# How precise must a satellite's orbit be?

I was thinking about the question on solid upper-stages and my feeling that a liquid upper-stage would allow a more precise orbit. But... how precise does an orbit actually have to be? I guess I mean the initial placement, and we can assume the satellite can make small corrections if needed. But there are also a lot of satellites without engines, like most cubesats, and they just tumble around wherever the dispenser throws them along the orbit of the primary payload, and it could be days before the operator is even given the official orbit parameters. So there has to be a certain amount of "slop" that can be tolerated, I would think.

• Perhaps a more useful question would be, "Which satellite applications require the most precise orbit?" – DrSheldon Jun 9 '19 at 1:20

## 1 Answer

This largely depends on purpose/application, but if you want it just to "be in orbit" then the answer is "not very precise at all.". Or quantifying better, the periapsis speed must be between the speed of circular orbit and escape speed which is $$\sqrt{2}$$ of that - so about 40% of "slop".

Of course satellites aren't of much use if you can't communicate with them, and you can't communicate with them if you can't locate them. So if you don't give your satellite some good orbit control capacity, it should at the very least have a good orbit determination capacity to be able to point a directional antenna at a ground station and announce where it got after the sloppy burn. And if your satellite is to be geostationary, or serve as a part of a global positioning system, or spy at a specific region of the world, or such, then you'll need a much more precise orbit. If you're trying to build a gravitometer out of a constellation of three satellites (like eLISA), their positioning will need to be ludicrously precise, millimeters on a ~million kilometer scale.

Summing up - the application often dictates high precision. Orbital mechanics alone though gives a plenty of wiggle room - if circular low orbit speed is 1, and escape speed is 1.41, anything in between is fair game.

• Some precision is necessary to get a circular low orbit and not an elliptical. If 400 km height was intended, an elliptic orbit with a minimum height of 100 km and a maximum height of 700 km would decay too fast. – Uwe Jun 9 '19 at 14:02
• In the spirit of the matter of precision, I think we can say that EVERY orbit is eccentric. A "circular" orbit might have an eccentricity within 1% of 1. Or 5%, or 10%... How close does it have to be before we go from "circular" to "dang it, we screwed it up"? Depends on the application, I suppose, like Dr. Sheldon mentioned. But eccentricity is also only one parameter. – Greg Jun 9 '19 at 16:08