Perspectives¶

Until now, we just talk about the coding, and we did not consider the carrier and time synchronization problem. As we have analysis in the first part, because of the long distance propagation from voyager 1 to the earth, the receiver of earth station receive a C/N0 = 22.32dBHz if the downlink data rate is 160bps, then the Eb/N0 is 0.28dB, which is very low. So, the carrier and time synchronization must be considered.

As we know that Up until 10 March 2016 the Voyager 1 has the distance of 20,086,725,880Km to the earth, and with a velocity of 17.062Km/s going away from the earth. According to X-band of 8420MHz, the Doppler frequency shift if
f’ = f0 * (v/c)
With f0 = 8420 MHz, v = 17.062 Km/s, c = 3*10^8 m/s, we get f’=478.8734667KHz. So, the Doppler frequency is so large than the bandwidth of Down link signal. Fortunately the Doppler shift change rate mainly comes from the earth revolution around sun. So the Doppler shift change rate can be predicted very accurately, so we can predict the Doppler shift as a function of time, and if we set the receiver PLL narrow enough, we can synchronize the Down link carrier very accurately. The same way is possible with the time synchronization. But this needs to unify designing of communication frame format.