Rangefinding with Eye-safe Light 1535nm wavelength

We may never know whether Iran deliberately used laser systems such as tank-mounted laser rangefinders to cause the cases of eye damage to Iranian soldiers which were reported during the 1980s Iran/Iraq war, but around the world there is now a trend towards the deployment of eye-safe laser rangefinders.

laser rangefinders emit a single pulse or series of pulses of coherent light towards the target. This light will be tightly controlled in dispersion, so the pulse may be only a metre in diameter at a range of several kilometres. When the light reaches the target, it is scattered in all directions, but enough energy is reflected back in the direction of the laser rangefinder to allow the returned pulse to be detected. By measuring the time taken for the pulse to complete the round trip, the laser rangefinder is able to determine target range.

The first laser demonstrated in 1960 used a synthetic ruby crystal as its working material, and by the early 1970s, ruby lasers had been fitted to the M551A1 Sheridan light tank, then to the M60A2 main battle tank. During the 1980s, laser rangefinders and target designators based on the more practical neodymium: yttrium aluminium garnet (Nd:YAG) technology were widely used by infantry, armour and artillery units, and were mounted in fixed-wing aircraft and helicopters to improve the accuracy of air-to-ground weapon delivery.

The laser had arrived as a practical tool for range measuring and target-designating, but it had an inherent problem which was a result of its operating frequency and the construction of the human eye.

While some military laser systems, such as the AN/VVG-2 laser rangefinder used in the M60A3, still used a ruby laser operating in the visible region of the spectrum at 0.69 microns, most used the newer Nd:YAG laser and operated at a wavelength of less than 1.4 microns.

Protecting the Eye

The anterior (front) portions of the eye have high transmittance and refractive power in the 0.4.- to 1.4-micron region of the spectrum (visible and near infrared), so laser energy in this part of the spectrum can reach the retina, and if powerful enough can do temporary or even permanent damage.

US Federal regulations require that the Nominal Ocular Hazard Distances (Nohd) -- the distance for each type of laser at which the beam has expanded to the point eye damage will not occur -- should be 10 to 20 km for laser designators seen with the naked eye, and 40 to 50 km for optically aided viewing. The Nohd for military laser rangefinders is generally between 5 and 10 km.

At shorter distances, these lasers can cause various levels of eye injury. For example, a ruby laser rangefinder could create a haemorrhage if seen at a range of less than 100 metres, and can cause a retinal lesion at distances of less than one kilometre if the victim is standing directly in the beam. Under the same conditions, a laser designator can inflict a retinal lesion within five kilometres. If the victim is using binoculars or some other form of optical viewing system, the danger zone can be significantly larger.

During the Iran/Iraq war, instances of laser eye injuries described as retinal burns and haemorrhages were reported. An August 1990 US intelligence report on Iraqi chemical, biological and laser capabilities stated that "The reported injuries could have been inflicted by a visible or near-infrared laser, most likely a tank-mounted ruby or neodymium/glass laser rangefinder".

"Our overall assessment is that laser eye injuries probably occurred as a result of the use of tank-mounted laser rangefinders or other laser systems. These systems possibly were used in an offensive, anti-personnel mode, with the explicit purpose of blinding troops. Hand-held laser rangefinders and designators associated with armour or artillery could be used in an attempt to dazzle, disorient or blind personnel in low-flying aircraft (fixed or rotor wing)."

Wavelengths longer than 1.4 microns (the mid to far infrared) are absorbed in anterior portions of the eye (mainly the cornea) so do not reach the retina. This has allowed the development of eyesafe lasers, which are now entering service in growing numbers, supplanting the older types.

How it is Done

Techniques used to obtain the new optical frequency include adopting erbium-doped glass as the lasing material, shifting the output of a Nd:YAG laser to 1.54 microns by means of a Raman cell, and using a technique known as optical parametric oscillation (OPO) to switch the laser's operating wavelength on demand between 1.06 and 1.58 microns.

