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

Outline Orbit Design Earth Coverage Constellation 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

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

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

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 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 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 S1 F1

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 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

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 - 16 Power - 1100W Lifetime - 7.5 years

Constellation Coverage

Street of Coverage Swath 2lstreet where coverage will be continuous

Street of Coverage Adjacent Planes

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

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