Tuesday, August 17, 2010

Project Planning - GPS

Expanding upon an earlier post - Measuring Systems Part 1 - Positioning, I would like to talk about planning. Perhaps the most significant factor in a successful collection is the GPS constellation.  Since the vehicle encounters many obstructions to the sky - trees, buildings, signs and other structures to name a few - it is desirous to collect when there is the highest likelihood of satellite visibility.  Sometimes this means our Mobile LiDAR crew is collecting in the early morning hours.

Utilizing simple planning software and an updated GPS almanac, we're able to input our project latitude and longitude coordinates to determine the times that we should be collecting - and when the crew should be doing something else.

The image above shows a 24-hour period for New Orleans, LA.  The number of visible satellites is but one thing we look at.  In addition, we look at the Position Dilution of Precision (PDOP) - a measure of strength of the geometry of the satellites.  Typically, the number of satellites has a direct correlation to PDOP (take a look at the dip in satellites around 08:30 above with the spike at the same time below).  However,  it does not necessarily mean that you will have a high PDOP if you have fewer satellites if the geometry of those satellites is strong - evenly distributed in the sky.

The successful completion of any project is planning.  The planning we perform for our Mobile LiDAR collections occur at many levels, each of which can vastly influence the results.

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