By the Doppler's sharp stare.

When United States citizens sing the line "and the rockets' red glare" from their national anthem, they recollect use of a high-technology weapon of the time during the 1797 War of Independence. Today's battlefields face not only the glare of rockets, but also the all-weather stare of the synthetic aperture radar modes used by modern fighter radars.

The simplest method of radar ground mapping is known as real-beam mapping. Widely available on fighter radars for many decades, it uses the main beam of the antenna to scan the terrain ahead of the aircraft. A small-scale radar map of the terrain ahead of the aircraft is shown on a head-down display.

The problem with real-beam mapping is the relatively poor resolution provided by a radar antenna. In a typical fighter radar of around 40cm diameter, the antenna beam width is around four degrees. The azimuth resolution available from such a beam is dependant on range, increasing with distance. At best, it will be measured in hundreds of metres, at worst in kilometres. The resulting crude imagery limited the usefulness of radar ground-mapping modes. Although useful as a navigation aid, it is unable to show individual targets.

For reconnaissance purposes, the technique known as Synthetic Aperture Radar (Sar) is used. Here, the radar antenna is directed to the side of the aircraft, often at right angles to the line of flight. Every second, an aircraft flying at a speed of 600 kt travels for a distance of around 300 metres. By integrating the radar returns received during this time, the radar can achieve the resolution of an antenna of up to 300 metres in diameter. In practice the resolution of a 300-metre antenna will not be achieved because of the inaccuracies in the aircraft's flight path, but a 200-metre antenna would give a beam width of about 0.01 degrees.

If the antenna used for Sar is pointed not at right angles to the line of flight, but at some intermediate angle between 90 degrees and the line of flight, Sar techniques will still provide an improvement in resolution that will be useful for ground-mapping purposes. Since the Sar technique relies on the radial velocity of the ground relative to the radar-equipped aircraft, this resolution will decrease as the antenna angle moves towards the direction of flight.

In the 1970s, the Westinghouse AN/APG-66 radar fitted to early-model General Dynamics F-16 fighters offered a Doppler Beam Sharpening (DBS) mode. It used Sar techniques and exploited the fact that since the radial velocity of the ground relative to the aircraft varies with the angle from the aircraft track, the antenna beamwidth could be considered as a series of angular sub-divisions, each having radar returns with a slightly different radial velocity, a characteristic that could be used to discriminate between them.

Doppler beam sharpening uses a coherent low-PRF radar mode. Resolution was related to aircraft ground speed and to antenna pointing angle with respect to the direction of flight, but typical azimuth resolution is around 0.5 degrees. When the antenna is pointing close to the aircraft track, there are not sufficient differences in relative ground velocity in the area being examined, so Doppler beam sharpening is ineffective within about 15 degrees of track.

The F-16 entered service equipped with the Westinghouse AN/APG-66. Its air-to-ground modes included real-beam ground mapping, expanded ground mapping and Doppler beam sharpening.

Expanded-beam real map mode provided an optional 4:1 magnification of a patch of terrain selected from anywhere within the radar's scan and range limits, while DBS provided a further x8 magnification over that in expanded real-beam map mode. Doppler beam sharpening relied on the processing of Doppler shift, so was only available at angles between 15 and 60 degrees off the aircraft's velocity vector. Should the aircraft's subsequent flight path bring the area being viewed to within 15 degrees of the aircraft centre-line, the radar would automatically switch to the normal ground-mapping mode.

More than 2300 examples of the AN/APG-66 were manufactured, and the set introduced many air forces to the capabilities of DBS. However, it reflected the electronics technology of the 1970s, and by the following decade, greatly improved performance was available.

By the mid-1990s, the first examples of F-16A and -16B aircraft rebuilt to a modernised standard adopted by Belgium, Denmark, Norway and the Netherlands were ,flying. This upgrade included modernising the AN/APG-66 to the improved APG-66(V)2A standard, with a new combined signal and data processor that provides seven times the speed and 20 times the memory of the older radar computer and digital signal processor line replaceable units. In this new variant, the displayed resolution in ground-mapping mode is quadrupled, and is reported to be close to that offered by Sar techniques.

Since 1984, the F-16C and -16D Block 30/40 models have been fitted with the improved APG-68, a set derived from the AN/APG-66(V)2. The -68 incorporates a software-controlled VHSIC processor handling signal and data processing. This provides the additional computing power needed for advanced radar modes including 8:1 and 64:1 high-resolution DBS ground mapping.

The resolution available from Doppler beam sharpening is limited by the antenna dwell time--the length of time for which the patch of terrain being observed is illuminated by the radar beam. If the antenna is set to scan that sector very slowly, or even to stop, flooding the target area with radar energy, the resulting radar returns can be integrated over a second or more. This provides an effective beamwidth of 0.5 degrees of better. This is similar to the beamwidth available from a sideways-looking Sar, so gives a much higher resolution, for example 0.5[degrees] or better.

