Homeland Security: Defending U.S. Airspace
CRS Report for Congress
Homeland Security: Defending U.S. Airspace
Specialist in National Defense
Foreign Affairs, Defense, and Trade Division
The September 11th attacks drew attention to U.S. air defense, and the 9/11
Commission Report recommended that Congress regularly assess the ability of Northern
Command to defend the United States against military threats. Protecting U.S. airspace
may require improvements in detecting aircraft and cruise missiles, making quick
operational decisions, and intercepting them. A number of options exist in each of these
areas. A variety of issues must be weighed including expediency, cost, and minimizing
conflicts with civilian aviation. This report will be updated.
In response to the Cold War threat of Soviet bombers and cruise missiles, the
Department of Defense (DOD) established the North American Air Defense Command
(NORAD) in1958.1 NORAD deployed a network of radars, fighter aircraft, and surface-
to-air missiles (SAMs) around the United States. The emergence of intercontinental
ballistic missiles in the 1960s drew attention away from U.S. air defenses. The air and
cruise missile threat appeared to decline further with the Soviet Union’s demise, and
growing U.S. superiority over other hostile air forces. Because an air attack on the United
States appeared unlikely, DOD relaxed its posture. By September 11, 2001, only 14 Air
Force fighters at 7 bases were assigned to protect the continental United States (CONUS)2
from air attacks. This number has subsequently been increased to over 100.
Today, NORAD operates radars in the United States and Canada, oriented outward,
to detect air attacks from foreign countries. NORAD augments these radars by
communicating with the Federal Aviation Administration (FAA), which operates its own
radars, and by flying E-3 AWACS aircraft. NORAD commands F-15 Eagle, F-16
Falcon, and Canadian CF-18 fighter aircraft flying combat air patrols (CAP) and on strip
alert (prepared to take off on short notice). NORAD’s Command and Control (C2) centers
1 For more information about cruise missiles and proliferation see CRS Report RS21252, by
2 Adam Hebert, “Ongoing Operations Made NORAD Response to Sept. 11 ‘Seamless,’” Inside
the Air Force, Dec. 21, 2001.
Congressional Research Service ˜ The Library of Congress
are located at Cheyenne Mountain Air Station (CO), Elmendorf AFB (AK), Tyndal AFB
(FL), and Canadian Forces Base, (Winnipeg Manitoba). The Air National Guard is also
a key organization in continental air defense. In July 2004, it was reported that The Air
Force had permanently transferred the homeland defense air patrol mission to the Air
National Guard, shifting more than $84 million from the Air Force operations and
Due to the September 11 attacks, and the growing threat of cruise missiles, some
policy makers are re-evaluating today’s modest U.S. air defenses. Improving defense of
U.S. airspace poses numerous challenges to defense planners, who must assess the pros
and cons of several military options. As part of its oversight role, Congress may be called
upon to assess these options and determine the most effective mix of systems employed.
Indeed, the 9/11 Commission Report [http://www.9-11commission.gov/] specifically
recommended that DOD and congressional oversight committees “should regularly assess
the adequacy of Northern Command’s strategies and planning to defend the United States
against military threats to the homeland.” Subsequent congressional legislation on
intelligence reform (H.R. 10 and S. 2845) do not directly address this recommendation.
Air Defense Challenges
Effectively protecting U.S. airspace requires detecting threatening aircraft and cruise
missiles, making decisions on how to address these threats (called “command and
control”, or C2), and negating these threats. On June 9, 2004, a small aircraft carrying the
governor of Kentucky flew into restricted airspace around Washington, DC. The
misidentified aircraft caused panic among Capitol Hill employees, and two F-15s were
scrambled to intercept the aircraft.4 This event suggests that 2½ years after the September
Surveillance. Detecting and tracking airborne threats to the United States are
complicated by environment and enemy tactics. The large volume of airspace that must
be surveyed presents one key environmental challenge. Airspace over the continental5
United States is estimated at approximately 3 million square miles. Enemy tactics could
include flying low to the ground, which makes detection difficult, or applying stealth
technology, which reduces an aircraft’s vulnerability to radar detection. As the September
11th hijackers demonstrated, turning commercial or civil aircraft into weapons is another
tactic that would make threat detection difficult.
