share article

Share on facebook
Share on twitter
Share on linkedin

Addressing GPS signal degradation for drone flights beyond-visual-line-of-sight


By Jeremy Bennington, VP of Position, Navigation and Time (PNT) Assurance, Spirent Communications

Beyond visual line of sight (BVLOS) drone flight is making steady inroads. Since 2018, the US Federal Aviation Authority has waived its CFR Part 107.31 rules over 70 times, authorising operators of small unmanned aerial systems (sUAS) to fly beyond BVLOS. For UAS weighing over 55 pounds, the FAA also offers Part 91 and 61 exemptions.

Among the Part 107 waiver recipients to date are Alphabet’s Project Wing, which is trialling drone package deliveries; Matternet, which is trialling drone delivery of medical samples; and ARE, which offers drone-based inspection for utility lines. These are all promising UAS applications, and the FAA is committed to enabling the “virtually limitless benefits” that operations like these can provide.


To get to a seamless scenario, there are several BVLOS challenges to address. First, FAA and its counterparts around the world must ensure drones can operate BVLOS without creating safety risks – either for other airspace users or people and structures on the ground. This huge task involves rethinking how airspace is segregated, how air traffic is managed when it includes millions of unmanned vehicles and traditional aircraft, and how drone safety and airworthiness are assessed and demonstrated.

That process is ongoing with the FAA BVLOS Aviation Rulemaking Committee (ARC), which will lead to formal regulation to come. In the meantime, any company wanting to fly BVLOS must apply for a CFR Part 107.31 waiver or Part 61/91 exemption. To obtain one, they must demonstrate their UAS safe-worthiness.

Reliable GPS is a fundamental safety condition for all BVLOS waivers: For Part 107 waivers, this starts with the FAA Order 8040.6 “Unmanned Aircraft Systems Safety Risk Management Policy”. A key requirement of this order relates to GPS reliability.

Every Part 107 waiver we’ve seen (all published on the FAA website) specifies that the drone must have a reliable GPS signal throughout its flight. Part 107 waivers granted to Wing, Matternet and ARE, for example, all include the following requirement: “Operations subject to this waiver must cease if, at any time: GPS signal is lost, or GPS location information is degraded”.

For Part 61/91 exemptions relating to larger and heavier drones, the FAA states that the flight must not even start if GPS reliability is not assured throughout the flight. The exemption issued to A-Cam Aerials, for example, states that: “The PIC may not begin or continue a flight if any global positioning system (GPS) outage, signal fault, integrity issue, Notice to Airmen in effect for any part of the planned operational area, or any other condition that affects or could affect the functionality or validity of the GPS signal”.

To highlight a related consideration of GPS reliability and performance, the waiver also includes: “Geo fencing relies on GPS accuracy. For this software to be effective as a mitigation, the PIC must know the GPS location certitude of its UAS system and factor that in to his or her planning. For example, if the GPS location certitude of the UAS is ±10 feet, then add 10 feet to the buffers between the UAS and filming personnel.”

These requirements point to a future where navigation will form a crucial part of any defined regulation. That may not be so much of an issue for operations in open rural spaces, but for services in urban or suburban areas, it could be a deal-breaker. That’s because the drone needs radio line of sight to at least four satellites – or six, if it’s also using an augmentation service like Wide Area Augmentation System (WAAS) or Satellite-based Augmentation System (SBAS) – to accurately calculate its position. If line of sight to those satellites is blocked or distorted by trees or buildings, even for a few seconds, the drone’s GPS receiver may temporarily lose lock on the signal.

GPS reliability

Clearly, GPS reliability could be a deal-breaker for UAS-based business models. If a drone has to land unexpectedly, the costs of recovery and mission failure could become punitive. Hence, a solution is needed to address GPS signal degradation. Without one, regulators may not be convinced that a drone can operate safely in certain areas, businesses may find it tough to gain approval to fly in those areas, and operations that do obtain authorisation for BVLOS flight may be disrupted if GPS problems occur.

As a provider of GNSS test, measurement and assurance solutions, Spirent Communications offers a two-fold solution to the GPS signal degradation, which were developed in close cooperation with the aviation industry:

Solution 1

GNSS availability forecasting solution, in the form of Spirent’s GNSS Foresight, which uses advanced 3D modelling and ray tracing to predict and map areas where GNSS signals might be degraded, and when. Drone operators can use Foresight before and during drone’s flight, to identify, anticipate and avoid areas of known GPS degradation.

Foresight is available as two separate solutions, depending on the requirement: Foresight Live calculates GNSS for every metre, every second, from 1-100m above ground, for the current time and days/hours in the future. In doing so it enables planning and real-time decision making to ensure GNSS continuity and reliability. Whereas Foresight Risk Analysis provides best-case, worst-case and 90th-percentile predictions over a given service area, to determine where GPS can always be assured.

Solution 2

The second solution relies on reliable test services for GNSS-reliant equipment. From GNSS chipsets to whole UAS, Spirent hardware and software solutions can test the equipment’s ability to navigate in areas of GPS degradation. Drone operators can combine this knowledge with data from GNSS Foresight to make intelligent decisions about where and when the drone can safely fly BVLOS.

For more on safe autonomous drone flight beyond visual line of sight, see Spirent Communications’s ebook “Achieving Reliable GNSS Performance for Autonomous UAS Navigation”.

Share this article

Share on facebook
Share on twitter
Share on linkedin

Related Posts

View Latest Magazine

Subscribe today

Member Login