The U.S. Navy continuously seeks to deploy new technology in order to increase the mission capabilities of surface ships and submarines. Shipboard electronic systems must support multiple mission scenarios. This includes intelligence gathering, reconnaissance, mine hunting, interdiction of enemy ships, as well as the transport of personnel. Integration of emerging wireless technology, such as network-centric warfare and precision weaponry, will enable successful execution of these diverse service requirements.

Modern warships feature multiple communications systems that are tailored for a particular mission. Achieving reliable operation of a wireless system that co-exists with other radio frequency (RF) platforms is a substantial design challenge. Comprehensive planning is required to determine the optimal locations for the multiple antennas mounted on a warship. Deploying the same communications systems across multiple vessels is complicated because the available space for mounting the antennas is not same on each warship. Recognizing the need to address a gap in antenna technology, the U.S. Navy has tasked the EMPF, along with its industry partners, to research new multifunctional capabilities for antennas in small or combined form factors. As a result, a new research program has been created to develop Flexible Antenna System Technology Insertion.

To establish the research goal,antenna count, placement of the antenna arrays, and the functional capabilities of each communications system utilized on the warships were considered. Requirements of the new system were also determined by this analysis. A flexible, multi-use antenna platform must be reconfigurable, as it will be utilized to counter anti-access threats.  With the present technology, multiple antennas are required to implement the complete communications solution.  Installation of multiple antennas would clutter the mast of the warship. In addition, co-location of antennas tends to cause interference problems and coupling between the transmitting and/or receiving elements. This can degrade the operation and integrity of the communications/radar/intelligence systems.

The EMPF has established the following goals for the Flexible Antenna System Technology Insertion research project:

• Assessmentofoperational requirements for the new antenna and capture of systems requirements.
  
  
• Antenna technology investigation – review commercial      off the shelf (COTS) and state-of-the-industry military antennas in addition to contemporary antenna technology research.
  
  
• Correlation of technology to the new ship acquisition requirements.

The EMPF strategy includes a partnership with leading technology providers in the defense industry. The EMPF will work with companies that have significant experience in the design of antennas, radio systems and ancillary equipment for maritime, terrestrial, and airborne platforms. The EMPF program will also incorporate the knowledge gained by companies that have contributed to the development of advanced, compact, multiuse antenna farms. As a result of this cooperation, a block diagram of proposed techniques for integration of the requirements and the associated technologies was developed.  Co-site issues (potential interference between closely spaced antennas) and a mitigation plan, were also identified.  Specific emphasis was placed on exploiting the current state-of-the industry in HF, VHF, and UHF technology.  Recent COTS antenna technologies were identified, including ultra wideband (UWB).  A correlation of the antenna technology and the new shipboard requirements resulted in multiple recommendations and approaches that were then narrowed down to a final approach, set of construction materials, and superstructure design for optimization.

The Goals for the Flexible and Affordable Antenna System are:

• Leverage the EMPF and industry partner findings to develop prototypes of critical hardware components such as antennas, filters, couplers and active cancellers to reduce the required number of antennas needed to support adaptable mission requirements.
  
  
• Define a standard interface for Navy shipboard communications equipment to allow compatibility with the new ship radio suite concept and proposed antenna solution.
  
  
• Design a single mast system and ship kit for integration into the ship along with the required documentation.

The findings from initial studies showed that no single specialized antenna met all requirements.  For the remainder of this initiative, the technical approach will be to investigate current state of the art antennas and specifications (i.e. return loss, frequency response, bandwidth, gain, radiation pattern, max available power gain, and antenna-to-antenna isolation) that were collected in the initial studies.  While the recommendations made in the initial studies were general in applicability due to the proprietary and competition-sensitive status of the new ship designs, the activities in the remainder of the initiative will follow these general recommendations to the specific dimensional and materials details for the selected sea frame concept.

The program will develop methods for reducing the antenna farm’s physical outline.  A computer model of the baseline antenna system will be constructed and simulations conducted on up to two candidate concepts.  By leveraging the EMPF’s industry partners capabilities, a prototype package will be developed and designed with a requirement to be survivable in a surface/marine environment.  The proof-of concept Antenna System Prototype will then be designed and verified electrically.  Functional tests will also be performed on the “electrical” prototype (with a near mechanical form factor) over a section of the 3 MHz – 2 GHz band.

As part of the review and refinement process of the initial study recommendations, requirements analyses (using data from the selected sea frame builder and systems engineering to refine the systems architecture) will be performed.  A market survey will be conducted in order to define other Navy qualified antennas as potential candidates for a stacked (extremely affordable, multi-use) architecture.  The resulting table of candidates will be compared to the baseline design and evaluated on whether they meet the minimum RF requirements and maximize cost benefit.  This chart of required performance as a function of cost will be invaluable for the required wideband operation from the HF to L-band.

Modeling and system level simulation will play a key role in determining the expected effectiveness of the antenna and digital pre-distortion system.  The modeling will include analysis to quantify the performance and interference aspects of the digital pre-distortion algorithms in the RF environment.  A co-site simulation analysis will then be done to establish parameters that define the utility of the proposed approach supporting the condensed antenna farm.  SIMULINK® will be used to model the digital pre-distortion algorithm and the receive-analog cancellation algorithm.  This model-based design platform will allow multi-domain simulation of dynamic systems in an interactive graphical environment.

The development program allocates time for prototype testing of critical system components. This includes the selection of antennas to perform mutual coupling tests based on system requirements.  The spiral development approach will be adopted to incrementally build up to the most performance-based design.  Testing of the prototype will take place and mutual coupling data will be generated.  Various options for pre-distortion and receive cancellers will be assessed along with digital sub-band filtering.
 
Mechanical design analysis will be performed to determine survivability of the antenna in a combat environment.  The mechanical characteristics of the system, such as stress, will be evaluated.  Particular attention will be focused on low life cycle cost, and the use of open standard interfaces and COTS components where applicable. As a result of the analysis, the system is expected to demonstrate compliance with the E3, SWaP, and shipboard environmental requirements.

After development of the prototype, electrical tests will be performed on the critical components to simulate the actual operation to the extent required to prove concept feasibility and determine the system performance.  Mutual coupling, digital pre-distortion, receiver cancellation, and digital sub-band cancellations options will be developed, modeled, and tested. During the whole development process, systems engineering activities will be conducted, which include requirements management, system design, systems integration and testing, and systems simulation and modeling.

Other relevant activities during the Flexible Antenna System Technology Insertion program are:

• Above Deck Antenna Integration – development of an RF penetrable shroud and RF bulkhead mountable antenna interface plate, ready configured for ship or test bed mounting.
  
  
• Below Deck Electronics Integration – development of an environmentally conditioned COTS rack containing the digital pre-distortion electronics, RF switches, and control hardware in a single mast solution.
  
  
• Transition Demonstration and Implementation – development and demonstration of a single mast system designed for a stealthy, space-optimized ship.

The US Navy’s expectation for the flexible antenna system technologies is extensive and demanding.  With the development of the new, affordable ship acquisition program, space conservation through the reduction of antenna count is a definite and necessary requirement.  At the same time, the reduced antenna count must be multi-functional to compensate for the additional antennas that are no longer present.  Therefore, the importance of the Flexible Antenna System initiative is paramount to the Navy to maintain current maritime dominance and effective joint operations.

REFERENCE
1. Walsh, Ed. The Next Step for Shipboard Electronics, Military & Aerospace Electronics. March 2006

2. Address by George W. Bush given at the U.S. Naval Academy, May 25, 2001