AIRBUS FLIGHT CONTROL LAWS

AIRBUS FLIGHT CONTROL LAWS

High AOA Protection	Load Factor Limitation	Pitch Attitude Protection
NORMAL LAW
High Speed Protection	Flight Augmentation (Yaw)	Bank Angle Protection

Low Speed Stability	Load Factor Limitation
ALTERNATE LAW
High Speed Stability	Yaw Damping Only

	Load Factor Limitation
ABNORMAL ALTERNATE LAW w/o Speed Stability
	Yaw Damping Only

DIRECT LAW

FLIGHT CONTROL LAWS SUMMARY
NORMAL LAW
Normal operating configuration of the system. Failure of any single computer does not affect normal law. Covers 3-axis control, flight envelope protection, and load alleviation. Has 3 modes according to phase of flight.
Ground Mode	Active when aircraft is on the ground. Direct proportional relationship between the sidestick deflection and deflection of the flight controls. Is active until shortly after liftoff. After touchdown, ground mode is reactivated and resets the stabilizer trim to zero.
Flight Mode	Becomes active shortly after takeoff and remains active until shortly before touchdown. Sidestick deflection and load factor imposed on the aircraft are directly proportional, regardless of airspeed. With sidestick neutral and wings level, system maintains a 1 g load in pitch. No requirement to change pitch trim for changes in airspeed, configuration, or bank up to 33 degrees. At full aft/fwd sidestick deflection system maintains maximum load factor for flap position. Sidestick roll input commands a roll rate request. Roll rate is independent of airspeed. A given sidestick deflection always results in the same roll rate response. Turn coordination and yaw damping are computed by the ELACs and transmitted to the FACs. No rudder pedal feedback for the yaw damping and turn coordination functions.
Flare Mode	Transition to flare mode occurs at 50' RA during landing. System memorizes pitch attitude at 50' and begins to progressively reduce pitch, forcing pilot to flare the aircraft In the event of a go-around, transition to flight mode occurs again at 50' RA.
Protections	Load factor Limitation Prevents pilot from overstressing the aircraft even if full sidestick deflections are applied. Attitude Protection Pitch limited to 30 deg up, 15 deg down, and 67 deg of bank. These limits are indicated by green = signs on the PFD. Bank angles in excess of 33 deg require constant sidestick input. If input is released the aircraft returns to and maintains 33 deg of bank. High Angle of Attack Protection (alpha): When alpha exceeds alpha prot, elevator control switches to alpha protection mode in which angle of attack is proportional to sidestick deflection. Alpha max will not be exceeded even if the pilot applies full aft deflection High Speed Protection: Prevents exceeding VMO or MMO by introducing a pitch up load factor demand. The pilot can NOT override the pitch up command. Low Energy Warning: Available in CONF 2,3, or FULL between 100' and 2,000' RA when TOGA not selected. Produces aural "SPEED SPEED SPEED" when change in flight path alone is insufficient to regain a positive flight path (Thrust must be increased).
ALTERNATE LAW
If Multiple Failures of Redundant Systems occur, the flight controls revert to Alternate Law. The ECAM displays the message: ALTN LAW: PROT LOST
Ground Mode	The ground mode is identical to Normal Law.
Flight Mode	In pitch alternate law the flight mode is a load factor demand law similar to the Normal Law flight mode, with reduced protections. Pitch alternate law degrades to pitch direct law when the landing gear is extended to provide feel for flare and landing, since there is no flare mode when pitch normal law is lost. Automatic pitch trim and yaw damping (with limited authority) is available. Turn coordination is lost. When pitch law degrades from normal law, roll degrades to Direct Law - roll rate depends on airspeed.
Protections	All protections except for load factor maneuvering protection are lost. The load factor limitation is similar to to that under Normal Law. Amber XX's replace the green = attitude limits on the PFD. A low speed stability function replaces the normal angle-of-attack protection System introduces a progressive nose down command which attempts to prevent the speed from decaying further. This command CAN be overridden by sidestick input. The airplane CAN be stalled in Alternate Law. An audio stall warning consisting of "crickets" and a "STALL" aural message is activated. The Alpha Floor function is inoperative. The PFD airspeed scale is modified: VLS remains displayed VALPHA PROT and VALPHA MAX are removed They are replaced by a red and black barber pole, the top indicating the stall warning speed VSW A nose up command is introduced any time the airplane exceeds VMO/MMO to keep the speed from increasing further, which CAN be overridden by the sidestick. Bank angle protection is lost. Certain failures cause the system to revert to Alternate Law without speed stability. Yaw damping is lost if the fault is a triple ADR failure.
ABNORMAL ALTERNATE LAW
Abnormal Alternate Law is activated if the airplane enters an unusual attitude, allowing recovery from the unusual attitude.
	Pitch law becomes Alternate (without autotrim or protection other than Load Factor protection). Roll law becomes Direct law with mechanical yaw control. After recovery from the unusual attitude, the following laws are active for the remainder of the flight: Pitch: Alternate law without protections and with autotrim. Roll: Direct law Yaw: Alternate law There is no reversion to Direct law when the landing gear is extended.
DIRECT LAW
Direct law is the lowest level of computer flight control and occurs with certain multiple failures.
	Pilot control inputs are transmitted unmodified to the control surfaces, providing a direct relationship between sidestick and control surface. Control sensitivity depends on airspeed and NO autotrimming is available. An amber message USE MAN PITCH TRIM appears on the PFD. If the flight controls degrade to Alternate Law, Direct Law automatically becomes active when the landing gear is extended if no autopilots are engaged. If an autopilot is engaged, the airplane will remain in Alternate Law until the autopilot is disconnected. There are no protections provided in Direct Law, however overspeed and stall aural warnings are provided. The PFD airspeed scale remains the same as in Alternate Law.
MECHANICAL BACKUP
In case of a complete loss of electrical flight control signals, the aircraft can be temporarily controlled by mechanical mode.
	Pitch control is achieved through the horizontal stabilizer by using the manual trim wheel. Lateral control is accomplished using the rudder pedals. Both controls require hydraulic power. A red MAN PITCH TRIM ONLY warning appears on the PFD.

