UAV Sensors and Communication Systems. Using a UAV to deliver aerial video to searchers has potential to support search and rescue efforts, but a number of technology and human factors problems must be overcome to make this practical (Cooper, 2008). The

SENSORS

Introduction

Search and rescue is a challenging problem because of the large areas and often rough terrain that must be searched. Using a UAV to deliver aerial video to searchers has potential to support search and rescue efforts, but a number of technology and human factors problems must be overcome to make this practical (Cooper, 2008). The biggest problem is how to manage flying the UAV from the ground station while simultaneously intepreting the information from all the sensors being relayed from the UAV to the ground station.

Typically, a UAV engaged in a search task requires either two operators or a single operator to fill two roles: a pilot, who “flies” the UAV, and a sensor operator, who interprets the imagery and other sensors (Cooper, 2008).

Search and rescue is a big challenge because it involves covering a vast area while looking for just a specific target. This is where the UAV plays a vital role. Using all the latest onboard sensors it helps a lot in search and rescue. In this part of the report I am going to look at these sensors in depth and how they help the sensor operator on the ground, who is responsible for analysing imagery and localizing potential signs of the target of the search and rescue mission.

The primary autonomous sensing methods used by the Unmanned Aerial Vehicle are:

Air data sensors
Inertial sensors
Imaging sensors
Radar sensors

All these sensors aid the navigation system to achieve optimal performance. As we are going to see the capabilities of the UAVs integrated navigation system blend the inputs of multisensory types to attain high levels of accuracy that allow new navigation and approach procedures to be used. This is important especially in today’s world where the sky is getting more and more traffic. This integrated system will assure safe and smooth traffic flow.

Let’s look at each of these sensors in more detail.

Air Data Sensor

Air data, as the name suggests, involve the sensing of the medium through which the aircraft is flying. Typical sensed parameters are dynamic pressure, static pressure, rate of change of pressure and temperature (Moir & Allan, 2003). The pressure parameters are sensed by two instruments known as pitot static probes and static sensors. These static probes are positioned on the side of the UAV to deal with the errors that may arise when the aircraft yaws or changes its angle of attack.

The temperature is sensed by positioning a device known as the total temperature probe in the airflow and sensing the change in resistance associated with temperature. It is important for the sensor operator to know the outside air temperature because it directly affects the aircraft’s performance. During take-off it directly affects the thrust available from the engine and the lift due to air density both of which are affect significantly the UAVs take-off distance and operational margins. In the cruise, engine performance and fuel consumption are affected (Moir & Allan, 2003).

These instruments are connected to an air data computer (ADC) which centrally measures air data and provides corrected data to the recipient subsystems. It can determine calibrated airspeed, Mach number, altitude, and altitude trend data from an aircraft's pitot-static system. It also measures the static air pressure and true air speed from the input of total air temperature.

The air data computer will be displayed on the ground station where the sensor operator will interpret the information given and use it for the safe operation of the aircraft.

Inertial Sensors

The UAV is fitted with what is known as the inertial navigation system (INS) which is essentially a navigation aid that uses a computer, accelerometers and gyroscopes to continuously calculate the position, orientation and velocity of the aircraft by means of dead reckoning. It is very suitable because it does not require any external references for it to determine the position, orientation or velocity of the UAV.

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Inertial navigation is accomplished by integrating the output of a set of sensors to compute position, velocity, and attitude. The sensors used are gyroscopes and accelerometers. Gyroscopes measure angular rate with respect to inertial space, and accelerometers measure linear acceleration, again with respect to an inertial frame (Stovall, 1997).

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The UAV will use what is known as the strapdown system. In this system, the inertial sensors are mounted directly to the aircraft’s structure and no gimballed platform is required. The inertial sensors are resolved mathematically using a computer prior to performing the necessary navigation calculations. The removal of the stabilized platform and the use of modern Ring Laser Gyro (RLG) sensor technology considerably increases the strapdown system’s reliability (Moir & Allan, 2003). Another advantage of this system is that it greatly reduces cost and eliminates what is known as gimbal lock. Gimbal lock is the phenomenon of two rotational axis of an object pointing in the same direction. Simply put, it means your object won't rotate how you think it ought to rotate (Andersson, n.d).

Imaging Sensors

The UAV’s payload of state of the art imaging sensors includes an Electro-optical sensor (EO), an infrared camera and a thermal imaging camera.

The Electro-optical sensor suite is the VERSATRON Skyball SA-144/18 quartet sensor. It consists of a PtSi 512 x 512 MWIR (Mid Wave IR) FLIR with six fields of view (to easily perform either detection or recognition or identification), a colour TV camera with a 10X zoom, a colour TV 9OOmm camera and an eye safe pulsed Er: glass laser rangefinder (this Er: glass laser could advantageously be replaced by an eye safe Er: YAG laser because YAG is a better heat sink than glass enabling a higher efficiency). The diameter of the Electro-optical sensor turret is relatively small-35cm. The turret has precision pointing with a line-of-sight stabilization accuracy of 10 prad (Schweicher, 1999).

