HyMAS (Hyperspectral Microwave Atmospheric Sounder) /Airborne Instrument
HyMAS is being developed by MIT/LL (Lincoln Laboratory) and the NASA Goddard Space Flight Center for a flight opportunity on a NASA research aircraft. The term "hyperspectral microwave" is used to indicate all-weather sounding that performs equivalent to hyperspectral infrared sounders in clear air with vertical resolution of approximately 1 km. 1) 2)
Key objectives of the HyMAS development are:
• Hyperspectral microwave (HM) sounding has been proposed to achieve unprecedented performance
• HM operation is achieved using multiple banks of RF spectrometers with large aggregate bandwidth
• A principal challenge is size/mass/power scaling
• Objectives of this work:
- Demonstrate ultra-compact (100 cm3) 52-channel IF processor (enabler)
- Demonstrate a hyperspectral microwave receiver subsystem
- Deliver a flight-ready system to validate HM sounding.
Figure 1: Illustration of HyMAS and ATMS instrument performances (image credit: MIT/LL, NASA)
Deploying the HyMAS equipped scanhead with the existing CoSMIR (Conical Scanning Microwave Imaging Radiometer) shortens the path to a flight demonstration. Hyperspectral microwave is achieved through the use of independent RF antennas that sample the volume of the Earth's atmosphere with various frequencies, thereby producing a set of densely spaced vertical weighting functions.
Performance simulations for the HyMAS 118/183 GHz system yield surface precipitation rate and water path retrievals for small hail, soft hail, or snow pellets, snow, rainwater, etc. with accuracies comparable to those of the ATMS (Advanced Technology Microwave Sounder). Further improvements in retrieval methodology (for example, polarization exploitation) are expected. The HyMAS scanhead will include an ultra-compact IFP (Intermediate Frequency Processor) module. The IFP is fabricated with materials made of LTCC (Low-Temperature Co-fired Ceramic) technology integrated with detectors, amplifiers, A/D conversion and data aggregation. The IFP will sample 52 channels of 16 bit data comprised of four nine-channel filter banks for temperature profiles and two eight-channel filter banks for water vapor.
An engineering prototype of a nine-channel IFP was flown on the MicroMAS (Micro-sized Microwave Atmospheric Satellite) CubeSat mission (launch July 13, 2014) and a next-generation ten-channel IFP will fly on the MiRaTA (Microwave Radiometer Technology Acceleration) CubeSat mission to be launched in 2016. The HyMAS airborne sensor is expected to be ready for flight validation in early 2015.
Some background: A number of recent technology advances driven in part by the gigabit wireless communications industry, the semiconductor industry, and the NASA Earth Science Technology Office has significantly and profoundly changed the landscape of modern radiometry by enabling miniaturized, low-power, and low-noise radio-frequency receivers operating at frequencies up to 200 GHz. 3)
These advances enable the practical use of receiver arrays to multiplex multiple broad-frequency bands into many spectral channels; the atmospheric sounding benefit of such systems is explored in this article. The term hyperspectral microwave refers generically to microwave sounding systems with approximately 100 spectral channels or more. In the infrared wavelength range, the term hyperspectral is used to denote the resolution of individual, narrow absorption features that are abundant throughout the infrared spectrum. In the microwave and millimeter wavelength range, however, there are substantially fewer spectral features and the spectral widths are typically broad, and an alternate definition is therefore appropriate.
The persistent observations afforded by a geostationary platform would allow temporal sampling over most of the viewable Earth hemisphere on time scales of approximately five minutes. The capability to sound in and around storms would significantly improve both regional and global numerical weather prediction models. — The tropospheric information content of infrared observations, however, is compromised by clouds, which attenuate the radiance to space from the atmosphere below the cloud level. The ability to model and forecast hurricanes would greatly improve with microwave measurements from geostationary orbit.
A hyperspectral microwave system can be constructed by using multiple receiver banks with a replicated, but frequency-shifted, version of the template channel set used to generate Figure 2. For example, eight receiver banks with eight channels each could be used to create a 64-channel system by progressively shifting the intermediate-frequency (IF) band of each receiver by 70 MHz.
Figure 2: Hyperchannel microwave operation (image credit: NASA/GSFC, MIT/LL)
Legend to Figure 2: Left: The template temperature weighting functions for channels near 118.75 GHz at nadir incidence span the range in altitude. The 1976 U.S. Standard Atmosphere over a nonreflective surface was used in the calculations. Right: The weighting functions for 52 channels near 118.75 GHz and the eight ATMS tropospheric temperature sounding channels near 50–57 GHz at nadir incidence show a similar coverage of all altitudes as in Figure 2 (left).
"Hyperspectral" measurements allow the determination of the Earth's tropospheric temperature with vertical resolution exceeding 1 km (~ 100 channels in the microwave region). Several recent enabling technologies make HyMW (Hyperspectral Microwave) sensing feasible:
- Detailed physical/microphysical atmospheric and sensor models
- Advanced, signal-processing based retrieval algorithms
- RF receivers are more sensitive and more compact/integrated
- The key idea: Use RF receiver arrays to build up information in the spectral domain (versus spatial domain for STAR systems).
Figure 3: Cloud penetration with microwave atmospheric sensing: The frequency dependence of atmospheric absorption allows different altitudes to be sensed by spacing channels along absorption lines (image credit: NASA/GSFC, MIT/LL)
Legend to Figure 3: There is a clear distinction between water vapor and no water vapor in the microwave/millimeter-wave absorption spectrum. Two calculations for the percent transmission (nadir view) using the 1976 U.S. Standard Atmosphere data are shown, one assuming no water vapor and one assuming 1.5 g/cm2 (15 mm).