The map image is presented as a 'still' after processing is complete. It provides enough detail to allow aircraft on an airfield to be located and counted from a long stand-off range, but the grazing angle must be large enough to avoid excessive masking of features. These high-definition Sar mapping modes can be used for navigational updating, target recognition, target selection and weapon aiming.

Inevitably, the sensors used to measure the position of the aircraft, and in the case of the higher-resolution Sar modes the actual position of the radar antenna, have small errors, and these errors result an error in the phase of the radar energy reflected by a target. By analysing the signal from a point target, these phase errors can be measured. Algorithms can then be used to correct these phase errors, sharpening the resolution of the resulting synthetic aperture radar image. This is known as 'autofocusing'. By measuring the errors from several point targets, resolution at long range can be reduced to sub-metre levels.

Another method of image improvement is to 'stack' a series of successive images. This process of superimposition strengthens the true target and helps minimise the effects of noise in much the same way that the rapid projection of a series of images by a cinema projector creates an image which is greatly superior to that which would be created by projecting a single image as a still picture.

In the case of a cine film, the successive frames contain an image whose position is near-constant; while the unwanted pattern created by the grain of the film emulsion differs from frame to frame, is therefore suppressed. In the case of Sar imagery, the successive images are taken at differing ranges and grazing angles, so each may show details not visible in the others.

The AN/APG-68(V)9 version introduced in 2000 for new-build F-16 Block 50 aircraft adds a high-resolution synthetic aperture radar mode. This version of the radar has a modified antenna subsystem incorporating a strapdown Inertial Measurement Unit (IMU), which is required for Sar operation. The Modular Receiver/Exciter (MORE), which effectively replaces the older Modular Low-Power Radio Frequency subsystem, is able to handle the wideband waveforms needed by the Sar mode.

The latest radar to be offered on the F-16 series is the AN/APG-80, the first radar for this aircraft to be fitted with an Active Electronically Scanned Array (AESA) antenna. Intended for use on Block 60+ aircraft, its first application will be the F-16 Block 60 Desert Falcon ordered by the United Arab Emirates. Designed to be interchangeable with the AN/APG-68(V)9, it will offer a higher level of performance, including an Ultra High Resolution Synthetic Aperture Radar (UHRSar) mode. Enhanced Sar/Automatic Target Cueing (ATC) and Moving Target Indicator (MTI)-on-Sar modes have been reported as options for future growth.

The F-15E Strike Eagle fielded during the 1980s needed a radar able to provide specialised ground-attack modes. In the resulting AN/APG-70 derivative of the -63 air-to-air radar used in the F-15A, B, and D models, Hughes used gate-array technology to develop a programmable signal processor (PSP) able to operate at almost 35 million operations per second, around five times the speed of the PSPs installed in earlier radars.

This processing power, teamed with custom-designed air-to-ground software, provided high-resolution mapping modes whose terrain imagery of near-photographic quality allows the detection 9f small targets from a range of 32 km. During early trials, the AN/APG-70 produced radar images with resolutions of less than three metres, allowing details such as buildings, streets, vehicles and even parked aircraft to be detected. The radar is now used by the F-15E, the F-15I and -15S models exported to Israel and Saudi Arabia, and on late-model F-15C and-15D aircraft.

At the time, for the McDonnell Douglas F/A-18A and -18B Hornet, Hughes developed the AN/APG-65. This offered real-beam ground mapping, plus two Doppler beam-sharpening modes. The first provided 19:1 expansion of a selected sector, while the second allowed a 67:1 expansion of a patch of terrain. In the 1990s, a modified version of this radar was fitted to the McDonnell Douglas AV-8B Harrier II Plus V/Stol fighters of the US Marine Corps and Italian and Spanish navies, while the licence-built AN/APG-65GY variant was used in Germany's F-4F Improved Combat Efficiency (Ice) programme, and on upgraded Greek F-4Es.

The follow-on F-18C and -D were the first to receive the improved Hughes (now Raytheon) AN/APG-73. Based on the older APG-65, this offered new air-to-ground operating modes such as terrain following and clearance, plus higher resolution ground mapping, features which made the aircraft a viable replacement for the subsonic Grumman A-6 Intruder carrier-based strike aircraft.

The synthetic aperture radar capability of the basic AN/APG-73 was limited by airframe flexing, since it relies on inertial reference data from the aircraft's Litton inertial navigation system. This was mounted in the centre part of the fuselage, a significant distance away from the radar's antenna.

