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Satellite Telemetry Timeframe Problem
A communication satellite onboard telemetry system samples 100 independent sensors on the spacecraft in a serial
sequence. Each sample is transmitted to an earth station as an 8 bit word in a time-domain-multiplexed (TDM)
frame. Additionally, 200 bits are added to the word message frame for synchronization and spacecraft status
data. This data is then transmitted at a 1kilobit per second rate using frequency-shift-keying (FSK) modulation
on a low power carrier wave. Given this scenario determine the following:
a) Find the time in which it takes to send the complete set of data from the satellite to the earth station?
b) Taking into account the propagation delay, what is the time period the earth station must wait between a
change in a parameter occurring with the satellite and the new data stream containing the new parameters
being received at the earth station? Let the path length equal 40,000km.
Solution:
a) A complete data stream would take the following time period:
(100 independent sensors) (8 bits / per sensor) + 200 additional bits = 1000 bits / per frame
So the data rate would be = 1kb / sec
Time = T = data / data rate = 1000 bits / 1kb/sec = 1 second
b) The propagation delay = 40,000km / 3E5 km/s = 0.133 seconds
Therefore, the longest time period the earth station would need to wait for the new data from the satellite would be:
1 complete frame time period + propagation delay = 1.0 + 0.133 seconds = 1.133 seconds
Satellite key words: transponder, LNA, sensors, status, health, telemetry, UT, TDMA, FSK, bit, bite, carrier,
low power, satcom, earth station, ES, data rate, propagation time, space craft, packet, header, buffer, transmission,
receive, frame, atmospheric, attenuation, kilobits per second, range, distance, time domain, frequency domain, frequency
shift keying, path length.
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