Integration of Satellite Based Broadband Data Service into the UH-60 Blackhawk

Sean Gannon SAIC, St. Louis MO

g_a_n_n_o_n_s_@_s_a_i_c_._c_o_m

David J. Kinney Project Lead, Aviation Applied Technology Directorate, Ft. Eustis VA

d_avid.k_inney_@_us_._army.mil

ABSTRACT Airborne Battle Command systems in the US Army require constantly increasing data communications bandwidth in order to maintain connectivity with other mobile and fixed Battle Command systems. Database exchanges, sensor data transmission, and Situational Awareness communications have created a need for mobile broadband data connectivity that has outstripped the capabilities of traditional military communications systems. Additionally, communications systems that have performed this function in the past have been terrestrially based, which limits their geographic coverage and requires substantial resources in-theater in order to operate effectively. Airborne Battle Command platforms such as the UH-60 Blackhawk hosted Army Airborne Command and Control System (A2C2S) therefore require Satellite based broadband data service in order to retain the efficacy of their embedded Battle Command Software Systems in environments where terrestrially based communications systems are not available. Challenges to integrating this capability into the UH-60 are signal blockage from the airframe, limited locations for antennas, radiation hazards to personnel, and weight/space/power limitations. Under the direction of the Project Management Office for A2C2S, the Aviation Applied Technology Directorate at Ft. Eustis, VA has developed a suite of equipment for the UH-60 that provides a secure satellite based transmission facility with bandwidth sufficient to carry multiple voice, video or data channels simultaneously. The final configuration provides the user with a rapidly installed kit that provides these services in all operational environments and with minimal impact to the weight, space, and power constraints of the host platform.

INTRODUCTION Airborne Battle Command systems are becoming increasingly dependent on high bandwidth data communications in order to maintain situational awareness, direct forces, and receive intelligence. The most advanced of these, the A2C2S, has 8 on-board computer systems that require continuous networked communications with other Battle Command systems. The A2C2S architecture has been primarily dependent on the Near Term Digital Radio (NTDR) for network connectivity with other Battle Command systems. The NTDR was developed to bridge the gap between the current need for tactical wireless networking and the fielding of the Joint Tactical Radio System (JTRS) and its Wideband Networking Waveform. Both the NTDR and the JTRS are terrestrially based communications systems which require Line Of Sight (LOS) between terminals in Presented at the American Helicopter Society 61st Annual Forum, Grapevine, TX, June 1-3, 2005. Copyright © 2005 by the American Helicopter Society International, Inc. All rights reserved.

order to complete a circuit. During thousands of hours of operation in Iraq, the A2C2S commonly lost connectivity with the NTDR network due to range limitations or terrain blockage. It quickly became clear that a terrestrially based network such as the NTDR or its replacement in JTRS is not a practical solution for rapidly moving mobile platforms such as the A2C2S in environments where forces are widely dispersed. The Program Management Office for A2C2S in conjunction with the User Proponent decided that a satellite based communications replacement for the NTDR was required. An initial requirement was set to provide 128 Kbps data service to the A2C2S without a dependence on a terrestrial network. This requirement is approximately equal to the realized data bandwidth of a NTDR radio in a tactical environment. A growth requirement was also set to permit up to 500 Kbps with minimal modifications to the aircraft hardware as new satellite services become available. The Aviation Applied Technology Directorate (AATD) was selected by the PMO A2C2S for development, performance testing, and rapid prototyping of a system that meets these requirements.

BACKGROUND A2C2S Description The A2C2S provides an airborne Battle Command platform for missions ranging from homeland security to deep operations in high intensity conflict. To accomplish this, the system must provide extensive robust communications capabilities, situational awareness, and computer systems, and must enable the commander and his staff to rapidly traverse battle space to critical places at critical times. The standard A2C2S communications suite includes UHF Satellite Communications (SATCOM) Voice and Data, UHF-AM, Havequick I/II, SINCGARS, VHF-FM, EPLRS, HF w/ Automatic Link Establishment, and Blue Force Tracking-Aviation (BFT-A). An additional 11 antennas are required to support the A2C2S mission kit (Figures 1 & 2), bringing the total number of antennas on the host UH-60 to 23.

