Skies, Spies, and Scientific Surveys – The Legal Aspects of Chinese Unmanned Balloon Flight Over American Territory

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Earlier this month, the US shot down an aerial object which had been seen traversing through parts of Alaska, Idaho, and Missouri. The Biden Administration claims that the object was an unmanned high-altitude surveillance balloon deployed by the PRC as ‘part of a larger Chinese surveillance-balloon program’. The PRC’s Ministry of Foreign Affairs has denied these allegations and purports that the unmanned balloon was ‘a civilian airship used for research, mainly meteorological, purposes’.

In this article, I shall answer a series of questions related to the legality of conducting flights over the territory of a foreign state for meteorological and surveillance purposes, respectively, under the 1944 Convention on International Civil Aviation (“Chicago Convention”).

I shall use the terms “espionage”, “reconnaissance” and “surveillance” interchangeably to denote the gathering of sensitive intelligence from, in, or regarding, the domain of a foreign state. Admittedly, international law does not always consider these terms to be synonymous. IHL treaties characterize “espionage” as an inherently covert activity (API, art. 46; HR, art. 29), whereas doctrine suggests that reconnaissance and surveillance may be conducted overtly (Valentino and Wurster, p. 93; Parkinson p. 247). However, the distinction between covert and overt activities has no material bearing on the legal aspects discussed herein.

Is an Unmanned Balloon an “Aircraft” for the Purposes of the Chicago Convention?

Yes, it is. Annex 7 to the Chicago Convention dictates that “aircraft” includes both manned and unmanned aircraft. Article 2 of Annex 7 clarifies that ‘[u]nmanned aircraft shall include unmanned free balloons’.

Is Obtaining Special Authorization Necessary Before Flying an Unmanned Balloon Over Foreign State Territory?

The answer is conditional on whether the balloon satisfies the definition of a “state aircraft”. The general rule, as coined in Article 3(d) of the Chicago Convention, is as follows:

[n]o state aircraft of a contracting State shall fly over the territory of another State or land thereon without authorization by special agreement or otherwise, and in accordance with the terms thereto.

Article 3(b) dictates that ‘[a]ircraft used in military, customs and police services shall be deemed to be state aircraft’.

There appear to be three general interpretations as to the scope of Article 3(b).

Firstly, the expansive interpretation. This school argues that Article 3(b) is not an exhaustive list. Instead, Article 3(b) merely serves as an identification of the types of aircraft presumed to be “state aircraft” (Wouters, para. 8). These scholars instead define ‘State aircraft as all aircraft owned and operated by the government’ (ibid, para. 1). This expansive thesis originated from the interpretation originally afforded to Article 3(b) by the Air Transport Committee of the International Civil Aviation Organization in ICAO Doc. 6895-AT/695 (dated August 1949) (Cheng, p. 495).  

Secondly, the exhaustive interpretation. This school opines that Article 3(b) is indeed exhaustive and, therefore, it applies exclusively to the types of aircraft listed therein (Wouters, para. 9). This interpretation is supported by, inter alia, the legislative history of the Chicago Convention, and the exhaustive list contained in Article 30 of the Paris Convention on Aerial Navigation of 1919 (the predecessor to the Chicago Convention), which served as the progenitor to Article 3(b) (Cheng, p. 495).

Thirdly, Cheng – who is arguably the leading authority in this field – believes that the true position with respect to Articles 3(b) and (d) falls somewhere in between the two extremes. He opines that ‘all flight craft engaged in activities implying the exercise of State jurisdiction, i.e., jure imperii, may not enter foreign territory without special permission’ (ibid, p. 496).

Under the first approach, any balloon, if owned and operated by a foreign government, would constitute “state aircraft” requiring special authorisation. The function of the balloon, i.e., whether it has been deployed for the purposes of surveillance, meteorological research, or both, would be immaterial for the purposes of classification.

Under the second approach, the classification of the balloon would be conditional on whether it can be deemed as being used in military services. Doctrine is divided as to what this phrase actually denotes (Wouters, para. 11). The prevailing thesis belongs to the “functional” school. It dictates that the phrase encapsulates aircraft actually deployed for conducting military activities, functions, or purposes (even if the aircraft was originally registered as a civilian aircraft and initially performed civil functions) (Bourbonniere and Haeck, p. 903).

