Unmanned military systems require higher performance electronic systems

By: Steve Munns, Military Aerospace Market Manager, Linear Technology Corporation (now part of Analog Devices)

Introduction

Modern unmanned military systems have become an integral part of armed forces around the world, and the defense industry continues to intensively develop these systems to enable them to perform a wide range of attack, surveillance and combat support roles. Unmanned systems are perhaps the most dynamic sector in the defense industry today, with global annual spending exceeding $5.5 billion and forecast to approach $10 billion by 2024[1].

One of the most striking aspects of the unmanned aerial vehicle (UAV) space is the sheer variety of systems, from tiny Nano UAVs (NUAVs), some weighing less than 20 grams, to medium-sized UAVs such as the Watchkeeper, which weighs 450 kg and has a payload capacity of 150 kg, to the MQ-9 Reaper (formerly Predator B), which has a takeoff weight of over 5,000 kg. Size

, Weight and Power (SWaP) are key considerations when balancing performance and mission endurance for UAVs of all sizes. There are a number of electronic systems that can be used, but for this article, the focus will be on the following areas when considering electronic systems:

Airborne Operation Safety and Autonomous Operation
Sensors and Data Processing
Communications and Information Security
Power Systems

Early Unmanned Systems Introduction

The origins of modern UAVs can be traced back more than 100 years, but the radio-controlled drones used for aerial target practice in the 1930s are probably considered the most well-known UAV ancestors. More than 400 of these aircraft were built in the United Kingdom, and they were known as the "Queen Bee" and are said to have coined the term "drone." These aircraft required that they be flown within the line of sight of the pilot who was flying the remote control aircraft. However, it was not long before autonomous flight beyond visual range was attempted. In 1940, Edward M. Sorensen patented his ground station invention, which used frequency modulation technology to control the aircraft and read back flight information beyond visual range. The patent was based on the recognition that there was a need for a fail-safe mode to keep the aircraft in level flight and establish a backup control system at the same time.

Unmanned military systems increased in complexity with the development of weapons payloads in wartime, and later reconnaissance platforms in the 1950s and 1960s. The early Ryan drones of the 1960s employed a basic guidance system consisting of a programmable timer, gyrocompass and altimeter to determine the departure flight level, heading and flight time, and also provided inversion and parachute-assisted landing capabilities. Although these were fairly basic functions, the strategic significance of imagery obtained with film cameras and the advantages of semi-autonomous systems were easy to see, so there was a desire to make more concerted efforts to develop them further.

Air Safety and Autonomous Operations

Clearly, the issue of flight safety is of paramount importance, and there has been extensive debate on how to regulate the skies so that the presence of UAVs does not affect the safety of existing air traffic, while at the same time allowing for unrestricted development of military and civilian UAV applications.

Small UAVs flying within visual line of sight rely on the pilot of the remote aircraft to determine whether a collision will occur, while larger UAVs operating autonomously or semi-autonomously require complex detection and avoidance systems to avoid mid-air collisions. A variety of sensors are being developed for this purpose, such as modified traditional aircraft transponders, visual and infrared cameras, Light Detection and Ranging (LiDAR) systems, and conventional radar systems. Converting the data from these sensor systems into a picture of the environment and then making autonomous flight decisions requires very complex software and hardware resources, and for UAVs sharing civilian airspace, it needs to operate under the requirements of existing agreements. When flying in friendly airspace, using ground radar and traffic mapping resources to reduce the complexity of the onboard system and extend the detection range may be an option, but this approach requires trade-offs in other issues such as data link reliability and latency. The ASTREA program shows that autonomous detection and avoidance technology can be used, but this technology is used on Jetstream aircraft, which do not have the power consumption, size and weight constraints of UAVs. Adapting this technology to work in most UAVs is a challenge, but it can be miniaturized using advanced field programmable gate arrays (FPGAs), digital signal processing (DSP), and high-performance analog electronics. Powering these electronic systems is also not a simple task. FPGAs require tight power accuracy as well as low voltage and high current, which requires careful design of the power chain to minimize power consumption and reduce heat generation. One approach is to use digital power system management (PSM) technology, which can reduce power consumption by dynamically adjusting voltage and frequency, thereby helping to extend the mission endurance of smaller UAVs. PSM also improves reliability and provides remote control and monitoring capabilities, as well as energy usage logging and "black box" fault logging capabilities.

Figure 1: Digital power systems manage

sensors and data processing

Even the smallest, manually-launched NUAVs can carry multiple cameras and still cameras for surveillance missions, and multiple versions of the MQ-9 Reaper can meet a variety of hunting and surveillance needs. Weapon-carrying versions may carry cameras, infrared night vision cameras, and synthetic aperture radar (SAR) for use in cloud or smoke, as well as laser rangefinders and target illumination systems for guided munitions. Versions that provide decoy and jamming capabilities have also been developed, while tactical data link systems can send targeting information and imagery data directly to manned aircraft. More development work is expected in the area of ​​signals intelligence (SIGINT), and as advances in SIGINT systems, longer-range versions will provide mission endurances of more than 40 hours. The rapid increase in onboard sensors and longer mission endurances has generated a large amount of data that must be compressed and stored or sent over real-time data links, which inevitably leads to certain trade-offs, such as bandwidth, quality, and possible imagery data loss.

