Manual Title:
LD7 Operations Manual
Ref.:
AB-V-MA-00555
Rev No:
A9
Page 43
“It is essential to assess the precision and reliability of each position in order to ensure the
quality of the DGPS measurements. Thus is recommends that the following processing steps
be implemented: -
w-test for outliers carried out for each position fix
F-test for unit variance carried out for each position fix
When no more outliers are identified in any fix, precision and reliability measures will
be calculated:
o
Precision:
a-posteriori error ellipse
o
Reliability:
external reliability (positional MDE using a power of test of
80%)”
Where accuracy and precision statistical parameters are generated these all represent a 95% (2
σ)
confidence region.
Appendix A of the UKOOA guidelines emphasises this
by listing ‘Suggested parameters to be
specified by a system user for typical marine survey operations’ and states that ‘In order to carry out
rigorous QC, the covariance matrix generated by the least squares computation should be used to
generate test statistics and quality measures’.
It recommends the following Test Statistics:
1. w-test
used to detect outliers
2. F-test
used to verify the model which is being used to account for ‘errors’ in
the DGPS observations
It recommends also the following Quality Measures:
1. Error Ellipse
an approximate graphical representation of the positional standard
deviation in two dimensions
2. External Reliability
the effect of the maximum MDE (Marginally Detectable Error) on the
computed position
These recommendations are particularly aimed at survey applications but could be applied equally to
DP applications.
