ATOMMS (Active Temperature, Ozone and Moisture Microwave Spectrometer)
ATOMMS is a NASA and NSF (National Science Foundation) funded limb-viewing occultation instrument - a technology demonstration project under development as of 2008 - representing a cross observation scheme between the GPS radio occultation (RO) and the Microwave Limb Sounder (MLS) techniques. ATOMMS is a cm and mm wavelength satellite-to-satellite occultation technique. The overall objective is to provide an unprecedented, long-term determination of the state of Earth’s troposphere and middle atmosphere - to address fundamental climate monitoring and process study needs. 1) 2) 3) 4) 5) 6)
The ATOMMS observations will yield a quantum step in performance relative to passive observations in terms of vertical resolution, precision and accuracy of moisture, ozone, temperature and pressure measurements in both clear and cloudy conditions. Typical precisions of the ~200 m vertical resolution temperature, geopotential height and moisture profiles will be ~0.4 K, 10 m and 1-3% respectively, extending from near the surface to the mesopause (ionospheric effects at mm-wavelengths are negligible). The 1-3% precision ozone profiles will extend from the upper troposphere into the mesosphere.
With additional signal frequencies, other trace constituents such as water isotopes, can be measured in the upper troposphere and above with similar performance. The system also measures cloud liquid water and ice content. Analysis indicates performance in cloudy conditions will be no more than a factor of two worse than clear sky performance. With averaging, the accuracy of the temperature and pressure profiles will be one to two orders of magnitude beyond the precision of the individual profiles. The accuracy of the moisture and ozone profiles will be at least as good as the individual profile precision and may be significantly better depending on our spectroscopic knowledge. The system is self-calibrating and experiences no drift because the signal source is viewed either immediately before or after each occultation.
Background: The ATOMMS concept has its roots in the AMORE (Atmospheric Moisture and Ocean Reflection Experiment) project proposed to NASA in 1998 as an ESSP (Earth System Science Pathfinder) mission (AMORE included the 22 GHz portion of the ATOMMS occultation concept). While AMORE was technically too immature for selection, NASA did fund in the same year the ATOMS (Atmospheric Temperature, Ozone and Moisture Sounder) instrument development, a joint effort between the University of Arizona and JPL (Jet Propulsion Laboratory) that included both the 22 and 183-195 GHz occultation concepts via its IIP (Instrument Incubator Program).
NSF has funded the ongoing development and refinement of the ATOMMS concept since 2001 including funding in 2007 of the ATOMMS prototype instrument development and the high altitude aircraft-to-aircraft occultation demonstration phase.
Partners in the ATOMMS project are the University of Arizona in Tucson, AZ, NASA/JPL in Pasadena, CA, The Aerospace Corporation in El Segundo, CA, and the Southern Research Institute in Birmingham, AL, USA.
After the ATOMMS performance has been properly demonstrated, the long-range concept foresees eventually a constellation of microsatellites in support of climate and NWP (Numerical Weather Prediction) services.
The measurement technique of ATOMMS is considered effective in particular in the UTLS (Upper Troposphere/Lower Stratosphere) regime where water vapor and ozone are radiatively very important constituents. The new observing system offers the capability of estimating several key climate state variables and to determine how the climate system is truly evolving, independently of atmospheric models. Key open questions in climate research can be addressed such as:
- Is the upper troposphere warming faster than the lower troposphere and the surface?
- Where is the transition between tropospheric warming and stratospheric cooling? - and the closely related question: How are lapse rates adjusting to the changes in vertical heating and dynamical feedbacks associated with climate change?
Challenges in the ATOMMS measurement concept development involve the following topics:
• Requires new transmitters in orbit
• Pointing (high SNR requires directional antennas)
• High amplitude stability
• Sampling density vs. cost of additional transmitters & receivers in space
• Enhanced sensitivity to turbulence
• Separate water vapor from liquid water clouds.
