Low Earth orbit (LEO) satellites are satellites at the lower end of possible orbits, usually defined as 1,200 miles (2,000 kilometers) or less. LEO typically extends to altitudes as low as 160km above Earth. In comparison, most commercial airplanes do not fly at altitudes greater than approximately 14 km. To remain in LEO, satellites have to travel at roughly 17,500 miles per hour (7.8 kilometers per second); at this speed, it takes around 90 minutes to complete an orbit of the Earth.
Unlike geosynchronous orbital (GEO) satellites, which match the rotational speed of Earth to remain in the same location above the planet, LEO satellites orbit faster than the Earth's rotational speed and move across the sky from the perspective of people on the surface. This means more routes are available for satellites in LEO.
With more orbital paths available, lower launch and manufacturing costs, and closer proximity, the majority of satellites that have been launched are LEO satellites. LEO satellites have a range of applications across imaging, communications, and defense. Proximity enables higher-resolution imaging and lower-latency communications. Communications LEO satellites are typically launched in constellations to ensure constant coverage. A single satellite moves across the sky and would not be continually in view of ground-based sites. Constellations of satellites operate together to create a "net" around the Earth, such that at least one is always in view.
A number of factors are considered when choosing a satellite's orbital altitude. This includes the radiation environment and the amount of space debris at that particular altitude. At many LEO altitudes, there is no significant debris, and the radiation environment is relatively benign, removing the need for radiation shielding. LEO satellites can also be manufactured at lower costs than higher-altitude satellites.
Orbits are possible due to the Earth's gravitational field and the relative motion of the satellite. Objects moving faster than the Earth's escape velocity (25,000 mph or 11.2 km/s) leave orbit. Different types of orbits are listed below.
- Low Earth orbit (LEO)—orbits with altitudes below 2,000 kilometers (1,200) miles.
- Medium Earth orbit (MEO)— geocentric orbits ranging in altitude from 2,000 kilometers to below a geosynchronous orbit at 35,786 kilometers (22,236 miles). This is also known as an intermediate circular orbit and is often used for global navigation satellite systems, such as GPS, GLONASS, Galileo, and BeiDuo.
- Geosynchronous orbit (GSO)—orbits matching the Earth's sidereal rotation period.
- High Earth orbit—geocentric orbit above the altitude of geosynchronous orbit (35,786 kilometers or 22,236 miles).
- Polar orbit and sun-synchronous orbit (SSO)—orbits that travel past Earth from north to south rather than east to west, passing roughly over the Earth's poles and are synchronized to always be in the same "fixed" position relative to the sun.
- Transfer orbits—a specialized kind of orbit to move a satellite from one orbit, usually when they are released from a launch vehicle to their final orbit. This is usually achieved through small built-in motors.
Examples of different orbit types with their typical latency.
The very low Earth orbit (VLEO) is typically classified as an orbit below approximately 450 kilometers in altitude and encompasses the lower part of the low altitude orbit. VLEO orbits have the potential to provide benefits to spacecraft operating in higher altitude orbits. The VLEO orbit offers improved spatial resolution, and the satellite can be manufactured with mass and volume savings while maintaining optimal performance. And, for radar and lidar systems, the signal-to-noise ratio can be improved with a VLEO orbit. Benefits include improved geospatial position accuracy, improvements in communications link-budgets, and greater launch vehicle insertion capability.
Example of a low Earth orbit satellite.
Low Earth orbit satellites are often used in constellations. Because of their rapid speed, a constellation allows a single satellite to pass along a workload to a following satellite in order to maintain service over a specified area. In order to develop these constellations, the satellites need to be small and low-cost. In lower orbits, the satellites do not require the same powerful amplifiers as other satellites in order to transmit signals. Due to the low radiation environment in LEO, the need for expensive radiation shielding of electronic parts allows for the lower cost of manufacturing these satellites. The reduced cost means launching a satellite into LEO is around USD$5,000/kg. More traditional, heavier satellites can cost at or greater than USD$7,000 per kilogram. The low cost of construction is also important as satellites age out, their orbits degrade, or they are damaged by debris or other collisions.
Starlink satellite launch seen from Earth.
With satellite constellation development, the satellites can include a phased array with RF beamforming and an embedded channelizer—a digital baseband processor for operators of a network of thousands of low earth orbit satellites, which allows these satellites to steer their beams. Being capable of steering beams allows a satellite constellation to steer to an area in need of more bandwidth and generally away from an area where less bandwidth is required. Traditional geosynchronous satellites broadcast with a large fixed beam bandwidth without the ability to switch the beam or bandwidth.
In order to keep costs down, there is a suggestion of manufacturing LEO satellites with internal shifting masses, which would shift the center-of-mass and, in turn, shift the location of the spacecraft through modulating, in direction and magnitude, the aerodynamic torques that allow the satellite reject aerodynamic disturbances. As well as developing navigation systems, engineers of satellites are working to develop and test advanced materials that can work to use atmospheric flow to control orientation and use an electric propulsion system that uses the residual atmosphere as a propellant. These systems are intended to develop satellites with the potential to keep the satellites in orbit indefinitely despite drag.
LEO satellites are used for different imaging applications because a satellite flying at a lower altitude can improve the resolution of optical sensors, radiometric performance (infrared or microwave sensors), and geospatial accuracy. Those sensing benefits can also reduce payload size—optical, radar, or communications systems—and overall cost. For example, Earth Observant's "Stingray" imaging satellite, a VLEO satellite, flies almost within Earth's atmosphere, which can provide a significant improvement in imaging categories. However, this low altitude also comes with downsides, including aerodynamic drag and strong gravitational pull that is significant enough to make an orbit decay in less than five years.