Eyesafe military laser rangefinders based on carbon dioxide lasers operating at 10.59 microns have also been fielded by military users. These can be teamed with thermal viewing systems operating in the same band, creating rangefinders which have a good performance in conditions of poor visibility and are able to measure the range of any Flir-visible target.

The sheer number of laser rangefinders currently being marketed, or being incorporated into different weapon and fire-control systems makes it impossible to comprehensively survey the range of products currently available. This article will look at typical rangefinders and systems, illustrating the general trends by giving details of a selection of `classic' systems, recently-introduced products and applications, and instances where retrofit programmes are replacing older non-eyesafe equipment with new eye-safe rangefinders.

Examples

The simplest form of laser rangefinder is the hand-held unit, which can be used for tasks such as ranging for anti-tank weapons, mortars, or artillery. In the US Army, the basic hand-held laser rangefinder is the AN/GVS-5, a 2.3 kg unit which is sighted using a single eyepiece. Manufactured by Radio Corporation and various vendors, it operates at 1.06 microns, and has an eye-safe range of 1.1 km.

Hand-held laser rangefinders are widely used, but most recent systems have reduced to weight to around 1.6 kg. OIP Sensor Systems, formerly the Belgian division of Delft Sensor Systems, offers the 1.6 kg MLR 30 and MLR 40 units for hand-held or tripod use. The MLR 30 is a Nd:YAG type operating at 1.064 microns, while the MLR 40 is an eye-safe version operating at 1.54 microns. Both are powered by eight 1.5V AA size batteries or an external 10 to 30V DC supply, have RS-232 and RS-422A serial interfaces, and can measure distances out to 20 km. Earlier this year, the company announced that it had been awarded a contract to supply laser rangefinders, thermal imagers and weapon sights to an undisclosed South American country.

Eloptro says that thousands of its LH30 Nd:YAG laser rangefinders are now in service with several armed forces around the world. This unit has x6 optics, and can measure ranges from 80 to 20 000 metres with a resolution of plus or minus 5 metres. It weighs less than 1.5 kg and is powered either by a single NiCd rechargeable battery providing around 700 shots per charge, or from an external power source of 10 to 30V DC.

The company recently announced that the British Army has selected the eye-safe LH40C to meet its Target Locating Equipment (TLE) requirement, and will take delivery of 226 units. Already in production, this single-pulse erbium:glass laser operates at 1.54 microns, weighs less than 1.65 kg, and offers an RS-232 connector able to supply range information to other systems. In UK service it will be linked to a GPS receiver.

Combining

Traditional laser rangefinders provided only range information, but the LH40C incorporates a digital magnetic compass, and acquires the range, bearing and elevation of the target, and when coupled to a GPS receiver, can provide target location data. This reduces both the workload on forward observers and the possibility of errors in calculating target co-ordinates. Fire correction can be done by ranging to the fall-of-shot, with the system automatically calculating and displaying the required offsets.

The motorised forward observers of the Swiss Army are to be equipped with a new observation and target-acquisition system. The Leica Geosystems ZVBA combines the Zeiss Optronik Halem laser rangefinder, Leica SG12 Digital Goniometer and Sagem Matis thermal imager, and will have a digital data link to the Intaff artillery command and control system. Qualification units are being prepared, and series production is planned for 2001.

Under the US Air Force Air Combat Command's Sure Strike programme, in 1996 Litton Laser Systems delivered 12 Mk VII portable laser target locators for use in Bosnia. This 1.91 kg unit combines 7.3 x 18 day optics, a 4 x 50 image intensifier night sight based on a Gen III tube, a Nd:YAG laser transmitter the output of which was shifted to an eye-safe 1.57 micron wavelength, an electronic inclinometer and a magnetoresistive electronic compass, as well as an RS-232 interface which supplied connection to a Precision Lightweight Global Position System Receiver (PLGR). This unit permitted forward air controllers to determine target azimuth, elevation and range, passing this via tactical radio to the pilot's head-up display in F-16 fighters. This information allowed the aircraft to fly single-pass attacks with conventional or guided weapons.

