When they appeared, mechanical gyroscopes were regarded as the panacea simply because there was nothing else short of the sextant. _This until the advent of the laser gyro.

• How does the laser gyro work and what are its limitations?
• Will the optical fibre beat the chunk of machined glass?
• What do electromagnetic fields techniques hold in store?

By Doug Richardson

Late 1998, intelligence reports claimed that China was asking Russia to provide the fibre-optic gyroscopes which are needed for China's latest generation of ballistic missiles - technology which could find its way into North Korea's ballistic missiles. Older Chinese missiles relied on traditional mechanical gyroscopes, but these proven systems have slipped from favour in China's missile-design laboratories. When Sextant Avionique of France and Rousskaya Avionica of Russia teamed in 1998 to offer an upgrade for the MiG-29 fighter, one of the features they proposed to add to the aircraft was "an improved navigation and weapon aiming system through state-of-the-art ring laser gyro inertial navigation system with embedded GPS (INS/GPS)". In Russia, whose technology often lags that of the West, the era of the mechanical gyroscope is coming to an end. In the West, the US Air Force and Navy have enthusiastically adopted ring-laser gyro (RLG) technology, installing equipment of this type in large numbers both on new-build aircraft and as retrofits. Western European air forces have been slower, so inertial navigation systems (INS) based on the traditional gimballed mechanical gyroscope are still in service in large numbers, although new-build and retrofit programmes are tending to adopt navaids based on ring lasers. Development of ring laser angular rate sensors (usually referred to as ring laser gyros, although they lack the traditional rotating mass of the mechanical gyroscope) started in the mid?1960s, when the technology was seen as a potentially more reliable replacement for the mechanical gyroscope. It was given a major boost by the decision in the early 1970s to explore the possibility of creating 'strap down' INS in which the gyros and accelerometers would no longer be mounted on a gimballed platform. This idea required gyros and accelerometers with a higher performance than was then available.

Making an INS needs accelerometers as well as gyros. Ferranti Technologies developed its FTS-10A accelerometer (arrow, to the right of a Marconi Electronics Systems RLG navaid where two of the separate laser units can be seen - one facing the camera, the second flat on top) as a European component which would be free of US export restrictions.

The gyros in a gimballed system had to measure rotations ranging from a few thousandths of a degree per hour to a few tens of degrees per hour, but those in a strap-down system would have to cope with the highest rotation rates possible within the full manoeuvring envelope of the aircraft. These could be several hundreds of degrees per second. Luckily for hardware designers, the improvement needed in accelerometers was much smaller - a factor of between two and five. Despite the best engineering efforts during the 1970s, the required level of performance could not be achieved using mechanical gyros. Only the newly-developed RLG could make strap-down INS practical. Ring laser angular rate sensors generally take the form of a laser operating within a ring-shaped conduit within a monolithic solid block of glass. This optical path is normally triangular in shape (but sometimes square), with a mirror at each corner to re-direct the light from one straight section to the next. This path is filled with a lasing medium, normally a helium/neon gas mixture. A high voltage is applied via a cathode and anodes to cause a discharge and the subsequent formation of two laser beams which propagate along the conduit, one with a clock?wise (CW) beam, the other counter?clockwise (CCW). Any rotation of the gyro about an axis perpendicular to the plane of the conduit causes a shift in the relative phase between the two waves due to the Sagnac effect, and this shift is proportional to the magnitude and sense of direction of the rotation. One of the mirrors is semi-reflecting, so allows a portion of both beams to escape to reach a photodetector where they form interference bands on the detector's sensitive surface. Any phase shift caused by rotation of the gyro is detected as a movement of these interference bands.

Over the past two decades, the RLG has evolved as a reliable and, to some extent, environmentally insensitive inertial rotation sensor able to withstand relatively large levels of mechanical shock without permanent degradation of its performance. Yet it is still far from perfect, and attempts are being made to develop improved designs. A laser designed to operate at one frequency may have an undesired mode operating at another unwanted frequency. Honeywell has devised a mirror arrangement in which the loss induced in the undesired mode exceeds the available gain of the laser. In a 1994 patent Method and structure for reflecting 633 nm radiation while eliminating 650 nm radiation in ring laser gyro mirrors it described an arrangement in which the first, second and third mirrors are comprised of alternating layers of materials having high and low indices of refraction respectively. The loss at the unwanted frequency is achieved by selection of the optical thickness proportional of these layers.

Lock-in and Dither Problems...

