Guidelines for the use of GNSS in surveying and mapping

7.4.4 System initialisation
Operational procedures are generally simple; with a
dual-frequency receiver, initialisation is fully
automatic and can be realised while static or while
moving (on-the-fly). Fully automatic initialisation,
under normal conditions, is usually achieved
reliably within a minute or so. To achieve centimetre
accuracies, the rover tracks carrier phase and code
phase (pseudorange) data and must initialise itself
with respect to the base station. Initialisation is the
term used to define the process of determining the
carrier phase integer ambiguities; at least five
common satellites must be tracked between the
base station and rover with a PDOP of better than
five. Before initialisation, the receiver may produce
a coarser solution, within about a metre, but can
converge within a minute or so to a few
centimetres. Once initialised, the system switches
to a fixed solution (ambiguities resolved) and
precision improves to about a centimetre.
7.4.5 Incorrect initialisations
The initialisation process is very reliable, but
incorrect initialisations can occur. Formal testing by
a manufacturer should indicate initialisation
reliability figures of at least 99.75 per cent. The
level of reliability may also be a function of time,
with less reliable initialisation figures being specified
for shorter initialisation periods. Different systems
respond in various ways, and this should be taken
into consideration when preparing quality
procedures. In all cases, best practice is to initialise
in completely open areas with no possibility of
multipath or other interference to satellite tracking.
For RTK initialisations, the production of three
distinct quality control parameters is recommended:
+ the ITRS89 co-ordinates of each initialisation
point, their accuracy at the 95 per cent
confidence level and the type of initialisation
(i.e. known point, on-the-fly, etc.)
+ integer bias values of the initialisation baseline
solution
+ ratio, variance, RMS of the initialisation
baseline.
These latter two items should be provided by the
software to give the surveyor an indication of the
reliability of each initialisation, and should be
checked after each re-initialisation with a duplicate
field point taken to provide a physical check. All
three should be logged as explicit quality control
parameters in the survey archive for each
initialisation and given in the final report if required.
7.4.6 Initialisation checks
A bad initialisation may result in position errors of
at least 20 centimetres. The receiver may
automatically detect and correct these events,
given suitable satellite availability and geometry.
This would normally take a few minutes, and this
time could be considered as an initialisation or selfchecking
window. The rare occurrences of bad
initialisations tend to be focused around lack of
redundant satellite coverage or poor PDOP (e.g.
five satellites only with a marginal PDOP), or at
heavily multipathed locations. Operational
procedures, such as satellite coverage planning or
checking against known points, are best practice in
these cases.
Similarly, observation periods where an initialisation
has been followed quickly by a loss of lock and
re-initialisation (i.e. within the time of a selfchecking
window) should be treated as suspect,
and two checks should be done at known points in
this segment.
A fixed solution, whilst requiring five satellites to
initialise, can be maintained through periods when
only tracking four satellites. In addition the fixed
solution can be maintained through periods when
the communications link is lost. The
communications link provides satellite observations
from the base station at the rover, in order that the
firmware can compute the initialisation baseline
solution. It is best practice to note when the
communications link is lost, and to re-process any
points captured, using post-processing software. In
most cases the RTK equipment will flag the
observations, but it is good practice to log these
points on a suitable booking sheet in the field. If
satellite lock is also lost when the communications
link is down, it is good practice to complete a
period of static initialisation according to the
medium-precision static criteria given in section
1.3.
It is best practice, however, to ensure that the
communications link is maintained for initialisations
and survey points.
This can be done by planning the survey to ensure
RTK baselines are kept short and the survey takes
54 | GUIDELINES FOR THE USE OF GNSS IN LAND SURVEYING AND MAPPING