World of Shareware - Software Farm 2/wosw_2.zip/wosw_2/GENERAL/SKY23NC.ZIP/SKYMAP.DOC

Text File | 1991-04-06 | 21KB | 402 lines

                              SkyMap Version 1.2

                          Documentation Draft Release

Introduction
------------

SKYMAP.EXE is a high-precision celestial mapping program for generating star
fields on an EGA/VGA monitor and/or an HP Laser Plotter (or any similar hard-
copy device that accepts the HewLett Packard Graphics Language).  Polar and
rectangular map projections are supported, which can be plotted at any scale
and centered on any coordinate.  Stars can be plotted at any epoch, with
defaults for Besselian 1950.0 (B1950) and Julian 2000.0 (J2000).  Proper
motion is included with precession.

SkyMap's star data files are based on the Smithsonian Astrophysical Observa-
tory Star Catalogue (SAO), epoch B1950.  The SAO contains some 259,000 stars,
but only those stars with visual magnitude brighter than 8.1 are presently
used by SkyMap.  This 51,000+ star subset was converted from a 150-byte ASCII
format to a 49-byte binary format by Rob Matson for use with SkyMap and other
astronomical software.  For ease of modem transmission as well as program
speed, these stars are sorted by right ascension and stored in 22 separate
files, each containing 2350 stars (115150 bytes) except the last which has
1773 stars (86877 bytes).  These files can be found in file area #6 of the
Rancho Palos Verdes bulletin board system operated by Dave Ransom.  SkyMap
also accepts a further compressed, 28-byte format, which can be created from
the 49-byte format files using a special program called 49TO28.EXE.  There
is no loss of precision with the 28-byte format, and run times can be cut by
as much as 30 percent over the 49-byte format (not to mention the megabyte
of disk space saved).  The difference between the two is that the 49-byte
format contains ALL the information found in the original 150-byte ASCII;
the 28-byte format contains only the right ascension, declination, visual
magnitude, and proper motion for each star.

Although SkyMap can be used simply to plot near-publication-quality star
maps, it's greatest feature is its ability to superimpose a time-tagged
satellite trajectory.  Using a standard NASA 2-line element set, SkyMap
can plot the trajectory of any low-earth orbit (LEO) satellite against
the stars as it would be seen from any location on earth.  (SkyMap uses
Space Command's SGP and SGP4 algorithms, which are designed to propagate
the element sets of satellites with periods less than 225 minutes.  Since
nearly all satellites that are visible to the unaided eye fall into this
category, the SDP4 deep-space routine has not yet been implemented.  SGP
and SGP4 will still make predictions for "deep-space" satellites, but with
somewhat decreased precision.)  As with the star catalogues, the NASA two-
line element sets can be found on the RPV BBS, file area #4.  These files
are of the form NASA8xx.TXT, and are updated approximately twice weekly in
the PAK compression format.

Similar to the way it handles satellites, SkyMap also supports trajectory
plotting of rocket boosters.  In this case, the 2-line ephemeris file is
replaced by a rocket trajectory file that contains a tabular listing of a
rocket's position as a function of time.  The format for each line of the
trajectory file is:  mission elapsed time (MET) in seconds, latitude in
degrees, longitude in degrees, and altitude in kilometers.  Negative values
are entered for south latitudes and west longitudes.  The MET values need
not be at regular intervals, but they must always increase.  SkyMap will
linearly interpolate the rocket's position for times occurring between two
tabular entries.

It should be pointed out that due to the limited resolution of monitors,
much of the labeling that appears in the hardcopy is not sent to the video.
For example, right ascension and declination grid labels, and satellite/
rocket date and time tags appear in the hardcopy only.  Also, SkyMap does
not (by itself) create hardcopy output; instead, it produces an HP-GL file
which is compatible with HP plotters such as the HP7475A.  With a raster-
izing program such as PRINTGL or Print-A-Plot, HP-GL files can be output
on an HP LaserJet II or similar printer.  (PRINTGL can be found on Dave
Ransom's RPV BBS).  Due to the large size of the HP-GL files that SkyMap
can produce (particularly for maps plotted to magnitude 8.1), your laser-
printer must have at least a megabyte of add-on memory to support plots
at the full 300-dpi resolution.  If your laserprinter has insufficient
memory, you will have to reduce the resolution to 150- or possibly 75-dpi
in the rasterization program for the file to be printed successfully.


