APG-76 MULTIMODE RADAR

ADAPTATION OF AN/APG-76 MULTIMODE RADAR TO THE SMUGGLING INTERDICTION MISSION * M. E. Tobin M. Greenspan Northrop Grumman Norden Systems Nonvalk, CT, USA 06856-5300 ABSTRACT The AN/APG-76 multimode radar was designed and developed for a multimode attack fighter application requiring rapid search, detection, identification, and precision location of both airborne and surface targets from long standoff ranges under adverse weather conditions. Unique is the radar’s ability to generate SAR images of a selected area while simultaneously detecting and tracking all-speed moving objects located within that imaged region. Northrop Grumman Norden Systems has recently upgraded and adapted this radar to smuggling interdiction and related law enforcement missions. These adaptations have added an integrated GPS/INS subsystem for enhanced self-navigation and target location accuracy, a long range wide-band digital data link and ground station for mission control and data dissemination, and 1 meter and 0.3 meter resolution SAR modes for positive target identification. Additionally, the radar has been installed into wing-mounted pods and adapted for side-looking and 360 degree coverage applications. Automatic target detection and enhanced-range sea-surveillance and air-targeting modes are also now available through the use of open architecture commercial processors and. nonproprietary transportable programming languages.

1.O INTRODUCTION In May of 1993, the Department of Defense issued a Systems Plan Report on Counterdrug Detection and Monitoring (D&M) Systems. This report was prepared in response to section 1043 of the 1994 National Defense Authorization Act which directed the Secretary of Defense to (a) “establish requirements for counterdrug D&M systems to be used by the DoD in the performance of its mission as the lead agency of the Federal Government for the detection and monitoring of the transit of illegal drugs into the United States”, (b) “identify and evaluate existing and proposed counterdrug D&M systems in light of those requirements”, (c) “prepare a plan for the development, acquisition, and use of improved counterdrug D&M systems by the Armed Forces”, and (d) report back to Congress on the requirements, evaluation results, and the resultant plan. The DoD Systems Plan focused on the movement of cocaine to the United States from South America and outlined how a combination of small general aviation aircraft, boats, and land vehicles were used to transport the cocaine from the source countries, through the transit zone, and finally into the arrival zone along the US Southwest border, Gulf states, Caribbean islands, and other coastal/border areas. Because of the immense size of the areas to be surveilled, the relatively small physical volume of * Presented at the IEEE 1996 National Radar Conference, Ann Arbor, Michigan, 13- 16 May 1996

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cocaine involved, the broad spectrum of potential drug transportation systems, as well as the flexibility and ingenuity of the drug traffickers, the report concluded that there was no clear, complete, and simple solution to the DoD D&M mission. It also concluded, however, that airborne assets were the most effective platforms for the mission and identified a complex combination of these existing available platforms and systems that, when applied together, could provide the operational flexibility to address the threat. The AN/APG-76(v) radar system described in this paper addresses this DoD D&M mission by attempting to combine air, sea, and land search, track, and identification capabilities within a single flexible airborne radar sensor system. 2.0 BASELINE AN/APG-76 CAPABILITIES The AN/APG-76 is a multi-mode, forward looking, tactical, airborne Ku Band radar system designed as an all-weather, air-to-air and air-to-surface surveillance and fire control sensor for the highspeed, high-maneuverability F-4E tactical strike fighter aircraft. In its baseline configuration, the AN/APG-76 radar has already demonstrated its high resolution SAR imaging capabilities as well as its ability to detect and track low flying light aircraft, small sea craft, and small, slow-speed land vehicles in high clutter environments. These same capabilities, available over wide areas from long ranges under all weather conditions, day and night, are considered crucial to supporting counterdrug detection and monitoring operations. Over the past several years, more than 50 of these radar systems have been produced and delivered. Norden Systems is now in the process of upgrading and adapting the system to further extend its capabilities and to more fully satisfy the unique requirements imposed by the smuggling interdiction mission. 2.1 SPECIFIC RADAR OPERATING MODES Operator selectable modes and capabilities include: Real Beam Plan Position Indication (PPI) Ground Map; High Resolution SAR Ground Map, including a 26 km (14 nmi) sector Doppler Beam Sharpening (DBS) mode and 9 km (5.0 nmi), 4.5 km (2.5 nmi), and 1.5 km (0.8 nmi) SAR patch modes; and High Resolution Ground Moving Target Indication (GMTI) modes. The system also provides high resolution fixed target, hard target, and ground moving target tracking; rotating antenna detection; air-toground ranging; and a full complement of air-to-air search, track, and air combat modes. As delivered, the baseline system provides Real Beam coverage over 120 deg. sectors at ranges up to 150 km (80 nmi), SAR resolutions as small as 3 meters (10 ft), detection and track of 2.5 to 30 m/s (5 to 55 kt) slow speed in-clutter moving targets, and the all-aspect detection and track of low cross-section airborne targets at ranges up to 75 km (40 nmi). The multi-mode capability of this system is ideal for the monitoring of coastline, border, roadway, airfield, shipping lane, port, and harbor activities. In addition, the high resolution SAR images allow for easy discrimination of terrain features, urban areas, variations in vegetation types, as well as small man-made objects. The AN/APG-76 provides the unique real time capability of simultaneous SAR and GMTI through the use of a multi-aperture antenna. Interferometric clutter cancellation techniques using a three port azimuth interferometer provide the ability to both detect slow speed movers in a high clutter environment and to locate them in their true ground position. GMTI detections are displayed as symbols superimposed at their true location relative to the simultaneously received and processed SAR or DBS background

