Communications: directive radio wave systems and devices (e.g. – Directive – Including antenna orientation
Reexamination Certificate
2001-10-12
2003-06-17
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including antenna orientation
C342S362000, C342S363000
Reexamination Certificate
active
06580391
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to an antenna pointing apparatus and method used in connection with aligning radio-frequency (RF) emissions from an antenna, and further relates to improving antenna performance in a satellite communication system using only relatively simple equipment.
2. Description of the Related Art
Typically, an installer desires to maximize or optimize, to the extent possible, a satellite earth station's performance, including the performance of the antenna which propagates/receives radio-frequency energy to/from one or more satellites. Determining whether the earth station's antenna's performance is optimized as much as possible under the particular circumstances is not always a simple matter. Several steps must be accomplished in making that determination. These include, but are not limited to, for example, aligning the earth station antenna in azimuth and elevation so that the antenna “boresight” is pointing directly at a particular satellite of interest, and also adjusting the polarization orientation of the earth station's antenna so that a cross-polarization isolation parameter measured at a hub station, for example, is acceptable for discriminating between signals having a different polarization than the polarization of a desired signal.
Other antenna parameters which may be of interest to installers and operators include gain, noise temperature, voltage standing-wave ratio (VSWR), power rating, receive-transmit group delay, radiation pattern, isolation, and so-called “G/T”, or antenna gain divided by system noise temperature. Many of these parameters are inherent in the fundamental antenna design, while others, such as cross-polarization isolation, may be adjusted to a specified value using an alignment procedure during antenna installation
The case of alignment of an earth station's antenna to a geostationary satellite is a simplified version of the general case of tracking a satellite in a non-geo-synchronous orbit. There are typically two angles of interest between the earth station and the satellite: the elevation angle, and the azimuth angle. The elevation angle is the angle of the satellite above the horizon, as measured from the earth station, and the azimuth is the angle between the line of longitude through the earth station, and the direction of a sub-satellite point, i.e. the point at which a line between the satellite and the center of the earth intersects the earth's surface. The angle of elevation depends on the latitude of the earth station, and will be relatively large towards the equator, and smaller toward the poles, while the azimuth angle is related to the latitude and longitude of the earth station. Thus, the elevation and azimuth angles define the “boresight” of the receiving antenna for a given receiver location, with respect to a particular geostationary satellite's orbital position.
Conventionally, in order to measure and/or adjust the cross-polarization isolation of an antenna at an earth station, or “remote station”, an installer must first align the antenna boresight to the specific satellite being used. This may be accomplished, for example, by merely roughly setting the azimuth and elevation angles based on a tabulation of angles corresponding to the satellite of interest, as a function of the latitude and longitude of the remote station. For more precise alignment, however, these settings could be used as an initial setting, and then a signal transmitted from the satellite could be monitored, and the azimuth and elevation angles could be more precisely adjusted based on “peaking” or maximizing the received signal, or simply by ensuring that the received signal is greater than a desired threshold signal value. Typically, the maximum signal strength achievable is used in finally adjusting the azimuth and elevation of the remote station antenna to ensure that the boresight of the remote earth station is accurately aligned with the satellite.
After the azimuth and elevation angles of the remote station antenna are set, the person installing the antenna would activate the transmitter at the remote station to uplink to the satellite, and the satellite would then, in turn, downlink to the hub station or network operations center. Typical communication satellites may have multiple transponders that receive signals at one frequency from one or more remote stations, and then retransmit these signals at a different offset frequency to the hub. Further, each of these transponders may be configured to receive a signal having a particular type of polarization, e.g. a polarization type that is one of the general cases of linear, circular, or elliptical polarization. More specifically, the satellite transponders may receive horizontal or vertical linear polarization, or right-hand or left-hand circular polarization. Further, the satellite transponders may retransmit to the hub using a different type of polarization than the type originally received from the remote station. For example, in the case of linear polarization, a transponder may receive a horizontally polarized signal from a remote earth station, and may retransmit a vertically polarized signal to the hub station. This same type of transponder approach may be used in the reverse direction, i.e., in transmitting from the hub to one or more remote stations through the satellite and appropriate transponders.
Therefore, it is not enough to merely align the boresight of the remote station antenna to the satellite of interest. The remote station must also account for and align to the polarization type that the satellite transponder is configured to receive and/or retransmit. If the polarization of the remote station antenna is not properly aligned to the satellite transponder's polarization (e.g. horizontal or vertical linear polarization), or is of the incorrect type, a reduction in signal strength most likely will occur at both the remote station and at the hub. In addition, cross-polarization interference may also occur between adjacent satellite transponders configured, in an interleaved fashion, to receive signals from other remote stations in the same frequency band, but with different polarizations, if the remote station is transmitting a signal with an undesired polarization. This interference will also then be received at the hub.
To preclude such cross-polarization induced interference problems, cross-polarization isolation may be determined at the hub by measuring, for example in the case of linear polarization, the signal strength of the desired linearly polarized signal being transmitted from one transponder on the satellite, and the signal strength of the undesired linearly polarized signal which is transmitted from an adjacent transponder on the satellite. The undesired linearly polarized signal is retransmitted by the transponder primarily due to misalignment of the remote station antenna polarization. If the remote antenna polarization is properly aligned, the undesired linearly polarized signal should theoretically be eliminated. Practically speaking, however, the undesired linearly polarized signal should be at least reduced by a typical value of at least−30 dB (1/1000th) from the signal strength of the desired linearly polarized signal, as measured at the hub. To be able to actually meet this typical value of isolation, the polarization at the remote station is usually iteratively adjusted until the cross-polarization isolation is within the specified value.
Unfortunately, this procedure requires that fine adjustments in the remote station antenna alignment must continue to be made until cross-polarization isolation, is at the desired or best-achievable value under the circumstances. Such a procedure necessarily requires dedicated communication (e.g. by telephone) between the installer at the remote station and hub station personnel. This process, as represented by the flow-chart of
FIG. 6
, takes time, satellite resources, and hub site resources, possibly re
Gaske Paul
Kepley Walter
Middeke Michael
Blum Theodore M.
Hughes Electronics Corporation
Sales Michael
Whelan John T.
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