Stingray VLEO Constellation
EOI (Earth Observant Inc.) Space plans to launch a VLEO (Very Low Earth Orbiting) constellation of “ultra high” resolution imaging satellites for both government and commercial use, known as Stingray. By operating in VLEO, the Stingray satellites will be closer to Earth than most other satellites, and will be capable of delivering the highest resolution imagery. The first satellite in the constellation is planned for launch in early 2024.
The VLEO constellation will operate closer to Earth than most other satellites, providing ultra-high resolution imagery for applications in real-time intelligence, asset monitoring, situational awareness, and more. EOI plans to launch up to 60 satellites in the constellation, with multispectral (MS) near-infrared (NIR) imagers delivering high resolution imagery.
Satellite operations from VLEO benefit from reduced payload power draw and improved signal to noise ratios (SNR), enhanced resolution for optical imagers, and lower latency for satellite communications. The high atmospheric drag in VLEO is capable of naturally deorbiting satellites and other space objects within weeks, ultimately reducing debris accumulation.
The NIR optical imager onboard Stingray will offer 0.15 m spatial resolution imagery with near-real time access, thanks to its proximity to the Earth and no need for ground processing stations – relaying imagery directly to the user. The imager can provide a maximum swath of 5 km x 50 km, with a revisit period of 1.5 days for a single Stingray and an estimated 10 to 15 minute revisit period for the whole constellation.
The constellation will make use of the HET-X Electric Propulsion System (EPS) to continuously maintain an VLEO altitude of 250 km. Stingray will match the performance of a much larger and heavier satellite with more expensive payloads flying at conventional LEO (Low Earth Orbit) altitudes, reducing costs on the satellite constellation and enabling competitively priced services to its customers.
Space and Hardware Components
The Stingray satellites will have a ‘space shuttle’ styled bus, to reduce the impact of high atmospheric drag in VLEO and to house solar panels on the wing structures. The spacecraft will have a mass of approximately 330 kg.
The first of EOI’s Stingray satellites is planned to launch in early 2024, with up to six satellites deployed to VLEO by the end of 2024. EOI envisions a constellation of up to 60 satellites with each spacecraft planned to remain in orbit for up to five years.
The startup company EOI (Earth Observant Inc.) Space, based in Louisville, Colorado, is developing a VLEO (Very Low Earth Orbiting) constellation of satellites to collect ultra-high resolution Earth imagery for both government and commercial use. EOI’s goal is to satisfy the growing demand for high resolution imagery with their “Stingray” constellation. 1)
The private startup company is composed of veterans of the satellite industry with expertise in propulsion systems. EOI Space began designing Stingray in 2018 and hopes to have a 60-unit VLEO imaging constellation that can provide highly accurate, timely imagery to military, government and civilian customers.
Using a proprietary electric propulsion system (EPS), EOI’s satellites can maintain a low altitude and remain in this orbit for up to five years. This reduced altitude increases data volume, improves imagery resolution, and reduces satellite production costs and risk of space debris impacts more common at higher altitudes. The HET-X proprietary EPS onboard Stingray has been tested in multiple iterations over the past few years in cooperation with the US Air Force (USAF). HET-X enables the Stingray constellation to maintain a consistent altitude of 250 km above the Earth. 1)
“VLEO can enable Earth observation at a fraction of the cost,” said Christopher Thein, EOI co-founder and CEO. “There’s huge potential because people want higher-resolution data.” EOI plans to be the first private company to operate in VLEO. EOI will collect very-high resolution imagery with a smaller spacecraft than required for LEO, and onboard image processing will allow for reduced data latency to provide faster data transmission.
VLEO satellites like Earth Observant’s Stingray constellation fly at an altitude of 300 km or lower. At the proposed altitude of 250 km, Stingray will still be in Earth’s atmosphere. That comes with some downsides like aerodynamic drag and strong gravitational pull, which are significant enough to make a spacecraft’s orbit decay in less than 5 years, requiring changes to traditional designs.
Estimates suggest that by 2025, the number of man-made objects sent into space annually will surpass 1,100, with most stationed in LEO. SpaceX offers a prime example: It has launched over 600 LEO satellites for its Starlink broadband internet constellation and plans to launch thousands. Space debris and defunct satellites in LEO can take hundreds of years to de-orbit naturally, and the increase of satellite launches with little concern for end of life (EOL) plans grows this issue. Spacecraft in VLEO are capable of naturally de-orbiting and reentering the Earth’s atmosphere to burn up in mere weeks.
The VLEO Stingray will have the capability to send near real-time imagery to a fixed ground station or mobile user within a few minutes. One of EOI’s goals is to skip the traditional ground station processing stacks, as the US Army and Air Force desire to receive data faster than current systems.