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map. The system provides a high probability of detection combined with a low false alarm rate through the use of low noise receivers and sophisticated signal processing techniques. These techniques include automatic constant false alarm rate thresholding, integration of correlated target reports prior to final target detection, and rejection of strong discretes from the mainlobe and sidelobes as well as saturated signals. The simultaneous SAWGTMI capability, available in all high resolution SAR modes , has been successfully utilized to detect ships, small boats, ship wakes, small slow moving land vehicles, and hovering helicopters. Also unique to this radar is an on-board real time motion compensation subsystem that blends a variety of sensor inputs. It enables focusing to be maintained under virtually all gusting and maneuvering flight conditions, reduces errors that would otherwise impact the location of the moving target symbol locations on the SAR background map, and significantly enhances the target tracking accuracy. The system has successfully demonstrated its ability to maintain track on slow moving vehicles under move/stop/move and turn around conditions. It was also shown to maintain a stable, accurate track on low cross-section stationary targets over periods of several minutes. The air-to-air mode of this radar system is a medium PRF search and track capability operating at ranges out to 75 km (40 nmi) over antenna coverages up to of +/-60 degrees in azimuth and +/-30 degrees in elevation. Range and search scan sector coverages are operator selectable and hand-control steerable. Ground moving target rejection is also provided, with 20 & 30 m/s (35 and 55 kt) limits selectable. This mode has been demonstrated to detect and track low flying aircraft under both platform and target aircraft high G maneuvering conditions. 2.2 RADAR SYSTEM ARCHITECTURE

The AN/APG-76 uses a high power wide band TWT-based coherent transmitter and a mechanically scanned, low-sidelobe, multi-aperture planar array. The array incorporates seven separate receive antenna apertures that include sum, delta azimuth, delta elevation, guard, and left, center, and right interferometer ports. Based on the specific selected radar mode, received signals from up to four of these seven apertures are simultaneously routed to independent matched receivers and analog-to-digital converters and then processed simultaneously to produce the needed real time digitall images and detected target reports. The current computer architecture for the radar signal processing and control portion of the radar is based on a Programmable Signal Processor (PSP) and a separate Radar Data Processor (RDP). The PSP contains both a Vector Processor (VP) and a Scalar Processor (SP). The SP is based on a MIL-STD1750A processor. Peripherals controlled by the SP include two MIL-STD-1553B bus interfaces, a floating point co-processor, and an interface to the VP ring bus. A 1 megaword non-volatile program store module maintains programs for executing each of the system modes. The VP consists of five parallel complex arithmetic pipeline processors, each capable of executing 100 million complex floating point instructions per second. The RDP is also a MIL-STD-1750A computer that is used to interface the AN/APG-76 to the operator’s Radar Control Panel and the rest of the aircraft avionics. In addition, the RDP provides the nine-state Kalman filtering used in the radar track modes. This fully militarized computer subsystem is currently in the process of being replaced with an open architecture commercial processor using a non-proprietary transportable programming language. This open commercial architecture is expected to significantly reduce costs while providing the means for readily customizing system operation and interfaces to satisfy individual customer requirements.

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3 .O EXTENDED AN/APG-76 CAPABILITIES Key system enhancements incorporated into the AN/APG-76(v) for the smuggling interdiction mission include improved navigation and target location accuracy, long range communication links for image data distribution and control, enhanced resolution for target identification, several options for compatibility with alternate platforms, and a virtual shopping list of new modes and capabilities that can be tailored to meet customer needs. Each of these extended capabilities will be addressed in the following paragraphs. 3.1 GPS AIDED NAVIGATION FOR PRECISION LOCATION AND TRACKING

