Antenna Pointing Techniques

User antennas must also be aligned with the desired satel­lites for signal reception. The satellite’s orbital position is specified by its ephemeris which describes the satellite’s alti­tude, eccentricity of orbit, inclination, longitude of ascension, and time of epoch. The satellite ephemeris and the user loca­tion are required to determine the antenna pointing angles. Typically, this information is transferred to the antenna con­trol unit where the required pointing angles are computed and the necessary commands to the antenna positioner are issued. The requirements for antenna pointing depend on the

antenna beamwidth, the uncertainty in the satellite’s loca­tion, and the orbital dynamics. For communication applica­tions, a pointing accuracy of one-tenth of a beamwidth is typi­cally specified to minimize signal loss caused by pointing errors.

In some cases, antenna pointing is trivial. For example, the antennas for personal communication systems are pur­posely designed with broad coverage antennas so that the user has no pointing requirements. Another common example is a small user antenna for direct broadcast satellite reception from geosynchronous satellites. These antennas are roughly aligned using the user’s geographic location, verified by signal peaking techniques, and rigidly secured. The combination of the relatively wide beamwidth from the small antenna and the stationary position of the geosynchronous satellite and its station keeping results in a fixed pointing requirement.

In other applications where satellites have inclined orbits or orbits that are not geosynchronous, the satellite undergoes dynamic motion with respect to the user, and a means must be provided to follow the satellite in orbit. For users with rela­tively broad beamwidths compared to the orbital uncertainty, the knowledge of the satellite ephemeris and the user location may be adequate to simply command the user’s antenna posi­tion. This open loop procedure is commonly referred to as pro­gram tracking.

If some uncertainty exists in the antenna pointing, such as ephemeris data that are not current, another open loop technique referred to as step track is commonly used to verify correct alignment with the satellite’s position. The user’s an­tenna is pointed at the nominal position of the satellite as might be obtained from program track. The antenna is then commanded to move in equal and opposite angular positions from this nominal pointing direction. If the antenna is cor­rectly aligned with the satellite, the received signal level should be reduced by the same amount at both angular off­sets. If the signal levels at both positions are not identical, the difference in the power level provides the angular correc­tion to the nominal position. This process is repeated in the orthogonal plane to properly position the antenna in two an­gular coordinates.

These two open loop antenna pointing techniques are com­monly used together. The step track procedure is periodically exercised to validate the correctness of the program track pointing. A significant advantage of these techniques is that minimal equipment is required. The antenna positioner is re­quired and simple software commanding is needed to execute the angular offsets. The power measurements at the different angular offsets may be obtained from a simple measurement of the receiver’s automatic gain control (AGC) voltage with the appropriate calibration and linearity verification.

When user antennas have beam widths that approach ten times the uncertainty in satellite position or a significant amount of orbital dynamics exist, closed loop tracking tech­niques are required. These techniques are commonly used in radar systems to locate targets and are referred to as mono­pulse designs. This term is derived from radar applications because the tracking information is obtained from each radar pulse. In radar applications, the received signal level varies as the target changes aspect angle and typically has a sig­nificant dynamic range. Thus, tracking information is derived from each radar pulse so that the dynamics of the target re­turn do not degrade antenna pointing performance. Mono­pulse systems for communication applications have easier re­quirements than those for radar systems. The received signal level has a relatively constant amplitude, so that the mono­pulse signals can be sequentially sampled reducing hardware requirements. The antenna pointing accuracy depends on aligning the antenna with sufficient accuracy to minimize sig­nal loss as compared to locating a radar target with the maxi­mum precision practical.

Monopulse systems operate by forming two types of an­tenna beam shapes: a sum beam that receives the desired sig­nal and a difference beam that has a null on the antenna boresight axis. The signal power in the sum beam is max­imized by positioning the antenna to the null of the difference pattern. The ratio of the signal levels in the sum and differ­ence beams is independent of the power density of the re­ceived signal, and provides a measure of the angular displace­ment from the antenna’s axis. This ratio can be used in a closed loop system so that the antenna will align to the re­ceived signal and will follow any variations in the signal lo­cation.

Two different types of feed designs are commonly used to generate the sum and difference beams. One feed design (27) uses a conventional feed for the sum beam and an additional four small feed elements to produce the difference beam. The second design (28) uses a higher order waveguide mode to produce the difference beam which is sampled by couplers. In both cases, the difference beam outputs are coupled into the sum channel and switched sequentially. The resulting AM modulation on the sum signal is processed to derive the an­tenna pointing measurements.