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The Benefits of Very Low Earth Orbit for Earth Observation Missions
N. H. Crisp, P. C. E. Roberts, S. Livadiotti, V. T. A. Oiko, S. Edmondson, S. J. Haigh, C. Huyton, L. Sinpetru, K. L. Smith, S. D. Worrall, J. Becedas, R. M. Domínguez, D. González, V. Hanessian, A. Mølgaard, J. Nielsen, M. Bisgaard, Y. -A. Chan, S. Fasoulas, G. H. Herdrich, F. Romano, C. Traub, D. García-Almiñana, S. Rodríguez-Donaire, M. Sureda, D. Kataria, R. Outlaw, B. Belkouchi, A. Conte, J. S. Perez, R. Villain, B. Heißerer, A. Schwalber
TL;DR
The paper examines how VLEO can support more capable and accessible Earth observation amid growing interest in lower-altitude operations. It reviews VLEO benefits and finds lower debris risk, while identifying thermospheric uncertainty and aerodynamic-control constraints as important limitations.
Problem
Growing interest in VLEO reflects efforts to reduce launch and operating costs while maintaining or improving Earth-observation data quality and availability.
Method
The paper provides a comprehensive overview and analysis of VLEO benefits for spacecraft operations, including Earth-observation applications.
Results
VLEO remains at lower relative collision risk than higher orbits that may become over-populated, while combined system MTF is largely altitude-independent.
Takeaways & Limitations
VLEO may support lower-cost Earth observation and improved imagery availability, but sustained operation requires further thermospheric understanding and propulsion development.
Takeaways & Limitations
Orbital-lifetime estimates remain uncertain because atmospheric-density models contain errors and depend strongly on difficult-to-predict solar activity.
Abstract
from arXiv · showhide
Very low Earth orbits (VLEO), typically classified as orbits below approximately 450 km in altitude, have the potential to provide significant benefits to spacecraft over those that operate in higher altitude orbits. This paper provides a comprehensive review and analysis of these benefits to spacecraft operations in VLEO, with parametric investigation of those which apply specifically to Earth observation missions. The most significant benefit for optical imaging systems is that a reduction in orbital altitude improves spatial resolution for a similar payload specification. Alternatively mass and volume savings can be made whilst maintaining a given performance. Similarly, for radar and lidar systems, the signal-to-noise ratio can be improved. Additional benefits include improved geospatial position accuracy, improvements in communications link-budgets, and greater launch vehicle insertion capability. The collision risk with orbital debris and radiation environment can be shown to be improved in lower altitude orbits, whilst compliance with IADC guidelines for spacecraft post-mission lifetime and deorbit is also assisted. Finally, VLEO offers opportunities to exploit novel atmosphere-breathing electric propulsion systems and aerodynamic attitude and orbit control methods. However, key challenges associated with our understanding of the lower thermosphere, aerodynamic drag, the requirement to provide a meaningful orbital lifetime whilst minimising spacecraft mass and complexity, and atomic oxygen erosion still require further research. Given the scope for significant commercial, societal, and environmental impact which can be realised with higher performing Earth observation platforms, renewed research efforts to address the challenges associated with VLEO operations are required.
1. Introduction
VLEO is being reconsidered for Earth observation because it may improve data performance and reduce mission costs, while atmospheric forces and limited operational heritage remain significant challenges.
- Earth observation supports environmental, maritime, security, agricultural, meteorological, and disaster-response applications with global societal and commercial significance.
- VLEO has generally been avoided because few spacecraft operate sustainably there, and atmospheric effects can increase development costs or significantly limit mission lifetime.
- Historical VLEO missions accepted short lifetimes or eccentric orbits to obtain high-resolution surveillance imagery, limiting either duration or imaging operations.
- Miniaturisation, NewSpace development, debris concerns, and more accessible launch opportunities have renewed interest in commercial VLEO operations.
- VLEO is typically bounded near 450–500 km, although the effective boundary varies with atmospheric conditions and the solar cycle.
- The paper reviews VLEO benefits for spacecraft operations and analyses how reducing altitude affects optical, radar, and infrared Earth-observation systems.
2. Benefits of Very Low Earth Orbits
Lowering orbital altitude changes coverage geometry and can improve Earth-observation spatial performance, but atmospheric and platform effects constrain the resulting system benefit.
- Orbit Geometry: For a fixed angular field of regard, reducing altitude decreases the footprint area and total available coverage.