Along with today's new technology, the size and weight of recently-introduced laser rangefinders has tumbled. The Brashear LP MLRF 100 mini-laser rangefinder weighs only 567 grams and is designed to be mounted on personal weapons. Developed for the US Army, this is an eye-safe unit operating at 1.54 microns

When coupled with a GPS receiver or other navaid, a laser rangefinder can rapidly determine the geographical co-ordinates of the target at which it is aimed. Since the location of the rangefinder is already known, target bearing and range can be used to determine target position, making the teamed systems a valuable tool of the digitised battlefield.

In 1997, the Israeli company Azimuth announced a Light module with a built-in C/A-code GPS receiver and electronic magnetic compass which could be mated to an existing handheld laser rangefinder allowing target co-ordinates to be calculated and displayed. Its RS232 and RS422 data interface sockets are intended to enable the unit to be connected to most types of laser rangefinder.

This capability is relatively simple to retrofit. Experience in Kosovo led the British Army to modify around 40 Scimitar tracked reconnaissance vehicles with a Tactical Navigation and Target Location System in which the laser rangefinder, which forms part of the vehicle's existing Avimo Spire (Sight Periscope Infra-Red Equipment) gunner's thermal sight, is linked with a KVH Industries TacNav fluxgate compass-based position and navigation system, a north-pointing sensor and a Rockwell PLGR GPS receiver.

In many cases this type of laser rangefinder is used to measure the distance to a target which will be engaged by some remotely-located weapon such as artillery, but it can also be used to improve the accuracy of an infantry weapon.

A laser rangefinder will form part of the Brashear LP fire-control system for the US Department of Defense's planned Objective Individual Combat Weapon (OICW). Expected to enter service around 2009 to replace some of an infantry squad's M16 rifles, the OICW will fire 5.56 mm bullets or 20 mm grenades. The latter projectile will have a maximum range of 1000 m. The grenades can be impact fuzed, or set to airburst close to the target. In the latter mode, the laser rangefinder will program the fuze to initiate at the correct distance downrange.

Fire Control

A laser rangefinder will also form part of a similar fire-control system for the Objective Crew Served Weapon (OCSW) which will the 7.62 mm medium and 0.50-inch heavy machine guns, and Mk 19 40 mm automatic grenade launchers of the US Army and US Marine Corps. This weapon will fire 25 mm grenades.

As part of its modernisation scheme for the M40 106 mm recoilless rifle, Bofors offers an LP101 laser sight with a built-in laser rangefinder and ballistics computer. This can be used to measure target range, and to calculate weapon elevation and the time of flight for improved 3A-HEAT-T (High Explosive Anti-Tank -- Tracer) rounds.

The Computing Devices Canada (CDC) Computerized LASer Sight (Class) can improve the accuracy of the M40 or of lighter weapons such as the shoulder-fired Carl-Gustaf. Class uses an eye-safe laser rangefinder with a range of up to 4000 metres, and a ballistic computer able to store data for up to ten ammunition types, plus a Gen III image intensification sight. Canadian army trials showed that soldiers using Carl-Gustafs fitted with the Class sight were able to achieve a 73 per cent first-round hit probability, and engage targets at 800 metres or more.

Given the growing use of semi-active laser-guided missiles, bombs and projectiles, the laser target designator is becoming a more common item of front-line hardware. While laser rangefinders are moving to eye-safe frequencies, designators still continue to operate at 1.06 microns, however, since the seekers of most laser-guided bombs and missiles operate in that band.

Ranging and target designation tasks can be combined in a single unit, and, as with rangefinders, new technology is having a dramatic effect of equipment size and weight. A typical 1980s system such as the Hughes AN/TVQ-2 had an all-up weight of 23 kg, so had to be vehicle mounted or carried by a team of two, but today's soldier wants a device small enough to be stowed in a rucksack, light enough to be an acceptable additional load and compact enough to be set up in any tactical location.

