Leica MOJO RTK Administrator Guide

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Page 1/9 Document #1060 rev1.0
HOW TO GUIDE: Base Range Optimisation
This document describes the steps you can take to improve the
communication range between the mojoRTK base station and console. This
is done by explaining the beneficial configurations, pitfalls and trade-offs
that exist when setting up radio communications systems that are
applicable to the mojoRTK base to console communications.
Description
The most basic (and possibly most common) base to console
communications scenario is to have the base station mounted on its tripod
and the communications to the console made over flat ground.
There are ways to improve the performance of the base to console
communications, some of which may be applicable to your situation and
some which are not. There as some things that you can do to improve
performance for one situation but make it worse for another.
Some background information will be provided that should help to
understand what actions you can take to improve communications
performance for the particular situation. Also, base station positions which
should be avoided will be described as they can adversely affect the radio
performance.
NOTE: This guide does not explain how to resolve issues with a
particular customer situation but provides information and concepts to
help avoid potential issues and to explain possible ways to improve
performance.
Major Points to Achieve Better Radio Performance
The major points here are listed as an initial guide and some are explained
in greater detail later.
Better performance with clear space between the base and the
console. This seems like a simple concept, however, there is
phenomena known as the Fresnel Effect or Fresnel Zones (Fresnel is
pronounced Fray-nell ). This is a complex topic and is further discussed
below. It is due to the Fresnel effect that mounting the base antenna
on top of a tower helps increase the base communications range.

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High gain antennas on the base and/or the console can improve
communications. High gain antennas do not provide any extra power
when transmitting or receiving. What they do is transmit into and listen
from a more directed field resulting in stronger signals to and from that
direction. It should be noted that there are cases where higher gain
antennas can cause problems with radio performance. Points to note
regarding high gain antennas on the base/console radio antennas:
You should avoid using a high gain antenna on the vehicle
when the vehicle is working on steep terrain (due to the effect
shown in Figure 3 later)
Also note that in some countries and regions higher gain
antennas are prohibited on certain radios or licenses may be
required to use them. When using high gain antennas it is
important to make sure that you are using the correct antenna
for the job. You should consult local communications
companies to guide you towards the right antenna.
Poor placement of the base station can result in radio reflections
which interfere with the direct signal. There are situations where the
base station can be placed in a position with a clear view of the sky for
good GPS reception but still be close to a tree, shed, silo, or with a hill
behind the base. The problem with having something behind the base
station is that the console can receive the direct signal from the base as
well as the signal reflected off the object behind the base. This can
cause interference patterns in the field which can be very disruptive to
the performance of the radio communications.
Use of good cables and connectors. When you are putting the
mojoRTK base station antenna at the top of a tower you should use
good quality cables so that the signal is not significantly weakened
before it reaches the transmitting antenna. Again, you should consult
local communications companies to guide you towards the right
equipment when extending RF cables. See the ‘How To Guide’ for fixed
base stations with external antenna.
Be aware of possible interference from other sources. Sometimes
the communications between the base station and the console may be
severely affected by radio interference from an external source or
possibly even another mojoRTK base station nearby. The first thing you
can try is to change the channel on the base station and console. You
have channels 0 to 3 to choose from with the European 868MHz
systems and 0 to 19 on the 900MHz North American and Australian
systems. Also note the following:

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Channels 10 to 19 on the 900MHz systems can help avoid
interference as those channels actually increase the over-the-
air data rate (not the total data, just how fast the bursts of
data are sent).
This helps to avoid interference as the radio requires less
transmission time to get the data through potentially avoiding
other bursts of transmissions from the interfering devices.
However, there is a downside to the higher data rate. The
higher data rate can shorten the range when the signal starts
to get weak. Its main purpose is to improve reliability when
within good range but suffering interference.
Basic mojoRTK Base to Console Communications Troubleshooting.
See the points below on general troubleshooting with the mojoRTK base
communications:
Check that the antennas are vertical and not folded over as
the standard mojoRTK antennas can fold down to prevent
breakage.
The 900MHz base stations sold in North America and Australia
provide a multi-transmit option. When turned on, the multi-
transmit sets the base radio to transmit each section of data
up to 3 more times. The base radio in the console is smart
enough to ignore any data that it receives more than once,
however, the multi-transmit increases the chance that the
console does actually receive the data at least once. To turn
multi-transmit on or off you need to go to Settings→Mulit-
Transmit in the base station menu and choose either Off, 1,
2, or 3. NOTE:

When you turn multi-transmit on, the base station
will use more power and will drain internal or
external batteries faster.

If you are using a repeater with the internal base
radio you should not be using multi-transmit as they
are not compatible.
Some radio installers are used to mounting antennas with a
ground plane. This should not be done with the standard
mojoRTK base radio antennas or any other dipole antenna (if
you are using high gain antennas for the base to console
antennas they will most likely be dipoles).
The 900MHz radios have different transmit mode when you
use channels 10 to 19 (explained above) and may be worth a
try to avoid interference.

