Micro Air Vehicle


The term micro air vehicle (MAV) or micro aerial vehicle refers to a new type of remotely controlled aircraft. The target dimension for MAVs today is approximately 15 centimeters (six inches) and development of insect-size aircraft is reportedly expected in the near future.
The Defense Advanced Research Projects Agency (DARPA) is the U.S. agency at the forefront of MAV development.

Types of MAV:

Three types of MAVs are under investigation:
  • Fixed wing models,
  • Rotary wing models and
  • Ornithopter (flapping wing) models.
Each type has different advantages and disadvantages, different scenarios may call for different types of MAV.
A fixed-wing aircraft, usually called an aero plane or airplane, is a heavier-than-air aircraft capable of flight whose lift is generated not by wing motion relative to the aircraft, but by forward motion through the air.
Fixed-wing MAVs can currently achieve higher efficiency and longer flight times, so are well suited to tasks that require extended loitering times, but are generally unable to enter buildings, as they cannot hover or make the tight turns required. The below figure shows the Fixed wing MAV.
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 A helicopter is an aircraft that is lifted and propelled by one or more horizontal rotors, each rotor consisting of two or more rotor blades.
Rotary-wings allow hovering and movement in any direction, at the cost of shorter flight time.
The below figure shows the Helicopter
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An ornithopter (from Greek ornithos "bird" and pteron "wing") is an aircraft that flies by flapping its wings. Designers seek to imitate the flapping-wing flight of birds, bats, and insects.
Flapping wings offer the most potential for miniaturization and maneuverability, but are currently far inferior to fix and rotary wing MAVs. The below figure shows Flapping wing MAV.
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Its requirements:
Response time limitation in human piloting of such vehicles points out the need for fully autonomous flight. Collision avoidance sensor capable of detecting obstructions and hazards to flight is an absolute requirement.
A fine resolution, short-range altimeter capability is needed to aid these, often fragile, micro vehicles in takeoff and landing, and to assist in hovering for surveillance applications. The envisioned operating environments for MAVs are quite complex and diverse, ranging from urban population centers to extreme wilderness environments.
Thus onboard sensor suites must be capable of maintaining a high level of performance in a wide array of scenarios and against very diverse targets. Candidate sensors must be extremely lightweight, occupy minimal space and must also have very low power consumption.
Additionally, the physical constraints of the MAV present significant challenges to the integration of sensor packages onboard the vehicle.
Ultra wideband (UWB) radar has emerged as a leading technology for MAV applications due to these technical considerations.

UWB: 

Ultra wideband (UWB) technology, well-known for its use in ground penetrating radar, has also been of considerable interest in communications and radar applications demanding low probability of intercept and detection (LPI/D), multipath immunity, high data throughput, precision ranging and localization.
After a very short introduction to the history and theory of ultra wideband technology, we describe the current state-of-the-art (within the United States) in this emerging field by way of examples of recently fielded UWB hardware and equipment.
Multispectral Solutions, Inc. (MSSI) is a pioneer and an established industry leader in the development of ultra wideband systems and has been actively involved in UWB hardware and system development since 1984.

AN (ULTRA) SHORT HISTORY OF UWB TECHNOLOGY:

The origin of ultra wideband technology stems from work in time-domain electro-magnetic begun in 1962 to fully describe the transient behavior of a certain class of microwave networks through their characteristic impulse response (Ross (1963, 1966)). ???????????? The concept was quite simple. Instead of characterizing a linear, time-invariant (LTI) system by the conventional means of a swept frequency response (i.e., amplitude and phase measurements versus frequency), an LTI system could alternatively be fully characterized by its impulse response h(t).
In particular, the output y(t) of such a system to any arbitrary input x(t) could be uniquely determined by the well-known convolution integral

However, it was not until the advent of the sampling oscilloscope (Hewlett-Packard c. 1962) and the development of techniques for sub-nanosecond (baseband) pulse generation

Uses of UWB technology:

