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GPS-Iridium Anti-Jam (AJ) Antenna Systems for Air Vehicle and Sea Vehicle Platforms


OBJECTIVE: Develop integrated military anti-jam (AJ) antenna systems with small footprint which can operate using both GPS navigation satellite signals and Iridium communication satellite signals. Antennas are desired for both air vehicle and sea vehicle platforms. An integrated antenna system offers the following potential benefits compared to the use of two separate antenna systems: reduced antenna footprint more suitable for platforms with limited size, weight and power (SWAP) constraints, reduced integration and test costs, reduced complexity of upgrade (e.g. an existing GPS antenna may be replaced with a GPS-Iridium antenna), integrated antenna provides AJ benefits to both types of systems, common antenna aperture for navigation and timing, e.g. facilitates compensation of Doppler effects associated with antenna lever arms and enables enhanced signal-to-noise ratio (SNR). DESCRIPTION: Signals broadcast by GPS satellites are susceptible to degradation or disruption by low levels of RF interference. One method to counter GPS interference is through the use of multi-element adaptive array antenna technology to either form an antenna beam null in the direction of interference sources, form beams in the direction of GPS satellites or perform a combination of these two operations. Current GPS antenna systems operate on the military L1 and L2 frequency bands. The inclusion of iridium signal functionality within a common adaptive array antenna system introduces significant challenges into the antenna design requirements. GPS AJ antenna systems must operate in the presence of multiple GPS satellites in view, e.g. up to 12 satellites, and also multiple GPS interferers in view having different line-of-sight (los) directions. Whereas older antenna technologies often operated on the principle of"nulling"the interferer signal, e.g. by forming nulls in the antenna gain pattern in the direction of the interferers, many of the newer antenna systems combine nulling with beam forming or beam steering, i.e. forming antenna beams in the direction to GPS satellites. The combination of nulling towards interferers and beam forming towards GPS satellites has the effect of attenuating the interference signals while amplifying the satellite signals. even when interference is not present, the effects of beam forming can provide increased SNR and link margin for processing GPS and iridium signals. The process of"nulling"can result in rapidly changing composite RF signal phases (often in an unpredictable fashion) in the GPS receiver and navigation algorithm. since some gps applications, e.g. aircraft landing systems, require the use of precise carrier phase information, the navigation function associated with these applications can be degraded when nulling occurs. If the complex antenna weights are controlled (or known) in the same processor as the navigation function, then the effect of the complex weights on the composite RF signal phase can be predicted. More advanced algorithms can combine adaptive antenna array and adaptive filter technologies in the time or frequency domains to perform space-time adaptive processing (STAP) or space frequency adaptive processing (SFAP). On some air vehicle and sea vehicle platforms the functions of beam forming and nulling must also compensate for aircraft kinematics, e.g. rotation of the antenna array plane. Thus modern adaptive antenna system signal processing may be tightly coupled with GPS and inertial navigation systems (INS) for improved performance. Designs which digitally form the antenna array beams within the receiver/navigation processor may also generate a different beam for each satellite signal processed, with the processing of each type of signal optimized for frequency band, signal power, signal and interference characteristics within the frequency band of interest, and signal type requirement (e.g. navigation or communication), etc. Solutions for both air vehicle and sea vehicle platforms are desired, including small form factors with diameters of four inches or less and depth of three inches or less (including antenna and antenna electronics). Both iridium signal transmission and reception functions are required. For air vehicles, AJ capability is desired for both the received GPS and iridium signals. For the sea vehicle version, AJ is required for the GPS signal reception and the GPS reception and iridium transmit/receive capability does not have to be simultaneous. Objectives include frequency bandwidth compatible with the GPS m-code (L1&L2 with at least 24 mhz bandwidth) and a 30 db interference rejection capability for at least three los directions while maintaining good signal availability for accuracy. Some of the technical challenges associated with an integrated GPS-iridium AJ array antenna system include: (1) the antenna system must address platform constraints and interface requirements for both gps and iridium signals, and must operate synergistically with GPS and iridium transceiver signal processing, (2) interoperability with other platform blue force rf emissions, (3) received iridium signal can be significantly stronger than received GPS signals, (4) the antenna must also support transmissions from the platform to the iridium satellites and the transmission signal is much stronger than the received signals -- thus the antenna system must provide good isolation to prevent the transmission signal from activating the nulling function, and to enable simultaneous reception of weak navigation and communication signals and transmission of strong signals, if applicable for platform mission requirements, (5) iridium is a LEO system and GPS is a MEO system so that the iridium satellites are subject to much smaller rise to setting time intervals, e.g. on the order of several minutes for iridium as compared to several hours for GPS satellites, and higher doppler and doppler rates for iridium than for GPS satellites, (6) iridium satellites do not have as much geometric diversity as the GPS satellites, e.g. whereas often eight or more GPS satellites are in typically in view, only four satellites are needed to enable a navigation solution. On the other hand, often only a single iridium satellite is in view and this satellite must maintain connectivity for the duration of the communications interval or until another satellite comes into view. Thus for a single iridium satellite in view, the los direction must be given a strong preference within the spatial processing algorithm. The beam forming may need to be coordinated with the iridium satellite communications function, including a coordinated switching to new iridium satellites at the appropriate time epochs. PHASE I: Perform analyses and trades to illustrate a strong understanding of the requirements and to predict performance. Develop designs for integrated GPS-Iridium antenna systems or antenna/transceiver navigation/communication systems that address the technical issues and challenges indicated above. Demonstrate how the antenna system integrates into air and sea vehicle platforms and operates synergistically with the GPS-Iridium and INS, if applicable, signal processing. The design should emphasize reuse of existing antenna technology and consideration of platform requirements and constraints, including interface and availability requirements. Document the design and predicted performance results in a report. For the sea vehicle version, a Phase I objective includes demonstration via breadboard or test bed. PHASE II: Develop the design into a mature bread board, test bed or prototype system, and demonstrate performance in an RF interference environment using simulated or real GPS and Iridium satellite signals. PHASE III: Develop operational GPS-Iridium integrated antenna systems or antenna/transceiver navigation/communication system, integrate and test the performance in air vehicle and sea vehicle platforms in the presence of interference. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: This technology may also be used for navigation on commercial air and sea vehicle systems, e.g. commercial aircraft. For example, because of the proliferation of low-cost GPS jammers, the FAA is becoming more concerned about the loss of GPS or Wide Area Augmentation System (WAAS) signals due to RF interference, such as recently occurred in the Newark airport area. In this case, the antenna design could be modified to include the WAAS signal as broadcast from WAAS satellites deployed at geosynchronous (GEO) orbit.
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