How has traffic been managed in the sky, on waterways, and on the road? Comparisons for space situational awareness (part 2)by Stephen Garber and Marissa Herron
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| A common theme throughout all these domains is that safety is about responsible use. |
One distinction between management of air traffic, whether piloted or not, and STM is that some conjunctions in space occur at a set frequency (i.e., every 90 minutes in LEO). While unmanned aircraft systems (UAS) may hover over a particular spot on the ground and air traffic controllers sometimes direct airline pilots into holding patterns, neither type of aircraft routinely flies in a circular, predictable orbit. Maneuvers in the air and on the road occur rapidly, as pilots and drivers can often either collide with or avoid each other within seconds. Typically conjunctions on the seas happen more slowly. Traditionally (non-automated and non-propulsive) satellites have had relatively long lead times to avoid conjunction, but if large constellations of automated and propulsive spacecraft do become a reality, the reaction time likely will be cut dramatically.
Another way to think about this is to consider what causes people to adhere to set rules. An interesting criminology paper proposes two reasons drivers do not break traffic rules: perceived likelihood of being caught and, perhaps more fundamentally, treatment by police. In terms of the latter, if motorists are treated with dignity and respect by the police, they are seen as more likely to see themselves as part of a larger social group and thus feel more of a “moral compunction” to obey the law.[1] In the absence of a legitimate STM enforcement authority, it may especially behoove actors to collaborate for the governance and technological reasons identified above, as well as the fact that space is a global commons and thus all space actors are part of the same global spacefaring community.[2]
A common theme throughout all these domains is that safety is about responsible use. For more details, we proceed to examine how traffic management has worked for airplanes (both piloted and unpiloted), ships, and motor vehicles.
Virtually since its beginning, air traffic control has had to address the issue of nations lofting planes whose locations and routes they did not want to disclose, for military reasons. The landmark 1944 “Chicago Convention” for international aviation law, which created the International Civil Aviation Organization (ICAO), specifically excluded “state aircraft” (military airplanes), and no such distinction exists for space objects.[3] The solution was the adoption of the “due regard” convention that has permitted nations not to disclose the whereabouts of military planes provided they didn’t endanger other aircraft. This “due regard” convention also “placed the full burden for the avoidance of collision on the state aircraft in exchange for the ability for those aircraft to operate outside the common rules, including the ability to be undetectable by other operators and service providers.”[4] A parallel argument to STM could certainly be made.
Modern air traffic control traces its origins back to a 1956 fatal mid-air collision between two planes operating over the Grand Canyon. The resulting 1958 Federal Aviation Act transformed the existing Civil Aeronautics Authority into the Federal Aviation Administration (FAA). This legislation created an independent, unified agency to promote and develop air safety for both civilian and military aircraft.[5]
Approximately 44,000 airplanes operate in US skies every day and, during busy periods, about 5,000 planes are in the air simultaneously.[6] Once a commercial flight takes off, the pilot activates a transponder, which broadcasts a signal indicating the plane’s “flight number, altitude, speed, and destination.”[7] US airspace is divided into 21 zones and then sectors. Also within each zone are Terminal Radar Approach Control (TRACON) airspaces that handle flights into and out of numerous airports. A useful analogy to planes being “passed off” to different flight controllers is a “zone” defense utilized by a football or basketball team.[8]
| In space, a traffic management system needs to handle both active (highly autonomous and less so) and inactive satellites (debris) and thus an integrated, rather than parallel, ecosystem seems to be a more apt metaphor. |
As of January 1, 2020, the FAA also has required that aircraft operating in most controlled US airspace be equipped with Automatic Dependent Surveillance-Broadcast (ADS-B) technology. ADS-B works by periodically, automatically, and actively broadcasting an airplane’s position information to ground-based air traffic controllers, satellites overhead, and other aircraft. More than just a very useful tool for controllers, it also provides aircraft-to-aircraft communications in time-critical situations. ADS-B In is an optional tool for pilots to increase their situational awareness by obtaining information about nearby aircraft that also have this equipment aboard. ADS-B is a key technology in the FAA’s Next Generation Air Transportation System (“NextGen”), which shifts more of air traffic control from ground-based radars to satellites.[9] In addition to being useful for air traffic control, ADS-B technology can be invaluable for search and rescue operations and is analogous to the Personal Locator Beacons (PLBs) that many backwoods hikers use.[10]