Required Files
--------------

To run SKYMAP.EXE, you will need several auxiliary files in addition to the
star catalogue and satellite ephemeris/rocket trajectory files.  Most
important is the configuration file, SKYMAP.CFG, which is discussed below.
SATNAMES.DAT is needed if you plan to use SKYMAP to plot satellite trajec-
tories.  Finally, ASTROCLK.CTY (or any city file that uses the ASTROCLK.CTY
format) is required if you plan to plot satellite or rocket tracks.  So, to
use all of SKYMAP's capabilities you should have 27 files (not including
this .DOC file):  SKYMAP.EXE, SKYMAP.CFG, SATNAMES.DAT, ASTROCLK.CTY,
NASA8xx.TXT, and 22 star catalogue files.


Configuration File
------------------

There are six lines in the configuration file SKYMAP.CFG.  These lines can
be modified using any text editor to represent your particular location and
operating mode.  Alternatively, SKYMAP gives you the option of changing the
values in the configuration file at run-time, which is somewhat easier.  The
configuration file format is as follows:

     Line 1:  28 or 49    (record size of star catalogue)
     Line 2:  Name of first star file (e.g. SAOM801.DAT)
     Line 3:  Name of 2-line elements or rocket trajectory file
              (e.g. NASA828.TXT)
     Line 4:  Name of city file (e.g. ASTROCLK.CTY)
     Line 5:  Name of your city or viewing site
     Line 6:  Longitude, latitude, altitude, and zone hour of site

The first star file name must be of the form [abcdef]01.[ghi], where the
bracketed characters are optional and can be of any length.  For example,
'STARS01.DAT', 'SAOM801.', and '01.X' are all acceptable; 'STAR02.DAT'
and 'SAO1.BIN' are not, since '01' must appear in the name.  On line 6,
longitude must be from -180 to 180 and latitude from -90 to 90 degrees.
Altitude is in meters, and the zone hour is the time difference from GMT.
Zone hour must be an integer from -12 to 12.  For the west coast, the
zone hour is -8 for Standard Time and -7 for Daylight Savings Time.


The City File
-------------

The city file can have any name, and although it won't be referenced unless
you plot a satellite or rocket track, the file still needs to "exist".  If
you plan to use SKYMAP to plot celestial maps only, just create an empty
file for SKYMAP to find and ignore!  (The same applies to the ephemeris
file -- in fact, you could create a file called NUL.DAT, and use this name
on lines 3 and 4 of SKYMAP.CFG).


Running SKYMAP
--------------

Type SKYMAP from your DOS prompt.  You'll first be asked to answer four
configuration questions, the defaults for which will appear in brackets
following each question.  If no changes are needed, just hit return after
each question.  SKYMAP will check for the existence of all files, and
indicate if any are missing.  If the configuration information checks
out, SKYMAP will move on to the REAL questions, most of which should be
self explanatory.  If you're interested in plotting a section of the sky
within 50 degrees of the equator, a rectangular map projection is probably
the way to go.  Otherwise, plot a polar projection.  The grid spacing
question refers to a celestial coordinate grid that will be overlayed on
the stars once they are plotted.  If you don't want a grid, enter negative
values for both the RA and Decl spacing.


Plot File
---------

A word on the plot file.  SKYMAP *always* produces a plot file, whether
you want it or not.  (This is an artifact of the program's evolution from
the days when screen output was not yet implemented.)  You can always
delete the file later if you don't need it, so just give it any name.
The program will check to see if the file already exists, and if so, give
you the option of writing over it or selecting another filename.


Screen Output
-------------

Screen output IS optional, since there are one or two people in the world
that don't have an EGA or VGA monitor.  At present, the default is NO
screen output, so don't hit return for default unless you mean it.  (This
will be changed in the next release of SKYMAP.)  For the user that selects
no screen output, when it comes time to plot the stars on the screen, the
program will instead output a running count (by hundreds) of the stars it
has processed.


Star Magnitude Limit
--------------------

The limiting magnitude of the catalogue is 8.1 at the present time.  If
you enter a star magnitude limit of 8.1 or higher, all stars that are
within the viewing window will be plotted.  Obviously, the higher the
limiting magnitude you enter, the fewer stars that will be plotted, but
the faster the program will run.


Star Map Epoch
--------------

The default is B1950, which appears in brackets.  However, you can enter
"J2000" for Julian 2000.0, or any other date.  For either of these cases,
SKYMAP calculates the proper motion of all stars, and then precesses the
new coordinates to the requested equinox.  Nutation is not handled by
SKYMAP at present.  You'll notice that SKYMAP runs somewhat faster using
the B1950 default epoch, since no proper motion or precession calculations
are needed.


