				INTRODUCTION

NPOE is an interactive shareware computer program for the IBM (tm) and true 
compatible computers which can model important orbital events and predict 
the long term evolution of satellites in Earth orbits. Program NPOE 
implements a special perturbation solution of orbital motion using a variable 
step size Runge-Kutta-Fehlberg integration method to numerically integrate 
Cowell's form of the system of differential equations. Orbital events are 
predicted using Brent's method for finding the root of a non-linear equation, 
and the user can control both the integration and root-finding convergence 
criteria.

Program NPOE can accurately predict the time, geometric and dynamic orbit 
characteristics for any physically realizable user-defined value of the 
following orbit parameters:

     - True anomaly
     - Argument of latitude
     - Flight path angle
     - Geocentric declination
     - East longitude
     - Geodetic altitude
     - Geodetic latitude
     - Orbital speed

The software can also determine the time and orbital characteristics of the 
following discrete events:

     - Ascending node crossings
     - Descending node crossings
     - Rise and set conditions of a satellite relative to a ground site
     - Visibility conditions of the Sun or Moon relative to a satellite
     - Earth and lunar shadow entrance and exit conditions

The NPOE software can model one or more of the following types of orbit 
perturbations:

     - Earth gravity
	 user-defined degree and order

     - Solar gravity
	 point mass

     - Lunar gravity
	 point mass

     - Atmospheric drag
	 U.S. Standard 1976 density model
	 Jacchia 1970 density model

     - Solar radiation pressure
	 user-defined reflective and geometric properties

The NPOE software includes the following useful features:

     - Osculating/mean orbital elements input and output
     - Event prediction with a numerical integration/root-finding procedure
     - Saving and recalling graphic images to and from disk
     - Interactive editing of NPOE input files
     - Flexible browsing of output data files
     - X-Y graphics with auto-scaling and a "smart" cursor
     - An interactive graphics plotting capability with user annotation
     - Printer support for data display pages and graphic images

			     SYSTEM REQUIREMENTS

Program NPOE requires an IBM-PC or compatible computer with 640K of 
conventional memory and DOS version 3.1 or later. The NPOE software will 
support EGA and VGA compatible video modes and NPOE will automatically 
detect the type of graphics available on your computer. A hardware numeric 
coprocessor is not required but is highly recommended. The NPOE software 
will support graphics and data hardcopy to Epson and HP Laserjet compatible 
printers connected to a parallel port. The NPOE editor will also support a 
Microsoft compatible mouse.

			     DISTRIBUTION FILES

The NPOE distribution disk contains the following executable and data files:

     NPOE.EXE - main executable program
     EDITOR.EXE - NPOE interactive editor
     NPOE1.IN - sample orbit propagation input data file
     NPOE2.IN - sample visibility input data file
     NPOE3.IN - sample Earth shadow conditions input file
     NPOE4.IN - sample orbital event input file
     NPOE1.OUT - sample orbital elements output file
     NPOE2.OUT - sample latitude, longitude, and altitudes output file
     NPOE3.OUT - sample state vector output file
     NPOE4.OUT - sample ground site-to-satellite visibility output file
     NPOE5.OUT - sample Earth shadow conditions output file
     NPOE6.OUT - sample orbital event output file
     NPOE.PIC - sample VGA graphics image
     NPOE.PAR - graphics parameter file for NPOE.PIC
     PRINTER.DAT - sample printer parameters data file
     GEM10B.DAT - NASA-Goddard GEM10B gravity model data file
     ATMOS76.DAT - U.S. Standard 1976 atmosphere data file

The main program is started by simply typing NPOE at the DOS prompt followed 
by the Enter key. The NPOE editor is normally activated by "shelling" to it 
from within the main program. However, it can also be run as a stand alone 
text editor by typing EDITOR at the DOS command line. This editor can also 
be used to modify printer parameter files from within NPOE.