At low rotation rates, laser gyros can suffer from a phenomenon known as 'lock-in'. The retroscatter from the mirrors within the optical path couples energy from one of the oscillating beams into the oppositely propagating beam. When this happens, the oscillating frequencies of the two counter-propagating beams lock together at a single frequency known as the lock point. The unit thus becomes insensitive to low rotation rates in a range known as the 'dead band'. One common solution to this problem is the use of a mechanical dither. Three different methods are used to apply dither. The first (favoured by Raytheon) causes the whole ring laser gyro structure to rotate at a fixed rotating speed. Honeywell devised a scheme in which the structure is energised with a periodic vibration, while the Northrop Grumman solution uses magneto-optical deviating mirrors. The shortcomings of the mechanically dithered gyroscope led several companies to devote extensive research to the possibility of creating an alternative RLG configuration known as a 'multi-oscillator', in which the clockwise and counter-clockwise beams would be at separate frequencies. In practice, this proved very difficult but some progress has been achieved.

… and Reciprocal Shifting

Litton has developed a multi-oscillator RLG which operates as a pair of two-mode ring-laser gyroscopes sharing a single cavity. Within this multi-oscillator light cavity are a substantially left circularly polarised (LCP) beam pair, comprising one beam circulating in the clockwise direction and the other in the counter-clockwise direction, as well as a substantially right circularly polarised (RCP) counter-propagating beam pair. Each beam pair acts independently as a two-mode ring laser gyroscope. In order to achieve independent operation of the two gyroscopes within the same cavity, there is a frequency difference (typically a few hundred MHz) between the LCP and RCP beam pair. This frequency separation is known as 'reciprocal splitting'. Each of the LCP and RCP gyros is still subject to the lock-in phenomenon. A second bias, known as 'non-reciprocal splitting' is used to prevent this. The technique developed by Litton applies non-reciprocal splitting to the multi-oscillator in a manner which results in the resulting bias shift in the LCP gyro being equal but opposite in sign to the bias shift in the RCP gyro. When the outputs of the two gyros are summed, the resultant signal is doubly sensitive to body rotation but independent of the magnitude of the applied bias. In this way, the differential nature of the multi-oscillator makes it inherently insensitive to bias variations that can be caused, for example, by changes in the magnetic field or temperature.

The Sextant Quasar INS unit from the Ariane space rocket was developed for peaceful purposes, but similar RLG technology or the even less expensive Fog hardware are being sought by nations attempting to build long-range ballistic missiles.

Usually, a resolution of one to two arc-seconds is adequate for navigation purposes; however, many new applications in the areas of pointing and tracking require even better angular resolution. These applications of the multi-oscillator ring-laser gyroscope require very high resolution outputs, down to 0.01 or even 0.001 arc-second. There are many such applications where it is desirable to point and track with an enhanced angular resolution, such as the positioning of a terrestrial- or celestial-based observatory and telescope. Several techniques for refining the resolution exist, particularly by state-of-the-art digital techniques. Such techniques depend upon sampled data systems and are susceptible to aliasing errors. Litton had devised a technique to enhance the resolution of the output signal from a multi-oscillator ring-laser gyroscope. This is based on a fast filter that uses sampling at rates of more than 2MHz and filtering of gyroscope data to provide effective resolution enhancement without causing substantial time delays. This fast filtering technique averages over many samples and therefore has good noise immunity. A sampling frequency chirp technique based on a phase locked loop has been developed which maintains long term frequency stability and substantially eliminates aliasing effects.

Being labelled "traverse" and "elevation" these two Curtiss-Wright Antriebstechnik optical fibre gyros leave no doubt on their application

Some other companies remain sceptical, and dither is far from being dead or even a dying solution. Writing in Vol. 4 No 3 of the GEC Review, Anthony King, chief engineer of the Marconi Electronic Systems navigation and electro-optic system division said in 1998 that "I suspect that any advantages of the multi-oscillator technique are balanced by corresponding difficulties and... the overall effect on the evolution of I.N. technology will not be too significant. Our own RLGs continue to use mechanical dither, which is robust and reliable". Although RLGs are widely believed to make navigation systems more reliable, King describes this as a fallacy. "Good spinning-wheel gyros today have mean time between failures (MTBF) - in an aircraft environment - of tens of thousands of hours, and virtually no life-limiting wear-out mechanisms. RLGs are not demonstrably better in either of these respects. In fact, it tends to be the reliability of the associated electronics that dominates an I.N. system's MBTF." Experimental and theoretical data has shown that the output power of a laser gyro is a function of both temperature and operating time. Output power diminishes slowly with time, eventually reaching a level below what is considered acceptable for use. In 1998 Honeywell patented a method of predicting laser gyro life based on the measurement of laser intensity, readout intensity, volts per mode (which is a derived quantity) and other gyro parameters. These parameters are monitored as a function of time over the life of the device, allowing the gyro unit to warn the inertial navigation system which uses it that failure is imminent. Different levels of warning can be sent, depending on how much of the estimated lifetime is left.

