MOSS Training

This training module is intended to suplement the moving target processing procedures by helping a Program Coordinator become familiar with the Moving Object Support System (MOSS). MOSS is the component of the HST ground system that is responsible for generating moving target ephemerides and observing windows. This module assumes thorough knowledge of the Solar System Targets chapter of the Engineering Phase 2 Instructions. If you are not already familiar with this chapter, read it now before proceeding.

Documentation

MOSS is a subsystem of the HST ground system. To see where MOSS fits into the ground system, see the Operational Data Flow Diagram. The MOSS subsystem consists a program called Percy and a set of ephemeris data for all planets and moons in the solar system. The Percy software is documented in the Percy User's Guide. It is important to become familiar with this document.

First, work though the tutorial. It will give a good basic introduction. Be sure to actually try the examples.

Next, it would be good to read the remainder of the Percy User's Guide except for the commands section. That section is intended as a reference. There is a fair amount of material in the Percy User's Guide, so you might want to skim some of it, but the following sections are most important:

• Tutorial
• File Redirection
• Environment Setup
• Configuration Files
• Take a look at the STScI operational configuration file
• Take a look at the STScI operational startup script
• Understanding Syntax Descriptions
• About Bodies
• About Constraints
• About Designated Objects
• About Graphics
• About Reports
• Take a look at some example reports.

As you read through the Percy User's Guide, try out the commands so that you become familiar with them. Especially be sure to try some FIND commands, some graphics, and some reports.

Write an Observing Program

Write a four visit phase 2 observing program. Choose any configuration, operating mode, spectral element, and exposure time, but make sure the four visits are targeted as follows:

1. Observe Io while it is between 355 degrees and 5 degrees orbital longitude.
2. Observe Enceladus
3. Observe the crater Copernicus on the Moon. Copernicus is located at planetographic latitude and longitude 7 degrees and 18 degrees respectively.
4. Observe Comet Forbes. Comet Forbes has a perihelion distance of 1.44601562 AU, an eccentricity of 0.568120461, an inclination of 7.1627986 degrees, a longitude of ascending node of 334.3677571 degrees, an argument of perihelion of 310.70361 degrees, a terrestrial dynamic barycenter time of perihelion passage of 4:23:37 on May 4 1999, a terrestrial dynamic barycenter osculation date of May 4, 1999, a radial component of non-gravitational acceleration of 4.707x10^-9 AU/day², and a component of non-gravitational acceleration lying in the orbital plane and parallel to the instantaneous velocity vector of -4.87x10^-10 AU/day². All of Comet Forbes' orbital elements are expressed in equinox J2000.

As you are defining each target in APT, notice that APT supplies some separation constraints by default:

These default separations do not apply if a body to be avoided is the target. For example, if the target is Europa, then the Europa separation constraint does not apply; but the Jupiter, Io, Ganymede, and Callisto separation constraints would apply.

These default separations may be modified by the observer overridden; but for the purpose of this excercise leave all of the default constraints in place for now.

Once the program has been written, check it into PLib using one of your test IDs.

MOSS Input Files

During PLib checkin, APT writes command script files that are used as the operational input to Percy. Look at these files. They are in /data/implementation/moss, and their naming convention is: <program number>_<target number>.per.

After the FIND commands that implement the constraints specified in the phase 2, the next part of the MOSS input file may contain some occultation constraints. Occultation constraints will be present in two cases. The first case is whenever the target is a moon of Uranus or Neptune. Since these planets do not have the default separation constraints, it is necessary to ensure that observations of their moons are not scheduled while the target moon is behind the planet. Second, whenever the target is a surface feature of any body, it is necessary to make sure that surface feature is not occulted during the observation. Any target where level 2 or level 3 is TYPE=PGRAPHIC, TYPE=PCENTRIC, or TYPE=MAGNETO is considered to be a surface feature for the purpose of determining the need for an occultation constraint.

Finally, there will be a CHEBY command and a CHEBY_WINDOW command. CHEBY generates the ephemeris file for the target. CHEBY_WINDOW generates moving target windows file for the target.

Problem Solving

These exercises will illustrate some problems that may be encountered operationally.

Now, run MOSS on all the targets of the test program. Some of the targets will have errors because not all of their constraints can be satisfied. To resolve these problems, determine which constraint(s) cannot be satisfied. Next, determine how the constraints may be modified so that they can be satisfied while making as few compromises as possible to what the observer originally requested. Use whatever methods you like. Some useful approaches might include:

• Set the MOSS Show Windows option in the target defintion in APT so that SHOW WINDOW commands are included in the input to MOSS.
• Generate graphic displays.
• Generate reports.

Repeat this process for each target that has errors.