Thales Z-Max Operation And Application Manual page 153

The handheld computer and application software may also be used to verify
operations. If these operations are successful, your Z-Max system is now
collecting GPS measurement data. No further interaction with the system is
required for the duration of data collection at this location.
Continue data collection until it is determined that enough data has been collected to
precisely position this point. The amount of data required is dependent on a number
of factors including:
• the quality of the satellite geometry (PDOP),
• the number of satellites above the elevation mask,
• any obstructions between the satellites and the GPS antenna, and
• the distance (or vector length) between the GPS receivers collecting data
simultaneously.
A general guide for good conditions is five or more satellites tracked continuously
during the occupation and PDOP less than 4.
For dual-frequency receivers, a reasonable occupation time for the Z-Max system is
10 minutes plus 1 minute for every kilometer of vector length. For example, if three
receivers are collecting data simultaneously and the longest vector being observed is
approximately five kilometers, the simultaneous observation time would be 15
minutes. Since it takes less time to collect additional data than to return to the site on
a different day, some safety factor (20% or more) added to observations is
recommended.
Optimistic observation times are generally adequate when vector lengths are less
than 30 km (19 miles), there are more than 6 satellites visible, obstructions are
minimal, and there is very little multipath (buildings or bodies of water that reflect
signals). If any of these factors is present, observation times should be increased.
Experience will give you the ability to take all factors into account and determine an
appropriate observation time for any given situation.
Vectors longer than 30km (19 miles) may require occupations longer than 60 minutes.
When surveying baselines in excess of 50km (31 miles), precise ephemerides should
be used. Refer to the user manuals for the GPS processing software for further details
about optimum parameters and procedures.
Also be aware that ionospheric activity can negatively impact GPS measurement
quality similar to the way rising heat from a hot road can negatively affect a long laser
distance measurement. Increasing occupation times can almost always compensate
for ionospheric activity the way a larger prism array can enable a longer laser distance
measurement on a hot day.
130 Z-Max Surveying System Operation and Applications Manual