BUGS recognized in SaVi
=======================
$Id: BUGS 148 2020-01-18 02:50:25Z lloydwood $

This list of BUGS describes caveats and problems with
use of the SaVi satellite visualization software.

This document contains the following sections:

1. Caveats in use of SaVi. Be aware of these.

2. Platform-specific issues.
   a. Mac OS X
       i. Mac compilation hints
          -- for SaVi
          -- for Geomview
       ii. 10.5 (Leopard)-specific issues
          -- for SaVi
          -- for Geomview
       iii. Other minor Mac-specific issues
          -- for SaVi
          -- for Geomview
   b. Cygwin on Microsoft Windows
          -- for SaVi
          -- for Geomview

3. A detailed SaVi wishlist. Notes on issues in these SaVi releases.
   a. Coding and build issues
   b. Scalability
   c. Scriptability
   d. User interface
   e. Coverage view improvements
   f. Fisheye view improvements
   g. Geomview interaction
   h. Dynamic texturemapping with Geomview
   i. Various
   j. Interaction with other 3D renderers
   k. Interaction with the network simulator ns
   l. Desirable constellations to simulate

4. Remaining integration work from unreleased SaVi 1.1.

5. Remaining items from the original SaVi 1.0 wishlist.



1. CAVEATS IN USE OF SAVI
=========================

Changing default simulation parameters (central body radius,
gravity, etc.) can cause unexpected behaviour; scripts generally
describe satellites in circular orbits via an altitude above the
body's surface. Use at own risk. If simulating a different planet
to Earth, we should force the -orbit-model J0 flag to avoid
simulating Earth's oblateness and environment. J0/J2 should be
selectable in the params dialog. Any changes should reset time
to zero - with a warning.

Two-line-element (TLE or 'elset') reading and constellation code
in tcl/constellations.tcl still needs considerable work. Reading
.tle files is experimental. This is not guaranteed for multiple
satellites of different epochs, so you can't really just expect to
download a .tle file, load it in and go.
Improving this and introducing clock time would make SaVi far more
practical.
(Sidereal time and the Earth's motion around the sun is not
simulated; noted by Tom Tessier.)

Selection: selected satellite is not visible as selected in Geomview
until a marker type is first selected or satellite orbital elements
are edited. Operations on multiple satellites (e.g. Cut) only work
on the last satellite selected.

Sunlight is approximated by having the sun orbit the Earth as a
distant satellite with the appropriate year-long period. This
should not be done as a J2 calculation, but should at least be J0.
Geomview's lighting rendering needs to be fixed to match this
coverage footprint.



2. PLATFORM-SPECIFIC ISSUES
===========================

a. MAC OS X

i. Mac compilation hints

-- for SaVi

SaVi and Geomview need Xcode (included in Developer Tools with the
Mac OS X system installer copy, or downloadable from Apple) to provide
the tools that can compile these and other programs.

As of Mac OS X 10.7, Xcode is now an app available from the Mac App
Store. As well as downloading and installing the Xcode app, it may be
necessary to explicitly enable command-line tools so that e.g. gcc
is available. The command line tools must be selected in:
Xcode > Preferences > Downloads > Components > Command Line Tools

Simply typing 'make' in a Terminal window may generate a dialog to
install the app and necessary command-line tools. Alternatively,
the command-line tools can be installed by typing:
    xcode-select --install

Although recent releases of Mac OS X include Tcl/Tk headers, they do
not include the X11 headers that Tk needs to compile. One way to
get these headers is to install the XQuartz package from
http://www.xquartz.org/
The XQuartz copies of X11 headers are looked for during compilation.

The Tcl/Tk libraries shipped with Mac OS X releases have some
user-interface problems and are prone to crashing. It is preferable
to install a later ActiveTcl release from
    http://www.activestate.com/activetcl
and then choose to build with that instead; to do so, uncomment the
second, commented, TCL_LIBS = line in src/Makefile_defs_macosx
before compiling savi by typing
    make ARCH=macosx

Mac Tcl issues fixed in ActiveTcl include:
- Mac OS X 10.5 Leopard menu problems
- accelerator key crashes
- coverage panel angle slider crashing Tcl due to a busy event loop
  when texturemapping, cone and footprint drawing are on.

-- for Geomview

On Mac OS X 10.8 or later, you will need to install XQuartz from
http://www.xquartz.org/ to provide an X11 environment.

To build Geomview on Mac OS X, you must first install a Motif clone
such as Lesstif (available via www.geomview.org's Downloads section).
In Lesstif's directory you will need to
    configure
    make
    sudo make install
and okay the sudo with a password to place Motif files where Geomview
expects to find them.