An extremely short duration pulse not only provides for fine radar range resolution (essential for meeting demanding autonomous flight and precision landing requirements); but also results in a low duty cycle waveform which can minimize the prime power demands on the vehicle.
For example, 10 kpps UWB radar operating with a 500 MHz instantaneous bandwidth has a pulse duty cycle of roughly 2x10-5. Thus, by time-power gating , the average power drain can be many orders of magnitude smaller than the peak power requirement for the radar.
Furthermore, low duty cycle emissions also result in low average power densities (Watts per unit Hertz) which are essential to minimize interference to other onboard electronics ? most importantly, the vehicle's flight control system and associated telemetry link.
Of course for military operations, a low power spectral density is of importance in making the vehicle less vulnerable to intercept and subsequent electronic countermeasures (ECM) attack. Another important feature of short pulse technology is the ability to establish precision range gates at user selectable distances.
These range gates allow the vehicle radar to eliminate detections outside of selected areas of interest and dramatically reduces the number of nuisance and false alarms. This is of particular advantage in high clutter environments such as urban centers or heavily forested terrain.
Additionally, since UWB-based radars function as presence sensors, they do not depend upon relative motion or Doppler information. Thus, they are suitable for a wide variety of operational scenarios including slow moving or hovering platforms. With its inherently large bandwidth waveform, UWB radar also provides an enhanced detection probability against complex and low radar cross section (RCS) targets such as suspended wires and utility poles.
Finally, since UWB radar designs are nearly all digital, with minimal RF and microwave electronics, low cost microminiaturization is possible through the use of custom application specific integrated circuit (ASIC) and radio frequency integrated circuit (RFIC) technologies.
In addition, the commonality of signal generation and processing architectures for both radar and communications permits the design of a multi-function unit that can encompass altimetry and obstacle avoidance as well as data link functions.

Working of MAVCAS:

MAVCAS system operation proceeds as follows. A programmable logic device (PLD) generates a transmit strobe to the low level impulse source when initiated by a trigger sequence from the system microcontroller. In the low power (25 mW) mode, the transmit pulse is directly produced by the output of a spectrally filtered, time-gated C-band oscillator; while, in the high power mode, an additional time-gated C-band power amplifier boosts the signal to the 0.8 W peak power output level.
Once the transmitter is triggered, the PLD immediately begins sampling conditioned, baseband (detected) pulses from the receiver tunnel diode detector circuitry. The tunnel diode is preceded by suitable bandpass filtering and an AGC-controlled, low noise amplifier (LNA) which is used to set the system noise temperature.
However, as the PLD clock frequency (in this case, 250 MHz) is insufficient to achieve a 2 nanosecond (1 foot roundtrip) resolution; the detector output is further subdivided into two separate streams, one of which is delayed by 2 nanoseconds from the other. This is accomplished using a precision, analog delay line which can be implemented in micro strip. Both pulse trains are then clocked into the PLD, and a 2-bit word results which contains information about events occurring on a 2 ns, rather than 4 ns, epoch interval.
Note that, in general, the radar resolution can be further improved by adding additional, finer time resolution, delay lines. The radar operates as a presence sensor, determining the distance of an object by simply measuring the roundtrip delay of the transmitted pulse. Sensitivity time control (STC) for MAVCAS is provided through digital control of both receiver RF gain (RF AGC) and detector bias.
RS-232 and RS-485 interfaces are provided for both user control and data output. MAVCAS currently utilizes a discrete RF design; however, it is readily adaptable to an RF integrated circuit (RFIC) implementation.
This, combined with the replacement of the discrete PLD and microcontroller with a custom application specific integrated circuit (ASIC), are the next steps necessary to fully enable commercial markets and to meet the final size and cost requirements for DARPA?s MAV program.

Frequency domain response of MAVCAS:


The frequency domain response of MAVCAS shows that frequency markers are placed near the edges of the FCC Part 15.209 non-restricted Band at 5.46 to 7.25 GHz, illustrating that the radar response falls entirely within this non-restricted band.
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Time domain response of MAVCAS:

The time domain response of MAVCAS shows that the radar is non-coherent (pulse-to-pulse); and that the pulse envelope is typical of a bandpass filter impulse response.
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Applications:

Potential military use is one of the driving factors, although MAVs are also being used commercially and in scientific, police and mapping applications.
Another promising area is remote observation of hazardous environments which are inaccessible to ground vehicles.
The military application for MAVs is primarily in the area of reconnaissance, and the projection is for tiny "spy planes" that a soldier can carry in a backpack, launch and use to scout ahead for enemy troops.
Aerial robots can aid military personnel or first responders in finding victims, safe passage ways, by exploring or securing an area, in search of bombs, chemical or biological agents.
Aerial robots can potentially aid in saving the lives of the rescuers and civilians in a terrorist attack or a disaster situation.
Applications have also been proposed which include use in search and rescue; remote nuclear, biological and chemical (NBC) sensing; monitoring of traffic patterns and airborne pollutants, etc.

CONCLUSION:

MICRO AIR VEHICLES provides soldiers a birds-eye view. The use of the MAV provided the platoon with better situational awareness, and led to less confusion during tactical operations.
Fine range resolution, high power efficiency, low probability of interference and low probability of detection, make UWB an excellent candidate technology for micro and organic air vehicle collision avoidance and altimetry applications.

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