To achieve stable and reliable satellite communications, ground terminal antennas must be precisely aligned with distant satellites. The pointing process is one of continuous searching and gradual refinement, with the goal of aligning the antenna beam center exactly with the satellite to maximize signal reception and transmission efficiency.
The antenna pointing technology of satellite communication terminals is the key to ensuring the stability of the satellite communication link. At present, the commonly used pointing modes mainly include the following five types:
- Beacon Tracking
- Carrier Tracking
- Reference Satellite Tracking
- Program Tracking
- Manual Tracking
This article will focus on one of the most common and efficient automatic pointing methods—Power-Detection Carrier Tracking.
Introduction to Carrier Tracking
Unlike Beacon Tracking, one of the five satellite terminal antenna pointing modes introduced in the previous section, Carrier Tracking does not rely on a dedicated beacon signal transmitted in the satellite downlink. Instead, it uses the traffic carrier signal for antenna pointing.
A beacon signal is typically a narrowband, unmodulated continuous wave (CW) signal, while a traffic carrier is a wideband, modulated signal. For example, in a communication link between two ground stations A and B, the traffic carrier signal transmitted by ground station A may have the following parameters:
- Information rate: 2 Mbps
- Roll-off factor: 0.25
- Modulation scheme: QPSK
- Coding rate: 1/2
As mentioned in Satellite Communication Link Budget II: Bandwidth and Power Balance, the symbol rate of this traffic data is 2 Msps (Mega Symbols per second), with an occupied bandwidth of 2.5 MHz. After the satellite transparently relays the signal, the downlink carrier still occupies the same 2.5 MHz bandwidth.
The antenna of Ground Station B, if operating in carrier tracking mode, uses the 2.5 MHz service carrier signal for tracking the satellite.
Carrier Tracking Mode
The antenna tracking system determines whether the antenna is aligned with the satellite by detecting the signal power of the traffic carrier. The antenna servo mechanism continuously adjusts the azimuth and elevation angles, and when the detected carrier signal power reaches its maximum value, the antenna is considered to be aligned with the satellite.
Some may ask: since the beacon signal is stable, continuous, and easy to detect, why use carrier tracking at all? After all, the carrier signal is more susceptible to service fluctuations, bit errors, interruptions, and other disturbances.
Characteristics of Carrier Tracking
Does not rely on an independent satellite beacon signal
Not all satellites provide a dedicated beacon signal. Reasons include:
- Some small commercial satellites do not transmit a beacon due to power constraints.
- Certain satellites keep beacon signals undisclosed for security reasons.
- Some satellites only transmit beacon signals in specific operating modes.
- Beacon transmitter failure onboard the satellite.
In such cases, beacon tracking cannot be implemented, while carrier tracking serves as an effective alternative.
Wide applicability
As long as satellite communication services are active, traffic carrier signals are present, giving carrier tracking broad applicability.
Accuracy limitations
Since traffic carrier signals generally have a wide bandwidth and lower power density per unit frequency, the tracking receiver requires higher sensitivity, which limits tracking accuracy.
Susceptibility to service variations
Changes in service data, bit errors, and signal interruptions directly affect tracking performance.
Composition and Workflow of the Carrier Tracking System
System Connection Structure
A carrier tracking system typically consists of the following components:
- Antenna
- LNB (Low Noise Block downconverter)
- Carrier Tracking Receiver
- Antenna Control Unit (ACU)
The connection method is similar to that of a beacon tracking system, except that the beacon receiver is replaced with a carrier tracking receiver. The specific structure is as follows:
Carrier Tracking Connection
Basic Process of Power-Detection Tracking
The carrier tracking process based on the power-detection mode is generally consistent with beacon tracking. The main steps are as follows:
Traffic Carrier Signal Reception
Ground station A transmits a modulated traffic carrier signal to the satellite, which is transparently relayed to ground station B. Ground station B receives the signal and feeds it into the carrier tracking receiver.
Operation of the Power-Detection Circuit
Inside the carrier tracking receiver, the power-detection circuit measures the signal power level according to the preset carrier frequency and bandwidth parameters, and feeds the results back to the ACU.
Antenna Scanning and Signal Detection
The ACU controls the antenna to perform small-range scanning in azimuth and elevation directions, recording the power values at different pointing angles. Parabolic antennas usually employ conical scanning, while phased-array antennas often use step scanning.
Peak Tracking and Alignment
The ACU records the power values at each pointing angle, identifies the maximum point as the optimal alignment position, and continuously fine-tunes to maintain satellite lock.
Key Parameters of Power-Detection Tracking
The performance of carrier tracking based on power detection depends on the following key parameters:
- Carrier Frequency: The center frequency of the satellite downlink service carrier minus the LNB local oscillator frequency.
- Symbol Rate: The symbol rate of the service data.
- Roll-Off Factor: Reflects the performance of the modem.
- Acquisition Range: Determines the frequency detection range.
- Update Period: Affects the real-time responsiveness of tracking.
