Limitations - Thales Z-Max Operation And Application Manual

done by the total station operator directing the rod person to the correct location. This
process is time consuming and requires two people. With RTK, the rover operator can
determine his/her current location by observing the handheld computer screen. The
coordinates of the point of interest are also known by the handheld computer. With the
RTK system knowing its current location and the location of the point to be staked, the
system can direct the user to the correct location. This capability makes RTK a very
effective stakeout tool. Any object can be staked out with GPS RTK including roads,
pipelines, DTM, and grids.

Limitations

Like other precision measurement methods, RTK does have limitations that affect its
ability to perform some of the survey tasks discussed above. Being aware of these
limitations will ensure successful results from your RTK surveys.
The limitations of RTK come from the GPS system itself, and not from GPS receivers.
As discussed earlier, GPS depends on receptions of radio signals transmitted by
satellites approximately 20,000 km from the earth. Being of relatively high frequency
and low power, these signals are not very effective at penetrating through objects that
obstruct the line of sight between the satellites and the GPS receiver. Any object that
blocks the path between the GPS receiver and the satellites will be detrimental to the
operation of the system. Some objects, such as buildings, will completely block out
the satellite signal. Therefore, GPS cannot be used in tunnels, under water, or any
indoor space where the signal level drops below detectable levels. Other objects will
partially obstruct or reflect/refract the signal, such as trees. Reception of GPS signals
is very difficult in a heavily forested area. While enough signal can often be observed
to compute a rough GPS position, surveying requires a higher positional accuracy and
thus, a larger quantity of clean data. RTK is usually not a tool for surveying in a dense
forest, just as an optical total station is not a tool for precisely measuring angles and
distances in the dark.
This does not mean that GPS RTK is only useful in areas with a wide-open view of the
sky. Advanced RTK systems like the Z-Max can be used very effectively and
accurately in partially obstructed areas. The critical factor is to be able to observe, at
any given time, enough satellite signal to accurately and reliably compute a position.
In the good parts of the satellite window (the part of the day when the most satellites
are visible) 5-10 GPS satellites may be visible and available for use in RTK surveying.
Accurate and reliable positions can be determined with 5 satellites distributed
throughout the sky (not clustered in one part of the sky). Therefore, an obstructed
location can be surveyed if at least 5 satellites can be observed. This makes RTK
useful in obstructed areas such as along a tree line or against the face of a building.
Advanced RTK systems such as the Z-Max report the precision of the position
Introduction 157