Background on VLEO
Very Low Earth Orbit (VLEO) is defined as altitudes between approximately 100 - 450 km above Earth’s surface. Satellite operations in this region provide numerous benefits for the public and private space sectors, inducing reduced payload power draw, improved Signal-to-Noise Ratios (SNRs) for synthetic aperture radar (SAR) imagery, enhanced spatial resolution for optical imagery, and lower latency for satellite communications. The high atmospheric drag at this altitude is capable of naturally deorbiting most objects in orbit within weeks, resulting in VLEO being ‘self-cleaning’ with respect to space debris. Deploying payloads into VLEO is also significantly easier and cheaper than into higher orbits. VLEO operations provide a business-positive means for satellite operators to strengthen their value propositions through reduced operational costs and improved payload function, whilst mitigating the impacts of increased satellite usage in Earth’s orbit. 2)
An increase in satellite usage in VLEO will reduce crowding in higher orbits, and thus reduce the risk of collisions and mitigate space debris accumulation by encouraging operations in a high atmospheric drag region that naturally enables rapid deorbiting.
The main advantage for satellite operators is that smaller, lighter platforms using cheaper payloads in VLEO can perform as well as larger, heavier and more expensive platforms in conventional LEO. This advantage allows satellite operators to reduce costs on their satellite constellations and therefore offer cheaper commercial services. Alternatively, the same platform and payload can be moved from LEO to VLEO, increasing the value gained from the platform and improving competitiveness of their value proposition to customers.
However, the environment of VLEO causes high atmospheric drag on spacecraft. This drag force increases with density and velocity, which only increases more at lower altitudes. Regular propulsion is required to compensate for the drag force and maintain an orbital altitude. Aerodynamic torque due to aerodynamic perturbations also affect spacecraft in VLEO and can result in reduced platform stability, an increase in disturbing torques when pointing, and increased attitude control actuator requirements.
The higher density of atomic oxygen in VLEO compared to higher orbits results in more damage and erosion to spacecraft surfaces. Atomic oxygen is a highly reactive form of oxygen only found in space where ultraviolet radiation can break apart O2 molecules into atomic oxygen, which upon collision with spacecraft results in chemical and physical changes to surfaces. This higher density increases interactions for spacecraft in orbit and thus the damage it causes. Flow-facing surfaces are particularly affected.
A satellite in VLEO will also have reduced coverage and accessibility due to a smaller coverage footprint by the sensors, a restricted elevation angle and atmospheric propagation constraints, reduced communication windows with ground stations, and impacts on data uplink and downlink. A spacecraft’s revisit time is also adversely affected in VLEO.
Previously the challenges of VLEO orbits were considered too great, and hence most satellites currently orbit in LEO or higher. However, with advances in propulsion technology and spacecraft design, VLEO is becoming a more achievable alternative to LEO, providing high quality data at low costs. 8)
Some examples of past and planned VLEO missions
- The GOCE (Gravity field and steady-state Ocean Circulation Explorer) mission of ESA remained at an altitude of 240-280 km from 2009 to 2013 with the help of xenon-fueled electric thrusters.
- The SLATS (Super Low Altitude Test Satellite) of JAXA (Japan Aerospace Exploration Agency), with a mass of approximately 400 kg, was flown at an altitude of approximately 200 km from 2017-2019, and was powered with xenon thrusters.
- Since November 1998, the ISS (International Space Station) of NASA, ESA, JAXA, Roscosmos and the Canadian Space Agency (CSA), the largest space structure flown with a mass of around 400 tons, resides at altitudes of around 400 km. Cargo vehicles and onboard thrusters help the ISS to maintain its orbit in VLEO.
- The company Planet lowered the altitude of its Earth observing SkySats from 500 km to 450 km in 2019 to improve the resolution of SkySat imagery. This borders on what is considered VLEO.
The Stingray satellites will have a mass of approximately 330 kg, and the spacecraft bus will be 2 m in length with deployment of solar panels. Their VLEO will enable capture of near real-time imagery with a 0.15 m resolution.
Stingray will send imagery via Ka-band (26.5 to 40 GHz) which fits into the multi-domain sensor linking TITAN (Tactical Intelligence Targeting Access Node) and AMBS (Airborne Battle Management System) systems that the Army and Air Force are respectively developing. The constellation has the potential to cut out ground processing stations, and relaying imagery directly to the customer would be of interest to these services.
To achieve near-real time transmission of optical satellite imagery, EOI is analysing and reworking the entire imagery chain, from tasking to image distribution. By performing the majority of image processing onboard the spacecraft, EOI will be able to provide more data directly to the user via its Ka-band high bandwidth transmitter. Elimination of ground stations will decrease the time between data collection and delivery and provide access to information faster.
