FASTRAK MANUAL
Rev. G
D-5
JUNE 2012
ACCURACY
The accuracy of electromagnetic 6DOF measurement instruments is a function of the
error involved in making measurements and is therefore expressed in statistical error
terminology. It should be noted here that the use of statistical error terminology is the reason the
accuracy of such instruments is generally specified in degrees RMS for attitude (orientation) and
in inches or centimeters RMS for position. As with resolution, accuracy will be considered here
from the point of view of the instruments as “black boxes.” When treating the instruments as
“black boxes,” all classic error terms such as linearity, repeatability, hysteresis and drift are
included.
One factor to be considered with any of the electromagnetic instruments is range or field-
of-regard. All instruments have a practical operating range for which accuracy is specified.
Operation beyond that range will degrade accuracy as a function of the degradation of the
system’s S/N ratio. Additionally, all electromagnetic 6DOF systems are affected somewhat by
the metallic environment in which they operate. As this is clearly an uncontrollable function of
the environment from the manufacturer’s viewpoint, accuracy is generally specified and/or
should be determined in a metallically clean environment.
The accuracy specified by manufacturers of electromagnetic 6DOF instruments is called
“Static Accuracy” as the measurements are made with both the transmitter and receiver in a fixed
and surveyed attitude and position condition within a specified motion box or field-of-regard.
POSITIONAL STATIC ACCURACY
The positional Static Accuracy may be determined by measuring the vector positions
(“X”, “Y”, “Z”) of a receiver positioned in a statistically valid number of fixed and known
locations throughout a specified motion box using a precise mechanical positioning instrument
with a precision gimbal. The X, Y, and Z error terms are recorded and the RMS values calculated
for each term. These resulting error values (one of “X”, one for “Y”, and one for “Z”) are the
system’s positional Static Accuracy at each given point within the specified motion box.
Obviously, an overall positional Static Accuracy for “X”, “Y”, and “Z” may be obtained by
calculating the RMS value for all positional Static Accuracy points taken within the specified
field-of-regard.
ORIENTATIONAL STATIC ACCURACY
Whereas a similar exercise is required to determine the orientational Static Accuracy, a
clear understanding of the orientation parameters is necessary to understand the meaning of the
specification and how it is measured. The electromagnetic instruments all measure and output
6DOF data in different optional formats including Cartesian coordinates of position and Euler
angles and/or direction cosines as orientation parameters. The azimuth, elevation, and roll (yaw,
pitch, and roll) angles are the more intuitive of the orientation parameters of the receiver and are
measured with respect to the alignment (or fixed transmitter) reference frame.
Euler angles are defined as the sequence of angles (azimuth, elevation, and roll) that
define the orientation of the receiver with respect to the X, Y, Z alignment reference frame.
Azimuth is a rotation of the receiver’s
x
axis projection in the X Y reference plane about the Z