Description about Satellite Orbits

1. Orbit and Constellation Design
Dr. Andrew Ketsdever
MAE 5595
Lesson 6

2. Outline
• Orbit Design
– Orbit Selection
– Orbit Design Process
– ΔV Budget
– Launch
• Earth Coverage
• Constellation Design
– Basic Formation
– Stationkeeping
– Collision Avoidance

3. Orbit Design
• What orbit should the satellite be put in?
– Mission objectives
– Cost
– Available launch vehicles
– Operational requirements
• Orbit Design Process
– 11 Step Process
– Wide variety of mission types which will be
unique in the orbit selection process

4. Orbit Design Process
• Step 1: Establish Orbit Types
– Earth referenced orbits
• GEO, LEO
– Space referenced orbits
• Lagrange points, planetary
– Transfer orbit
• GTO, Interplanetary
– Parking orbit
• Temporary orbit for satellite operational checks,
EOL

5. Orbit Design Process

6. Orbit Design Process
• Step 2: Establish Orbit-Related Mission
Requirements
– Altitude
• Resolution  Lower altitude is better
• Swath width  Higher altitude is better
– Inclination
• Ground station coverage
– Other orbital elements
• J2 effects on RAAN and AoP
– Lifetime
– Survivability (ambient environment)
• Must be able to survive the entire orbital profile (e.g. transit
through Van Allen Radiation Belts)

7. Orbit Design Process

8. Orbit Design Process
• Step 3: Assess Specialized Orbits
– Typically set some orbital parameters (e.g.
semi-major axis, inclination)
• Geosynchronous, Geostationary
• Semi-synchronous
• Sun synchronous
• Molniya
• Lagrange points

9. GEO
• 24 hr period – co-rotation with Earth
• Solar/Lunar pertubations
– N/S drift
– Pointing requirements
• J2 perturbations
– E/W drift
– 10% of ΔV required for N/S
– Important due to neighboring slots
• Early 1990’s
– 50% of launches to GEO
• Overcrowding is a constant issue

10. Sun Synchronous Orbit
• J2 causes orbit to rotate in inertial space
– Rate is equal to average rate of Earth’s
rotation around the sun
– Position of Sun relative to orbital plane
remains relatively constant
– Sun Synchronous orbits can be achieved
around other central bodies
• Usually near 90º inclinations

11. Molniya Orbit
• Highly elliptical orbit with i=63.4º
(zero rate of perigee rotation)
• Can have a large (up to 99%)
fraction of the orbit period between
• Any orbital period can be obtained
 change apogee altitude
• Satellite constellations in these
orbits can provide very efficient
coverage of high (or low) latitudes

27090 ≤≤ν

12. Orbit Design Process
• Step 4: Select Single Satellite or Constellation
Architecture
– Single satellite
• Advantages
– Reduced overhead (single system)
– More capability per copy
• Disadvantages
– Limited coverage (potential)
– Reliability
– High cost
– Constellation
• Advantages
– Enhanced coverage
– Survivability
– System simplicity
• Disadvantages
– Higher operational and launch costs (potential)
– Limited capability

13. Orbit Design Process
• Step 5: Mission Orbit Design Trades
– How do orbital parameters affect the mission
requirements?
– How are satellites in a constellation phased
throughout the orbital plane(s)?
– Constellations
• Typically at the same altitude and inclination
• Drift characteristics
– At different altitudes and inclinations, satellites in a
constellation will drift apart

14. Orbit Design Process
• Step 6: Evaluate Constellation Growth and
Replenishment or Single-Satellite Replacement
Strategy
– Constellation
• Growth
– Time consuming (several months to years)
– Operational without full constellation
• Graceful degradation (reduced level of service)
• Replenishment
– Single Satellite
• Single point failure
• Degradation
• Replacement

15. Orbit Design Process
• Step 7: Assess Retrieval or Disposal
Options
– Retrieval is typically not an option (Shuttle)
• On-orbit servicing
– Disposal (De-orbit) may be a requirement
soon
• Orbital debris
• Limited useful operational orbits
• Re-enter (LEO)
• Disposal orbit (GEO)

16. Orbit Design Process
• Step 8: Create a ΔV Budget
– Orbital maneuvers
• Launch Vehicle may not get you directly to the
desired orbit
• Transfer orbit ΔV
– Orbit size
– Orbit inclination
– RAAN
• Stationkeeping
• Rephasing
• De-orbit

17. The ΔV Budget
• Maneuvers requiring ΔV
– Orbital transfer
– Plane change
– Drag make-up
– Attitude control
– Stationkeeping
– Rephasing
– Rendezvous
– De-Orbit

18. The ΔV Budget
• Start with the ideal rocket equation
• Mpropellant for a particular burn is the difference in initial mass
and final mass
• High Isp is desirable, but it must be weighed versus the
“cost” of the higher value (e.g. higher power, higher dry
mass, etc.)
• Investigate concepts that reduce ΔV requirements
– Aerobraking
– Solar Sails
– Tethers
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19. Environment Interactions

21. Orbit Design Process
• Step 9: Assess Launch and Orbit Transfer
Cost
– Availability of LV
– Cost
– Mass to particular orbit
= $$$

22. Orbit Design Process
• Step 10 and 11: Document and Iterate

23. Earth Coverage
• Earth coverage refers to
the part of the Earth
that a spacecraft
instrument can “see”
• Field of View: Actual
area the instrument can
“see” at any moment
• Access Area: Total
area on the ground that
could potentially be
seen at any moment.

