CONFORMAL PHASED ARRAY WITH BEAM FORMING FOR AIRBORNE SATELLITE COMMUNICATION H. Schippers ([email protected]), J. Verpoorte , P. Jorna, A. Hulzinga (National Aerospace Laboratory NLR, Amsterdam), A. Meijerink, C. Roeloffzen, (University of Twente, Enschede), R.G.Heideman, A. Leinse, (LioniX bv, Enschede), M. Wintels (Cyner Substrates, Utrecht) ABSTRACT • Electronically steered phased array antennas (see Figure For enhanced communication on board of aircraft novel 7). antenna systems with broadband satellite-based capabilities • Hybrid steered antennas; such antennas are normally are required. The installation of such systems on board of mechanically steered in one direction (e.g. azimuth), aircraft requires the development of a very low-profile and electronically in the other direction (e.g. elevation). aircraft antenna, which can point to satellites anywhere in the upper hemisphere. To this end, phased array antennas which are conformal to the aircraft fuselage are attractive. In this paper two key aspects of conformal phased array antenna arrays are addressed: the development of a broadband Ku-band antenna and the beam synthesis for conformal array antennas. The antenna elements of the conformal array are stacked patch antennas with dual linear polarization which have sufficient bandwidth. For beam forming synthesis a method based on a truncated Singular Value Decomposition is proposed. 1. INTRODUCTION Figure 1 Mechanically steered reflector antenna For enhanced communication on board of aircraft novel antenna systems with broadband satellite-based capabilities Mechanically steered reflector and array antennas are required. The technology must bring live weather reports require a large radome for reasons of protection and to pilots, as well as live TV and high-speed Internet aerodynamic behaviour. Such a radome increases the connectivity to passengers. Satellite communication services aerodynamic drag and requires additional cuts and stiffeners can be provided by Low Earth Orbiting (LEO) systems and in the fuselage structure. Therefore, since many years Geostationary (GEO) systems. Today there are more than research is being performed to develop electronically steered 300 operational geostationary satellites. These satellites are phased array antennas (or hybrid antennas) on singly curved being used for television broadcasting, communications and structures, for instance conformal to the fuselage of an weather forecasting. In general, receive and transmit aircraft. These conformal antennas should have similar antennas of ground stations do not need to track antenna characteristics as mechanically steered antennas, geostationary satellites. These antennas are installed at a with high scan capabilities at high latitudes. In addition the fixed location and are much less expensive than tracking antenna should be able to withstand severe environmental antennas. However, for a mobile terminal for satellite conditions such as temperature, pressure and vibration. communication (for instance when applied on an aircraft) a In the Netherlands, a consortium (consisting of the tracking antenna is required in all circumstances. Many University of Twente, Lionix BV, the National Aerospace studies are going on worldwide to employ these Ku-band Laboratory NLR and Cyner Substrates) is developing geostationary satellites for communication with mobile technology for such a conformal airborne antenna. The terminals on cars, trains, ships and aircraft. hardware for this antenna consists of a broadband conformal Ku-band terminals on board of moving platforms Ku-band phased array antenna (developed by NLR and require antennas with high gain of which the main beam can Cyner) and a broadband optical beam forming network with continuously be steered to geostationary satellites. To this True Time Delays (developed by the University of Twente end the following types of antenna systems can be and Lionix). Beam forming synthesis methods are being identified: developed by NLR to control the beam shape and direction. • Mechanically steered reflector antennas (see Figure 1). This technology is developed in the national project • Mechanically steered array antennas (beam of array is FlySmart. The objective of this paper is to present research fixed, but mechanically steered). results of two key topics: the broadband conformal phased array antenna and the array antenna beam forming synthesis method. 