In my opinion, aircraft deployed for surveillance purposes would satisfy this interpretation. International treaties and doctrine generally recognise aerial reconnaissance as a military “natured” activity during both peacetime and war (ibid, pp. 902-903). For instance, in relation to the former, the Treaty on Conventional Armed Forces in Europe includes within the definition of “combat aircraft”, ‘aircraft which performs other military functions such as reconnaissance’. Moreover, at the dawn of the Space Age, both Soviet and American jurists characterised satellite reconnaissance as an inherently military activity (Soraghan, p. 463). In relation to the latter, the HPCR Manual on International Law Applicable to Air and Missile Warfare designates aircrafts used for reconnaissance as legitimate military targets.

In relation to meteorological research, the position is even less clear. Cheng has commented on this topic, albeit exclusively in the context of wartime. He notes, ‘in time of war, when the information is required for the planning of day-to-day operations … meteorological flights [may be considered to] constitute an integral part of a belligerent’s military efforts’ (Cheng, p.496). Based on this definition, one may reliably deduce that, during peacetime, meteorological studies conducted as part of planning an armed attack would constitute a military activity. However, it is unclear whether a genuine meteorological survey, undertaken entirely for scientific purposes, but conducted under the auspices of a military branch (say, the air force), would equally be characterised as a military activity.

Under the third approach, one needs again to evaluate the balloon’s function. If the meteorological services performed by the balloon can be characterised as jure imperii they will require special authorisation (Cheng, p.496). Moreover, this approach commands that aircraft used for aerial reconnaissance shall invariably require special authorisation (ibid). Thus, in the author’s opinion, dual-purpose balloons that may perform meteorological services jure gestionis while simultaneously engaging in surveillance, would not enjoy exemption from the ambit of Article 3(d).

The main difference between approaches two & three vis-à-vis approach one is that, according to the former, state-owned aircraft may still be classified as, and enjoy the privileges of, “civil aircraft” (ibid).

Can Article 3(d) Authorisation be Presumed?

Authorisation can indeed be presumed, albeit, under the right circumstances (Cheng, p.496). It is reported that a presumption of acquiescence already exists in relation to flyovers during peacetime, of meteorological balloons without photographic or videographic capabilities, in so far as they are deployed in furtherance of ‘the general promotion of human knowledge’ (ibid). This appears to be the case even if they are operated by a branch of the armed forces (ibid, p. 497). However, no such acquiescence can be identified in relation to aircraft actually or allegedly engaged in surveillance (ibid, p.496).

Does International Law Allow for the Use of Force Against Civilian or Unauthorised State Aircraft?

The use of force against civilian aircraft is generally prohibited. Article 3 bis, as introduced by the Chicago Convention’s 1984 Protocol, dictates that ‘contracting States recognize that every State must refrain from resorting to the use of weapons against civil aircraft in flight’. Article 3 bis is widely recognised as a codification of CIL (ICAO Doc. LC/38-WP/2-2, para. 5.1.4). Notably, in Resolution 1067 (1996), the Security Council condemned ‘the use of weapons against civil aircraft in flight as being incompatible with … the rules of customary international law as codified in article 3 bis of the Chicago Convention’.

Article 3 bis caveats that this prohibition is without prejudice to ‘the rights and obligations of States set forth in the Charter of the United Nations’. The act of shooting down a civilian aircraft is regarded as a use of force against that aircraft’s State of registration (Dörr, para. 23). Thus, the use of such force is also prohibited by Article 2(4) of the Charter, but, wrongfulness may be precluded if force was exercised in self-defence (Trapp, para. 5).

On this point, I wish to briefly engage with a clever argument articulated by Mačák in a previous post. He pleads that it is doubtful whether ‘the act of shooting down a foreign plane over one’s own territory’ would truly be ‘inconsistent with the Purposes of the United Nations’. I believe that it would, most certainly, be inconsistent. The destruction of state-owned aircraft, or aircraft accommodating foreign nationals, is inherently antithetical to the maintenance of peace and security (Article 1(1)). This was made clear by various delegations in response to the shooting down of Korean Air Lines Flight 007 (for statements by delegates from the Republic of Korea, US, Japan and Zaire, see UN Doc. S/PV.2470, paras. 22, 27, 69, 144). Furthermore, in the case of unmanned aircraft, not owned by the state (but still registered therein), I believe that the use of force would nevertheless be in contradiction to the proliferation of the peaceful settlement of disputes which might lead to a breach of peace.

Although Article 3 bis does not apply to “state aircraft”, international law does not recognise an independent right to use force against “state aircraft” exclusively on the grounds of their uninvited or unauthorised presence in a foreign State’s airspace (Wouters, para. 6). The use of such force is likewise governed by the provisions of the Charter.

