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Old 14th September 2010, 18:37   #1411
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@dexkid, your assumptions are probably not correct. I completely agree with DerAlte's post above.

As I have already mentioned, GPS error is not a completely "random error" type error that will fluctuate between one extreme to another extreme in a very small amount time (the sampling time).

If your assumption that the error can fluctuate from +25m to -25m in two consecutive samples were to be true, then leave alone speed measurement, it will create a havoc in using the GPS device even for basic navigation. For example, it will jump my position by 50m in the map suddenly and if I am close to a turn that I have to take, this error may result in the device thinking that I have missed the turn (it will move me ahead of the turn by 50m) and cause chaos! Also, when I am close to a turn, it gives me guidance such as the turn is 100m ahead, 50m ahead, 20m ahead etc. One would observe complete randomness in such guidance if the position error can change by 50m in 5 seconds.

I have extensively used GPS navigation in cars and have never seen such erratic behavior from the device in any situation.

Last edited by Dr.AD : 14th September 2010 at 18:44.
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Old 17th September 2010, 21:15   #1412
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Quote:
Originally Posted by tsk1979 View Post
From what I know, horizontal accuracy is dependent upon HDOP.
With WAAS enabled, and a HDOP of around 1.0, you can get around 5-7m accuracy.
With no WAAS, with a HDOP of 1-1.5 you can easily expect around 15m of accuracy.

As for speed, after you stop accelerating, GPS speed readout is very very accurate.
HDOP is an estimator. Not absolute.
Has WAAS been implemented in the Indian context?

Quote:
Originally Posted by dexkid View Post
By 50m accuracy, I meant +- 25m.

This means that at time t1 when you are supposed to be at exactly spot s1, then raw gps coordinate data is g1 = s1 +- 25m. So at time t2 when the vehicle has moved to spot s2, the raw gps coordinate data is g2 = s2 +- 25m.

Actual velocity is av = (s2 - s1)/(t2 - t1).

(worst case)
If suppose g2 = s2 + 25 and g1 = s1 - 25
then gps velocity gv
= (g2 - g1)/(t2 - t1)
= (s2 + 25 - s1 + 25) / (t2 -t1)
= (s2 - s1 + 50) / (t2 - t1).

If sampling is done every 5 secs then
at av = 40 kmph = 11.11 m/s
worst case gv = (55.55m + 50) / 5 = 105.55 /5 = 21.11 m/s = ~76 kmph

at av = 80 kmph = 22.22 m/s
worst case gv = (111.11 + 50) / 5 = 32.22 m/s = ~116 kmph

Inferences -
1> Position does matter when calculating velocity.
2> At lower velocity you will get more inaccurate results.
3> Longer the duration of sampling at constant speed, the more accurate velocity you will get. But it is stale velocity data. So approximations are applied to gps positions in software and hence you achieve the accuracy that everyone here is talking about. But the core hardware will still give you +-25m accuracy.

To my knowledge, only defence forces have actual +- 1m accurate GPS devices.

I agree that I don't have complete info. If someone has the correct information about GPS working, then please let me know and help me correct my misconceptions.
If you take a trackplot recorded at 1 sec interval, and calculate individual leg speeds with v= ds/dt, you will get wildly fluctuating readings. This is because the time part is truncated. So your dt=1s has an error component of equal magnitude. For a trackplot recorded at 5 sec interval, individual leg speeds should be pretty OK.

If you are moving very slowly/ walking, well thats a different ballgame.

For instantaneous speed readings, GPSrs normally use (or give high weightage to) the doppler speed.

MTK has brought out a chip (MTK2) which updates at 5Hz.

