Monitoring the Hydrologic Cycle With the PATH Mission

INV ITEDP A P E R

Monitoring the HydrologicCycle With the PATH MissionAn Earth satellite system designed to be controlled from the ground,

will aim to improve the accuracy of weather forecasting and

prediction of hurricanes and severe storms.

By Bjorn Lambrigtsen, Shannon T. Brown, Todd C. Gaier, Linda Herrell,

Pekka P. Kangaslahti, and Alan B. Tanner

ABSTRACT | The Precipitation and All-weather Temperature

and Humidity (PATH) mission is one of the NASA missions

recommended by the NRC in its recent Earth Science ‘‘Decadal

Survey.’’ The focus of this mission is on the hydrologic cycle in

the atmosphere, with applications from weather forecasting to

climate research. PATH will deploy a microwave sounder, a

passive radiometer that measures upwelling thermal radiation,

in geostationary orbit and will for the first time provide a time-

continuous view of atmospheric temperature and all three

phases of water under nearly all weather conditions. This is

possible because microwave radiation is sensitive to but also

penetrates both clouds and precipitation, as has been demon-

strated with similar sensors on low-earth-orbiting satellites.

Data from those sensors, despite observing a particular

location only twice a day, have had more impact on weather

prediction accuracy than any other type of satellite sensor, and

it is expected that PATH will have a similar impact with its

ability to continuously observe the entire life cycle of storm

systems. Such sensors have also played an important role in

climate research and have been used to estimate long-term

temperature trends in the atmosphere. An important applica-

tion of PATH data will be to improve the representation of

cloud formation, convection, and precipitation in weather and

climate models, particularly the diurnal variation in those

processes. In addition to measuring the three-dimensional

distribution of temperature, water vapor, cloud liquid water,

and ice, PATH also measures sea surface temperature under

full cloud cover. Such observations make a number of

important applications possible. Depending on the application

focus and the geostationary orbit location, PATH can serve as

anything from a hurricane and severe-storm observatory to an

El Nino observatory. A geostationary orbit offers many

advantages, as has been demonstrated with visible and

infrared imagers and sounders deployed on weather satellites,

but those sensors cannot penetrate clouds. It has not been

possible until now to build a microwave radiometer with a large

enough antenna aperture to attain a reasonable spatial

resolution from a GEO orbit. A new approach, using aperture

synthesis, has recently been developed by NASA at the Jet

Propulsion Laboratory, and that is what makes PATH possible.

Key technology enabling the large array of receivers in such a

system has been developed, and a proof-of-concept demon-

strator was completed in 2006. The state of the art in this area

is now such that PATH mission development could start in 2010

and be ready for launch in 2015, but the actual schedule

depends on the availability of funding. An option to fly PATH as

a joint NASA-NOAA mission is being explored.

KEYWORDS | Aperture synthesis; atmospheric sounding; geo-

stationary; hurricanes; hydrologic cycle; interferometer; mi-

crowave; radiometer; tropical cyclones

I . INTRODUCTION

The U.S. National Oceanic and Atmospheric Administra-

tion (NOAA) has for many years operated two weather

satellite systems, the Polar-orbiting Operational Environ-

mental Satellite system (POES), using low-earth orbiting

(LEO) satellites, and the Geostationary Operational

Environmental Satellite system (GOES), using geostation-

ary earth orbiting (GEO) satellites. Similar systems are also

Manuscript received March 18, 2009; revised July 22, 2009. First published

April 23, 2010; current version published May 5, 2010. This work was

carried out at the Jet Propulsion Laboratory, California Institute of Technology,

under a contract with the National Aeronautics and Space Administration.

The authors are with the Jet Propulsion Laboratory, California Institute

of Technology, Pasadena, CA 91109 USA (e-mail: [email protected];

[email protected]; [email protected]; [email protected];

[email protected]; [email protected]).