F-117A Nighthawk Stealth Fighter, United States of America

The F-117A Nighthawk stealth fighter attack aircraft was developed by Lockheed Martin after work on stealth technology, and the predecessor test demonstrator aircraft, Have Blue, was carried out in secret from 1975.

Development of the F-117A began in 1978 and it was first flown in 1981, but it was not until 1988 that its existence was publicly announced.

"The outer surface of Nighthawk is coated with a radar-absorbent material (RAM)."

The Nighthawk was the world's first operational stealth aircraft. The first aircraft was delivered in 1982 and the last of the 59 Nighthawks procured by the US Air Force was received in 1990.

The F-117A aircraft is also known as the Frisbee and the Wobblin' Goblin. The mission of the aircraft is to penetrate dense threat environments and attack high-value targets with high accuracy. Nighthawk has been in operational service in Panama, during Operation Desert Storm, in Kosovo, in Afghanistan and during Operation Iraqi Freedom. The Nighthawk is only used for night-time missions.

F-117A stealth fighter replacement and retirement

The F-117 was to be replaced in the USAF by the F-22 Raptor before the F-22 programme was cancelled in 2009 and replaced with the cheaper and more versatile F-35 Joint Strike Fighter. The first 10 of the 55 F-117 aircraft in service were retired in December 2006. A formal retirement ceremony took place at Wright-Patterson AFB in March 2008.

The F-117s are being stored in hangars at an airfield in the Tonopah Test Range, Nevada. The wings and tails are being removed for storage, but some aircraft will be able to be rapidly recalled to flight if required. The last four F-117 aircraft flew to Tonopah on April 22nd 2008.

Nighthawk stealth fighter design

The surfaces and edge profiles are optimised to reflect hostile radar into narrow beam signals, directed away from the enemy radar detector. All the doors and opening panels on the aircraft have saw-toothed forward and trailing edges to reflect radar.

The aircraft is mainly constructed of aluminum, with titanium for areas of the engine and exhaust systems. The outer surface of the aircraft is coated with a radar-absorbent material (RAM). The radar cross-section of the F-117 has been estimated at between 10cm² and 100cm².

The F-117A has four elevons on the inboard and outboard trailing edge of the wing. The V-shaped tail, which controls the yaw of the aircraft, acts as a flying tail, which means that the whole surface acts as a control surface. The elevons do not act as flaps to reduce the rate of descent for touchdown, so the landing speed of the F-117A is high, at about 180mph to 190mph, and a drag parachute is used.

