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how to use doppler effect on bt747

husel's picture
Thu, 10/01/2009 - 13:17 — husel

Hi,

I understood, that the bt747 has the option to use 5Hz and the doppler effect to determine exact speed. What settings do I have to select to achive that and to get exact height values?

The application is to use the bt747 in a rc plane and to log speed and altitude.

Regards, Rainer

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Thu, 10/01/2009 - 15:18 — mdeweerd
mdeweerd's picture

BT747 is a program and you

BT747 is a program and you need a logger to achieve this.  BT747 can configure the logger.

What is the logger that you have?

You basically need to log time, lat, lon, speed, height and preferably also an indicator of precision - in your case PDOP.  PDOP indicates the 3D precision.  You can log VDOP to have the precision related to the height.  You should expect an error of several meters.

Further you set the Fix period to 200 ms and I would set the speed condition to 3 km/h.  The plane surely goes faster than that.

Thu, 10/01/2009 - 18:19 — husel
husel's picture

Sorry to be unprecise: I got

Sorry to be unprecise: I got the i-BLUE 747 GPS LOGGER. Still the question: is there an option to enable / disable doppler measurement of speed.

Thu, 10/01/2009 - 19:52 — mdeweerd
mdeweerd's picture

I suppose that the speed is

I suppose that the speed is always measured - I am not certain about the method used.

It can be activated on this screen. Simply select speed and click 'Set Format ...'.

Tue, 12/01/2009 - 22:44 — fabwes
fabwes's picture

  >You basically need to log

 

>You basically need to log time, lat, lon, speed, height and preferably also an indicator of precision - in your case PDOP. >PDOP indicates the 3D precision.  You can log VDOP to have the precision related to the height.  You should expect an >error of several meters.

Huh? From my experience with the MTK and also other chipsets, especially the height may be off by several 10m, even with a valid 3D fix and a larger # satellites, it especially likes to drift slowly over that large range! (I know this from several logged competitive runs ("jogging") where I can compare the logged height with Google... and other height data)Regarding the doppler speed measurement, interestingly nobody seems to have checked the validity/origin of the speed data from the MTK loggers up till today!? Also, from GPS analysis software I know of (Jasta, GPSanalyze...) all seem to base their current speed in magnitude and vector only on the difference of logged absolute positions, which gives a very unstable result, while the doppler effect usually is claimed to be usable down to much better than - say - 1km/h

Tue, 12/01/2009 - 23:17 — mdeweerd
mdeweerd's picture

I agree that the height can

I agree that the height can drift a lot, but you do need to apply the height correction offered by BT747 before concluding about the error.  The height recorded by the logger is referenced to WGS84 .

On www.gpspassion.com there are some comparisons of speed.  In my experience the speed reported by the logger is rather good, but I admit that I never made formal measurements.

Regarding the SW you reference: do you mean that this SW indicates that the loggers base their speed measurement on the difference between logged positions?