Geomview works well with OpenGL, but the OpenGL header files needed
are not included in Xcode by default. They were an extra optional
package provided with the Mac OS X system installer. Building Geomview
with
    ./configure --without-opengl
    make
is a fast way to get started, but the OpenGL rendering is recommended
for coverage texturemapping. To build Geomview with OpenGL once
these optional header files are installed:
    ./configure --with-opengl=/usr/X11R6
    make

and then optionally to make Geomview available from the command line
everywhere:
    sudo make install


ii. 10.5 (Leopard)-specific issues

-- for SaVi

The Tk 8.4.7 supplied with Mac OS X 10.5 (Leopard) has problems
displaying SaVi's menubar.

To work around this, menu buttons can be put in each SaVi window with:
    savi -redrawn-menus
This is not required on Mac OS X 10.6 (Snow Leopard) or 10.4 (Tiger),
where the Mac menubar can be used correctly.

The ./savi launch script attempts to detect 10.5, and adds the
-redrawn-menus flag to make SaVi usable for Leopard users.
This avoids having to add the parameter at the command line each
time SaVi is launched.

Installing ActiveTcl, uncommenting the TCL_LIBS line before compiling,
as described above, and removing the -redrawn-menus flag from the
./savi launch script is the preferred way to work around the user
interface problems of the Tk supplied with 10.5.

-- for Geomview

If doing make of Geomview fails to link at end with a
'cycle in dylib re-exports with /usr/X11/lib/libGL.dylib'
error, use the following single-line, no-breaks command:
export LDFLAGS="-Wl,-dylib_file,/System/Library/Frameworks/OpenGL.framework/Versions/A/Libraries/libGL.dylib:,/System/Library/Frameworks/OpenGL.framework/Versions/A/Libraries/libGL.dylib"
then repeat the previous instructions. This problem is caused by a
linking bug in Leopard.


iii. Other minor Mac-specific issues

-- for SaVi

- Menu shortcuts are disabled on the Mac unless -redrawn-menus is
  used. This is to prevent a freeze on using a keypress to open
  a window (seen in 10.6 with Tk 8.5.7). 10.7 is unaffected,
  and shortcuts can be enabled for ActiveTcl by editing disable_mac
  in tcl/utils.tcl.
- On compilation, two binaries are produced: bin/SaVi-macosx.bin and
  bin/SaVi. You can rm bin/SaVi-macosx.bin to get just 'SaVi' in the
  menubar as the application name.
- Under the Tcl/Tk supplied with 10.5, coverage colour buttons are
  only fully coloured when another application is frontmost.
- Under the Tcl/Tk supplied with 10.6, menu highlight colours are
  incorrect; highlighted text remains black.

-- for Geomview

- With X11 rendering, Geomview does not appear to be able to control
  its background colour, which is always grey. OpenGL rendering is
  fine.


b. CYGWIN ON MICROSOFT WINDOWS

SaVi and Geomview can be installed as packages by the Cygwin installer.
32-bit Cygwin and the newer 64-bit Cygwin can coexist on a Windows PC.
32-bit Cygwin has often had better Geomview interaction in the past,
and so was preferable for SaVi.

If both 32-bit and 64-bit Cygwin are installed:
- Depending on which XWin the Start menu Cygwin-X folder launches,
  the other can be launched when that is not running by launching
  the appropriate terminal, then typing
    startx -- -multiwindow -clipboard &

- The Task Manager will indicate whether 32-bit or 64-bit Xwin.exe
is running.

- uname -a in a terminal indicates whether that terminal is
32-bit or 64-bit; x86_64 is 64-bit.

-- for SaVi

When expanding the coverage window to show a tiled coverage map, there
can be two lines of pixels, one to the left of the map, one to the right,
where coverage is not drawn on the background. Satellites and maps
are drawn correctly. Oddly, this only appears under the 32-bit
Cygwin, not in the newer 64-bit Cygwin.
This has not been seen on any platform other than 32-bit Cygwin.

Cygwin support isn't seamless, and may need some editing of
tcl/Makefile to support the version of Tcl Cygwin uses.
This caveat only appears to apply to very old versions of Cygwin;
not Cygwin 1.5.6-1 and later.

Under cygwin, specifying -mno-cygwin to gcc in src/Makefile in
order to build using MinGW generates errors. We're a long way from a
standalone Windows executable.
  
-- for Geomview

SaVi and Geomview can be installed as packages by the Cygwin installer.
The SaVi/Geomview combination took a while to become functional under
the new 64-bit Cygwin introduced in March 2013, with OpenGL and
piping problems.

Under 64-bit Cygwin, you can try building Geomview with:
MOTIFLIBS='-lDtPrint -lMrm -lXm -lXt -lXmu -lXp -lXext -lX11 -lSM -lICE'
./configure --with-motif=/usr --with-opengl=/usr CPPFLAGS="-DglBindTextureEXT=glBindTexture -DglDeleteTexturesEXT=glDeleteTextures"
make



3. A DETAILED SAVI WISHLIST
===========================

This is an additional wishlist of improvements for SaVi
originally accumulated during work for the SaVi 1.2 release during
October-November 2001, and kept up-to-date with later development.