Command & Control. Expediently identifying airborne threats, and accurately
verifying that they are not civilian or friendly military aircraft is a key air defense
challenge. The large amount of air traffic within CONUS will likely make separating
“friend from foe” difficult. FAA data show that on a given day, over 80,000 distinct
domestic commercial aircraft movements (e.g., departures, overflights) take place over
3 Cynthia Di Pasquale, “Air Sovereignty Alert Becomes Permanent Air Guard Mission,” Inside
the Air Force, July 16, 2004.
4 David Hughes, “Capitol Hill Investigates King Air Security Incident,” Aviation Week & Space
Technology, July 12, 2004.
5 R.W. Rogers, “Terrorists Exploited U.S. Air Defense,” Newport News Daily Press, Oct.7, 2001.
CONUS.6 These 80,000 aircraft movements do not include international flights, or the
approximately 200,000 civil aircraft in the United States that fly some 24 million flight
hours annually. Nor does this number include military aircraft that fly within both civilian
and military airspace. Air defense C2 over CONUS is further complicated by the fact that
decision making will not be a solely military enterprise. Civil entities such as the FAA,
and the U.S. Customs Service, and military authorities will require seamless
communications and hardware interoperability to make effective decisions.
Intercept. Anti-aircraft artillery, surface-to-air missiles (SAMs), or military aircraft
can shoot down enemy aircraft and cruise missiles. In dire situations, hijacked civilian
aircraft may also need to be shot down, although negating this threat in other ways will
likely be preferred. Minimizing civilian casualties both in the air and on the ground may
be a key challenge, especially if the threatening aircraft or missile carries weapons of mass
destruction. Adequately covering the large number of assets (e.g., cities, nuclear power
plants, military facilities, national buildings and monuments) will also be challenging.
Options and Issues
Following the terrorist attacks of September 11, 2001, DOD increased the resources
devoted to CONUS air defense by deploying an aircraft carrier to New York harbor and
by flying fighter CAPs over major cities. NATO allies contributed to this effort (called
Operation Noble Eagle) by flying AWACS aircraft over CONUS. Although these efforts
were welcome, they appear unsustainable in the long term, as Operation Noble Eagle7
costs DOD about $3 billion per year. DOD must still develop a long-term plan for
improving air and cruise missile defense of CONUS. When considering air defense
options DOD may evaluate factors such as expediency, potential impact on commercial
and civil air traffic, potential competition with other military needs, and minimizing
collateral damage and civilian casualties. Designing a defense that can address the whole
range of potential threats (e.g., enemy bombers, stealthy cruise missiles, and hijacked
commercial aircraft), yet be optimized to address the most likely or most dangerous threat
may also be a key challenge.
Cost is another key consideration. Estimated costs for air and cruise missile defense
of CONUS vary widely depending on assumptions regarding the threat (e.g., number of
attackers, flight characteristics, and payload), what is to be protected, system effectiveness
(the number of “leakers” that is acceptable) and the exact mix of systems deployed. A
attack would cost on the order of $70 billion. A 1989 study estimated that fielding a
system that could defend the 20 largest U.S. cities and 50 military installations from a
large scale air and cruise missile attack would cost between $54 billion and $170 billion,
7 Elizabeth Rees. “USAF Forced to Pay ‘05 Noble Eagle Bill; Money not in Supplemental. Inside
the Air Force. Feb. 11, 2005.
8 Barry Blechman. “The Macroeconomics of Strategic Defenses,” International Security, Winter
depending on the exact mix of forces deployed.9 A more contemporary study suggests
that an air and cruise missile defense system for CONUS could cost in the neighborhood
of $30 billion, with annual operating costs on the order of $1 billion.10
Surveillance. Surveillance radars can be divided into three categories: ground-
based, airborne, and space-based. The primary advantage of ground-based radars is that
they tend to be less expensive to field and operate than other radars. A shortcoming of
ground-based radars is that they tend to have trouble detecting low flying aircraft.
Features such as mountains and buildings block or “clutter” the radar picture, and the
Earth’s curvature leaves gaps in coverage that low-flying threats can exploit. NORAD
already operates a network of ground based radars, and it will likely serve as one
component of a CONUS defense surveillance system. Improvements in this network may
be considered however, including upgrading the radar to improve its ability to detect
stealthy threats, deploying more radars to cover gaps in coverage, and fielding radars that
survey airspace within CONUS, to augment today’s outward looking radars.