Pilots of small planes (“general aviation”) often operate under visual flight rules (VFR) (“see and avoid”) and are encouraged, but not required, to file flight plans with the FAA. Larger commercial flights are equipped to fly in more inclement weather and thus operate under instrument flight rules (IFR) and are separated by the FAA’s formal air traffic control system. Largely due to federal deregulation of the airline industry in the 1970s, air travel has significantly increased in the last 50 years. Since the construction of new airports and runways hasn’t kept pace with the number of flights, the FAA and NASA have utilized new technologies, such as GPS, to automate the system with limited infrastructure.[11]
In recent years UAS have proliferated exponentially: in 2019, various experts estimated that there would be 700,000 UAS in US airspace in 2020 and three to six million by 2021 with millions flying daily.[12] UAS are also called unmanned aerial vehicles (UAV), remotely piloted aircraft systems, and, most simply, drones. Traffic management of UAS presents a cogent analogy to SSA of remotely controlled Earth-orbiting satellites.[13] Just as with spacecraft, there are diverse groups (i.e.: large, small, commercial, nonprofit, federal, and international organizations) of pilots who operate UAS. Because of the very rapid growth in UAS among governments, companies, and hobbyists, the FAA and NASA have been working intensively to devise a suitable UTM system. In 2017, a (likely unintentionally ironically named) UTM Pilot Program began at three sites.[14]
Additionally, NASA researcher Parimal Kopardekar received a UTM patent that includes a concept called National UAS Standardized Testing and Rating to assess the maneuverability of a particular vehicle and thus which one should move given a potential conflict. The issue of maneuverability is very applicable to STM.[15]
As with some new constellations of satellites, the FAA-NASA UTM system is designed to be largely automated and “cloud-based” in which Machine to Machine (M2M) or Vehicle to Vehicle (V2V) communication will be key so that UAS can autonomously detect and avoid other UAS. There will also be the periodic need for “dynamic restrictions” when, for example, law enforcement or medical flights would take priority over hobbyists’ flights. Additionally, a NASA site notes that “Each user will have the same situational awareness of airspace.”[16]
 Parimal Kopardeker and Eric Mueller, “The Science of Drones” presentation (undated), available here, p. 12. |
UTM is often depicted as a separate, parallel “ecosystem” to air traffic management. Because of this, some experts have suggested that future UAS be “dually capable” to operate in both UTM and ATM environments, leading to a longer-term goal of an integrated National Airspace System.[17] In space, a traffic management system needs to handle both active (highly autonomous and less so) and inactive satellites (debris) and thus an integrated, rather than parallel, ecosystem seems to be a more apt metaphor.
One Department of Homeland Security (DHS) site also mentions that “Anyone flying in the UTM system will need an interface to a UAS Service Supplier (USS) to submit flight intent to other users and receive authorizations for specific access.”[18] The FAA can penalize operators who fly UAS too close to airports, but it doesn’t have criminal enforcement authority and other federal departments, such as the Department of Defense and Department of Homeland Security, can interfere with illegal drones in limited circumstances.[19] In addition, local and state law enforcement have the authority to ensure public safety and security by policing UAS appropriately at these levels. For example, if a UAS is flying over a crowd or violating local voyeurism laws, local police have full authority to deal directly with this use of technology to break laws and report it to the FAA later.[20]
In December 2019, the FAA proposed new regulations to permit the tracking of virtually all drones. The regulations would require all UAS weighing more than about half a pound to carry remote ID technology that broadcasts over the Internet, enabling federal authorities to identify the UAS’ owner/operator. UAS operators of this size have been required since 2015 to register with the FAA, but not to include remote ID technology.[21]
Analogous to remote ID technology on UAS or ADS-B, a recent development in SSA is the concept of an electronic “license plate” for space objects. A Los Alamos National Lab team has created a device less than a square inch in size containing a laser that blinks a unique identifying code multiple times each second. Such extremely low-resource optical identifiers (ELROI) require no external power source so could be attached to rocket bodies and would work even if a satellite was no longer operational.[22] This could be a more convenient alternative to registering satellites internationally to help identify space objects.[23]
| Could satellite operators employ a technology similar to AIS, at least for active satellites, that would help avoid collisions in space? |
A similar concept of a GPS transponder was proposed a couple of years ago.[24] The ELROI or GPS transponder concept is also somewhat analogous to an airplane’s transponders, which a pilot can turn off, or perhaps more precisely to the underwater locator beacon in a plane’s “black box.” Another limitation of technologies such as ELROIs or GPS transponders is that they only potentially work where you put them—that is, an owner/operator can’t put such a tracking device on every potential piece of space debris.
