH X 4 hours = 400 miles). It WILL give you something like 8 hrs because it ASSUMES you will be traveling just under the 55 MPH posted speed (due to many factors) over the balance of your trip , this despite the fact you are looking at 100 on the speedo....

 

hopefully this clear this topic up

 

 

I thought a GPS system was able to calculate speed ? Why should you have to enter an avg speed ?

When you display the Vehicle Dynamics Screen and use the speedo, where does that signal come from ? Is there a way to get the speed based on a GPS calculation to be displayed ?

 

I found this article, which I thought I would share :

GPS Speed:

How accurate is it? How fast can I go? How HIGH can I go?

 

GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed.

 

HOW ACCURATE IS THE SPEED READING?

 

 

From the NAVSTAR GPS User Equipment Introduction document Section 3.7:

 

GPS receivers typically calculate velocity by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent, (multipath, obstructed sky view from the dash of a car, mountains, city canyons, bad DOP) but 0.2 m/sec per axis (95%) is achievable for PPS and SPS velocity accuracy is the same as PPS when SA is off.

 

Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on.

 

HOW FAST CAN I GO AND HAVE MY GPS READOUT MY SPEED?

All Currently manufactured CONSUMER GPS receivers we know about will measure speed to 999 miles per hour.

 

Garmin (uniquely as far as we know) manufactured the G-38, G-40, G-45, and the G-II (not PLUS) units which had a 90 mph speed readout limit. These units are manufacturing discontinued and we know of no other manufacturer who made a unit which would not go to 999mph.

 

HOW HIGH CAN I GO AND HAVE MY GPS READOUT ALTITUDE?

Defense department regulations prohibit standard consumer GPS receivers from functioning above 60,000 feet and 999mph (simultaneously). Most GPS receivers seem to set hard limits at EITHER 999mph or 60,000 feet.