- Spatial Resolution: For a fixed aperture diameter, lowering altitude improves both ground resolution distance and ground sample distance.
- Spatial Resolution: Lower altitude permits a smaller aperture while maintaining a given spatial-resolution performance, enabling potential mass and volume savings.
- Modulation Transfer Function: The combined modulation transfer function is largely altitude-independent, although lower altitude can worsen motion effects and atmospheric performance at wide coverage angles.
- Modulation Transfer Function: Additional atmospheric-density disturbances at lower altitudes may degrade modulation transfer function through the platform’s vibrational response.
2.3. Radiometric Performance
Lower altitude generally improves radiometric performance by shortening the target range, while off-nadir atmospheric paths can offset that benefit.
- For active sensors such as radar, received power varies with the fourth power of target range, making altitude reduction especially consequential.
- A smaller collection aperture can maintain similar radiometric performance at lower altitude.
- At significant off-nadir viewing angles, increased atmospheric path length can degrade signal-to-noise performance.
- Reducing orbital altitude improves received power and radiometric performance because the spacecraft is closer to the target.
- Maintaining the same aperture at lower altitude improves both radiometric performance and spatial resolution.
2.4. Temporal Resolution
Temporal resolution in VLEO depends strongly on altitude, orbit type, and constellation configuration, leaving only selected altitude windows suitable for effective operation.
- Revisit time is a key Earth-observation design factor, with low maximum revisit time generally desired alongside global coverage.
- Repeating ground tracks in VLEO require aerodynamic compensation or control because residual atmospheric interaction causes orbital decay.
- Some altitude ranges have poor temporal resolution because orbital-period and Earth-rotation resonances produce incomplete coverage or long repeat patterns.
- VLEO can provide close-to-optimal maximum revisit time in selected altitude windows, but other windows restrict effective operation.
- In non-sun-synchronous orbits, approximately 250–475 km can provide low maximum revisit time.
2.5. Ground Communication and Link Budget
Lower altitude can improve communication radiometric performance through shorter propagation range, but access time and pass frequency can constrain total data transfer. VLEO also generally supports post-mission lifetime compliance with IADC requirements, although lifetime estimates remain uncertain.
- Communications performance: Shorter range in lower orbits reduces free-space loss and improves signal-to-noise ratio for antennas with the same size and EIRP.Received radiated power depends on range, receiving antenna diameter, and antenna efficiency.
- Communications performance: Lower-altitude links may be constrained by allowable elevation angles, increased orbital velocity, and fewer daily ground-station passes.These effects can reduce effective access time or the total volume of data transferred despite improved radiometric performance.
- Deorbit requirements: Orbital-lifetime estimates are difficult because thermospheric-density models and long-term solar-activity predictions contain substantial uncertainty.Calculated lifetime also varies with propagation fidelity, included perturbations, input data, solar flux, altitude, and spacecraft ballistic coefficient.
- Deorbit requirements: VLEO satellites below 450 km generally have post-mission lifetimes under 25 years across solar environments and satellite sizes and masses.This supports compliance with IADC guidelines without additional deorbit hardware or propulsion, reducing potential complexity, cost, and mass.
2.7. Debris Collision Risk Resilience
VLEO has a lower debris spatial-density profile than higher LEO and is comparatively resilient to debris accumulation and cascade events. Lower-altitude radiation exposure also offers potential mission-lifetime and component-cost benefits, although the debris model omits planned megaconstellations.
- Debris environment: Debris generated in or entering VLEO decays faster because atmospheric density is higher at lower altitude.This faster decay contributes to lower persistence of debris in the VLEO regime.
- Debris environment: VLEO below 500 km has lower debris spatial density than higher LEO and appears resilient to predicted 700–1000 km debris buildup through 2055.The projection uses ESA MASTER-2009 under a business-as-usual scenario.
- Caveat: MASTER-2009 projections omit planned or recently launched megaconstellations, although their planned higher altitudes leave VLEO collision risk low under the stated assumptions.The remaining VLEO risk concerns spacecraft that de-orbit, fail, or naturally decay through the regime.
- Collision risk: VLEO remains at lower relative collision risk than potentially over-populated higher orbits, including under a Kessler-syndrome-type cascade scenario.Risk can still increase slightly because de-orbiting spacecraft and debris transit VLEO.
- Radiation environment: At 300 km, peak proton flux is reduced by an order of magnitude relative to 600 km, while substantial proton-flux distribution also decreases.Electron peak magnitude does not decrease, but the geographic distribution of high-energy electrons does.