ATOMMS airborne instrument on NASA's WB-57F high altitude aircraft:
ATOMMS represents a new class of active, airborne, limb-viewing spectrometer that is a cross between Global Positioning System (GPS) occultations and NASA’s MLS (Microwave Limb Sounder). The objective is to characterize atmospheric water vapor and ozone by actively probing via radio occultation the absorption lines at 22.2 GHz, 183.3 GHz and 195 GHz, respectively. The analysis shows that ATOMMS will profile tropospheric and middle atmosphere water vapor and middle atmosphere ozone to 1-5%, temperature to 0.5 K, and geopotential heights to 10-20 m, all with ~200 m vertical resolution, in both clear and cloudy air. This unprecedented performance will improve significantly with averaging. Because the occultation signal source is observed immediately before or after each occultation, ATOMMS is self-calibrating, which eliminates long-term drift. These capabilities will fulfil crucial needs for climate change monitoring, research and policymaking. 7)
Two ATOMMS prototype instruments have been developed at the University of Arizona (with NSF funding) for the WB-57F high altitude aircraft to demonstrate their performance. The ATOMMS instrumental configuration is depicted in Figure 1. The ATOMMS system consists of 5 elements:
1) The ATOMMS microwave instruments with 13 GHz, 22 GHz and 183 GHz transmitters and receivers
2) ATOMMS precise positioning system which is a combination of hardware consisting of a GPS receiver and a 3 axis precision accelerometer on each aircraft combined with precise positioning system software from JPL
3) The two WB57F aircraft
4) The WAVE gimbal built by SRI for NASA that points the ATOMMS microwave instrument and
5) The ATOMMS retrieval software system under development at the University of Arizona.
During an occultation, each ATOMMS microwave transmitter radiates several monochromatic signal tones that pass through the atmosphere to the receiver on the opposite side of the atmosphere which digitizes and records the signals. The ATOMMS transmitters and receivers are designed to simultaneously sample water vapor at both the 22 and 183 GHz lines to create the dynamic range needed to profile water vapor from the surface into the mesosphere as well as measure ozone at 195 GHz in the upper troposphere and middle atmosphere. The ATOMMS signal processing system later derives the phase and amplitude of the signals and combines them with the precise knowledge of the transmitter and receiver positions (from the ATOMMS precise positioning subsystem) and physical constrains such as the hydrostatic equation to derive profiles of atmospheric moisture, ozone, temperature and pressure.
Figure 1: Block diagram of the ATOMMS A aircraft. With the exception of the 22 & 183 GHz transmitter and receiver pairs, ATOMMS B is identical (image credit: University of Arizona)
Accuracy and Vertical Resolution of Temperature and Water Vapor Profiles: ATOMMS unique observations & parameter retrievals are very well suited for monitoring climate change and will provide a new window into the atmosphere strongly constraining thermodynamic & dynamic processes needed to assess and improve the realism of climate models. The unprecedented combination of performance includes:
• High precision profiles of temperature to 0.4 K, water vapor to 1-10% and geopotential height to ~10-20 m extending through the free troposphere to the mesopause and ozone to 1-10% through the middle atmosphere, whose accuracy should be better by an order of magnitude or more when many profiles are averaged.
• ~200 m vertical resolution, as demonstrated by GPS occultation missions, that exceeds the vertical resolution of passive systems (e.g., AIRS, IASI, AMSU and MLS) by approximately an order of magnitude or more.
• Self calibration because ATOMMS measures differential absorption and the signal sources are measured immediately before or after each occultation which eliminates drift and should provide absolute accuracy.
• Retrievals in both clear and cloudy conditions with performance in clouds expected to be within a factor of 2 of clear sky performance, thus eliminating clear sky biases that plague other remote sensing systems.
• Full sampling of the diurnal cycle every orbit with a satellite constellation like the COSMIC GPS RO (Radio Occultation) mission.
• Refinement of the spectroscopy from orbit.
• ATOMMS ability to estimate the climate state independent of atmospheric models, an achievement that is simply not possible with passive radiometric sensors, and yet is fundamental to both determining the true climate state and quantifying climate and weather model performance and realism.
