Flying small uncrewed aerial vehicles (UAVs aka drones) in proximity and even contacting structures during flight is laden with risk and dangers. There is a reason in aviation history aircraft do not fly near, or contact, structures: it is very dangerous. In addition to the behaviors of environmental variables, such as wind and barometric pressure, creating difficult flight conditions close to structures there are a host of technical issues that can negatively impact drone flights, such as magnetic, electrical, and radio frequency interference. UAV operators need to recognize a drone is a tool and like any tool is best suited for its intended use. Therefore no one should fly a drone near or try to fly it to contact a surface or structure unless the drone has been designed for that type of use or specific use case.

Drones Need: to Know their Location, Direction, and Elevation

To fly a drone safely there are, in essence, three things at a minimum a drone needs to know in real-time 1) location or where it is in the world 2) direction or where it is headed, and 3) altitude or how far is it from the ground.

• Location – typically the drone uses the onboard Global Positioning Satellite (GPS) receiver to determine its location in the world. Including. However, in many industrial locations or behind assets such as above-ground storage tanks or large cargo, or other ships, the satellites signals are not able to be fully received resulting in degraded or denied GPS.

• Direction – the magnetic compass onboard the drone is used to find north which allows the drone to know its heading and directional orientation. However, in an environment filled with large metal steel objects, compass readings are inaccurate or unavailable. In essence, the magnetic field from the large metal objects says hey look at me I’m north.

• Elevation – the drone, just like commercial aircraft, uses barometric pressure to determine its distance from the ground. However, when in close proximities to structures barometric readings are often skewed or incorrect.

When any of these information flows are compromised the UAV autopilot can fail or potentially lose control. UAVs are remarkable and fantastic, yet highly complex, machines. Degradation or false information and data can be catastrophic.

Solving For Compromised Flight Data

Some companies, for example, those that complete close visual inspection (CVI) of live powerlines on power transmission towers, overcome some of these issues by building a Faraday cage or shield around some of the drone’s electronics. A Faraday cage is a grounded metal cover or screen surrounding a piece of equipment to exclude electrostatic and electromagnetic influences.

A more complex solution comes from another company that builds custom-designed and manufactured systems that includes a robotic arm/rod that protrudes from the drone body past the propellers. At the end of the robotic arm, known in the robotics world as an end-effector, holds a probe tip that makes physical contact with a surface. To enable the system to make physical contact with structures during the flight they use a multimodal array of various sensors, an onboard computer, and a set of complex integrations, including a lot of software, to allow automated precision flight control close to structures. These systems flights are controlled by the onboard computer and software and make 20 or more micro-adjustments to the flight per second, something impossible for human pilots.

There is no single solution that is best in all instances for every potential use case. One way to think of it is the drone is a tool in the toolbox and as such should be used when appropriate and when it is the best tool for the job. Just as you would not use a hammer when a screwdriver is needed you should not use a generic drone designed for flying far and flying fast to fly slow and close to or come in contact with, structures.

Solving For Environmental Factors

Environmental Factors that can negatively affect drone flight safety include:

• Atmospheric/Barometric pressure inaccuracies

• Winds/Breezes

• Venturi effect - Winds around structures

• Magnetic interference - KP Index

• Radio-frequency interference (RFI) sometimes called Electromagnetic interference (EMI)

Atmospheric/Barometric pressure inaccuracies: Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure of Earth’s atmosphere roughly measured as the difference in air pressure from the mean sea level. As altitude increases, atmospheric pressure decreases allowing one to calculate the atmospheric pressure at a given altitude. However, barometric inaccuracies occur near structures as both wind and air pressure are disturbed. Temperature and humidity also affect atmospheric pressure, but temperature and humidity sensors can correct this. Another way to help increase altitude measurement accuracy is with a laser altimeter that measures the distance to the ground vs the barometric altimeter measuring air pressure. Having both a laser and a barometric altimeter sensor is also helpful as it allows for data fusion which heightens the strengths of each data stream while minimizing the weaknesses. 

