Moving Target Ephemerides
Details on how JWST and APT obtain and use moving target orbital elements are covered in this article. It includes how a proposer can obtain ephemerides using JPL Horizons and how JWST's orbit affects ephemeris accuracy.
On this page
Moving target tracking
Using the orbital elements specified in APT and the apparent positions of the guide stars (GS) and the moving target as a function of time, the ground system generates 5th-order polynomial that describes the path of each GS across the field of view of the Fine Guidance Sensor (FGS) such that the moving target will remain stationary in the science instrument reference frame (modulo dither offsets). See the Moving Target Acquisition and Tracking article for more details. The ephemerides for multiple guide stars are uplinked to the observatory, with the ensemble of their usability windows spanning the scheduled time for observation execution, including margin in case the observation executes earlier or later than planned.
Once an observation begins, the FGS measures the GS position and flight software compares that to its pre-computed ephemeris position in the FGS. It then updates the observatory pointing to force the star to follow the track specified by the polynomials. Ephemeris data is not included in the FITS headers, but guiding information obtained every 64 ms is included (in the level-1b "_uncal" files), enabling a reconstruction of the target's motion across the sky. The heliocentric and observer distances are also included in the FITS headers.
JWST in JPL Horizons
Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.
The observatory orbit in the JPL Horizons ephemeris generation system is the post-launch, post-insertion solution. Station-keeping to maintain the observatory's orbit about the L2 point is performed roughly once every 2 or 3 weeks. Shortly after each station-keeping maneuver, the updated JWST orbit is sent to JPL and incorporated in Horizons. A description of the orbit and the station-keeping procedures can be found on the JWST Orbit page.
There are 2 ways to specify JWST as the Observer Location in Horizons: @jwst, or the observatory code, 500@-170. Figures 1–3 provide a quick walkthrough for specifying the Observer Location in Horizons.
Step 1. JPL Horizons home page
On the main page of the JPL Horizons web interface, locate the Observer Location row, as highlighted in Figure 1. Click on change.
Figure 1. JPL Horizons home page
Click on the figure for a larger view.
Step 2. Observer Location page
After clicking on change, you will be redirected to the Specify Observer Location page. Locate the Specify Origin: Named Body or Site section and the search box. Input either @jwst or 500@-170, and click the Search button. Then, you will be redirected back to the main page.
Figure 2. Specifying the observer location
Click on the figure for a larger view.
Step 3. Check that Observer Location was updated
After being redirected to the main page, the Observer Location should have updated to read "James Webb Space Telescope (JWST) Spacecraft [500@-170], as highlighted in Figure 3.
Figure 3. Observer location set to JWST
Click on the figure for a larger view.
Ephemerides and the APT Visit Planner
Video Tutorial: 
APT Visit Planner
See also: APT Visit Planner
The Visit Planner in the Astronomer's Proposal Tool (APT) use orbital elements directly from JPL Horizons to construct ephemerides and determine observing windows. See the Tutorial on Creating Solar System Targets in APT for step-by-step instructions on obtaining orbital parameters for any moving target.