Chapter- 2(Lecture-3) Maximum Frequency Range, Doppler effects and Maximum Doppler Shift [Satellite Communication Engineering].

Discussion

This question is as same as question 2 of chapter 2 in satellite communication Engineering (322 km circular orbit) carries a 300 MHz transmitter.

a. Determine the maximum frequency range over which the received signal would shift

due to Doppler effects if received by a stationary observer suitably located in space.

Note: the frequency can be shifted both up and down, depending on whether the satellite

is moving towards or away from the observer. You need to determine the maximum

possible change in frequency due to Doppler (i.e. 2Df).

b. If an earth station on the surface of the earth at mean sea level, 6,370 km from the

center of the earth, can receive the 300 MHz transmissions down to an elevation angle of

0^o, calculate the maximum Doppler shift that this station will observe.

 Note: Include the earth’s rotation and be sure you consider the maximum possible Doppler shift for a 322 km circular orbit.

Solution

a. The highest Doppler shift would be observed in the plane of the satellite at the orbital

height of the satellite: the satellite would be coming directly at the observer or directly

away from the observer. The maximum Doppler shift would therefore be the sum of

these two values.

From eqn. (2.5), the orbital velocity v = (m/r)^1/2 = (3.986004418x10^5 / 6700.137)^1/2 =7.713066 km/s = 7,713.066 m/s.

The orbital velocity was calculated in question 2 as 7,713.066 m/s.

Using equation (2.44a), Df / fT = VT / vp , where Df is the Doppler frequency, fT is the

frequency of the transmitter at rest, VT is the component of the transmitter’s velocity

directed at the observer, and vp is the phase velocity of light. Since the observer is at

orbital height, the component of the transmitter’s velocity towards the observer is the

actual velocity of the satellite. 

Thus Df = (7,713.066x300,000,000) / 2.9979x10^8 =7,718.468928 Hz.

 The maximum Doppler shift therefore = 7,718.468928x2 =15,436.93786 Hz = 15,436.94 Hz.

b. It is best to draw a diagram to see what the set up looks like. Below is a view from

above the orbit of the satellite (orthogonal to the orbital plane).

The important element in this part of the question is the component of the satellite’s

velocity towards the earth station.

Summary: After Completion of this lecture you will be able to Determine the maximum frequency range over which the received signal would shift due to Doppler effects if received by a stationary observer suitably located in space, other than that also able to calculate the maximum Doppler shift that this station will observe, So stay tuned for more.