The potential utility and benefits of Unmanned Aircraft Systems (UAS), or drones, are well known. They can fly to accident sites ahead of first responders and send video to help them prepare. They can reduce congestion and carbon emissions by taking package delivery airborne. They can inspect utility and oil line infrastructure faster and more safely than traditional methods, all at much lower costs. For these use cases and many others to be fully realized, the drone must be able to navigate safely and accurately beyond visual line-of-sight (BVLOS) of a human pilot.
To navigate, almost all drones rely on signals from global navigation satellite systems (GNSS) like GPS, Galileo and BeiDou. These signals are freely available and highly effective almost everywhere and almost all of the time. But they can be intermittently obscured by buildings and terrain, creating patches of degraded reception that can affect navigation performance – and this creates challenges for UAS. In order for a UAS to fly BVLOS, the operator should determine if an area always has acceptable GNSS, or determine where/when degradation occurs so they can fly around those areas at those times.
Establishing the likelihood of GPS/GNSS degradation or failure
Spirent’s GNSS Foresight Risk Assessment service provides quantitative data to determine the likelihood of GPS/GNSS degradation/failure based on the flight environment at any time. For example, as seen below, a Foresight risk analysis was performed for the rural area of Outer Banks of North Carolina. The heatmap areas are colored based on the worst GPS/GNSS performance 95% of the time. This corelates directly with how GPS/GNSS is tested in ADS-B, IFR avionics, and other FAA/IACO requirements and this risk assessment provides the quantitative data in relation to the FAA Order 8040.6 Risk Matrix. If the operator stays in the green and yellow shaded areas of the heatmap the likelihood of failure is extremely remote.
Risk analysis for urban and suburban areas
The Foresight GNSS Risk Analysis can be performed for urban, suburban and rural zones to identify areas that are always safe to fly, and areas that have more frequent degradation and would require a specific day/time forecast to avoid GPS degradations. As shown below, downtown Indianapolis, Indiana has a number of areas that fall into the worst-case degraded category. Essentially the risk analysis tells us that GPS degradation is too frequent for safe flight in the orange and red without more detailed analysis. The operator can simply avoid these areas, or if needed, take additional steps to determine when and where degradation occurs to avoid it on a per-flight basis.
Spirent GNSS Foresight Risk Analysis heatmap for urban district
In order to fly in the areas that are not always safe (worst-case red and orange), the best-case GNSS coverage map can determine if it is ever safe to fly (seen in #2 below). The best-case scenario coverage map shows that there are times in most of the downtown areas where GPS coverage is safe for flight. However, since there are frequent degradations, a Risk Analysis is not sufficient for safety, and a GPS coverage analysis is needed for each flight. To address this requirement, GNSS Foresight Live provides a forecast for the current day plus 48 hours with 1 second granularity. GNSS Foresight Live is a cloud-based service accessed directly or through a UTM provider, that determine the safest areas or routes in a suburban or urban area.
As shown below, the Foresight solutions can determine where GPS coverage is always good and where it is variable (#1); if it is variable, where it is never safe to fly (#2), and for any given time in the next 48 hours where the degradations are located that must be avoided (#3). The goal of all of these solutions is to quantify GPS performance and likelihood of degradation or failure. This directly conforms to the FAA Order 8040.6 safety risk management framework.
FAA waiver requirements for GPS safety risk management of UAS operations
To be authorized to fly in any zone within the United States, the UAS operator must obtain an FAA waiver. The FAA’s Order 8040.6 establishes the policy requirements for UAS Safety Risk Management (SRM). Order 8040.6 codifies a methodology for conducting SRM for UAS requests to operate. For those submitting requests for a UAS to operate beyond current regulation, like BVLOS, the applicant must provide a verifiable framework for analysis of GPS degradation or failure, and the impact this has on risk.
The severity definition varies depending on the flight environment and aircraft equipage. When determining the severity, the applicant’s analysis must account for how GPS is used not only for navigation, but also for flight stability, surveillance, geofencing, and collision avoidance. GPS is typically the primary source of navigation, and it is the sole source of position information sent by ADS-B and the most common source used for remote identification (Remote ID).
(Note: ADS–B [Automatic Dependent Surveillance–Broadcast] is a surveillance technology in which a larger aircraft determines its position via GPS and periodically broadcasts it, enabling it to be tracked by air traffic control (ATC) and other aircraft.)
Identifying the hazards
As detailed in Appendix A of Order 8040.6, the outcome of GPS degradation is a potential collision in the air, on the ground, or with a person. A list of common hazards provides a critical reference point, where all hazards applicable to the operation must be identified and accounted for in an FAA EAS waiver submission.
An operator must mitigate Appendix A’s list of hazards to be granted a waiver. GPS and other positioning-related issues are identified and require the operator to determine the severity and likelihood of the risk. The categories of hazard causes related to GPS listed in Appendix A include:
US Leaves Planned Route
GPS Signal Degradation
Defining the severity and likelihood risks
The level of risk severity and likelihood for UAS is categorized in Table C1 Severity Definitions:
Minimal – 5. Negligible safety effect
Minor – 4. Physical discomfort to persons; slight damage to aircraft/vehicle
Major – 3. Physical distress or injuries to persons; Substantial damage to aircraft/vehicle
Hazardous – 2. Multiple serious injuries; fatal injury to a relatively small number of persons (one or two); or a hull loss without fatalities
Catastrophic – 5. Multiple fatalities (or fatality to all on board) usually with the loss of aircraft/vehicle
Table C2 outlines the likelihood definitions for general aviation operations, small aircraft, and rotorcraft:
Figure C1 displays the risk matrix for general aviation operations, small aircraft, and rotorcraft:
For an FAA waiver to be approved, the applicant must minimize the hazards from Appendix A using the risk matrix above. The bottom line from the FAA is: “Operations subject to this waiver must cease if, at any time: GPS signal is lost, or GPS location information is degraded.”
Facilitating optimal GPS risk assessment
Spirent’s GNSS Foresight Risk Assessment provides a best- and worst-case scenario for a given route or geographical area, showing where safe flight can always be achieved; where it cannot, the Foresight Live service is needed to pin-point degradation in areas that are not always safe. This authoritatively facilitates the ability to plan routes and locations for reliable operation. Foresight can quantify the degradation of the signals at any time in any location, to quantify and directly correlate with the FAA’s Table C2 (seen above) which outlines the likelihood definitions for BVLOS.
Not only does Foresight address the questions in FAA Order 8040.6, it unlocks BVLOS flight in areas previously thought to be unsafe. Without quantifiable proof of where GPS works, flight in many urban and suburban areas would be variable, unknown, and unsafe. With GNSS Foresight it is possible to prove when and where GPS will perform and meet the risk requirements.
To date, Spirent’s GNSS Foresight system provides more detailed insight than any other GNSS forecast currently on the market. Unlike tools that predict how many satellites are in the clear-sky above, Foresight uses 3D maps, terrain, geospatial data, and precise orbital models as a cloud-based service to unlock BVLOS flight. Choosing the right state-of-the-art – and constantly evolving – data service supplier with the technology capable of addressing all the requirements outlined in this blog can be the key to success for new UAS BVLOS flight.
To learn more about realism in GNSS testing, read Spirent’s eBook – Achieving Reliable GNSS Performance for Autonomous UAS Navigation.