Communications: directive radio wave systems and devices (e.g. – Directive – Including a steerable array
Reexamination Certificate
1998-04-13
2001-08-28
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a steerable array
C342S374000, C342S422000
Reexamination Certificate
active
06281839
ABSTRACT:
This application is the national phase under 35 U.S.C. §371 of prior PCT International Application No. PCT/DK96/00434 which has an International filing date of Oct. 11, 1996 which designated the United States of America, the entire contents of which are hereby incorporated by reference.
The present invention relates to a method and a system for communicating electromagnetic signals, and more particularly to a method and a system for stabilizing an antenna for tracking an electromagnetic energy source. The invention also relates to a communication method and system for simultaneously receiving and transmitting signals.
In communication via a satellite to and from a moving vehicle such as a ship or car, a mobile terminal installed on the vehicle is required. Usually mobile INMARSAT terminals are composed of one part being installed on a vehicles platform which platform is in a fixed position relative to the vehicle. This platform will hereafter be designated “moving platform” and the part of the terminal that is installed on it is designated EME (external mount equipment). Furthermore, the terminal may comprise electronics that is installed in the wheelhouse of the vehicle. This part of the terminal is designated IME (internal mount equipment).
There is a well-known problem associated with stabilization of the antenna of the EME in such a way that specifications for G/T (Antenna Gain/Receive system noise Temperature) for the receiving system in the direction of the satellite as well as EIRP (Equivalent Isotropic Radiated Power) for the transmit system are met, as long as vehicle motions such as pitch, roll and turn rate are within specified limits. This stabilization problem becomes more serious when the gain of the applied antenna in the mobile terminal gets higher. Because of adoption of new modulation techniques and launching of satellites having a much higher EIRP and G/T at the L-band, it has become possible to reduce the antenna gain and hence the size and cost of the EME. This invention shall be seen in the light of this ongoing process of reducing cost, size and complexity of EME.
Up till now at least three types of mobile terminals employing a stabilized-antenna are defined by INMARSAT, namely INMARSAT A, INMARSAT B and INMARSAT M. Antenna stabilization is typically performed by one of two methods namely, 1) the so-called passive stabilization or 2) the socalled active stabilization, the former being the most simple and the latter being the solution showing the best performance. However, with the reduction of size and weight for future terminals, method 2) should bring the best solution since fly-wheels with high momentum of inertia become increasingly difficult to accommodate in the structure of an electromechanical antenna stabilizing system. In INMARSAT A, B and M, many different types of stabilization mechanisms have been used with the number of rotation axes ranging from two to four. Within the field of antenna stabilization it has been common practice to use gravity as a reference to measure inclination of the moving platform and to use either the magnetic field of the earth or the information from a gyro (e.g. ships' gyro) as a azimuth reference. With these two references it is possible to generate a set of control signals to be fed to the various motors that control angular rotation about the mechanical axes. Generation of a vertical reference can be done by an inclinometer that is made insensitive to horizontal accelerations. However, the azimuth reference usually present a problem since the magnetic field of the earth is affected by the structure of the vehicle and since the inclination may be high, i.e. close to 90°, so that a precise projection onto a plane parallel to the surface of the earth becomes increasingly difficult to achieve. On the other hand, a reference from a gyro is very reliable but requires the vehicle to be equipped with an expensive apparatus. Also, installation of the terminal is complicated by the need to interface to a gyro or other exterior devices.
A system using active stabilization, method 2, is described in U.S. Pat. No. 4,881,078. This patent discloses a tracking system with a beam switching antenna. The tracking system is used for tracking a stationary satellite, and a phased array is used for an antenna mounted on an automobile. The phased array antenna has a sharp beam which is switched between two different directions in azimuth. The antenna beam is switched between the two directions periodically by control of phase constants in a feeding circuit of the antenna and comparison is made in strength between signals received before and after the beam switching to obtain an error signal as an azimuth error signal. Then, the antenna is mechanically moved according to the error signal until the error signal becomes zero.
However, the tracking system of U.S. Pat. No. 4,881,078 only comprises a receiver system. Hence, there is a need to have a combined receiver and transmitting system using active stabilization for tracking an electromagnetic energy source such as a communication satellite or a repeater satellite.
In communication systems utilizing phase modulation (such as the new INMARSAT systems) there is, however, a serious problem associated with beam switching in that the phase of the transmit signal and hence the transmit frequency spectrum may be disturbed by the beam switching when this is performed on transmit frequencies as well as receive frequencies as is the case in conventional beam switch systems utilizing phase shifters located in the signal path common to both receive and transmit signals coming from and going to the antenna elements in e.g. a phased array.
Another disadvantage of conventional beam switching is that there is a typical 0.4 dB loss of transmit power due to the fact that the direction of maximum transmission is switched a few degrees from physical bore sight of the antenna. Furthermore in conventional systems, considerable loss of transmit power may occur in the switching diodes of the phase shifters. The switching diodes must therefor be bulky which in turn leads to higher parasitic components such as parallel capacitance and series inductance. This in turn makes it difficult to match the antenna and duplexer to the low noise amplifier (LNA) so that the noise figure may be increased and even worse, the varying LNA gain and noise figure may vary when switching diodes are turned on and off. In tracking systems based on beam switching, LNA gain and noise FIG. must be kept absolutely constant to ensure good tracking performance.
Thus, it is an object of the present invention to bring a solution to the above mentioned problem. Accordingly, the present invention provides a method and a system for two way communication between a first station and a second station where each of the stations comprises a receiving means and transmitting means for receiving and transmitting electromagnetic communication signals. The first station has an array antenna for transmitting and receiving the electromagnetic communication signals to and from the second station, and the array antenna has a direction of optimum transmission or direction of electric boresight of transmission which is substantially constant in relation to a physical boresight axis of the antenna or an axis perpendicular to a plane mainly comprising the array antenna. The array antenna is coupled to the receiving means and transmitting means of the first station by electrical feeding means.
In the method according to the invention one or more signals is/are transmitted from the first station to the second station, and the direction of the physical boresight axis of the antenna of the first station is controlled so as to reduce or minimize pointing errors of the antenna in relation to the second station. In order to obtain signals for controlling the physical boresight axis of the antenna, the direction of optimum reception or electric boresight of reception of the antenna of the first station is electrically changed or switched in one or more directions
Mull Fred H.
Tarcza Thomas H.
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