Tracking Accuracy of Satellite Antenna: Definition, Measurement, and Calculation

In the previous chapter on Pointing Accuracy of Satellite Antenna: Definition, Measurement, and Calculation, we introduced the concept of antenna pointing accuracy. Today, we will discuss another important performance parameter for satellite communication antennas: tracking accuracy.

What is Tracking Accuracy of Satellite Antenna?
Tracking accuracy of an antenna refers to the ability of its servo control system to real-time and stably align with a target satellite during dynamic operations. In simpler terms:

“Can the antenna ‘stick’ to the target when either the target or the antenna is moving?”

Tracking accuracy is usually expressed as a statistical measure of angular deviation, such as RMS (root mean square) or maximum instantaneous deviation, and the unit is degrees (°).

In What Scenarios Does Satellite Antenna Need to Track a Satellite?
There are often questions like:

Question 1: Only mobile antennas need tracking accuracy, right? Fixed antennas pointing to GEO satellites don’t need it?
Question 2: If either the satellite (or target) or the antenna is moving, then the antenna needs tracking and thus has a tracking accuracy specification, right?

In fact, whether an antenna needs to track a satellite does not depend on whether the antenna is “moving” or “stationary”; it depends on the relationship between the antenna’s beamwidth (HPBW) and the relative angular motion of the target.

Engineering guideline: If the target’s motion causes the antenna pointing to deviate by more than 10%–20% of the beamwidth, an automatic tracking system must be used, and tracking accuracy becomes a critical performance parameter.

For example, the beamwidth of a parabolic antenna can be approximately calculated as:

θ ≈ 70 * λ / D

Where:

θ = beamwidth (degrees)

λ = operating wavelength (meters)

D = antenna aperture (meters)

Based on this principle, we can consider several typical scenarios:

Scenario 1: Satellite Antenna is stationary, satellite is moving
This mainly applies to LEO (Low Earth Orbit) and MEO (Medium Earth Orbit) satellites. These satellites move rapidly relative to the ground, with pass durations ranging from a few minutes to tens of minutes.

Ground antennas must track the satellite smoothly, quickly, and accurately. Any lag or jitter in tracking can cause a sharp drop in signal quality or even communication loss.

Tracking accuracy determines whether the communication link can remain stable during satellite passes.

Scenario 2: Large fixed ground satellite antenna tracking GEO satellites
For GEO satellites, the situation is more nuanced. GEO satellites are not perfectly stationary relative to Earth; their orbits experience small perturbations, usually within ±0.1°. From a ground observer’s perspective, the satellite traces a tiny figure-eight in the sky over 24 hours.

Small-aperture ground antennas (e.g., 1.2 m at Ku-band, λ = 0.025 m) have a beamwidth:

θ≈70 * 0.025 / 1.2≈1.5°

The GEO satellite drift (±0.1°) is much smaller than the beamwidth, so signal degradation is minimal. Typical operation:

Initial programmatic tracking aligns the antenna to the satellite.

Servo stops; only reactivated when switching satellites.

Large-aperture ground antennas (e.g., 10 m at Ku-band) have a beamwidth:

θ ≈ 70 * 0.025 / 10 ≈ 0.175°

Here, satellite drift (±0.1°) is comparable to the beamwidth, which can cause significant gain loss. Thus, automatic tracking (e.g., monopulse tracking) is required. The ACU periodically or based on signal quality (e.g., AGC voltage drop) triggers tracking adjustments. This runs continuously to compensate for slow satellite drift.

Authority Standards:
Intelsat IESS-207 (Ku-band): Antennas ≥ 3.5 m must have automatic tracking.

Intelsat IESS-308 (C-band): C-band Standard A antennas ≥ 7 m must have automatic tracking.

Intelsat IESS-601 (Ka-band): Antennas ≥ 1.2 m are recommended or required to have automatic tracking; for large gateway stations (>3.5 m), it is mandatory.

Scenario 3: Target is stationary, satellite antenna is moving (Satcom On-The-Move, SOTM)
This is the typical SOTM scenario, where antennas mounted on moving platforms track GEO satellites. The servo system must not only point accurately but also compensate for platform motion (pitch, roll, sway) in real-time.

Scenario 4: Both satellite antenna and target are moving
Here, the antenna is on a moving platform tracking LEO or MEO satellites.

Sources of Tracking Error
Tracking errors mainly come from:

External environment

Servo system itself

Signal carrier-to-noise ratio (C/N)

Satellite ephemeris errors
1. External environment errors:

Wind: wind pressure creates torque on the antenna structure. If servo torque is insufficient, continuous pointing error occurs, especially with gusts.

Atmospheric effects: refraction shifts the apparent satellite position, particularly at low elevation angles, causing tracking error.

2. Servo system errors:

Mechanical errors:

Motor and gearbox backlash: creates response delay and dead zones, especially at low speed or frequent reverse corrections.

Structural deformation: thermal expansion, wind loading can deform reflector, back structure, or feed support arm, altering phase center and pointing.

Antenna pedestal misalignment: non-orthogonality of azimuth/elevation axes, bearing runout, etc.

Control loop errors:

Servo bandwidth limit: system cannot respond to fast disturbances (gusts) or rapid LEO passes, causing lag.

Algorithm errors: different tracking algorithms introduce different error types.

Sensor errors:

Angular encoder: resolution, eccentricity, quantization errors convert directly into pointing error.

IMU (gyros, accelerometers): bias drift, scale factor errors, noise cause attitude reference errors.

3. Signal carrier-to-noise ratio:
High C/N is essential; low C/N increases tracking error, reduces stability, or may cause complete signal loss.

4. Satellite ephemeris errors:
In programmatic tracking of LEO/MEO satellites, tracking is based on predicted satellite position from downloaded ephemeris. Any prediction error accumulates over time, directly contributing to tracking error.

Measurement and Calculation of Tracking Accuracy
On-site testing system consists of tracking system, controlled antenna system and spectrum analyzer, as shown in Figure.

Test Steps
When testing the step tracking accuracy, you should select a Satellite with small drift and stable beacon as the aim Satellite.

Make the antenna point to the aim Satellite (Observe by the spectrum analyzer and make the tracking signal maximum), read and record the antenna Az position Pa0and El position Pe0at this time.
Set up the tracking signal loop;record the tracking signal frequency F0. Adjust the gain of the tracking loop to make the tracking level as about 6V, and also set this signal level as the maximum level.
Control antenna Az and El to deviate from the Satellite appropriately, then start step tracking. When the step tracking becomes in waiting state, read and record antenna Az position Paiand El position Pei, So
Az tracking error at this time (Considering the secant compensation)

Dai=|(Pai-Pa0)cosPe0|,

El tracking error at this time

Dei=|Pei-Pe0|.

n（n³10）times repeat the Step 3, Az and El should be deviated to different direction, and get a group of tracking error data, so the tracking accuracy is