3 ms·

> the spacecraft couldn't simply receive and retransmit the signal, because the Doppler shift from the upwards journey would be lost. How is that true? Isn't "

> the spacecraft couldn't simply receive and retransmit the signal, because the Doppler shift from the upwards journey would be lost. How is that true? Isn't "simply receiving and retransmitting" the signal just the same as a reflective surface, which works just fine for Doppler relative speed measurements? (I'm ignoring the retransmission increasing the signal strength over a simple reflection.) My guess would be that the frequency difference makes it easier to detect the faint return signal over the strong transmitted signal.

dtgriscom 5y ago
Ah: I read further. I believe you meant that it couldn't just send the signal back at a fixed frequency; the frequency had to be based on the incoming frequency.
kens 5y ago
It depends which signal they retransmit. If they amplified and retransmitted the raw 2 GHz signal, you are correct that it would be the same as a reflection, but stronger. Unfortunately, if the spacecraft is transmitting and receiving on the same frequency at the same time, the outgoing signal will overwhelm the signal you're trying to receive and you'll get feedback on the spacecraft. Kind of like getting feedback from a microphone and speaker. And it's also going to be very hard to detect the signal on the ground, as you mention. Alternatively, they could extract the pseudo-random signal from the carrier and retransmit it at a different, fixed frequency. In this case, the problem is that the ground station doesn't have the spacecraft's transmission frequency for comparison, so it's hard to extract the Doppler shift. You could put an extremely precise frequency reference on the spacecraft and a matching frequency reference on the ground, but that's a lot harder than shifting the frequency in the transponder. Also, this approach only gives half the Doppler shift of the transponder approach, since you lose the Doppler shift in the upward direction.

dtgriscom 5y ago

Ah: I read further. I believe you meant that it couldn't just send the signal back at a fixed frequency; the frequency had to be based on the incoming frequency.

kens 5y ago

It depends which signal they retransmit. If they amplified and retransmitted the raw 2 GHz signal, you are correct that it would be the same as a reflection, but stronger. Unfortunately, if the spacecraft is transmitting and receiving on the same frequency at the same time, the outgoing signal will overwhelm the signal you're trying to receive and you'll get feedback on the spacecraft. Kind of like getting feedback from a microphone and speaker. And it's also going to be very hard to detect the signal on the ground, as you mention. Alternatively, they could extract the pseudo-random signal from the carrier and retransmit it at a different, fixed frequency. In this case, the problem is that the ground station doesn't have the spacecraft's transmission frequency for comparison, so it's hard to extract the Doppler shift. You could put an extremely precise frequency reference on the spacecraft and a matching frequency reference on the ground, but that's a lot harder than shifting the frequency in the transponder. Also, this approach only gives half the Doppler shift of the transponder approach, since you lose the Doppler shift in the upward direction.