"The Nighthawk's surfaces and edge profiles are optimised to reflect hostile radar."

Cockpit

The cockpit has a Kaiser Electronics head-up display (HUD) and the flight deck is equipped with a large video monitor, which displays the infrared imagery from the aircraft's onboard sensors. The cockpit has a full-colour moving map developed by the Harris Corporation. The fly-by-wire system is supplied by BAE Systems Aircraft Controls.

Weapons

The aircraft can carry a range of tactical fighter ordnance in the weapons bay, including BLU-109B low-level laser-guided bomb, GBU-10 and GBU-27 laser-guided bomb units, Raytheon AGM-65 Maverick and Raytheon AGM-88 HARM air-to-surface missiles.

In January 2004, an F-117 successfully released a JDAM (JDAM) 2,000lb bomb for the first time. The integration of JDAM and other precision-guided weapons on the F-117 is coupled with the block II software upgrade and achieved initial operating capability (IOC) in 2006.

FLIR and DLIR sensors

For stealth, the F-117A does not rely on radar for navigation or targeting. For navigation and weapon aiming, the aircraft is equipped with a forward-looking infrared (FLIR) and a downward-looking infrared (DLIR) with laser designator, supplied by Raytheon. The aircraft uses a Honeywell inertial navigation system.

The aircraft has multi-channel pilot static tubes installed in the nose. Multiple ports along the length of the tubes provide differential pressure readings. The flight control computers compare these in order to provide the aircraft's flight data.

Flight management

Before flight, mission data is downloaded on to the IBM AP-102 mission control computer, which integrates it with the navigation and flight controls to provide a fully automated flight management system.

After take-off, the pilot can hand over flight control to the mission programme until within visual range of the mission's first target. The pilot then resumes control of the aircraft for weapon delivery.

"The F-117A aircraft is also known as the Frisbee and the Wobblin' Goblin."

The aircraft is equipped with an infrared acquisition and designation system (IRADS), which is integrated with the weapon delivery system. The pilot is presented with a view of the target on the head-up display, first from the FLIR and then from the DLIR.

The weapon delivery and impact is recorded on the aircraft's internally mounted video system, which provides real-time damage assessment.

F-117A engines

The F-117A is powered by two low-bypass F404-GE-F1D2 turbofan engines from General Electric. The rectangular air intakes on both sides of the fuselage are covered by gratings, which are coated with radar-absorbent material.

The wide and flat structure of the engine exhaust area reduces the infrared and radar detectability of the aft section of the engine. The two large tail fins slant slightly outwards to provide an obstruction to the infrared and radar returns from the engine exhaust area.

F-35 Specs

Autonomic Logistics (AL)
Because logistics support accounts for two-thirds of an aircraft's life cycle cost, the F-35 will achieve unprecedented levels of reliability and maintainability, combined with a highly responsive support and training system linked with the latest in information technology. The aircraft will be ready to fight anytime and anyplace. Autonomic Logistics (AL) is a seamless, embedded solution that integrates current performance, operational parameters, current configuration, scheduled upgrades and maintenance, component history, predictive diagnostics (prognostics) and health management, and service support for the F-35. Essentially, AL does invaluable and efficient behind-the-scenes monitoring, maintenance and prognostics to support the aircraft and ensure its continued good health.

Commonality Commonality is the key to affordability – on the assembly line; in shared-wing platforms; in common systems that enhance maintenance, field support and service interoperability; and in almost 100 percent commonality of the avionics suite. Component commonality across all three variants reduces unique spares requirements and the logistics footprint. In addition to reduced flyaway costs, the F-35 is designed to affordably integrate new technology during its entire life cycle.

Distributed Aperture System In a joint effort with Lockheed Martin Missiles and Fire Control, Northrop Grumman Electronic Systems will provide key electronic sensors for the F-35, which includes spearheading the work on the Electro-Optical Distributed Aperture System (DAS). This system will provide pilots with a unique protective sphere around the aircraft for enhanced situational awareness, missile warning, aircraft warning, day/night pilot vision, and fire control capability.

Diverterless Inlet The F-35's diverterless inlet lightens the overall weight of the aircraft. Traditional aircraft inlets were comprised of many moving parts and are much heavier than newer diverterless inlets. The diverterless inlet also eliminates all moving parts.

