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

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?

• Just go to the bare url of the site - it is a web page. ssd.jpl.nasa.gov/horizons.cgi At the beginning there are links. Two clicks away is a page called "documentation". – uhoh Nov 3 '18 at 11:08
• The first paragraph is laden with additional links, here's the text: The JPL HORIZONS on-line solar system data and ephemeris computation service provides access to key solar system data and flexible production of highly accurate ephemerides for solar system objects ( 789366 asteroids, 3539 comets, 190 planetary satellites, 8 planets, the Sun, L1, L2, select spacecraft, and system barycenters ). HORIZONS is provided by the Solar System Dynamics Group of the Jet Propulsion Laboratory. – uhoh Nov 3 '18 at 11:12
• I think I found something inside the output itself (specifying " OBJ_DATA='YES' "): "orx_180801_181203_181030_od055-N-AM3A-P-AM4_v1" could probably mean "data updated on 2018/10/30 after AAM3 and valid till AAM4", but I could be wrong. – jumpjack Nov 3 '18 at 11:17
• Said like that, it wouldn't be, but that's not what I did. Instead, I've written nearly 200 words, and I've recommended you go to the website and find out how much great documentation exists there for you. I've given the link too. I've also voted to close your four-question question as too broad. MAKE_EPHEM is there in your url, so reading about what an ephemeris is would be your first step. I've included a link to the Wikipedia page for that as well. After a bit of research you may have some narrower and better defined questions. – uhoh Nov 3 '18 at 11:27

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

• Thanks. As far as I can understand, last line in the table says that the file contains data from 2018-Aug-01 to 2018-Dec-03 but it has been updated on 2018/oct/30, which is exactly the date of Asteroid Approach Maneuver 3 (AAM3, or just AM3). But I can't understand the meaning of "od055", "N" and "P" . – jumpjack Nov 5 '18 at 20:17
• od055 just means the 55th orbit determination solution they ran. I don't know what the N or P means. – Mark Adler Nov 5 '18 at 22:24

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:

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:

*******************************************************************************
*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