As with a UTM system, an “STM system must accommodate safe operations in the presence of non-participants.” Space is, of course, inherently international with a very significant national security component. Military and intelligence personnel, as well as international or domestic rogue actors, likely do not want to disclose fully their spacecraft’s positions, maneuvers, and capabilities. “While conjunctions between participating and non-participating spacecraft cannot be addressed using the STM system, standards could be developed to encourage information sharing…To be politically palatable, an STM system cannot compel O/Os [owner/operators] to cede operational control of their spacecraft to space traffic management service suppliers (S3s) or a government entity.”[25]
Figurative “rules of the road” actually predate roads and have evolved over centuries for boats and ships traveling on water. This long history of maritime navigation, particularly on the international high seas, likely makes it another good potential analogue for STM. Certain basic rules always apply, such as an international rule that “every vessel shall at all times maintain a proper look-out by sight and hearing as well as by all available means” to avoid collisions.[26] The 1982 Law of the Seas treaty calls for, among other things, ships to be seaworthy, be registered, have trained crews, use signals to avoid collisions, and to reduce and control marine pollution as much as possible—the parallels to STM are notable.[27] A general hierarchy applies to the right of way: an overtaken vessel has top priority down to power driven vessels having the lowest priority. Similarly, motor boats should yield to rowing shells. In other words, as in other domains, more capable vehicles yield to less capable vehicles.[28]
An interesting connection between UTM and maritime navigation is self-identification. Historically, ships have flown flags of their countries of registration and, more recently, the electronic Automatic Identification System (AIS) for ships has become widely used. AIS uses VHF transponders onboard the ship so they are self-identifying and communicating. AIS not only identifies a ship, it also delivers navigation information to nearby users. The information enables the operators to communicate and coordinate their navigation intentions directly with one another.[29] The benefit of direct operational communications is to enable efficiency and accuracy during time critical events. Of course, the point of this communication is to avoid maritime collisions, which would damage the marine environment and potentially cause human injuries or even deaths.
Could satellite operators employ a technology similar to AIS, at least for active satellites, that would help avoid collisions in space? To some extent, existing services such as SDA already provide a similar situational awareness capability. These collision avoidance services facilitate operator-to-operator communication based primarily on ground sensors, with some operator data. In other words, the satellites do not communicate directly with one another. Furthermore, the service relies on knowing the identity of the operators associated with the active satellites. Is it possible to have a future where the satellites can communicate and de-conflict conjunctions onboard with one another? For now, the addition of self-identifying technology is worth consideration. The implementation of an AIS-like system in space will rely on easily implementable, cost-effective, self-identifying detection and communication technology for users. The IMO Convention of the Safety of Life at Sea requires AIS on transport ships and passenger ships. Similarly, AIS could be initially implemented on a select category of satellites.
Perhaps satellite owner-operators also can learn from legal frameworks regarding shipwrecks. The Nairobi International Convention on the Removal of Wrecks, which went into force in 2015, addresses three basic questions: who is responsible for a wreck, what measures can be taken based on that responsibility, and how can the responsibility be enforced? In short, the convention mandates that ship owners are financially liable for wrecks, whether they pose a navigational or environmental hazard, and requires them to have insurance to cover the costs of wreck removal. Generally speaking, if a shipowner does not adequately or promptly mark or remove a wreck, the relevant coastal nation can do so at the registered owner’s expense. While the US is not a signatory to this convention, the 1987 Abandoned Shipwreck Act provides authority for the relevant coastal state to handle wrecks. Canada is a signatory to the Nairobi convention and, additionally, under Canadian law, if the owner is unable financially to pay for removal of a shipwreck, there is a “fund of last resort” for marine oil spills that potentially could be used.[30]
| The “one size does not fit all” analogy to STM is apt: just as motor vehicles and boats are diverse, so too are satellites. Thus uniform assessments and analyses are not always appropriate. |
In this context, shipwrecks are analogous to orbital debris, with which operators obviously want to avoid colliding, and thus perhaps new kinds of financial incentives to remediate orbital debris are needed. One legal scholar notes that not only do maritime salvage principles apply to space debris in the limited sense that actively removing debris would protect the owner of the debris from third-party liability, but that the 1989 Salvage Convention explicitly permits salvors to remove wrecks to prevent or minimize environmental damage, a direct parallel to the global commons of space.[31]
International maritime collision avoidance regulations (COLREGs) were last updated in 1972 and took effect in 1977 after problems with traffic separation in the English Channel. From 1956 to 1960, there were 60 collisions in the Channel. In 1967, a new traffic separation scheme became voluntary and in 1971 the International Maritime Organization passed a resolution making it mandatory. After the new regulations took effect, the number of Channel collisions dropped significantly.[32]
An important consideration for how travel is governed on waterways is that US Coast Guard navigation “rules do not grant privileges or rights, they impose responsibilities and require precaution under all conditions and circumstances.”[33] Thus prudence, common sense, and prioritizing safety should prevail so that all boats and ships may share waterways, a common resource.