SINCGARS 2SINCGARS 1SATCOM/GPS

BFT-Aviation

Figure 1. A2C2S Antennas (Upper)

EPLRSSINCGARS 3SINCGARS 4HQ 2

NTDRHQ 2

Figure 2. A2C2S Antennas (Lower)

The commander’s environment (Figure 3) includes 5 user workstations, supporting Army Battle Command Systems, BFT-A client, and other mission profile software. The commander and his staff, through the A2C2S, are able to see, understand, act, and finish decisively while rapidly traversing the battle space. The A2C2S began Rapid Deployment fielding with 3 systems in 2002 to support Operation Iraqi Freedom and continues with Low Rate Initial Production in 2005.

FireSupport

Commander

Operations

Intel

MasterOperator

Figure 3. A2C2S Maneuver Commander’s Environment

Available Aviation Satellite Services Satellite service options considered for meeting the broadband requirement included Department Of Defense operated communications networks as well as commercially operated networks. Only networks that are supported by currently available airborne antennas and terminals were considered. Table 1 details each of the services considered.

System Military / Commercial

RF Band

Bandwidth / channel

UFO Military UHF 56 Kbps Iridium Commercial L 10 Kbps Inmarsat Swift 64

Commercial L 64 Kbps

Inmarsat BGAN

Commercial L 432 Kbps

Boeing Connexion

Commercial Ku 5 Mbps (down) 256 Kbps (up)

Table 1. Satellite Service Options The UHF Follow On (UFO) system, which has been in operation for over a decade, was designed specifically for mobile users and works well on helicopters. The terminals are relatively small and lightweight and require only a simple omni-directional antenna. Unfortunately, simultaneous operation of multiple channels on the aircraft would require multiple sets of equipment, and therefore growth to 500 Kbps is impractical. Additionally, the UFO network is oversubscribed which makes it difficult to obtain access to the system. The Iridium system operates only narrowband channels which were developed primarily for voice communications. When these channels are adapted to data communications, the data bandwidth is not adequate to support A2C2S requirements.

The Inmarsat Swift 64 service is a popular choice for many military and commercial aviation users. The airborne terminals are capable of accessing up to 4 channels simultaneously which delivers up to 256 Kbps of bandwidth. When the Broadband Global Area Network (BGAN) service becomes available in 2006, these terminals can be easily modified to access BGAN services which will increase data bandwidth to 432 Kbps per channel. Connexion by Boeing offers by far the highest available data bandwidth for aviation users. Unfortunately, the airborne antennas and terminals are intended for Boeing 737 and larger fixed wing aircraft, and therefore are generally too large and heavy for integration onto helicopters. It was decided that operation in the L-Band spectrum offered the best compromise between available bandwidth and practicality of integration onto the UH-60. Inmarsat was the only service offered in the L-Band spectrum that could meet both the current and future bandwidth requirements. Inmarsat Description Inmarsat is a privately held company which operates a global satellite system comprised of its second and third generation satellites. The current system supports voice and data communications to mobile users in the maritime, aeronautical and land mobile markets. Inmarsat’s Swift-64 service is designed primarily for the airborne user. This service is provided by the Inmarsat-3 (I3) Satellites. With 2 channels in operation the Swift-64 service is capable of meeting the near term A2C2S broadband requirement. The Inmarsat BGAN service is aimed at delivering multi-media services to personal, mobile and portable terminal users. The BGAN service with data compression and network acceleration will easily meet the growth requirement of 500 Kbps.

SYSTEM COMPONENTS Inmarsat System Architecture The Inmarsat system architecture (Figure 4) consists of an airborne element within the A2C2S, a satellite communications network provided by Inmarsat, a terrestrial data network, and a Tactical Network Interface. The airborne element consists of all mobile equipment required to receive and transmit with the satellite, process data, multiplex audio channels, and encrypt/

decrypt information. The satellite communications network consists of the Inmarsat I3 constellation and the Land Earth Stations which bridge the communications into the Terrestrial network. The Terrestrial network consists of the Public Switched Telephone Network (PSTN). Lastly, the Tactical Network Interface takes the communications from the Terrestrial network, processes the same functions as were performed in the aircraft, and then connects into the Army’s Tactical Internet.