Digital Object Identifier: 10.1109/JPROC.2009.2031444

862 Proceedings of the IEEE | Vol. 98, No. 5, May 2010 0018-9219/$26.00 �2010 IEEE

operated by other nations. The POES satellites have beenequipped with both infrared (IR) and microwave (MW)

atmospheric sounders, which together make it possible to

determine the vertical distribution of temperature and

humidity in the troposphere even under cloudy conditions.

Such satellite observations have had a significant impact

on weather forecasting accuracy, especially in regions

where in situ observations are sparse, such as over the

southern oceans. In contrast, the GOES satellites have onlybeen equipped with IR sounders, since it has not been

feasible to build the large aperture system required to

achieve sufficient spatial resolution for a MW sounder in

GEO. As a result, and since clouds are almost completely

opaque at infrared wavelengths, GOES soundings can only

be obtained in cloud free areas and in the upper

atmosphere, above the cloud tops. The prevalence of

clouds is so highVtypically more than 80% at the spatialscales relevant to sounders [14], [22]Vthat this has limited

the effective use of GOES data in numerical weather

prediction. Full sounding capability with the GOES system

is highly desirable because of the advantageous spatial and

temporal coverage that is possible from GEO: while POES

satellites provide coverage in relatively narrow swaths, and

with revisit times of 12–24 h or more, GOES satellites can

provide continuous hemispheric or regional coverage,making it possible to monitor highly dynamic phenomena

such as hurricanes and severe continental storms. Clearly,

a combination of infrared and microwave is the most

desirable configuration, merging the cloud penetrating

capability of the microwave sounder with the higher

accuracy and spatial resolution of the infrared sounder.

The ultimate goal is therefore to put both sensors in

geostationary orbit, as has been done in low-earth polarorbit.

The microwave observations are also important for

climate and atmospheric process studies, and NASA has

been in the forefront of developing new capabilities to

enable such research. A prime example is the Aqua satel-

lite [25], which is a mission to study the hydrologic cycle.

Launched in 2002, it carries the Atmospheric Infrared

Sounder (AIRS) suite, which consists of the AIRS IRsounder and a duplicate of the Advanced Microwave

Sounding Unit A (AMSU-A) flying on the NOAA POES

satellites. AMSU-A is primarily a temperature sounder, but

the suite also includes a microwave humidity sounder, the

Humidity Sounder for Brazil (HSB)Va near duplicate of

the AMSU-B humidity sounders operated by NOAA. The

temperature and water vapor fields derived from the AIRS

suite have similar accuracy as balloon-borne radiosondeobservations, which are viewed as the Bgold standard[ in

atmospheric measurements, but AIRS provides near-global

coverage every dayVunlike radiosondes, which are

primarily launched from easily accessible land stations.

The AIRS observations are now enabling a number of new

investigations in atmospheric and climate research (e.g.,

[26]) and are also being used in numerical weather

prediction to great effect [7]. Research applications have sofar focused on atmospheric process and climate variability

studies, but as the data time series is extended, it will also

be possible to study climate trends.

One of the limitations of IR sounders is that most

clouds, i.e., those with an optical thickness greater than

3–5 are effectively opaque at infrared wavelengths.

Therefore, IR observations can be used only indirectly to

study most cloud processes, convection and precipitation.Partly transparent clouds, such as cirrus, can be studied in

the infrared [9], and efforts to derive microphysical

parameters from IR radiometer observations are continu-

ing. It is also possible to infer information about

precipitation and other properties from the cloud top

height and temperature of opaque clouds [28], and

knowledge of the state of the atmosphere in clear areas

surrounding cloud systems is very useful as well. Forexample, important information about weather systems

can be obtained from the GOES IR sounder [21], and these

observations are routinely used to derive atmospheric

stability indices, such as convective available potential

energy (CAPE) and lifted index (LI), in the clear areas

surrounding clouds. The stability indices are used to

analyze the development of severe weather systems, but it

is not possible to use infrared systems to gain a view insidesuch systems. Methods have been developed to enable IR-

based soundings in partially cloudy scenes [35], with so-

called cloud clearing techniques that employ MW

observations in combination with the primary IR observa-

tions, and recently those methods have been extended to

using ancillary information other than the MW observa-

tions that are usually used for that purpose [19]. Progress is

being made in this area, but cloud clearing only marginallyimproves the scope and coverage of IR sounders and still

leaves fully cloudy areas and storms unobserved.