Development takes place at:
http://savi.sourceforge.net/develop/

Many suggestions (such as the user interface and fisheye
modifications) should be straightforward if you have some Tcl/Tk/C
experience and the spare time and interest. Others are difficult.



a. CODING AND BUILD ISSUES

- make install is needed. That should probably match the current debian
  package. Early creation of it may have been able to prevent the previous
  package problems caused by maintainers unfamiliar with the savi tree,
  described below.

- Binary packages are not created by the SaVi authors. You can always
  download SaVi from http://savi.sf.net/ and compile it to get
  all the files created for SaVi.

- The savi-1.4.3 package available from getdeb.net from December 2009 onward
  (and available in Ubuntu's Synaptic package manager) initially removed the
  textfiles and manual from SaVi. This package installed SaVi under
  /usr/share/savi, while the COPYRIGHT file was edited, renamed 'copyright'
  and placed under /usr/share/doc/savi. This broke SaVi's help
  system by removing all help information. Packaging was fixed in later
  revisions. These packages were not initially compiled using zlib, although
  and making zlib a package dependency is straightforward. More details
  are in the PACKAGING textfile.

- $HOST or $HOSTNAME? One or both have the machine info we need
  for the compile version info preserved in Help/About...

- gv_utils.c warning: implicit declaration of functions fileno and fdopen
  because they're POSIX, not ANSI C, even though they're normally in stdio.h;
  could set -std=gnu99 flag to relax restriction if this mattered.

- The executable bin/SaVi (named for viewing the process name, e.g. in
  the Mac OS X menubar) is considered the same name as the savi script
  on case-insensitive file systems (Mac OS X, Windows). This may lead
  to confusion, particularly if src/Makefile is ever made to build in
  the savi directory.

- Improve open/close filehandling and catch errors.

- Under error catching, if SaVi is launched from a terminal window
  and backgrounded with the & metacharacter, and that terminal
  window is then closed, writing status messages to stderr, e.g.
  when loading a new constellation, fails due to a broken pipe.
  The resulting Tk error dialog stalls SaVi until it is closed.
  Option to remove ALL output sent to stdout? -quiet? Would need
  printf varargs wrapper. Could log to file or to a Tk log window
  instead - but little point sans timestamps?

- We should really be using snprintf instead of sprintf.

- We should examine jerkiness when used with Geomview, particularly
  under Cygwin. utils.c::millisleep() calls select() without reference
  to the state of the pipe to Geomview; how can this be made smarter?
  Should we sleep after gv_end()/gv_stop() ceases nesting?

- Command line handling to note what options are set should note more
  for hint in About dialog. For debugging purposes, could allow override
  of existing global variables by writing support for
    -tclvar <variable1> <value> -tclvar <variable2> <value> 
  by setting them near the end of init.tcl init code. Check existence
  of value set and warn if not already present.

- Menu shortcuts don't work while any menu is dropped down. This looks
  like a Tcl failing crossplatform.

- -fisheye and -coverage flags to spawn those windows at startup?
  How do we set size? -large-map/-snmall-map and -levels handles that...
  -camera <n> to spawn n geomview cameras at startup? 2nd/3rd could
  be north/south pole views. How to position windows on screen?

- Rewrite README (for building SaVi) and README-TEXTUREMAPPING into
  README (for building), SAVI-USE, and GEOMVIEW-USE for better online
  help.

- Filehandling: why does the SaVi package installed as a deb say 'File
  not found' when a file parameter is passed?



b. SCALABILITY

- Scaling up.
  Can SaVi detect available CPU power and e.g. scale number of
  segments in drawn orbit circles/number of sides to a coverage cone
  appropriately for Geomview? A slider or command switch giving some
  control over this would be very useful. Scaling up the number of
  segments when drawing highly-elliptical orbits has been done, but
  ORBIT_SEGMENTS defined in include/constants.h needs to be passed
  to Tcl as a variable.
  More use of Geomview binary format (discussed below) may help.

- Scaling down.
  Geomview rendering is optional; you can conserve CPU by running
  SaVi standalone. SaVi 1.2 and later don't do Geomview calculations
  when run standalone, although 1.0 did.
  Why shouldn't Tk and a graphical display also be optional?
  You do need a Tcl parser - that's essential - but we
  can execute scripts to build satellites and get results; we can
  make use of command-line switches, and we can pipe graphics to
  stdout for a variety of uses, including building web cgis
  (a la Chris Meenan's footprint generator).



c. SCRIPTABILITY

As well as scripts that get results there needs to be a
scriptable interface to record tracefiles. Scriptable recording of
satellite-related values over time periods would be very useful;
the self-scheduling Tcl procedure model that is used in the
network simulator ns-2 (see below) may be worth emulating.

