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NGS User Guidelines





TPC's GNSS averaging is based on User Guidelines for Single Base Real Time GNSS Positioning by National Geodetic Survey, Version 2.1, August 2011.


Page 2 of the guide reads,

"RT positioning of important data points cannot be done reliably without some form of redundancy. As has been shown in the NOAA Manual NOS NGS-58 document “GPS Derived Ellipsoid Heights” (Zilkoski, et. al., 1997), and NOAA Manual NOS NGS-59 document “GPS Derived Orthometric Heights” (Zilkoski, et. al., 2005), GNSS positions can be expected to be more accurate when one position obtained at a particular time of day is averaged with a redundant position obtained at a time staggered by three or four hours (and thus with different satellite geometry and multipath effects). The different satellite geometry commonly produces different results at the staggered times. The position—all other conditions being equal—can be accurately obtained by simple averaging of the two (or more) positions thus obtained. Redundant observations are covered in the Accuracy Classes of the Field Procedures section, where most of the RT Check List items, found below, are also discussed."

Other discussions in the guidelines include

Accuracy vs Precision pg 28

Multipath pg 29

Position Dilution of Precision (PDOP) pg 30

Accuracy Classes

The guide reads, "The term “accuracy,” in this case, actually refers to the precision from a base station, correctly set over a monument held as truth. The accuracy of the rover positions will be less than the accuracy of the base station’s alignment to the user’s datum".

The classes RT1, RT2, RT3 and RT4 are defined and discussed, with accuracy classes RT1 and RT2 requiring redundant GNSS observations.  The guide further reads,

For Accuracy Classes requiring redundant locations, in addition to obtaining a redundant location at a staggered time, use this procedure for each location to prevent blunders: 1. Move at least 30 m from the location to create different multipath conditions, invert the rover pole antenna for 5 seconds, or temporarily disable all satellites in the data collector to force a re-initialization, then relocate the point after reverting to the proper settings. 2. Manually check the two locations to verify the coordinates are within the accuracy desired or inverse between the locations in the data collector to view the closure between locations. (This operation can be automated in some data collectors). Each location should differ from the average by no more than the required accuracy. 3. Optionally, after losing initialization, use an “initialization on a known point” technique in the data collector. If there was a gross error in the obtained location, initialization will not occur. 4. For vertical checks, change the antenna height by a decimeter or two and relocate the point. (Don’t forget to change the rover’s pole height in the data collector!

Page 52 includes an Accuracy Class Summary Table which provides helpful information on class definitions.

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