That is where the need for a MW sounder is the

greatest and will have the largest impact. Millimeter-wave

radiometers are also extremely sensitive to scattering

caused by ice above precipitating systems and can be used

to estimate rain rates and convective intensity [2]Va

capability of great importance for the study of tropicalcyclones and mesoscale convective systems as well as for

general atmospheric and climate research. It is for reasons

such as these that there has long been and continues to be a

strong interest in and desire for a GEO MW sounder,

despite the development of the hyperspectral IR sounders.

A microwave sounder is able to measure all three phases of

waterVvapor, liquid, and solidVand is sensitive to all

forms of precipitation. It is possible to solve for all of theseparameters simultaneously even under conditions of light

to moderate precipitation [6], except in deep convection,

where it may not be possible to estimate temperature and

vapor profiles. Nevertheless, a microwave sounder covers a

very large portion of the hydrologic cycle in the atmo-

sphere, and that is the basis for calling PATH a hydrologic-

cycle mission.

Lambrigtsen et al.: Monitoring the Hydrologic Cycle With the PATH Mission

Vol. 98, No. 5, May 2010 | Proceedings of the IEEE 863

II . NRC DECADAL SURVEY

The National Research Council, an arm of the U.S. Na-

tional Academies of Science, recently released a Bdecadal

survey[ of NASA and NOAA Earth space missions [1].

Among the 15 missions recommended for NASA to under-

take was one called the BPrecipitation and All-weather

Temperature and Humidity[ mission (PATH). A BMW

array spectrometer[ was identified as the recommended

instrument payload for PATH. Such an instrument, called

the Geostationary Synthetic Thinned Aperture Radiometer

(GeoSTAR), is being developed at NASA’s Jet Propulsion

Laboratory (JPL). Due to the very large antenna aperture

needed for a microwave sounder to provide the required

spatial resolution, it has not been possible to develop such

instruments for GEO, although efforts have been under way

for many years. Examples are the Geostationary Microwave

Observatory (GEM) and the Geostationary Observatory for

Microwave Atmospheric Sounding (GOMAS) [4]. These

and earlier designs use relatively large parabolic reflector

antennas to achieve high spatial resolutionVbut the aper-

ture cannot be made large enough to achieve the required

resolution in the crucial 50-GHz temperature sounding

band, provide the high beam quality that is required for

sounding, and allow for physical scanning. As a result, only

infrared sounders have been feasible, but their effective-

ness is hindered by cloudsVwhich is not a problem for

microwave sounders. GeoSTAR overcomes the technical

difficulties by synthesizing a large aperture. This new ap-

proach was clearly viewed by the NRC as a very important

breakthrough. At the same time, the NRC’s assessment of

the maturity of the technology needed to implement the

GeoSTAR concept led the panel to assign the PATH mission

to the third tier of decadal-survey missions and a late

projected launch date. However, progress in this area has

been very rapid, and it is now feasible to implement

GeoSTAR and PATH in the near future.