- Radiation environment: Reduced radiation exposure at lower altitudes may support longer missions using non-radiation-hardened components and potentially cheaper consumer components.The proposed benefits arise from reduced lifetime dosage and could decrease mission costs and development time.
2.9. Access to Orbit
Lower-altitude insertion can increase launch capability and reduce some Earth-observation geospatial errors, while also allowing relaxed pointing requirements. The trade-off is that off-nadir operations may require greater spacecraft position knowledge, and usable temporal-performance windows remain altitude-dependent.
- Access to Orbit: Approximately 10% to over 50% improvement in launch capability is available when insertion altitude decreases from 600 km to 300 km, depending on vehicle.The comparison covers Falcon 9, Antares, Electron, Pegasus, and Vega for SSO insertion.
- Access to Orbit: Lower-altitude insertion can deliver more satellites per launch or decrease shared-launch unit cost, improving access to VLEO missions.Greater payload capability also increases the number of vehicles able to launch a given spacecraft.
- Geospatial position accuracy: Onboard clock or timing errors do not depend on orbital altitude.Other geospatial error sources include satellite position, pointing, target altitude, and Earth-rotation uncertainty.
- Geospatial position accuracy: Geospatial position accuracy generally improves as orbital altitude decreases, and platform pointing requirements can consequently be relaxed.Pointing-related mapping errors have a stronger relationship with target range than satellite position errors.
- Geospatial position accuracy: Off-nadir pointing increases the position-knowledge requirement as spacecraft-to-target range decreases.Satellite position and onboard-clock errors show an inverse relationship with pointing error as range decreases.
2.11. Aerodynamic Control
Aerodynamic forces become substantially stronger at lower altitudes, creating opportunities for aerodynamic attitude and orbit control. However, low lift-to-drag ratios constrain control use unless improved materials or propulsion offset drag while preserving orbital lifetime.
- Aerodynamic forces: More than 200-fold increase in aerodynamic force can result from reducing altitude from 600 km to 300 km.The increase follows higher atmospheric density and a small increase in orbital velocity at lower altitude.
- Aerodynamic control: Stronger aerodynamic forces can increase the effectiveness or efficiency of aerodynamic attitude and orbit control methods.These methods exploit aerodynamic forces and associated torques for control and manoeuvring.
- Aerostability: Aerostability is possible up to approximately 500 km, with optimal results demonstrated below 450 km.Aerodynamic stiffness depends on atmospheric density relative to perturbing torques such as solar radiation pressure, magnetic dipoles, and gravity gradient.
- Control trade-off: A lift-to-drag ratio of about 0.1 is generally found in rarefied LEO flow, constraining aerodynamic control while maintaining reasonable orbital lifetime.Higher ratios may require specular-reflection materials or propulsion to counteract drag.
2.12. Atmosphere-Breathing Electric Propulsion
Atmosphere-breathing electric propulsion (ABEP) collects atmospheric gas as propellant, potentially extending spacecraft lifetime and reducing launch mass in VLEO. Its benefits are bounded by intake, thrust, power, atmospheric-density, and atomic-oxygen constraints.
- ABEP concept: ABEP collects oncoming atmospheric gas and uses it as propellant for an electric thruster, eliminating onboard propellant storage.This principle can significantly extend spacecraft lifetime beyond current designs.
- System trade-offs: Removing propellant storage offers lower spacecraft and launch mass, but intake, compressor, thruster, and power-system mass can offset the saving.Additional deployable solar arrays may also be required.
- Aerodynamic and intake constraints: Additional ABEP components increase drag and therefore the thrust requirement, while intake efficiency decreases as intake collection area grows relative to thruster inlet area.This efficiency trend has been checked against concept intakes from JAXA and BUSEK.
- Operating range: An optimal ABEP altitude range exists because higher altitudes reduce propellant mass flow, while lower altitudes increase required thrust and onboard power.The preferred range depends on system performance.
- Atomic oxygen: Atomic oxygen can erode accelerating grids, electrodes, and discharge channels, degrading thruster performance over time.Inductive plasma thrusters are cited as a more erosion-resilient contactless alternative.
3. Earth Observation in Very Low Earth Orbits
Lowering orbital altitude improves several Earth-observation performance measures, including optical resolution, radiometric performance, radar and lidar signal strength, and SAR sizing. These gains remain subject to coverage, resolution, power, antenna, and aerodynamic trade-offs.