				 MAIN MENU

NPOE software features are selected via the following main menu:

	< 1 > Display an NPOE graphics disk file

	< 2 > Browse an NPOE output file

	< 3 > Edit an existing NPOE file

	< 4 > Select an input data file and run NPOE

	< 5 > Run NPOE interactive plotter

	< 6 > Quit NPOE and return to DOS

A selection is made by simply pressing the appropriate number key followed 
by the Enter key. The following is a brief discussion about each program 
option.

The first menu option displays a list of NPOE graphic disk files. The 
keyboard arrow or cursor keys can be used to highlight the file of interest. 
Pressing the Enter key will recall the file and display it on the monitor 
screen. The graphics image can then be annotated, printed or saved to disk 
under a user-defined filename.

The second main menu option allows the user to recall and browse any NPOE 
output file. The keyboard arrow or cursor keys can be used to highlight the 
file of interest. Pressing the Enter key will recall the file and display it 
on the monitor screen. You can move around the data file by using the cursor, 
Home, End, Page Up and Page Down keys. To leave this option press the Esc key.

The third main menu option will invoke the NPOE interactive editor. This 
simple editor has two pull-down menus called System and Editor. These menus 
can be invoked by using a mouse or the Alt S or Alt E keys combinations. The 
Editor menu has four options called Load, Save, Save As and Print. The Load 
option will allow the user to load an existing NPOE input file for editing. 
The Save option will resave the file to disk with the original filename. The 
Save As option will save the current file to disk under a new, user-defined 
filename. The Print menu option will dump the current file to your printer. 
You can move around the file using either the cursor keys or the mouse 
controlled vertical scroll bar. 

To leave the NPOE editor, select Quit from the System menu. The editor will 
ask for confirmation with a yes or no prompt.

Main menu option 4 will display a window filled with existing NPOE input 
files. The file of interest is highlighted with the cursor keys and selected 
with the Enter key. The interactive NPOE plotter is invoked with main menu 
option 5.  

				INSTALLATION

To install NPOE make a subdirectory called NPOE on your hard disk and copy 
the entire contents of the NPOE disk(s) into this subdirectory. NPOE expects
all files to be in this NPOE subdirectory. If you received NPOE in "zipped" 
form, be sure to unzip all files using PKUNZIP after copying them to the NPOE
subdirectory.

				REGISTRATION

The registration cost for NPOE is $50 and includes a written copy of the
user's manual, one or more diskettes containing the latest version of the 
software, first class postage, program updates, and technical support via 
Compuserve, Internet and written mail. Registration also entitles you to a 
version of NPOE called NPOE386 which requires a computer with the equivalent 
of a 80386 CPU with math coprocessor. NPOE386 is a FORTRAN version of the NPOE 
"computation engine" and is almost an order of magnitude faster than the 
QuickBASIC shareware version. NPOE386 does not provide graphics capability. 
However, the ASCII data files generated by NPOE386 can be viewed, plotted, 
printed and saved to disk with the shareware version of NPOE. NPOE386 can also
use ASCII input files which are compatible with the shareware version of NPOE.

Registration also provides you with a copy of the JGM-2 (Joint Gravity Model, 
version 2) gravity model. JGM-2 is an Earth gravity model developed jointly by 
Goddard Space Flight Center (GSFC) and the University of Texas of order and 
degree 70. Please note that the shareware version of NPOE can accomodate a 
gravity model up to degree and order 40. NPOE386 can accomodate a gravity 
model as large as degree and order 70.

The NPOE registration fee to countries outside the United States is $60. To 
register NPOE please send a check or money order (no credit cards please) in 
U.S. dollars and payable to "Science Software" to the following address:

     Science Software
     P.O. Box 2188
     Reston, VA 22090-0188

     Attention: David Eagle

Please send all technical questions and bug reports to the above address or 
via e-mail to

     Internet --> eagled@iia.org
or
     Compuserve --> 74561,606

The following floppy disk formats are available:

     (1) 1.2 MB - 5 1/4"
     (2) 720 KB - 3 1/2"
     (3) 1.44 MB - 3 1/2"

Please indicate the disk format when registering.

October 29, 1994
