The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008 
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Space Objects Simulator is one of the core sub-systems in V- 
RSS. It provides multiple analytical and numerical propagators 
(Two-body, J2, J4, SGP4) to compute satellite position data in a 
wide variety of coordinate types and systems. For the novice, 
Space Objects Simulator provides the Orbit Wizard to guide the 
user through quick creation of commonly-used orbit types such 
as geostationary, circular, critically-inclined, sun synchronous, 
molniya, and retrograde. For ground-base objects, this module 
also has the ability to calculate their position and velocity for 
analysis, visualization. 
The Space Environment Simulator provides the cabebility of 
numcal caculation for the basic space environment elements 
such as atmosphere, Magnetosphere, Ionosphere, and etc. 
Figure 1. Major Software Modules & Their Interrelationships 
The Space System Analysis Module is another core sub-system 
in V-RSS. It has the ability of access, chain and coverage 
analysis. V-RSS allows user to determine the times one object 
can see, another object. In addition, user can impose constraints 
on accesses between objects to define what constitutes a valid 
access. The Chain module allows users to extend the pair-wise 
analysis capabilities to include accesses to and from remote 
sensing satellite constellations, ground station networks, groups 
of targets and multiple sensors. The Coverage module allows 
user to analyze the global or regional coverage provided by a 
single remote sensing satellite or a constellation of satellites 
while considering all access constraints. Specific results are 
generated based on detailed access computations performed to 
grid points within an area of coverage. 
The last important module is the 3D Display Engine. This 
Display Engine brings 3-D viewing capability to the V-RSS 
environment. This engine takes advantage of today's readily 
available workstations graphics engines to provide state-of-the- 
art interactive graphics performance. It displays all near-earth 
space information from V-RSS using realistic and dynamic 3-D 
views of space, airborne and terrestrial assets, space 
environment, and orbit trajectories. 
The only database in V-RSS is the Satellite Orbit Database. 
This database uses the STK satellite database as its data source. 
STK satellite database is available on its web site 
(www.STK.com) and updated every week. The most extensive 
database, from which all of our other databases are derived, is 
an up-to-date catalogue of over 8,000 objects. This database is 
maintained by U.S. Space Command. It contains data on Two- 
Line Element sets (TLEs), Space Surveillance Catalog (SSC) 
numbers, common names for satellites, launch dates, launch 
times, apogee, perigee, activity state and more. 
4. OBJECTS OF V-RSS 
Objects are the building blocks of a V-RSS simulation 
scenario. Platforms in V-RSS include satellites, grounds 
stations, airplanes, ships as well as earth, moon and sun. 
Figure 2 contains the objects diagram for the V-RSS. 
We build the space system using object-oriented methodology. 
The basic space object types - Space base and Ground base 
objects, Sphere (celestial bodies) and Stars - are all derived 
from the Space-object, which is the root object for the system. 
Figure 2. Object Definition 
The sun, moon and earth objects are derived from the Sphere 
object. The Earth object is considered to be at the origin of the 
V-RSS and the magnitude of its position is always zero. The 
physical constants describing the Earth include mass, mean 
equatorial radius, flattening coefficient and rotational rate. The 
Sun and Moon objects are internally predefined and the user has 
limited control over their initial conditions. They are 
propagated using algorithms based on general perturbation 
theory as described in [Bayliss 1971]. The propagation modules 
are accurate enough to allow modeling of solar and lunar 
eclipses. 
The NORAD Space Object defines circular or elliptical orbits 
about the earth by using the NORAD SGP4 or SDP4 orbit 
propagators, while the keplerian space object also defines orbits 
by using analytical and numerical propagators (Two-body, J2, 
J4,etc.)