Electro-Optical Targeting System Lockheed Martin Missiles and Fire Control and Northrop Grumman Electronic Systems are jointly providing key electronic sensors for the F-35 to include the Electro-Optical Targeting System (EOTS). The internally mounted EOTS will provide extended range detection and precision targeting against ground targets, plus long range detection of air-to-air threats.

Helmet Mounted Display System Vision Systems International, LLC (VSI) is developing the most advanced and capable Helmet Mounted Display System (HMDS) for the F-35. Utilizing extensive design experience gained on successful production Helmet Mounted Displays (HMD), the F-35 HMDS will replace the traditional Head-Up-Display (HUD) while offering true sensor fusion.

Integrated Communications, Navigation and Identification Avionics
Northrop Grumman Space Technology's integrated avionics satisfy the requirements for greatly increased functionalities within extreme space and weight limitations via modular hardware that could be dynamically programmed to reconfigure for multiple functions. This "smart"-box approach delivers increased performance, quicker deployment, higher availability, enhanced scalability and lower life cycle costs.

Interoperability
The F-35 will have the most robust communications suite of any fighter aircraft built to date. The F-35 will be the first fighter to possess a satellite communications capability that integrates beyond line of sight communications throughout the spectrum of missions it is tasked to perform. The F-35 will contain the most modern tactical datalinks which will provide the sharing of data among its flight members as well as other airborne, surface and ground-based platforms required to perform assigned missions. The commitment of JSF partner nations to common communications capabilities and web-enabled logistics support will enable a new level of coalition interoperability. These capabilities allow the F-35 to lead the defense community in the migration to the net-centric warfighting force of the future.

Low Observability
An integrated airframe design, advanced materials and an axisymmetric nozzle maximize the F-35's stealth features.

Multi-Function Display System
An 8"x20" Multi-Function Display System (MFDS) will be the panoramic projection display for the F-35. MFDS employs leading edge technology in projection engine architecture, video, compression, illumination module controls and processing memory – all of which will make the MFDS the most advanced tactical display. One-gigabyte-per-second data interfaces will enable the MFDS to display six full motion images simultaneously. The adaptable layout will be easily reconfigurable for different missions or mission segments. Projection display technology will provide a high-luminance, high-contrast, and high-resolution picture with no viewing angle effect.

Multi-Mission Active Electronically Scanned Array (AESA) Radar
Northrop Grumman Electronic Systems is developing the Multi-Mission Active Electronically Scanned Array (AESA) Radar for the F-35. This advanced multi-function radar has gone through extensive flight demonstrations during the Concept Demonstration Phase (CDP). The radar will enable the F-35 JSF pilot to effectively engage air and ground targets at long range, while also providing outstanding situational awareness for enhanced survivability.

Propulsion
The F-35 Propulsion Systems are the most powerful fighter/attack turbofans in the world. There are two manufacturers with propulsion systems currently being tested. The propulsion systems are interchangeable and both will power the F-35. There are two major engine variants for the F-35. One engine will power the CTOL and CV versions of the aircraft, while the other will power the STOVL version. The F135 engine is made by Pratt & Whitney.

F135
The Pratt & Whitney F135 family of advanced propulsion systems utilize cutting edge technology to provide the F-35 with higher performance than conventional fighter aircraft. The engine consists of a 3-stage fan, a 6-stage compressor, an annular combustor, a single stage high-pressure turbine, and a 2 stage low-pressure turbine.

The F135 is currently in the SDD phase. The F135 is using the lessons learned from the F119 engine core and the JSF119 during the CDA stage to reduce risk in SDD. During SDD the F135 test engines will undergo a range of ground and flight tests to simulate various mission profiles. In these tests the system demonstration engines will be run for hours throughout various flight envelopes to ensure they meet performance requirements. One of the vital milestone tests occured at the end of 2003 with the first F135 engine to test.

The first CTOL F135 engine test occurred on 11 October 2003. The first STOVL F135 engine test occurred on 14 April 2004. To date over 2,000 hours have been accumulated on the F135 test engines.

Rolls-Royce Lift System
Rolls-Royce is subcontracted to Pratt & Whitney on the F135 to provide the Lift System for the F-35. The Lift System is comprised of the Lift Fan, Clutch, Drive Shaft, Roll Posts and the Three Bearing Swivel Module (3BSM).