Airborne radars offer some advantages over ground based radars: they are more
mobile. Because they operate tens of thousands of feet above the Earth, they are not as
subject to radar clutter, and are thus well suited to detect low flying, and in some cases
stealthy, aircraft. The E-3 AWACS and E-2C Hawkeye surveillance aircraft are examples
of current airborne sensors. Their main disadvantage is that they cost more to field and
operate than ground-based radars. The Air Force estimates that the E-3 alone costs $123
million in 1998 dollars. The FPS-117 long range air search radar that forms the backbone
of NORAD’s North Warning System, in contrast costs between $5.8 and $22 million.11
Operating costs for aircraft are similarly higher than operating costs for ground systems.
Unmanned Aerial Vehicles (UAVs) use has increased militarily and commercially.
Some suggest that UAVs could help conduct surveillance over CONUS for enemy aircraft
and cruise missiles. While UAVs cost less to field and operate than manned aircraft,
concerns exist about operating these aircraft over populated areas or in airspace heavily
used by civilian aircraft. The FAA currently prohibits UAVs from flying in commercial
U.S. airspace, although these restrictions could be changed. Also, today’s UAVs operate
sensors optimized for ground surveillance, not air surveillance. Using UAVs for air
defense would require replacing the sensors on current UAVs or fielding new UAVs.
Radars deployed on aerostats, tethered, unmanned balloons, are less expensive than
surveillance aircraft, and can also detect low flying aircraft and cruise missiles. Aerostats
are attractive because of their long “on-station time.” They can remain aloft for months.
Aerostats cannot fly, cannot be moved rapidly, and may prove some hazard to civilian
aircraft. Aerostats are currently deployed by DOD for military purposes, and by the U.S.
Customs Service to search for drug smuggling aircraft and boats. Deploying radars on
manned or unmanned airships (blimps) may be a middle ground between aircraft and
9 Arthur Charo, Continental Air Defense: A Neglected Dimension of Strategic Defense, CSIA,
Occasional Paper no. 7, 1990, p. 43.
10 Protecting the American Homeland, Brookings Institution, Chapter 2, 2002.
11 Paul Mann, “New Air Defense Pact Provides Canadian Takeover of DEW Line,” Aviation
Week & Space Technology, Mar. 25, 1985.
aerostats: costs and flexibility lie somewhere between the two. NORAD and has
reportedly expressed interest in using airships for homeland defense.
DOD is also studying deploying radars on satellites designed to detect and track
moving ground targets, not airborne targets. In FY2005, appropriations conferees cut
$252 million from the Air Force’s $327 million request for space based radar funding.12
DOD is requesting $266 million for FY2007. The first satellite launch of the system is
currently planned to occur in FY2015. Space-based radars applicable to air defense
surveillance may be developed in the long term, but questions about technical feasibility
and cost effectiveness remain.
Command & Control. Several options exist for improving NORAD’s air defense2
C capabilities. One menu of options focuses on improving NORAD’s ability to detect,
identify, and track threats originating from CONUS. DOD has engaged in a $30 million
upgrade of NORAD’s computers to better integrate FAA and military airspace
management systems.13 Other options that might be pursued would be to make2th
permanent, ad hoc C relationships devised after September 11 to integrate NORAD
radars with Customs Service aerostats and with the Navy’s AEGIS ship radars. NORAD
may also wish to find ways to leverage the Civil Air Patrol for air defense. The Civil Air
Patrol is an auxiliary of the Air Force and typically flies disaster relief, search and rescue,2
and counter drug surveillance missions. Properly integrated with NORAD C, however,
the Civil Air Patrol might perform niche air defense functions.
A second menu of options pertains to improving NORAD’s ability to counter an
attack by low flying and stealthy cruise missiles. DOD has attempted to improve theater
air and cruise missile defenses by promoting interoperability among the services and
creating a Single Integrated Air Picture. DOD may consider expanding these efforts to
make them applicable to CONUS air defense. It is not clear that the C2 improvements
designed to counter cruise missile attacks would also help detect and counter threats
originating from inside CONUS. Another option for improving C2 would be to mandate
improved Identification Friend or Foe hardware and procedures for civilian aircraft that
operate near high risk areas. This could help reduce the number of accidental incursions
into restricted airspace (which require a military response and risk downing a civilian
aircraft) but would likely be resisted by civilian pilots due to increased costs.
