February 2, 2021
The biggest effect on robust satellite operation performance is RF interference. A growing problem in wireless communications, it significantly increases costs for satellite operators. A survey by the Satellite Interference Reduction Group (now known as the Satcoms Innovation Group) found 93% of satellite operator respondents suffer satellite interference at least once a year. More than half experience it monthly, and 17% see interference continuously in their day-to-day operations.
All these factors make it imperative for satellite operators to understand RF interference causes. Selecting appropriate test solutions is one critical step, as are implementing the proper evaluation and maintenance processes. By doing so, signal interruption can be reduced and debugging can be done more efficiently, for reduced associated expenses and downtime.
Satellite earth stations (figure 1) form the ground segment of satellite communications. They contain one or more satellite antennas tuned to various frequency bands, making them susceptible to a variety of interference.
Let’s outline a list of the potential causes of interference in satellite communication systems:
- Adjacent frequency emissions from other signals – Due to the extreme distance between satellite and earth station, the incoming power flux density of the satellite signal at the earth station is very low and susceptible to interference signals with higher power levels.
This is particularly prevalent with the rollout of 5G. Figure 2 illustrates the band separation between a 5G deployment and satellite frequencies. The top shows the range of legacy low-noise block downconverters (LNBs) while the lower image is a bandpass filter blocking the 5G carriers.
- Aircraft – Airplane altimeters generate signals in the 4200 MHz - 4400 MHz band, adjacent to the C-band downlink frequency of 3400 MHz - 4200 MHz for fixed satellite service stations (FSS). Even though there is no frequency overlap, aircrafts are much closer to the earth station than the satellite. Therefore, altimeter signals can overdrive the satellite feed system’s low noise amplifier (LNA), distorting the signal. Also, the LNA and LNB generally operate at frequencies wider than the authorized band, making them susceptible to out-of-band signals.
- High-power radar – S-band radars are used in high-power weather, air traffic control, surface ship surveillance, and similar applications. Given the intermittent (pulsed) nature of radar signals, interference can appear irregular. For example, in the case of surface ships, the interfering ship may disappear as it passes along the coast. Satellite ground stations near airports or coastal ports are especially prone to interference from these radar signals.
- Broadcast FM transmitters – Signals from nearby transmitters can overload the LNA at the ground station. Due to low power levels of the satellite signals, LNAs are optimized for gain. Satellite signals can be easily compressed by the LNA due to other signals present in the LNA bandwidth.
- Adjacent satellite interference (ASI) – As more satellites launch, slot spacing becomes more crowded, with only 2 degrees between many geostationary satellites. At the same time, the number of portable and mobile terminals is growing rapidly. These two conditions increase ASI.
- Satellite ground stations – Examples include unwanted emissions from other satellite ground stations which may either be spurious or out-of-band, as well as unauthorized transmissions (piracy) and intentional jammers
Real-time Spectrum Analysis
Satellite operators require a portfolio of test instruments to mitigate interference problems. Some solutions are designed for long-term spectrum monitoring. Others address issues associated with equipment upgrades and interference from RF services operating in spectrum adjacent to satellite frequencies.
For example, a real-time spectrum analyzer (RTSA) with frequency range of 9 kHz to 54 GHz, such as the Field Master Pro™ MS2090A (figure 3), is ideal for monitoring downlink signals to search for interference and noise. Wide frequency coverage is necessary because there are more than 2,000 active satellites orbiting the earth that communicate with the ground through dedicated earth stations utilizing sub-6 GHz and up to 50 GHz bands.1
Additionally, interfering signals can be intermittent in nature and difficult to track. An RTSA samples spectrum continuously, ensuring that no interferer will go undetected
Spectrum Clearing Importance
The United States Federal Communication Commission (FCC) has launched an accelerated spectrum clearing program for the C-band for 5G. The clearing will take place over the lower 300 MHz range (3.7 GHz – 4.0 GHz) with the top 20 MHz used as a guard band. Similar efforts for C-band clearance are taking place at many other international locations.
While the repurposed spectrum is being cleared of legacy signals, field engineers and technicians must ensure that remaining satellite frequency bands are free of 5G interference. A Mobile Interference Hunter (MIH) with a spectrum clearing mode allows field engineers and technicians to set a go/no-go threshold. This number, in combination with a minimum hold capability, allows for efficient localization of good areas and those that need assistance.
Another initiative that requires a MIH is signal compression. For earth station operators, efforts are underway to adopt advanced signal compression technology to pack content from a 500 MHz band into just 200 MHz. Ground infrastructure, such as antennas and filters, will need to be deployed to protect the remaining bandwidth from interference.
Time Difference of Arrival
Signals generated by unauthorized satellite operators are another common interference source. The satellite signal may be generated without proper authorization (piracy) or by experiments being conducted by nearby research labs and private enterprise. Jamming may also be due to hostile transmissions designed to intentionally interfere with communications.
One method traditionally used by satellite operators for localization of interference signals is Time Difference of Arrival (TDOA). Operators use the satellites themselves to triangulate an interference signal, however, there are many sources of error associated with TDOA when pinpointing the interferer position. A MIH is a preferred alternative, as it uses audio signals and mapping for the driver, so it can make hundreds of measurements per second to guide the user to the signal.
Satellite operators often require round the clock spectrum monitoring. A Remote Spectrum Monitor (RSM) can provide up to 24 RF inputs to monitor multiple earth station dishes simultaneously (figure 4). A high-speed multiplexer can be integrated into the RSM to switch between each dish RF signal.
To learn more about improving satellite communications, download the Anritsu application note – Resolving Interference Issues at Satellite Ground Stations.
1United Nations Office for Outer Space Affairs (UNOOSA)