Regards
Sutripta
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Old 21st September 2010, 19:35   #1413
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Default How to mount it well

I went through the entire thread for good mounting options of a Garmin Nuvi. To summarize, what are the options that are convenient, don't look odd, don't attract muggles? In US, I stuck the suction cup bang in the middle of the windscreen, and didn't care about the dangling power chord. But here I don't want the power chord to be hanging around, and middle of windscreen is ... annoying. Also unmounting every time you leave the car is also painful, I want to do it only when parked in unsafe areas, not everyday (but at the same time don't want to attract someone's attention).
  • Put it where the sun visor will be. Top right corner, with a small stem suction cup. Like Google nexus one mount: http://i.i.com.com/cnwk.1d/i/tim//20...6/car_dock.jpg Also I'll bring up a wire with miniusb head via the driver side A pillar.
  • Put it on top of the dash on right corner. Maybe the suction cup can be mounted on the triangular glass. Again, wire need to run via inside.
  • On dash, somehow on left A/C vent of the driver. There should be a 4-prong fix on the vent holding the mount bracket. This is near to Cig lighter socket so short exposed cabling is possible (but the position of miniusb port on Garmin Nuvi is on the back and not on side).
  • Keep it somewhere below the A/C knobs near the gear lever. The chances of losing satellite signal is high, as well you really have to look down for navigation.
  • Sitting like a duck on top of dash with a friction mount. Can be a mess. In case of accident, a flying GPS unit inside the cabin is a big hazard.
Any other suggestions? Any tried/tested position?
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Old 21st September 2010, 22:33   #1414
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Quote:
Originally Posted by kousik View Post
  • Put it where the sun visor will be. Top right corner, with a small stem suction cup. Like Google nexus one mount: http://i.i.com.com/cnwk.1d/i/tim//20...6/car_dock.jpg Also I'll bring up a wire with miniusb head via the driver side A pillar.
  • Put it on top of the dash on right corner. Maybe the suction cup can be mounted on the triangular glass. Again, wire need to run via inside.
  • On dash, somehow on left A/C vent of the driver. There should be a 4-prong fix on the vent holding the mount bracket. This is near to Cig lighter socket so short exposed cabling is possible (but the position of miniusb port on Garmin Nuvi is on the back and not on side).
  • Keep it somewhere below the A/C knobs near the gear lever. The chances of losing satellite signal is high, as well you really have to look down for navigation.
  • Sitting like a duck on top of dash with a friction mount. Can be a mess. In case of accident, a flying GPS unit inside the cabin is a big hazard.
Any other suggestions? Any tried/tested position?
I have mounted my MapmyIndia Vx140 at the top middle position of the dashboard. It is not mounted on dashboard, but instead mounted on the windshield using a suction cup, and the mount has a horizontal arm that brings the screen a bit forward. The pictures of my installation can be seen here:

http://www.team-bhp.com/forum/test-d...ml#post2058462 (Beast prevails over beauty. Story of my SX4-ZXi VVT)

I am pretty happy with this position. The main advantage is that the screen is just a bit below the eye level (it is at the same level as the dashboard), and hence it is very easy to glance at the screen quickly without taking the eyes off the road for a long time. Also, the screen is within a reachable distance (for my finger tips) in case I need to use the touchscreen. Further, the 12V socket in my car is right below the AC controls and hence it is very convenient to take the wire from the socket to this mount position without getting in the way of the gear shift lever or any other important equipment. The wire stays below the eye level and does not obstruct the view at all.

However, since you mentioned that the mini-USB port in your device is at the back, this position may not be optimal. In my case, the mini-USB port is on the RHS of the device, and hence it is perfectly right position for me to pull out or plug in the wire from the driver's seat.

By the way, you did not mention which car you are talking about. This position may not work in all cars because if the windshield is too forward and sloping (such as in Civic), then the windshield mounted device will be too far away from the driver. In case of SX4, the windshield is a bit upright and that helps in this case.

Regarding other positions you mentioned, I think the first option (right next to sun visor) is good provided you can take care of routing the wire through A-pillar and taking it out of the eyesight. However, this position may interfere with the sun visor and/or may require you to look up to read the screen. This is not very ergonomic because you will have to take eyes off the road to look up and read the device. I am not sure how easy that will be.

The point you mentioned about safety in case of an accident is a very good point. However, I don't know if there is a way to address this, because regardless of where you mount it, it may become dangerous in case of an accident depending on the angle and direction of impact.

