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Satellite Missions Catalogue

GLOWS (Global L-band Observatory for Water Cycle Studies)

Jan 18, 2024

EO

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NASA

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Considered

Global L-band Observatory for Water Cycle Studies (GLOWS) is a new mission concept proposed by NASA that will use an L-band radar and radiometer to measure ocean salinity, sea ice thickness, vegetation water content, and ocean surface winds. GLOWS will address the need for a future L-band mission to maintain data continuity at the same resolution and accuracy as the European Space Agency’s (ESA) Soil Moisture and Ocean Salinity (SMOS) and the Soil Moisture Active Passive (SMAP) missions.

Quick facts

Overview

Mission typeEO
AgencyNASA
Mission statusConsidered

GLOWS mission

Overview

L-band observations have proven useful for estimating soil moisture and ocean salinity variables in previous missions such as the European Space Agency’s (ESA) Soil Moisture and Ocean Salinity (SMOS) and the Soil Moisture Active Passive (SMAP) missions. L-band refers to a portion of the electromagnetic (EM) spectrum with frequencies ranging from approximately 1-2 GHz, or 30-15 cm in wavelength. NASA has proposed a new mission concept, the Global L-band Observatory for Water Cycle Studies (GLOWS), which aims to provide L-band data continuity at the same resolution and accuracy as SMOS and SMAP, from a lower cost and smaller platform.  1) 2)

GLOWS is being developed by a team comprised of groups from Brigham Young University (BYU), NASA Goddard Space Flight Center (GSFC), MMA Design LLC (MMA), and Agile RF Systems LLC (ARS). 4)

The climate dynamics of the global soil moisture system are heavily influenced by the global water and energy cycles. GLOWS aims to estimate global water and energy fluxes at the land surface at SMAP resolution and accuracy. GLOWS, like SMAP, is required to achieve less than 0.4 m3/m3 (1-σ) uncertainty level in soil moisture measurements. The data from GLOWS will provide a greater understanding of the processes that link terrestrial water, energy and carbon cycles, and allow for improved predictions about floods, droughts, weather and climate forecasts. Soil moisture observations have also been shown to be important for other applications including agricultural productivity forecasts, terrain mobility, weather, wildfire risk, and water resources. 5) 6)

Spacecraft

GLOWS has been designed to match the resolution and accuracy of SMAP and SMOS at a substantially lower cost, size, and weight. Most significantly, the GLOWS satellite is projected to be less than half the weight of SMAP in terms of both mission and antenna mass. The key to maintaining the scientific value of the observations is the retention of the full six-metre antenna aperture. This is achieved by employing a deployable reflectarray meta-lens antenna with a compact and lightweight design. The compact design of the GLOWS satellite greatly simplifies the satellite bus requirements. Additionally, the satellite design utilises improvements in radar electronics to minimise the size, weight, and power (SWaP) of the radar component of the active/passive GLOWS instrument system.

Table 1: Comparison of SMAP and GLOWS missions. *Mission/Instrument Power are based on published SMAP data and GLOWS projections. The GLOWS power budget includes a 25% margin. GLOWS volume and mass budgets are preliminary and include ~50% margin.

 

SMAP

GLOWS

Mission Stowed Volume (m3)

3.81

<0.74*

Mission Power beginning of life (BOL) (kW)

2

1.5

Average Power* (W)

448

<500

Mission Mass (kg)

944

<400

Radar/Radiometer/Antenna Mass (kg)

356

124.1

Launch

Delta II Payload

Falcon-9 four-metre ESPA

Launch

Like SMAP, GLOWS will be launched into a Low Earth Orbit (LEO) of 685 km. GLOWS will perform measurements at a consistent local time of day (6am/6pm), and have global coverage every three days. The satellite will be launched on a SpaceX Falcon-9 four-metre EELV Secondary Payload Adapter (ESPA). 5)

Mission Status

  • June 20, 2023: A development status update on the completed testing for the GLOWS instrument was presented by NASA at the Earth Science Technology Forum. The GLOWS instrument has completed an instrument study, with some future design and structural improvements planned. 7)
  • 2021: GLOWS concept of operations was presented at the IEEE International Geoscience and Remote Sensing Symposium in Brussels, Belgium 1)

Sensor Complement

The GLOWS instrument is a low-cost deployable reflectarray lens antenna that will include both an L-band radar and radiometer. GLOWS will employ a novel deployable antenna system that will provide both active (radar) and passive (radiometer) L-band measurements of the Earth. The compact design of the instrument reduces spacecraft size, complexity, and cost.

