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A data acquisition system or analog instrumentation recorder may not
always be available at the telemetry station to produce PCM data streams
for system checkout and operator training. Therefore, it is highly desirable
to simulate identical PCM data streams produced by the acquisition subsystem.
Simulators vary in performance;
some produce a simple static frame at fixed rates, while others create
the most complex frames and data rates
to match the decommutator’s capabilities. Describing the frame format
for setup may not be required since the telemetry system can produce it
from the decommutator’s setup definition. The simulator produces major
and minor frames, including super-commutated, sub-, and sub-sub frames;
and multiple embedded asynchronous data streams. The PCM output signal
is available in any of the standard IRIG codes and levels. Simulators
and encoders also provide MSB or LSB word orientation, programmable synchronization
words, and support for format switching. Measurands can be simulated
statically
either as user-defined constants and wave shapes via a CVT or as multiple
function generators (square, sine, ramp, triangular) at different data
rates and amplitudes. While the data changes, it is not considered dynamic.
Dynamic simulation uses real-time data from external sources and measurand
simulators as products of data bus, vehicle, or satellite constellation
models. These dynamically simulated streams are desirable for training
and system test. A dynamic simulator is, in effect, a PCM encoder. You
can produce a new PCM stream by extracting words from incoming PCM stream(s)
or external data sources for applications such as commanding or forwarding
data to another site. An example of the former is to control the operation
of a satellite, while the latter is for an airborne-based ground station
to forward key measurands to the ground station during flight tests (see
figure below). The airborne ground station not only selects all instances
of individual parameters, but may compress them (e.g., averages values
or combines multiple measurands, as in processed parameters).
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