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1 mile visibility and clear of clouds.
1,000-foot ceiling and 3 miles visibility.
I don't think there are limits to how slow an aircraft can fly in any airspace.
By radar or GPS.
ATC clearance is authorization to proceed under specified traffic conditions in controlled airspace.
There's no single answer for this. When a presidential aircraft moves, the government declares airspace restrictions. (If the airspace belongs to a country other than the one the president runs, that government will declare airspace restrictions as a diplomatic gesture.)
Airspace in the US is classified into the following categories: A, B, C, D, E, and G. The airspace is ordered from Class A-Tightest control to Class G-No control. Class A airspace is anywhere from 18000 feet to flight lever 600 (approx. 60000 feet); all aircraft in class A airspace must fly IFR (instrument flight rules) and stay in contact with ATC. The required separation is also greater in class A airspace. Class B airspace surrounds larger airports, and consists of 3 or more "shelves". Class B airspace looks somewhat like an upside down wedding cake. In Class B airspace, all aircraft require a specific clearance to enter or fly through the airspace. Class C airspace Surrounds smaller airports, and looks like an upside down cake with only two layers. Aircraft entering class C airspace need only establish two way radio contact with ATC before entering. Class D airspace has the same restrictions as class C, but has only one layer. Class E airspace encompass everything from 1200 feet to 18000 feet that is not Class B, C, D, or, G. Sometimes the base elevation is lower or higher that 1200 feet, but 1200 is the normal base. Class E also encompasses everything above Flight Level 600. VFR (visual flight rules) aircraft are not required to contact ATC in class E airspace. IFR aircraft maintain ACT contact in class E. Class G airspace is uncontrolled, and usually extends to anything below 1200 feet. Even IFR aircraft do not contact ATC in class G airspace.
Yes, the airspace over Washington DC is restricted. It is classified as a Special Flight Rules Area (SFRA) and is highly regulated to ensure the security of the capital. Pilots must obtain specific clearance and follow specific procedures when flying in this airspace.
Military aircrafts don't disable their transponder beacon when flying in enemy airspace, they change the settings on it though.
The Lockheed U-2 flown by Gary Powers .
It depends on the national state of emergency. If there is a state of national emergency then all civilian aircraft are usually grounded and the military have precedence.
1,000 feet and 3 miles.