9.3.3
Multi-Session Network Processing
SECONDARY NETWORK ADJUSTMENT:
THE
MODELS
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Two
levels of complexity can be identified (as has been done in section 9.2.4):
- The Arithmetic
Approach: The trivial form of "adjustment"
where there
are no redundancies, based on simply adding the single session
solutions
together to propagate from the single known (datum) station out
to the
other stations in the network. The VCV matrices are concatenated
into a
single network VCV. The final VCV structure consists of full block
diagonal submatrices for each session within a matrix containing null
submatrices
for the covariances between stations occupied in separate
sessions.
- The Least Squares Approach: The
more common type of
adjustment can accommodate redundancies, to produce
an optimal solution
with any degree of constraint, from the minimal
(single datum), to the
over-constraint (many fixed stations, as discussed
in section 12.1.2).
The latter approach, because of its flexibility, is the one
universally
favoured and will be referred to here by the generic label of
"secondary
network adjustment approach".
Choosing a network adjustment
program.
Some features to
consider:
- Can it automatically read
output files of GPS baseline reduction
software?
- Can it handle vector correlations?
- Can
it handle multi-baseline input?
- Can it handle
non-GPS observation types?
- Can it carry out
conventional 2-D adjustments as well?
- Coordinate
Geometry (COGO) capability?
- Does it use a geoid
model? Can it be changed?
- Can it solve for
transformation parameters?
- Can it hold several
stations fixed in an adjustment?
- Can it carry out
sequential processing (of subnets)?
- Does it have
graphical output?
- What features for altering input
VCVs are there?
- Result presentation? Variety of
coordinate systems?
- Maximum number of
parameters?
- Maximum number of
observations?
- Computer resources required?
- Training requirements? Support and
upgrade?
There are many additional features
that can aid Quality Control,
error checking and general
"trouble-shooting". Some of these are:
- What statistical testing can it do?
- Compute Tau values?
- Error
ellipse/ellipsoid computations?
- What observation
editing features are there? Error checking
features? Blunder
detection?
- Can it detect singularities?
- Can it detect multiple networks?
- Can it
flag "no check" measurements?
- Can it
output vectors and residuals in projection system?
- Can it aid users in detecting antenna height errors?
- Can it sort stations by name?
- Can it
sort baselines by length?
- Can it sort coordinates by
duplicate station
names?
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The output of the
primary GPS data processing of an observation session
is similar to reduced
field data in conventional terrestrial surveys. The
fact that the GPS
carrier phase observations have already gone through a
Least Squares
process to determine the baseline parameters makes little
difference to the
development of a network adjustment model, except that
the functional and
stochastic model of these "reduced" session
observations must be
correct. There are two options:
- The output of single
baseline solutions consists of the baseline
vector
and associated 3x3 VCV matrix.
- The output of multi-baseline
processing is a set
of n station coordinates and
the associated 3(n-1)x3(n-1) VCV matrix.
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© Chris
Rizos, SNAP-UNSW, 1999