Figure 4. Inmarsat Architecture

Airborne Element Architecture The airborne element is comprised of several basic hardware groups (Figure 5). All of this equipment is located on the aircraft. The Inmarsat Terminal Assembly receives and transmits via the antenna and does all signal processing such as channel assignments, subscribing to the Inmarsat service, and providing Integrated Services Digital Network (ISDN) connections to the Networking Subsystem. The Autonomous Steering Unit continuously updates the Inmarsat Terminal with the location, altitude, roll, pitch and yaw data of the aircraft in order to keep the antenna pointed at the satellite. The equipment selected for the Inmarsat Terminal Equipment Subsystem consists of an EMS HSD-128 Terminal, LITEF Aircraft Heading Reference System, flux valve compass, and a GPS/ WAAS Receiver. The Networking Subsystem consists of all equipment required to interface the ISDN connections provided by the Inmarsat Terminal Subsystem to the A2C2S mission kit. These functions include multiplexing voice and data, network acceleration, data encryption/decryption, and media conversion. The multiplexing device allows dynamic allocation of the available data bandwidth to either voice or data

functions. There are 4 telephone connections available for the user which can be used for either secure or non-secure telephone calls. The network acceleration device uses features such as compression, application acceleration, and traffic discovery to improve the apparent bandwidth of the Satcom network.

Figure 5. Airborne Element Architecture

Aircraft Antenna Selection All Inmarsat antennas must be approved by Inmarsat before being sold to end users for integration into aircraft. Inmarsat ensures that the antennas have sufficient gain, beam width, and can operate in the temperature and vibration extremes of an aircraft. The selection of an Inmarsat Antenna for A2C2S therefore does not so much depend on the operational characteristics of the device but rather on the weight, space, and ease of integration of the device. There are two basic types of Inmarsat antennas for aircraft; mechanically steered (Figure 6), and electronically steered (Figure 7). The mechanically steered antennas are gimbaled in 2 axes and are physically pointed at the satellite by an Antenna Control Unit. These antennas are mounted inside an enclosure called a Radome which protects the antenna from the environment but is transparent to the radio signals. In order for the antenna to maintain LOS to the satellite while being mechanically steered, it must protrude from the Outer Mold Line (OML) of the aircraft. The electronically steered antennas, instead of being physically steered toward the satellite, are electronically reconfigured so that the main beam of the antenna is shaped in the direction of the satellite. These antennas do not have any moving parts and generally do not stick out above the OML of the aircraft by more than a few inches. They come with an integral outer cover which forms the radome over

the antenna elements. These antennas are generally heavier than the mechanically steered types and also require a larger surface area on the aircraft to mount.

Figure 6. Mechanically Steered Inmarsat Antenna

Figure 7. Electronically Steered Inmarsat Antenna From a performance point of view, these two types of antennas generally have similar electromagnetic characteristics when steered near the zenith. When steered near the horizon, however, the electronically steered antennas have a diminishing aperture and lower antenna gain numbers. The mechanically steered antennas maintain the same electromagnetic characteristics regardless of which direction they are pointed. The electronically steered antennas are generally preferred for fast moving fixed wing aircraft since they minimally change the OML of the aircraft. To compensate for off axis performance, often two of these antennas are installed on either sides of the aircraft. This ensures that one antenna is able to link with the satellite without requiring high off axis angles. With 23 antennas already installed, the A2C2S has minimal available “real estate” remaining, and it was decided to use a mechanically steered antenna which would have a much smaller footprint than the electronically steered antenna. The AMT-50 mechanically steered antenna from EMS Technologies in Ottawa Canada was selected by the design team. The AMT-50 already had been successfully integrated onto the CH-53 and the CH-47 helicopters. EMS was also able to offer an off the shelf radome for the AMT-50 which could be easily adapted to the UH-60. These factors significantly reduced program risk.

