Antenna Pointing Accuracy in Satellite Communication

Antenna Pointing Accuracy – Definition, Measurement, and Calculation
In the technical specifications of satellite communication antennas, the parameter antenna pointing accuracy is almost always specified. Typically, it is expressed as: Antenna Pointing Accuracy ≤ 1/10 of the Half-Power Beamwidth (HPBW)
What Is Antenna Pointing Accuracy?
Basic Concepts
Before discussing pointing accuracy, it is necessary to understand several related fundamental concepts.
• Beamwidth
Beamwidth usually refers to the Half-Power Beamwidth (HPBW) of an antenna.
It is defined as the angular separation between the two directions on either side of the main beam where the antenna gain drops by 3 dB from its maximum value (i.e., where the radiated or received power is reduced to half of the peak).

Beamwidth is the most important reference when evaluating pointing accuracy.
The narrower the beamwidth, the higher the requirement for antenna pointing accuracy.
• Beam Center
The beam center is the direction in which the antenna radiates or receives maximum power, corresponding to the maximum antenna gain.
All pointing errors are measured relative to this direction.
• Pointing Error
Pointing error is the angular difference, at any given moment, between the antenna’s actual pointing direction and the desired target direction.
• Pointing Accuracy
Pointing accuracy describes the deviation between the actual pointing direction of the antenna main beam and the desired pointing direction (the target satellite).
Pointing accuracy is not an instantaneous value, but a statistical quantity evaluated over a specified time period. It is commonly expressed as a root mean square (RMS) error.
In essence, pointing accuracy measures how accurately an antenna can point toward a theoretically calculated direction (in this case, the satellite’s theoretical azimuth and elevation angles) under open-loop conditions.
In one sentence: pointing accuracy indicates how precisely the antenna can be aimed at the target.
The main factors affecting pointing accuracy include:
Antenna installation accuracy (base leveling, azimuth reference)
Calibration accuracy
Resolution of encoders or sensors
Structural deformation under gravity and steady wind load
• Tracking Accuracy
Tracking accuracy refers to the performance of the antenna’s automatic servo control system in maintaining alignment with a moving target (such as a satellite) during operation. It measures the error between the actual beam center and the satellite’s real-time position during closed-loop tracking.
Factors affecting tracking accuracy include:
Servo system response speed
Control algorithms
Beacon receiver sensitivity
System latency
Dynamic wind loads (gust-induced vibration)
Mechanical backlash in the drive system
Why Is Pointing Accuracy Important?
• Maximizing Antenna Gain and Ensuring Link Quality
Using the most common parabolic antenna in satellite communications as an example:
A parabolic reflector achieves high gain by concentrating electromagnetic energy into the main beam.
If the beam center is not precisely aligned with the satellite, the transmitted or received signal deviates from the region of maximum gain, resulting in reduced signal strength (carrier-to-noise ratio, C/N) and degraded communication quality, potentially even causing link outages.
For example:
For an antenna with an HPBW of 1°, a pointing error of 0.5° may result in a gain loss exceeding 3 dB.
• Avoiding Adjacent Satellite Interference
Geostationary orbit (GEO) orbital resources are extremely limited. The angular separation between adjacent satellites is typically only 1° to 3°.
If a ground antenna is inaccurately pointed, its sidelobes may illuminate neighboring satellites, causing uplink interference.
Uplink interference degrades the performance of adjacent satellite systems
It may also lead to serious international coordination issues or frequency disputes
Therefore, ITU-R Recommendation S.734-1 explicitly states:
For systems operating above 10 GHz, antenna pointing accuracy should be better than one-tenth of the half-power beamwidth.
This requirement is widely adopted across the industry.
Sources of Antenna Pointing Errors
The causes of antenna pointing errors can be classified into three categories:
1. Static Errors
Caused by antenna structure, installation, and environmental conditions, including:
Gravitational deformation: changes in reflector shape and feed position under gravity at different elevation angles
Installation misalignment: deviation between antenna azimuth/elevation reference zero points and true north or the horizontal plane
Thermal deformation: uneven solar heating causing differential thermal expansion
2. Dynamic Errors
Mainly caused by external dynamic loads:
Wind load: the dominant source of dynamic pointing error, inducing structural deflection and oscillation
Mechanical vibration and transient disturbances during rotation
3. Other Error Sources
Atmospheric refraction: bending of electromagnetic wave paths through the atmosphere
Satellite position error: inaccuracies in ephemeris data leading to satellite position prediction errors
Measurement and Calculation of Pointing Accuracy
For satellite communication ground stations, the most commonly used and practical method is the satellite beacon method.
Measurement Procedure
1. Satellite Selection
Select a stable in-orbit geostationary communication satellite transmitting a continuous-wave (CW), unmodulated beacon signal. The satellite’s orbital longitude is precisely known.
2. Calculation of Theoretical Pointing Angles
Based on:
Satellite orbital longitude
Ground antenna location (latitude and longitude obtained via GPS)
Calculate the theoretical azimuth (Az₀) and elevation (El₀) angles of the antenna.
3. Initial Antenna Pointing via the Servo System
Based on the antenna’s GPS-determined location, the servo control system drives the antenna to the calculated target angles. The positioning accuracy of the control system has a direct impact on the initial pointing accuracy.
4. Scanning
Perform a small-area scan around the target direction using:
Cross-scan
Conical scan
5. Signal Recording
Record the received beacon signal power during the scan.
6. Peak Detection
Identify the azimuth (Azmax) and elevation (Elmax) corresponding to the maximum received beacon power. This point represents the antenna’s actual electrical pointing direction.
7. Deviation Calculation
Compute the difference between the measured peak point (Azmax, Elmax) and the theoretical values (Az₀, El₀).

Pointing Accuracy Calculation
Practical Data Processing Steps
Data Acquisition
During the scan, record a set of data pointsi:
Azᵢ, Elᵢ: actual azimuth and elevation angles (read from axis encoders)
Pᵢ: received beacon signal power at (Azᵢ, Elᵢ)
Electrical Axis Determination
Perform surface fitting (e.g., two-dimensional Gaussian fitting) on the collected (Az, El, P) data
The peak of the fitted power surface corresponds to the antenna’s electrical axis pointing direction (Az_peak, El_peak)
Error Calculation
Azimuth error: ΔAz = Az_peak − Az₀
Elevation error: ΔEl = El_peak − El₀
Az₀ and El₀ represent the theoretical azimuth and elevation angles derived from the ground antenna location and the accurate orbital coordinates of the beacon satellite.
Pointing Accuracy Calculation
The total pointing accuracy is commonly calculated as:F_Total = sqrt( (ΔAz * cos(El))² + ΔEl² )