SX BLUE GPS iSXblue II+ GNSSTM, User Manual

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SXblue Series User Manual 54
frequency receiver. The maximum baseline recommended for dual-frequency
operations is about 25-30 km.
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One limitation of the base/rover setup is the type of radio used. The
broadcasting range of a radio is dependent upon a number of factors,
including transmission power, free space loss, ionospheric state, surface
conductivity, ambient noise, etc. Spread spectrum 1 Watt radios operating
in the 900MHz or 2.4GHz offer usually very short range (500 to 1000m) but
are relatively very inexpensive compared to UHF radios. They usually do
not require licensing from local authorities. UHF radios, in the 430-470MHz
range are more commonly used in the surveying industry but are also
bulkier and more expensive. A 2 Watt transmitter can cover about 1-3 km
whereas a 35 Watt transmitter can reach up to 12 km. The key to success
for both types of radios is to set the transmitting antenna as high as possible
above the ground to increase the propagating distance. UHF radios
transmitting at high power (usually in excess of 2-4 Watts) require a license
from the government authorities.
The expected range of a broadcast also depends upon the conductivity of
the surface over which it travels. A signal will propagate further over a
surface area with high conductivity than a surface with low conductivity. The
line-of-sight wave has a substantial impact on the performance of the UHF
and Spread Spectrum radios.
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The radio range limitation in a base/rover setup can be overcome by using
the Internet on a cellular connection. Cellular phones do not allow direct
TCP connections between them in the field, but a TCP server at the office
can act as a relay between the base and the rover. A typical setup for such
a configuration would require an SXblue Server installed on a PC in the
office with the Mirror feature activated, an SXblue base station with a cell
phone PDA running SXblue Mirror and sending the corrections to the office,
SXblue receivers in the field all running SXblue RTN and connected to the
mirror port of the SXblue Server. The advantages of such a solution are: no
line-of-sight requirement for the RTK transmission from the reference
station to the rover, baseline length can easily be increased but subject to
adequate cellular phone coverage, cleaner, lighter and cheaper solution.
2.4.2 RTK Networks
Many authorities and private companies have installed Internet networks of
reference stations that broadcast RTK corrections to users of this system. With the
increasing utility of these networks for surveying applications, there is an
increasing trend towards such a technology. Some of these networks are free of
charge and others require a subscription. These networks follow a standard
protocol called NTrip (Networked Transport of RTCM via Internet Protocol).