Shaft Driven Lift Fan (SDLF)
Lockheed Martin developed the idea for a Short Take-Off Vertical Landing (STOVL) lift system that uses a vertically oriented Shaft Driven Lift Fan (SDLF). A two-stage low-pressure turbine on the engine provides the horsepower necessary to power the Rolls-Royce designed Lift Fan. The Lift Fan generates a column of cool air that provides nearly 20,000 pounds of lifting power using variable inlet guide vanes to modulate the airflow, along with an equivalent amount of thrust from the downward vectored rear exhaust to lift the aircraft. The Lift Fan utilizes a clutch that engages the shaft drive system for STOVL operations. Because the lift fan extracts power from the engine, exhaust temperatures are reduced by about 200 degrees compared to traditional STOVL systems.

The SDLF concept was successfully demonstrated through a Large Scale Powered Model (LSPM) in 1995-96 and during the flight-testing of the X-35B during the summer of 2001. The Lift Fan, a patented Lockheed Martin concept, was developed and produced by Rolls-Royce Corp. in Indianapolis, Indiana and in Bristol, England.

Robust Structure
Continuous tailhook-to-nose-gear structure and catapult-compatible nose gear launch system are strengthened for catapult and arresting loads.

Sophisticated Cockpit The F-35 provides its pilot with unsurpassed situational awareness, positive target identification and precision strike under any weather condition. Mission systems integration and outstanding over-the-nose visibility features are designed to dramatically enhance pilot performance.

Weapons Integration The F-35 will employ a variety of US and allied weapons. From JDAMs to Sidewinders to the UK Storm Shadow, the F-35 has been designed to carry either internally or externally a large array of weapons.

Laser Gyroscopes

1.          Introduction

Laser gyroscopes offer considerable advantages over their mechanical predecessors.  By utilising properties of wave mechanics and light, more durable^[1] devices with superior accuracy^[1] than the previous mechanical generation may be produced, with no moving parts^[1] and no maintenance required^[1].  These exhibit a high dynamic range for measurements of rotation about their central axis, often from thousandths of a degree per second[*]^[2] to tens^[1] of degrees per second, with typical sample rates near the order 100Hz^[1].

2.          Ring-laser gyroscopes (RLGs)

One common setup is a ring-laser, and (despite its name) often has a square or triangular cavity.  As light propagates in both directions around the "ring", the ring becomes a resonator.  Due to the finite speed of light, a continuous rotation of the ring alters the distances that each beam must travel to make a full circuit^[1].  In order to resonate, the frequencies of both beams must change to account for the altered circuit lengths^[1].  This change in path lengths can be detected by the spatial interference between the beams as they lose phase cohesion, or by the temporal interference due to the mixing of two slightly different frequencies (resulting in low-frequency “beating”)^[1].  This change in frequencies can be treated classically, as it is not a result of any relativistic Doppler effect (which cancels out due to the symmetry of the device^[3]).  Omissions between this simple model and practical implementations cause some minor problems, however these are easily solved without significantly increasing the complexity of the device.

While sources other than lasers have been used^[8] in passive gyroscopes, lasers provide narrower bandwidths than their alternatives, resulting in measurements that are more precise. The active gyroscopes, being ring-lasers, would not be possible without laser technology.

Now we will build a mathematical model of this device and derive some important equations that describe its operation.

2.1. Light resonating in a ring

Consider a beam of light, resonating around a closed ring (containing a gain medium) of circumference L, enclosing area A (Fig. A):

Fig. A             Light travelling around a circular path

A quick refresh of geometry reminds us:

The time taken for the light to make one pass round the loop, t_pass, is given by:

 ,
where c is the speed of light in the ring.

Now we mark a point along the ring α (which is at a distance r from a rotational axis).  If the ring is rotated at a constant angular velocity ω about the centre (Fig. B), the distance that the light must travel to make a full round-trip ‑ starting from α and finishing at α ‑ will be lengthened or shortened by the distance that α travels during that time.

Fig. B             Light travelling around a rotating circular path[†]

As the ring is a resonator, it must satisfy (periodic) boundary conditions, which requires it to fit an integral number of wavelengths of the beam into one rotation:

Therefore, a shift in resonant frequency must occur under rotation, to satisfy the new path length:

2.2. Two counter-propagating beams resonating in a rotating ring

We now introduce a second beam, almost identical to the first, but travelling in the opposite direction around the ring (Fig. C).  With no rotation, these will form simple standing waves in the ring, analogous to those formed in a typical laser cavity[‡]. 

