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Products
CamNav
Mapper
RTGIS
Live
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Microwave
Downlinks
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BlueGlen’s
RTGIS Live™
mapping system uses specialised equipment from Navtech Systems Ltd
to support the needs of operators of airborne observation
platforms who need to get their video images to the ground in real
time.
Aircraft
The
airborne ‘camera’ provides a ‘standard’ video signal in
PAL or NTSC format. This is fed to a data encoding system,
developed by Navtech, called Telenav IIä,
which ‘invisibly’ inserts digital data from a GPS receiver
into the signal. The signal is thus ‘tagged’ with the position
of the aircraft (note that this system does not require the use of
audio or other separate data channels – the data is carried
‘free’ and invisibly by the video signal at full transmission
power).
If
a ‘secure’ link is required then a video encryption unit
(‘scrambler’) is added between the camera and the Telenav data
encoder.
The
GPS - encoded signal is then fed to the microwave transmitter.
Here the video signal is ‘added’ to the high power microwave
‘carrier’ signal that the transmitter generates, ready for
transmission through the antenna. This antenna is normally an
‘omni-directional’ type, meaning that the signal is
transmitted equally in all directions. (For very long-range
operations a high gain, ‘directional’ system can be fitted to
the aircraft)
Standard
links normally operate successfully up to about 80 kilometers
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| Aircraft
Equipment
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Camera
platform (video source)
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Telenav
II data encoder (GPS into video)
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Microwave
transmitter
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Power
supply (DC/DC)
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Transmit
antenna
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Options
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Antenna
actuator (to deploy the antenna clear of the aircraft)
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Video
encryption (enable secure transmission)
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LCD
TFT video displays
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Permanent
or portable installations
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This
portable Aircraft Transmit Case is designed to be used in
conjunction with a Transtenna unit, the case is
self-contained in order to minimise the number of
connections between Telenav II, aircraft systems and the
antenna.
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Ground
On
the ground the signal is picked up by the receive antenna, either
a simple omni-directional unit if operating over short ranges (for
example 1 to 8 kilometers
), or a high-gain, directional unit for long range operation ( in
excess of 8
kilometers). If a directional antenna is used then a
‘tracking’ system is required to keep the antenna pointing
towards the transmitting aircraft. Navtech has developed its own
unique GPS-based system, called Microtrackerä,
for this purpose.
The
transmitted microwave signal passes from the receive antenna to
the microwave receiver where the original video signal is
‘extracted’ from the ‘microwave carrier’. This is then
‘decrypted’ (unscrambled), if required, before passing to the
Telenav decoder that forms part of the tracking system. This
Telenav unit reads the encoded aircraft GPS positional data to
enable the tracking platform to steer the antenna towards the
aircraft.
Also
at this stage, BlueGlen’s RTGIS
Live™
receives the decoded signal. This allows ground personnel to see
the transmitted pictures alongside a moving map of the local area,
with an icon indicating the position and track of the transmitting
aircraft.
The
video signal is then free for normal distribution and display.
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| Ground
Equipment
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Receive
antenna
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Tracking
platform (for long range systems)
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Microwave
receiver
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Power
supply (mains or DC)
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Options
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Signal
boosters (for long cable runs)
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Video
monitors and distribution
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Video
decryption
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Fixed
or portable display
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number of variations of Microtracker exist for fixed and
mobile ground units.
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Fixed
ground units are designed for permanent installation on a
rooftop or mast. A
larger antenna maximises the system’s gain.
Three
mobile variations exist.
The first is designed for mobile command posts, which
can be deployed to any suitable location in proximity to an
incident or where an aircraft may be operating out of range
of a fixed site.
The
second variation is the compact Combi, able to fit in the
rear of a vehicle and capable of being ready to track an
aircraft within minutes of set up.
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Finally,
the Grab & Go ground receive case designed to be carried
by patrol cars, integrating a receiver, Hi-8 VCR and 10.4”
LCD TFT display.
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Practical
Cosiderations
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Line
of sight
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Clear,
unobstructed line of sight between the transmitting and
receiving antennas is a prerequisite.
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Frequency
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Local
authorities generally determine the frequency of the
microwave carrier transmission.
A range of 1.5GHz to 4.5GHz
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Range
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The
range over which pictures will be successfully received
depends on several factors:
Transmitter
power
Microwave
frequency
Transmit
antenna gain
Receive
antenna gain
Receiver
sensitivity
Signal
loss in cables and equipment
Level
of atmospheric attenuation and path loss
Definition
of acceptable signal and picture quality
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Installations
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Mount
system for transmit antenna on the aircraft can have a
significant effect on overall system performance.
In most cases performance is increased by mounting
the antenna on an actuator.
This is used when in flight, to deploy the antenna
beneath the aircraft on the end of a pole, so that it is
clear of the aircraft underside and skids/undercarriage.
This
allows the signal to be transmitted unrestricted and allows
a degree of flexibility in the banking manoeuvres that the
aircraft can perform without signal break-up.
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Antenna
gain
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Gain
refers to the ability of directional antennas to behave as
if there is a stronger signal.
An
omni-directional antenna operates in all directions.
The gain of the antenna refers to the effective
increase in input power level necessary for an
omni-directional antenna to provide the same effective
signal strength at a fixed receive measurement point.
A
directional antenna is focused in one direction therefore
maximising efficiency and link performance.
This type of antenna is generally required for long
range operations.
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Polarisation
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Microwave
antennas transmit a signal in a particular orientation.
For downlinks, circular polarization is generally
used. One of
the main advantages of using circular polarization is that
any signal that gets reflected from an obstruction has its
polarization reversed in the process.
This reflected signal will then not match the receive
antenna orientation, thereby further minimizing multi-path
interference.
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Power
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The
more power transmitted, the greater the range.
Power amplifiers are available up to approximately
40W. However,
costs escalate significantly when range requirements
increase beyond 80 kilometers.
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Download
pdf
Microwave
Downlinks information sheet
(325kb)
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