ATOMMS provides the combined vertical resolution and precision critical to resolving the 1.5 km scale height of water vapor and fundamental vertical structure such as ubiquitous layering in the troposphere with vertical scales of a few hundred meters. Only RO can globally determine temperature and lapse rates at the sharp vertical scales at which they vary and can do so in both clear and cloudy conditions. While accurate GPS-RO temperatures are limited to the upper troposphere (by moisture) through the mid-stratosphere (by the ionosphere), ATOMMS will accurately determine temperature and vertical stability from the free troposphere through the mesosphere.
Another key point is that GPS-RO measures temperature or water vapor, not both. GPS-RO has shown some of the potential for RO observations to measure water vapor in the warmer regions of the lower and middle troposphere with accuracies of 0.2 – 0.5 g/kg. ATOMMS will extend this dynamic range by orders of magnitude to precisely profile water vapor over mixing ratios ranging from several percent in the lower troposphere to a few ppm at the mesopause while simultaneously profiling temperature to sub-Kelvin precision over the same altitude interval. With averaging, the project anticipates the absolute accuracies will be better by an order of magnitude or more (depending on spectroscopy which will be refined with ATOMMS).
The ATOMMS microwave instrument has been designed to take advantage of off-the-shelf telecommunications technology whenever possible, particularly for the 22 GHz channel. The basic instrument design uses very similar circuits for all channels. Figure 2 shows the block diagram of the 22 GHz transmitter and receiver. The transmitter uses eight separate phase locked YIG oscillators to generate the tones. These tones are individually power-monitored before they are power combined. A single amplifier then amplifies these eight tones to a level of ~100 mW per tone.
Figure 2: The ATOMMS 22 GHz transmitter (top) and receiver (bottom) subsystems. Two of the eight channels are shown in each block diagram for clarity (image credit: University of Arizona)
Since ATOMMS measurements are effected by differential amplitude noise, a common power amplifier is used for all tones to attenuate differential amplitude fluctuations. The receiver amplifies all eight received tones simultaneously for the same reason. The amplified signal is then power divided into eight channels. Bandpass filters in each channel isolate a single received tone. These tones are then mixed with LO signals generated by YIG phase locked oscillators fed with a reference from a DDS synthesizer. This synthesizer is used to offset the frequency of the LO, generating a ~ 40 kHz IF frequency. The low frequency IF is then low pass filtered and amplified with a low noise audio frequency amplifier. The IF is then fed into a National Instruments Compact RIO (Reconfigurable Input/Output) realtime data acquisition system, where the time domain waveform is digitized and recorded.
This data acquisition system has been shown to operate at ambient pressure in the WB-57F in previous experiments. The 13 GHz reference tone transmitter and receiver are identical to the 22 GHz system, but with a single transmitted and received tone rather than eight.
Figure 3: Block diagrams of the ATOMMS 183 GHz transmitter (top) and receiver (bottom), image credit: University of Arizona
The 183 GHz subsystem is based on a two tone transmitter and subharmonically pumped Schottky mixer receiver front end from Virginia Diodes. The transmitters each provide 40 mW of power from 180-203.5 GHz, and are power combined using a waveguide magic tee. Power monitoring diodes before the magic tee record the transmitted power level of each channel, for later removal of differential amplitude effects. After power combining, the transmitted power is ~20 mW per tone. The subharmonically pumped Schottky receiver has a measured noise temperature of ~1100K, and is flat across the band. A low noise amplifier with a 1-12 GHz bandwidth relays the IF signal to a downconverter module. The receiver IF downconverter is identical in architecture to the 22 GHz receiver system with the exception that tunable synthesizers are used to generate the LO signals rather than fixed tuned oscillators. Block diagrams of the 183 GHz subsystem are shown in Figure 3. Figure 4 shows the 183 GHz transmitter system mounted to the ATOMMS-A rear plate.
Figure 4: Photo of the ATOMMS 183 GHz transmitter system and 22 GHz receiver feed (image credit: University of Arizona)
The ATOMMS antenna system uses a pair of coaxially mounted feedhorns to illuminate a single 30 cm diameter high density polyethylene lens, anti-reflection grooved for operation at 183 GHz.
