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

2. Outline Orbit Design Earth Coverage Constellation 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

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
Enhanced coverage
Survivability
System simplicity
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

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 Footprints
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
GHz (F2)
GHz (S2)
GHz (F1, S1)
Uplink Ku-Band
GHz
Services
Audio/Video Broadcasts
Telephone Relay
Internet Access
Business Teleconferencing S1 F1

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 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 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 Number of Satellites Example
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 W Lifetime years

42. Constellation Coverage

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

44. Street of Coverage
Adjacent Planes

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

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