PATH will provide a number of measurements that are

crucial for the monitoring and prediction of hurricanes and

severe stormsVincluding hemispheric 3-D temperature,

humidity and cloud liquid water fields, rain rates and

storm totals, tropospheric wind vectors, sea surface tem-

perature, and parameters associated with deep convection

and atmospheric instabilityVeverywhere and all the time,

even in the presence of precipitating clouds. These param-

eters will be derived from a continuous stream of 2-D

radiometric Bsynoptic snapshots[ provided by GeoSTAR

and covering the entire visible disc. With these capabil-

ities, GeoSTAR will become a prime hurricane sensor. In

addition, it will provide the basic sounding functions re-

quired by Boperational[ agencies for regional weather

prediction and key observations needed for research re-

lated to the hydrologic cycle. In particular, with GeoSTAR

the diurnal cycle can be fully resolved, and atmospheric

processes related to cloud dynamics and convection can be

studied without the diurnal temporal sampling biases that

are prevalent with polar-orbiting sun-synchronous satellitesensors, i.e., where a satellite always passes overhead at the

same local time and therefore only samples the diurnal

cycle at two points 12 h apart. As has been demonstrated in

LEO, microwave sounders are excellent tools for climate

applications, with their superior stability and lack of

sampling bias. Much of the technology risk of this new

measurement concept was retired with the development of

the prototype; in addition, technology development as wellas application studies are under way, funded by NASA and

NOAA. (This work is still in progress, and thus has not yet

been published.)

The primary objective of PATH is to provide contin-

uous soundings and precipitation measurements under

both clear and cloudy conditions, with very rapid refresh

rates. Those observations will be used to improve and

constrain atmospheric models, which is expected to lead tosignificant improvements in storm forecasts as well as

improved understanding of atmospheric processes related

to the hydrologic cycle.

The NRC stated in its report that current numerical

weather prediction models are widely recognized as having

an inadequate representation of the processes of cloud

formation, evolution, and precipitation. The models rely

on simplified parameterization schemes and an incompleteunderstanding of the underlying cloud microphysics to

represent the most rapidly changing weather phenomena.

The PATH measurements will impose powerful new

constraints on, and are expected to lead to greatly im-

proved models for boundary layer, cloud, and precipitation

processes. The availability of continuous observations will

also significantly mitigate the requirements on weather

prediction models because they will be able to be fre-quently re-initialized by observations. These observations

will also enable major scientific advances in the under-

standing of El Nino, monsoons, and the flow of tropical

moisture to the U.S. The NRC report suggests that Baccom-

modation of an all-weather sensor suite on future GOES

GEO platforms is the most promising option in the next

ten years.[ In the climate arena, the situation is similar in

the sense that the models are recognized as being inade-quate, particularly in their treatment of clouds, convec-

tion, and precipitation. The largest modeling uncertainties

are related to boundary layer clouds, as discussed in the

latest Intergovernmental Panel on Climate Change (IPCC)

report [34]. While an AMSU-like sounder like GeoSTAR

cannot resolve the boundary layer, it will nevertheless

produce important new observations of the hydrologic

cycle and lead to improvements in climate models as well.The PATH measurements require penetration well into

clouds, and that requires temperature sounding in the 50–

60 GHz oxygen absorption band and water vapor sounding

at the 183 GHz water vapor line. These bands are also

suitable for precipitation observations. Although the NRC

report does not specify a particular payload design,

numerous references make it clear that the study panel


864 Proceedings of the IEEE | Vol. 98, No. 5, May 2010

envisioned that this would be a mission based on theGeoSTAR concept and recognized that known alternative

concepts would not be able to satisfy the measurement

requirements. This is reflected by the report identifying a

BMW array spectrometer[ as the presumed payload.

III . ATMOSPHERIC SOUNDING

Atmospheric sounding is based on the fact that the thermalradiation received by a radiometer originates at wave-

length-dependent depths in the atmosphere. This is caused

by a nonuniform absorption spectrum, particularly by

molecular absorption lines. (Note that in an atmosphere

that is in thermal and radiative equilibrium, emission

equals absorption. If that were not the case, the atmo-

sphere would either cool down or heat up until balance is

reached.) At wavelengths near the peak of such a line,absorption may be so strong that most of the underlying

atmosphere is opaque, and only the top of the atmosphere

is Bseen.[ Conversely, at wavelengths far away from the

lines, often called a Bwindow[ region, the atmosphere may

be nearly transparent, and the surface or the bottom of the

atmosphere is seen instead.