Miniaturisation vs. Reliability

Although the gas laser is the basic of virtually all RLGs, it has several disadvantages. The laser structure is mechanically and thermally sensitive, and the degree to which it can be miniaturised is limited by the need for a certain minimum amplifying length. High voltages are required for operation of the gas discharge, while the overall efficiency of the laser is very low. Creating mirrors that produce the extremely low levels of scattered light needed to avoid 'lock-in' is very expensive. One solution is to move to a diode-pumped solid-state ring laser gyro which has no movable parts, is insensitive to external disturbances, has a simplified mechanical construction better suited to miniaturisation and offers a measuring resolution and measuring precision similar to that of a high-quality HeNe ring laser gyro. In 1997, researchers from Daimler-Benz Aerospace (now DaimlerChrysler) patented a diode-pumped monolithic solid-state ring laser whose laser medium consists of doped crystal or glass material which can be of square or rectangular shape. Two ring waves rotate in opposite directions within a common solid-state material and are simultaneously amplified. The rate of rotation of the resulting gyro is derived from the difference frequency of the two ring waves.

Fibre Optic Gyro

The Sagnac effect is also used in a second type of light-based sensor - the fibre-optic gyroscope (Fog). Here the optical path takes the form of a coiled optical fibre into which oppositely rotating light waves are launched using relatively inexpensive opto-semiconductors. Light from a semiconductor laser diode is divided into two beams of almost equal intensity by the beamsplitter such as a semi-transparent mirror. One of these moves clockwise through the fibre coil, while the other moves counter-clockwise. After passing through the coil, the two light beams are recombined by the beamsplitter then passed to a photodetector where the Sagnac phase shift caused by rotation is measured. Although the result is a useful gyro, the fibre coil has significant thermal and mechanical sensitivity, and the angular resolution of a Fog is low.

		This diagram from Curtiss-Wright Antriebstechnik describes the operating principle of the optical fibre laser gyro
Translation LASER = laser DETECTOR = sensor VERSTÄRKER = amplifier TREIBER = driver POLARISIERER = polariser DIREKTIONALER KOPPLER = directional coupler DEMOD. = demodulator ANALOGER AUSGANG = analogue output SIGNALPROZESSOR = signal processor GLASFASERSPULE = optical fibre coil DIREKTIONALER KOPPLER = directional coupler OSZ. = oscillation DIGITALER AUSGANG = digital output SERIELLE SCHNITTSTELLE serial interface

Like all precision measurement devices, fibre-optic gyros exhibit errors. In this case, these include angle random walk, bias instability and bias offset. "Angle random walk is caused by the presence in the gyro's output signal of broadband random noise from 'shot noise' or thermal noise in the photodetector, while bias instability creates a fluctuation in indicated angle rate when the gyro is not rotating.

The low cost of fibre-optic gyros makes inertial navigation a guidance candidate even for low-cost weapons such as the Aerospatiale Matra AASM (Armement Air-Sol Modulaire) guided bomb.

Bias instability (sometimes referred to as bias drift) should not be confused with bias offset - a small but false rotation rate indicated by the gyro when the latter is not rotating about its sensitive axis. In an application such as a land navigation system, bias offset can be estimated by measuring the indicated rotation rate when the vehicle is stationary. Once its value is known, it can be subtracted from the gyro output to yield the true rotation rate. One optical gyro manufacturer tells the story of how company engineers were puzzled by a persistent but unexplained indication of rotation from a high-quality and highly sensitive gyro during the early days of this technology. Told of this phenomenon, an outsider recognised that its value corresponded with what could be expected from the Earth's rotation. In precision applications, this can result in outputs of up to 15 degree/hr.

Obtaining high quality SAR radar images like these from a fighter radar may require fitting a fibre-optic sensor to the radar or its antenna.