Having Tcl/C interaction daisy-chained as keywords off the 'satellites'
keyword is awkward, particularly for Geomview interaction, where a
'geomview' keyword would seem more intuitive. Perhaps a 'C' keyword?



d. USER INTERFACE

A user manual has been written and placed in the manual/ directory,
but more detail is needed. The manual is not yet directly
accessible from SaVi; could SaVi find and spawn a web browser?
Building a web browser into SaVi seems a bit much.

The Tk procedures in tcl/tcl_utils.tcl (inherited from dsTool)
are straightforward to use, but are not that efficient in
using screen real estate. This makes SaVi cluttered, even on a
1024x768 display. SaVi 1.2 has improved things, but this still
needs more work.

* Main panel:
  A realtime push-hold button next to Time Interval would be useful;
  removes Rendering menu (->Geomview menu) when Geomview is not
  present.  Move current time into reworked playbar line instead?
  Cutting to remove a satellite jumps to end of (long) list, unless
  start of list is shown.
  Bindings are needed for cursor up/down so that satellite selection
  moves.

* Coverage panel:
  Some wasted space here. When -large-map is selected the playbar
  at end is drawn below the bottom of a 1024x768 screen, which
  is a problem if you can't move windows from available edges -
  which is why the buttons are also listed as commands in a menu.
  Reaching the coverage angle options also becomes tricky.
  Clicking and dragging the map sideways to shift the view would
  be cool - esa's Space Trajectory Analysis v3.0 does this.
  Bindings are needed for cursor left/right (forwards/back) and
  shift-cursor left-right (single step forwards/back)

* Edit satellite:
  Dialog layout can be improved, with better alignment of captions,
  text boxes and sliders.
  Apply button was removed since values do update live - but does
  that also hold for older versions of Tcl?
  There's no easy way to select multiple satellites so that
  you can change a checkbox setting for all of them.
  Panel can be left open when inappropriate, e.g. when a new
  constellation is loaded in. Should switch to editing a new
  satellite.

* Rosette and crude star generators:
  Dialog layout can be improved, with better alignment of captions,
  text boxes and sliders.
  Slider positions do not update to match text value changes.


Create overriding 'Show all {satellites | orbits | coverage }'
checkboxes that, when checked, force related checkbox (in satellite
Edit parameters) on and the feature visible for all satellites.
When checked off, disable the feature for all satellites. These would
be entries on the Rendering menu, under 'Real-time mode'.

Provide menu of colour choices for pane backgrounds, highlights etc.

It's possible to type into the help menu text windows. That should be
prevented. But that is not simple in Tk: http://wiki.tcl.tk/1152

Ballard/crude star generators need to create info text in "About
constellation" window.  Look at how the initial LEO satellite text
is created. Need to create info text separately from function that
we may in future call from scripts that we collapsse down to Ballard?



e. COVERAGE VIEW IMPROVEMENTS

Now that we have satellite names, an option to show them in the
coverage view would be useful. The satellite plot scripts do this
for print via xfig.

Could blend coverage maps somehow with the detailed map, but that
means uncompressing it and blending on a pixel-by-pixel basis,
or setting alpha transparency in Geomview for a coverage sphere
over the earth sphere.

Decay/sunlight options are really mutually exclusive, and could
be done via a popup menu:
No coverage (all-white background)
Coverage (white background)
Coverage and decay (default)
Coverage and sunlight (two shades of decay)
Coverage over land and sea (blue background)
but we need masking for all supported projections first.

Drawing the Earth map last in each rastering is nice for sending
to Geomview earlier, where the vector Earth can be overlaid instead -
but means that green satellite cross markers are behind and occluded
by continent/country outlines. Ideally we have subsatellite nadir
markers drawn by Geomview, with bitmap crosses drawn after the
bitmap Earth maps, and not sent to Geomview either. Could fix coverage
satellites drawn behind map bitmap outline problem. In map-drawing
section, test value in lookup table, i.e. that it's not green,
before plotting. Or could move loop of detect-and-draw-green-crosses
after map drawing loop, at cost of not drawing them in Geomview.

If "use sunlight" is on, why is change shading only restricted to
land/satellite coverage only? For land/sea both should change;
debatable for others. But labels need to update to show 1/2 sats etc.

Could implement 0 satellites in view/0 intervals as two white boxes,
not given in color swaths, but colors set individually by clicking
buttons. Possibly better as menu option spawning color box? Set
uncovered map color? Should intervals off default to 4+ colors, not 1?

Add greyscale option for colors? diversity is white? Unless land/sea?

As well as allowing coverage angles to be set per satellite, having
multiple fooprints per satellite at set elevation angles (5 degrees,
10 degrees) would be very useful. Either approach could be used
to build traditional geostationary iso-coverage maps, marked out
with diversity colours.

