|
Description:
|
The problem of observing a given set of Earth target sites within an assigned time
frame is examined . Attention is given mainly to visiting these sites as sub -satellite
nadir points . Solutions to this problem in the literature require thrusters to continuously
maneuver the satellite from one site to another . A natural solution is proposed .
A natural solution is a gravitational orbit that enables the spacecraft to satisfy the
mission requirements without maneuvering . Optimization of a penalty function is
performed to find natural solutions for satellite orbit configurations . This penalty
function depends on the mission objectives . Two mission objectives are considered :
maximum observation time and maximum resolution . The penalty function poses
multi minima and a genetic algorithm technique is used to solve this problem . In
the case that there is no one orbit satisfying the mission requirements , a multi -orbit
solution is proposed . In a multi -orbit solution , the set of target sites is split into
two groups . Then the developed algorithm is used to search for a natural solution
for each group . The satellite has to be maneuvered between the two solution orbits .
Genetic algorithms are used to find the optimal orbit transfer between the two orbits
using impulsive thrusters . A new formulation for solving the orbit maneuver problem
using genetic algorithms is developed . The developed formulation searches for a mini mum fuel consumption maneuver and guarantees that the satellite will be transferred
exactly to the final orbit even if the solution is non -optimal . The results obtained
demonstrate the feasibility of finding natural solutions for many case studies .
The problem of the design of suitable satellite constellation for Earth observing
applications is addressed . Two cases are considered . The first is the remote sensing
missions for a particular region with high frequency and small swath width . The second
is the interferometry radar Earth observation missions . In satellite constellations
orbit's design , a new set of compatible orbits , called the "Two -way orbits" ,whose
ground track path is a closed -loop trajectory that intersects itself , in some points ,
with tangent intersections is introduced . Conditions are derived on the orbital elements
such that these Two -way Orbits exist and satellites flying in these orbits pass
the tangent intersection points at the same time . Finally , the recently proposed concept
of observing a space object from onboard a spacecraft using a star tracker is
considered . The measurements of the star tracker provide directions to the target in
space and do not provide range measurements . Estimation for the orbit of the target
space object using the measurements of the star tracker is developed . An observability
analysis is performed to derive conditions on the observability of the system states .
The Gaussian Least Squares Differential Correction Technique is implemented . The
results obtained demonstrate the feasibility of using the measurements of the star
tracker to get a good estimate for the target orbit within a period of measurements
ranging from about 20 percent to 50 percent of the orbital period depending on the
two orbits . |