aerosols

Schematic diagram of the many interactions of aerosols in the environment and with clouds.

Focus Area Lead - Steve Ghan

The role of aerosols in climate forcing is a critical factor in assessment and prediction of climate changes, as well as in advancing climate modeling frameworks. Aerosol forcing has two major components, direct and indirect. Direct effects of aerosols are the influence of the aerosols on the planet's radiation balance by the scattering of solar radiation, which result in the cooling of the Earth's surface. Aerosols can also have an indirect effect on climate, based on the ways in which aerosols interact with surrounding clouds. Direct effects of aerosols on the climate system are much better understood and quantified than the indirect effects.

kinetic energy

The kinetic energy of an object is the extra energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its current velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. Negative work of the same magnitude would be required to return the body to a state of rest from that velocity.

Kinetic energy for single objects is completely frame-dependent (relative). For example, a bullet racing by a non-moving observer has kinetic energy in the reference frame of this observer, but the same bullet has zero kinetic energy in the reference frame which moves with the bullet. The kinetic energy of systems of objects, however, may sometimes not be completely removable by simple choice of reference frame. When this is the case, a residual minimum kinetic energy remains in the system as seen by all observers, and this kinetic energy (if present) contributes to the system's invariant mass, which is seen as the same value in all reference frames, and by all observers.

PICTURE: The cars of a roller coaster reach their maximum kinetic energy when at the bottom of their path. When they start rising, the kinetic energy begins to be converted to gravitational potential energy. The sum of kinetic and potential energy in the system remains constant, assuming negligible losses to friction.

Directed-energy weapon

A directed-energy weapon (DEW) is a type of weapon that emits energy in an aimed direction without the means of a projectile. It transfers energy to a target for a desired effect. Some of these weapons are real or in development; others are at present only science fiction.

The energy can come in various forms:

• Electromagnetic radiation (typically lasers or masers).
• Particles with mass (particle beam weapons).
• Sound (sonic weaponry)

In science fiction, these weapons are sometimes known as death rays or rayguns and are usually portrayed as projecting energy at a person or object to kill or destroy.

Some lethal directed-energy weapons are under active research and development, but most examples appear in science fiction (non-functional toys, film props or animation).

[edit] Tactical considerations and problems

Lasers have several main advantages over conventional weaponry:

• Laser beams travel at the speed of light, unlike projectile weapons, so there is no need in terrestrial applications to aim ahead to allow for the target moving while the shot travels as the transit time over such distances is virtually zero.
• The speed of delivery means that the target has no chance to detect or evade (in contrast with enemy aircraft targeted with anti-aircraft missiles), and that some third object does not have the time to accidentally move into the trajectory while the energy is delivered.
• Light's short transit time also nearly eliminates the influence of gravity, so long range projection does not require compensation for such. Other aspects such as wind speed can be ignored.
• Lasers can provide a level of pin-point accuracy that cannot be matched by a projectile.
• Some lasers run on electricity which can be cheaply generated, reducing the need for expensive and finite ammunition, possibly replacing it with smaller batteries that would hold many more shots. However, building portable electric power sources of sufficient energy capacity is a problem.
• Because light has a practically nil ratio (exactly 1 / c) of momentum to energy, lasers produce negligible recoil.
• Laser beams do not "betray" themselves when emitted, either by eyesight or by sound. Unlike missiles ( e.g. ICBMs) there is no system to track and contain them.
• Design of laser weapons does not have to consider forces that the classic ammunition causes during firing.
• The laser could have much longer range than firearms without need of a long barrel or rifling

Since lasers can theoretically defeat artillery and missile attacks, any group fielding an effective laser system will gain decisive advantages in ground, air and space combat. Under radar control, lasers have shot artillery shells in flight, including mortar rounds. This suggests that a primary application of lasers might be as part of a defensive system.

The main difficulty with currently practical lasers is the high expense and fragility of their mirrors and mirror-pointing systems.