The use of self-defence is of course contingent on the condition of an “armed attack” (Charter, Article 51). In the context of a balloon genuinely and exclusively engaged in meteorological surveys, I cannot imagine that this condition would ever be satisfied (even if it is operated by a branch of the military). The condition could perhaps be “bypassed” where the survey is a precursor to an imminent armed attack, and force is used to prevent the imminent armed attack (Trapp, para. 5).

What about a reconnaissance aircraft? Would surveillance activities meet the threshold of an “armed attack” or the use of “armed force”? The general consensus is that activities restricted to the gathering of sensitive data do not meet this threshold. An early 20th-century view regarding aerial surveillance can be summarised as follows; ‘[a]erial reconnaissance, while illegal, is not armed attack’ (Wright, p. 851). Moreover, in relation to cyber espionage, contemporary scholarship suggests, ‘[i]t is undisputed that cyber operations that solely carry out spying and data-gathering activities cannot reach the threshold of intensity required to qualify as an armed attack’ (Delerue, p. 341).

It matters not whether these activities are conducted by civilian aircraft, or military “state aircraft”. Military intelligence flyovers which do not involve a physical use of force are considered to fall short of amounting to an armed attack (Brownlie p. 364). This is corroborated by Dörr, who comments, ‘it is very doubtful whether, for example, the violation of airspace … by units of armed forces that remain unopposed, or even undetected—so that no actual use of force occurs—is covered by the prohibition of the use of force’ (Dörr, para. 19).


If the Chinese balloon was engaged in surveillance, its unauthorised incursion would constitute a violation of US airspace sovereignty. On the contrary, a balloon, engaged in meteorological studies exclusively for scientific purposes, is unlikely to require express authorisation, and thus its presence is less likely to be unlawful. However, it should be acknowledged that the US does not come with “clean hands”. The US had previously deployed surveillance aircraft over PRC territory, disguised as meteorological balloons, as part of Project Genetrix, during the 1950s. It is unclear whether this could have any bearing on precluding wrongfulness on the part of the PRC, given the disputed status of the “unclean hands” defence in international law.

Moreover, there appears to be no lawful basis for the use of force by the US (even if the balloon was deployed for surveillance purposes). This is the case, even if the operation was part of a consistent ‘pattern of Chinese behaviour’, as alleged by NATO Secretary General Jens Stoltenberg. The inherent, but “intangible”, harm caused by the mere unlawful collection of sensitive data, even if excessive, is not sufficient to reach the threshold. Instead, espionage must either: (a) inflict a parallel “tangible” harm to, or: (b) be inseparable from a subsequent “armed attack” against, the compromised state. This was recognised in relation to cyber espionage in the Tallinn Manual, which identified two instances where the threshold could be met.  Firstly, where the method or means of carrying out the espionage results in damage to, or destruction of, the compromised state’s cyber infrastructure (Tallinn Manual 2.0, p. 335). Secondly, where espionage was an indispensable component of an “armed attack” (ibid, p. 171).

In his previous post, Mačák cleverly argues that force may be exercised against reconnaissance “state aircraft”, because such aircraft will be considered as engaged in a “hostile mission”. To justify his conclusion, he relies on the shooting down of an American U-2 reconnaissance aircraft by the USSR, in 1960. Regarding the permissibility of the use of such force, he places great emphasis on the fact that ‘the US did not issue a protest against the attack’.

Respectfully, I disagree with his conclusion. Period literature underlines that to justify the use of force, the hostility should extend beyond a pure intent to gather sensitive intelligence through aerial reconnaissance (Wright, p. 851). Moreover, one should not rely on silence on the part of the US as an acknowledgement of the permissibility of the use of such force. Cold War literature has attributed the lack of condemnation by the US – and its decision to accept the wrongfulness of its actions (ibid, p. 850) – specifically to an intention to avoid nuclear escalation (Geelhoed, p. 102, see also here). Moreover, the US never acknowledged that its use of reconnaissance was a “hostile act” (despite its objective wrongfulness). Indeed, this is reflected in an official statement delivered by US Ambassador Henry Cabot Lodge, where he declared that the reconnaissance was devoid of aggressive intent, and merely conducted to monitor possible Soviet escalation (Wright pp. 841-842).

