Go to https://ssd.jpl.nasa.gov/horizons/app.html. The HORIZONS web interface shows the current settings, all of which can be modified using the “Edit” buttons. Previously-used settings seem to be remembered, so we first recommend that all inputs should be set to their default using the “custom” settings. The inputs should then be set as follows:
- Ephemeris Type should be Observer Table (default).
- Set Target Body to the relevant solar-system body.
- The Observer Location is most easily set to ALMA using the “Lookup the Specified location” option. Entering “ALMA” will enable a search and the entry that should be chosen is Atacama Large mm/sub-mm Array (ALMA, Center)@399. Note that geocentric locations should no longer be used.
- The Time Specification should be set as needed. The ephemeris should contain at least one row for every hour and, except for fast objects, one row every 20 minutes is sufficient. The online system will linearly interpolate between rows in an ephemeris when determining the antenna pointing and propagation delay.
- Of the many possible Custom settings, only Astrometric RA & DEC and Obsrv range & rng rate are required - there are options 1 and 20. It is also recommended that “Extra precision” be selected from the additional settings. Do not select the elevation cutoff option as this is likely to fail.
Once the inputs have been checked and found to be okay, the ephemeris can be generated. This will take the form of text in the browser window which can be downloaded as a text file and then read into the Science Goal or SB.
The ALMA OT Team, 2024 May 21