According to this answer, using "coord@10" as a target code for JPL's Horizons represents the motion of a hypothetical location on the Sun's surface; presumably as a way to represent the rotation of the Sun's surface.
So far I have not been able to access "coord@10" as either a target, or an origin for a coordinate system.
I'd like to try this out to reproduce the explanation. How can I access this point using Horizons?
From the tutorial:
To select a point on the surface of a target body (limited to those with an IAU rotational model), use this form (units are degrees and km):
{g: E.Long, latitude, h@}BODY [geodetic/planetographic coords]
{c: E.Long, DXY, DZ@}BODY [cylindrical coordinates]
For example: "g: 348.8, -43.3, 0 @ 301" specifies the crater Tycho on the Moon (body 301), at geodetic (planetographic) coordinates 348.8 degrees east longitude, -43.3 degrees latitude (south), and zero km altitude with respect to the IAU reference triaxial ellipsoid.
So the 10 is the body id for the sun, and you can type coordinates before the @ sign.
Try for example: g:0,0,0@10
The Coord@ comes in to play when you are getting the values to run a query not using the web interface. It also seems they have upgraded the interface a fair bit since I used it first. In Vector mode, you could not set the SSB as an option for the center, or at least, it never seemed to work when I attempted it. I think it also pre-filled in the coordinates when I first used it, although it is hard to know what a website did in the past, especially one that saves the last query you did. It is possible that this was also set because I searched for a target from the list of cities, which might have pre-populated the observational location. If you have the coordinate filled out for the object type, and look at the "Batch" mode configuration, which was my starting point, you get the following:
!$$SOF
COMMAND= '499'
CENTER= 'coord@10'
COORD_TYPE= 'GEODETIC'
SITE_COORD= '0,0,0'
MAKE_EPHEM= 'YES'
TABLE_TYPE= 'OBSERVER'
START_TIME= '2018-03-21'
STOP_TIME= '2018-04-20'
STEP_SIZE= '1 d'
CAL_FORMAT= 'CAL'
TIME_DIGITS= 'MINUTES'
ANG_FORMAT= 'HMS'
OUT_UNITS= 'KM-S'
RANGE_UNITS= 'AU'
APPARENT= 'AIRLESS'
SUPPRESS_RANGE_RATE= 'NO'
SKIP_DAYLT= 'NO'
EXTRA_PREC= 'NO'
R_T_S_ONLY= 'NO'
REF_SYSTEM= 'J2000'
CSV_FORMAT= 'NO'
OBJ_DATA= 'YES'
QUANTITIES= '1,9,20,23,24'
!$$EOF
The coord@10 is the coord@ I was referring to. By looking in to the documentation, and taking advantage of updates they seem to have made since I first was playing with the interface, I discovered I have much better results without the "coord" in the center, as it won't track a point on the sun, but rather the center of the sun.