These sensors all relay images in real time or nearly real time to the sensor operator in the ground station. They are invaluable in the search and rescue process and enable the sensor operator to be able to detect and identify the target with precision.

Radar Sensors

The UAV carries a number of radar sensors that permit the aircraft to derive data concerning the flight of the aircraft. The principle radar sensors are:

Synthetic Aperture Radar (SAR)
Weather-sensing radar system
Radar altimeter

Synthetic Aperture Radar (SAR)

Synthetic Aperture Radars (SAR) typically provides a two-dimensional representation of the scatters over an extensive area that has been illuminated with microwaves. These two dimensions are called slant range and azimuth resolution. It is known that SAR systems obtain high range resolution transmitting signals with a large bandwidth (González-Partida, 2008).

This radar is very useful in search and rescue because of the need to identify objects on the ground from a relatively high altitude. The high resolution images obtained from the SAR help in detecting, classifying and identifying them. To achieve high resolution, it is necessary to transmit a large bandwidth. The range resolution is inversely proportional to the RF transmitted bandwidth. The system works in the millimeter-wave band. Thus, a large bandwidth can be more easily transmitted because the relative bandwidth is low (González-Partida, 2008).

The SAR provides all-weather surveillance capability and can detect objects that infrared or electro-optical cameras cannot. It relays the images in real-time or nearly real-time to the ground station and thus offers the semsor operator and pilot a real-time view of what is on the ground below the UAV.

With the advent of the miniSAR which weighs less than one-fourth the weight and one-tenth the volume of SARs that currently fly on larger UAVs this technology will be very suitable for our specific UAV since it greatly cuts on weight. It has the same capability as the larger SARs of making fine-resolution images through weather, at night, and in dust storms while still being smaller and cheaper.

The miniSAR consists of two major subsystems: the Antenna Gimbal Assembly (AGA) - the pointing system that consists of the antenna, gimbal, and transmitter - and the Radar Electronics Assembly (REA) - the signal generator, receiver, and processors. The AGA transmits and receives the radar signal. The REA is the electronics package that generates the radar signals, controls the system, processes the data, and transforms it into an image (Sandia National Laborotories, 2005)

Weather-Sensing Radar Systems

The weather radar is used to alert the pilot of adverse weather or terrain in the aircraft’s flight path. It radiates energy in a narrow beam with a beam width of -3° which may be reflect from clouds or terrain ahead of the UAV. The radar beam is scanned either side of the aircraft centre-line to give a radar picture of objects ahead of the aircraft (Moir & Allan, 2003).

It is fitted with a feature that can detect turbulence ahead of the aircraft through a process known as Doppler processing. This is a very key feature because it is known that maximum wind shear does not necessarily occur when there is heaviest precipitation.

The sensor operator can see the radar picture on either a dedicated radar display or it might be overlaid on the navigation display. It is displayed in colour making it easier to interpret and it has various selectable range markers which are referenced to the aircraft heading.

As useful as the information from the weather radar may be, the sensor operator and pilot must be able to interpret it well enough while on the search and rescue mission. Additional information from air traffic controllers may help as well.

Radar Altimeter

A radar altimeter is a limited scale aircraft altitude measurement instrument that indicates absolute altitude or the aircraft's exact height above the ground. The instrument achieves this by directing radio waves directly down at the ground and reading the reflected signals. The time lapse between transmitted and received signals is then used to calculate the height above ground level (Scott, 2011). This makes the radar altimeter a very useful tool as it can be used as part of the UAVs ground proximity warning system (GPWS). The GPWS is a type of equipment that warns the pilot when the UAV is at a dangerously low altitude and in danger of crashing.

There is a newer version of the radar altimeter which is half the size of the original one; weighing only 1.5 kg and about 35%-50% cheaper. It is known as the Miniature Radar Altimeter and it will be most suitable for our UAV.

COMMUNICATION SYSTEMS

The sensors described in the previous section do not require the assistance of a third party and are fully autonomous to the UAV. However, there are other systems which the UAV uses for communication that fully depend on external equipment such as transmitters and beacons. As the principle purpose of these aircraft is to obtain quality, high bandwidth data and move that information into the hands of the sensor operator and the pilot as quickly as possible, the communication link is a key element of the UAV system.

The communications system comprises:

Satellite Communication (SATCOM)
Traffic collision and avoidance system (TCAS)
Automatic Dependent Surveillance Broadcast System (ADS-B)

Satellite Communications (SATCOM)

This form of communication provides a very reliable method of communication because it uses the International Maritime Satellite Organization (INMARSAT) satellite constellation. The principle in which it works is quite simple. The UAV communicates via the INMARSAT constellation and remote ground earth station by means of a C-band uplinks and downlinks to or from the ground stations and L-band links to or from the UAV.