Fig. C            Two counter-propagating beams in the ring

When the angular velocity of the ring is non-zero, a difference in perceived resonator-length emerges between the two beams, as shown in equation (115).  This causes the resonant frequencies for each direction of propagation to diverge according to equation (119).

For example, a device with a path in the shape of an equilateral triangle with 10 cm sides: A = 43 cm², L = 30 cm, using a HeNe gain medium (λ = 632.8 nm) will respond to a rotation rate of 0.01 ° s^-1 with a beat frequency of 16 Hz (beat period of 62 ms).

2.3. Realistic design for a ring-laser gyroscope

In reality, light does not typically travel in circular paths, and a completely closed loop would make measurements somewhat difficult. By splitting the "loop" as shown in Fig. D, the beating may be measured by a detector with a suitably high temporal resolution.

Fig. D       A typical laser gyroscope, with rotation Ω. Adapted from [6]

Here, the two beams are initially in-phase and have the same wavelength.  On application of a steady rotation, the resulting difference in path-lengths between the beams results in a phase difference^[4], which the lasing medium converts to a frequency difference^[5], causing the beating at the detector.

2.4. Enter the third dimension

If the axis of rotation is not perpendicular to the plane of the ring, the measured rate of rotation will be less than the actual rate of rotation, specifically:

Where the axis of rotation is unknown, three independent laser-gyroscopes may be mounted at right angles to each other, to provide triaxial rate-of-rotation measurements.  An example of such a device is pictured below (Fig. E):

Fig. E             "Three axis laser gyro", Prof. A E Siegman, Stanford University, retrieved on 10-Dec-2010 from:
http://www.stanford.edu/~siegman/ring_laser_gyros/Three%20axis%20laser%20gyro%20med.jpg

3.          Design Considerations

The model used so far is very simple, and lacks certain important features.  The effects of these omissions can cause the device to respond non-linearly, or not to respond at all to some small rotations.  Imperfections in the cavity are mostly responsible for the former problems, while the latter are due to interactions between the beams.

Fig. F             Different deviations from the ideal response curve for different sources of error
Adapted from [6]

3.1. Lock-in

Slight backwards scattering of the beams results in a small interaction between them in the resonator.  When the angular velocity of the system is small, the frequency difference between the beams is also very small, allowing this interaction to cause unintended mode-locking, where both beams will lock to the same frequency, as shown in Fig. G.  While this threshold rotation value can be calculated^[6]_, little can be done to prevent the mode-locking from occurring within that threshold.

Fig. G            The frequency shift (Δυ) vanishes for small rates of rotation (Ω) due to mode-locking.
Acquired from [6]

Application of an electric field across the lasing medium may utilise the Faraday effect to alter the polarisation^[6] of the beams in order to reduce their interaction, however this also alters the frequencies of the beams^[5], resulting in another source of potential error.

A different approach is to continuously rotate the system by a fixed amount (much larger than the threshold value), in order to keep the device's angular velocity out of mode-locking range.  Mechanical rotation introduces a lot of error in the measurements^[5], as the rate of mechanical rotation is not known exactly, or perfectly steady[§].  A more successful approach is "dithering": apply a small oscillation to the angular velocity.  There is no net measured effect of this when averaged over several oscillation cycles, as the clockwise rotation cancels out the anticlockwise rotation.  This oscillation reduces the amount of time that the angular velocity of the system is within the mode-locking threshold, allowing smaller rates of rotation to be measured.  One negative effect of this method is the production of non-linear "dead-bands", where the rotational frequency is an integer multiple of the "dithering" frequency^[5].

Finally, by oscillating the position of one of the cavity mirrors slightly, a Doppler shift can be introduced, increasing the spacing between the frequency of a back-scattered wave and the resonating wave propagating in the same direction^[6].

3.2. Null-shift

In a gas laser, atoms in the cavity circulate^[7][5], resulting in a slight variation of refractive index depending on the direction of a beam through the medium^[5].  This causes a slight frequency-shift between the two beams and a non-zero beat frequency when the system is stationary^[7].  This displaces the response curve of the device^[1].