- Optical systems: A 50% reduction in altitude improves diffraction-limited optical resolution by a factor of 2 for a fixed aperture.Aperture diameter can also be reduced proportionally while maintaining the same diffraction-limited resolution.
- Optical systems: Lower altitude reduces optical footprint area for a given angular field of regard, while off-axis viewing further decreases achievable resolution.The field-of-regard effect is greater at higher altitudes.
- Optical systems: Lower altitude permits smaller optical apertures at a given SNR or improves dynamic range and SNR for a fixed sensor and aperture.Lower-cost panchromatic and multispectral platforms can therefore reduce mass and integration requirements while improving resolution and SNR.
- Real-aperture radar: For real-aperture radar, angular resolution improves with reduced target range, whereas range resolution is independent of target distance.Together these define a resolution cell for distinguishing multiple targets.
- Real-aperture radar: Radar received power and SNR improve with decreasing range, following an inverse-fourth-power relationship that can reduce transmitter power while maintaining similar SNR.A monostatic radar’s SNR also improves with the square of antenna area.
- Synthetic aperture radar: SAR along-track resolution improves as antenna size is reduced, while cross-track resolution can improve with increasing off-nadir viewing angle.Theoretical cross-track resolution reaches its maximum at 90° off-nadir viewing.
- Synthetic aperture radar: SAR SNR improves with lower altitude and larger antenna area, while minimum antenna area decreases at lower altitude for the same incidence angle.SAR sizing trades required resolution against available platform power and must account for antenna and solar-array drag.
- Lidar: Lidar returned power improves by a fourth-power relationship as target range decreases, although range resolution remains set by the timing measurement chain.Returned power also depends on beam divergence, atmospheric and system transmission, and target backscatter.
4. Impact and Applications of Very Low Earth Orbits for Earth Observation
VLEO can improve Earth-observation resolution, radiometric performance, communications link budgets, and revisit opportunities while enabling smaller or more numerous spacecraft. These capabilities may support commercial, societal, sustainability, and environmental applications.
- System-level benefits: VLEO’s principal EO benefits are enhanced ground resolution, improved radiometric performance, and improved communications link budgets.Equivalent capability may be achieved with smaller spacecraft than traditional higher-altitude systems.
- System-level benefits: Smaller VLEO spacecraft could be deployed in larger numbers and less traditional orbits, increasing revisit frequency and temporal resolution.This follows from the possibility of reducing system cost through smaller spacecraft.
- Applications: Higher-resolution and more timely imagery can support commercial users and global societal, sustainability, and environmental objectives.The cited applications include border security, maritime surveillance, humanitarian assistance, and crisis management.
- Economic and industrial impact: Demand for VLEO systems may support growth in Earth-observation markets and upstream industries such as spacecraft development, manufacturing, and launch.Operating effectively in VLEO also requires new technologies and encourages innovation.
5. Conclusions and Recommendations
VLEO Earth observation offers substantial commercial and societal promise, but sustained operation depends on resolving atmospheric, materials, propulsion, and economic challenges. Engineering and business modelling are needed to identify viable concepts and guide implementation.
- Lower-altitude Earth observation can deliver commercial and societal benefits, motivating renewed research into VLEO operations.
- Atomic oxygen degrades optical sensor surfaces and thermal coatings, driving research into erosion-resistant, drag-reducing materials.
- Atmospheric-density and thermospheric-wind measurements are needed to support aerodynamic control and high-precision VLEO operations.
- Atmosphere-breathing electric propulsion could extend spacecraft lifetime by providing drag compensation without onboard propellant.
- Quantitative assessments and business models remain necessary to establish VLEO systems' economic and commercial potential across Earth observation applications.
- Engineering, system, and business models can identify promising VLEO concepts and define exploitation and implementation roadmaps.
Nomenclature
The nomenclature defines aerodynamic, communications, orbital, velocity, receiver, and synthetic-aperture variables used in the paper's VLEO analysis.
- A_ref denotes aerodynamic reference area, while V_rel denotes velocity relative to the atmosphere.
- B_n denotes receiver noise bandwidth, and L_a denotes the transmission path loss factor.
- The nomenclature section also cites prior work on sustained low-altitude Earth observation satellites and VLEO system analysis.
- L_SAR denotes synthetic aperture length, while r_s denotes orbit or satellite radius.
- T_r denotes receiver noise temperature, T_E denotes effective noise temperature, and V_E denotes Earth equatorial velocity.