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I have come across the terms "ground station lock" and "ground station unlock". For example here and here. I guess that is about phase locking with the uplinked signal.

I understand how phase difference between the uplinked and (phase-locked) downlinked signals can be used for very precise ranging but why does it also allow faster communication?

From the second link

STEREO-A has largely two modes of normal operation. High rate data while in lock with a ground station and low rate data when not in lock and sending deep space beacon data including the compressed images used for space weather forecasting.

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  • $\begingroup$ When in lock most of the bandwidth may be used for data transmission and only very few to maintain lock. Without lock data must be transmitted together with much more clock signal bandwidth to allow data decoding, the separation of data and clock from the received signal. $\endgroup$
    – Uwe
    Jul 5, 2023 at 15:54
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    $\begingroup$ @Uwe what exactly is "lock"? I'm looking at amsat.org/amsat/articles/g3ruh/127.html and the caption of Figure 13 which mentions "Deep Space Network (DSN) mode" but I don't understand exactly what is locking to what. $\endgroup$
    – uhoh
    Jul 5, 2023 at 22:48
  • $\begingroup$ @uhoh there's this space.stackexchange.com/a/33258/6944 $\endgroup$ Jul 5, 2023 at 23:58
  • $\begingroup$ @OrganicMarble oh, so it's not that the spacecraft's communications system locks to the ground station, the term is only about what happens inside the receiver. The radio at the ground station achieves "lock" for a radio transmission that requires "lock" to make sense of. The other transmission mode can be more easily received without such a fancy phase-sensitive demodulator on the ground perhaps. Or something like that? $\endgroup$
    – uhoh
    Jul 6, 2023 at 0:06
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    $\begingroup$ @uhoh Lock is the start of the frequency and phase synchronization of the symbol clock generated in the ground station to the received signal. $\endgroup$
    – Uwe
    Jul 7, 2023 at 5:57

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"Lock" oftentimes has three distinct meanings with regard to ground reception of transmissions from vehicles in space:

  • Transmission lock, oftentimes achieved using a phase-locked loop receiver.
    The frequency of a broadcast from a space vehicle typically varies for multiple reasons. The Doppler effect has a significant effect vehicles in low Earth orbit. A space vehicle is moving toward the ground-based receiver at a good clip when the space vehicle appears over the horizon. Later, when the space vehicle is about to disappear below the horizon the space vehicle is moving away from the ground-based receiver. Other effects include the signal passing through the Earth's ionosphere and not quite perfect transmitters and receivers. A phase-locked loop accounts for these effects by slightly and constantly adjusting the receiver. To accomplish that, the receiver must first "lock" on to the broadcast frequency.

  • Bit lock, oftentimes achieved using a bit synchronizer.
    Some kind of encoding / modulation is used to overlay a data stream onto a broadcast frequency. In many cases (most cases nowadays), that underlying signal is digital. Where does a bit (or in some cases, a group of bits) start and end? The purpose of a bit synchronizer is to rebuild the digital signal in the form of a bit stream from the analog signal that is transmitted by the spacecraft. Once the bit synchronizer has accomplished that rebuilding of the digital bit stream, the ground-based receiver is said to have achieve bit lock.

  • Frame lock, often achieved using a frame synchronizer.
    Transmissions from space are typically organized into frames that consist of a header, a data portion, and possibly a trailer. The header typically contains a sequence of pseudo-random bits that are always the same for that specific transmission. The frame synchronizer looks for this pseudo-noise pattern. This is complicated by two issues. One is that the data portion might happen to contain that same pseudo-noise sequence. Another is that the bit synchronizer might have gotten a bit or more wrong in the pseudo-noise pattern. The first issue can be overcome by using fixed-length frames, which is what most transmissions from space vehicles use. The second issue can be overcome by allowing for some bit mismatches in the pseudo-noise portion of the header. This gives a handle on the quality of the received transmission. Once the frame synchronizer has found the pseudo-noise pattern repeating itself at the expected rate, the ground-based receiver is said to have achieve frame lock.

The transmissions to and from space vehicles typically follow standards issued by the Consultative Committee for Space Data Systems (CCSDS). The CCSDS model closely conforms with the Open Systems Interconnection (OSI) model, much more so than does the TCP/IP model used by the internet. The three concepts of locking outlined above represent the OSI physical layer and data link layers.

Regarding faster communication, using compression techniques in conjunction with communications that are only somewhat reliable is not a good idea. Those compression techniques are fragile and require very good and consistently reliable communications. You can see this on your cable or satellite TV when communications become somewhat less than reliable. What you get is psychedelic colored blocks instead of imagery, and in extreme circumstances, that is all you get.

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