One simple solution to this problem is to use two discharge tubes, discharging in opposite directions with respect to the beam path:

 

Fig. H            Two discharge tubes in the ring laser, with discharges directed in opposite directions in order to cancel out the frequency shift due to flowing of gain atoms.
Acquired from [5]

The frequency shifts from each tube are equal and opposite, cancelling out the null-shift.

3.3. Other error

Even with the previous corrections, noise and non-linear optical behaviour may cause the response of the device to deviate from a straight line.  A well-calculated choice of materials and good quality optics can reduce these errors.

4.          Fibre-optic gyroscopes (FOGs)

By not having a gain medium in the ring, the change in path-lengths will manifest as phase displacements rather than frequency shifts.  From equation (114) and the wave equation, the effects of the interference between these two shifted waves at the detector can be calculated:

[5]

Unfortunately, while high-precision detectors capable of detecting small changes in intensity may exist, very long paths may be needed in order to produce detectable changes in the intensity unless a fast rotational rate is expected.  For example, Michelson^[8] used a device with A = 0.21 km² to accurately measure the change in interference patterns due to the rotation of the Earth.

4.1. Theory

One simple way to vastly increase the enclosed area, without increasing the volume of the device to impractical values is to stack several loops:

 

Fig. I               Several broken loops, stacked and joined to form a coil

Fibre-optic cables provide a very cheap and convenient way to stack these areas; a coil of N turns, with each single turn enclosing area a provides an effective area A of Na.  The reduction in the number of optical components (particularly corner mirrors) combined with the wave-guiding properties of the fibres allows the creation of simple, robust devices requiring no calibration to find their zero-point.  As they use small changes in detected amplitude (due to interference), rather than variations in the time-domain (beating) to make measurements they are more susceptible to noise, however they are considerably cheaper to manufacture^[9] and suffer from no (or almost no) lock-in compared to ring laser gyroscopes.

As the laser source is external to the ring, solid-state lasers may be used in place of gas-lasers, considerably reducing the power consumption and voltages required for operation.

To provide an area similar to that of Michelson's device (see previous page), using loops enclosing 25 m² (approx 5.64 m in diameter), the number of turns needed would beN = 8,400.  As we would be using sensitive photodetectors and lasers with very narrow bandwidth and naturally high spatial coherence (instead of forcing coherence with slits as Michelson did), the number of turns needed could be reduced considerably.  Even without this, 8400 turns of a 0.4 mm thick[**] cable results in a coil depth of 3­­­–4 metres; this is far more convenient than Michelson's 0.21 km² single-loop[††].

4.2. Error sources in FOGs

Besides the usual sources of error in an optical system^[5], some error is due to power transfer between polarisations in the optic fibre, combined with birefringence, which causes a beam to interfere with itself^[5].  A highly birefringent fibre may be employed to reduce propagation of the unwanted polarisation.  Due to changes in the refractive index as light reaches the end of the fibre, the fibre may act as a weak resonator^[5].  This effect can be reduced by gradually changing the refractive index in several steps^[5].  The optical Kerr effect (interactions between the electric fields of the beams and the fibre medium) may cause varying phase changes that introduce more error: as with the ring laser gyroscope, a good choice of materials and optical components is essential.

While HeNe lasers typically have very narrow line-widths, diode lasers are being more commonly used in FOGs due to their lower power requirements, smaller size, better durability and reliability.

5.          Examples of modern laser gyroscope systems

5.1. Military

Due to the accuracy, robustness and low maintenance needs of optical gyroscope systems, they have been a large success with the military.  The notorious AC-130U gunship uses ring-laser gyroscopes^[10] in addition to standard GPS systems in order to provide fast directional information.

The primary inertial-reference system on the Boeing 777 uses Honeywell laser gyroscopes^[11], as do some of Boeing's other commercial planes including the 757 and the 767^[17].

Another interesting use is on a USAF anti-satellite missile: ASM‑135 ASAT, which also uses a Honeywell ring laser gyroscope^[12].

5.2. Commercial

An example of a discrete (fibre) laser gyroscope device is the Northrop Grumman LN-251^[13]: a durable six-kilogram device requiring a power of only twenty-five watts. It uses a single solid-state diode laser, and owes its accuracy to fibre-optic gyroscope(s) ^{[13], page 4}.

