NASA JSC Active Response Gravity Offload System

ARGOS is the gravity offload facility at NASA JSC: an overhead bridge crane carrying a motor-driven cable that holds a constant upward force on a test article while following it through the workspace, so the article moves as though it weighed a lunar, Martian or microgravity fraction of its Earth weight [1][4]. Three translational axes are actively driven, which is the difference between it and the earlier Pogo rig described on the reduced gravity testing page, and the reason it can be used for walking rather than only for standing tasks [2].

It offloads at one point. Everything the following sections say about fidelity follows from that: the cable knows the total weight it is holding, and nothing about how the mass is distributed across the limbs beneath it [3].

Every number in the capability table is the operator’s stated specification. Nothing published reports a measured offload force accuracy, a bandwidth, a residual horizontal force or a cable angle error against an independent reference [1][4].

Laboratory data

Parameter	Value
Operator	NASA JSC [4]
Location	Houston, Texas, United States [4]
Commissioned	Third iteration of the system as described in 2017; earlier dates not published [1]
Type	Active cable gravity offload rig, human rated and two fault tolerant [1]
Floor area	Bridge crane structure 41 x 24 x 25 ft (12.5 x 7.3 x 7.6 m) [4]
Capabilities	Offload envelope, gimbals
Simulant or terrain	None installed. Terrain is brought in by the test [1]
Instrumentation	Displacement and force sensing on X, Y and Z; AIBEL motion tracking [4]
Ground truth	Not published. No independent verification of delivered offload is reported [1][4]
Fidelity limits	Single attachment point; limbs stay at 1 g; three rotations passive [2][3]
Access	Test support offered through the JSC analogs and mockups program; no lead time published [4]
Cited by	Robonaut 2 [5]

Capabilities

Offload envelope

Parameter	Value
Working volume	Over 340 m3: 11.0 m Y, 5.8 m X, 5.5 m working height [1]
Test article limits	340 kg (750 lb) payload [1][4]
Vacuum	None. Ambient pressure in a high bay [1]
Temperature	Not controlled. Breathing air and suit cooling water are supplied [4]
Illumination	Not published. No solar simulator
Slope	Not applicable. Terrain is whatever the test brings in
Gravity offload	Vertical, at one point, set in software from 1 g to microgravity [1][4]
Instrumentation	Redundant sensors, real-time safety model, independent overspeed detection [1]

Speed and acceleration trade against payload mass: 3.3 m/s and 24.4 m/s2 at 140 kg, falling to 1.4 m/s and 10.2 m/s2 at 340 kg [1]. A separate horizontal-system maximum of 2.0 m/s and 6.1 m/s2 is quoted alongside those figures without being reconciled with them [1]. The operator’s own catalog gives the limits in the form a test director needs, as tracking rates by configuration: 4 ft/s vertical and 6.5 ft/s horizontal with a suited subject, 11 ft/s vertical and 6.5 ft/s horizontal unsuited [4]. Motion faster than that is outside the offload envelope, which is a constraint on the task, not on the rig.

Vertical offload load sensitivity is quoted as below 0.0005 kg, and horizontal tracking sensitivity as under 0.1 degree of cable angle, both without a bandwidth, a condition or a measurement method [1]. Safety faults are detected in under 20 ms with two fault tolerance, and a lifting cable device limits the shock load into the payload during a high-speed stop [1].

The two published descriptions disagree on the workspace. The 2017 description gives 11.0 by 5.8 m at 5.5 m height, over 340 m3 [1]; the operator’s catalog gives 13 by 30 by 15 ft, about 4.0 by 9.1 by 4.6 m or 167 m3, inside the 41 by 24 by 25 ft crane structure [4]. Neither states which convention it uses, and the difference is a factor of two in usable volume, so a test that needs the full run length should confirm it with the facility rather than from either number here.

