Orbit and Constellation Design

Dr. Andrew KetsdeverMAE 5595Lesson 6

Outline

• Orbit Design– Orbit Selection– Orbit Design Process– ΔV Budget– Launch

• Earth Coverage• Constellation Design

– Basic Formation– Stationkeeping– Collision Avoidance

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

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

Orbit Design Process

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)

Orbit Design Process

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

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

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

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

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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

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

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

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)

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

The ΔV Budget

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

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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Environment Interactions

Orbit Design Process• Step 9: Assess Launch and Orbit Transfer

Cost– Availability of LV– Cost– Mass to particular orbit

= $$$

Orbit Design Process

• Step 10 and 11: Document and Iterate

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.

Footprint

Hellas-Sat 2

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

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

Sirius Radio

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

Earth Coverage

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.

Swath Width

Earth Coverage

Earth Coverage

(-) for subscript 1, (+) for subscript 2

Earth Coverage

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

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

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

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

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 - 16Power - 1100WLifetime - 7.5 years

Constellation Coverage

Street of Coverage

Swath 2street where coverage will be continuous

Street of Coverage

• Adjacent Planes

Iridium (Atomic No=77)

66 active satellites, 6 planes

Reduced to 6 orbital planes (from a proposed 7) byIncreasing the orbital altitude slightly.

Iridium Satellite Constellation

Constellation Design

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

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

Collision Avoidance

Microsatellite Constellations