How many satellites would there have to be to actively monitor the entire world?

To actively monitor the entire worlds surface, how many satellites would a nation have to have?
To monitor the land?
To monitor the oceans?
And by monitor I mean like reconnaissance. Knowing about troop movements and being able to see missile launches.
 
Theoretically, 4 I think. Read somewhere that 4 was the ultimate minimum necessary to watch over the earth, since a triangular pyramid can encircle a sphere.
 

Insider

Banned
Theoretically, 4 I think. Read somewhere that 4 was the ultimate minimum necessary to watch over the earth, since a triangular pyramid can encircle a sphere.

Theoretically yes. In reality it depends on sensors you have, and what you demand from them. If the mission is detecting something as energetic as the nuclear tests, the sensor can be very basic, and satelites can have very wide orbits. That would theroetically allow to use even three... but then you would have limited coverage over the poles.

If you want the satelites to detect troop movements, that would require it to fly low. For active monitoring that would require hundreds of satelites. It is unfeasible. A dozen or two sattelites could get a close look on any place on earth within 24 hours. They may not spot troop movements themselves but can register the traces that they left.
 

jahenders

Banned
A fairly small number (4-6) might theoretically be able to see any spot on the earth's surface at some point. However, that wouldn't be anywhere near enough to have either continuous monitoring or the kind of detail necessary.

To do what you suggest, assuming today's technology, I think you need a minimum of several dozen powerful satellites. You also need a strong ground-based system to collect and analyze the data, and watch for specific things.

To actively monitor the entire worlds surface, how many satellites would a nation have to have?
To monitor the land?
To monitor the oceans?
And by monitor I mean like reconnaissance. Knowing about troop movements and being able to see missile launches.
 
To actively monitor the entire worlds surface, how many satellites would a nation have to have?
To monitor the land?
To monitor the oceans?
And by monitor I mean like reconnaissance. Knowing about troop movements and being able to see missile launches.

Six would be enough. Gerard.
 
It especially depends on whether you know exactly what to look for, resp. whether you want to monitor a given small piece of real estate (e.g. A certain military convoy) or whether you want to see every little piece of ground simultaneously. In the latter case it is a question whether you have enough cameras per satellite - and for each camera you would need 4 people to observe and interpret in shifts. The smaller the objects you monitor visually, the smaller the area will be and therefore you would need more cameras and more satellites.
For monitoring undirected radiation, e.g. Radio communications, you don't need focusing and can get by with far less satellites.
To monitor visually all populated areas with a single person resolution you would need a significant fraction of the worlds population just to look at the pictures.
 
for permanent continuous monitoring, a minimum of 4 or 5 if you use special tricks, AIUI, which are theritically possible but not yet deployed, AFAIK. (note that 4 in a pyramid is not stable with classic configuration,because of orbital mechanics, if I am not mistaken)

If you want full coverage but accept not to look everywhere all the time, one is enough.
 
I think 4 or 5 recon satellites are enough for that task, but that's the tip of the iceberg of the surveillance that occurs. The US currently has 8 IR satellites that monitor missile launches, plus whatever number listen to electronic communications and other emissions.

The military also uses the take of civilian satellites to beef up their own picture.
 
3 Geostationary satellites can cover all but the poles. To fix that gets tricky.

If you got 4 sats at that altitude that did north/south excursions (e.g. their ground tracks would look like a figure 8 centered on the equator), you MIGHT be able to schedule them so every part of the globe is in sight at once.

If you had six, I suspect it would be 'easy'. 5??? Don't know.
 
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