Gimbals

Parameter	Value
Working volume	Not applicable. The gimbal is the payload interface
Test article limits	Suited, unsuited and unmanned interfaces available [4]
Actively controlled axes	Three translations. The three rotations are passive [2]
Instrumentation	Center of gravity board with four-corner force transducers, for pivot setup [3]

Roll is limited to about 30 degrees by the parallelogram linkage and pitch by interference with the subject [2]. Gimbal mass is a facility variable with a measurable effect: replacing the 105 lb all-metal claw gimbal with a 53 lb gimbal adapted from EMU and MkIII suit hardware measurably improved subject-rated simulation quality, the lower mass and inertia being the stated cause [2].

What it does not reproduce

The limbs. The gimbal attaches at a single point on the portable life support system mockup, so arms and legs move against full 1 g. It is worst in prone postures, where the entire lower torso lies outside the offloading cable region [3]. Any result about limb effort in a suited task at ARGOS is a 1 g limb result. This is a fidelity limit stated by the operators, not a measurement of the error it causes, and no published source quantifies it.

Gait timing. Suited walking at 1/6 g offload gave a 50/50 swing and stance split, the same as 1 g unsuited ground walking, but the mid-swing point occurred 10 percent earlier and the cause was not established [2]. Different subjects were used in the two environments, so the comparison is not paired.

A pivot point that does not have to be tuned. Across 68 configurations and 14 xEMU subjects of 175.1 +/- 8.4 cm stature in lunar offload, the functional zone for the gimbal pivot is 2.5 to 6.4 cm above and 1.3 to 2.5 cm aft of the system center of gravity [3]. Suited subjects notice fore-aft shifts as small as 0.6 cm, while up-down shifts of 2.5 cm produce minimal performance difference [3]. A simulation run without that tuning is measuring the rig. Setup uses a center of gravity board with four-corner force transducers and the hard upper torso hatch angle: standing neutral came out at 58 to 62 degrees from vertical, standing tip-over at 47 to 52 degrees, one-knee kneeling tip-over at 15 to 27 degrees, with a standing sway range of 0.5 to 8.5 in and a static center of pressure offset of 3 to 3.5 in [3].

Soil. The rig offloads the test article. The ground beneath it stays at 1 g, so a mobility or excavation result taken here inherits terrain stronger than the destination’s [1]. The reasoning is on the reduced gravity testing page.

Duration is the thing it does reproduce. Against a parabolic aircraft, which gives the highest offload fidelity of the analogs but only about 30 s per parabola in a constrained cabin volume, ARGOS trades fidelity for unlimited run time [2]. Against Pogo, which drove only two axes and carried significant overhead inertia from the lifting mechanism, it trades mechanism complexity for a third driven axis [2].

Campaigns run there

Robonaut 2 with climbing legs, 2014. A nearly 300 kg two-legged Robonaut 2 configuration placed its end effector to under 1 cm of error over a reach of more than 1 m, with both legs stiff at a natural frequency of 20 and a damping ratio of 1.2, base leg joint torque limited to 100 and reaching leg to 20 in units the source does not state, and the embedded impedance loop integrator enabled [5]. The result carries the facility’s own limit: only the base leg was effectively offloaded, so the reaching leg worked against 1 g [5].

xEMU pivot point characterization, reported 2023 [3]. The 68-configuration study above is a campaign about the facility rather than about a test article, and it is an unusual thing for an offload rig to publish. One dedicated run tested 10 pivot settings on a single experienced subject in a day [3].

Suited lunar gait measurement, reported 2020. Inertial measurement units mounted on the exterior of a Mark III suit recorded the swing and stance split at 1/6 g offload [2].