Finally, let us imagine that espionage, of the level of an “armed attack”, had in fact transpired. Would the US response be deemed “necessary” and “proportionate”? Force is “necessary” where the “armed attack” could not have been diverted without resort thereto (Greenwood, para. 27). The balloon was unmanned. Furthermore, the PRC claims it had no directional control over it. Consequently, the US could not have commanded the aircraft to divert its course. Nor could it have demanded that the PRC remotely withdraw the balloon from its airspace. The only non-forcible option available to the US would have been to wait for air currents to possibly steer the balloon away naturally. Surely, this would not be a reasonable expectation of a state subjugated to an “armed attack”. “Proportionality” should be assessed in relation to the harm inflicted upon the injured state (Higgins, p. 231). It is difficult to assess this limb without delving deep into the realm of hypotheticals. However, it would have invariably proved favourable to the Americans’ cause, that the balloon was unmanned, and thus the use of force did not, and could not, endanger human life (onboard). For the foregoing reasons, and despite contentions by the PRC to the contrary, it is entirely plausible that the use of force could have been construed as both “necessary” and “proportionate”, had the espionage reached the threshold of an “armed attack”.

Photo: ‘Sailors assigned to Explosive Ordnance Disposal Group 2 recover a high-altitude surveillance balloon off the coast of Myrtle Beach, South Carolina, Feb. 5, 2023’ (U.S. Navy Photo by Mass Communication Specialist 1st Class Tyler Thompson).

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Claudine Bailey says

March 6, 2023

Interesting, balanced and compelling discourse.

Richard Gardiner says

March 8, 2023

Thank you for a very interesting study.
Is section 10 of Chicago Annex 7 relevant? It states (unless later amended) that the provisions of the Annex do not apply to ‘meteorological pilot balloons used exclusively for meteorological purposes or to unmanned free balloons without a payload’.
If a balloon is civil and used ‘mainly’ but not ‘exclusively’ for meteorological purposes, wouldn’t the requirement in the Annex to carry the prescribed identification plate apply? If not being exclusively used for meteorological purposes, and if there is no plate, could one conclude that a balloon is not civil (or is non-compliant)?
A further thought: would it be relevant to know when the provision in section 10 was included in the Annex and why? If it was to clarify an uncertain situation, and to exempt only balloons ‘exclusively’ used for meteorological purposes from the Chicago Annex regime, would that suggest an implied exemption for only those balloons from the prohibition in article 8 of the Chicago Convention of overflight by pilotless aircraft without special permission?
If so, would the limitation to balloons used ‘exclusively’ for meteorological purposes indicate that other civil balloons do require special permission and, a fortiori, state operated balloons likewise require permission?
Apologies that these are all questions, but I do not have answers!

Batuhan Betin says

March 8, 2023

Dear Richard,

Thank you for your high-level comments and for engaging with the post!

It is a most joyous coincidence, given that I am currently engaging heavily with your treatise on treaty interpretation for a forthcoming book chapter I am authoring!

Formalities aside now, I will divide my answer into four parts, each corresponding to one of your paragraphs. Capitalization is used for emphasis because I cannot underline words in the comments section.

Part I
Firstly, in my opinion, the balloon does NOT fall within the class of instruments contained in section 10 of Annex 7. In my opinion, the term “pilot balloon” refers to a particular type of meteorological instrument (known also as a “ceiling balloon”). It is a more primitive instrument, typically much smaller in stature, and which performs limited functions (such as assessing the altitude of clouds). Albeit not the most conventionally authoritative sources, please find below elucidations regarding the physical characteristics and functions of such balloons.

See also,

The PRC’s balloon appears to have been far more sophisticated than a typical “pilot balloon”, and thus, in my opinion, does not fall within the first limb. As for the second limb of section 10, i.e., the absence of a payload, it is widely reported that the balloon did support a payload weighing over 1000 lbs. (or approximately 450 KG).'s%20payload%20weighed%20more,Command%20and%20U.S.%20Northern%20Command

Thus, the second limb is also not satisfied.

Had section 10 intended to cover all variations of unmanned meteorological balloons, surely, I say, the draftsmen would have omitted the term “pilot”.

Part II
With respect to your first question, yes, I agree that the requirement would apply. However, in relation to the second, I would say two things. First, the question of classification (civil vs state) would depend on whether the auxiliary uses (i.e., the non-meteorological uses) fall within the ambit of Article 3(b). Secondly, the absence of plates or markings would not, necessarily, have a bearing on the classification, but of course it would suggest that the aircraft is not compliant with Annex 7.

