November 8-10, 1991 - Klofas.com

normal. A more complete discussion of the data analysis phase
can be found in [11.
COMPONENTS OF THE MODEL
The statistics given in Table 3 have been incorporated into
the simulation model. Particular attention has been given to
parameters associated with file server system operations, since
the file server is the primary communications resource. The
arrival rates for successful and unsuccessful file server
connections, file server transaction service times, byte counts
per connection, and data-link layer response times have all been
included in the model with distributions based on those determined
from the analysis phase.
The model should be considered valid only for the case of
directional, computer-pointed downlink antenna systems. For the
directional antenna case, there is usually enough Eb/NO margin to
counteract intermittent periods of fading. Even though the model
does not include the effects of the link margin on BER, it does
perform link margin computations and displays the effect of the
satellite slant range at the simulated clock time. The simulator
user has the option of having the link margin computations based
on the use of either a directional or omni-directional downlink
antenna.
Spacecraft telemetry of some type must be broadcast
periodically on the downlink, therefore the simulation allows a
list of frame types, frame lengths, and transmission intervals to
be specified by the user. This component of the model is useful
because there may be times when a higher-than-usual telemetry
transmission rate must be used. It would then be desirable to
know the effect of the higher rate on other downlink traffic.
For example, how much file broadcast traffic would be preempted
by the higher telemetry rate?
The highest priority is given to file server transaction
processing during execution of the simulation program. Telemetry
traffic has the next highest priority and file broadcasting has
the lowest priority.
The simulation clock and other program variables are
assigned values from an orbital prediction data base file
maintained separately. This particular approach has been used
for two reasons: (1) Since part of the model validation process
requires checking on how well the model predicts the past, it is
convenient to make simulation runs for exactly the same orbits as
found in the system measurement data; and (2) If the simulator
were to be run to model some yet-to-be-Iaunched satellite system
it would still be possible to generate prediction "history" based
on pre-launch orbital elements supplied by the launching
authority.
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