The ATOMMS instrument package mechanical aspects are as highly engineered as the electronics. Past experience in flying complex research instruments in the WB-57F aircraft have shown that a fairly sophisticated minimum level of integration of structure, power, thermal, vibration, low pressure and various other design factors are required to build a successful instrument.
The ATOMMS instrument design, shown in Figure 5, was engineered down to the level of fasteners, connectors and wiring using 3D Computer Aided Drafting (CAD) software before any manufacturing. Figure 6 shows the ATOMMS-A instrument, completely assembled and awaiting system testing.
Figure 5: CAD model of one of the ATOMMS instruments in the SRI WAVE gimbal (image credit: University of Arizona)
Figure 6: The fully assembled ATOMMS A instrument. Visible components are labeled. The 183 GHz Tx and 22 GHz Rx modules are not visible (image credit: University of Arizona)
Data acquisition duties are handled by National Instruments Compact RIO systems. These small chassis can be loaded with up to eight multifunction interface modules to handle science signal and housekeeping digitization, digital I/O and accelerometers readout. Each Compact RIO system communicates with a PC over two dedicated Ethernet cables. These PCs are mounted in a partially pressurized part of each aircraft, just behind the moving portion of the gimbal. Each PC is equipped with large capacity solid state hard drives, and runs Labview Realtime OS. These computers receive the data collected by the Compact RIO system mounted directly on the ATOMMS instruments and record the data to disk. They also are responsible for collecting the GPS observables from the JPL provided GPS receiver used as part of the Precise Positioning System.
PPS (Precise Positioning System): The ATOMMS system will profile atmospheric temperature, humidity and pressure. Air temperature and barometric pressure, in a dry atmosphere, are derived from a profile of refractivity that is derived from a profile of bending angle derived in turn from a profile of Doppler shift versus time. The determination of the atmospheric absolute humidity profile requires the analysis of the vertical profile of atmospheric water vapor, and this is the most important contribution of ATOMMS when compared to the GPS-RO technique that probes only the real part of the atmospheric index of refraction.
In the aircraft to aircraft occultations, the atmospheric Doppler shift is much smaller than for the spacecraft occultation case because the aircraft move much slower (~200 m/s) than the spacecraft (several km/s). At the uppermost altitudes, just below the altitude of the aircraft, the atmospheric bending angle is quite small. Therefore the atmospheric Doppler shift is quite small. In order to precisely determine atmospheric temperature and pressure, the ATOMMS system must measure very small bending angles at high altitudes. The system goal is to estimate the motion of the aircraft to an accuracy of 0.1 mm/s.
The ATOMMS PPS consists of accelerometers and GPS receiver on each aircraft. Positions can be estimated very accurately from the GPS receiver data about every 100 seconds. In profiling the atmosphere via the ATOMMS occultations, the project determined to use integration times of ~10 seconds or less. To achieve the high vertical resolution and performance over these short intervals, it was determined that low-noise and very accurate accelerometers must be used. Essentially the precise reconstruction of the time-varying aircraft positions and velocities will integrate the acceleration measured by the accelerometers to obtain the velocities of the two ends of the ATOMMS instrument. The GPS receiver data will essentially be used to estimate the bias and scale factor of the accelerometers. Extremely low-noise accelerometers (Endevco Model 86), developed for seismic research, were selected for the ATOMMS experiment after extensive analysis by the ATOMMS team at the University of Arizona and JPL.
High performance GPS receivers have been selected that could satisfy the ATOMMS requirements that were also familiar to JPL. The receivers already in the WB-57F aircraft were deemed insufficient to deliver the quality of phase data needed. JPL suggested a high performance Ashtech receiver that they use for other applications.
NASA/SRI (Southern Research Institute) WAVE Gimbal Pointing System: The ATOMMS experiment takes advantage of NASA’s WAVE (WB-57F Ascent Video Experiment) system, designed to optically image the Space Shuttle during launch. This system is a complete replacement nose for the WB-57F, containing a 2-axis gimballed pointing system capable of 0.25º pointing accuracy. The system also contains an optical telescope with a high definition video camera and recorder. The ATOMMS microwave sensors replace this optical imaging package, but still use the replacement nose and gimbal. The optical window will be replaced with a microwave-transparent radome manufactured by Nurad corporation.