Other offload approaches, and what the soil does

ARGOS is one of several ways to remove part of a test article’s weight, and the others differ in what they offload and what they leave at 1 g. A 1966 NASA Langley study put suited subjects in an inclined-cable lunar gravity simulator to measure self-locomotion, which is the earliest offload method among the sources cited here [9]. A master’s report from Michigan Technological University describes the IRGO system for a lunar simulant sandbox: it tracks an infrared beacon on a rover to remove inertial and friction forces on two horizontal axes, offloads part of the weight with a passive counterweight as the third axis, and lists an active vertical axis and a lightweight gimbal as future work [6]. That rig is a laboratory build for rovers, not a flown or operational system. A legged-robot study used a constant-force spring offload to run locomotion experiments in lunar gravity, and measured 12.2 W for a power-optimized controller against a baseline 36 % higher in that setup [8]. The spring is passive, so it holds one force level and cannot be commanded across gravity fractions as ARGOS can.

All of these offload the test article, not the ground. Parabolic flight tests with a Mars simulant showed a statistically significant average traction reduction of 20 % at lunar gravity and 5 to 10 % at Martian gravity against 1 g, with increased sinkage in both [10]. A dissertation comparing Earth-based methods against parabolic reference runs found traction reductions of 20 % or more and sinkage increases up to 40 % relative to reduced-weight testing, and under 10 % error for granular scaling law wheel tests [11]. A mobility result from a cable-offload rig therefore carries terrain that is stronger than the destination’s, which the offload cannot correct.

Requirements the facility is used to bound

Suit mass limits for Mars are set in part from offloaded suit tests. A 2026 study recommends 230 lbm as a starting maximum Mars suit mass, against an xEMU just under 400 lbm, and notes that studies involving Martian offloading are limited [7].

What is not established

No cited source reports measured offload force accuracy, control bandwidth, residual horizontal force or friction for ARGOS [1][2][3][4]. Reported fidelity evidence is subjective simulation-quality ratings from two and three subjects, a gait comparison across different subjects, and a pivot-point heuristic with no lunar or parabolic ground truth [2][3]. The two published workspace descriptions disagree [1][4]. The corpus does not hold 30 curated works on this facility; the held, curated works on gravity offload are cited here and nothing further has been read.

References

Valle, P. S. (2017). Reduced Gravity Testing of Robots (and Humans) Using the Active Response Gravity Offload System. ntrs.nasa.gov/citations/20170008856

BibTeX

@misc{valle2017reduced,
  title = {Reduced Gravity Testing of Robots (and Humans) Using the Active Response Gravity Offload System},
  author = {Valle, Paul Steinar},
  howpublished = {Presentation at the Institute of Electrical and Electronics Engineers},
  year = {2017},
  url = {https://ntrs.nasa.gov/citations/20170008856},
  abstract = {No abstract available}
}

Bekdash, O. S., Valle, P. S., Kim, K. J., Jarvis, S. L., Dunn, J. T., Norcross, J. R. and Abercromby, A. F. J. (2020). Development and Evaluation of the Active Response Gravity Offload System as a Lunar and Martian EVA Simulation Environment . International Conference on Environmental Systems, 20205003610. Source

BibTeX

@inproceedings{bekdash2020development,
  title = {Development and Evaluation of the Active Response Gravity Offload System as a Lunar and Martian EVA Simulation Environment},
  author = {Bekdash, Omar S. and Valle, Paul S. and Kim, Kyoung Jae and Jarvis, Sarah L. and Dunn, Jocelyn T. and Norcross, Jason R. and Abercromby, Andrew F. J.},
  booktitle = {International Conference on Environmental Systems},
  number = {20205003610},
  institution = {NASA},
  year = {2020},
  url = {https://ntrs.nasa.gov/citations/20205003610},
  abstract = {In preparation for future exploration missions, NASA seeks the ability to simulate partial-gravity operations  for  use  in  ground-based  research,  crew  training,  and  engineering  design  evaluations.  The  Active  Response  Gravity  Offload  System  (ARGOS)  at  the  Johnson  Space  Center (JSC) is designed to simulate reduced gravity environments, such as lunar, Martian, or  microgravity,  using  a  robotic  system  similar  to  an  overhead  bridge  crane.  ARGOS  continuously offloads a portion of a suited human’s weight during all dynamic motions within the test facility, which can include basic functional movements such as walking, running, and jumping, as well as a wide range of planetary surface activities. This system will be used as part of a metabolic-rate task characterization study to determine the workload associated with partial-gravity  extravehicular  activity  (EVA).  Pilot  testing  was  conducted  using  the  MKIII  prototype planetary space suit and two gimbal designs to determine the ability of the ARGOS test environment to simulate planetary EVA operations. This paper will describe the lessons learned from the feasibility testing,  simulation-environment  mockup  design,  and  the results from  the  pilot  tests  and  their  influence  into  the  final  study  design.  Being  able  to  effectively  simulate partial-gravity environments and characterize the performance of crewmembers will have an impact on multiple domains including suit design, task design, thermal models, and life-support-system capacity verification plans, among others.}
}

