Microwave Downlinks
- A new user’s guide
Introduction
Navtech Systems Ltd has over ten years experience in supporting the needs of
operators of airborne observation platforms who need to get their video images
to the ground in real time. We design and manufacture the specialised equipment
necessary to achieve this and have extensive operational field experience in
different countries around the World.
Navtech works closely with, but is independent from, the major ‘microwave’
companies. We are able to select from them the most appropriate equipment to
combine with our own technology to meet the unique demands of any particular
customer. Our background and expertise in all the necessary technologies allows
Navtech to offer operators the best solutions to their requirements and provide a
unique, unbiased service.
We have produced this brief guide to microwave downlinks for the benefit of
those operators who have not used the technology before.
Basic System Elements
An air-to-ground video system contains the following items :
Aircraft equipment
1.
2.
3.
4.
5.
camera platform ( video source )
Telenav data encoder ( GPS into video )
microwave transmitter
power supply ( DC/DC converter )
transmit antenna
options : a) antenna actuator ( to deploy the antenna clear of the aircraft )
b) video encryption ( to enable ‘secure’ transmission )
c) LCD TFT video displays
d) ‘permanent’ or ‘portable’ installations.
Ground Equipment
1.
2.
3.
4.
receive antenna(s)
tracking platform ( for long range systems )
microwave receiver
power supply ( mains or DC )
Options :
a)
b)
c)
d)
e)
signal boosters ( for long cable runs )
video monitors and distribution ( LCD or CRT )
video decryption
moving map display
‘fixed’ or ‘portable’ installation
Basic principles and equipment functions
The equipment listed previously performs the following functions:
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,
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 )
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 5 miles ), or
a high-gain, directional unit for long range operation ( in excess of 5 miles). 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 MICROTRACKER, 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 which 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. It also enables a
unique software package (RTGIS ) to operate. 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.
Practical considerations :
‘Line of sight’
The most fundamental aspect concerning microwave linking is that the system is
restricted to ‘line of sight’ operation. There must be a clear, unobstructed line
between the transmitting and receiving antennas. Obstructions cause the signal
to be interrupted resulting in picture ‘break-up’ or complete picture loss. Even
trees ‘obstruct’ the signal.
‘Frequency’
The frequency of the microwave carrier transmission is generally determined by
the ‘allocation’ dictated by the local authorities. For air-to-ground operations the
frequencies most often used lie within the range 1.5GHz to 4.5GHz. Range
performance ( assuming certain factors are fixed ) decreases with increasing
frequency but so does the physical size of the necessary antennas. A good
compromise is to operate in the region of 2.5GHz.
‘Range’
The range over which pictures will be successfully received depends on several
factors :
a) Transmitter power
b) Microwave frequency
c) Transmit antenna gain
d) Receive antenna gain
e) Receiver sensitivity
f) Signal loss in cables and equipment
g) Level of atmospheric attenuation and ‘path loss’
h) Definition of acceptable signal and picture quality
( general ‘surveillance’ operations can normally accept slightly lower levels
than TV outside broadcasters )
With current technology and regulations, standard links can normally operate
successfully up to 30 Miles ( but in certain circumstances, local conditions may
reduce this ) and given appropriate conditions exceed this. ( Navtech has been
involved in operations where pictures were received from in excess of 60 miles ).
Operators must appreciate the way in which ‘local atmospheric’ conditions can
alter the performance level of their system at the longer ranges. For ranges in
excess of 30 miles careful consideration need to be given to equipment choice.
‘Installations’
Equipment can be provided in a form suitable for ‘permanent’ installation ( e.g.
fixed, roof-top receive site ) or in ‘Pelicases’ for portable, rapid deployment
situations. In all cases, it is important to minimise the lengths of microwave (rf)
interconnecting cable to keep signal loss to a minimum.
The mounting of the transmit antenna on the aircraft can have a significant effect
on overall system performance. Aircraft type determines what is actually
possible, but as a general rule performance is increased by mounting the
antenna on an ‘actuator’. This device 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.
‘Antenna gain’
Antennas are not ‘active’ devices i.e. they do not increase the actual power
level of the signal. The term ‘gain’ sometimes leads to confusion – it simply refers
to the ability of ‘directional’ antennas to behave as if there is a ‘stronger’ signal :
A standard omni-directional antenna, as implied by its name, operates in all
directions. Thus for example 1W of input power is spread over a wide area. If a
directional antenna is used then this 1W can all be ‘focused’ in one direction. 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.
On the transmit side of the link a directional antenna would enable the
transmitted energy to be ‘sent’ in a particular direction only, thereby maximising
efficiency and link performance. On the receive side, a high gain directional
antenna gives a higher received signal output level for a given transmitted signal
strength compared to a zero-gain omni antenna, but only when directed towards
the transmitting source.
Directional antennas also have the significant benefit of minimising ‘multi-path’
interference ( caused by signal reflections from buildings for example ).
‘Polarisation’
Microwave antennas are normally ‘polarised’ which means that the transmitted
signal is of a particular ‘orientation’ . For downlinks, ‘circular’ polarisation is
generally used ( either Right-Hand or Left-Hand ). The transmit and receive
antennas need to match for effective operation – one of the advantages of
circular polarisation is that any signal that gets ‘reflected’ from an obstruction has
its polarisation reversed in the process. This reflected signal will then not match
the receive antenna orientation, thereby further minimising ‘multi-path’
interference.
‘Power’
The more power that is transmitted, the greater the range that can be achieved.
Power amplifiers are available up to approximately 40W. However costs escalate
significantly when range requirements increase beyond about 50 miles. In many
cases range performance is effectively limited by restrictions placed on
transmitted power levels by government authorities ( ‘ERP’ levels ).
Questions and advice
Navtech engineers are always willing to answer any questions and give advice or
guidance on air-to-ground linking systems. In this specialised area, we are
independent and unbiased, with the goal of providing the system operator with
the best possible levels of performance and service.
Contact details :
UK :
Navtech Systems Ltd
The Hollies
Naseby Road
Clipston
Market Harborough
Leicestershire
LE16 9RZ
USA : Navtech Systems (USA) Ltd
3181 Kinross Circle
Herndon
Virginia 20171
USA
tel : +44 (0)1858 525454
fax : +44 (0)1858 525462
e-mail: sales@navech.demon.co.uk
tel : + (703) 478 3250
fax: + (703) 478 3252