Low Earth orbit satellites

Low Earth orbit (LEO) satellites are satellites in an orbit described as a low orbit, meaning the altitude is usually less than 1000 kilometers and can be as low as 160 kilometers above Earth. This LEO range is low compared to other orbits, but most commercial aircraft do not fly at altitudes greater than 14 kilometers. The best examples of LEO satellites include: the Hubble space telescope, the International Space Station, the Spot family of satellites, and military observation satellites.

The low Earth orbit is often described as orbits below 2,000 kilometers (1,200 miles) and as low as 160 kilometers. When below 450 kilometers in altitude, the orbit can be classified as a very low Earth orbit (VLEO). The low Earth orbit makes it useful for various applications that benefit from the proximity to Earth. Most satellites in a LEO travel at a speed around 7.8 km per second, which means a satellite takes 90 minutes to circle Earth. Unlike satellites in geostationary or geosynchronous orbits, LEO satellites do not have to follow a particular path, such that their plane can be tilted, which consequently makes available more routes for satellites in LEO.

Factors in the choice of altitude include 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, which means the necessary radiation shielding is not needed, and satellites in this orbit can be manufactured at a lower cost than their higher altitude peers.

Image of different orbit types.

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.

Often low Earth orbit satellites are 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 often need to be small and low-cost, and the lower orbits in turn help build these smaller satellites. In lower orbits the satellites do not require the same powerful amplifiers of 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 developing satellite constellations, 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 which allows the satellite reject aerodynamic disturbances. As well as developing navigation systems, engineers of satellites are working to develop and test advanced materials which can work to use atmospheric flow to control orientation and use an electric propulsion system which use 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, and 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. Traditional 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 the navigation. Further, using a low Earth orbit can reduce path loss and make navigation systems more resilient than jamming.

As well, 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. As well, since signals can travel rapidly through the vacuum of space when compared to fiber-optic cables, LEO satellites have the potential to rival or exceed ground-based networks. SpaceX has largely driven the popularity of LEO satellite-delivered internet. As of March 2021, SpaceX has launched 1,500 of their Starlink satellites. Meanwhile, China Aerospace Science & Industry Corp. has proposed a network of 156 satellites, and Amazon has requested permission to launch 3,236 satellites.

With internet delivery by signal, the expectation (especially for widespread user adoption) is that the signal latency will be near or similar to that offered by fiber-optic internet delivery. OneWeb, a London-based company, launched a washing machine-sized LEO satellite with an average latency of 32 milliseconds, in July 2019. SpaceX has stated publicly that they are aiming for an initial latency target of 20 milliseconds, which they hope to halve as more satellites are delivered. Expectations around video streaming and conferencing will also need to be considered. OneWeb has already demonstrated live, full high-definition streaming video, and Telesat has demonstrated videoconferencing services. With the advent of 5G, delivery speeds with high bandwidth and low latency should improve and offer new avenues for the use of satellite internet for smart cars and IoT technology in remote areas.

One consideration with the advent of LEO satellites delivering internet will be user terminal pricing and availability, such that these terminals should be reliable and physically robust, while also being easy to install and use. There is also the question of whether the LEO satellite internet providing industry will see expansion or consolidation; this would be either regulatory bodies allowing companies to launch individual satellite constellations, or much like current telecom towers, constellations would be required to carry multiple service providers. This consideration also includes the increasing regulations in satellite constellations, especially with the risk of orbital debris. These possible regulations could be around deployment rate, frequency allocation, orbital debris mitigation, and de-orbit procedures. Another consideration for the development of LEO satellite internet provider constellations would be the possibility of service interruptions in the case of orbit degradation or collisions with other satellites or debris and how those will be handled.

Satellite IoT have 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. As well, 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 type of IoT connectivity services pathways:

Direct satellite and satellite gateway services visualization.

Comparable to GSM or WiFi backhaul service, the IoT gateway backhaul over satellite emerges as a new application. This is through 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 cost 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. As well, they can enable new applications such as:

• 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 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, geodetic positioning of platforms emitting at radio frequencies, and 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 have 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.

A major question for the continued development of LEO satellites for military use is whether the military will develop and launch their own LEO satellite constellations, or if they would include military assets and payloads on commercial LEO satellites in order to reduce the overall cost of military applications while simultaneously improving the resiliency of military operations in space.

Earth Observant's conceptual overview of it's Stingray imaging satellite constellation.

Earth Observant won a development contract with the Air Force's AFWERX to advance the company's design for a small, very low Earth orbital optical imaging satellite—the company's Stingray satellite. These satellites present a chance for reduced signal interruptions and reduced possibility of collisions while offering faster signal processing, beneficial for the sometimes quick-paced nature of military operations. Furthermore, to offer greater benefit to the military, these satellites work to offer near real-time imagery to Air Force or Army use. This is done through the on-board processing of raw optical data using edge computing methods rather than sending the raw data to a ground station to be processed.

In April 2020, Lockheed Martin was awarded a $5.8 million contract for the first phase of a satellite integration on DARPA's Blackjack program. This program looks at the proliferation of LEO constellations and the development of surrounding technology for the commercial space and works to bring those to achieve a number of military goals. The program is also a shift toward on-orbit processing, where the Blackjack constellation can carry the processing burden of ground-based systems.

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 which are more familiar with space investments and therefore have deep knowledge of the sector.