Jarvis, S. L., Vu, L. Q., Gupta, G., Benson, E., Kim, K. H., Rhodes, R. and Rajulu, S. L. (2023). Development of ARGOS (Active Response Gravity Offload System) Offloading Assessments and Methodology for Lunar EVA Simulations . International Conference on Environmental Systems, 20230002430. Source

BibTeX

@inproceedings{jarvis2023development,
  title = {Development of ARGOS (Active Response Gravity Offload System) Offloading Assessments and Methodology for Lunar EVA Simulations},
  author = {Jarvis, Sarah L. and Vu, Linh Q. and Gupta, Garima and Benson, Elizabeth and Kim, K. Han and Rhodes, Richard and Rajulu, Sudhakar L.},
  booktitle = {International Conference on Environmental Systems},
  number = {20230002430},
  institution = {NASA},
  address = {Calgary},
  year = {2023},
  url = {https://ntrs.nasa.gov/citations/20230002430},
  abstract = {The Active Response Gravity Offload System (ARGOS) at NASA Johnson Space Center (JSC) is an analog environment that can offload pressurized suited subjects for various reduced gravity simulations. The suit is suspended from a robotic overhead crane by a cable connected to the suit via a gimbal with an adjustable pivot point (i.e. offload attachment). There has been increased interest in providing planetary pressurized suited training at ARGOS in preparation for lunar missions. Determination of the appropriate gimbal pivot point location for a given subject is vital for a high-fidelity functional lunar simulation. Interactions between the pivot point location and human-spacesuit center of gravity (CG) can result in righting moments that may lead to artificially stable or unrealistically challenging
configurations. Changing the pivot point location is time consuming and repeated adjustment can result in loss of valuable pressurized suited time. This paper aims to share knowledge obtained from the offloading characterization efforts during pressurized suited testing at ARGOS and document the ongoing process to define an appropriate pivot point location through iterative quantitative and qualitative assessments. Human-spacesuit CG locations for the ARGOS lunar simulation were estimated using a 3D body scan and density model combined with spacesuit hardware CAD and specifications. Early pilot testing of the gimbal revealed that setting the pivot point coincident with the modeled CG location was not always possible due to the current gimbal design, and small pivot point shifts had noticeable effects on subject stability. Fourteen subjects performed a series of CG-related tasks in the
Exploration Extravehicular Mobility Unit (xEMU) to assess simulation characteristics. Through iterative testing, this task list evolved to streamline the process needed to efficiently identify a suitable pivot point for a given subject. The developed methodology will be critical for pivot point selection during astronaut training in the ARGOS environment.}
}

NASA Johnson Space Center. (2024). JSC Analogs and Mockups. nasa.gov/reference/jsc-analogs-mockups

BibTeX

@misc{nasajscanalogs,
  title = {JSC Analogs and Mockups},
  author = {{NASA Johnson Space Center}},
  organization = {nasa.gov},
  year = {2024},
  url = {https://www.nasa.gov/reference/jsc-analogs-mockups/}
}