EOI’s first satellite of the constellation is planned to launch in early 2024 on a SpaceX Falcon 9 rocket, as a part of their Rideshare program. The Stingray satellite will deploy from the Falcon 9 at approximately 500 km and work down towards VLEO. 3)
- August 6, 2020: EOI was awarded a contract under the US DoD SBIR (Small Business Innovative Research) program and is working with AFWERX (of the USAF), the Space and Missile Systems Center (SMC), and the Air Force Research Laboratory (AFRL). EOI is eager to apply the SBIR award toward development of the Stingray VLEO constellation. 7)
- August 28, 2020: Earlier this month, EOI announced it had won a development contract with the Air Force’s AFWERX technology incubator to advance its design for a small, very low Earth orbit (VLEO) optical imaging satellite. 6)
- February 18, 2021: Earth Observant completed tests of their Hall-effect Thruster-X (HET-X) at the US Air Force Research Lab’s vacuum chamber facility at Edwards Air Force Base over a period of two weeks. The HET-X technology uses traditional electric propulsion (EP) fuel sources and the thruster is capable of maintaining mission operations from VLEO with high efficiency and minimal erosion. This technology enables the required satellite manoeuvrability in VLEO, as well as “drag makeup”, orbital plane changes, upper stage payload ferryng, and controlled reentry. The new propellant type used with HET-X benefits from reduction of cost, the need for high-pressure storage vessels and in situ propellants. The thruster weighs less than 2 kg, and tests showed an output of 20 - 150 millinewtons and specific impulse between 1300 - 2200 ISP when supplied with an input power between 350 - 2500 W. 5)
- September 14, 2022: EOI Space announced a deal with SpaceX to launch their first Stingray satellite using SpaceX’s rideshare program. The first satellite of the constellation will be a technological demonstration, manifested to launch in early 2024 on a SpaceX Falcon 9 rocket. Validation of EOI’s technology will ensure the satellite meets the desired mission applications. 4)
Sensor Complement (Stingray Optical Imager)
The satellite will house a four-band multispectral (MS) near-infrared (NIR) optical instrument from an outside provider, that EOI will integrate into the satellite bus. Through the development and implementation of the Stingray constellation, EOI plans to introduce its own optical payload as well as a linked GPU (Graphics Processing Unit) compute cluster, as well as its own propulsion system. EOI hopes to become as vertically integrated as possible with in-house development of Stingray. Providing imagery at a 0.15 m (6-inch) spatial resolution will enable EOI to deliver worldwide high-resolution imagery and location data rapidly, never previously seen from commercial satellites. 3)
Stingray Imagery Specification
Resolution (All spectral bands)
450 - 800 nm
450 - 520 nm
520 - 600 nm
630 - 690 nm
Revisit period (one satellite)
Revisit period (complete constellation)
10 - 15 minutes
Scene Size (max)
5 km x 50 km
1) ”Earth Observant- Creating new opportunities,” EOI, 2020, URL: https://www.earthobservant.com/
2) Chan Yuk Chi, Indraji Ushantha Wanigaratne, David Dubinsky, ”How low can you go: Advocating very low Earth Orbit as the next frontier for satellite operation,” Proceedings of the 8th European Conference on Space Debris (virtual), Darmstadt, Germany, 20–23 April 2021, published by the ESA Space Debris Office ,Ed. T. Flohrer, S. Lemmens & F. Schmitz, (http://conference.sdo.esoc.esa.int, May 2021), URL: https://conference.sdo.esoc.esa.int/proceedings/sdc8/paper/109/SDC8-paper109.pdf
3) “Satellite Imagery Startup EOI Space Has Crossed the Valley of Death by Flying Low,” Forbes, 14 December 2022, URL:https://www.forbes.com/sites/erictegler/2022/12/14/satellite-imagery-startup-eoi-space-has-crossed-the-valley-of-death-by-flying-low/?sh=1b6048134beb
4) “Earth Observant Inc (EOI Space) Inks Deal with SpaceX,” EOI Space, 14 September 2022, URL: https://eoi.space/spacex/
5) ”Earth Observant Inc. Successfully Tests Next-generation Propulsion Technology to Support Future Very Low Earth Orbit Missions,” EOI Space, 18 February 2021, URL: https://eoi.space/propulsion/
6) Eric Tegler, ”How Low Can Satellites Go? Air Force Bets Very Low Earth Orbit Will Give It More Capabilities,” Forbes, 28 August 2020, URL: https://www.forbes.com/sites/erictegler/2020/08/28/how-low-can-satellites-go-air-force-bets-very-low-earth-orbit-will-give-it-more-capabilities/
7) “Earth Observant Inc. Wins Air Force Contract to Further Development,” PR Newswire, 6 August 2020, URL: https://www.prnewswire.com/news-releases/earth-observant-inc-wins-air-force-contract-to-further-development-301107313.html
8) Nicholas Crisp, “The Benefits and Challenges of Very Low Earth Orbits,” The University of Manchester, 18 November 2020, URL: https://discoverer.space/wp-content/uploads/2020/11/Crisp_DISCOVERER_Masterclass_Benefits-and-Challenges.pdf
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