As with travel on water, drivers on land are expected to abide by certain international customs such as driving more cautiously when other vehicles are present, in poor weather, and at night. Also, drivers of motor vehicles should be careful around and yield the right of way to less capable vehicles such as bicycles. Rules such as which driver may proceed first when two or more simultaneously approach a four-way intersection may be clear in theory (the driver to the right may proceed first) but are not necessarily adhered to in practice. When entering traffic on a highway, drivers are expected to yield to the ongoing traffic and merge when there is sufficient time (given their vehicle’s ability to accelerate) and indicate their intentions to maneuver (with a turn signal). A parallel could easily be made to planned satellite maneuvers, particularly when transitioning through different altitudes or joining a constellation.
Moriba Jah also points out that we regulate vehicles on various criteria such as size, maneuverability, weight, hazard potential, and so forth. “Trucks carrying hazardous fuel are regulated differently than Vespa scooters, Oil Tankers on our seas are regulated differently than kayaks and canoes. So, why would we treat all Resident Space Objects as the same thing…cannonballs?” The “one size does not fit all” analogy to STM is apt: just as motor vehicles and boats are diverse, so too are satellites. Thus uniform assessments and analyses are not always appropriate.[34]
Another aspect of roadway travel that is relevant to other domains like space is the use of automating technologies. For example, newer cars tend to have more safety features such as blind spot avoidance when shifting lanes, anti-tailgating automatic braking technology, and backup cameras that turn on when a driver shifts into reverse. Typically higher levels of automation are tested first in restricted areas before being allowed to integrate with existing, lower levels of technology. Similarly, the advent of self-driving vehicles presents both challenges and opportunities. Some municipalities are experimenting with self-driving vehicles for multiple passengers as a potential new form of public transportation.[35] Could the approach we are taking to integrating new technology into our daily lives be applicable in space, as with the SpaceBees situation of employing new tracking technology on difficult-to-track satellites or the proposed increased autonomy of satellites?
One caveat about the roads analogy is that they are geographically constrained areas typically under national jurisdiction (as well as sometimes state and local jurisdiction), rather than an inherently international environment such as space or the high seas.[36] Nevertheless, it still may provide some useful touchstones.
 Comparison of key issues in various types of traffic management. See full chart. |
So what can we apply from this discussion of analogous “rules of the road” to the changing nature of space operations and thus SSA, as well as STM regulatory oversight and policy coordination? A strong argument can be made that the federal government’s role should be to focus on setting STM guidelines and developing standards, rather getting too involved in operational details for the disparate community of space operators. The latter would be impractical, if not impossible, given the many foreign nations, consortia, and commercial companies now operating in space. As one analyst notes, negotiating international “standards of behavior for the purpose of collision avoidance” is an inherently governmental function.[37] Just as NASA and then the federal government have promulgated standard practices for orbital debris mitigation, so too could the government “promote efficient and effective space safety practices”[38] that encourage responsible activity in space. NASA and other federal organizations can also enable the development of tools to manage “big data” for large constellations of satellites, essential to do conjunction assessments with many satellites. NASA and other federal organizations can and do also promote data quality and technical standards in a variety of areas, including STM.[39] Crafting national policy will need to be done carefully, with due consideration of various domestic viewpoints including the distinction between rules and standards or guidelines, as demonstrated by the recent controversy over the FCC’s updating of some of its rules regarding orbital debris.[40]
| An important consideration for UTM, ATM, and STM is whether a “top-down” or “bottom-up” system is more feasible for each of these domains. |
In previous years, some people have argued for a centralized, international body to handle STM akin to the ICAO, which sets international standards for ATM that individual nations’ governments implement; however, changes in the commercial space sector and other factors no longer make this approach realistic.[41] As a former air traffic controller who now studies SSA noted, safety on the high seas (a domain, like space, that no nation-state owns) relies on “sea faring nations of the world to enforce the agreed upon standards.” She also contends that while space technology is more similar to that of aviation, “international maritime agreements may provide the more instructive model” for STM.[42]
An important consideration for UTM, ATM, and STM is whether a “top-down” or “bottom-up” system is more feasible for each of these domains. A top-down system entails a centralized body with enforcement authority, while a bottom-up system relies on decisions made by lower-level participants. For piloted aircraft, there is a civil national air traffic control system run by the FAA and an international one run by the ICAO that works reasonably well with centralized oversight and decentralized execution. The Pilot in Command rule still applies: he or she has ultimate responsibility for the actions of his or her aircraft.[43] This rule also applies to ships and to satellites. The SDA is representative of a bottoms-up approach, as commercial satellite owner-operators needed a collision avoidance service, so they banded together and created one in the absence of a regulatory regime. As one author noted eight years ago, but after SDA’s creation, “commercial space entities themselves have incredibly large incentives for safe operations…and thus industry has even create[d] self-regulation to fill in gaps in government regulation in certain instances.”[44]
At another level, rules certainly should be, but are not always, followed. Police officers can ticket and hold accountable speeding drivers, for example, but law enforcement and regulatory authorities cannot identify all infractions in all domains. Society typically accepts that these infractions occur at a sufficiently infrequent rate that more stringent accountability mechanisms are not necessary. The awareness of a potential ticket and the desire for safe passage are often, but not always, adequate to encourage proper behavior. When that minimum level of negative or positive incentivization is no longer adequate, then stricter accountability mechanisms (e.g.: radar guns) are imposed. Similarly, SSA operators will commit infractions and not all infractions will be caught but most are incentivized towards responsible behavior. Of course, the safest way to prevent a collision is never to fly in space, just as the safest way to avoid a car accident is never to get in a car.