Deployable Reflectarray Lens Antenna

The membrane lens antenna is lightweight with a flat geometry, and can therefore be deployed from a very small package, which greatly simplifies requirements for the hosting spacecraft compared to SMAP. The reflectarray antenna can also provide beam steering. The meta-lens reflectarray technology can support other large aperture antenna missions. 6) 

Figure 1:  Illustration of the GLOWS launch and deployed configurations. Image credit: NASA.

The antenna contains high isolation diplexers that multiplex the radiometer spectrum from 1400 to 1427 MHz, with the radar band spanning 1217 to 1298 MHz. It incorporates a tensioned aperture of flat membranes that shape and direct the Radio Frequency (RF) energy of the instruments through passive phase-shifting, which steers the beam to a 35.5° angle from the flat membrane. The six metre diameter flat antenna spins at 14.6 rpm about the nadir axis.

Table 2: Key GLOWS instrument requirements.

Parameter

Radiometer

Radar

Measurement bandwidth

1400-1427 MHz

1 MHz instantaneous bandwidth within 1217-1298 MHz

Polarisation

HH, VV, T3, T4

HH, HV, VV (not polarimetric)

Swath width

1000 km

Ground resolution

30 km, 39 km × 47 km footprint

250 m ×400 m

Antenna efficiency

90%

Pulse Repetition Frequency (PRF)

 N/A

2.85 kHz

Transmit peak power

 N/A

500 W

Relative accuracy

<1.5 K on 40 km grid

1-1.5 dB on 3 km grid

Antenna beam rotation rate

14.6 rpm

 

Ground Segment

The radar data will be processed on the ground using both a low-resolution scatterometer mode over the full swath and coherent synthetic aperture processing over the outer swath.

References  

1) Long D, Bindlish R, Piepmeier J, De Amici G, Bailey M. Global L-band Observatory for Water Cycle Studies (GLOWS). 2021 IEEE International Geoscience and Remote Sensing Symposium IGARSS, Brussels, Belgium, 2021, pp. 7779-7782. Published online 2022. doi:10.1109/igarss47720.2021.9554845, URL: https://ieeexplore.ieee.org/document/9554845 

2) NASA. Electromagnetics - NASA Science. URL: https://science.nasa.gov/learn/basics-of-space-flight/chapter6-3/ 

3) Bailey M, Long D, Bindlish R, Piepmeier J, De Amici G. Enabling Big Science in a Small Satellite - The Global L-band Observatory for Water Cycle Studies (GLOWS) Mission. 36th Annual Small Satellite Conference. 2022; SSC22-III-08, URL: https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=5315&context=smallsat 

4) Smith JN. Systems Engineering of the Global L-Band Obser Systems Engineering of the Global L-Band Observatory for W y for Water Cycle Studies. Brigham Young University BYU ScholarsArchive . Published online December 4, 2022, URL: https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=10402&context=etd 

5) Long D, Bindlish R, Piepmeier J, Bailey M. The Global L-band Observatory for Water Cycle Studies (GLOWS) - SMAP Continuity Mission. IGARSS 2022 - 2022 IEEE International Geoscience and Remote Sensing Symposium, Kuala Lumpur, Malaysia, 2022, pp. 7379-7382. Published online 2022. doi:10.1109/igarss46834.2022.9884742, URL: https://ieeexplore.ieee.org/document/9884742 

6) Long DGLG, Bindlish R, De Amici G, Piepmeier J, Bailey M. Global L-band Observatory for Water Cycle Studies (GLOWS): L-band Active/Passive Ocean Observations. IOVWST 2022. April 2022, URL: https://mdc.coaps.fsu.edu/scatterometry/meeting/docs/2022/2.04%20-%20IOVWST2022_GLOWS.pdf 

7) NASA. Enabling Big Science in a Small Satellite. June 20, 2023. URL: https://esto.nasa.gov/forums/estf2023/Presentations/S3P5_Bailey_GLOWS_ESTF2023.pdf 

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (eoportal@symbios.space).

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