User's Manual

450 RTK Performance
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12.5 kHz versus 25 kHz Bandwidth
There are two possible bandwidths that can be set in the
450 MHz RTK radio:
25 kHz Wideband
12.5 kHz Narrowband
The 25 kHz bandwidth delivers twice the baud rate of the
12.5 kHz. Wide bands have more data capacity and faster
message transmission than narrow bands. In the RTK
system, this results in a more robust data link. However,
the disadvantage to using a larger bandwidth is that fewer
licensed frequencies are available to applicants. To more
efficiently use the available frequencies, some spectrum
authorities are transitioning from older wideband licensing
plans to narrowband licensing plans.
For example, in the United States the FCC has established
January 1, 2013 as the deadline for migration to 12.5 kHz
technology. Applications for wideband operations (25
kHz channels) will be accepted until January 1, 2011. All
existing Part 90 radio systems operating on frequencies
between 150512 MHz have eight years to convert those
systems either to 12.5 kHz bandwidth or to a technology
that provides one voice path per 12.5 kHz of bandwidth or
provides a data rate of 4800 bps per 6.25 kHz.
L2 versus L4 GFSK Modulation
The 450 MHz RTK radio is a digital transmitter. There are
two possible modulation techniques that can be set in 450
MHz RTK radio: Level 2 (L2) or Level 4 (L4) Gaussian
FrequencyShift Keying (GFSK).
When transmitting with L2 modulation, 1’s and 0’s are
sent by shifting the frequency between two possible
states about the center frequency within the bandwidth of
the radio. The “Gaussian” term refers to a technique of
smoothing the transition between the two frequencies.
When transmitting with L4 modulation, the radio shifts
between four frequencies about the center frequency.
This allows four different symbols to be sent during a
single transmission: 00,01,10,and 11. This results in 2x
the overtheair speed as the L2 modulation technique.
The drawback is that the signal to noise is lower with
the L4 technique relative to the L2. This is because the
vehicle radio must differentiate between 4 frequencies
instead of only 2.
The default modulation for the 450 MHz RTK is L2. This
provides the greatest RTK range. L4 will be an advantage
in selected applications. One example is operation near
the base where small objects frequently block the line
of sight. Here the signal strength is sufficient for L4.
The higher baud rate results in short transmission times.
These messages then have a lower probability of being
blocked.
Operation of Vehicle Next to Base
Depending on the base station setup, operation of
vehicles directly under or next to the base may be limited.
The 450 MHz system has both higher power and higher
signal sensitivity than standard RTK. A consequence of
this is that radio signals in the immediate vicinity of the
transmitter may be too strong for the receiving radios. For
base stations that serve vehicles at both ≥20 km and ≤
1 km from the base, it may be necessary to add inline
RF attenuators to the rovers operating directly under
the base. Attenuator specifications: RF load of 15 dBm,
capable of 2W, inline female TNC to male TNC.
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121509
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