2.8 Orbit Types

Question 1

LEO

Answer 1

Low Earth Orbit

• Average latitude: less than 2,000 km
• Typical period: 90 - 120 mins
• Applications: manned spaceflight (ISS), communication satellites constellations, earth observing (weather, earth sensing, ISR)
• 67 satellites needed for total coverage
• Advantages: ease of access (it’s cheaper to launch into), and being close to the surface provides good resolution for observation satellites, and low power requirements for communications and related systems, protection from solar radiation.
• Disadvantages: short dwell time over any spot on the Earth’s surface (generally minutes) due to the high orbital velocities resulting in short orbital periods (generally 1.5-2.0 hours), atmospheric drag issues requiring onboard propellant to keep the satellite’s orbit from degrading, and the relatively narrow field of view (footprint) for any sensors on the satellite bus.

Question 2

MEO

Answer 2

Medium Earth Orbit
- Average altitude: >2,000km but <35,876km
- Typical Period: ~2hrs-23h56m4.09s
- Applications: Position, Navigation, and Timing (PNT)
Communications
Persistent Earth Observation
- 30 satellites needed for total coverage
- Advantages: larger field of view, less satellites needed for total coverage
- Disadvantages: higher power needed to overcome attenuation (loss of signal due to higher altitude)

Question 3

Semi-Synchronous Orbit

Answer 3

A “special” orbit that repeats its ground track every day
Period: ~12 hours (11:58:02)
Semi major axis: 26,562 km (20,191 km)
Mission:
Precise Navigation and Timing (GPS, Glonass)

Question 4

GEO

Answer 4

Geosynchronous Orbit
- Average altitude: 35,876km
- Typical Period: 23h56m4.09s
- Applications:
Communications, weather, missile warning
- 3 satellites needed for total coverage
- Advantages: less satellites needed for total coverage, Constant viewing angle
- Disadvantages: increases demand for comm systems power; limits resolution for ISR missions.
3 satellites cannot give coverage over polar regions81+ latitude

Question 5

Geostationary

Answer 5

• i=0 e=o (dot) satellite to appear hovering in orbit

- Figure 8, e=0; inclination and eccentricity is not zero

Question 6

SSO

Answer 6

SunSynchronous Orbit
Special, near polar inclination with retrograde motion. Passes over the same target at the same time of day, same sun angle every orbit
Applications: ISR, weather, earth sensing, pattern of life of a building/area
i=98, exploits nodal regression

Question 7

HEO

Answer 7

Highly Elliptical Orbit
- Specialized orbit with long dwell times typically over northern latitudes
Approximately 8 hours of a 12 hour orbit
- Max coverage at higher latitudes
– Covers the gap left by a GEO constellation
Utilized critical inclination, cancelling the effects of apsidal rotation due to J2, 𝜔̇=0
𝑖 ≈63.4°
𝑖 ≈116.6°
- Applications: communications/SIGINT, ISR, missile warning, METOC.

Question 8

Field of Regard

Answer 8

Portion of earth’s surface geometrically visible from the satellite’s position in space
As altitude increases, so does FOR

Question 9

Field of View

Answer 9

Coverage area capable of being seen by the payload

Question 10

Dwell Time

Answer 10

Length of time a satellite has line of sight to a point of earth

Question 11

Revisit Period

Answer 11

The length of time that passes for a satellite to be visible over a target area