Part of the usefulness of LEO satellites is imaging for monitoring weather. This is especially useful in a polar orbit or sun-synchronous orbit rather than a GEO orbit. Often satellites in a sun-synchronous orbit are operated in pairs with one making a morning pass and the other making an afternoon pass to ensure every spot on the Earth is observed at least every six hours and often every four hours. The passive microwave imagery capable on LEO satellites is also capable of seeing through non-raining clouds and viewing rainbands, eyewalls, and storm eyes, even when obscured by upper-level clouds. This can provide good data and information on precipitation and clouds at low and medium levels and can give important details concerning the intensity and position of storms.
Example of imagery produced by the National Oceanic and Atmospheric Administration (NOAA)'s low Earth orbit satellites.
Along with more traditional navigation systems, low Earth orbit imaging satellites offer benefits for space utilization in air traffic control management. These benefits can include the development of more efficient air traffic routes to reduce travel times, which, in turn, reduces the amount of carbon dioxide emissions through the commercial airspace industry. Further, these low Earth orbit satellites can track commercial aircraft in remote areas or air traffic blackout areas.
Example of air quality monitoring from a low Earth orbit satellite.
The use of low Earth orbit satellites for imaging can also be used for environmental monitoring across a broad spectrum of environmental parameters, with improved capabilities and reduced latencies to better serve environmental monitoring needs. Satellites in polar orbits and lower inclination orbits can also add new and different data compared to traditional satellite environmental monitoring and imaging.
A traditional navigation system, such as the Global Position System, is a component of systems including power grids, wireless communications, and aircraft management. These systems are based on GEO satellites. With the increase of satellites in low Earth orbit, there is a chance to include payloads allowing those LEO satellites to act as navigation satellites. The number of satellites can provide better geometry than GPS, and the lower radiation environment also means lower cost components for navigation. Further, using a low Earth orbit can reduce path loss and make navigation systems more resilient than jamming.
Also, because a lot of LEO satellites use different frequency bands, the data provided to navigation systems can be changed to applicable bands depending on the area. These systems can be paired with traditional GPS satellite data. Meanwhile, similar to the promise of broadband internet provision to remote or rural areas, those areas where GPS systems are less reliable can be serviced by LEO satellites.
One of the more well-known use cases for low Earth orbit satellites is the possibility of delivering high-speed internet coverage for clients such as governments, mining corporations, residences, and shipping conglomerates in regions where they lack internet infrastructure and, therefore, access to the internet. Contrasted with satellites in a geosynchronous orbit, LEO offers a shorter and faster trip for an internet signal and thereby reduces latency.
Satellite IoT has been used for a while with traditional mobile sat systems (MSS) dominant in the M2M or IoT market, using an L-band spectrum and with a focus on mobile and maritime applications. Also, fixed sat systems (FSS) have developed M2M and IoT services over Ku or Ka band, with higher bandwidths well-suited to satellite-based IoT and backhaul services connecting terrestrial local area IoT networks from high-density sensor networks to the internet.
Newer low Earth orbit satellites offer reduced costs compared to traditional satellite IoT solutions, based on new CubeSat technology (which uses a range of UHF, VHF, S-band, and Ku-band services) and lower power modems to connect to ground sensors. These orbits also do not have the level of capex burden that incumbent satellite network operators have been saddled with as well. The use of satellite IoT can offer connectivity services for transportation and logistics services, including while at sea or in remote areas. These satellite IoT connectivities for navigation and logistics can be used for environmental monitoring systems, air traffic control infrastructure, agricultural monitoring and connectivity, and disaster management services. There are two main types of IoT connectivity service pathways:
Direct satellite and satellite gateway services visualization.
These are comparable to GSM or WiFi backhaul services. Gateway services use the low-cost terrestrial radio transmission standards for IoT, such as LoRa, Sigfox, LTE-M, or NB-IoT. These networks come with low-cost gateways and low-cost, low-orbit satellites to increase their possible reach.
This type of service, especially with low-cost and low-power options, is ideal for wide-area sensor networks, with sensors dispersed over a geographical territory. This service is especially important in remote areas where the low possible costs can enable massive networks with new data points to feed data analytics in a range of industries. With the increase in reliability and the decrease in latency, the direct to satellite services have become more valuable. Services can include tracking, tracing, logistics, insurance, and performance monitoring for remote assets. They can also enable new applications such as the following:
- Process monitoring and grid management in the energy sector
- Asset management in the mining sector
- Wide area monitoring applications for public structure monitoring
- Monitoring in smart agriculture for food, water, environment, and security
Low Earth orbit satellite IoT applications
Low Earth orbit satellites have found use in intelligence, surveillance, and reconnaissance (ISR) applications. These systems include image intelligence (IMINT), signals intelligence (SIGINT), and measurement and signatures intelligence collection systems (MASINT). These systems provide photographic coverage over denied territory and geodetic positioning of platforms emitting at radio frequencies, and they have been used for early warning systems for strategic intercontinental ballistic missiles. With their lower altitude and faster traveling speed, they have been used for defending against shorter-range ballistic missiles and for estimating launch points to enable counter-attacks against mobile targets.
Outside of imaging and detection, the use of LEO satellites has been tested for improving long-range precision fire of missiles and missile defense systems. With signal-sourcing capabilities, these satellites offer a chance for the military to understand where jamming and spoofing is originating from, in order to increase their situational awareness.
Partnerships in low Earth orbit satellites
In addition to grants, partnerships, and project money, there is a large investment in low Earth orbit satellite companies, especially those working to expand broadband, imaging, or IoT services. However, many of the venture capital firms investing in the LEO space are internet- or software-focused firms, as opposed to firms that are more familiar with space investments and therefore have deep knowledge of the sector.