In 1995, work began on the AN/APG-73 Phase II radar, which fitted a small navigation system based on fibre-optic gyros within the radar, using the free space created by redesigning a power supply. Attached to the bulkhead of the radar rack directly aft of the antenna, the measurement sensing subsystem, provides accurate antenna position and vibration information needed to create higher resolution Sar-based ground maps and improved air-to-surface weapon designation accuracy.

Another measure which improved the resolution of ground imagery was the addition of a stretch generator within the radar receiver unit. This 'stretched' the radar waveform using a linear-frequency modulator to enable the system to provide higher resolution imagery.

As a result of these changes, the upgraded AN/APG-73 matches the range and resolution performance of the AN/APG-70 in the F-15E and the Hughes ASar in the U-2R.

The AN/APG-73 is also fitted to the Boeing F/A-18E and -18F, but in the second half of this decade Lot 27 and subsequent aircraft will use the Raytheon AN/APG-79 AESA radar. Some Hornets could also be retrofitted with the AN/APG-79. Furthermore, reports suggest that Lot 26 aircraft will be built with provision for the newer radar.

As its designation suggests, the AN/APG-79 AESA will incorporate an active electronically-scanned array antenna. It will also have a new receiver/exciter section, and make extensive use of commercial off-the-shelf processing. The new set will offer high-resolution, long-range Sar modes that will allow the all-weather targeting of stand-off missiles.

DBS technology was also to feature in the Lockheed Martin AN/APG-67(F) which was developed for retrofitting into light fighters such as the Northrop F-5. The basic radar offered 40:1 high-resolution Doppler beam sharpening, but some versions of the radar were offered with synthetic aperture imaging.

When developing its Phantom 2000 upgrade in the 1980s, Israel turned to Norden (now part of Northrop Grumman) for the aircraft's AN/APG-76 Multi-Mission Radar System. The original specification for this high-resolution radar was very demanding, and intended to give the modernised aircraft an ability to fly attack mission in all weathers, delivering smart weapons. The requirement proved over-ambitious, resulting in programme delays. The specification was finally relaxed in some areas. Air-to-ground modes are reported to include real-beam ground mapping, DBS, high-resolution Sar and ground mapping with simultaneous Ground Moving Target Indication (GMTI).

The AN/APG-77 fire-control radar that Northrop Grumman has developed for the US Air Force's Lockheed Martin F-22 Raptor has no air-to-ground modes. The F-22 was developed as a pure air-superiority fighter, but under the new Global Strike Task Force concept will also be tasked with strike missions. There have even been proposals for a dedicated light bomber variant of the aircraft.

When the Block 5 version of the AN/APG-77 software enters service, this will add a number of air-to-ground modes, including moving target indication automatic target detection and classification and a long-range Sar mode with three-metre resolution. Also planned is the ability to merge radar data with Digital Terrain Elevation Data to create a three-dimensional perspective view.

This ability to register the radar imagery with a map or aerial photograph is the next major step in air-to-ground targeting, says Elta Electronics, which plans to add this to future fighter radars.

For the Lockheed Martin F-35 Joint Strike Fighter, Northrop Grumman is developing a multi-mission AESA radar. Scheduled to have begun rooftop tests in August 2003, it is due to fly in testbed configuration in March 2005, and as a complete radar in February 2007.

Air-to-ground modes will include high-resolution Sar mapping for target location, large-area Sar mapping and automatic target cueing. High-resolution Sar will be used to positively identify targets at long range, says Northrop Grumman, while a 'zoom' facility will allow a portion of the Sar image to be examined in greater detail. The radar's GMTI facility will be effective even against slow-moving vehicles.

In its original form, the Thales Airborne Systems (formerly Thomson-CSF) RDY radar for late-model Mirage 2000 fighters includes a DBS air-to-ground mode and optional high resolution Sar mapping. In 1997, the company announced the development of a new Sar-based high-resolution mode that could be used to provide a reconnaissance capability. In this mode, the antenna is slewed to its azimuth gimbal limit, while the aircraft flies a straight course. The radar can gather an image of a swath of terrain around 15 km wide and with a resolution of between one and two metres. A second spot mode can provide sub-metric detail of an area a few square kilometres in size.

Image processing is done on the ground using Sar data recorded in flight, so the new mode requires only slight hardware modifications to the radar. The company said that it was studying the feasibility of transmitting the Sar data over a datalink.

Work on a Sar mode for the Thales Airborne Systems Antilope V radar carried by the Mirage 2000N and -D is reported to have begun in 1996, but there are no indications of whether this upgrade has entered service.

Originally developed by NIIR Moscow (Scientific Research Institute for Radio Engineering, Moscow)--now part of the Phazotron-NIIR Scientific & Production Company--the Zhuk family of multi-purpose radars have air-to-ground modes that include real beam ground mapping, DBS and synthetic-aperture.

