Some Major Orbit Types
Steven Dutch, Natural and Applied Sciences,University of Wisconsin - Green Bay
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Latitude of View Longitude of View
Scale (km/px) Time Step (min) Auto-Select Scale
Show Ground Tracks Show Satellite Motion
Select Orbits
Types of Orbit
- Geostationary: Equatorial circular orbit. A satellite in this orbit keeps pace with Earth's rotation and appears to remain stationary in the sky. An interesting factoid is that at the equinoxes, these satellites have brief blackout periods when they are in line with the Sun.
- Molniya: From the Russian word for "lightning." 12-hour highly elliptical and inclined orbit. Because of the Earth's size, geostationary satellites are not visible north of 55 degrees, which includes much of Russia. Molniya orbits fill the gap. Their great eccentricity means they travel slowly at apogee, allowing long periods of contact. Three Molniya satellites allow continuous coverage. The 12 hour period means there is one apogee period over Russia, another over North America, making them also useful as spy satellites for both sides. The orbital inclination is 63.4 degrees, which results in zero precession of the major axis of the orbit, so the apogee remains fixed above the northern hemisphere.
- Navstar: Navigation satellites travel in circular 12-hour orbits with 55 degree inclinations. There are six orbital planes in use.
- Tundra: More interesting theoretically than practically. A highly eccentric 24 hour orbit with the magic 63.4 degree inclination to prevent precession of the major axis of the orbit. Sirius satellite radio is the only current user of Tundra orbits since Molniya orbits are nearly as practical and closer to Earth.
- Inclined Geosynchronous: Allows the user to experiment with other geosynchronous orbits. Details like precession of the orbit plane and perigee are not shown.
- Sun - Synchronous: Circular, near polar and slightly retrograde orbits with inclinations of 90+ degrees. Orbits are chosen so precession of the orbital plane matches the Earth's motion around the Sun. When a sun-synchronous satellite passes southbound over the daylight side of the Earth, it is always late morning, optimum time for photography. Early morning haze has typically burned off and afternoon cumulus has not built up. Orbits are also chosen to pass over the same tracks at regular intervals, generally 2-3 weeks. Sun-Synchronous orbits are used by most Earth observation systems like Landsat.
- ISS (Space Station): Circular orbit with 51.6 degree inclination. Nominally the altitude is 350 kilometers, but the station is a big sucker and atmospheric drag causes the station to spiral in and require periodic boosts to maintain altitude. Actual altitude is 300 to 400 kilometers.
- User-Defined Orbits: Up to three orbits can be custom designed. If you want to show multiple Molniya or Navstar orbits, for example, you can create them here.
Instructions
- Latitude of View: Input the latitude of your viewpoint. 90 is north polar, 0 is equatorial, -90 is south polar.
- Longitude of View: Input the longitude of your viewpoint. West longitude is negative, east is positive. Applies to drawing static views, although the box will display the current longitude when the earth is rotating.
- Auto-Scale: Selects optimum scale and time step for the selected orbits.
- Scale (km/px): Input the scale in kilometers per pixel. For low orbits a scale of 25 km per pixel is suitable, for geosyncronous orbits a scale of 200 is necessary to show the entire orbit.
- Time Step (min): Input the time step in minutes for animations. For low orbits a step of 2-5 minutes is best, otherwise satellites flick by too quickly to see. For geosynchronous orbits, 15-20 minutes is acceptable.
- Show Ground Tracks: Shows the sub-satellite point on the ground as well as the radius to the satellite.
- Show Satellite Motion: Shows the actual motion of a satellite.
- Draw: Draws a static picture of the earth centered on the chosen latitude and longitude, with selected orbits, satellites and ground positions. No motion.
- Animate: Shows a moving view, with earth rotation and satellite motions.
- Halt: Stop the animation temporarily. Pressing Animate will restart it.
- Clear Figure: Erase the picture.
- Orbit: Check the box to select the desired orbit.
- Perigee-Apogee: Perigee is the closest point, apogee is the most distant. Distances are km above the Earth's surface. Perigee = (Mean Radius)(1-Eccentricity)-6371. Apogee = (Mean Radius)(1+Eccentricity)-6371.
- Mean Radius km: Average distance in kilometers from the center of the earth. Also equal to half the length of the long axis of the orbit (semi-major axis). Equal to 6371 + (Apogee+Perigee)/2
- Inclination: Inclination of the orbit plane with resect to the equator. 0 is equatorial, 90 is polar, inclinations greater than 90 are retrograde (opposite the earth's rotation). For a true view of an orbit, select a latitude equal to 90-Inclination (26.6 for Molniya).
- Node: Where the orbit crosses the earth's equatorial plane. On this page, it defines the orientation of the orbit relative to the line of sight. A node of zero means we are looking at the orbit straight on, and 90 means we are looking at the orbit edge on.
- Eccentricity: How elliptical the orbit is. Most orbits are nearly circular but Molniya and Tundra orbits are highly elliptical. The eccentricity of the Molniya orbit, 0.741, means the center of the earth is 0.741 of the way along the major axis from the center of the ellipse to the end.
- Perigee Angle: The azimuth of the perigee measured in the plane of the orbit. 0 means the perigee is at the equator. The -90 values for the Tundra and Molniya orbits mean the perigee is in the Southern Hemisphere, so the out of view period is short.
- User-Defined Orbits: Users can input their own values for orbital parameters. If Perigee and Apogee are input, the values for Mean Radius and Eccentricity will be adjusted. If Mean Radius and Eccentricity are selected, the values for Perigee and Apogee will be adjusted. Perigees below 200 km will result in short-lived satellites due to atmospheric friction and of course perigees below zero will intersect the earth.
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Created 14 February 2012, Last Update 21 September 2016
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