What is the diversity/decay value of the pixel that we have clicked?
Report it next to fisheye lat/long coords. Unlike coords, this
would need to be updated in real time. If fisheye is open, then we
likely already know it from within the red circle, and can just
repeat it in coverage? Useful reality check by comparing the two.

Should opening coverage panel turn on vector Geomview map overlay?

Saving maps now supports .gif, as that's integral to Tcl. Support
for handling .png requires a non-buggy version of Tcl 8.6 or
later, and produces larger non-indexed files, so it's not
a menu option - but if .png is given in the filename, an
attempt to save as .png will be made. May need to revisit
filetypes here at some much later point.

There are alternatives to plotting full circular footprints:
- just plotting only edges of footprints - hollow circles.
- plotting imaging swath widths, i.e. a line perpendicular to
  the orbit inclination, width diameter footprint.
- plotting complex spotbeams, again aligned with respect to
  to the orbit inclination. (Chris Meenan's footprint generator
  demonstrated hexagonal arrays.)
This would involve making changes to e.g.
src/stats_utils.c::fill_grid() and intensity_circle_footprint()

Look at masking SaVi logo onto coverage maps - this makes most sense
in corners of spherical and sinusoidal maps, but area used would
need to be discounted from coverage calculations.

Update all Earth maps to track more recent political boundaries.
A couple of pixels of west Mexican coastline are missing from the
1024 unprojected map.

Consider producing better outline and landmass mask maps across
all projections, and even new projections, by generating them
using the d3.js map drawing functionality. Unfortunately, this
means first understanding d3.js.

The coverage slider position should be updated when a script
loaded from the menu changes the coverage angle.

Threshold altitudes for enabling coverage footprints and cones
are useful for elliptical orbits where satellites only
transmit at apogee, but is currently only a command-line option.
This may be more useful for a mix of orbits if specified as an
angle around apogee, rather than as an altitude.

Similar thresholds could be implemented for latitude and longitude,
based on satellite position, and implemented as command-line options
to start with, or a Coverage constraints coverage panel menu option.
-coverage-latitude-max, -coverage-latitude-min,
-coverage-longitude-max, -coverage-longitude-min, with
-coverage-altitude-max, -coverage-altitude-min renaming the
transmit-altitude-max/-min already implemented to fit the set.

Longitude calculation for spherical fisheye clicks are inaccurate.
Edge calculation methods have visible differences (as does
sinusoidal). Area calculation hasn't been attempted; calculations
of areas in general for all projections are poor estimates anyway.
Using sinusoidal coverage map, full coverage reads from 99.6% to 99.8%
due to rounding at curved edges of map. Coverage calculations for all
projections are estimates at best, reflect the map projection in use
by changing with latitude, and are limited to not exceed
the 100% threshold. Calculations should really reflect degree of
coverage overlap and be consistent across all views. Working
off stats_utils.c::footprint_circle_radius() when that is computed,
not off a distorted map, can give total coverage for each small
circle that coverage generates. These can be summed to give
overlapping coverage.

Calculating amount of coverage overlap for 1/2/3 etc. satellites
is possible by counting pixels, but still depends on choice of
projection and latitude of satellite. For handling overlap, see
Kantsiper and Weiss, An Analytic Approach to Calculating Earth
Coverage, AAS 97-620, while they were at MIT, for an analytical
integration approach using the inclusion/exclusion principle.
Also Brian Kantsiper, A Systematic Approach to Station-Keeping
of Constellations of Satellites, PhD dissertation, MIT, Feb 1998.
See also A. Gonzalez, Measurement of areas on a sphere using
Fibonacci and latitude-longitude lattices, Mathematical
Geosciences, January 2010, pp. 42-49.
http://arxiv.org/abs/0912.4540

The correct way to count coverage overlap is debatable. Counting
pixel intensities and saying X% of the earth has single satellite
coverage, Y% of the earth has double coverage... could be useful -
and these percentages could then be overlaid on the colour key wells.
But counting pixels from a map still introduces some distortion.  

When choosing new colour shades after animating forwards then
stopping, the satellites move on an interval unexpectedly as
the map is updated with new colours. Is this due to computed
coverage finally being shown by tk_update()?

Project groundtracks for the next n hours only projects forwards in
time, even if the simulator is currently being run backwards - is
this the least confusing approach? Enter negative number to project
backwards.

When altering the coverage map and tiling, remember to run the
Teledesic (840) simulation to check that the top and bottom
of the map are visible as a single line of uncovered (blue or
white) pixels in cylindrical view. Tk does require a unit offset.

Add option to fade groundtracks to background over time, just
as interval decay works for coverage areas. When recording
groundtracks, erase the cross arms and leave only the
subsatellite point.

