The reliability of derived outputs may be more variable than the
reliability of positions. This could arise for reasons possibly related to the absence of
standards for such outputs and for the algorithms used for deriving such outputs. The
quality of damping of ARPA vectors Is analogous. Heavily damped vectors appear more
comforting to the untrained eye, while lightly damped vectors are more likely to give an
early indication of the change of course by a tracked target.
At the present time, the primary derived outputs appear to be
constrained to Course and Speed over the ground. However, it is possible to achieve other
outputs as indicated below:
Velocity
While a general idea of velocity can be obtained by comparing the track
between successive positions, it is also possible to obtain instantaneous measures of
velocity by measuring the doppler shift of the carrier signal from multiple satellites.
The resulting output gives the velocity in three dimensions. The accuracy of such velocity
measurements are less dependent on change of position and are therefore more likely to be
usable in the navigational environment.
Acceleration
As such velocities can be obtained several times per second, near
instant measures of acceleration can be derived. by differentiating such velocity
measurements over small time intervals. In the maritime environment these can be used to
optimise ships speed performance and autopilot characteristics to aid both economy and
safety.
Heading
Installing multiple sensors can provide the capability to determine the
azimuth between sensors. This can be done either by having two separate receivers
controlled to capture the same satellite configuration and spatially separated at over 100
metres. Differential level accuracies of relative position can be obtained in such
instances, providing thereby a heading measure accuracy of about 2.5 degrees. Greater
accuracies can be obtained either by increasing the distance between such sensors or
utilising carrier phase differences between sensors separated by as little as 1 metre. As
the wavelength of the L1 carrier is about 19 cms, relatively small changes in heading will
produce significant changes in phase. Accuracies of the order of fractions of a degree are
feasible.
Yaw, Roll and Pitch
The use of an antenna array as described above can be used to obtain
real time measurements of yaw, roll and pitch as well as heave. The accuracies of these
derived measures may be presently untested, but with the advent of improved receivers,
processors and computational methods, can be expected to yield usable results in the near
future., The crucial issues relate to the limitations and applications of such resources.
These are discussed in the following pages.
Error Characteristics of Derived Outputs
The errors which occur in derived outputs can be significantly
different in terms of causative origins than GPS position errors. Damping and
computational algorithms can vary dramatically between receivers, and this tendency may be
enhanced by the absence of international standards and specifications for such outputs.
The imposition of standards at this time may be counterproductive to exploration and
evolution of GPS but care must be taken in using such derived outputs.