Another notable FOG is the KVH Industries DSP-1500^[14]: its sensing element is only 1.5 inches (~38 mm) in diameter^[14], coiled to a thickness of approximately 20 mm^[14], resulting in a unit weighing just forty grams^[14]!  Despite this, the device has a very wide range of operating environments^[15] and error of less than five degrees per hour^[15].  A two-axis variant of this device is also available^[14].

Toyota is also developing navigation systems for cars that utilise ring-laser gyroscopes^[16].

5.3. Scientific

The Canterbury Ring Laser (C-I)'s successor, C-II, encloses and area of one square metre^[18] by a cavity filled with helium and neon (forming a HeNe ring laser)^[18].  This was a prototype for a larger^[19] project (enclosing 16 m²) ­­— the "Gross Ring", which measures fluctuations in the rotation rate of the earth^[19] to within one part per billion^[19].

6.          Conclusion

As currently available technology is already capable of providing measurements to higher resolutions than are usually required and the laser gyroscopes are typically combined with other systems^[20] (such as GPS/GLONASS), large improvements of the underlying technology are unlikely to be of much importance in future development of it.  Instead, improved efficiency and miniaturisation are more likely, as the technology becomes embedded in increasingly mass‑produced consumer products such as cars and (size-depending) satellite-navigation devices and mobile phones.

With this in mind, the simpler power requirements, lower cost and lower part-count of FOGs (in comparison to ring‑laser devices) are likely to contribute far more to their popularity than their inferior resolution^[20] will do to impede it, while ring-laser devices will enjoy increasing use in high-precision situations such as scientific research.

Introduction to INS

Introduction to INS
A technology known as INS, or Inertial Navigation System originally developed in the mid 60s for Missile Guidence systems has undergone an extensive evolutionary process and has now been introduced into Civil Aviation. Aircraft such as the Boeing 747 are now fitted with INS systems. INS coupled with GPS (Global Positioning Systems) can be used to navigate by global Lattitude and Longitudinal co-ordinates and also allows for real-time calibration of the INS.


How does INS work?
Gimballed INS
Early IN systems used gravity and momentum. Gyros and accelorometers were mounted on rotating plates known as Gimballed Inertial Platforms, these gyros and motors can measure the change in angle on various axis,

Strapdown INS The 70s introduced Strapdown INS, which was similar in principal to the Early IN systems but had no moving parts, infact the gyros and accelerometers were fixed i.e strapped down, to the chassis/cicuitry of the equipment. The strapdown systems suffered from a major flaw in that power consumption was so high, thermal problems were introduced, making it unreliable.

and hence using electronic circuitry can calculate the position relative to the start, thus they cannot determine their initial location, just the change relative to it. The early IN systems relied heavily on a precision engineered mechanism. Left: Simplified Schematic Diagram of a Gimballed INS Mechanism. © GEC Marconi Above: Photo of Internals of a Gimballed Inertial  Platform INS Mechanism. © GEC Marconi

Ring Laser Gyro INS
RLG INS (Ring Laser Gyro INS), is another form of Strapdown INS (i.e has no reliance on a physical mechanism as such). RLG uses a solid state glass block with three drilled tubes, mirrors placed at each coner act as optical resonators and reflect the beam.

The tubes are filled with a Helium/Neon Halogen mixture and a High voltage (about 1Kv) is applied, just like a Tube-light or Cathode Ray tube in a Telivision set. RLG systems use two counter beams around the tube initially at the same frequency, as the unit is moved the distance each photon (sub atomic particle responsible for light) has to travel (to the next mirror) changes, resulting in a subtle change in Frequency of that beam. The frequency of the beam can be measured then the change in angle be calculated from Delta

Above: Simplified Schematic Diagram of a RLG INS System. © GEC Marconi

Frequency  (in this case the change in Frequency from before and after the unit was moved).

Left: Internals of the RLG System. Above: Housing and Circuitry of the RLG INS unit. © GEC Marconi

Current INS in Civil Aircraft
In the recent 10 years or so, precision engineering of mechanical parts has
improved dramatically making it possible to create accurate gimaballed
IN systems, which are currently being used in Civil Aviation Applications.
The computing power behind that of this INS is equivelant to the 68040
or 80486 (486) processor, and is far less than that of todays standard
home PC! - Bear that in mind next time your flying by 747!!

References / Acknowledgements
Special thanks to Anthony D. King of GEC Marconi for his excellent White paper and to the IEE (Institute of Electrical Engineers).