In gyros designed for precision inertial navigation systems (with drift rates of less than 1 degree/hr), the Faraday effect caused by the presence of strong magnetic fields can affect the laser light in the sensing coil, creating non-reciprocal light paths. In less sensitive applications the mu-metal shielding can prevent this. KVH tells prospective users of its E-Core fibre optic gyroscopes that "If your application results in mounting the gyro near a very strong magnetic field (>50 gauss), a practical test to determine sensitivity is suggested. No effects are expected in typical vehicle installations". The output of a Fog tends to be noisy, partly due to noise introduced by the optical fibres, and partly due to the amount of amplification needed to convert the signal representing phase shift into something which is usable for control and guidance purposes.

Digital Interface

The basic output from the Fog is analogue, but most of today's navaids use digital electronics. D. Cesaretti of Curtiss-Wright Antriebstechnik explains the digital interface used in that company's Cwat laser gyro. "The bus-interface used allows the synchronised reading of all sensors linked to the bus with an update-rate of eight kHz. This results in maximum transmissable frequency of four kHz according to Niquist theorem.The transmission protocol used allows the detection and damping of transmission errors. This ensures that there is no deterioration of the signal quality during the transmission of signals from the gyro. "While analogue transmission allows a resolution of 12 or - with very high efforts - a maximum of 16 bits, our bus offers 24 bits per sensor. This allows an extremely fine resolution of rotational rates of up to 1000 degrees per second without overruns. "A further advantage of digital transmission is the accurate recognition of error functions. In an analogue signal transmission, a rate of zero can mean two things: either the system is idle, or there has been a transmission failure (due, for example, to a broken cable). The consequence of an undetected transmission failure can be very unpleasant as the consequence is a so-called open-loop reaction - the turret or weapon being stabilised by the gyro moves in an uncontrolled manner. With a protocol-based digital transmission system, even if the gyro output is zero, this information is transmitted at regular intervals in digital form. If a cable breaks or is cut by combat damage, this digital data is no longer received, and the system can react appropriately."

Enter GPS

The growing trend of linking Global Positioning System (GPS) satellite-navigation receivers and INS was originally done as a method of improving navigation accuracy, with INS remaining the primary sensor and GPS a secondary source of updates. However, if the availability of GPS can be guaranteed, it can be used as the primary navaid, with INS taking over if the satellite signals are briefly lost. Such a role can in many cases by filled by a less accurate sensor than the traditional full-specification INS, requiring gyros with an accuracy of 0.1 to 1 degree/hour, rather than 0.002 to 0.0l degree/hour needed for a traditional INS. This is an excellent role for the Fog, which is much cheaper to manufacture than the RLG.

Resonating Alternatives

Meanwhile the search for alternatives to the RLG and FOG continue. Honeywell has successfully developed its Hemispherical Resonator gyro, a technology which has also been developed in Russia by the Ramenskoye Design Company, but this solution was not been widely adopted. It is possible that new forms of optical gyro may emerge. "It is notable that all successful optical gyroscopes to date are based on the Sagnac effect where the light propagates around a closed circular path", wrote two Russian researchers from Nizhny Novgorod in a 1996 patent (US Patent No. 5,517,306). "Because of this, there is virtually no precedent for rotation sensors based on other possible manifestations of the effects of rotation on electromagnetic wave propagation in rotating systems. Consequently, in nearly all the publications which describe RLGs and Fogs, there is no description of rotation sensors in which the light propagates in directions other than around closed paths which are generally perpendicular to the axis of rotation. "The concept disclosed here does not depend on the Sagnac effect in that the light or electromagnetic radiation does not propagate along a closed planar path but rather in a direction parallel to the axis of rotation of the gyroscope. Therefore, there appears to be little precedent in the traditional prior art of ring laser or fibre-optic gyroscopes. The apparatus excites an electromagnetic field in a resonator such that an electromagnetic beam propagates along the propagation axis parallel to the object rotation axis. The apparatus then detects a field component of the electromagnetic beam. The detected field component of the electromagnetic beam indicates the angular velocity of the object." If its inventors are correct, and can obtain financial backing, it is possible that a new form of optical gyro will emerge to rival the RLG and Fog. But we must be cautious - not every promising idea results in an end product. Nuclear magnetic resonance (NMR) gyros have been the subject of research since the 1970s. Although some limited work may still be still going on, there is no sign that NMR gyros it will result in any practical sensor in the foreseeable future. One thing is certain, however. To use a Scots expression, the mechanical gyro is approaching "the end of its days".