Some believe that mirrors or other countermeasures can reduce the effectiveness of high energy lasers. This has not been demonstrated. Small defects in mirrors absorb energy, and the defects rapidly expand across the surface. Protective mirroring on the outside of a target could easily be made less effective by incidental damage and by dust and dirt on its surface. However protective measures have been considered that would evaporate off the surface and reduce the intensity of the beam, such as ablative armor.

Retransmission Homing Guidance

Retransmission Homing Guidance

A more unusual example of homing guidance is the retransmission method. This technique is largely similar to command guidance but with a unique twist. The target is tracked via an external radar, but the reflected signal is intercepted by a receiver onboard the missile, as in semi-active homing. However, the missile has no onboard computer to process these signals. The signals are instead transmitted back to the launch platform for processing. The subsequent commands are then retransmitted back to the missile so that it can deflect control surfaces to adjust its trajectory.

This method is also sometimes called "track via missile" (TVM) since the missile acts as a conduit of tracking information from the target back to the ground control station. The advantage of TVM homing is that most of the expensive tracking and processing hardware is located on the ground where it can be reused for future missile launches rather than be destroyed. Unfortunately, the method also requires excellent high-speed communication links between the missile and control station, limiting the system to rather short ranges. Retransmission homing guidance is used on the Patriot surface-to-air missile.

HOMING GUIDANCE

HOMING GUIDANCE

Homing guidance is the most common form of guidance used in anti-air missiles today. Three primary forms of guidance fall under the homing guidance umbrella--semi active, active, and passive. We will discuss each of these in turn, as well as a more unusual form called retransmission or track-via-missile homing.

Passive radiation homing

Passive radiation homing

Many missiles employing this type of guidance have an extra trick up their sleeves; If the target does attempt to jam them using some kind of ECM, they can in effect turn into an anti-radiation missile and home in on the target's radiation passively. This makes such missiles practically immune to ECM, in addition to removing the second disadvantage. Since they already have the radar receiver on board, this should not be a difficult feature to add (at least, it requires extra processing logic but little extra hardware).

Operation

Active radar homing is rarely employed as the only guidance method of a missile. It is most often used during the terminal phase of the engagement, mainly because since the radar transceiver has to be small enough to fit inside a missile and has to be powered from batteries, therefore having a relatively low ERP, its range is limited. To overcome this, most such missiles use a combination of command guidance with an inertial navigation system (INS) in order to fly from the launch point until the target is close enough to be detected and tracked by the missile. The missile therefore requires guidance updates via a datalink from the launching platform up until this point, in case the target is maneuvering, otherwise the missile may get to the projected interception point and find that the target is not there. Sometimes the launching platform (especially if it is an aircraft) may be in danger while continuing to guide the missile in this way until it 'goes active'; In this case it may turn around and leave it to luck that the target ends up in the projected "acquisition basket" when the missile goes active. It is possible for a system other than the launching platform to provide guidance to the missile before it switches its radar on; This may be other, similar fighter aircraft or perhaps an AWACS.

Active radar homing

Active radar homing is a missile guidance method in which a guided missile contains a radar transceiver and the electronics necessary for it to find and track its target autonomously. NATO brevity code for an active radar homing missile launch is Fox Three.

Advantages

There are two major advantages to active radar homing:

• Because the missile is tracking the target, and the missile is typically going to be much closer to the target than the launching platform during the terminal phase, the tracking can be much more accurate and also have better resistance to ECM. Active radar homing missiles have some of the best kill probabilities, along with missiles employing track-via-missile guidance.
• Because the missile is totally autonomous during the terminal phase, the launch platform does not need to have its radar enabled at all during this phase, and in the case of a mobile launching platform like an aircraft, can actually exit the scene or undertake other actions while the missile homes in on its target. This is often referred to as fire-and-forget capability and is a great advantage that modern air-to-air missiles have over their predecessors.

Disadvantages

There are two major disadvantages to active radar homing:

• Since the missile has to contain an entire radar transceiver and electronics, it was until recently difficult to fit all of this into a missile without unacceptably increasing its size and weight. Even with today's miniaturisation making this possible, it is quite expensive to make these missiles since the sophisticated electronics within the missile are inevitably destroyed upon impact.
• There is very little chance that targets with any sort of decent radar warning receiver would be unaware that an incoming missile is approaching them. This gives them sufficient time to take evasive action and deploy countermeasures. However, given the accuracy of this homing method, unless the target is especially maneuverable or the missile is not, there may not be much they can do to avoid being intercepted.
• These types of missiles with this mounted equipment is only effective in long range confrontations.

