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A new class of drones for hover-in-place communications


Fixed-wing, airplane-style drones have been the standard in the skies for many years, widely used for military applications where unmanned flight is advantageous, particularly for long-duration missions. But, whilst fixed-wing drones can traverse airspace at high speed, they aren’t equipped for applications requiring stationary aerial flight, which is especially important for persistent airborne surveillance and communication relays.

Tethered aircraft

Early experimentation with tethered, unmanned blimps revealed their relative inability to remain stationary in high-wind conditions. A new approach to unmanned aircraft systems (UAS) was needed, and Dragonfly Pictures Inc (DPI) developed a new class of drone, the hover-in-place tethered drone.


Unlike battery-powered multirotor drones which require battery changes every 20 minutes, tethered drones receive their power through an electrical cord connected to a base station. This enables them to stay aloft for hours, even days.

DPI’s tethered multirotor drones are designed to track and follow mobile host platforms including ships, boats, trucks and other unmanned surface and ground vehicles. They offer several benefits over fixed-wing drones, including vertical take-off and landing, without needing a launcher or recovery equipment.

Unlike a blimp, they also offer the ability to achieve persistent, stationary positioning – even during turbulent weather and fast-changing wind speeds.

Tested extensively in real-world operating conditions, DPI tethered drones are qualified for use by the US Navy, in marine and maritime environments for intelligence, surveillance, reconnaissance, communications and video applications. They are commonly employed as mobile masts or “instant towers”, to support RF-based communications over long distances to unmanned vessels monitoring for security threats.

Overcoming extreme design challenges

DPI military and industrial-grade Unmanned Multirotor Aerial Relay (UMAR) tethered multirotor drone is weatherproofed for rain, snow, dust and heat, and has been fortified for saltwater marine environments. The system has been designed to withstand harsh temperatures, from 0 to 120 degrees Fahrenheit.

These drones are especially beneficial in that they can provide 400+ hours of non-stop uptime and operations, at altitudes to 500 feet, because of the continuous power supplied by the tether. Nevertheless, there are significant design challenges inherent to the tethered architecture. The power needs to be delivered from the host vessel to the drone at high voltage and low current, to allow for the thinnest and lightest possible tether, which in turn enables greater drone mobility and larger airborne payloads.

Operating at power levels of 8-10kW, the UMAR is extremely powerful and rugged. This challenge is often compounded by turbulent waters impacting the positioning of the host vessel. Therefore, the drone requires the power capacity and agility to accelerate rotor speeds for lift and yaw in short or prolonged bursts as needed to maintain their altitude – with instantaneous responsiveness.


Dragonfly UMAR drones can provide 400+ hours of non-stop uptime and operations

Inside the drone’s electronics, the high-voltage conversion must be achieved in the smallest possible footprint and lightweight profile. The eight onboard independent rotors require sophisticated, interconnected PCB circuitry, so any space savings enabled at the power component layer can be repurposed for other value-add components.

Using Vicor power modules, the weight of all components onboard the drone has been reduced, which allows for higher altitudes and airspeed whilst carrying a greater mission payload.

As an airborne RF antenna, the UMAR extends the ship radio line-of-sight from eight to 30 miles; see Figure 5. Communications data transmitted and received by RF radio equipment mounted on the tethered multirotor drone is relayed through the tether to and from the host vessel, allowing beyond-line-of-sight communication with surrounding unmanned support vessels. This communications architecture presents EMI challenges for the power delivery network architecture. Given the critical role these drones play in maritime defence and surveillance applications, the integrity of communications data is of crucial concern.

Drone power architecture

For its drones, DPI used Vicor’s ultra high-voltage (UHV) BCM VIA modules, which offer conversion to 98%, with only 2% losses from 800V to 50V.

Two sets of four UHV BCM4414 modules are co-located within two waterproof aluminum enclosures and are board-mounted to the host PCBs in a setup that allows the low-profile, flat-sided UHV BCMs to be cooled from both sides. The compact footprint and mounting versatility of the UHV BCMs were particularly valuable, since they enable very power-dense board configuration.

The Vicor BCM4414 in a VIA package is a high-efficiency fixed ratio bus converter, operating from a 500-800VDC high-voltage bus to deliver an isolated 31.3-50.0VDC. Measuring 110.55 x 35.54 x 9.40mm, this ultra-low-profile module provides up to 776W/in3 power density whilst incorporating DC-DC conversion, integrated EMI filtering and PM Bus commands and controls in a chassis- or board-mount forms.

An array of eight BCMs powers the UMAR’s eight independent rotors, which can also share power among them in parallel for increased redundancy; see Figure 6. An additional onboard UHV BCM powers the avionics, autopilot and payload functionalities.

The high-efficiency UHV BCMs greatly reduces the heat dissipation within the drone’s avionics modules, which are sealed in aluminum enclosures to prevent moisture ingress from saltwater and rain. The avionics modules leverage a system of heat pipes and sinks for passive cooling when the drone is grounded. When airborne, the spinning rotors provide additional active cooling.

A low-voltage-side-referenced PM-Bus-compatible telemetry and control interface provide access to the BCM VIA configuration, fault monitoring and other telemetry functions. This PM Bus capability allows the UMAR operators to monitor system temperatures, voltage and currents in real time, which is particularly valuable in hot climates.

The UHV BCMs’ integrated EMI filtering capability minimise noise that could compromise RF communications between drone and host. They offer a very clean EMI signature with very few harmonics below typical EMI standard levels. A small available companion filter reference design outside the VIA package cuts conducted EMI levels to near noise-floor signature well below typical requirements.

The sky’s the limit

In addition to its current trial deployments with the US Navy, DPI technology is being evaluated by a host of government agencies, contractors and other entities. It shows significant promise for additional applications like first-response disaster relief and large-area monitoring (public events, stadium security, etc.). Anywhere a hover-in-place communications and surveillance presence might be needed, DPI multirotor drones could be readily deployed.

With Vicor UHV BCMs at the heart of the UMAR systems, DPI is assured of a power-dense, thermally-adept power system for high-voltage power conversion from tether to drone. As DPI multirotor drone technology takes flight, we can expect to see many more of them on the horizon.

By Tom Curatolo, Senior Principal Applications Engineer, Vicor

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