The Litton Laser Systems AN/PEQ-1A Special Operations Forces Laser Marker (Soflam) weighs less than 5.2 kg, but can measure range to the target and mark the target for all Nato Band I and Band II PRF codes. Developed for the US Army Special Operations Command, it has been used in operations in the former Yugoslavia.

Another unit in this weight class is the Pilkington Optronics LF28 laser target marker (known as the LF25 while under development). The laser and the x10 optical sight share a single optical aperture, and the former can be used either to measure target range out to 10 km, or to mark the target for attack. The device weighs 6 kg, and can be fitted with a clip-on thermal imager or image intensifier for use at night.

Erbium et Al

Most new-generation main battle tanks use Nd:YAG laser rangefinders, but a shift to other forms of laser has begun. The Hughes Laser Rangefinder used in the M1 Abrams was a Nd:YAG system. Although the US Army recognised that this could be hazardous to the unprotected eyes of soldiers, and had made an eye-safe replacement a `desired' feature for many years, a formal requirement did not emerge until 1994.

Under a US Army project, Hughes was asked to develop a new eye-safe laser rangefinder incorporating a Raman resonator which shifts the wavelength from 1.06 to 1.54 microns. The ELRF contract award was made 79 days after receipt of proposals, and the basic concept of the project was that the contractor would receive the existing M1 laser rangefinders and upgrade them to the ELRF configuration.

For upgrading the M1 tanks for the US Army and some export users, Litton Systems now manufactures an erbium: glass eye-safe laser rangefinder which operates at 1.54 microns. It can range from 200 to 7990 m, with an accuracy of plus or minus 10 m.

For its Stabilised Aiming, Vertical sensing And Navigation (Savan) gunner's multi-channel stabilised sights used on the Leclerc tank, Sagem can supply either a Nd:YAG or eye-safe laser rangefinder, while the Raytheon Gunner's Primary Tank Thermal Sight (GPTTS) in service on South Korea's K-1 MBT uses a carbon dioxide laser range-finder. Although the Leopard 2 uses an Eltro CE628 Nd:YAG laser rangefinder, when Sweden adopted the Leopard 2A5 it specified that a eyesafe Raman shifted Nd-YAG laser be fitted to its vehicles.

Kollsman offers a family of erbium: glass-based eye-safe laser rangefinders which includes the Eyesafe Integrated Laser Periscope (Elip) for armour and light armour applications. Variants of the Elip design may be used as a cost effective, eyesafe upgrade of ageing laser rangefinder systems for light armoured vehicle, fixed wing and turreted payload applications.

A laser rangefinder is a key component in the modernisation of the Swiss Army's Skyguard fire-control units. Used to control twin 35 mm anti-aircraft guns, the updated Skyguards began to enter service late last year, and a total of 66 will be modified by the end of 2001. The engagement sequence has been automated, and can be based on data either from an upgraded radar, or from electro-optical tracking with range data from a high-power laser rangefinder. The FCU also has a new C2003 computer with software that should provide a better hit probability.

Laser rangefinders also form a key component of many other modernisation schemes for anti-aircraft guns, including widely-used ex-Soviet types. Saab Dynamics' LVS fire-control system is a private-venture scheme which uses a x7 day sight incorporating a laser rangefinder plus an optional image intensifier or thermal imager for use at night. Successfully demonstrated on the ZSU-23-2 and the Russian 57 mm S-60 towed AA gun, it has been adopted by several users of the Bofors 40 mm L/70 anti-aircraft gun. The Swedish Army has taken delivery of more than 200 systems, the Swedish Navy adopted the LVS to update some of the 40 mm L/70 AA guns used by coastal artillery units and the first export order was from the Royal Thai Army.

