[X] My Assistant

[elakdawalla]

I've also been thinking about the best way to visualize MRO's operations--not easy. As for the 17-day groundtrack, here's one approach (click for larger image and to see the faint ground tracks): If you zoom in you can see that MRO (the red marker) is just beginning to overlap just to the left of the tail of the track, where it started 17 days earlier. It's interesting to note that the field of view of HiRISE at this scale is only 1/2th the width of the groundtrack line. So even the CTX FOV is just a few line widths (pixels). This was generated from a SPICE kernel & so should be fairly accurate. Did you want to see that on a sphere? Still, it's kind of busy.

This map is great -- it is busy but it should be, it illustrates the network of the ground tracks nicely. Thanks also for the details about the relative scales of the FOVs.

Your video is really cool -- too big for my website but really cool!

--Emily

[helvick]

No, wait a minute, you're right. I was assuming that the only way to put the orbit into Celestia was to supply orbital elements. I now see that there's an option to use "SampledOrbits" which are just the XYZ locations at some time spacing. Assuming that Celestia wouldn't choke with, say, a few months' worth of 5-minute or 1-minute spacing, one could write a program to extract that from the SPICE files. Then there's just the wrinkle of figuring out the coodinate system.

Sorry I wasn't clear about that - the Celestia forum guys have some tools for this.
This Celestia forum thread delves into the topic

[jmknapp]

Emily -- glad you can use the map.

Helvick -- do the Celestia xyz files use J2000 coordinates?

[mcaplinger]

Helvick -- do the Celestia xyz files use J2000 coordinates?

I couldn't find any documentation on this. The orbital elements are pretty clearly given in some central-body-equatorial system, but where the X vector is pointed wasn't clear to me. While possible, it seems unlikely that the Celestia developers settled on the IAU_MARS frame (IAU vector and Mars equinox of 2000)

BTW, there's a small MGS Mars map at http://www.msss.com/allmars.jpg (1440x720) so you don't have to keep using that old Viking map I'll see if I can't put a bigger one out soon.

[helvick]

Emily -- glad you can use the map.

Helvick -- do the Celestia xyz files use J2000 coordinates?

Yes - The data (with some reformatting) is the same as that produced by http://ssd.jpl.nasa.gov/horizons.cgi although on the only occassion that I used it I used the email option as per the following:

Here's a sample template for an e'mail message to request xyz coordinates
from Horizons for use with Celestia. You will have to edit the returned
message into the format that Celestia uses.

The message must be mailed to the address "horizons@ssd.jpl.nasa.gov"
The subject must be "JOB" ( do not include the quotes in either )

The body of the message should consist of the following text which
is between the lines of =====. Don't include those lines.
The lines starting with "!$$" are required, however.
Replace the fields ending in "here" with appropriate values.
Don't touch any of the other lines until you've read and understood
Horizons' documentation.
==========================================================
!$$SOF (ssd) JPL/Horizons Execution Control VARLIST
EMAIL_ADDR = 'your address here' ! 'you@there.com'
COMMAND = 'object name or id# here' ! e.g. 'TRITON'
OBJ_DATA = 'NO'
MAKE_EPHEM = 'YES'
TABLE_TYPE = 'VECTORS'
CENTER = '@10'
REF_PLANE = 'ECLIPTIC'
SITE_COORD = '0,0,0'
START_TIME = 'starting date here' ! e.g. '1989-Aug-23 09:00'
STOP_TIME = 'ending date here' ! e.g. '1989-Aug-27 09:00'
STEP_SIZE = 'sample period here' ! e.g. '10m' or '2d'
REF_SYSTEM = 'J2000'
OUT_UNITS = 'KM-D'
VECT_TABLE = '1'
VECT_CORR = 'NONE'
TIME_DIGITS = 'MIN'
CSV_FORMAT = 'NO'
VEC_LABELS = 'NO'
ELM_LABELS = 'NO'
R_T_S_ONLY = 'NO'
CA_TABLE_TYPE= 'EXTENDED'
CALIM_SB= '0.1'
CALIM_PL= '.1, .1, .1, .1, 1.0, 1.0, 1.0, 1.0, .1, .003'
!$$EOF++++++++++++++++++++++++++++++++++++++++++++++++++++++
==========================================================

[jmknapp]

BTW, there's a small MGS Mars map at http://www.msss.com/allmars.jpg (1440x720) so you don't have to keep using that old Viking map I'll see if I can't put a bigger one out soon.