The Zhuk first saw service on the MiG-29M in 1988. It was followed by the Zhuk-27 for Su-27 upgrades and the Su30, the Zhuk-M for the MiG-29K and other MiG-29 upgrades and the Zhuk-Ph, which was the only model to use a phased-array antenna rather that a conventional mechanically-scanned unit.

In the upgraded MiG-29SMT, the original Sapfir-29 radar has been upgraded to N-019MP Topaz standard. This includes a Sar mode for ground mapping. This new mode provides higher-accuracy targeting information for air-to-surface weapons such as the Kh-31A television-homing and Kh-31P anti-radiation missiles.

For the MiG-35, Phazotron developed the RP-35 multimode airborne radar. Originally intended to use an electronically-scanned phased array antenna, this was reconfigured to use a conventional mechanically-scanned antenna, reportedly as a cost-saving measure. Air-to-surface modes include real-beam ground mapping, DBS and Sar. Similar features are offered by the Sokol radar developed for late-model Su-27s.

In the late 1990s, the MiG design bureau was known to be working on a planned upgrade to the MiG-31 'Foxhound' heavy interceptor. Intended to give the aircraft a multi-role capability, plus the ability to conduct missile attacks against Mach 6 targets, this would have modified the nose-mounted Zaslon-A radar to provide real-beam and synthetic-aperture ground-mapping modes. Given Russia's current economic problems, which have enforced what is essentially a 'procurement holiday' for the rest of the current decade, the upgrade is unlikely to be adopted.

Phazotron has developed several types of radar for export, most of which incorporate high-resolution mapping modes. The Kopyo radar being fitted to Indian Air Force MiG-21bis fighters as part of the MiG-21-93 upgrade, and offered in pod-mounted Kopyo-25 form for use on the Su-25TM has real-beam ground mapping, 10:1 DBS, and a Sar mode. Similar modes are available on the Moskit (Mosquito), a smaller and lighter radar being offered for use on advanced trainers and in fighter upgrades.

Israel's Elta Electronics' EL/M-2032 is being marketed customised for different aircraft. It forms part of Romania's Lancer retrofit to the MiG-21, and the WZL-2/IAI upgrade for the Sukhoi Su-22M4 'Fitter-K' fighter-bomber. The performance of the radar will depend on antenna size, but all versions offer real-beam mapping, DBS and high-resolution mapping (Sar) modes.

The Mk 3 version of the Ericsson PS05/A radar will be introduced on the Batch 2 version of the Gripen. (Batches 1 and 2 aircraft have the Mk 2 version.) This offers real-beam, Doppler Beam Sharpening and high-resolution Sar ground mapping.

The Mk 4 version being developed under the Greta (Gripen Radar Enhanced Target Acquisition) project for service in 2008 will involve minimal hardware changes, mostly to the receiver section, allowing Mk 3 radars to be upgraded to the new standard. New air-to-surface modes will be DLH (high-resolution Sar), DLVH (very-high resolution Sar) plus a Sar-GMTI mode able to detect surface targets located within the Sar patch such as combat bridges, vehicles and parked aircraft. Greta also involves a wide-band passive (Ralf) mode and an LPR look-up mode for long-range detection of air targets.

Under the Nora (Not Only A Radar) project, several further-improved variants are planned. Nora-PX will have better high-resolution Sar facilities, while the Nora 3 will be fielded around 2010 as the Mk 5 version of the radar. Designed for use in extreme EW conditions, this will have an AESA antenna plus additional wingtip units. Flight tests of demonstration hardware are expected to begin next year.

A follow-on Nora/EIRA (Ericsson Integrated RF Avionics) project is seen as a potential Gripen mid-life upgrade. This will create an all-new radar.

The attack radar for the Tornado interdiction/strike aircraft was designed in the 1970s by that was then Texas Instruments, and combines a ground-mapping radar and a terrain-following radar. BAE Systems has proposed an upgrade which would add DBS and Sar capabilities.

Air-to-surface modes of the Euro-Radar Captor radar (formerly known as the ECR-90) radar carried by the Eurofighter Typhoon include real-beam mapping, Ground Moving Target Identification (GMTI) useful for picking out moving surface targets such as armour and Sar spot mapping. Tranche 1 Eurofighters will have a Sar resolution one metre, but this is likely to be reduce to 0.3 metres on Tranche 2 aircraft.

Since the Thales RBE2 radar of the Rafale uses a passive electronical scanned antenna it is able to interleavests operating modes, so functions such as terrain following can be used at the same time as ground mapping modes. The latter include a high-resolution able to display individual buildings. A Sar mode is reported to be under development to provide weapon targeting and reconnaissance functions.

Captor and RBE2 could both exploit AESA technology when the time comes for a mid-life upgrade. This could use technology being developed under the Airborne Multi-mode Solid-state Active-array Radar (AMSar) technology-demonstration programme.