Coverage areas of the selected satellite(s) can be shown in
different shades - possibly shades of green.

Add optional latitude/longitude lines. Could add equator line
to match showing equator in fisheye view.

Earth outline on sinusoidal map is drawn using different algorithm
to backdrop. Overlap is poor; both should be drawn on the
cached map, rather than on each coverage map drawn. This matters
less when the backdrop is set to black to match the outline.

How much work would be required to enable multiple coverage windows
to be spawned, so that multiple projections could be seen, and
any available cylindrical or unprojected projection is used for
texturemapping if desired? Probably a lot.

Opening the coverage window when animating means that the coverage
playbar is not immediately updated with playbar status, and
playbars only sync when the next playbar button is pressed.

Menu options invoking popup windows to control project groundtrack,
coverage angle slider, and minimum transmit altitude slide would
be useful on small screens where these controls are offscreen
at the bottom. Minimum transmit altitude is of minority interest,
does not require slider to always be onscreen.



f. FISHEYE VIEW IMPROVEMENTS

Now that the fisheye window is resizable, having controls partially
obscured at small sizes is messy. Can this be improved? (Not resizing
the window when recording tracks would be a deliberate choice, given
the complexity of the alternative.)

New cross location in coverage map is not removed immediately when
checkbox is turned off.

Add slider to select size of fisheye crosshairs; will need to adjust
text positioning slightly. Can we add this on buttonbar?

When equatorial exclusion zone is shown, satellites are not indicated
as within or outside it. This needs to be figured out. Equator
altitude should be figured out from first principles in case
simulation constants get changed and we're not simulating Earth.

We support minimum elevation of the terminal, i.e. whether the
terminal can see each satellite, but do not consider half-angle of
the satellite, i.e. whether each satellite can see the terminal.
Considering both would be ideal, and useful as a prelude to
connectivity/link work.

Could create fisheye orbit markers to show trajectory of currently
selected satellite, if any. Would need to be a separate satellite
marker list, since parallels table wraps around. Do we space markers
in RAAN or time? Would need to cut down on satellite info to just
the minimum and create a special marker group, then simplify equator
code to use it? Would this even make sense with HEO orbits, where
markers won't be spaced evenly? Could we create markers for each
satellite as lists hanging off the Satellite in the satellites
list? Hard to keep updated.

How much work would be required to enable multiple fisheye windows
to be spawned? Probably a lot, if pass times are to be recorded in the
SL list structures on a per-view basis.

A fisheye view that wandered (randomly?) over time would be nice
to have, as it could show the equator and latitude lines moving
smoothly. This would probably require a simulation event scheduler
to be implemented so that commands could be executed on repeat
intervals.

The -debug flag outputs information about satellite positions, but it's
verbose, showing angle to all satellites, even those below the horizon,
and lots of debugging information for visible satellites. A decent log
format for visible satellites and their angles above the horizon is
needed.

Saving bitmaps would be nice to have as an option. Postscript can
always be converted to a bitmap via GhostScript:
gs -dNOPAUSE -sDEVICE=png256 -r300x300 -sOutputFile=out.png -q -dBATCH out.ps
but Ghostscript is a tad unwieldy, and since you'll end up cropping the
results anyway... it's simpler to just take a screen capture.



g. GEOMVIEW INTERACTION

Geomview coverage cones and footprints are dynamically updated when
the coverage view angle slider is moved. For cones, this is computationally
laggy for large constellations. Speed it up by forcing a delay and
scheduling a later update after a timer expires? We're already
collapsing it in BEGIN/END. Should gv_end force a texturemap update
for after e.g. loading a constellation?

Should footprints be turned on if dynamic texturemapping and switching
to a projection that cannot be texturemapped?

Fisheye location(s) should be shown in Geomview - another colored line?

Now that we have satellite names, an option to show them in Geomview
would be useful. This would be even harder than showing in the coverage
view - look at how The Orrery, another Geomview module, renders fonts
and text.

Could make markers distinguish satellites - a satellite struct field
holding a number mapping to dot/sphere/box/sat or colour (since
shades of black aren't useful), and text column saying marker type
if geomview is running. Selecting marker could ask for all, or
operate on highlighted selection of satellites. This looks hard.

Look at altering sleep code to give longer sleeps when Geomview
is running and animating, so that more CPU time is devoted to
Geomview and overall performance is made more responsive.

Being able to control and program camera position, to e.g. show
exactly what a satellite sees at nadir as the satellite moves,
would be very useful to have.

Geomview's light source positioning appears to be buggy at present,
as shown by savi -geomview -sun-lighting. Could implement a
terminator plane at right angles to the sun pointer, reusing
equatorial plane code.

Quitting SaVi as a Geomview module can leave the blue Earth sphere
behind. This was identified as a bug in Geomview exposed by chosen
-O3 optimisation when building, which vanishes when -O2 is chosen -
see the January 2012 geomview-users archives.