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Page 4/9 Document #1060 rev1.0
Clear Space Between Base and Console
This section is mainly about elevating the base station antenna by placing
the base antenna at the top of a tower. Try to think of the unobstructed
space required between the base antenna and the console antenna as a
large ellipsoid shown below in Figure 1. This space is known as a Fresnel
Zone and there are many more Fresnel zones which wrap around the first
zone, each with a greater radius that the previous zone.
(Visit http://en.wikipedia.org/wiki/Fresnel_zone for a highly technical explanation)
Figure 1 – First Fresnel Zone and its obstruction
Figure 1 shows two radio towers communicating with each other in two
situations. The first situation shows no obstruction of the first Fresnel zone
and the second shows some obstruction of the first zone.
Some obstruction of the Fresnel zones can often be tolerated, and as a
rule of thumb the maximum obstruction allowable is 40%, but the
recommended obstruction is 20% or less.
Figure 2 shows two scenarios for using the mojoRTK base station and
console over flat terrain. The first scenario shows the base station set up
on the standard tripod. The second shows the base station antenna at the
top of a tower for increased antenna height.

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Figure 2 – mojoRTK base radio antenna on the ground and elevated
Note: it is possible to have too much clearance as the radio reflections in
the second Fresnel zone interfere with each other in a way which reduces
the signal at the receiver. However, when setting up the base radio on a
tower it is unlikely that you will get the base antenna high enough to suffer
this problem.
Table 1 shows a list of antenna heights for 40% and 0% obstructions of
the first Fresnel zone over different distances between the mojoRTK base
station and the console.
It is likely that you won’t have a choice as to how high you can mount your
base antenna on an existing tower. Use the table below to work out if your
antenna height will be below the height for 40% obstruction or above the
0% obstruction height. It would be better to be above the 0% height than
below the 40% height but going above the 0% height will introduce more
of the disruptive reflections off the ground. This may be somewhat
complicated by the fact that you have to work these numbers out over a
range of distances and further complicated again because this table is for
flat terrain only.
In any case, Table 1 has been provided help make informed decisions and
for problem diagnosis.

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Page 6/9 Document #1060 rev1.0
Table 1 – base antenna heights for 40% and 0% obstruction of the first
Fresnel zone over flat ground.
Distance (km) 40% Obstruction
Antenna Height
(
m
)
0% Obstruction
Antenna Height
(
m
)
21626
42236
62741
83354
10 38 61
Distance
(miles)
40% Obstruction
Antenna Height
(feet)
0% Obstruction
Antenna Height
(feet)
14472
2 63 105
3 80 134
4 94 154
5 108 176
6 119 193
US Standard/Imperial Units (868MHz and 900MHz)
Metric (868MHz and 900MHz)
NOTE: the calculations in the table were made using an online Fresnel Zone calculator and the antenna heights
listed are:
0% Obstruction Antenna Height = twice the sum of the 1
st
Fresnel Zone radius and the midpoint ground
elevation due to Earth curvature
40% Obstruction Antenna Height = twice the sum of the 40% Obstruction Radius and the midpoint ground
elevation due to Earth curvature
You should also think about obstructions that you may get when trying to
receive the signal past objects like trees, sheds, silos and so on. Think of
these elliptical zones from a top down view as well. Just because you have
direct line of sight to the base station from the vehicle looking past an
object doesn’t mean that you have unobstructed base communications.

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Page 7/9 Document #1060 rev1.0
High Gain Antennas
The first thing to note about antennas with higher gain is that they do not
produce higher transmit powers, instead they transmit the same power
into a more directed field. However, when receiving, high gain antennas do
produce more power when the signal is received from its higher gain
direction, and lower power when receiving signals from other directions.
Figure 3 shows the basic concept of the difference between antenna gain
and antenna directivity. In the diagram, the darker the signal the stronger it
is. The low gain antenna spreads the transmitted signal over a greater area
and is therefore weaker than the signal from the high gain antenna which
transmits the same signal into a smaller area.
You can often obtain the gain pattern of an antenna from the
manufacturer or the supplier. The gain patter specification provides a plot
of gain versus direction. These plots can be difficult to read but generally
provide a general guide as to which directions they don’t work well at.
Figure 3 – Directivity of antennas with different antenna gains
NOTE: although the high gain antenna pushes a stronger signal further out it may cause problems when trying to
receive the signal when you are too close to the antenna and it is mounted high up a tower. Also note that the
diagram may exaggerate the reduction in transmission area and was drawn only to explain the concept.

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Page 8/9 Document #1060 rev1.0
Select the Right Location for the Base Station
One problem that you may encounter with the base radio is interference
when the signal is reflected off an object close to the base station. This
can be very difficult to measure or diagnose as the cause of any
communications problems but as long as you are aware of the issue you
can take action to avoid such interference.
Objects that may cause these reflected signals include silos, sheds, or
trees. Even a hill close behind the base station can cause reflections if the
base station is setup near the top of a hill but not quite at the top.
For the purpose of understanding the effects of the reflected signals Figure
4 shows an extremely simplified interference pattern due the reflections
from an object close to the base station. This diagram is drawn as though
there are two signal sources, the first is the original signal from the base
station and the second is the reflected signal off the silo. The diagram also
loosely defines areas of weak signal and strong signal due to the
interference, however, note that the diagram doesn’t show the signals
getting weaker as you get further away from the base and the silo. Also,
the reflected signal off the silo would be weaker than the original from the
base.
Figure 4 – Basic Interference Pattern due to reflections from an object
near the base

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Page 9/9 Document #1060 rev1.0
When the original signal and reflected signal reach a location in phase with
each other they combine in a way that strengthens the signal. However,
when original and reflected signals reach a location out of phase they
combine to produce a weak signal and theoretically can cancel each other
out completely to produce no signal.
It is important to understand that reflected signals don’t always cause the
desired signal to be weakened. If it appears as though the signal is good in
some patches and very poor in others when there is no significant
difference in distance from the base you may be experiencing the effects
of reflection interference, possibly even an interference pattern from
another source such as a mobile/cell tower.