Wind and Breezes: Winds can negatively impact drone flights. Not only does the drone have to work harder to fly in winds, which shortens flight time and battery life, but wind can also negatively impact stability. It is not recommended to fly drones in strong winds especially when you are flying close to structures. One strategy is to wait for the winds to calm down and then fly. This is similar to a “rain day” or “weather day” sometimes inserted into commercial drone operation contracts. Another strategy, that works for limited winds, is to use a system with autonomous flight systems that compensate or even take over flight controls from the manual input by a pilot on the ground. Various drone companies do this with various levels of success, partly dependent on the physical characteristics of the drone and dependent on wind speed and gusts. For example, flying a drone when there is constant wind is much easier than one where there are gusts of wind. Depending on the job being performed some variation of the drone flight can be okay. If flying in open air well away from the ground and structures while recording a video while drifting slowly a few feet off course and returning to the planned flight path as the winds slow can be perfectly acceptable. However, if you are flying close to a structure or contacting a surface during the flight a very small deviation from the intended flight path can be catastrophic.

Venturi Effect (for wind): While flying in winds can be challenging, flying close to structures during wind is fraught with even more danger and complexity. The Venturi Effect refers to a change in pressure when fluid enters a constricted area within a pipe. The Venture Effect also applies to air and winds. When air flows through a confined area it becomes constricted and the velocity increases. If you are familiar with drone flights and operations, you are probably aware of ground-level turbulence where air from the drone propellers hits the ground and disperses horizontally while also creating a “bubble” of air pushing vertically from the ground. The Venturi Effect for wind is similar as there may be wind speeds of 5 mph a few meters from the structure but when the wind hits the structure and “whips” around it the wind speed could be 10 or 15 mph.

In addition to the Venturi and canyon effects for winds, they also behave differently around structures than in an open environment as flying near structures the drones’ propellers can generate turbulence and “push back” from the structure similar to ground-level turbulence. The best way to mitigate this is with software flight controls, especially precision flight control systems. For example, if the drone is flying based on onboard sensors and computers it can, as discussed earlier, make 20 or more micro-adjustments to the flight per second, something impossible for human pilots.

Magnetic Interference: Electromagnetic interference is any disturbance that negatively impacts an electrical circuit due to electromagnetic induction or electromagnetic radiation. These disturbances, emitted external sources, can interrupt, obstruct, or otherwise degrade or limit the effectiveness of electronic equipment. Drones are crunching a ton of numbers each second just to say flying in the best of circumstances. Magnetic interferences can negatively affect the drone’s navigation when it is looking to magnetic north for direction. Flying next to an above-ground storage tank or a big piece of metal close to the drone and it starts shouting to the drone “hey, I’m north”.

Mitigating for magnetic interference can be as simple as flying in locations where there are no power lines, mobile phone towers, or structures with Ferrous materials. Even certain rocks with high metal content can be enough to degrade your drone’s flight performance. In addition, there are other mitigation strategies including shielding the electronics of the drone, flying drones that use onboard sensors and computers allowing for multiple micro-adjustments to the flight per second, or a combination of both.

Radio-frequency interference (RFI): Radio-frequency interference (RFI) and Electromagnetic interference (EMI) occur when there is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction. The disturbance may degrade the performance of the circuit or even stop it from functioning. Drones are heavily reliant on data from integrated circuits that are part of the accelerometers, inertial measurement units (IMUs), barometers, magnetometers, autopilots, onboard computers, and more. Further, control of the drone often is from the radio (RF) controller manually operated by the remote pilot on the ground. These RF controllers also contain a multitude of electronic devices venerable to RFI and EMI.

In the case of data flowing to and from these electronic components, these effects can range from an increased error rate to a total loss of the data. These can cause a loss of control that can be catastrophic for drones. There are a plethora of both man-made and natural sources generating changing electrical currents and voltages that can cause RFI and EMI including mobile phones and phone networks, motors and generators, lightning, solar flares, and auroras such as the northern/southern lights. Preventing RFI and EMI issues enabling safe drone flights can be done with shielding such as a Faraday cage, building in redundancies, using hardened electronic components, enabling autonomous flight without input from a ground controller, and other methods.

Conclusion – The Right Tool for the Right Job Enables Safe Flights

Drones are sophisticated electronic systems made up of a complex set of system integrations operating via embedded software. In addition, flight directions are provided to the drone from onboard sensors and computers, an operator on the ground, or both. This set of interdependent functionalities allows for the drone to do amazing things – like almost defying physics.

If you are flying these systems in proximity or contacting structures during flight, there is an additional set of risks and dangers. These dangers include variables, such as barometric pressure errors, wind and rain, magnetic, electrical, radio frequency interference, and more. UAV operators should recognize a drone is a tool and like any tool is best suited for its intended use. Therefore no one should fly a drone near or try to fly it to contact a surface or structure unless the drone has been designed for that type of use or specific use case.

Fly safe, be safe, and have fun.

Return to the Blog