Running under Geomview with -fake-geomview flag on as well
leads to weird behaviour, so don't do that. One or the other!

Should be able to scale one or all satellites in size. Being able
to scale one spherical satellite to large could be useful for
simulating the Moon. But would need to match position to J2 model.

Rotation of satellite markers as they approach the poles is inaccurate;
detailed satellite solar panels should be at right angles to the
ecliptic and rotate to follow the sun (if there's a sun...), rather
than pointing east-west. (Face on to the sun at right angles to the
ecliptic is at 67.5 degrees to the equator, which is 22.5 degrees to
the ecliptic.)

That's reality; for visualization, having panels at right angles to the
planes increases visibility - when the planes are drawn.
Could use inclination of orbit.

Make this unwieldy SaVi/Geomview combination run in root window under
Xscreensaver, in long tradition of overdone OpenGL hacks. To do that,
more command switches must be passed to SaVi through Geomview.

Can central blue sphere rendering be rotated so that lat/long
lines map to barely-visible rendering of sphere segments?
Can we draw latitude/longitude lines that rotate with the
central body as an option? This would be like Geomview's
'Draw Unit Sphere Projection' Camera menu option, but rotating
with the Earth.

Replace equatorial plane with more detailed sectored concentric
circles, or with a translucent circle sized to the current
stationary orbit as set in parameters.

Translucent surfaces (e.g. yellow selection cubes) may not
be that visible in some renderers.

Add a 'Sky view' rendering option that combines the following:
  - switches Geomview to spherical geometry, so we're looking
    out from the centre of the Earth;
  - turns off central body, so that we look through outline
    continents.
  - turns on footprints, turns off spotbeams.

Can Geomview postscript output be improved and made useful by
tweaking SaVi's oogl? This probably requires work on Geomview

On Mac OS X, Geomview does not support the Mac menubar, but
uses its own Motif menus in its main window. This means that
switching between SaVi and Geomview is a little disruptive
to the user experience. This cannot be fixed only in SaVi.

Check deleting central body and turning off simple earth.
only works in one order, should work in both.

Loading in constellation updates texturemapped coverage slowly.
Because we need to change earth types?


Geomview 1.8.1 points:

These are unlikely to be dealt with, as 1.8.1 no longer
builds with modern compilers.

With Geomview 1.8.1, the large list structures used in writing
footprints and cones cause complaints about not being able to 'seek
back far enough in pipe.' This is related to how piping is handled,
and the piping code rewrite by Claus-Justus Heine in Geomview 1.8.2
release candidates for the 1.9 releases appears to have fixed this.
An alternative to the oogl used in:
footprints.c::write_footprints_geom()
cones.c::write_cones_geom()
would be good to have for remaining Geomview 1.8.1 users.
Footprints and cones are off by default to avoid this problem.

Geomview binary format is only used in one place at present (the
default satellite marker if the architecture permits it), but
offers a scaling advantage. Note that e.g. Geomview 1.8.0 (Linux,
some glibc) has problems with handling binary format; selecting
a more complicated marker type than the default green dot avoids
use of binary format entirely, but an explicit option would be
better.



h. DYNAMIC TEXTUREMAPPING WITH GEOMVIEW

Dynamic texturemapping is described in the file
README-COVERAGE-TEXTUREMAP
which can be found in the same directory as this BUGS file.

Dynamic satellite coverage passing over the poles shows an interesting
'circle' effect where coverage diversity appears to be evaluated once
for the entire around-the-pole circular region. This may be dependent
on Geomview rendering options.

Coverage texturemap is not immediately updated when turning 'project
groundtracks forward in time' on or off, so it is out of sync with
coverage window until the next animation step.



i. VARIOUS

Constellation generation tools should be working at a higher level
of abstraction: generating constellation to match input properties
for behaviour and characteristics, rather than just taking orbital
parameters as input without calculating what is needed and then
generating those parameters to give a desired outcome.

Implement per-satellite coverage angles, as a property of each
satellite when it is created, like the name. But global coverage
properties for constellations are useful, and the global slider will
be hard to give up - how do we do both? Do we need to store
half-angle/mask setting on a per-satellite basis?

Improve time display, so that groundtrack drawing has useful
'elapsed time', and the time that the fisheye and coverage
displays are stopped can be easily read from their respective
windows. Average coverage display needs to count number of intervals
sampled, since time between intervals can be changed.

Implement a -start <+/- seconds> switch, so that a simulation can
be loaded at a specific point. Also -coverage, -fisheye, -edit to open
up the various windows on startup; can these be scripted?

