Astrodynamics. Brief History of Orbital Mechanics Basic Facts about Orbits Orbital Elements Perturbations Satellite Coverage

Transcription

1 Astrodynamics Brief History of Orbital Mechanics Basic Facts about Orbits Orbital Elements Perturbations Satellite Coverage References P&M Ch 3 SMAD Chs 6 & 7 G&F Ch 4 BMW Vallado

2 A Brief History of Orbital Mechanics Aristotle (384-3 BC) Ptolemy ( AD) Nicolaus Copernicus ( ) Tycho Brahe ( ) Johannes Kepler ( ) Galileo Galilei ( ) Sir Isaac Newton ( )

3 Kepler s Laws I. The orbit of each planet is an ellipse with the Sun at one focus. II. The line joining the planet to the Sun sweeps out equal areas in equal times. III. The square of the period of a planet is proportional to the cube of its mean distance to the sun.

4 Kepler s First Two Laws I. The orbit of each planet is an ellipse with the Sun at one focus. apoapsis periapsis II. The line joining the planet to the Sun sweeps out equal areas in equal times.

5 Kepler s Third Law III. The square of the period of a planet is is proportional to the cube of its mean distance to the sun. 3 a T = π µ Here T is the period, a is the semimajor axis of the ellipse, and m is the gravitational parameter (depends on mass of central body) µ µ sun = GM = GM = sun = km s 11 3 km s

6 Mean Motion The Mean Motion is defined as µ 3 a The period can be written in terms of the mean motion as The Mean Anomaly is defined as M n = T = π / where t p is the time of periapsis passage n = n( t t p )

7 Earth Satellite Orbit Periods Orbit Altitude (km) Period (min) LEO LEO MEO GPS GEO

8 Newton s Laws Kepler s Laws were based on observation data: curve fits Newton established the theory Universal Gravitational Law GMm F g = r r Second Law M r F = m& r m Universal Gravitational Constant 6.67 G = 10 m kg s

9 Elliptical Orbits Planets, comets, and asteroids orbit the Sun in ellipses Moons orbit the planets in ellipses Artificial satellites orbit the Earth in ellipses To understand orbits, you need to understand ellipses (and other conic sections) But first, let s study circular orbits: A circle is a special case of an ellipse

10 Circular Orbits Speed of satellite in circular orbit depends on radius µ v c = r a=r If an orbiting object at a particular radius has a speed < v c, then it is in an elliptical orbit with lower energy If an orbiting object at radius r has a speed > v c, then it is in a higher-energy orbit: elliptical, parabolic, or hyperbolic v

11 The Energy of an Orbit Orbital energy is the sum of the kinetic energy, mv /, and the potential energy, -µm/r Customarily, we use the specific mechanical energy, E (i.e., the energy per unit mass of satellite) µ µ E v = r a From this definition of energy, we can develop the following facts E<0 orbit is is elliptical or circular E=0 orbit is is parabolic E>0 orbit is is hyperbolic E =

12 Properties of Ellipses b a r ν vacant focus ae a-p focus p=a(1-e ) a a(1+e) a(1-e)

13 Facts About Elliptical Orbits Periapsis is the closest point of the orbit to the central body r p = a(1-e) Apoapsis is the farthest point of the orbit from the central body r a = a(1+e) Velocity at any point is v = (E+µ/r) 1/ Escape velocity at any point is v = (µ/r) 1/

14 Orbital Elements Equatorial plane ^ K ν ω ^ I Orbital plane Ω ^n ^ J i Orbit is defined by 6 orbital elements (oe s): semimajor axis, a; eccentricity, e; inclination, i; right ascension of ascending node, Ω; argument of periapsis, ω; and true anomaly, ν

15 Orbital Elements(continued) Semimajor axis a determines the size of the ellipse Eccentricity e determines the shape of the ellipse Two-body problem a, e, i, Ω, and ω are constant 6 th orbital element is the angular measure of satellite motion in the orbit angles are commonly used: True anomaly, ν Mean anomaly, M In reality, these elements are subject to various perturbations Earth oblateness (J ) atmospheric drag solar radiation pressure gravitational attraction of other bodies

16 Instantaneous Access Area λ R e H IAA= K K A A K A = πr (1 cosλ) e = Re cosλ = Re + H Example: Space shuttle = 6378 km R e IAA 8 km H = 300 km cosλ = λ = 17.4 o IAA = 11,476,68 km

17 Elevation Angle Elevation angle, ε, is measured up from horizon to target Minimum elevation angle is typically based on the performance of an antenna or sensor R λ 0 λ ε D η ρ IAA is determined by same formula, but the Earth central angle, λ, is determined from the geometry shown The angle η is called the nadir angle The angle ρ is called the apparent Earth radius

18 Geometry of Earth-Viewing R λ 0 λ ε D η ρ Given altitude H, we can state sin ρ = cos λ 0 = R / (R +H) ρ+ λ 0 = 90 For a target with known position vector, λ is easily computed cos λ = cos δ s cos δ tt cos L + sin δ s sin δ tt Then tan η = sin ρ sin λ / (1- sin ρ cos λ) And η + λ + ε = 90 and D = R sin λ / sin η

19 Algorithm for SSP, Ground Track Compute position vector in ECI Determine Greenwich Sidereal Time θ g at epoch, θ g0 g0 Latitude is is δ -1 s = sin -1 (r 3 /r) Longitude is is L -1 s = tan -1 (r /r 1 )- θ g0 g0 Propagate position vector in the usual way Propagate GST using θ g = θ g0 g0 +ω (t-t 0 ) where ω is is the angular velocity of the Earth Notes:

20 Ground Track This plot is for a satellite in a nearly circular orbit ISS (ZARYA) latitude longitude

21 Ground Track This plot is for a satellite in a highly elliptical orbit B latitude longitude

22 Error Sources

23 Error Budgets

24 Initial LEO orbit has radius r 1, velocity v c1 Desired GEO orbit has radius r, velocity v c Impulsive v is applied to get on geostationary transfer orbit (GTO) at perigee µ µ µ v1 = r r + r r 1 Coast to apogee and apply another impulsive v µ µ µ v = r r r + r 1 Hohmann Transfer 1 1 r v c v v r 1 LEO v c1 v 1 v 1 GTO GEO

25 10 Earth Oblateness Perturbations Earth is non-spherical, and to first approximation is an oblate spheroid The primary effects are on Ω and ω: Ω & = ω& = 3J nr a (1 e e 3J nr 4a (1 e e ) ) cosi (4 5sin i) m r M The Oblateness Coefficient 3 J = 1.086

26 Main Applications of J Effects Sun-synchronous orbits: The rate of change of Ω can be chosen so that the orbital plane maintains the same orientation with respect to the sun throughout the year Critical inclination orbits: The rate of change of ω can be made zero by selecting i 63.4