Precision target location requires target position to be determined in absolute coordinates with respect to an earth-referenced coordinate frame, not just relative to the targeting aircraft. This allows the targets location to be precisely passed to a command and control center for distribution to other network assets. Precision targeting requires accurate knowledge of own-ship position in all three axes within the reference coordinate frame. In 1994, Norden Systems integrated the AN/APG-76 radar with a Global Position System (GPS) aided Ring Laser Gyro (RLG) Inertial Navigation System (INS). This RLG/GPS INS provides the increased navigation accuracy in position, velocity, and attitude required for precision targeting. This unit provides (by specification) own-ship position, velocity, and attitude error in the order of 16 meters SEP, 0.03 meterdsecond, and less than 0.1 milliradians respectively. These “tightly coupled” systems also provide relative immunity to external interference since they have a narrow band tracking loop. Precision targeting algorithms refine the baseline target tracking and motion compensation algorithms to take advantage of the increased navigation accuracy of the RLG/GPS INS system. These algorithms have been developed and applied to flight data using ground processing. Increased ground target tracking accuracies beyond those inherently obtained from the conversion to an RLG/GPS INS system have been demonstrated by Norden Systems using APG-76 Gulfstream I1 flight test data and these algorithms.

3.2 DATA LINK / GROUND STATION FOR COMMAND AND CONTROL Several types of standard wideband digital data links are available for use with the AN/APG-76(v) radar. A low-cost L-band system that is currently used on both U.S. Navy and U.S. Air Force pod installations, as well as on the Northrop Grumman BAC 1- 11 airborne test aircraft, provides virtually hemispheric coverage to a range in excess of 185 km (100 nmi) using only an omnidirectional whip antenna on the ground-based receiver. The down-linked digital data consists of both RS-170 formatted radar video frames as well as accompanying digital data packets that contain information on the platform’s position, velocity, and orientation, as well as specific operating mode, field of view, and other operating parameters. This realtime digital down-linked data has been used during representative field exercises to detect and identify unusual formations or movement of ground targets as well as air and sea assets. Figure 1 illustrates the ability of the real-time data linked imagery to provide remote sites with real-time imagery that shows both land and sea-based activities. Also, if helicopters or other low flying aircraft had been present within the imaged area, they would also have appeared in the scene as symbols at their true ground coordinates.

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3.3 ENHANCED RESOLUTION FOR TARGET IDENTIFICATION Recently demonstrated upgrades to the baseline system have extended its capability to provide up to 0.3 meter resolution imagery, polarization diversity for image enhancement, and focused moving target imaging (important for ship detection and identification). Figure 2 illustrates the benefit of 1 meter vs 3 meter resolution for the identification of ship targets. Both 1 meter an 0.3 meter resolution SAR images are currently available in flight in real time with the AN/APG-76(v) and provide the level of detail needed for positive target identification in most targeting situations. These enhanced resolution SAR modes have been extensively flight tested, and both the hardware and software developed for this mode has been found to be mature and robust. Automatic target recognition and identification routines designed to run in real time in a commercial workstation environment have also been developed and are currently being interfaced to the real-time radar data in preparation for field demonstrations. 3.4 ALTERNATE PLATFORM INSTALLATIONS Although originally intended for nose mounted fighter installations, the AN/APG-76(v) has recently been adapted to external wing-mounted pod applications. The installation shown on the Viking S-3 in Figure 3 utilized a standard U.S.Navy equipment pod and an un-modified F-4 radome assembly. The installation shown in-flight on the F-16 in Figure 4, used a standard 2.3 cu. meter (600 gallon) fuel tank. Other alternate system installations that have been studied and proposed include steerable sidelooking arrays and a chin- or belly-mounted radome assembly providing 360 degree azimuth coverage. 3.5 OPEN ARCHITECTURE GROWTH CAPABILITIES The replacement of the existing digital processing subsystem with an open-architecture commercial processor that supports standard non-proprietary software development languages and operating environments is expected to significantly simplify the development and maintenance of unique system modes and interfaces as required by specific customer communities. Capabilities that would be of special interest to the smuggling interdiction mission community would be the addition of a simple automatic target detection and cueing capability as well as the ability to merge external intelligence data that indicates the location of established commercial shipping lanes and air corridors, the location of specific known commercial ships and aircraft, as well as other ancillary data such as the locations of known roads, airfields, docks, and other important landmarks. Earthreferenced (north-up) displays could also be provided to simplify the comparison with other sensor information and to standardize the hand-off of sensor data to other network uses and agencies. Additionally, high performance sea surveillance modes could be provided to optimize the detection of slow or stationary vessels in rough seas. Similarly, long range air target detection and tracking modes with significantly increased target sensitivity could also be configured to take advantage of any external air targeting cues that could be used to narrow the air search space. 5.0 SUMMARY / CONCLUSIONS

Modern multi-mode surveillance and targeting radars provide many of the capabilities needed to support smuggling interdiction and related civilian missions. Adaptation of such radars to new platform and operating environments is not only very possible, but highly practical as well.

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Marine Vehicle Activity on-thslow Beach .

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Figure 1. AN/APG-76(v) SAR & SAWGMTI Down-Linked Imagery

Figure 2. AN/APG-76(v) Images of Ship in Delaware Bay

Figure 3 : S-3 Pod Installation

Figure 4. F- 16 Pod Installation

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