The "spoofing" theory is not so unbelievable though. That could easily cause a lot of mischief.
No it could not. The GNSS positional accuracy is (where required) confirmed both by the airborne sending unit, and independently via the ground station (where in coverage) receive timing. Also, don't forget that even if an ADS-B GNSS derived position is not (temporarily) broadcast as part of the 1090 message, the A,C and S data still is, and will be plotted where Multilat (non-dependent) and/or Mode S radar coverage exists (Capital city and surrounds).
Back to the sub-subject of GPS availablity/reliability
http://frwebgate.access.gpo.gov/cgi-...19346&TYPE=PDF
[/b]§ 2281. Global Positioning System[/b]
(b) SUSTAINMENT AND OPERATION FOR CIVILIAN PURPOSES.— The Secretary of Defense shall provide for the sustainment and operation of the GPS Standard Positioning Service for peaceful civil, commercial, and scientific uses on a continuous worldwide basis free of direct user fees.
In doing so, the Secretary—
(1) shall provide for the sustainment and operation of the GPS Standard Positioning Service in order to meet the performance requirements of the Federal Radionavigation Plan prepared jointly by the Secretary of Defense and the Secretary of Transportation pursuant to subsection (c)
(2) shall coordinate with the Secretary of Transportation regarding the development and implementation by the Government of augmentations to the basic GPS that achieve or enhance uses of the system in support of transportation;
(3) shall coordinate with the Secretary of Commerce, the United States Trade Representative, and other appropriate officials to facilitate the development of new and expanded civil and commercial uses for the GPS;
(4) shall develop measures for preventing hostile use of the GPS in a particular area without hindering peaceful civil use of the system elsewhere; and
(5) may not agree to any restriction on the Global Positioning System proposed by the head of a department or agency of the United States outside the Department of Defense in the exercise of that official’s regulatory authority that would adversely affect the military potential of the Global Positioning System.
(c) FEDERAL RADIONAVIGATION PLAN.— The Secretary of Defense and the Secretary of Transportation shall jointly prepare the Federal Radionavigation Plan. The plan shall be revised and updated not less often than every two years. The plan shall be prepared in accordance with the requirements applicable to such plan as first prepared pursuant to section 507 of the International Maritime Satellite Telecommunications Act 1 (47 U.S.C. 756). The plan, and any amendment to the plan, shall be published in the Federal Register.
ACCESS TO GLOBAL POSITIONING SYSTEM
Pub. L. 104–106, div. A, title II, § 279, Feb. 10, 1996, 110
Stat. 243, provided that:
‘‘(a) CONDITIONAL PROHIBITION ON USE OF SELECTIVE AVAILABILITY FEATURE.—Except as provided in subsection (b), after May 1, 1996, the Secretary of Defense may not (through use of the feature known as ‘selective availability’) deny access of non-Department of Defense users to the full capabilities of the Global Positioning System.
‘‘(b) PLAN.—Subsection (a) shall cease to apply upon submission by the Secretary of Defense to the Committee on Armed Services of the Senate and the Committee on National Security of the House of Representatives of a plan for enhancement of the Global Positioning System that provides for—
‘‘(1) development and acquisition of effective capabilities to deny hostile military forces the ability to use the Global Positioning System without hindering the ability of United States military forces and civil users to have access to and use of the system, together with a specific date by which those capabilities could be operational; and
‘‘(2) development and acquisition of receivers for the Global Positioning System and other techniques for weapons and weapon systems that provide substantially improved resistance to jamming and other forms of electronic interference or disruption, together with a specific date by which those receivers and other techniques could be operational with United States military forces.’’
Have a read of the full legislation. It makes quite clear why the US is not interested in any civilian 'unreliability' issues.
Quite apart from the non-US Galileo and other interoperable constellations coming on line.