Infrared (IR)

Infrared (IR) radiation is electromagnetic radiation whose wavelength is longer than that of visible light (400-700 nm), but shorter than that of terahertz radiation (100 µm - 1 mm) and microwaves (~30,000 µm). Infrared radiation spans roughly three orders of magnitude (750 nm and 100 µm).

Direct sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux, which includes infrared (47% share of the spectrum), visible (46%), and ultra-violet (only 6%) light. Bright sunlight provides luminance of approximately 100,000 candela per square meter at the Earth's surface.

PICTURE: Image of two human bodies in mid-infrared ("thermal") light (false-color)

Ultraviolet (UV) light

Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV. It is so named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the color violet.

UV light is found in sunlight and is emitted by electric arcs and specialized lights such as black lights. As an ionizing radiation it can cause chemical reactions, and causes many substances to glow or fluoresce. Most people are aware of the effects of UV through the painful condition of sunburn, but the UV spectrum has many other effects, both beneficial and damaging, on human health.

PICTURE: False-color image of the Sun's corona as seen in deep ultraviolet by the Extreme ultraviolet Imaging Telescope

Chemical lasers

Chemical lasers

Chemical lasers are powered by a chemical reaction, and can achieve high powers in continuous operation. For example, in the Hydrogen fluoride laser (2700-2900 nm) and the Deuterium fluoride laser (3800 nm) the reaction is the combination of hydrogen or deuterium gas with combustion products of ethylene in nitrogen trifluoride. They were invented by George C. Pimentel.

Spectrum of a helium neon laser

Spectrum of a helium neon laser showing the very high spectral purity intrinsic to nearly all lasers. Compare with the relatively broad spectral emittance of a light emitting diode.

A helium-neon laser

A helium-neon laser demonstration at the Kastler-Brossel Laboratory at Univ. Paris 6. The glowing ray in the middle is an electric discharge producing light in much the same way as a neon light. It is the gain medium through which the laser passes, not the laser beam itself, which is visible there. The laser beam crosses the air and marks a red point on the screen to the right.

Gas lasers

Gas lasers

Gas lasers using many gases have been built and used for many purposes.

The helium-neon laser (HeNe) emits at a variety of wavelengths and units operating at 633 nm are very common in education because of its low cost.

Carbon dioxide lasers can emit hundreds of kilowatts[14] at 9.6 µm and 10.6 µm, and are often used in industry for cutting and welding. The efficiency of a CO2 laser is over 10%.

Argon-ion lasers emit light in the range 351-528.7 nm. Depending on the optics and the laser tube a different number of lines is usable but the most commonly used lines are 458 nm, 488 nm and 514.5 nm.

A nitrogen transverse electrical discharge in gas at atmospheric pressure (TEA) laser is an inexpensive gas laser producing UV Light at 337.1 nm.[15]

Metal ion lasers are gas lasers that generate deep ultraviolet wavelengths. Helium-silver (HeAg) 224 nm and neon-copper (NeCu) 248 nm are two examples. These lasers have particularly narrow oscillation linewidths of less than 3 GHz (0.5 picometers),[16] making them candidates for use in fluorescence suppressed Raman spectroscopy.

lidar scanner

This lidar scanner may be used to scan buildings, rock formations, etc., to produce a 3D model. The lidar can aim its laser beam in a wide range: its head rotates horizontally, a mirror flips vertically. The laser beam is used to measure the distance to the first object on its path.

laser rangefinder

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A laser rangefinder is a device which uses a laser beam in order to determine the distance to a reflective object. The most common form of laser rangefinder operates on the time of flight principle by sending a laser pulse in a narrow beam towards the object and measuring the time taken by the pulse to be reflected off the target and returned to the sender. Due to the high speed of light, this technique is not appropriate for high precision sub-millimeter measurements, where triangulation and other techniques are often used.