Preliminary results from evaluating a Garmin OEM GPS module...

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- pbreed
  
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posted
18 years ago

Wed, Aug 31, 2005 6:00 PM

I've been evaluating the new Garmin 5Hz OEM gps. I want to use it to build a low cost IMU for ameateur rocketry. I wanted to evaluate it's dynamic behvior at 5Hz.

You can see some notes on the hardware setup at:

formatting link

I used the unit for 4 short trips in the car yesterday looking for anomalyous data points.

The first run that included driving under a gas station cover and stopping under a freeway overpass due to traffic generated some signifcant jumps in position: The other three sample sets had no jumps in position that exceeded 30 ft.

My measurement was very simple. I read the GPGGA sentence and calculated the distance moved between samples.

On the very first trip the position moved as much as 770 ft between samples. This jump was associated with a change in the Satellites being tracked.

All the jumps > 35ft in this run are below: sats is the number of sats tracked for the two fixes. and fix is the fix quality for the two fixes 1= normal GPS 2= WAAS gps.

moved 770.970449 from 54838.600000 to 54838.800000 sats 7 and 8 fix 2,2 hdop=2.300000 2.600000 moved 201.671901 from 54843.600000 to 54843.800000 sats 9 and 9 fix 1,2 hdop=3.000000 3.000000 moved 134.095680 from 54848.400000 to 54848.600000 sats 8 and 8 fix 2,2 hdop=1.700000 1.700000 moved 597.091140 from 54848.600000 to 54848.800000 sats 8 and 8 fix 2,2 hdop=1.700000 2.100000 moved 604.211826 from 54849.000000 to 54849.200000 sats 8 and 8 fix 2,2 hdop=2.300000 2.300000 moved 391.617132 from 54849.400000 to 54849.600000 sats 8 and 8 fix 2,2 hdop=2.500000 2.500000 moved 290.874850 from 54849.800000 to 54850.000000 sats 8 and 8 fix 2,2 hdop=2.500000 1.700000 moved 149.955833 from 54850.000000 to 54850.200000 sats 8 and 8 fix 2,2 hdop=1.700000 1.800000 moved 49.052521 from 54850.200000 to 54850.400000 sats 8 and 8 fix 2,2 hdop=1.800000 1.900000 moved 145.764867 from 55400.000000 to 55400.200000 sats 7 and 7 fix 2,2 hdop=1.800000 1.800000 moved 150.236276 from 55404.800000 to 55405.000000 sats 7 and 7 fix 2,2 hdop=1.600000 1.600000 moved 111.018301 from 55732.000000 to 55732.200000 sats 10 and 10 fix 2,2 hdop=1.500000 1.500000 moved 137.976873 from 55741.400000 to 55741.600000 sats 9 and 9 fix 2,2 hdop=1.700000 1.700000

I have not parsed the speed course or other information yet.

I will try an correlate the jumps to geographic position in the next few days.

Paul

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- Tom
  
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posted
18 years ago

Mon, Sep 5, 2005 9:29 AM

In that case, I wouldn't let the rockets off under a gas station cover or a freeway. Not without more accurate navigation ;-)

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- Paul Keinanen
  
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posted
18 years ago

Mon, Sep 5, 2005 6:34 PM

under a freeway overpass due to traffic

The GPS receiver measures the distance from each four (or more) satellites to the receiver and from that tries to find the point were the radii from the various satellites intersect.

If you do not have a direct view of the satellites, but the signal is reflected from a surface outside the cover or overpass to the receiver antenna, this extra propagation path is added to the radius and the calculated position is somewhere under the ground, below the point of reflection.

Paul