Badger, J. M., Hulse, A. M., Taylor, R. C., Curtis, A. W., Gooding, D. R. and Thackston, A. (2014). Model-based Robotic Dynamic Motion Control for the Robonaut 2 Humanoid Robot . IEEE-RAS International Conference on Humanoid Robots, 20140000410. Source

BibTeX

@inproceedings{badger2014model,
  title = {Model-based Robotic Dynamic Motion Control for the Robonaut 2 Humanoid Robot},
  author = {Badger, Julia M. and Hulse, Aaron M. and Taylor, Ross C. and Curtis, Andrew W. and Gooding, Dustin R. and Thackston, Allison},
  booktitle = {IEEE-RAS International Conference on Humanoid Robots},
  number = {20140000410},
  pages = {62-67},
  institution = {NASA},
  year = {2014},
  doi = {10.1109/humanoids.2013.7029956},
  abstract = {Robonaut 2 (R2), an upper-body dexterous humanoid robot, has been undergoing experimental trials on board the International Space Station (ISS) for more than a year. R2 will soon be upgraded with two climbing appendages, or legs, as well as a new integrated model-based control system. This control system satisfies two important requirements; first, that the robot can allow humans to enter its workspace during operation and second, that the robot can move its large inertia with enough precision to attach to handrails and seat track while climbing around the ISS. This is achieved by a novel control architecture that features a joint-level embedded impedance control law which is tightly interfaced with a kinematic and dynamic coordinated control system that resides on centralized processors. This paper presents the integrated control algorithm as well as several test results that illustrate R2's safety features and performance.}
}

Wavrunek, T. A. (2021). Active Gravity Offloading System with Infrared Tracking for Rover Testing . Michigan Technological University. Source

BibTeX

@mastersthesis{wavrunek2021active,
  title = {Active Gravity Offloading System with Infrared Tracking for Rover Testing},
  author = {Wavrunek, Travis A.},
  publisher = {Michigan Technological University},
  school = {Michigan Technological University},
  type = {Master of Science report, Mechanical Engineering},
  year = {2021},
  doi = {10.37099/mtu.dc.etdr/1213},
  abstract = {Gravity offloading is a tool used to test how different gravitational forces will impact the mobility of rovers bound for Lunar or Martian expeditions. Previous approaches have been successful in simulating partial gravity environments, and this report details how the Infrared- Gravity offload (IRGO) system, developed for the Planetary Surface Technology Development Laboratory (PSTDL) and lunar simulant sandbox, has a similar aim. Through a series of iterations, IRGO has been developed to actively track an infrared beacon and follow a rover within the test chamber to eliminate inertial and friction forces along two horizonal axes. A portion of a rover’s weight is offloaded using a passive counterweight system to provide a third translational degree of freedom. Future plans to incorporate a lightweight gimbal as well as an active vertical axis are also discussed as solutions to improving the IRGO system.}
}

Ogilvie, R., Swartout, B., Hoffmann, B., McFarland, S. and Hickox, L. (2026). Mars Spacesuit Mass Requirements . International Conference on Environmental Systems. Source

BibTeX

@inproceedings{ryan2026,
  title = {Mars Spacesuit Mass Requirements},
  author = {Ogilvie, Ryan and Swartout, Ben and Hoffmann, Bradley and McFarland, Shane and Hickox, Lauren},
  booktitle = {International Conference on Environmental Systems},
  publisher = {International Conference on Environmental Systems},
  year = {2026},
  doi = {10.32865/2346/108847},
  abstract = {The gravity on Mars is 3/8th of Earth gravity. The latest exploration Extravehicular Mobility Unit (xEMU) designed for lunar exploration is just under 400 lbm. While it would be reasonable for a crewmember to operate the xEMU on Earth’s Moon with 1/6 gravity, related studies have demonstrated that this same suit would not be feasible to operate on the surface on Mars. Studies involving Martian offloading are limited and the few available have demonstrated difficulty in suit operation with current advanced suit prototypes. Furthermore, deconditioning due to a long-term journey to Mars is expected to affect the overall suit mass that a crewmember can operate. This initial study aims to provide an initial mass requirement to bound the Mars spacesuit design within a reasonable range. It is expected that detailed Mars-simulation testing will subsequently refine the estimate to support Martian mission timelines. Considerations such as crew deconditioning due to the long exposure to microgravity, pressure garment system (PGS) weight offloading, restricted mobility increasing metabolic rate, and human fitness and size are combined into an initial assessment. Based on preliminary considerations, 230 lbm is recommended as a starting maximum Mars suit mass, which will be split between the PGS and Portable Life Support System (PLSS) subsystems.}
}

