SOC keyhole documentation / double and donwlink-only pass handling

Introduction and background

Double (and sometimes triple) passes occur, because some of the DSN antennas are only able to downlink ("D/L") from SOHO (for instance, they do not have uplink transmitters in the S-band used by SOHO). Some antennas in turn are not able to downlink due to the keyhole conditions, but have transmitters able to uplink to SOHO ("U/L"). In situations where no uplink and downlink capable antenna is available, it may be possible to either pair a downlink-only and an uplink-only antenna to achieve both commanding and telemetry/dumping capabilities, or a downlink-only connection (no commanding) has to be tolerated to improve the likelihood of successful dump of the recorder(s). Note, that commanding (uplink) is as a baseline not done in the blind, hence commanding in practice requires also a downlink as well.

Examples of both double and downlink-only pass handling can be found in Stein Haugan's document outlining the 2006 March keyhole.

Double passes

Double pass handling

If the beginning and ending times of the uplink and downlink passes are identical (the two passes overlap completely), no actions on or modifications to the initial keyhole plan are required. If the the overlap is partial, some editing may need to be done, depending on the type of overlap(s).

Overlap	Actions/changes required
U/L starting before D/L	No problem. The U/L prior to D/L is not used except possibly for a few minutes to sweep into the receiver blindly (34 m keyhole, 34 m downlink). Large amounts of time can be released if the D/L antenna cannot be arranged to begin earlier. If the keyhole schedule is very tight, every little bit may be valuable and it may be worthwhile to try extending the D/L antenna's coverage from the beginning.
D/L starting before U/L	The downlink-only portion at the beginning must as a baseline not be relied upon. That portion can hence be given up and the pass shortened from the beginning unless the detailed planning shows the continuity of VGM to be at risk. It is therefore prudent to first shorten the pass in the keyhole plan, but not formally give up the time until more detailed planning has been made. See below.
U/L ending before D/L	The FOT must time-tag the end-of-pass activities and there needs to be enough time for it, otherwise no problem. The FOT needs to check the adequacy of the time.
D/L ending before U/L	Release the extra U/L time, no use for it unless the D/L time can be extended.

Notes and comments

The most common thing to fix with double passes involves a D/L station starting just a little before the corresponding U/L station.
If both begin and end times differ, treat with both separately according to the guidelines given in the table above.
When you change the keyhole plan pass entries, be careful to
- leave the original information in comments
- mark when the change was made
- especially if more than one person works on the keyhole plan, mark who made the change
if pass time is marked to be released, keep notes on which passes and which time periods are marked for release or which have been released.

Downlink-only (D/L) passes

The key issue with the downlink-only passes is: what data are planned to be dumped on a downlink-only station must not be overwritten until it has been verified that the data have been downlinked successfully. To achieve that those data shall be recorded onto the tape recorder (TR) instead of the SSR and kept on the TR until it has been dumped completely. A couple of things need to be kept in mind here:

Use the SWAP commands to use the TR
If the SSR is in use in the beginning of an intended TR dump pass, insert the DUMPALL command into the beginning of that (high-rate) pass. Add DUMPALLs to as many consequtive passes as are needed to empty the TR.
Keep in mind the contingency planning for losses of single passes — loss of a pass may kick on the VGM contingency and this needs to be checked by killing passes as needed.