Celestial Map Only
------------------

After the stars have all been plotted, SKYMAP will ask if you want to add
a satellite or rocket trajectory to the plot.  The default is YES, but
enter 'N' if the map is all you need.  SKYMAP will ask for a title to
print below the map, which can be up to 70 characters.  Hit return if
you don't want a title.  You then come to the final input:  whether to
save changes to the configuration file.  The default is no, and if you
hit return SKYMAP.CFG will remain unchanged.  If you made configuration
changes that you want to save, enter 'Y'.  That's it!  You're done!  Your
SKYMAP HP-GL file is ready to plot.


Plotting a Satellite Trajectory
-------------------------------

Plotting a satellite trajectory requires a bit of preparatory work BEFORE
you run SKYMAP, and perhaps a little trial and error.  SKYMAP doesn't have
the "brains" to know when a particular satellite is going to make a favor-
able pass over your location, but there are plenty of other programs that
were specifically written for this purpose.  I use my own program called
VIEWSAT (which I may release later to the public), but others such as Dave
Ransom's STSORBIT and Paul Traufler's TRAKSAT are available on various
bulletin boards, and are easy to use.  Furthermore, they use the same two-
line orbital elements that SKYMAP does.

Before running SKYMAP, try one of these programs and write down the date
and time of the satellite pass you wish to plot.  Also note the range of
celestial coordinates covered during the pass.  If the satellite is in a
near-polar orbit, the right ascension probably won't change much except
near the pole, so a polar plot will probably work best for you.  As an
example, if the right ascension starts between 15 and 16 hours and ends
between 3 and 4 hours, and the declination starts at -10 degrees, increases
to 85 and ends at 70, a polar plot centered on RA 15.5 hours, Decl 50 deg,
with a vertical field of view of 120 degrees would cover the entire orbit
track.  (In practice, when using a polar projection it is best to stay away
from the celestial equator so that distortion is minimized).

With the proper satellite position and time information, you should be able
to plot a part of the sky that the satellite will pass through.  Once the
stars are plotted, hit return twice to select the satellite track mode,
and enter the date of the satellite pass.  The date will be interpreted as
local or GMT depending on the form of the start time (entered later).


Ground Site Information
-----------------------

The next input is your ground site name, where the default in brackets is
the site name found on line 5 of the configuration file.  If you select the
default, the coordinates which appear on line 6 of SKYMAP.CFG will be used
WHETHER OR NOT THEY ARE THE ACTUAL COORDINATES FOR THAT SITE.  (If your site
name is San Diego, CA, and the entries on line 6 are -77, 39, 0, -5, the
satellite's celestial coordinates will be from the vantage point of Washing-
ton, D.C.!)  If you don't want the default site, enter the name you want or
a search string.  SKYMAP searches through your city file (e.g. ASTROCLK.CTY)
for a city name containing the string you entered.  If a match is found,
SKYMAP will display the complete city name containing the match, giving you
the option of accepting it or continuing to search for another match.  If
the entire file is searched without an acceptable match, SKYMAP will display:

     Site name not found in ASTROCLK.CTY (or your city file name)

You will then have to enter the site latitude, longitude, altitude, and
local timezone hour difference.  The only default offered is the timezone,
which SKYMAP will guess at based on the site longitude.  For most cities,
SKYMAP guesses correctly for standard time, but not for Daylight time.  (A
notable exception is Atlanta).  Once site information has been entered, the
user is given the option of adding the site to the city file.


Satellite Selection
-------------------

Satellites are selected by either their five-digit NORAD designation (e.g.
16609 for Mir), or their name.  Numbers have an advantage over names in
that they are unambiguous.  But names are easier to remember.  Selecting
satellites by name is similar to searching for a city name in the city
file.  To find a particular satellite, SKYMAP does not need the whole
name; just enough of the name to distinguish it from other satellites.
As an example, entering "Mir" for the satellite name is not sufficient to
distinguish the Mir Space Station from the Miranda satellites.  However,
entering "Mir s" clears up the ambiguity.

When SKYMAP finds a match, it displays the full name of the satellite it
has found, and gives you the option of accepting it, or continuing to
search for further matches.  If no acceptable matches are found, SKYMAP
will report "Name string not found.", and ask you to enter another name
or number.


SATNAMES.DAT file
-----------------

The file SATNAMES.DAT is used to connect satellite names with their NORAD
numbers.  Since new satellites are continually being launched, you may
want to occasionally add entries to this file.  As with the city file and
configuration file, this can be accomplished with any text editor.  Just
follow the format of the other entries in the file, and keep the NORAD
numbers in increasing order.  The first six characters of each line of
the file must be the 5-digit NORAD designator followed by a space.  The
next four characters are the satellite's standard visual magnitude, which
is described below.  The name you want to use to identify the satellite
begins in column 11.  Feel free to use any name you like, and to modify
any of the names already in the file.  (You can also delete file entries
for satellites that don't interest you).  You'll notice that for satellites
which don't yet have a name, I've used the NORAD number and letter scheme.


