Treaty on Open Skies

The Treaty on Open Skies is a multilateral arms control and confidence-building agreement that establishes a programme of unarmed aerial observation flights over the entire territory of participating states. It is designed to enhance mutual understanding and confidence by allowing each state, regardless of size, to gather information about military forces and activities of concern to it. The treaty entered into force on 1 January 2002 and, following successive accessions and later withdrawals, involves several dozen states, largely from the Euro-Atlantic area. It represents a distinctive form of cooperative security, enabling mutual transparency through shared surveillance rather than secrecy.

Historical background

The underlying concept of mutual aerial observation emerged during the early Cold War. At the Geneva Summit in 1955, President of the United States Dwight D. Eisenhower proposed to Soviet Premier Nikolai Bulganin that the United States and the Soviet Union permit each other to conduct surveillance overflights of their respective territories. This Open Skies proposal was intended to reduce fears of surprise attack and alleviate mutual suspicion by providing direct, verifiable evidence of military activities. The Soviet leadership, wary of Western intelligence-gathering, rejected the idea, and it remained dormant for more than three decades.
The proposal resurfaced in the final years of the Cold War. In 1989, United States President George H. W. Bush, a former Director of Central Intelligence, revived Open Skies as part of a broader effort to build confidence and security between NATO and Warsaw Pact states. An international Open Skies conference, involving all NATO and Warsaw Pact members, was convened in Ottawa, Canada, in February 1990. Subsequent negotiation rounds took place in Budapest (Hungary), Vienna (Austria), and Helsinki (Finland).
The Treaty on Open Skies was signed in Helsinki on 24 March 1992 by the United States, Russia, and other states from both alliances. After completion of ratification procedures, including those in Russia and Belarus, the treaty entered into force on 1 January 2002. In the years that followed, additional states acceded to the treaty, including several post-Yugoslav states and Baltic countries. In November 2020 the United States withdrew, and in 2021 Russia announced and then completed its own withdrawal, citing concerns about data handling and the implications of the earlier United States decision.

Objectives and principles

The central objective of the Treaty on Open Skies is to promote transparency in military activities and stability in international relations. By authorising unarmed observation flights over the entire territory of each state party, the treaty seeks to:

• reduce the risk of misunderstanding and miscalculation, especially during periods of tension
• reassure potential adversaries that large-scale offensive preparations are not under way
• provide mutual verification of compliance with other arms control and security commitments
• involve smaller states directly in the gathering of security-relevant information.

The principle underpinning the treaty is that openness and verifiable access to information can serve as a substitute for suspicion and worst-case planning. Aerial observation is therefore used not as a tool of unilateral intelligence advantage but as a cooperative mechanism available to all parties on defined, reciprocal terms.

Membership and depositary states

The treaty has 32 state parties drawn primarily from Europe and North America. These include Belarus, Belgium, Bosnia and Herzegovina, Bulgaria, Canada, Croatia, the Czech Republic, Denmark (including Greenland), Estonia, Finland, France, Georgia, Germany, Greece, Hungary, Iceland, Italy, Latvia, Lithuania, Luxembourg, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Turkey, Ukraine and the United Kingdom. Kyrgyzstan has signed but not ratified, and thus is not a state party.
Canada and Hungary act as depositary states for the treaty, in recognition of their particular contributions to the Open Skies process. Depositary states maintain the official treaty documents, circulate notifications and amendments, and perform related administrative and legal functions.
The treaty is of unlimited duration and is open to accession by other states. Post-Soviet successor states that were not original parties retain the right to join at any time. Applications from other interested countries are subject to a consensus decision by the Open Skies Consultative Commission, ensuring that all existing parties agree on further expansion.

Territorial scope and observation regime

The Treaty on Open Skies covers the entire territory over which each participating state exercises sovereignty. This includes mainland areas, islands, and internal and territorial waters. The treaty explicitly requires that the whole territory be open to observation; states may not designate exclusion zones on national security grounds.
Observation flights can be restricted only for reasons of flight safety, such as air traffic control constraints, hazardous weather, or technical issues. Even in such cases, the observed state is obliged to propose alternative flight plans that still meet the objectives of the observing party. This broad territorial openness is a key element of the treaty’s transparency function, preventing states from shielding particular regions or facilities from legitimate scrutiny.

Aircraft, certification and operating arrangements

Observation flights are carried out using specially configured aircraft that meet treaty standards for safety, equipment and sensor capabilities. Aircraft may be provided either by the observing state or by the observed state. When the observed state provides the aircraft, this is commonly referred to as the “taxi” option, permitting states without suitable aircraft to conduct flights.
All aircraft and their onboard sensors must undergo joint certification and pre-flight inspection to ensure compliance with treaty limitations and to build confidence in the reliability of the equipment. Certification confirms that only permitted sensors are installed, that their technical performance (especially resolution) does not exceed agreed limits, and that there are no unauthorised systems.
Examples of aircraft used under the treaty include:

• the United States OC-135B Open Skies aircraft
• Canadian C-130 Hercules aircraft equipped with a SAMSON sensor pod, owned and operated by a multinational consortium (Belgium, the Netherlands, Luxembourg, Canada, France, Greece, Italy, Portugal and Spain)
• Antonov An-30 aircraft operated by Bulgaria, Romania, Russia and Ukraine
• Tupolev Tu-154 monitoring aircraft formerly used by Russia and Germany
• Swedish Saab 340 (OS-100) aircraft
• a range of aircraft used by the United Kingdom, including the Hawker Siddeley Andover in earlier years and, later, various chartered or allied platforms
• an Airbus A319 acquired by Germany as a future dedicated Open Skies aircraft.

