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Nov 17 2015

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A primer on how to get to GEO

Buried in the middle of this article on all things RPO is an excellent primer on what it takes to get a satellite to Geosynchronous orbit (GEO).

This is such a good primer, I’ll just quote it directly:

 

At this point it is important to pause and discuss how satellites get to GSO, which is a more difficult and lengthy process than getting to LEO. The geostationary belt is by definition 35,786 kilometers (22,236 miles) above the Earth’s equator and at zero degrees inclination (i.e., following the Equator.) This means all satellites going to GSO need to conduct several maneuvers to get there, and there are many different approaches that are used. The fastest, but most energy-expensive, method is to launch the payload(s) and an attached upper stage into a LEO parking orbit of a few hundred kilometers altitude. Shortly thereafter, the upper stage executes a large burn to place itself and the payload(s) into a highly elliptical geosynchronous transfer orbit (GTO), which has its perigee at the LEO parking orbit and apogee at or near GSO. After several hours, the satellite will reach apogee and execute another large burn, either itself or with an apogee kick motor, to circularize its orbit near GEO. This is the method typically used by the Briz-M upper stage.

GSO profile

Typical five-burn, nine-hour flight profile for a Briz-M to inject a payload into GEO. Image credit:International Launch Services, Inc.

The slowest method to get to GSO, but one that is increasingly popular, is to use a GTO that has a very high apogee, sometimes on the order of 100,000 kilometers (60,000 miles). The reason for this is that it makes it much more efficient to change the inclination—the more elliptical the orbit, the slower a satellite is moving at apogee and the less energy it takes to conduct a plane change maneuver. The satellite then conducts many burns over a much longer period of weeks or months to slowly lower apogee, raise perigee, and zero out its inclination to reach GSO. This technique has risen in popularity due to the increased use of electric propulsion systems on communications satellites, which are less powerful than traditional chemical thrusters but much more fuel efficient. Some companies are even switching to all-electric satellites that forgo chemical propulsion entirely.

Once a satellite has reached the GEO region, it’s far from done. GSO can be thought of as a large, circular racetrack with all the cars going in roughly the same direction at roughly the same speed. A newly launched satellite needs to “merge into traffic” and find a slot in the lineup, sort of like a car coming back into the race from pit road. In the case of a GSO satellite, the slot is the equatorial longitude over which it wants to provide services. If the satellite is a communications satellite, this slot will often be registered with the International Telecommunication Union (ITU) to keep enough distance between other satellites transmitting on the same frequencies in order to prevent radio frequency interference (RFI).

A GSO satellite can change its slot, or longitude, by raising or lowering the altitude of its orbit. Raising the altitude of its orbit means the satellite will move slower relative to the rest of the GSO satellites, and thus appear to drift westward through the GSO belt (from the perspective of an observer on Earth). Lowering its altitude means the satellite will move relatively faster, and thus appear to drift eastward. Satellites are sometimes maneuvered into one slot for initial checkout, and then maneuvered again to reach their operational slot. Sometimes a satellite may be moved to a new location to replace another satellite that is being retired or serve new customers. And in some cases, multiple satellites occupy the same slot in what’s called a cluster. Usually, all the satellites in a cluster belong to a single satellite operator and they follow a very closely-choreographed dance to prevent collisions.

Even after a GSO satellite has reached its operational slot, it is still constantly maneuvering. While in a slot, a GSO satellite is continually affected by orbital perturbations due to the bulging of the Earth and the gravity of the Sun and Moon that work to pull it north-south or east-west. Most active GSO satellites execute periodic station-keeping maneuvers to counteract these perturbations and keep it in the orbital “box” for its mission. At the end of their life, satellites are also supposed to conduct a set of maneuvers to boost their orbit into the graveyard region at least 235 kilometers (146 miles) above the active belt in accordance with the IADC space debris mitigation guidelines.

So there you go. I’ll link this primer post to my References page.

Much thanks to reader Vic Moberg, who provided the link to the RPO article — an article so good, it led to two posts today.

Permanent link to this article: http://www.newspaceraces.com/2015/11/17/a-primer-on-how-to-get-to-geo/

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