Standard Visual Magnitude
-------------------------

The standard visual magnitude appears in columns 7-10, and is meant to
represent the approximate visual magnitude of the satellite if it were at
a range of 1000 km and a phase angle of 90 degrees (half-illuminated).
My VIEWSAT program uses this value to predict the visual magnitude of a
satellite as a function of its actual range and phase angle, but SKYMAP
makes no such calculation at the present time.  It is included only for
compatablity reasons, but I've mentioned it here for the curious.  Most of
the standard magnitude values in SATNAMES.DAT come from the N2L-xxx type
element sets, courtesy of Ted Molczan.  In general, I've found these
values to be slightly pessimistic, and in a few cases I've made changes
to reflect observations that I've made.  Entries of "99.9" are used for
satellites which appear in the NASA bulletins, but not in Ted's N2L-type
files.  Ted's two-line element sets concentrate on satellites which CAN be
visible to the unaided eye, so those satellites with standard magnitude
entries of 99.9 are either too dim or too recently launched for a measure-
ment to have been made.


Propagation Model and Lighting Constraints
------------------------------------------

If SKYMAP successfully locates your satellite, it will next ask for the
propagation model to use.  SGP and SGP4 (the default) are supported, with
results differing slightly between the models.  SGP4 should theoretically
give more accurate predictions, so it is always the default model for
SKYMAP.  SGP's results are generally quite similar, with the greatest
variations occurring along track rather than cross-track.  If speed is a
concern, SGP does run somewhat faster than SGP4.

Once you've selected the propagation model, you'll be hit with a barrage
of lighting constraint questions.  The lighting constraints control what
portions of an orbital track are plotted.  For most users, the bracketed
default inputs will work fine for you.  The default constraints only
require that the satellite be in sunlight.  It is possible to set the
constraints such that tracks will only be plotted when the satellite is
in sunlight and the sun is below the horizon at your site.  You can also
place lunar lighting constraints on the satellite if desired.  Since the
questions asked by SKYMAP are fairly straight-forward, they won't be
covered in any more detail here.

One final note about lighting constraints.  If you plan to plot multiple
satellite tracks on the same star map, you'll be pleased to know that
you only have to input the date and lighting constraints once.  The same
date and constraints will be used for all subsequent satellite plots.


Time Information:  Start times, duration, resolution, and timetags
------------------------------------------------------------------

The user-input start time tells SKYMAP when to start the orbital propagation.
This will be the time of the beginning of the satellite track on the map,
assuming that the satellite happens to be within the map window.  I try to
pick a time around 10 minutes before the satellite will appear above my site
horizon, to ensure that none of the track is missing on my star map.  Times
can be UTC or local time, where local time is referenced to the zone hour of
your site.

The duration of orbit propagation tells SKYMAP how far into the future to
extend the orbital track.  For a single pass at a location, a half-hour to
an hour is sufficient.  If you propagate too far into the future, you may
end up plotting multiple passes unintentionally.

The resolution of the trajectory plot tells SKYMAP how frequently to calcu-
late the satellite's position.  The shorter the time resolution, the longer
it takes SKYMAP to run, but the smoother the track will look.  Tracks are
fairly linear over 10-second periods, but if you are plotting a very small
portion of the sky you will want even more frequent positional information.
When selecting the resolution, you should also be thinking about how fre-
quently you'll want the orbit time-tagged.  Timetags must occur at some
integer multiple of the trajectory resolution.  (Note that timetags CAN
occur at every trajectory point if desired).  As an example, if a satellite
is going to take 30 seconds to cross your map, I would select a resolution
of 1 second, and a timetag resolution of either 1 or 2 seconds.  Be careful
not to select so short a time between tags that they begin to overlap one
another.  With a little practice, you'll get a feel for these time-related
inputs.


Plot Title
----------

Once a trajectory is complete, SKYMAP gives you the option of adding
more satellite tracks to the same map.  When you have no more tracks to
add, enter No.  As with the star-only map, you can add a title below the
plot.  The default title is the name of the last satellite you plotted.
To select the default, hit return, or enter your own title.  Your last
option is to save changes to your configuration file (the default is no).
The program then ends, and your output file is ready to be plotted or
rasterized and printed.


Plotting Rocket Trajectories
----------------------------

This capability will be documented at a later time.  FIREBIRD.TRJ is a
sample of what a rocket trajectory file should look like.