The rapid growth of activity in Low Earth Orbit (LEO), driven largely by mega-constellations such as Starlink and OneWeb, has transformed space into an increasingly crowded and complex environment. More than 20,000 additional satellites have been proposed for launch. As a result, operators must manage not only the risk of physical collisions but also the growing threat of radio-frequency (RF) interference.
Although RF spectrum is invisible, it is a limited resource. Coordinating its use has therefore become essential to mission success and the long-term sustainability of space operations.
International Spectrum Coordination
The International Telecommunication Union (ITU) is the primary international body responsible for coordinating spectrum use and preventing harmful interference. Satellite operators work through their national administrations to notify the ITU of their planned use of radio frequencies.
This process can take several years. It generally includes coordination under Article 9 of the ITU Radio Regulations, followed by notification under Article 11. Once the necessary requirements have been met, the frequency assignment may be recorded in the Master International Frequency Register (MIFR).
Non-geostationary satellite systems, including most LEO constellations, operate under specific obligations. For example, Footnote No. 5.484A of the ITU Radio Regulations states that non-geostationary systems operating in certain frequency bands must protect geostationary satellite networks and cannot claim protection from them.
Regulatory Changes Introduced at WRC-19
The rapid expansion of LEO constellations led to new regulatory measures at the 2019 World Radiocommunication Conference.
One major concern was “spectrum warehousing,” in which operators reserve frequencies for large constellations without deploying the systems they proposed. To discourage this practice, ITU Resolution 35 introduced deployment milestones for non-geostationary satellite systems.
After launching at least one satellite within the initial seven-year regulatory period, an operator must deploy:
- 10% of the proposed constellation within two years;
- 50% within five years; and
- 100% within seven years.
When an operator fails to meet a milestone, the registered size of the constellation may be reduced to reflect the number of satellites actually deployed.
Resolution 32 created a simplified process for small satellites with missions lasting less than three years. It can reduce the notification period to approximately six to nine months and requires less information from qualifying academic or non-commercial operators.
Protecting Geostationary Satellite Networks
Non-geostationary systems must also comply with the equivalent power flux density, or e.p.f.d., limits established in Article 22 of the ITU Radio Regulations. These limits are designed to prevent LEO constellations from causing harmful interference to geostationary satellite networks.
Traditional power flux density limits generally consider interference from a single source. By contrast, e.p.f.d. limits account for the combined interference generated by all satellites in a non-geostationary constellation that may be visible to a geostationary earth station at the same time.
Article 22 specifies the maximum equivalent power that a geostationary receiver may experience from a non-geostationary system. When a LEO constellation complies with these limits, it may be permitted to operate in shared frequency bands without completing formal coordination with individual geostationary networks.
Operators can use several methods to remain within the limits. These include steering beams away from the geostationary arc, reducing transmission power, and duty cycling their radios.
Radio Duty Cycling
Radio duty cycling controls how long a satellite transmitter remains active compared with how long it remains inactive. Instead of transmitting continuously, a satellite switches its radio on and off according to operational, technical, or regulatory requirements.
Several approaches can be used.
Scheduled blackout windows: Network control systems can create periods during which satellite transmissions are intentionally suspended. These windows may be used when a satellite passes through an area where its signals could interfere with a protected service.
Geographic silence: Satellites can be programmed to become electronically silent over sensitive locations. For example, satellite coverage cells may be disabled while passing over radio astronomy facilities.
Automated scheduling: Modern constellations use automated planning and optimization systems to manage transmission windows. A satellite may transmit only when it is in contact with an authorized ground station, effectively cycling its radio according to its orbital schedule.
Transmission-time limits: Operators can also impose limits on the percentage of time a transmitter may remain active. This creates periods during which other systems can operate without interference and can also help manage power or thermal constraints.
Technologies for Dynamic Interference Management
In addition to regulatory compliance, LEO operators are adopting technologies that allow them to respond to interference more dynamically.
Software-Defined Radios
Software-defined radios allow operators to change center frequencies, bandwidths, waveforms, and modulation methods through software rather than hardware modifications. This flexibility can help satellites move away from an interfering signal or adapt to changing spectrum conditions.
More advanced systems may use cognitive-radio techniques, in which satellites or ground systems monitor the RF environment and collectively determine how to use available spectrum more efficiently.
Spot Beams and Frequency Reuse
Satellites can use narrow spot beams to concentrate signal power within specific geographic areas. Because the beams cover smaller regions, the same frequencies can be reused in locations that are sufficiently separated from one another.
This approach increases spectrum efficiency while reducing the likelihood of overlapping signals.
Electronic Beam Steering
LEO operators can combine advanced antenna technology with spacecraft-orientation maneuvers to direct signals toward intended users and away from protected regions, including the geostationary arc.
Many modern LEO satellites use phased-array antennas. These antennas contain numerous small radiating elements whose signal phases are controlled electronically. By adjusting those phases, the system can steer a beam almost instantly without physically moving the antenna.
Some systems use metamaterial-based or holographic beamforming. In these designs, software changes the electromagnetic pattern produced by the antenna surface, allowing beams to be steered across a wide field of view, potentially extending toward the horizon.
Efficient Coding and Modulation
Advanced coding, decoding, and multiplexing systems allow satellites to transmit more data within a given amount of spectrum. Increasing the number of bits transmitted per hertz reduces the bandwidth required to support a particular data rate.
Adaptive Coding and Modulation, or ACM, goes a step further by adjusting the transmission method in response to current link conditions. When interference increases or signal quality falls, the system can select a more robust modulation and coding scheme. Under better conditions, it can use a more efficient scheme to increase throughput.
Detecting and Locating Interference
Identifying the source of interference is often more difficult than detecting the interference itself. New commercial systems are being developed specifically to monitor the RF environment.
Constellations such as HawkEye 360 are designed to detect, characterize, and geolocate radio-frequency signals around the world. These capabilities can help identify unauthorized transmissions, jamming, or other sources of harmful interference.
Information sharing also plays an important role. Organizations such as the Space Data Association facilitate the exchange of ephemeris data, which describes the positions and trajectories of satellites. Accurate orbital information can help operators coordinate transmissions and reduce both physical and spectral overlap.
In especially sensitive cases, satellites may be programmed to stop transmitting while passing over protected areas, such as radio astronomy observatories.
Optical Communications as an Alternative
As demand for RF spectrum continues to grow, many satellite projects are exploring free-space optical communications. These systems use lasers rather than radio waves to transmit information.
Optical links can provide much higher data rates and do not create conventional RF interference. They can therefore reduce pressure on congested radio-frequency bands.
However, optical communications introduce other challenges. Laser links require extremely accurate pointing, acquisition, and tracking. They may also be affected by clouds, atmospheric turbulence, and other environmental conditions when communicating with ground stations.
Despite these limitations, optical communications are likely to become an important part of future LEO networks. Combined with improved regulation, dynamic spectrum management, advanced antennas, and better coordination among operators, they offer a promising path toward maintaining reliable communications in an increasingly congested orbital environment