Each new payload capability adds a greater burden on the power system. Fortunately, however, there have been advances in the development of PCB-level power solutions, with power density improving significantly in recent years, and Linear Technology's µModule® (micromodule) regulator solutions are one such advancement. Each small module contains a complete, high-efficiency power supply in a form factor suitable for applications with very tight size requirements, and with very high reliability. Figure 2 shows an example.

Figure 2: LTM4644 µModule Regulator

Communication and Information Security

The communication link of a UAV can be divided into two parts:

the flight control data link - used for remote commands (uplink) and telemetry (downlink) information to monitor the UAV as it responds to operator instructions or flies autonomously according to GPS coordinates to execute the mission plan. Typically, a 56kbps link using spread spectrum technology can meet the needs of the flight control data link, and the uplink can be protected with 128-bit encryption and forward error correction.

The communication link that carries the payload sensor information - is considered a separate communication link, and high-definition video may require up to 10Mbps bandwidth while running COFDM, MPEG-4 or similar modulation schemes. Large UAVs such as Reapers typically use a combination of leased, dedicated satellite relay links (Ku-band) and terrestrial (C-band) communications links with ample space for large antennas, while other types of drones may operate in industrial, scientific and medical (ISM) bands such as 2.4GHz (WLAN) and 5.8GHz.

Integration with air traffic control systems and protocols is another barrier to fully autonomous UAV operation, as UAVs need to respond to voice commands, provide heading and flight level information, and confirm that commands have been received via VHF radio channels and synthetic voice confirmation systems.

Information security risks include intentional or accidental interference; spoofing or interception of command and control signals; and degradation of communication channels. In conventional manned flight, the pilot can immediately take control of the aircraft to avoid any closely approaching aerial objects, but in the case of UAVs, the pilot obviously always relies on the stable operation of the communication link and onboard sensors.

Risks can always be mitigated, and even very small UAVs flying to a set of GPS coordinates can increase their flight altitude to recover lost GPS signals and automatically return to a base station when their departure endurance limit is reached. As a fallback, anti-spoofing GPS systems use a combination of GPS receivers and inertial measurement units, and statistical analysis of received GPS signals can also help determine if someone is trying to spoof the system.

Of course, all of these communication systems require power, and sensitive radio receivers need some very low noise power so that radio sensitivity is not degraded by the power supply. New chip process technologies and novel IC design methods have led to a series of groundbreaking products that provide unprecedented high efficiency and very low noise, such as the LT8640 Silent Switcher® and LT3042 ultralow noise, ultrahigh PSRR RF linear regulators.


Figure 3: LT8640 Silent Switcher Regulator

Power System

Earlier in this article, some IC-level power technology advances have supported the continued changes in UAVs and sensor payloads, but the choice of onboard power source is also a core factor affecting overall performance. As people focus on developing lower cost, smaller size and lighter weight UAVs, the appeal of internal combustion power sources has decreased, and fuel cell technology has become a possible choice, especially for missions with long endurance and low average power requirements. A

fuel cell being tested in the Puma series of small UAVs has extended flight time from 150 minutes (when using LiSO2 batteries) to nearly 5 hours. The entire fuel cell system weighs about 2 kg, with a power-to-weight ratio of about 1kW/kg.


Figure 4: Power-to-Weight Ratio of UAV Power SourcesPower

-to-Weight Ratio: Power-to-Weight RatioSolar
PV: Solar Photovoltaic CellsLithium
-Ion Battery Types: Lithium-Ion
Fuel Cells: Fuel CellsPiston
/Radial Engines: Piston/Radial EnginesTurbofan
/Turboprop Engines: Turbofan/Turboprop EnginesIncreasing
Complexity and Cost:

Fuel cells are positioned between battery and internal combustion engine solutions and offer environmental advantages, but do face some fuel handling and storage issues, which can be overcome by storing pelletized hydrogen in replaceable fuel cartridges.

Small UAVs and NUAVs will most likely continue to use lithium-ion batteries, and a single battery can fly a NUAV for about 30 minutes, depending on the configuration. Longer endurance and larger models will require multi-cell designs, which can benefit from cell balancing techniques implemented with ICs such as the LTC3300 to maximize system runtime. High-flying pseudo-satellite UAVs, such as those being developed by Google and others to provide Internet services in the future, could also use solar power instead of batteries. Such systems need to operate reliably in environments where single-event upsets may occur due to enhanced radiation, which increases complexity and may require the ICs used to be specially characterized and tested.

Conclusion

Unmanned operating systems now play an integral role in the armed forces, and significant military funding has led to rapid development of such systems, with a particular focus on smaller, less expensive UAV systems.

As sensor payloads and UAV platform electronics become more complex, the efficiency of the power chain and onboard power sources becomes critical to providing adequate operational performance, and new IC power solutions are helping to achieve SWaP goals.

High-flying UAVs and very long-endurance missions are driving the need for new power sources such as solar and fuel cells, which in turn means new ICs.

References

[1] http://www.tealgroup.com/index.php/about-teal-group-corporation/press-releases/118-2014-uav-press-release