Part III
I cannot DEFINITEVELY identify the date on which section 10 was introduced. Unfortunately, the ICAO has not made the previous editions of Annex 7 publicly accessible (at least to me). However, my best estimation, is that it was first included in the fourth amendment (1981). The ICAO claims that the ‘fourth amendment, adopted in 1981, introduces provisions related to registration and nationality marks for unmanned free balloons’. I have had sight of the table of contents to the 1st edition of Annex 7 (as originally drafted) and can confirm that section 10 was absent. This could suggest that the EARLIEST date at which section 10 could have been introduced was 1963, the date of the second amendment.

Turning to the WHY. I cannot identify any relevant commentary on section 10. However, in my opinion, it was intended to address dual use balloons. The second, third and fourth amendments to Annex 7 were all made during the Cold War. It is against this background that I think section 10 should be considered. In the article by Cheng (cited throughout my study), it is reported that during the Cold War, states started to deploy espionage balloons under the guise of genuine meteorological research balloons. For example, in 1956, the US Air Force announced “Project Moby Dick” where it released a series of balloons carrying ‘meteorological instruments, including cameras and radio equipment’ over various European states, claiming that they were genuine research balloons. It was later revealed that “Project Moby Dick” was indeed a covert surveillance mission.

I will address the questions relating to Article 8 and implied exemptions in Part IV below.

Part IV
Thank you for bringing this to light. In hindsight, I should have addressed Article 8 in my original post.
Firstly, there is a factual debate as to whether the balloon was remotely controlled. As mentioned in the conclusion, the PRC denies that it had any directional control over the balloon, whereas the US claims that the balloon was equipped with technology suggesting that it was capable of being maneuvered remotely ( and's%20payload%20weighed%20more,Command%20and%20U.S.%20Northern%20Command=)

Is this relevant for the purposes of Article 8, specifically in relation to whether the balloon was without a PILOT? It does not appear to be. With respect to UAVs (albeit not unmanned balloons), in ICAO Doc 9854 AN/458, the ICAO commented:
An unmanned aerial vehicle is a pilotless aircraft, in the sense of Article 8 of the Convention on International Civil Aviation, which is flown without a pilot-in-command on-board and is either remotely and fully controlled from another place (ground, another aircraft, space) or programmed and fully autonomous.

Implying that pilotless aircraft, for the purposes of Article 8, includes those which are "remotely piloted".

Separately, Appendix 4 of Annex 2 says:
An unmanned free balloon, other than a light balloon used exclusively for meteorological purposes and operated in the manner prescribed by the appropriate authority, shall not be operated across the territory of another State without appropriate authorization from the other State concerned.

However, Annex 2 is comprised entirely of standards and recommended practices. Thus, this is not a legally binding requirement, per se. Annex 2 is also primarily concerned with ensuring safety of flight, and not necessarily concerned with preserving the territorial sovereignty of states.

Undoubtedly, these two sources support the notion that, yes, pilotless aircraft (civil or state), even if remotely operated (or remotely piloted), requires special authorization pursuant to Article 8.

However, Cheng comments that pilotless flight, absent of express authorization, may still be allowed if permissible under CIL (see pp. 501-505). He goes so far as saying that the entire governance of pilotless flight should be regulated by CIL (see p. 504). On this premise he argues that, generally, so far as a state exercises control over the flight, and that the flight does not pose a risk of injury to the state over whose territory it is conducted, such flight can be presumed permissible.

I also want to highlight that during the International Geophysical Year (1957-1958), many states operated on the permissibility of the legality of similar flyovers. In relation to space objects, the Ad Hoc Committee on the Peaceful Uses of Outer Space reported that during the IGY ‘and subsequently, countries throughout the world proceeded on the premise of the permissibility of the launching and flight of space vehicles which were launched, regardless of what territory they passed “over” during the course of their flight through outer space’ (A/4141 pp. 63-64). Of course, space objects are distinct legal entities from aircraft. However, from Cheng’s account of state practice from the 1950s (including official statements by the US and the USSR), one could also conclude that a similar presumption may have existed in relation to aircraft used genuinely, and exclusively, for scientific studies (see pp. 488, 497).

In short, I would summarize the final position as follows: despite Article 8, in relation to certain types of civil aircraft, depending on their function and the circumstances of their flight, special authorization is to be presumed. On this basis, I would also say that the presumption of exemption applies to a wider class of aircraft than that which you have proposed in paragraph 3 of your comment.

I hope these comments are helpful.

Richard Gardiner says

March 9, 2023

Many thanks for your extremely informative analysis. It will be interesting to see whether this incident prompts any investigation of these issues at ICAO or elsewhere.