Intercept. Similar to the options for air defense surveillance, options to intercept
aircraft and cruise missiles can be divided into surface- and air-based, each offering
strengths and weaknesses. Fighter aircraft are well suited to shoot down other aircraft and
cruise missiles. They are inherently deployable and flexible. They also tend to cost more
to procure and operate than other intercept options. Immediately following September
11th, the Air Force began 24 hour combat air patrols over New York and Washington, and
intermittent patrols over other major cities. Cost estimates of these patrols vary between
$100 million to $200 million per month.14 These costs, the strains they put on pilots and
12 H.R. 4613 (108-622) July 20, 2004. p. 325.
13 Gail Kaufman, “Small Job to Shape USAF Network Plans,” Defense News, Jan. 6, 2003.
14 Eric Schmitt, “U.S. to End 24-Hour Fighter Jet Patrols Over New York,” New York Times,
other personnel, and the unanticipated wear and tear they put on fighter aircraft have led
some to recommend reducing these patrols and search for other intercept solutions.15
The costs of using combat aircraft for air defense might be reduced in three ways.
First, combat aircraft could be kept on 15 minute strip alert, rather than having them fly
patrols. During the Cold War, NORAD kept aircraft on strip alert at over 100 sites.16
Some loss of responsiveness would be expected. On January 6, 2002 a private aircraft
flew into an office building in Tampa, FL, passing over MacDill AFB in the process. The
Air Force’s inability to intercept the aircraft before it crashed suggests how strip alert may
be less responsive to intercept needs than fighter CAP. Another way to reduce the cost
of using combat aircraft for air defense would be to design aircraft specifically for this
mission. One company claims it can build an interceptor for $4 million, a fraction of the
cost of modern fighters.17 The feasibility of building such a low cost combat aircraft is
still unproven. A third potential way of reducing aircraft costs would be to field air-to-air
missiles on UAVs. The Air Force is currently experimenting with the Stinger on its
Predator UAV, which reportedly engaged in a dogfight with an Iraqi fighter aircraft.18
DOD operates many SAM systems. The Army’s Patriot, the Marine Corps’ Hawk,
and the Navy’s ship-based Standard Missile, are examples of SAMs that could be part of
a CONUS air defense. SAMs tend to be less expensive than combat aircraft, and carry
more missiles. The Hawk, for instance, costs approximately $25 million, and a battery can
fire 48 missiles.19 SAM warheads are generally larger than air-to-air missile warheads,
which provides more destructive power. Unlike aircraft, SAMs cannot chase enemy
aircraft and cruise missiles, and their deployment must be carefully planned. Unlike
combat aircraft, SAMs cannot visually identify a target and determine if it is hostile.
Regardless of which systems are deployed, a CONUS air and cruise missile defense
system will likely be made up of layered elements. A mix of fighter aircraft and SAMs
(or other options) is typically more attractive than deploying only fighters or only SAMs.
Similarly, defense planners will likely lean toward a mix of surveillance platforms and
sensors rather than just one type. A mix of systems reduces the chance of “single point
failure,” complicates an adversary’s attack planning, and can make a more effective
system. Determining the best mix, however, may be critical.
Mar. 18, 2002.
15 In the two years following September 11, 2001, NORAD scrambled fighters or diverted patrols
more than 1,500 times. Source: Catherine Tsai, “Two Years After Terrorist Attacks, Northcom
Poised to Become Fully Operational,” Army Times.com, Sept. 11, 2003.
16 Adam Hebert, “DoD Weighs Air Defense options as Patrols Become Unsupportable,” Inside
the Air Force, Jan. 25, 2002, p. 1.
17 Greg Griffin, “Colo. Firm Designs Patrol Jet,” Denver Post, Feb. 28, 2002, p. 2D.
18 “The Predator,” 60 Minutes II, aired on CBS, Jan. 9, 2003.
19 “Hawk Surface-to-Air Missile System,” Fact File, [http://www.hqmc.usmc.mil].