The benefits of advanced Unmanned Aircraft Systems (UAS) operations can only be enabled with near-term implementations of federated Unmanned or Universal Traffic Management (UTM) systems. Industry and government regulators often delay system implementations until they address future predicted densities and operations. We need to stop worrying about future densities so we can implement UTM systems quicker. The aviation system did not wait for advanced Air Traffic Control (ATC) automation platforms, datalink communications, and ADS-B to start managing the commercial airline traffic in the 1920s, instead they started with simple procedural, strategic ATC. Predictions of great densities are often proven wrong. Remember when Very Light Jets were going to darken the skies in the early 2000s?
Instead of solving problems that do not yet exist, we must focus on the minimum viable product (MVP) that meets the current need for advanced operations, in terms of both UTM and the Unmanned Aircraft (UA). UTM needs to evolve like the internet did since it is basically the internet of the sky. We only need the necessary pieces to support safe operations and digital interoperable data exchanges that are built for future scalability. One key tenant of a minimum viable product approach is that you don’t know what all the requirements are until you start using a system, so you need to start using it sooner and improving it over time. The developers of the original internet wouldn’t have imagined the scale which it is used today to enable mobile streaming services and secure banking transactions; neither could the early ATC pioneers envision the air transportation system in existence today.
Strategic separation and UA performance
The UTM approach to strategic separation, as trialed during NASA UTM TCL4 in 2019, the FAA’s UTM Pilot Program Phase 2 (UPP2) this year, and being implemented by the ASTM F38 UTM group, is inherently flexible when it comes to UA performance. Deconfliction is done by separating operational volumes horizontally, vertically, and in time (4 dimensions – 4D). This is an inherently performance-based approach to separation, which is sensor and UA performance agnostic. The minimum size of each operational volume is based on calculated Total System Error (TSE) for the UA and its environment. This TSE is based on a combination of path planning error, position error, navigation error, and can further include environmental effects such as turbulence or winds aloft data uncertainty. The most important part of this UTM separation scheme is not the UA performance itself, but properly characterizing and bounding its error.
So, how can this approach lead to near-term implementations? The key is starting with larger 4D volumes, and then make those smaller over time. Right now, there are almost no true Beyond Visual Line of Sight (BVLOS) UAS operations, so initial operations can take up more space than they will in the future. Operators can start by working with their USS to define an overly conservative TSE and much larger operational volumes with more time uncertainty than the true TSE (i.e. 3-5X larger as a conservative starting point). They can operate in these larger than needed volumes which bound the errors while the operator and USS collect relevant data to better characterize their errors. This will enable refinement of the TSE estimate over time, leading to the gradual reduction in the size of the operational volumes. Additionally, new positioning and navigation technologies and weather forecasts will improve over time, resulting in lower TSE, smaller volumes, and more network/airspace throughput. Big data and analytics can help determine needed volume sizes based on predicted and observed vehicle performance, winds, etc. These predictions can then be delivered to the operator via UTM enabled data services and integrated into their aircraft systems. This is similar to how RAIM can be used to predict GNSS performance during pre-flight planning activities.
A similar performance approach was taken for Reduced Vertical Separation Minimum (RVSM) airspace and Required Navigation Performance (RNP) routes, where larger separations were used until operators were able to better define and limit their TSE. Like with RVSM and RNP, we will also need a set of standards to help UAS operators and their USS characterize, determine, and verify the TSE for UA operational volume exchanges.
An important aspect of UTM separation is that the 4D operational volumes are flexible in their shape and size. They can represent an entire operation, a trajectory-based operation, an area-based operation, or a combination. UAS do different missions than manned aircrafts, so are less likely to do point-to-point flights. They may follow a trajectory to a search aream, then fly random patterns within that area, before returning along a trajectory at an uncertain time. An operational intent made up of 4D volumes can capture that complexity.
Unlike traditional aviation routes and flight levels, UTM 4D volumes don’t need to be pre-defined for an airspace. They capture both the separation provided by airspace structure (routes and flight levels) and aircraft performance (RNP, RVSM, etc.). Also, unlike traditional airspace structures, UTM volumes can be updated at any time, including in fight, based on observed conditions or vehicle condition. This is similar to the Dynamic RNP concept (https://faaco.faa.gov/index.cfm/attachment/download/49458), but operator managed instead of ATM directed. If an onboard system degrades, the operational volume can be increased to account for the degradation.
To provide strategic separation, these 4D volumes cannot overlap in both space and time. In the draft ASTM F38 UTM standard, operational volumes can abut each other with no external buffer. Thus, the level of safety is set by defining how the volumes are defined and created. Regulators, with airspace safety in mind, need to help define acceptable safety. Are two volumes based on 95th percentile TSE abutting safe for this strategic layer of the ICAO conflict management model from ICAO Doc 9854, Global Air Traffic Management Operational Concept, or is a more rigorous 99.999th percentile TSE needed? This safety analysis requires additional work, but should assume that there would be additional tactical mitigations available if a vehicle exits its operational intent volumes.
Given that initial 4D volumes will be large to conservatively capture all elements of TSE, there will be a need to reduce them in 3D size and time uncertainty as airspace densities increase and there is additional needs to fly more precise routes over the ground to address ground risk and noise considerations. Industry or government airspace managers can reduce the allowed UTM volume sizes in areas where the density warrants. Market forces can also be used to incentivize smaller volumes. If problems arise with operators “hogging” the airspace, additional UTM services can be introduced to address negotiation, flight plan overlaps, fairness, and equitable access to airspace. However, solutions to those problems are not necessary today when a simple, first-reserved, first-served principle will work adequately. The need for advanced UTM airspace management services should be demand-based, not based on existing airspace classes since those classes were established based on current manned aircraft operations.
We built the existing ATM system over time – first with flags, then radios, procedural control, radar control, and now NextGen operations. The internet was also built over time with increased capabilities added as technologies evolved and demand warranted. We should allow UTM to evolve in a similar manner, with simple, safe separation mechanisms—the minimum viable product—implemented now with large operational volumes. Smaller volumes and additional services can be added in the future. As demand occurs, we can implement solutions with the best technology of the day instead of provisioning for all future operations and densities now. We shouldn’t plan for the future with today’s technologies. Instead, we must enable advanced BVLOS operations today utilizing current UA performance and UTM strategic separation services based on the ASTM F38 UTM standard, leaving the solution open-ended enough to incorporate future needed enhancements and technologies.