Space is considered to be a common heritage and is not owned or controlled by any government , but how far above sea level is considered to be sovereign territory? Is there any such specification such as 20,000 kilometers above the sea level?
Generally speaking, the Karman line, which is about 100km/62mi, represents the level at which "space" begins. The Karman line is the lowest point that an object can orbit the Earth. For the most part, trying to restrict orbits legally is an exercise in futility. Sooner or later you're going to pass over another country, but at the Karman line, you're moving more than 17,000 miles/hr, making enforcement when the object is in your "airspace" sometimes seconds long. This is is why the Outer Space Treaty says
outer space, including the Moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means
Be aware that nobody has really challenged this, but that's largely due to the fact that so few had rockets that could put things in orbit. With the nascent civilian space rocket industry gearing up, that could change. For instance, some equatorial countries asserted they own geostationary orbits above them. These claims were never recognized by any spacefaring nation.
Right now, it's 100km/62 miles by international agreement of the major nations. That may change as the situation changes. The major nations also agreed not to station weapons in orbit, specifically nuclear weapons.
This echoes back to the concept that a nation's sovereignty existed 12 miles out into the ocean that surrounds the nation. This was a practical limit, as it was beyond what shore based cannon of that day could reach. You control what you can enforce. Today, the commercial limit is considered to be 200 miles, but even that is being tested, what with China building artificial islands and claiming a 200 mile sovereignty around them.
At the time the 100km line was established, humanity could barely get anything into orbit, let alone doing anything about the satellite once it was there. Like the 12 mile limit, 100km was also much higher than surface to air missiles of that day could operate, so this was a practical limit as well. Today, several nations have demonstrated the ability to destroy satellites in orbit, and for a while, the US even had the ability to capture a satellite and return it to earth via the shuttles.
There was an interesting case recently regarding very small satellites that illustrates how the situation is changing faster than the agreements. A company put a number of very small satellites into space, and the US Federal Communications Commission is all over them for doing so despite being denied approval for the launch. The idea being, we don't want a lot of tiny satellites that are cheap to put into low earth orbit, adding to the space debris orbiting the earth.
Note the governing body: the FCC, which governs transmissions of radio signals only in the US. It was the FCC that beat this US based company over the head for putting up satellites that transmit radio signals over the US without approval, because there are no other laws, international or otherwise, regarding what can be put up into space. Also, in the US, the Federal Aviation Administration governs launching rockets from the US into orbit, but that's only in the US.
So we're getting into a gray area that current agreements don't cover, because new technology changes have opened up new possibilities, and potentially new abuses. The US FCC doesn't issue approval for other nations to orbit radio transmitting devices, only people in the US, so that's really a stopgap measure, and it shows where future problems may arise. If you're creative enough, and have the money, you can even put a car into space and enjoy the very high speed limit. 300,000 km/second... it's not just a good idea, it's the law.
This illustrates how technology changes, specifically low cost orbital launches, pose a problem for which current international treaties don't cover. Whether or not that affects the current 100km limit, remains to be seen. But, as launches become even less expensive, new problems or abuses of the concept will continue to crop up.