Last edited by Dr.AD : 21st September 2010 at 22:34.
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Old 22nd September 2010, 10:52   #1415
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You can buy a circular plate on ebay for around 6$.
This sticks to your dashboard via a very strong glue.
You basically mount the GPS suction cup on this plate instead of windscreen.
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Old 22nd September 2010, 12:09   #1416
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BOX for Garmin Nuvi has 2 -3 mounting disks. You can fix them and use it for attaching the unit.
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Old 22nd September 2010, 12:21   #1417
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Use the friction mount. This sits on any flat area of the dashboard. I love it.
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Old 22nd September 2010, 12:22   #1418
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Yes the garmin unit came with a small disc that can be glued to dash. Unfortunately the area I'd like to mount on dash is not level, so not sure if it'll be stable. Plus I have to solve exposed electrical chord problem too. Somehow not able to decide on most discreet solution, hence seeking advice.

@Dr.AD Thank you for your note, I see your MMI fits pretty well in your setup. My car is an Innova, so middle of the windscreen is kind of a point far away from convenience.

@sgiitk Sir won't it vibrate on bad roads? Will it get hurled in case of accidents?

Last edited by kousik : 22nd September 2010 at 12:24.
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Old 22nd September 2010, 12:41   #1419
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Quote:
Originally Posted by sgiitk View Post
Use the friction mount. This sits on any flat area of the dashboard. I love it.
Its a risk in case of accident. It will fly towards windscreen might bounce back to hit passengers.
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Old 22nd September 2010, 14:51   #1420
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The base is quite heavy with a rubber underside. It is essentially a 'heavy flat bean bag with rubber on the underside'. Considering it is a common accessory in the US and Europe I think we can assume it to be safe.

I used it to and from Gwalior (270km each way) incl. about 20km each way on what can only be called a Tractor-able Road, more fit for a Lunar Rover. Never needed to readjust the position on the Civic dash - on the flat between the speedo and the tacho!

In a Civic with the windscreen so far away, it is the only option, to keep the GPS visible to the driver.
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Old 26th September 2010, 12:25   #1421
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My Garmin Nuvi 320 is mounted on the extreme right on the windscreen with a suction-cup mount just above the dash. I can view the GPS screen with the corner of my eye while driving. That gives me the entire frontal view from the windscreen without any obstruction.

The external GPS antenna mounted on top of the roof enables the unit get its fix instantly and almost everywhere..

And no, it doesn't vibrate on a bumpy road. A small tip - Put a drop of water on the suction mount, spread it evenly and then fix it on the windscreen. The thin film of water displaces the air more effectively creating a better vaccum.

Last edited by gd1418 : 26th September 2010 at 12:28.
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Old 27th September 2010, 16:30   #1422
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i second this - my nuvi is mounted similarly but I prefer it to be on my left just next to and slightly above the instrument panel cluster. no troubles at all with road vision or anything else. the water tip from gd1418 is a good one! thanks!

I dont have an external GPS antenna. Maybe I should fix one. Since my device didnt come with an external, Im wondering whether I can get on aftermarket. And even if I do fix one, how would it connect and feed the data to the GPS device without an actual physical connection from antenna to the device. OR is it just to boost the satellite signal so it makes life easier for the GPS to locate a satellite when it is searching.

Would greatly appreciate this info.


Quote:
Originally Posted by gd1418 View Post
My Garmin Nuvi 320 is mounted on the extreme right on the windscreen with a suction-cup mount just above the dash. I can view the GPS screen with the .......

The external GPS antenna mounted on top of the roof enables the unit get its fix instantly and almost everywhere..

And no, it doesn't vibrate on a bumpy road. A small tip - Put a drop of water on the suction mount, spread it evenly and then fix it on the windscreen. The thin film of water displaces the air more effectively creating a better vaccum.

Last edited by shankar.balan : 27th September 2010 at 16:31.
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Old 27th September 2010, 16:41   #1423
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Garmin Nuvi devices have an antenna port, you will have to connect cable from antenna to the device.
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Old 27th September 2010, 16:43   #1424
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you can buy any 11-12$ active antenna with relevant cable(MMCX is very common) and it will boost signal tremendously.
I use one and even in low signal areas I get a cold fix in couple of minutes. For example in San Francisco I was getting a cold fix in 10-12 minutes in busy areas(high rises), but with antenna I could get fix in 2 minutes
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Old 28th September 2010, 11:51   #1425
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thanks netfreak and tsk for the inputs
will scout about for an external and the cable of course. next challenge will be to mount it without harming my vehicle bodywork
cheers
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