Zeiss Optronik developed its CE 658 laser rangefinder specifically for antiaircraft applications. Based on a Nd:YAG laser transmitter which uses Raman shifting to convert its output to an eye-safe 1.54-micron wavelength, the CE 658 is already in service with several unidentified customers. The unit weighs 18 kg, is liquid and air-cooled, and has a maximum range of 40 km and an accuracy of plus or minus 5 m.

Surface-to-Air Missile (Sam) systems which depend on active radar for target tracking are vulnerable to counter-attack by anti-radiation missiles. This has increased interest in passive forms of tracking such as using a thermal imager. In such cases, a laser rangefinder is needed to obtain target range.

An example of this approach can been seen in the French and German plans to upgrade the Roland self-propelled Sam system. The current optical sight is replaced by a Glaive optronic system containing a thermal camera and optical television channel for target tracking, an infrared localiser to track the missile and laser rangefinder. The first firing tests of the upgraded system began earlier this year in south-west France, with the modified Roland system destroying a C-22 target drone with a Roland 2 missile.

Submarine periscopes and optronic masts often use passive rangefinding techniques such as split-image and stadiametric ranging, but in some cases laser rangefinding is being used. The German and Italian navies both selected the Zeiss Optronik Sero 15 stabilised modular attack periscope for their new Type 212 submarines. Already in service with the Ula class submarines of the Royal Norwegian Navy, this includes an eye-safe laser rangefinder, plus low-light TV and 3- to 5-micron thermal channels.

The electro-optical facilities of the Kollmorgen Model 90 optronic periscope system include a CCD TV camera, thermal imager, 35 mm photographic camera and a laser rangefinder, while Lomo offers 1.06- or 1.54-micron rangefinders as optional fittings to supplement the visual, television and thermal imaging channels of the periscope it supplies for the Russian Navy's Kilo and Amur class submarines.

The US Navy's new Virginia class attack submarines use a non-penetrating Kollmorgen Photonics mast whose sensor suite includes an eye-safe laser rangefinder able to range targets observed by the colour and monochrome high-definition television channels or the staring infrared array thermal imager.

Existing periscopes can also be retrofitted with a laser rangefinder. While Eloptro's upgrade package for the M41 search periscope upgrade fitted to Daphne-class submarines relies on a passive split image rangefinder, the company says that it could re-design the unit to include a laser rangefinder.

The first optronic weapon director to be deployed on US Navy warships was the Mk 46 Mod 0, which entered service aboard the destroyer Arleigh Burke on 4 July 1991. This included a CCD TV camera and a thermal imager, but did not have a rangefinder. The US Naval Surface Warfare Center has now awarded Kollmorgen a contract worth $ 5.5 million to produce the first examples of an improved Mk 46 Mod 1 version with an eye-safe laser rangefinder, a colour camera and a 3- to 5-micron thermal imager. This will be used to support the Mk 34 gun weapon system on Aegis destroyers, enabling these to use either radar or E/O target tracking.

The Royal Australian Navy is retrofitting its six Adelaide class frigates with Radamec System 2500 electro-optical tracking and fire-control systems. These have a British Aerospace Australia LRTS 3-5[Mu]m thermal imager and an eye-safe laser rangefinder.

The original version of the British Aerospace Systems and Equipment (Base) Gun System Automation Mk 8/ General Purpose Electro-Optical Director (GSA 8/GPEOD)EO sensor systems procured by the Royal Navy were delivered with a 1.06-micron Nd:YAG laser rangefinder which was not eye-safe, but the final examples used a Saab Dynamics 1.54-micron Low Eye Hazard Laser which was to be retrofitted to the 20 systems already delivered.

A laser rangefinder can also improve the accuracy with which even a simple aircraft can deliver air-to-ground weapons. This was graphically illustrated at the 1999 Paris Air Show when a fully armed Slovenian Pilatus PC-9M was displayed by the Israeli company Radom Aviation Services. The aircraft had new avionics for the light strike role, and was shown with armament options which included an FN Hersta112.7 mm gun pod, FN LAU-7A unguided rocket pod, an Alkan practice bomb dispenser and a laser rangefinder pod.

