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Terms & Concepts
to both the rover and base station receivers when the
baseline distance is short. By removing these common errors
through RTK double difference processing, centimeter level
accurate vectors can be calculated between the base and
the rover.
When the double difference ambiguities are estimated
as real numbers, a float solution is obtained. When the
ambiguities can be constrained to fixed integers, a fixed
solution is achieved. Determining the correct integer
ambiguities of a fixed solution is a statistical process and the
probability of fixed solutions providing inaccurate position
solutions increases in poor environments.
Multipath, the reflections of GNSS signals from nearby
structures and objects (including the ground), combine
with the direct GNSS signals from the satellites to the GNSS
receiver and shift their signals. It’s as if your measuring
tape is bent around an obstacle like a tree instead of a free
and clear line of sight between two points. The worst case
is when the receiver doesn’t receive the direct signal at all;
e.g., the satellite is behind a building, but it’s still receiving
the signal reflected off of the nearby structure. Such Non-
Line-Of-Sight (NLOS) signals can be strong, unhelpful and
misleading.
The other aspect impacting the veracity of a fixed solution is
when there are weak GNSS signals. Frequently, weak signals
are due to their penetration directly through tree canopy.
While J-Field can’t move the obstacles that are creating
multipath out of the way, its sophisticated engineering is
designed to handle even the weakest signals like no other
system with its RTK Verification and Validation System (patent
pending).
Ellipsoids
Ellipsoids (Spheroids) are mathematical models of the
general shape of the Earth. An ellipsoid is formed by an
ellipse rotated about the semi-minor (polar) axis and is
defined by semi-minor axis and semi-major axis or semi-
minor axis and inverse flattening.
Geoids
Geoids model the local effect of Earth gravity. Geoids
are models and improve as gravitational measurements
and density of measurements improve. Geoid models
are important to land surveyors because these models
allow GNSS determined positions to accurately determine
orthometric heights related to sea level (such as might be
determined by differential leveling) instead of being related
to an imaginary shape such as the ellipsoid. Differences in
orthometric heights between two points, based on a geoid
model, will determine the way in which water will flow
between them.
Transformations
Coordinates for a point on the Earth can vary depending
on when and how those coordinates were determined.
Reference Frames, or Datums, refer to the time and basis
of a coordinate system. A transformation is a mathematical
process that converts a coordinate related to one datum
to another. The geodetic team of Javad GNSS elected to
set the
J-Field
internal database to the ITRF2008 reference
frame. This means that any coordinate input as a NAD83_
adjustment xx (which is set in the project settings) is
transformed internally to ITRF2008. When a position
is recalled from the database, the ITRF2008 position is
transformed back to a NAD83_adjustment xx position (or