343 978-1-4244-1757-5/08/$25.00 c 2008 IEEE The antenna front-end consists of a broadband The phased array antenna shall maintain the proper conformal phased array antenna (bandwidth 2 GHz) (linear) polarization during all attitudes and at all positions operating in Ku-band. The antenna elements are stacked of the aircraft (also at high latitudes). patch antennas with dual linear polarization. First results An antenna to be used on aircraft has to be able to indicate sufficient bandwidth in input impedance and operate in severe environmental conditions concerning radiation pattern. Special attention is paid to the temperature, pressure, vibration and humidity. The manufacturability of the antenna by selecting specific environmental requirements for civil airborne equipment are materials and appropriate fabrication techniques. given in RTCA DO-160 or EUROCAE ED-14 [6]. Once the global design of the conformal phased In general the antenna system consists of a phased array has been established, further research concerns the array antenna, electrical-to-optical conversion, optical beam verification of its beam forming capabilities. For instance, is forming (or beam steering) and optical-to-electrical it possible to find an optimum set of element excitations for conversion (Figure 2) the antenna elements of the array to best satisfy the radiation pattern demands in some well-defined sense and to steer the beam in the direction of the geostationary satellites? The National Aerospace Laboratory NLR has developed an adapted least squares pattern synthesis that yields an efficient tapering for conformal phased arrays (see [1]). For the steering of the beam of the conformal phased array a squint-free, continuously tunable mechanism is proposed that is based on a fully integrated optical beam Figure 2 System design of Ku-band receive antenna with forming network (OBFN) using cascades of optical ring Optical Beam Forming Network (OBFN) resonators (ORRs) as tunable delay elements [2]. The phased array antenna will be a conformal 2. SYSTEM ASPECTS (singly-curved) 2-D array of dual linear polarised broadband In the ITU Radio Regulations [3] portions of the Ku-band antenna elements. Each antenna is followed by a Low Noise are allocated to aeronautical services: Amplifier (LNA) and down-converter (together a Low • AES receive band 1: 10.70 – 11.70 GHz (primary Noise Block converter, LNB). The Local Oscillator (LO) allocation to fixed satellite service) signals of the LNBs are synchronised to maintain an • AES receive band 2: 12.50 – 12.75 GHz (primary appropriate phase relation between the OBFN channels. The allocation to fixed satellite service) Intermediate Frequency (IF) signal from the LNB is • AES transmit band: 14.00 – 14.50 GHz (secondary subsequently fed to optical modulators which perform the allocation to mobile satellite service) electrical-to-optical conversion. In the Optical Beam The Aeronautical Earth Stations (AES) have to comply with Forming Network (OBFN) each individual signal is ITU-R RECOMMENDATION M.1643 [4] and with ETSI attenuated and delayed in order to shape and direct the EN 302 186 [5], a harmonised European Norm for satellite antenna beam. The sum of all signals is converted back from mobile Aircraft Earth Stations (AESs) operating in the the optical to the electrical domain. 11/12/14 GHz frequency band. The tracking algorithm will use the aircrafts In addition, reception of commercial satellite position and attitude to determine the appropriate broadcasts is of interest: polarization and azimuth and elevation of the antenna beam. • Satellite TV: 11.70 – 12.50 GHz (primary allocation to To reach the objective of a 2 GHz bandwidth, both broadcast satellite service) the antenna front-end and the beam forming network should In the Dutch FlySmart project, only the receive have broadband characteristics. Therefore, the antenna antenna system will be developed. The objective is to front-end consists of an array of stacked patch antennas. The develop a conformal phased array antenna having an beam forming network consists of an optical network with True Time Delays (TTD) which have inherently a large instantaneous bandwidth of 2 GHz, covering the whole o frequency range of 10.7 to 12.75 GHz. bandwidth. To have a 2 beamwidth and high gain antenna Satellites operating in these bands are geostationary (approx. 36 dB), a large array antenna is needed. The array satellites spaced 2o apart in the United States and 3o in size may be in the order of 40 by 40 antenna elements. Europe. In order to be able to receive these satellites also at high latitudes (e.g. during inter-continental flights) the antenna system should have sufficient performance at low elevation angles. Therefore the antenna system is required to have a small beamwidth (to discriminate between the satellite signals) and a high gain (>30 dB) also at low elevation angles. Since gain of the antenna is related to the effective aperture of the antenna in the direction of the satellites, a conformal antenna also covering side parts of the fuselage could be an advantage. 344 2008 International ITG Workshop