You mean for the map that has the ground tracks? I used a Celestia map done by praesepe the original of which looks like the one you mention--must be the one it's based on. For the animation I used your 256 pixel/deg maps.

[mcaplinger]

[font=Courier New]Yes - The data (with some reformatting) is the same as that produced by http://ssd.jpl.nasa.gov/horizons.cgi although on the only occassion that I used it I used the email option as per the following...

Note that Horizons is outputting everything in heliocentric XYZ. If you did that for a Mars-orbiting spacecraft, then if Celestia's idea of where Mars is would have to match JPL's exactly, or the orbit would come "unstuck" from the planet. I'm pretty sure that the XYZ coordinates that Celestia uses should be Mars-centered. I'll have to get the source to see if it's oriented in J2000, but I seriously doubt that it is.

[Sym05]

Note that Horizons is outputting everything in heliocentric XYZ. If you did that for a Mars-orbiting spacecraft, then if Celestia's idea of where Mars is would have to match JPL's exactly, or the orbit would come "unstuck" from the planet.

Try to investigate jpleph.dat for Celestia, I found it in a thread about New Horizons to Pluto add-on. They had a similar problem.
http://www.shatters.net/forum/viewtopic.php?t=8730


In this topic the author of Celestia, Chris Laurel, give some details: http://www.shatters.net/forum/viewtopic.ph...er=asc&start=15

Have a look at http://www.shatters.net/~claurel/celestia/files/jpleph/

There are three JPL ephemeris files. The .405 files cover a timespace of 50 years. The .406 file covers 300 years from 1800-2100. The max interpolation error of the DE406 ephemeris is slightly greater (up to 25m versus 1mm for DE405), but this shouldn't matter for Celestia. To experiment with the JPL ephemerides, just copy one of the files (probably unxp1800.406) into Celestia's data directory as jpleph.dat. Then, change the CustomOrbits in solarsys.dat from vsop87-<planet> to <planet>-jpl. There's still a slight discrepancy between the positions Horizons returns and the positions I compute from the JPL ephemerides--for Jupiter, it's up to 200km. At this point, I suspect the problem may be related to a slightly different time standard.

[mcaplinger]

I'm pretty sure that the XYZ coordinates that Celestia uses should be Mars-centered. I'll have to get the source to see if it's oriented in J2000, but I seriously doubt that it is.

I found the following about Celestia's SampledOrbits:

"The origin is the center of the parent object, whether it's a star or a planet. If the parent is a planet, the +z axis is the planet's north pole and the xy plane is the planet's equatorial plane at the reference epoch J2000. The x axis points at the ascending node of the equatorial plane on the ecliptic."

I believe this is the same inertial frame as the NAIF Toolkit's MARSIAU frame (not to be confused with the rotating IAU_MARS frame, alas.)

edit: for those of you still reading this obscure discussion, I think I was wrong: the Celestia frame is what we usually call Mars Mean Equator and Equinox of J2000, which is different from MARSIAU. Back to the drawing board.

[Mike Loucks]

I'm sorry I didn't see this until now. STK easily does all of what you want.

The statement that "STK isn't likely to do what you want" is way off. STK does all of this stuff, and in my opinion does it better than any of the other tools mentioned here.

It can absolutely use SPICE files, although it doesn't have to because it has it's own numerical integrators that will create a real ephemeris if you like.

The Astrogator module of STK has flown libration point trajectories at GSFC and comes from the GSFC 'Swingby' code that was used to fly Clementine, WIND, SOHO and Lunar Prospector.

STK does ground tracks in 2D and 3D orbits in any frame you can think of. We regularly use it to show planetary fixed, and inertial frames, along with spacecraft relative (formation flying) and rotating libration point frames.

Repeating ground track stuff is easy, at any planet. Animations are also easy. STK runs on Unix and PC platforms.

That said, STK isn't free (like Celestia). But if you work for an organization that owns it, you can do an awful lot of stuff.

[mcaplinger]

The statement that "STK isn't likely to do what you want" is way off.

By that I meant the free version of STK. I don't think many people reading this will be able to afford the needed add-on modules to read SPICE files; last I looked they were well over $10K.

BTW, the current version of Celestia can read SPICE SPK files. The support's not all there yet, but it's improving rapidly.

[Hari]

You can also try xplanet. It can draw satellite orbits, ground tracks in many different projections, and read spice kernels.

See this page for an example.