HyMAS overview and system components:
• HyMAS comprises multiple receivers at 118.75GHz (oxygen absorption line) and 183.31 GHz (water vapor absorption line)
• Independent RF antenna/receiver arrays sample same volume of the earth's atmosphere at slightly different frequencies
• Yields a set of dense finely spaced vertical weighting functions via frequency multiplexing
• HyMAS will be integrated into a scanhead compatible with the NASA/GSFC CoSMIR to facilitate demonstration and performance characterization
• The limited volume of the existing CoSMIR scanhead requires an ultra compact receiver system (the ultra compact 52-channel IF Processor is a key technology development).
• Hyperspectral microwave operation is achieved by replicating an 8-channel receiver multiple times with slight frequency shift
• Channel center frequency is shifted by 70 MHz
• Template weighting function of single receiver replicated into an aggregate set of eight receivers.
Figure 4: Block diagram of HyMAS (image credit: NASA/GSFC, MIT/LL)
Figure 5: Photo of the 10-channel CoSMIR scanhead assembly (image credit: NASA/GSFC, MIT/LL)
IFP (Intermediate Frequency Processor) module:
• IF processor functions
- Amplify, channelize and detect 18-29 GHz IF bands (52 channels)
- Post-detection filtering, A/D conversion, data processing
• Scalable in number of channels, processing capability
• LTCC (Low-Temperature Co-fired Ceramic) microwave filters for high performance, small size
- Assess state of technology for more aggressive (frequency, bandwidth) designs and more compact structures
• COTS parts for availability, low cost
- Microwave MMICs (Monolithic Microwave Integrated Circuits)
- Analog/digital ICs and passives
• Ultra-compact form factor (10 x 10 x 1 cm3) and low DC power requirement (<100 mW/ch) drives the architecture and design
- Leverage high performance miniature microwave filters, COTS MMICs, electronics packaging.
IFP 9-channel prototype summary:
• Demonstrated 9-channel IF processor
- Scalable to larger numbers of channels
- Post-detector analog processing and noise characterization
- Data conversion and processing and software
• High frequency LTCC technology characterized for microwave filters
- Tolerances within expectations confirming viability of representative circuits up to ~ 30 GHz
- Very good measured SIW filter results
Figure 6: Prototype PCB of a 9-channel IFP (image credit: NASA/GSFC, MIT/LL)
Legend to Figure 6:
• A PCB (Printed Circuit Board) is composed of FR4 and Rogers RO4350B material
• Top: digital, low frequency analog (post-detector), DC power filtering
• Bottom: 18-30 GHz RF (IF) amplifier, multiplexer, and detectors with cavities for LTCC filters, MMICs, and GaAs passives.
Receiver FEE (Front End Electronics):
Figure 7: HyMAS system overview (image credit: NASA/GSFC, MIT/LL)
• Four F-band receivers (108 – 119 GHz)
- 9 IF Channels each
- 22.6 GHz DRO
• Two G-band receivers (172 – 183GHz)
- 8 IF channels each
- 38.5 GHz DRO
• Each receiver has an integrated IF amplifier with passband 18 – 29 GHz
• Four COTS F-band low noise RF amplifiers (Noise Figure < 5 dB)
• G-band low-noise amplifiers
- Space allocated in design
- GSFC internal development
- SBIR development through Virginia Diodes, Inc.
Figure 8: Photo of the HyMAS receivers (image credit: NASA/GSFC, MIT/LL)
HyMAS antenna subsystem:
• One at 183 GHz
- Bandwidth 172-183 GHz
- Beamwidth: 3.1 – 3.3o over the bandwidth
- Sidelobes: ~30 dB below main lobe
- VSWR: <1.5:1
- Polarization: dual linear.
• Two at 118 GHz
- Bandwidth 108-119 GHz
- Beam width: 3.1 – 3.3o over the bandwidth
- Side lobes: ~25 dB below main lobe
- VSWR: <1.5:1
- Polarization: dual linear.
Figure 9: Gaussian optics lens antenna with wire grid to separate polarizations (image credit: NASA/GSFC, MIT/LL)
Figure 10: HyMAS scanhead computer configuration (image credit: NASA/GSFC, MIT/LL)
Figure 11: HyMAS scanhead mechanical integration (image credit: NASA/GSFC, MIT/LL)
• The hyperspectral microwave receiver offers profound atmospheric sounding performance in a small package
• IFP technology offers two order of magnitude improvement in the size of the radiometer back end
- Enables CubeSat/smallsat implementation
- Enables hyperspectral microwave operation with very large aggregate bandwidth (necessary for optimum performance)
• Complete airborne sensor ready for demonstration flights in early 2015
• Technology infusion already started (MicroMAS, MiRaTA, NAST-M, and others).
1) W. Blackwell, C. Galbraith, L. Hilliard, P. Racette, E. Thompson, "Technology Development for a Hyperspectral Microwave Atmospheric Sounder (HyMAS)," ESTF 2014 (Earth Science Technology Forum), Leesburg, VA, USA, Oct. 28-30, 2014
2) L.M. Hilliard, P. E. Racette, W. Blackwell, C. Galbraith, E. Thompson, "Hyperspectral Microwave Atmospheric Sounder (HyMAS) Architecture and Design Accommodations," IEEE Aerospace Conference, Big Sky, MT, March 2-9, 2013, URL of presentation:http://tinyurl.com/lodaeg4
3) William J. Blackwell, R. Vincent Leslie, Michael L. Pieper, Jenna E. Samra, "All-Weather Hyperspectral Atmospheric Sounding," Lincoln Laboratory Journal, Volume 18, Number 2, 2010, URL: https://www.ll.mit.edu/publications/journal/pdf/vol18_no2/18_2_2_Blackwell.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 (firstname.lastname@example.org).