ATOMMS does present several challenges for pointing and integration with the WAVE system. The ATOMMS instrument must be adequately balanced, and within mass limits for the gimbal. More importantly, ATOMMS is not an imaging detector, so pointing cannot be done with image recognition. In addition, the atmospheric attenuation effects we wish to measure will not allow pointing based on feedback from the microwave signal strength. Any atmospheric fluctuations would be interpreted as a pointing error, and would cause the pointing loop to become unstable. We have therefore developed, jointly with SRI (Southern Research Institute) a pointing system based on GPS coordinates (Ref. 7).
1) E. R. Kursinski, D. Ward, A. Otarola, C. Groppi, S. Albanna, M. Shein, “The Active Temperature Ozone and Moisture Microwave Spectrometer (ATOMMS) Climate Observing System,” XXIX URSIGA 2008 (International Union of Radio Science - General Assembly), Chicago, Illinois, USA, Aug. 7-16, 2008, URL: http://cnfrs.institut-telecom.fr/pages/pages_ursi/URSIGA08/papers/GFp5.pdf
2) E. R. Kursinski, D. Ward, A. Otarola, B. Herman, C. Groppi, C. Walker , M. Schein, S. Albanna, D. Rind, “The Active Temperature, Ozone, and Moisture Microwave Spectrometer (ATOMMS) - A LEO-LEO Occultation Observing System,” COSMIC Workshop, Boulder CO, USA, Oct. 24, 2007, URL: http://www.cosmic.ucar.edu/oct2007workshop/pdf/kursinski_24.pdf
3) E. R. Kursinski, C. Groppi, C. Walker, D. Ward, W. Bertiger, H. Pickett, M. Ross, “Active Temperature, Ozone and Moisture Microwave Spectrometer (ATOMMS),” July 2007, URL: http://map.nasa.gov/documents/CLARREO/ATOMMS_2page_Jul2007.pdf
4) E. R. Kursinski, D. Ward, A. Otarola, B. Herman, C. Groppi, C. Walker , M. Schein, S. Albanna, D. Rind, “The Active Temperature, Ozone, and Moisture Microwave Spectrometer (ATOMMS) - A LEO-LEO Occultation Observing System,” COSMIC Workshop, Boulder, CO, USA, Oct. 24, 2007, URL: http://www.cosmic.ucar.edu/oct2007workshop/pdf/kursinski_24.pdf
5) E. R. Kursinski, “ATOMMS: A Next-Generation Occultation System for Earth,” URL: http://www.atmo.arizona.edu/personalpages/kursinsk/NextGenOccEarth.htm
6) E. R. Kursinski, D. Ward, A. Otarola, K. Sammler, R. Frehlich, D. Rind, C. Groppi, S. Albanna, M. Shein, W. Bertiger, H. Pickett, M. Ross, “The Active Temperature Ozone and Moisture Microwave Spectrometer (ATOMMS),” ECMWF GRAS SAF Workshop on Applications of GPS Radio Occultation Measurements, June 16-18, 2008, Reading, UK, URL: http://www.ecmwf.int/publications/library/ecpublications/_pdf/workshop/2008/gras_saf/Kursinski.pdf
7) E. Robert Kursinski, Abram Young, Angel Otarola, Michael Stovern, Brian Wheelwright, Dale Ward, Kate Sammler, Robert Stickney, Christopher Groppi, Sarmad Al Banna, Michael Schein, Steve Bell, Willy Bertiger, Mark Miller, Herb Pickett, “Laboratory and Ground Testing Results from ATOMMS: the Active Temperature, Ozone and Moisture Microwave Spectrometer,” 21st International Symposium on Space TeraHertz Technology, Oxford, UK, March 23-25, 2010, URL: http://www.nrao.edu/meetings/isstt/papers/2010/2010186194.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.