Tactical Network Interface (Ground) The Tactical Network Interface forms the other end of the communications link. It mirrors the equipment in the A2C2S Networking Subsystem. ISDN lines come in from the terrestrial service provider and are processed identically as is done in the aircraft Networking subsystem. The Ground Node is located near an existing Tactical Internet processing facility.

AIRCRAFT INTEGRATION Antenna Location Options With the system architecture and major electronics components defined, the most challenging issue became where to place the AMT-50 antenna on the A2C2S equipped UH-60. The location for a SATCOM antenna on an aircraft must provide a clear LOS between the antenna and the Satellite. With geosynchronous (also referred to as geostationary) satellites, the satellite remains fixed above a given point on the earth. As the earth rotates on its axis, the satellite orbits above the earth at the same rate. Geosynchronous satellites are therefore very popular for fixed base communications systems because the ground antenna can be pointed to a fixed position and then left alone. On an aircraft, however, the antenna must constantly adjust to compensate for changes in the aircraft’s attitude and to maintain the correct bearing to the satellite as the aircraft moves over the earth. The ideal location for a SATCOM antenna on an aircraft would be a location that provides a horizon to horizon unobstructed view of the sky even as the aircraft moves over its full range of roll and pitch angles. SATCOM antennas are commonly installed on fixed wing aircraft in either the top of the fuselage or on the top of the vertical stabilizer. In these locations, the path between the Inmarsat antenna and the satellite is rarely obstructed by the airframe. In helicopters, however, the main rotor system limits the number of locations for a SATCOM antenna. Locating equipment above the main rotor system as is done in the OH-58D and AH-64D is complex and expensive. The only practical locations on a rotorcraft therefore suffer from some blockage from the airframe and main rotor system. An initial list of candidate locations was generated based and an evaluation performed on the integration feasibility of each. The locations and Pros / Cons for each is detailed in Table 2. Of these 7 candidate locations, 3 were considered low-risk and also did not interfere with existing UH-60 and/or A2C2S systems: the External Stores Support

System (ESSS) Wing, the Forward Avionics Bay Door (Nose), and the Engine Access Door. Only these three locations would be further evaluated as possible locations for the Inmarsat Antenna.

Candidate Location

Airframe impact, Integration complexity, and Programmatic

Considerations Top of Vertical Stabilizer

↓ Structure not robust enough ↓ High Vibration Environment

Above Main Rotor

↓ High cost ↓ Lengthy development schedule ↓ High Technical Risk

Forward Maintenance Access Door

↓ Interference with operation of Wire Strike Protection System

↓ Cable wear when operating door. APU Access Door

↓ Blocks Infrared Countermeasures system

↓ Potential Electromagnetic Interference with GPS Receiver and ATC/IFF Transponder

↓ Location crowded with other antennas

ESSS Wing (Wing)

↑ Requires minor modifications to aircraft

↑ Low development cost and risk ↓ Forces ESSS configuration at all

times Forward Avionics Bay Door (Nose)

↑ Existing Weather Radar Location ↑ Low development cost and risk

Engine Access Door (Engine)

↑ Requires minor modifications to aircraft

↓ Potential structural issues Table 2. Initial List of Candidate Locations for

Inmarsat Antenna & Pros/Cons

Performance Analysis of Antenna Locations An analysis was conducted on the three acceptable antenna locations to quantify the expected performance of the Inmarsat system at each. The analysis had two primary purposes; 1) determine the amount of airframe Blockage for each location, and 2) determine the Radiation Hazards to Humans for each location. The airframe blockage analysis assumes that the Inmarsat Antenna will operate properly when it has a visual Line of Sight to the Satellite. This is considered a worst case assumption since with diffraction of the Electromagnetic Wave there would be more of a gradual attenuation of the radio signal as the antenna moved into the shadow of the airframe rather than an abrupt interruption of the signal. With this worst case assumption, a high degree of confidence is built into the analysis that if visual LOS is maintained between

the antenna and the satellite, the Inmarsat Link will be maintained. In order to perform the LOS blockage analysis, the aircraft was simplified into 3 rectangular shapes; main rotor system, upper fuselage, and main fuselage. For each antenna location a geometric analysis was performed to determine the azimuth and elevation angles at which LOS blockage occurred (Figures 8, 9, & 10).