In years past, DoD was the reluctant leader in SSA because the SSN/CSpOC had the only viable capabilities, yet providing SSA information to all sorts of space operators was not considered (and is not) an inherently military mission. More recently, however, with the ready availability of advanced analytics, the growth of commercial space, and other factors, individual companies’ and consortia’s SSA capabilities have begun to outstrip the military’s. The advent of commercial capabilities offers the opportunity for a traditionally, inherently governmental function now to be accomplished by non-governmental entities.
Thus, DoD has begun to ride the train driven by the commercial sector, to use another transportation metaphor. As a commercial provider notes, “SSA is not the core function of the CSpOC, and 90 percent of the SSA warnings issued apply to commercial or international satellites rather than military”[45] and so the military and civilian segments of the federal government presumably have welcomed the commercial sector’s increased role. Taking this notion even further, General John Raymond, head of the new US Space Force, recently testified that “We are absolutely reliant on commercial space capabilities today, and I think we’re going to be more reliant on it in the future…They have operational capabilities that are relevant, and we are eager to develop an architecture that capitalizes on that.”[46]
| Just as driver’s education in the classroom and behind the wheel is incentivized through decreased insurance premiums, might educational incentives for safe and responsible space operators be a good idea? |
One area that the federal government can do, does in other sectors, and probably should do here is conduct more basic research on specific SSA technologies, as well as systems of tools. As one expert argues, “While the commercial sector is already innovating to a certain degree, there is still a strong need for research into future technologies to improve SSA and tackle emerging challenges such as large constellations, tracking and identification of CubeSats, and increasing the accuracy of conjunction assessments.” In effect, NASA and perhaps other federal organizations could “seed” the development of SSA technologies that the commercial sector would then develop, adapt, and adopt.[47] Another research topic that the federal government could pursue is system-level STM simulation, just as NASA and the FAA have conducted air traffic control research and simulations for many years. This could be especially important given constellations of autonomously maneuverable satellites.[48]
Overall, more voluntary coordination is needed, both for SSA operations and for STM policy in the rapidly changing space environment, but many questions remain. Just as pilots file flight plans about their intended paths, a spacecraft’s insertion and intended orbit is outlined during the licensing process, but there is no comprehensive mechanism for communicating future orbital maneuvers to all relevant space operators. Just as driver’s education in the classroom and behind the wheel is incentivized through decreased insurance premiums, might educational incentives for safe and responsible space operators be a good idea? If so, how do we oversee and hold accountable the users and nations within the global commons of space? Perhaps a voluntary code of conduct or set of best practices for space owners/operators would be useful, but would this suffice? Carefully looking at precedents of how traffic has been managed in other domains should provide policymakers and other space stakeholders with guidance on the best mixes of regulatory coercion and norms-based cooperation, carrots and sticks, in the increasingly complex STM environment. Ultimately, rather than dictating a complex and burdensome regime, a successful STM system will enable and encourage responsible behavior and use of the space domain.
Thank you very much to Bill Barry, Brad Brewington, Alvin Drew, Anna Gunn-Golkin, Diane Howard, Moriba Jah, Dana Johnson, Parimal Kopardekar, Jerry Krassner, Bhavya Lal, Adele Luta, Michael Mineiro, Steve Mirmina, Mark Mulholland, David Murakami, Michele Ostovar, Matthew Schaefer, Robert Sivilli, Tiffany Smith, Bryan Tipton, Quentin Verspieren, and Brian Weeden for all their helpful comments and insights.
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