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Manufacturer of active RF components and modules primarily
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74 Visit http://mwj.hotims.com/28487-105
ered only after the spacecraft had been
launched, making a solution difficult
to apply. In the end, the orbital plan of
Cassini was altered in order to hold the
Doppler shift in the communication
channel from the Huygens craft within
restricted parameters, thereby preserving most of the mission’s goals.
These environmental effects have
historically been difficult to simulate in
a laboratory, especially in an electrically realistic and physics-compliant way.
The channel simulator was created to
address exactly this issue. When the
channel simulator is placed into the
communication link of the system under test, it “passes” (RF or cable connections at RF/IF) the signal from the
transmitter on to the receiver, adding
impairments that match link conditions
that exist in the actual operational environment. This allows every part of the
device under test including antennas,
amplifiers, converters, transponders,
modems, demodulators, and even data
recovery and firmware to be tested under realistic RF signal conditions.
Programming of the environmental conditions can be accomplished in
multiple ways, from simple point-by-point input of variables to sophisticated modeling of terrain, velocity, antenna locations, interference, weather
and background noise to name a few.
When this latter methodology is used,
the resulting signals can be so realistic
that operators would find them indistinguishable from the actual in-motion
test. This level of sophistication and
realism is achieved by coupling sophisticated motion, terrain, and link
budget modeling software to a channel
simulator allowing it to add environmental effects to the input signals, as
determined by the simulation. The result is a realistic test signal appropriate
for detailed parametric transmit- and
receive-chain testing. Another aspect
of this realistic test signal is that it can
also be used for training operators of
the equipment being tested.
Figure 1 is an example of an actual
flight plan programmed into a modeling
software. The simulation can include
data such as the aircraft flight characteristics and path, along with the terrestrial features, antenna parameters, link
budget parameters, and even antenna
placements on the aircraft and ground.
This information is then translated into
a continuous stream of control parameters, which allows the channel simulator to mimic the RF perturbations en-
s Fig. 1 Aircraft flight modeled at T = 0 on
Analytical Graphics Inc. STK SW.
countered continuously throughout the
flight. For example, if this were the first
flight of an experimental aircraft, for
which the communication link in question would relay all operating parameters of the first flight, the flight could
be flown virtually over and over before
any aircraft or personnel were put at
risk. By utilizing the actual communication equipment, along with a channel
simulator creating realistic waveform
impairments, the system can be optimized, insuring reliable communication
throughout the actual maiden flight.
The yellow line in the figure represents the communication link between
the aircraft and the receiving antenna.
As the aircraft proceeds around the
flight path (shown in green), the running scenario passes the appropriate
programming variables to the channel
simulator. The channel simulator in
turn adds exact RF perturbations to
the signal as the physics of the motion
would impart in an actual flight. Some
of the communication link variables
being modified at the instant the figure
was captured include: Doppler (aircraft
moving away from the receiving antenna); delay (time for signal to travel the
distance between aircraft and receiving
antenna); power levels (influenced by
distance); and line-of-sight connectivity (terrain and antenna placement on
the aircraft are affecting transmission).
Other variables to add realism can be
included in this scenario, such as radio
transmissions, modulation type, data
rate, signal-to-noise ratios and radar
signals to name a few. It is also important to point out that this can be done
with full motion simulation in real-time, accelerated time, or slowed time,
not just as a static test. If, for example,
the tracking station on the ground was
using the time delay of the communication path as a backup measurement
of distance from the receiver, the delay imparted by the channel simulator
would mimic the delay, even though
the test may be running in the lab.
Channel Simulator BloCk