Question

A signal transmitted from an Earth station bounces back from a geostationary satellite, with an altitude of 35864 km from the Earth's surface. The range from the Earth station is 38200 km. The round trip time is about: (excluding processing delays and assuming c = 3 × 10⁸ m/s)
1. 255 ms
2. 127 ms
3. 239 ms
4. 120 ms

Answer 1

Correct Answer - Option 1 : 255 ms

Concept:

Round Trip Time

In telecommunications, the round-trip delay (RTD) or round-trip time (RTT) is the amount of time it takes for a signal to be sent plus the amount of time it takes for an acknowledgment of that signal to be received.

• This time delay includes the propagation times for the paths between the two communication endpoints.
• The expected propagation delay will be a little more than 100 ms per hop.

In general, the relation between speed and distance is:

Speed = Distance / Time

Calculation:

Given that exclude all the processing delays and speed = 3 × 10⁸ m/s and the given range is 38200 km.

Now, distance is counted two times, one is for uplink and another for downlink. So, the total distance is:

Distance = 38200 × 2 km

Distance = 76400 km

Round trip time is:

\(Time = \frac{{38200\; \times \;2\; \times \;{{10}^3}}}{{3\; \times\; {{10}^8}}}\) 

If it is 381 then it is divisible by 3 exactly but the numerator is slightly more than that so the answer will be slightly more than 127 × 2 = 254

Conclusion:

Since the ISRO do not allow calculator so, we are doing the approximate calculations.

From the options, it is clear that the total round trip time is 255 ms