Laser rangefinders and designators play a major role in the avionics suite of fixed-wing aircraft and helicopters. Although the designators must retain their 1.06-micron operating frequency to remain compatible with laser-guided weapons, the greater emphasis on peacekeeping operations has made eye-safe laser rangefinders desirable. French Air Force Mirage F1CT ground attack fighters were originally fitted with TMV 630 laser rangefinders and TMV 634 laser spot trackers (LST), but in mid-1997 the service ordered the new Thomson-CSF Optrosys, an 18 kg unit which combines a 1.54-micron eye-safe laser rangefinder with a 1.06-micron laser-spot tracker.

Kollsman developed the SELRD (Switchable Eyesafe Laser Rangefinder/Designator) using components developed and tested for the Laser Designator/Rangefinder of the RAH-66 Comanche. Intended as an upgrade for the OH-58D Kiowa Warrior, this uses a diode-pumped laser to provide high power laser energy at 1.06 microns for target designation, but an optical parametric oscillator (OPO) shifts this to 1.57 microns to allow eye-safe laser range finding.

New applications for laser rangefinders continue to emerge. For example, DRS Optronics is under contract to Boeing to provide the Helrif (High Repetition Rate Eyesafe Laser Rangefinder) for the Visual (Virtual Imaging System for Approach and Landing) system, an aircraft carrier landing aid. This programme is currently in the engineering manufacturing and development stage. The laser will be integrated into a complete ship-mounted sensor unit including Flit and TV sensors. It will give the ship's Landing Signals Officer detailed information on the glide scope, position and speed of approaching aircraft.

In its classic form, a laser rangefinder determines only one target parameter -- range. However, given the high resolution of a laser rangefinder compared with a microwave radar, work is already under way to extract other target-specific information from the returning pulses so as to have some form of target identification.

The US Department of Defense Target Acquisition ATD which ended in FY98 was intended to develop and demonstrate an extended-range, multisensor target acquisition suite for future tank, cavalry and scout vehicles, and to provide technologies for the M1A2 System Enhancement Package (SEP) P3I. It included a multifunction laser able to fit within the space constraints of the current M1 laser.

Developed by Fibertek, the resulting Multifunction Laser System (MFLS) uses diode-pumped solid-state laser technology. It offers three modes of operation:

* eye-safe rangefinding at 1.54 microns

* eye-safe target profiling within a narrow field of view for aided target recognition at 1.54 microns,

* laser designating at 1.06 microns

In rangefinding mode, the MFLS operates at 1 to 20 Hz, but this is increased to 200 Hz for target profiling.

Future complex operating modes will probably require more accurate ranging techniques. The task of timing the interval between the transmission of a laser pulse and its reception is often done by starting a counter when a laser pulse is transmitted and stopping it when return pulse is detected. In another technique, electronic circuitry begins a ramp function as the pulse is transmitted, and halts it when the return pulse is received.

Both techniques suffer from threshold uncertainties and a variety of other problems, says Lockheed Martin which has recently patented a third solution which uses a digital filter known as a finite impulse response filter. This measures the correlation between the return pulse and a template signal embodying a set of predetermined coefficients expected to describe the return pulse.

The system can determine the time of the return pulse by determining which set of digitised samples from the receiver have the highest correlation to the template. From the transmission and arrival time, the system then calculates the range.

Given this sort of technology, it seems certain that some future laser rangefinders will use digital technology to extract the maximum amount of information from the returned pulses. The boundary between a laser rangefinder and a laser radar (Lidar) is beginning to blur.

In short

* "Normally used to measure distances, laser rangefinders are alleged to have been exploited as blinding systems"

* "New techniques are being implemented to alter laser frequencies so that they do not reach the eye retina"

* "Optical parametric oscillation, Raman, erbium, Nohd etc, are the discipline's new buzz words."