How can I know if NASA Horizons output is measured, calculated, estimated or what-else?

Question

This link allows downloading from NASA Horizons server data about the distance of a body (OSIRIS-REX spacecraft ) from the Sun (see link here):

How can I know how the distance is obtained?

1. Is it the distance it should have on specified date according to mission schedule?
2. Is it measured from Earth by radar?
3. Is it an onboard telemetry?
4. Is it calculated by the time the light takes to go from Earth to S/C and back?

How can I know whent it was last updated and on which basis?

Answer 1

1. No, these are from measured orbit determinations. However the data does go into the future somewhat, as you can see from the table below. The "od" you see in the rows means "orbit determination", and refers to an update to the navigation state based on a series of radiometric tracking activities over time.

2. No, if you mean bouncing radio signals passively off of a spacecraft and trying to detect that back at Earth. There is no way to get enough signal back that way. (Note that radar signal strength goes as $1\over R^4$.)

3. No, spacecraft generally don't do their own orbit determination.

4. Yes, that and the Doppler shift of the returned signal. This is the typical radiometric tracking of spacecraft, and depends on the spacecraft receiving, amplifying, and returning the signal, with a rational frequency multiplier. Returning a signal actively gives a $1\over R^2$ signal strength.

In this table, the third data in each name refers to, I believe, the navigation data cutoff date. So the trajectory is based only on radiometric data received on or before that date.

  Trajectory name                                   Start (TDB)    Stop (TDB)
  ----------------------------------------------    -----------   -----------
  orx_160908_231024_pgaa3_day06m60_v1               2016-Sep-08   2016-Sep-09
  orx_160909_171201_170830_od023_v1                 2016-Sep-09   2017-May-01
  orx_170501_180710_171005_od027_v1                 2017-May-01   2017-Sep-23
  orx_170923_180710_180125_od030_v1                 2017-Sep-23   2018-Mar-01
  orx_180301_181203_180921_od044-N-AM1-F-AM4_v1     2018-Mar-01   2018-Aug-01
  orx_180801_181203_181030_od055-N-AM3A-P-AM4_v1    2018-Aug-01   2018-Dec-03

Answer 2

Partial answer to the last part of the question:

How can I know when it was last updated and on which basis?

Compare the "revision date" in output header to the "Start (TDB)" date of last item in the table of ephemeris files:

Output header:

MRO around Mars:

*******************************************************************************
 Revised: Oct 17, 2019    Mars Reconnaisance Orbiter (MRO)  / (Sun)         -74
                            http://marsprogram.jpl.nasa.gov/mro/

Ephemeris files table:

Trajectory files (from MRO Nav., JPL)      Start (TDB)         End (TDB) 
--------------------------------------  -----------------  -----------------
mro_cruise                              2005-Aug-12 12:42  2006-Mar-10 22:06
mro_ab                                  2006-Mar-10 22:06  2006-Sep-12 06:40
misc reconstruction (mro_psp1-51)       2006-Sep-12 06:40  2019-Jul-01 01:01
mro_psp_rec                             2019-Jul-01 01:01  2019-Oct-12 22:41
mro_psp                                 2019-Oct-12 22:41  2019-Dec-16 14:58

Revision date is 2019/10/17, file spans from 2019/10/12 to 2019/12/16

---

LRO around Moon:

Output header:

*******************************************************************************
 *Revised: Aug 13, 2019          LRO Spacecraft / (Moon)                     -85
 *                              http://lro.gsfc.nasa.gov/

Ephemeris files table:

Trajectory name                 Start (TDB)         Stop (TDB)
---------------------------- -----------------  -----------------
Reconstructed trajectory     2009-Jun-18 22:16  2019-Mar-15 00:01
Concatenated predict updates 2019-Mar-15 00:01  2019-Aug-07 00:01
558day_20190807_01.V0.1      2019-Aug-07 00:01  2021-Feb-15 00:01   

Revised 2019/08/13, ephemeris file starts from 2019/08/07 and ends in 2021/02/15, hence some values are real, others are forecast.

---

Sometimes prediction/real is stated right before the ephemeris files table:

MRO around Mars:

 SPACECRAFT TRAJECTORY: 
  Trajectory starts after Centaur launch stage separation (cruise phase).
  Trajectory after 2019-Oct-12 is PREDICTION.

Other times it's written before the table or in first column of table itself:

LRO around Moon:

SPACECRAFT TRAJECTORY: 
  Updated irregularly (on Horizons) or by request. 
  Concatenated historical (reconstructed) trajectories are from PDS. 

  Trajectory name                 Start (TDB)         Stop (TDB)
  ---------------------------- -----------------  -----------------
  Reconstructed trajectory     2009-Jun-18 22:16  2019-Mar-15 00:01
  Concatenated predict updates 2019-Mar-15 00:01  2019-Aug-07 00:01
  558day_20190807_01.V0.1      2019-Aug-07 00:01  2021-Feb-15 00:01