Figure 8. Geometric Analysis of Airframe Blockage to Wing Mounted Antenna

Figure 9. Geometric Analysis of Airframe Blockage to Nose Mounted Antenna

Figure 10. Geometric Analysis of Airframe Blockage to Engine Door Mounted Antenna

Radiation hazard analysis was performed by plotting areas in and around the aircraft where the Inmarsat antenna could point and where the radiation field strength level exceeded the FCC limit for controlled exposure. If any of these plotted areas coincide with an operator location within the A2C2S (red shaded areas in Figures 11, 12 & 13), then software modifications would need to be made to the Inmarsat terminal in order to inhibit transmitting while the antenna is pointed into these areas. Inhibiting transmitting while in these regions effectively creates another blocked region, further limiting the performance of the Inmarsat system. Radiation hazard areas for areas outside the aircraft are also plotted (areas marked with solid red line in Figures 11,

12 &13). These are areas where personnel should be restricted during ground use of the Inmarsat system.

Figure 11. Radiation Hazard Region -

Wing Mounted Antenna

Figure 12. Radiation Hazard Region -

Nose Mounted Antenna

Figure 13. Radiation Hazard Region -

Engine Door Mounted Antenna

The LOS blockage and the Radiation Inhibit region must be then overlaid on top of each other to determine the overall effective blockage for a particular antenna location.

The effective blockage (actual plus radiation inhibit) only impacts the operation of the system if the antenna actually needs to steer into the blocked region to maintain link with the satellite. In order to determine if this will occur, the elevation angle to the satellite for each geographic location must be determined. Since the geosynchronous satellite stays fixed above a point on the earth along the equator, there is one geographic location on the earth where a particular satellite is directly overhead. Moving radially away from that location, the SATCOM antenna must point to decreasing elevation angles in order to remain pointing at the satellite. Eventually the angle to the satellite goes below the horizon. Since the I3 satellite constellation consists of 4 spacecraft, it is rare that an angle above the horizon of less than about 10 degrees is required.

Since the A2C2S is being equipped for operation in specific locations around the world it seemed appropriate to predict the blockage for those specific locations rather than generically for worldwide operation. There were 7 locations selected for the analysis where A2C2S aircraft were expected to operate in the next few years: Ft. Drum, NY; National Training Center, CA; Joint Readiness Training Center, LA; Germany; Korea; Kuwait; and Northern Iraq (Mosul). While operations in any part of Iraq are possible, the Northern and Southern extremes of Mosul to Kuwait provide a reasonable range of Satellite elevations with which to predict system performance. Germany and Korea were modeled at the approximate geographic center of each country. The critical factor for determining blockage is the elevation angle to the satellite. The closer the satellite is to the horizon, the more likely that the airframe will cause blockage. For all antenna locations, blockage is possible only for a range of aircraft azimuth bearings (i.e. when the main rotor mast or other airframe component is between the antenna and the satellite). For this reason, the actual azimuth location of the satellite is ignored and the analysis plots azimuth bearings relative to the front of the aircraft.

Since the blockage is a function of the antenna angle relative to the aircraft rather than to the earth, a baseline aircraft attitude, and a dynamic range of aircraft attitudes needed to be selected. The baseline attitude adjusts the elevation angle relative to the earth to an angle relative to the aircraft. These numbers would be typical for straight and level flight for the aircraft in an A2C2S configuration. The baseline roll attitude selected is 0 degrees. The baseline pitch attitude selected is -3 degrees for ESSS wings installed and -1 degree without ESSS. Since elevation angles to the satellite are the driving factor towards operational performance, yaw attitude is ignored as it has no impact on antenna elevation angle.

The dynamic range of aircraft attitudes is selected to approximate the range of attitudes expected from a UH-60 performing a Battle Command mission. The roll range variation is +/- 15 degrees, and the pitch range variation is +/- 5 degrees. These angles are represented as vertical arrows (Figures 14, 15, & 16) to show variation in blockage as a result of dynamic flight. Since the nose mount antenna location is on the center butt line of the aircraft, changes in the roll angle of the aircraft have minimal effect and are ignored. Also, since the engine door and ESSS wing locations are approximately along the same longitudinal station as the main rotor mast, changes in pitch angle have minimal effect and are ignored for those locations.