The figure below shows the typical parameter specifications of a carrier tracking receiver:
|
Detection Mode |
Frequency Range |
950 MHz ~ 2150 MHz |
|
|
Input Power |
-80 dBm ~ -25 dBm |
|
|
|
Symbol Rate |
0.2 ~ 200 Msps (configurable) |
Step size 1 kHz, expandable up to 500 Msps |
|
|
Acquisition Frequency Offset |
0.1 ~ 5 MHz (configurable) |
Step size 1 kHz |
|
|
Amplitude Stability |
≤ 0.25 dBm |
|
|
|
Receiver Status Update Period |
8 ~ 60 ms (configurable) |
Step size 1 ms |
Carrier Tracking Receiver Parameters
Carrier Frequency
Carrier frequency = Satellite downlink service carrier center frequency − LNB local oscillator (LO) frequency. For example, if an LNB with 11,300 MHz is used and the downlink service carrier center frequency is 12,450 MHz, the set frequency is 12,450 − 11,300 = 1,150 MHz.
The carrier frequency is generally set within the range of 950 MHz to 2,150 MHz, consistent with the modem’s received signal range.
Symbol Rate and Roll-off Factor
The symbol rate Rt is related to the service information rate Rb, modulation scheme, and coding scheme. The formula is as follows:
Where:
MM is the modulation order;
Rc is the code rate.
In “Satellite Communication Link Budget Part 2: Bandwidth and Power-Bandwidth Balance”, we provided examples of symbol rates under different modulation modes and coding modes:
The following table shows the calculated symbol rates for different modulation schemes under the condition of an information rate of 1 Mbps, Rb=1 Mbps and code rate r=1/2:
|
Modulation Scheme |
Modulation Order N |
Transmission Rate after Coding Rt*=*Rb*/*r |
Symbol Rate Rs*=*Rt*/log2*(N) |
|
BPSK |
2 |
2.000 Mbps |
2.000 Msps |
|
QPSK |
4 |
2.000 Mbps |
1.000 Msps |
|
8PSK |
8 |
2.000 Mbps |
0.667 Msps |
|
16APSK |
16 |
2.000 Mbps |
0.500 Msps |
|
16QAM |
16 |
2.000 Mbps |
0.500 Msps |
|
64QAM |
64 |
2.000 Mbps |
0.333 Msps |
Relationship between Symbol Rate and Modulation/Coding
The user only needs to calculate the symbol rate based on the service information rate, modulation scheme, and coding scheme, and then configure it into the carrier tracking receiver.
Furthermore, the occupied carrier bandwidth is closely related to the roll-off factor:
In *Satellite Communication Link Budget Part 2: Bandwidth and Power-Bandwidth Balance*, we also introduced occupied carrier bandwidth and the roll-off factor.
For the same data symbol rate, if the roll-off factors of modems from different manufacturers are different, the resulting occupied bandwidth of the modulated signal will also differ. This is one of the core specifications for each manufacturer’s product.
We can use the following diagram to represent the relationship between the symbol rate and the occupied carrier bandwidth:
Relationship between Symbol Rate and Occupied Bandwidth
Relationship between Roll-off Factor and Occupied Bandwidth
Where
α is the roll-off factor.
The occupied carrier bandwidth is closely related to satellite transponder resources, which require payment for usage. If a manufacturer’s modem has a smaller roll-off factor, the same symbol rate will occupy less satellite transponder bandwidth, thereby reducing costs.
In practice, the occupied carrier bandwidth is typically calculated as follows:
where
α is the roll-off factor of the modem.
Conventional modems generally have a roll-off factor around 0.25. Higher-performance modems can achieve a roll-off factor below 0.2, while some manufacturers claim their modems can reach roll-off factors as low as 6.1%.
Roll-off Factor
The roll-off factor is a critical parameter in satellite communication modems. The roll-off factor of Ground Station A’s modem can be configured as a parameter for the service carrier in the carrier tracking receiver.
The roll-off factor is typically set between 0.1 and 0.35 and must match the parameter of the transmitting-end modulator.
Frequency Search Range
For beacon tracking, the beacon signal is generated by the satellite. The clock frequency on the satellite is highly accurate, resulting in minimal frequency offset. In contrast, for carrier tracking, the service carrier is generated by Ground Station A. The frequency of the service carrier depends on the local clock frequency of Ground Station A. If the local clock frequency of Ground Station A is inaccurate or unstable, it will introduce frequency deviations during carrier tracking at Ground Station B.
Additionally, if Ground Station A is moving at high speed, such as when installed on a high-speed train or aircraft, the signal received by the satellite from Ground Station A will experience a Doppler effect due to the high-speed movement of the vehicle/aircraft. This can also cause significant frequency deviations in the service carrier.
Therefore, the frequency search range for carrier tracking is generally wider than that for beacon tracking (hundreds of kHz vs. tens of kHz).
Summary
This article detailed the working principles, characteristics, advantages, and key parameter configuration methods of power detection-based carrier tracking. As a satellite alignment method that does not rely on beacon signals, it is suitable for various satellite communication scenarios. Particularly in situations where beacon signal support is unavailable, it effectively ensures automatic antenna alignment.
In the next section, we will introduce another satellite alignment method based on service carriers—DVB tracking.
