The U.S. Global Positioning System (GPS), as well as similar systems from other
countries, are collectively known as global navigation satellite systems (GNSS).
These systems make it possible for us to determine locations extremely accurately.
When determining positions with geodetic quality GNSS equipment, you can reduce
errors and obtain high-quality data by following best practices. Even small
problems in setup and data collection can have big impacts on results. This is
equally true for both static and real-time data collection. GNSS equipment
works best when it receives unobstructed signals from the satellites. Obstructions
such as trees and buildings will impact signal propagation. Signals can also
bounce off of surfaces such as billboards, chain link fences, automobiles,
and buildings en route to the receiver. This effect, known as multipath,
can reduce accuracy. As much as possible,
GNSS receivers should be set up with clear access to the open sky, and out of
the range of potential multipath interference. Satellite geometry, the
spatial arrangement of the visible satellites, is important.
Several well spaced satellites, covering the entire sky, is preferable to
satellites grouped together. Often simply waiting for the satellite constellation,
and therefore the geometry, to change will improve positioning accuracy. The
position dilution of precision (PDOP) is an indicator of the level of precision
achievable for a specific time and location, based on the satellite geometry.
The lower your PDOP, the better your results. Your planning software
provides information on the PDOP and the best times to collect data.
Electrical and solar storms can compromise your data as well. Check with
NOAA Space Weather Prediction Center for geomagnetic storm warnings, alerts,
and watches.
When collecting data, carefully set up your tripod over your point to minimize
centering errors. The antenna should be level and it must remain stable. To
ensure your antenna is level, the bubble should be in the center of the circular
level vial. As part of your routine maintenance, make sure your circular
level vial is in adjustment. On most fixed height poles, you can check your adjustment by
turning the level vial 180 degrees, and verifying that the
bubble stays in the center. It is important to know the height of the
antenna's reference point (or ARP) above the point you are observing. The ARP is
usually the bottom of the antenna body, not including adapters. This is called
the true vertical antenna height, and any error in this measurement directly
affects your final point elevation. When using a standard tripod you may need to
measure the antenna height from the mark to the edge of the antenna. This
measurement, called a "slant height" can be a source of error. Slant heights must be
corrected to a true vertical antenna height. Some vendor software may do this
for you, but confirm this to be sure. Using a fixed height tripod or range pole
can reduce or eliminate height measurement errors by removing the need
to physically measure the antenna height. However, the tripod tip should be
inspected periodically, and it should be replaced if it is worn or damaged. For
optimal accuracy, most antenna manufacturers recommend orienting a
specific point on the antenna to the north.
Finally another common error source can occur if an incorrect antenna model is
selected when post processing the data. Choosing the wrong model may result in
incorrect electronic phase center variations and offsets. This could
introduce a significant error in the height, directly translating to your
point elevation. Both the vendor of your equipment and NOAA's National Geodetic
Survey website can provide more information about antenna phase center
offsets as well as the antenna reference point and North orientation feature.
In general, the longer you observe, the better your accuracy will be. With a longer
session you'll have a better probability of fixing ambiguities and canceling some
of the errors in GNSS. The longer the session duration, the lower the
root-mean-square deviation (RMSD) In other words, the higher the degree of
horizontal and vertical accuracy achieved. While it won't directly reduce
your error sources, a second independent observation is a helpful way to check
your results.
Remember, even when doing real-time positioning, you can always verify the
accuracy of your equipment and procedures by collecting data on a
geodetic control mark. Post-processing tools such as NOAA's online positioning
user service, known as OPUS, will solve for a position. However these tools will
not detect important external errors, such as entering a poor antenna height,
or selecting the wrong antenna model for processing. Following these simple
techniques will allow you to get the most out of your GNSS equipment, and...
position you for success!