Arm, P., Fischer, O., Church, J., Fuhrer, A., Kolvenbach, H. and Hutter, M. (2025). Efficient Learning-Based Control of a Legged Robot in Lunar Gravity . arXiv. Source

BibTeX

@article{arm2025efficient,
  title = {Efficient Learning-Based Control of a Legged Robot in Lunar Gravity},
  author = {Arm, Philip and Fischer, Oliver and Church, Joseph and Fuhrer, Adrian and Kolvenbach, Hendrik and Hutter, Marco},
  journal = {arXiv},
  year = {2025},
  doi = {10.48550/arxiv.2509.10128},
  abstract = {Legged robots are promising candidates for exploring challenging areas on low-gravity bodies such as the Moon, Mars, or asteroids, thanks to their advanced mobility on unstructured terrain. However, as planetary robots' power and thermal budgets are highly restricted, these robots need energy-efficient control approaches that easily transfer to multiple gravity environments. In this work, we introduce a reinforcement learning-based control approach for legged robots with gravity-scaled power-optimized reward functions. We use our approach to develop and validate a locomotion controller and a base pose controller in gravity environments from lunar gravity (1.62 m/s2) to a hypothetical super-Earth (19.62 m/s2). Our approach successfully scales across these gravity levels for locomotion and base pose control with the gravity-scaled reward functions. The power-optimized locomotion controller reached a power consumption for locomotion of 23.4 W in Earth gravity on a 15.65 kg robot at 0.4 m/s, a 23 % improvement over the baseline policy. Additionally, we designed a constant-force spring offload system that allowed us to conduct real-world experiments on legged locomotion in lunar gravity. In lunar gravity, the power-optimized control policy reached 12.2 W, 36 % less than a baseline controller which is not optimized for power efficiency. Our method provides a scalable approach to developing power-efficient locomotion controllers for legged robots across multiple gravity levels.}
}

Spady, A. A. J. and Krasnow, W. D. (1966). Exploratory Study of Man's Self-Locomotion Capabilities with a Space Suit in Lunar Gravity . NASA Langley Research Center, NASA-TN-D-2641. Source

BibTeX

@techreport{spady1966exploratory,
  title = {Exploratory Study of Man's Self-Locomotion Capabilities with a Space Suit in Lunar Gravity},
  author = {Spady, Amos A., Jr. and Krasnow, William D.},
  number = {NASA-TN-D-2641},
  institution = {NASA Langley Research Center},
  year = {1966},
  url = {https://ntrs.nasa.gov/citations/19660006606}
}

Niksirat, P., Daca, A. and Skonieczny, K. (2020). The effects of reduced-gravity on planetary rover mobility . IEEE Transactions on Nuclear Science, 7. Source