In this way, communications are routed from the aircraft via the satellite to the ground earth station and on to the UAV’s ground station where the pilot and sensor operator are. The pilot or sensor operator sends communications to the aircraft in the revers fashion. Therefore, so long as the UAV is within the area of coverage of a satellite then communication may be established (Moir & Allan, 2003).

Traffic collision and avoidance system (TCAS)

As a means of identifying the UAV and to facilitate the safe passage of the aircraft through controlled airspace, an air traffic control (ATC) transponder is fitted into it and allows the ground surveillance radars to interrogate the aircraft and decode data, which enables correlation of a radar track with the UAV.

A collision and conflict avoidance system for autonomous unmanned air vehicles (UAVs) uses accessible on-board sensors to generate an image of the surrounding airspace. The situation thus established is analysed for imminent conflicts (collisions, TCAS violations, airspace violations), and, if a probable conflict or collision is detected, a search for avoidance options is started, wherein the avoidance routes as far as possible comply with statutory air traffic regulations. By virtue of the on-board algorithm the system functions independently of a data link. By taking into account the TCAS zones, the remaining air traffic is not disturbed unnecessarily. The system makes it possible both to cover aspects critical for safety and to use more highly developed algorithms in order to take complicated boundary conditions into account when determining the avoidance course (Heni, Knoll, & Beck, n.d).

Automatic Dependent Surveillance-Broadcast (ADS-B)

This is a new type of technology that is very suitable for our search and rescue UAV. It is essentially advanced GPS-based technology that’s optimised when deployed in formation. The system uses signals from the Global Navigation Satellite System which provide the air traffic controllers and the pilot with much more accurate information that will help keep aircraft safely separated in the sky and on runways.

ADS-B works by having aircraft transponders receive satellite signals and using transponder transmissions to determine the precise locations of aircraft in the sky. The system converts that position into a unique digital code and combines it with other data from the aircraft’s flight monitoring system - such as the type of aircraft, its speed, its flight number, and whether it is turning, climbing, or descending. The code containing all of this data is automatically broadcast from the aircraft’s transponder once a second. Aircraft equipped to receive the data and ADS-B ground stations up to 200 miles away receive these broadcasts. ADS-B ground stations add radar-based targets for non-ADS-B-equipped aircraft to the mix and send all of the information back up to equipped aircraft - this function is called Traffic Information Service-Broadcast (TIS-B). ADS-B ground stations also send out graphical information from the National Weather Service and flight information, such as temporary flight restrictions - this is called Flight Information Service-Broadcast (FIS-B) (Federal Aviation Administration, 2007).

With this new system, the pilot an the air traffic controller will see the same real time displays of air traffic. At night and in poor visual conditions, pilots will also be able to see where they are in relation to the ground using on-board avionics and terrain maps. This will greatly increase the safety of the UAV in the sky.

These communication systems need some degree of tailoring in order for us to obtain best performance on the UAV. Some of the design issues include:

Antenna Placement - the number of potential antenna mounting points is limited. “Shadowing” or blocking of the antenna is routinely a problem, as the single most common mission is flying in circles while monitoring a fixed or slowly moving point below. For many antenna locations, there will be some part of the orbit in which the antenna is shadowed by the aircraft fuselage, resulting in a link dropout. Use of antenna space diversity techniques in the aircraft can greatly improve shadowing performance.
Self-Jamming - Almost every UAV has receivers for GPS and as well as for command and control for the payloads and the aircraft navigation systems. Care is required to ensure that the downlink transmitter does not de-sensitize these receivers, which typically involves analysis of transmitter emissions and receiver selectivity as well as care in antenna placement.
Asymmetric Transmissions - for most UAVs the downlink data transmission bit rate is typically much greater than the uplink rate. Coupled with the fact that aircraft power supply constraints limit the amount of transmitter power, this means that the downlink is substantially disadvantaged compared to the uplink. Techniques such as downlink antenna space diversity provide gain in multipath channels, and are one way to recover some of this disadvantage.
Relay Capability — it is easier to acquire good imagery when operating at low altitudes, since there is less atmospheric haze and shorter focal-length lenses are lighter and have less stringent stabilization requirements. Operational altitudes from 2,000-5,000 feet are commonplace, but these altitudes cut line-of-sight range to about 50-100 miles when the terrain is smooth, or even less in hilly or mountainous terrain. Using a second high-flying UAV to relay the ISR data can extend the range substantially. (Gardner, 2009)

Therefore taking all these things into consideration, a well-conceived system approach can provide an interoperable system that can be tailored to a range of specific mission scenarios.

Bibliography

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