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BVLOS vs BLOS for UAS and Drone Operations: What’s the Difference?


Line of sight is a critical concept in drone operation, and acronyms relating to it are starting to abound. We break down the essential distinctions between BVLOS and BLOS.

The world of unmanned aircraft systems (UAS, or drones) is evolving fast, bringing a whole new set of acronyms for drone operators and aviation regulators to get to grips with.

One of the most important concepts in drone operation is “line of sight” (LOS). Many of the current rules hinge on whether a drone is being operated “within” line of sight or “beyond” it. But within the overall concept of line of sight there are a number of important distinctions, and terms that may superficially seem to mean the same thing actually have quite different meanings.

One such distinction is between the terms BVLOS (beyond visual line of sight) and BLOS (beyond line of sight). They’re often used interchangeably, but in reality they’re not the same thing. Let’s look at each in turn.

What is Beyond Visual Line of Sight (BVLOS)?

The term you’re most likely to see regulators employ is Visual Line of Sight, or VLOS. This relates to the ability of the drone operator – or, in some cases, a secondary observer – to see the drone with their own eyes at all times during the flight.

VLOS is a requirement for almost all commercial and recreational drone flights today. The thinking is that if the person or people responsible for operating the drone can see it, they can take appropriate action if the drone gets into trouble – if it’s caught in a gust of wind, or looks like it might collide with something.

Beyond Visual Line of Sight (BVLOS), then, refers to operations where the person or people responsible for operating the drone can’t physically see it during some or all of its flight.

Types of BVLOS flight vary enormously, but they include flights where the drone is required to:

  • Fly over long distances: e.g. to inspect an oil pipeline for damage, or to deliver medical supplies to remote areas

  • Fly in spaces that are dangerous or inaccessible for humans: e.g. to survey an area of wildfire or flooding, or to inspect a ship’s fuel tank

  • Fly at an altitude that’s too high to be clearly seen by the naked eye: e.g. to survey fire damage to a very tall building

  • Fly in an area with lots of visual obstructions: e.g. to fly ahead of first responders to a downtown accident site, or for package delivery in urban and suburban settings

These are all critical use cases for future drone use. They largely require the drone to have a navigation system that allows it to make decisions in line with airspace regulations, and a system to automatically detect and avoid aircraft in its path (DAA).

Such systems are evolving fast, with the result that regulators are cautiously approving some BVLOS flights. As early as 2018, for example, US-based Airobotics received FAA approval for BVLOS flights to improve the safety of mining operations. And in 2021, the UK’s Civil Aviation Authority granted sees.ai a BVLOS licence to trial autonomous drone flights using its AI technology.

What are EVLOS and TBVLOS?

Two more acronyms you may hear in the VLOS category are extended visual line of sight (EVLOS) and tactical BVLOS (TBVLOS).

An EVLOS permit may be granted if an observer – as opposed to the operator – has VLOS to the drone, as long as that observer can communicate effectively with the drone operator. This makes longer-distance drone flights possible without autonomous navigation.

TBVLOS authorisation may be granted in urgent situations where the operator is likely to lose visual line of sight to the drone for some of the flight. These tactical waivers are sometimes granted by the US FAA to emergency responders, for example.

What is Beyond Line of Sight (BLOS)?

Although BLOS is often used interchangeably with BVLOS, it is not necessarily the same thing. In this acronym, LOS can relate to radio signal transmission and reception, and the line of sight is not between the human operator’s visual perception and the drone, but between the drone and any radio equipment it relies on for operation.

Until recently, If a drone was being controlled remotely from the ground, the remote control unit would typically have been using the 900 MHz, 2.4 GHz or 5.8 GHz frequency band to exchange data with the drone. Because RF signals propagate within an ellipsoid shape (known as the Fresnel zone) rather than point to point, environmental factors within that zone could hamper RF line of sight – even if seeming not to be in the direct path between the control unit and the drone.

Trees in the Fresnel zone can block or weaken the signal, for example, and signals can also reflect off tall structures in the zone, creating “multipathed” signals that can interfere with clear reception. It’s therefore possible for a human operator to have clear visual line of sight to the drone, but for the radio control unit – which is as important as the human operator – to have its line of sight impaired. Essentially, if a human operator can see the drone but can’t control it because radio line of sight is lost, that clearly presents a safety risk.

BVLOS, BLOS and autonomous drone flight

With the shift to 4G and 5G for data exchange, the significance of BLOS – in this traditional sense – for autonomous drone flight is diminished. With the density of beacons in urban areas, and the range through unbroken sightlines outside of urban centres, it would be uncommon for a drone to be outside of LOS of the control unit. However, BLOS does take on a greater significance when applied to RF from GNSS satellites. Precise positioning and navigation is critical for BVLOS, and this typically depends on GNSS. A drone’s GNSS receiver needs radio line of sight to at least four navigation satellites (six if using an augmentation service like WAAS or SBAS) to calculate its position accurately.


A drone’s GNSS receiver needs radio line of sight to at least four navigation satellites (six if using an augmentation service like WAAS or SBAS) to calculate its position accurately.

With this in mind, it’s clear that both BVLOS and BLOS are important terms and concepts for UAS use cases. FAA part 107 BVLOS waivers, up to this point, have all referenced a requirement for accurate and reliable GNSS positioning for operations to continue. Thus, to operate BVLOS users will need to ensure LOS to a minimum of four satellites at all times.

That does limit the potential for autonomous BVLOS drone flight in areas where satellite signals can be intermittently obscured by surrounding buildings and other structures. That includes built-up urban and suburban areas – which, frustratingly, is where many of the most promising drone use cases lie.

It’s an area where Spirent is helping to make safe BVLOS flight possible for autonomous drones through our GNSS Foresight solution. It forecasts the reliability of GNSS signals both before and during a flight, allowing flight planners – and the drone itself – to avoid areas where LOS to GPS or GNSS satellites may be degraded or lost.

VLOS and LOS will become important distinctions as autonomous drone flight nears

Today, little distinction is often made between BVLOS and BLOS. But as drone use becomes more widespread, standards and regulations develop, the distinction between visual line of sight and RF line of sight will become a crucial one.

As the world’s leading provider of GNSS test, measurement, and assurance solutions, Spirent is working with navigation system designers, drone manufacturers and airspace regulators to help ensure safe and effective drone navigation for BVLOS. If you’d like to learn more about enabling safe autonomous navigation for drones and other UAVs, check out our eBook - Achieving reliable GNSS performance for autonomous UAS navigation.

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