Russia has worked to replace its older An-30 and Tu-154MON aircraft with Tu-204/Tu-214ON platforms. At various stages certain new sensor suites have been the subject of technical and political scrutiny within the treaty framework, illustrating the sensitivity of sensor capabilities in maintaining the agreed balance between transparency and security.

Sensors and technical limitations

Open Skies aircraft carry a combination of commercially available sensors designed to observe and record military-relevant information within agreed limits. The permitted sensor categories include:

• optical panoramic and framing cameras for daylight photography
• video cameras for continuous imaging
• infrared line-scanning devices providing day-and-night coverage
• synthetic aperture radar systems enabling day-and-night, all-weather imagery.

Sensor performance is constrained so that imagery provides sufficient detail to identify major military equipment, such as distinguishing a tank from a truck, without enabling the same fine-grained level of intelligence gathering as advanced national systems. Imagery resolution is limited to approximately 30 centimetres, enough to ensure meaningful transparency while preserving a balance between openness and national security concerns.
New sensor types and improvements in capability may be introduced only by agreement of all state parties, allowing the treaty regime to evolve with technology while keeping changes under collective control. A fundamental rule is that all sensors used in Open Skies operations must be commercially available to every signatory, preventing any state from gaining a unique technological advantage within the treaty framework.

Quotas, flight rights and obligations

The treaty operates through a system of passive and active quotas.

• A state’s passive quota specifies the number of observation flights it is obliged to accept over its own territory each year.
• Its active quota represents the number of flights it may conduct over other states and may not exceed its passive quota.

This reciprocity is intended to ensure fairness in the distribution of observation opportunities. During the first three years after entry into force, each state was required to accept no more than 75 per cent of its passive quota, allowing a phased introduction of the regime.
Certain states operate group arrangements, such as the Russian Federation and the Republic of Belarus, which function jointly for quota allocation purposes. States may schedule flights according to agreed procedures, and detailed flight plans are submitted and refined through pre-mission consultations. Observation missions are typically flown with representatives of both the observing and observed states on board, further contributing to mutual confidence.

Data sharing and transparency mechanisms

A central feature of the Treaty on Open Skies is the collective sharing of data obtained from observation missions. Imagery and related information collected on any Open Skies flight must be made available to any other state party upon request, usually at the cost of reproduction. This arrangement means that even states with limited active quotas, or those lacking their own aircraft, can still gain access to a broad set of observational data.
The pooling of information effectively multiplies the transparency benefits of each flight and creates an incentive for cooperative use of quotas. It also reinforces the multilateral, rather than purely bilateral, character of the treaty, aligning it with broader European and Euro-Atlantic security architecture.

Open Skies Consultative Commission and implementation

The Open Skies Consultative Commission (OSCC) is the principal implementing body of the treaty. It consists of representatives from each state party and meets monthly at the Vienna headquarters of the Organisation for Security and Co-operation in Europe (OSCE). The OSCC:

• reviews compliance with treaty provisions
• approves technical updates and sensor certifications
• settles questions of interpretation and implementation
• takes consensus decisions on requests for accession by new states
• coordinates practical aspects of flight quotas and mission planning.

In addition to the OSCC, national agencies are responsible for training inspection personnel, maintaining certified aircraft and sensors, and organising observation missions. In the United States, for example, the On-Site Inspection Agency (later part of the Defence Threat Reduction Agency) was tasked with training, management and support for Open Skies operations, conducting numerous trial and training flights prior to and after entry into force. Other states maintain comparable structures to ensure that they can host and conduct flights in accordance with treaty rules.

Significance and subsequent developments

The Treaty on Open Skies is widely regarded as an important confidence- and security-building measure in the post-Cold War era. It complements other arms control arrangements by providing an independent, cooperative verification tool that can confirm declared levels of forces and observe military exercises and deployments. For smaller states, the treaty offers a rare opportunity to participate directly in sophisticated aerial observation activities on a multilateral basis.
At the same time, the regime has faced political and technical challenges. Disputes have arisen over flight restrictions, sensor upgrades and the broader strategic value of the treaty in changing geopolitical circumstances. The withdrawal of the United States in 2020, followed by that of Russia, raised questions about the long-term viability and geographical focus of the regime. Remaining parties, however, continue to view Open Skies as a useful instrument for transparency and stability, particularly within Europe, and the treaty’s open-ended duration leaves room for future adaptation and potential expansion as security conditions evolve.