24. Footprint

25. Hellas-Sat 2

26. Footprint
• ASTRIUM Eurostar 2000+ Platform
• Payload
– 30 x 36 MHz transponders, onboard
– 8 x 36 MHz redundant
• 12 on fixed beam F1, 6 on fixed beam F2,
up to 12 on beam S1 and 6 on beam S2.
• Footprints
– Fixed over Europe
– Steerable over Southern Africa, Middle
East, Indian subcontinent, South East
• Frequencies
– Downlink Ku-Band
• 10.95-11.20 GHz (F2)
• 11.45-11.70 GHz (S2)
• 12.50-12.75 GHz (F1, S1)
– Uplink Ku-Band
• 13.75-14.50 GHz
• Services
– Audio/Video Broadcasts
– Telephone Relay
– Internet Access
– Business Teleconferencing
F1
S1

27. Footprint
• Geostationary Operational
Environmental Satellites (GOES)
– GOES 8: Decommissioned
– GOES 9: Operational (Japan)
– GOES 10: Operational, Standby,
Drifting
– GOES 11: Operational, West
– GOES 12: Operational, East

28. Sirius Radio

29. Earth Coverage
• Earth Coverage Figures of Merit
– Percent Coverage: Number of times that a point is
covered by one or more satellites divided by a time
period
– Maximum Coverage Gap: Longest of the coverage
gaps (no coverage) encountered for a particular point
– Mean Coverage Gap: Average of the coverage gaps
(no coverage) for a particular point
– Mean Response Time: Average time from a random
request to observe a particular point

30. Earth Coverage

31. A Different Kind of Gap?
• A U.S. Government Accountability Office report on a new polar-
orbiting environmental satellite program has concluded that cost
overruns and procedural difficulties could create a gap in
important national weather data derived from the satellites that
could last at least three years, beginning in late 2007.
• Polar-orbiting environmental satellites provide data and images
used by weather forecasters, climatologists and the U.S. military
to map and monitor changes in weather, climate, the oceans and
the environment. The satellites are critical to long-term weather
prediction, including advance forecasts of hurricane paths and
intensity.
• The current U.S. program comprises two satellite systems - one
operated by the National Oceanic and Atmospheric
Administration, and one by the Department of Defense - as well
as supporting ground stations and four central data processing
centers. The new program, called the National Polar-orbiting
Operational Environmental Satellite System, or NPOESS, is
supposed to replace the two systems with a single, state-of-the-
art environment-monitoring satellite network.
• NPOESS - to be managed jointly by NOAA, DOD and NASA -
will be critical to maintaining the continual data required for
weather-forecasting and global climate monitoring though 2020.
The problem is the last NOAA polar-orbiting satellite in the
existing program is scheduled to be launched in late 2007, while
the first NPOESS launch will not be until at least late 2010. If the
earlier satellite fails, its data capability would be difficult, if not
impossible, to replace during the interim.

32. Swath Width

33. Earth Coverage

34. Earth Coverage
(-) for subscript 1, (+) for subscript 2

35. Earth Coverage

36. Constellation Design
• Constellation: Set of satellites distributed
over space intended to work together to
achieve a common objective
• Satellites that are in close proximity are
called clusters or formations
• Constellation architectures have been
fueled by recent development of small, low
cost satellites

38. Constellation Design
• Coverage
– Principle performance parameter
– Minimize gap times for regions of
interest
• Entire Earth
• North America
• Colorado
• US Air Force Academy
• Number of Satellites
– Principle cost parameter
– Achieve desired coverage with the
minimum satellites
• Example
– GPS requires continuous coverage
of the entire world by a minimum of
four non-coplanar satellites

39. Constellation Design
• Number of Orbital Planes
– Can be a driver for coverage
– Satellites spread out (typically
evenly) throughout plane
– Plane changes require large
amounts of propellant
– Meet requirements with the
minimum number of orbital
planes
– Rephasing can be
accomplished with less
propellant in a single plane

40. Constellation Design
• Constellation Build-Up,
Replenishment, and End of
Life
– Typically a constellation is in a
“less-than-complete” form
– Build-up can be a several year
process with multiple launches
– Re-Configuration of the
constellation is necessary
when satellites fail
– Dead satellites need to be
removed from the active
constellation
• Collision avoidance

41. GlobalStar
• LEO Cellular Phone
Constellation
• 48 satellites in 8 planes
• h=1414km
• i=52º
• Latitude coverage: ±70º
• 7 Boeing Delta II Launches
• 6 Russian Soyuz Launches
• Each launch vehicle
carried 4 satellites
• On-orbit spares
• Two additional Deltas were
purchased to ferry spares
to the constellation
General Characteristics:
Total weight - 450kg,
Number of Spot beams - 16
Power - 1100W
Lifetime - 7.5 years

42. Constellation Coverage

43. Street of Coverage
Swath 2λstreet where coverage will be continuous

44. Street of Coverage
• Adjacent Planes

45. Iridium (Atomic No=77)
66 active satellites, 6 planes
Reduced to 6 orbital planes (from a proposed 7) by
Increasing the orbital altitude slightly.

46. Iridium Satellite Constellation

47. Constellation Design

48. The Walker Constellation
• Symmetric
• T = total number of satellites
• S satellites evenly distributed in each of P orbital
planes
• Ascending Nodes of the P orbital planes are
uniformly distributed about the equator
• Within each plane, the S satellites are uniformly
distributed in the orbit
• Relative phase between satellites in adjacent
planes to avoid collisions

49. Stationkeeping
• Approaches to perturbations
– Leave perturbation uncompensated
– Control the perturbing force the same for all satellites
in the constellation
– Negate the perturbing force
• Example: h=700 km, i=30º and 70º
– Node rotation rate of 2.62º /day and 6.63º /day
– Relative plane movement of 4º /day
– Makes construction of long term constellation difficult
• Coverage requirements
• Active rephasing may be necessary

50. Collision Avoidance

51. Microsatellite Constellations