BibTeX

@article{niksirat2020effects,
  title = {The effects of reduced-gravity on planetary rover mobility},
  author = {Niksirat, Parna and Daca, Adriana and Skonieczny, Krzysztof},
  journal = {IEEE Transactions on Nuclear Science},
  volume = {39},
  number = {7},
  pages = {797-811},
  publisher = {SAGE Publications},
  year = {2020},
  doi = {10.1177/0278364920913945},
  abstract = {One of the major challenges faced by planetary exploration rovers today is the negotiation of difficult terrain, such as fine granular regolith commonly found on the Moon and Mars. Current testing methods on Earth fail to account for the effect of reduced gravity on the soil itself. This work characterizes the effects of reduced gravity on wheel–soil interactions between an ExoMars rover wheel prototype and a martian soil simulant aboard parabolic flights producing effective martian and lunar gravitational accelerations. These experiments are the first to collect wheel–soil interaction imagery and force/torque sensor data alongside wheel sinkage data. Results from reduced-gravity flights are compared with on-ground experiments with all parameters equal, including wheel load, such that the only difference between the experiments is the effect of gravity on the soil itself. In lunar gravity, a statistically significant average reduction in traction of 20% is observed compared with 1 g, and in martian gravity an average traction reduction of 5–10% is observed. Subsurface soil imaging shows that soil mobilization increases as gravity decreases, suggesting a deterioration in soil strength, which could be the cause of the reduction in traction. Statistically significant increases in wheel sinkage in both martian and lunar gravity provide additional evidence for decreased soil strength. All of these observations (decreased traction, increased soil mobilization, and increased sinkage) hinder a rover’s ability to drive, and should be considered when interpreting results from reduced-load mobility tests conducted on Earth.}
}

Daca, A. (2023). Predicting Planetary Rover Mobility in Reduced Gravity Using 1-g Experiments . Unpublished. Source

BibTeX

@phdthesis{daca2023predicting,
  title = {Predicting Planetary Rover Mobility in Reduced Gravity Using 1-g Experiments},
  author = {Daca, Adriana},
  journal = {Unpublished},
  school = {Concordia University},
  address = {Montreal, Quebec, Canada},
  type = {PhD thesis},
  month = {February},
  year = {2023},
  doi = {10.13140/rg.2.2.12718.28485},
  abstract = {Traversing granular regolith, especially in reduced gravity environments, remains a potential challenge for wheeled rovers. Mitigating hazards for planetary rovers requires testing in representative environments, but direct Earth-based testing fails to account for the effect of reduced gravity on the soil itself. Here, experimental apparatus and techniques for reduced-gravity flight testing are used to systematically evaluate three existing Earth-based testing methods and develop guidelines for their use and interpretation: (i) reduced-weight testing, (ii) matching soil testing instrument response through soil simulant design, and (iii) granular scaling laws (GSL). \nExperimentation campaigns flying reduced-gravity parabolas, with soil and wheel both in lunar-g, have shown reductions in net traction of 20% or more and increases in sinkage of up to 40% compared to Earth-based testing methods (i) and (ii). Scaled-wheel testing, according to GSL (method iii) has shown better agreement with reduced-g tests (less than 10% error) and also tends to err on the side of conservative predictions. \nLimitations of GSL are investigated including a recently proposed cohesion constraint (that the wheel radius ratio must be the inverse of the gravity ratio) and the effects of wheel size and aspect ratio on GSL’s accuracy. It was found that the cohesion constraint can most likely be ignored for mildly cohesive soils such as lunar regolith. Limits on wheel sizes and aspect ratio variation are also proposed. \nThe application of GSL to planetary rover testing is demonstrated through two studies undertaken in collaboration with NASA’s Jet Propulsion Laboratory. One study compares wheel designs for a skid-steer lunar rover in single-wheel tests scaled by GSL, demonstrating that diagonal grousers improve turning performance without requiring larger wheels. The second study involves application of GSL to the design of two reconfigurable test platforms for evaluating steep-terrain mobility performance. \nAnother aspect of rover mobility testing—normal force control in single-wheel testbeds—is also investigated. An improved method for single-wheel testing, using a 4-bar mechanism, essentially eliminates normal force oscillations from frictional vertical sliders. \nFinally, guidelines for conducting and interpreting 1-g mobility tests for lunar rovers are presented, and potential avenues for future research are outlined.}
}