Allow switching between a set of default values for various planets
(e.g. Venus, Earth, Mars) with matching surface maps, and even a
default Constellations menu for each planet. This wuold involve
changing the body gravity/spin defaults and possibly adding a
finetuned special-case per-planet orbit model to handle the
equivalent of Earth's J2 precession. A different default colour
scheme to indicate 'not Earth' is probably also a good idea.
A number of scripts - Clarke geostationary, Draim, possibly
Molnya/Tundra - could compute from the defaults and generalise
to any planet; three-satellite Mars stationary would be the first
target. Other constellation scripts would be Earth-specific.

More accurate Earth gravitational models than J2, e.g. J20, could be
implemented via the new -orbit-model flag and the switch cases in
orbit_utils.c::oe_time_to_oe(). Related to that, the faked 'sun'
satellite should always be running off the simplest Keplerian J0,
to avoid amplifying Earth oddities.

Accurate gravitational models for each planet would be useful.
Maps for each planet can be obtained from
e.g. Stuart Levy's The Orrery for Geomview, or from
James Hastings-Trew at http://planetpixelemporium.com/
JHT's maps are unprojected, so require
STSPHERE CYLINDRICAL 1 0 0 0
rather than { : numesh_h } in oogl/earth.oogl, which means at least
Geomview 1.9.0 for support. They're large, even once run through
jpegtopnm and compressed, so would have to be optional.

A way of drawing coverage on a translucent sphere just over the
planet map (but just under e.g. the Earth boundary vectors) would
be useful, so that colourful maps could be combined with coverage
and diversity. But this needs alpha rendering support; easier to
do the maths and mapping upfront when drawing a single coverage
map.

TLE loading code is inelegant; a new popup function tying menu name
and url to load together would be desirable and easier to maintain.

Makefiles, header files and #include dependencies could be much better.
There's no attempt to optimise with judicious use of inlines,
but since we spend most of our time in the Tcl interpreter,
gains would be minimal and we can't inline functions in
other files anyway.

Adding -O2 in src/Makefile (and turning off -g debug) has produced a
smaller and faster executable.



j. INTERACTION WITH OTHER 3D RENDERERS

Are there other 3D renderers with piped input for which SaVi could
add support?

An alternative is to simply render the constellation in a web browser
using d3.js. This is shown to be possible by the animations in:
Ashlee Vance, The New Space Race: One Man's Mission To Build
A Galactic Internet, Bloomberg News, 23 January 2015.
http://www.bloomberg.com/news/features/2015-01-22/the-new-space-race-one-man-s-mission-to-build-a-galactic-internet-i58i2dp6
so scripts in Javascript would replace Tcl, and d3 driving
the browser would replace Geomview.



k. INTERACTION WITH OTHER SIMULATORS

network simulator ns-2 - http://www.isi.edu/nsnam/

SaVi can generate output in a format suitable for Lloyd Wood's perl
ns-2 satplotscripts. This really requires working intersatellite-link
functionality to be useful; drawing satellite positions alone doesn't
give much over the coverage panel, other than satellite names,
unless you need a postscript map.

ns-2 could write the simplest SaVi format that describes satellites
(the simple Tcl scripts that SaVi saves files as...) to allow SaVi
to show where ns-2 satellites are. Over time the ns-2 and SaVi simulations
will diverge if SaVi is using the more complex J2 orbital model -
the simpler J0 model should be preferable in this case.

The default ns output format for Lloyd Wood's perl ns-2
satplotscripts describes long/lat position, but does not include
orbital information such as altitude, making it useless for SaVi
to simulate from files in that format as it stands. A new format
would be needed.

The later ns-3 does not yet include satellite simulation functionality.



l. DESIRABLE CONSTELLATIONS TO SIMULATE

Many newly-announced large satellite constellations can be simulated,
and there's a backlog of FCC filings with technical supplements to
find, read, and create outline simulations from.

Add a Chinese BeiDou-2 navigation constellation simulation script.
The complete perfect BeiDou-2 constellation has five geostationary,
three inclined geosynchronous, and 27 medium earth orbit satellites.



4. REMAINING INTEGRATION WORK FROM UNRELEASED SAVI 1.1
======================================================

This work requires detailed knowledge of relevant geometry,
satellite requirements, SaVi and Geomview in order to build on
Patrick Worfolk's efforts.

- Intersatellite link simulation in SaVi and Geomview - although
  this really needs a good scripting interface, and must also
  consider visibility, handoff, and configuration.



5. REMAINING ITEMS FROM THE ORIGINAL SAVI 1.0 WISHLIST
======================================================

- Ability to input satellite positions correctly in two-line-element
  format, allowing multiple satellites with different epoch
  times to be simulated correctly simultaneously.

- Incorporation of actual time into program so that the evolution
  of real satellites can be monitored.

- Full incorporation of NORAD SGP4/SDP4 propagation models for
  NORAD two-line element sets for added realism.


Lloyd Wood <lloydwood@users.sourceforge.net>