Summary of Blockage Analysis

The blockage analysis reveals that all locations suffer from some blockage due to the airframe. The wing location has blockage from the upper fuselage and the main rotor mast (Figure14). In level flight, the upper fuselage presents the most significant blockage (100 degrees wide in azimuth) and impacts performance in Korea, Germany, and NTC. Since the analysis has the antenna on the left ESSS wing, the blockage increases during 15 degree left turns enough that the Mosul, Ft. Drum, and JRTC locations would have reduced system performance. During 15 degree right turns there is no antenna blockage. The nose location has substantial blockage from the main fuselage due to the relatively low waterline location of the antenna (Figure 15). In level flight all geographic locations except for Kuwait have a 120 degree wide blockage. Changes in aircraft pitch angle do not materially impact the overall performance. The engine door location has only minor blockage due to the main rotor mast (Figure 16). This blockage is 45 degrees wide in azimuth and impacts performance for Ft. Drum, JRTC, Korea, Germany, and NTC. Since the analysis has the antenna on the left engine door, the blockage increases during 15 degree left turns enough that the Mosul, Korea, and Germany locations would have reduced system performance. During 15 degree right turns there is only minimal blockage from the main rotor mast for Korea, Germany, and NTC.

Figure 14. Wing Mounted Antenna

Blockage and Radhaz Regions

Figure 15. Nose Mounted Antenna

Blockage and Radhaz Regions

Figure 16. Engine Door Mounted Antenna

Blockage and Radhaz Regions Summary of Radiation Hazards Analysis At the wing location, the radiation hazard region occurs in the cockpit for the co-pilot and in the main cabin for A2C2S operators at the two left workstations. The left gunner position is inside the radiation hazard region but is shielded by the wing structure. The cockpit and main cabin radiation hazards do require transmit to be inhibited but do not dramatically change the system performance since the radiation hazard regions occur at such low elevation angles. At the nose location, the radiation hazard region extends well into the cockpit of the aircraft. In order to ensure safe electromagnetic field levels, transmitting of the Inmarsat system would need to be inhibited

approximately 10 degrees above the angle where airframe blockage occurs. The inhibiting of transmissions in this region substantially adds to the blockage due to the airframe. Due to the relatively high waterline of the engine door antenna location, there is no radiation hazard region for occupants of the aircraft. At this location, the AMT-50 antenna is not capable of illuminating into either the cockpit or the main cabin. All three locations present a radiation hazard region outside of the aircraft and proper procedures would need to be followed to ensure personnel safety while operating the Inmarsat system on the ground. Selection of Antenna Location Based on the results of the blockage analysis, radiation hazards analysis, and operational constraints, the engine door location was selected for test, evaluation, and initial fielding. This location offers good performance in the primary theater of operation and does not require any mitigation of Radiation Hazards. Additionally, this location places minimal restrictions on the platform or tactics of employment since the system can be operated with or without ESSS wings installed. Initial structural analysis of the engine access door concluded that the materials and mounting method for the door system is adequate to secure the AMT-50 antenna, associated electronics, and the radome.

CONCLUSION The Inmarsat design developed by AATD allows the A2C2S to un-tether from terrestrial data networks while meeting the user’s requirements for broadband data connectivity. Program risk was reduced by performing quantitative analysis of possible antenna locations to predict overall system performance on the A2C2S/UH-60 platform. A prototype of this system will undergo testing on an A2C2S aircraft in March and April of 2005. If test results confirm the analysis, then 11 A2C2S aircraft will be equipped with this Inmarsat design in calendar year 2005. The design presented can be easily modified to support the Inmarsat BGAN service which will increase data bandwidth by more than triple. With BGAN, this system is expected to meet the requirements of the A2C2S well into the future.

ACKNOWLEDGEMENTS

The technical work presented in this paper was performed by and funded by the Aviation Applied Technology Directorate. SAIC’s role in this effort is limited to the professional services provided under contract DAAH10-02-F-007. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation hereon.