Through spectral sampling, i.e., by measuring narrow

spectral bands, or Bchannels,[ it is then possible to probeinto different depths of the atmosphere. A Bweighting

function[ describes which portion of a channel’s signal

strength originates from different depths. For a radiometer

looking down into the atmosphere from a high-flying

aircraft or a satellite, a typical weighting function reaches a

peak at a certain depth, which is characteristic for that

channel. A set of channels is selected so that the respective

weighting functions are evenly distributed through theatmosphere.

It is possible to separate the effects of different at-

mospheric molecular species, by using channels in spectral

regions where absorption from one species dominates. For

example, AIRS uses a large number of narrow CO2

absorption lines in the infrared spectral region to measure

temperature profiles. AMSU-A, on the other hand, uses a

few O2 absorption lines in the microwave region between50 and 60 GHz. To measure water vapor profiles, AIRS

uses a number of narrow H2O absorption lines throughout

its spectral range, while AMSU-B uses a single H2O

absorption line near 183 GHz. Since the vertical distri-

bution of CO2 and O2 are both stable and quite well

known, the CO2 and O2 channels allow the temperature

distribution to be determined. With that known, the H2O

channels allow the vertical distribution of water vapordensity to be determined. These parameters are usually

derived simultaneously.

Fig. 1 shows the MW absorption spectrum in the

sounding region, and Fig. 2 shows the nominal AMSU-A

weighting functions in the 50-GHz band. Fig. 3 illus-

trates the different Bbrightness temperature[ views of

Earth through a transparent channel versus an opaque

one. The continental outlines are clearly distinguishable

in the former (left panel) but barely detectable in thelatter (right panel).

Fig. 1. Microwave absorption spectrum due to oxygen,

water vapor, and liquid water (attenuation vs. frequency).

Fig. 2. The 50-GHz band temperature-sounding weighting functions.



Liquid droplets and ice particles make most cloudscompletely opaque in the infrared band, but in the micro-

wave region they are partially transparent. The microwave

spectral absorption features of liquid water therefore make

it possible to determine the vertical distribution of the

liquid water in clouds from microwave sounder measure-

ments [30]. Scattering caused by the larger ice particles,

such as those created through deep convection, is also

easily measured by MW sounders, and that makes itpossible to infer precipitation rate and convective intensity

[3]. It is even possible to form a three-dimensional picture

of the structure of tropical cyclones, similar to that

obtained with radar systems [17].

The geophysical parameters are Bretrieved[ by invert-

ing the radiative transfer equation, which describes the

absorption, emission, and scattering that the thermal ra-

diation undergoes as it travels from the source to the sen-sor and expresses the magnitude of the observed radiation

as a function of the geophysical state of the atmosphereVtemperature, water vapor, liquid water, particle size and

distribution, etc. Inverting this equation yields the geo-

physical parameters as functions of the measured radia-

tion. It is a highly nonlinear underdetermined equation,

and the inversion is an ill-posed problem. It therefore has

many possible solutions, and much effort has gone intodeveloping stable Bretrieval[ methods that produce solu-

tions close to the Btruth.[ A common method is based on

the Boptimal estimation[ procedure [29]. Such methods

are called Bphysical[ retrieval methods, since they are

based on inverting an equation that describes the physics.

The Microwave Integrated Retrieval System [6] is an ex-

ample. Other approaches use statistical methods to infer

the complex relationship between the observations and theunderlying geophysical state. An example is a precipitation

method based on neural networks [8].

IV. GeoSTAR CONCEPT

The Geostationary Synthetic Thinned Aperture Radiome-

ter (GeoSTAR) concept was first conceived of at the Jet

Propulsion Laboratory in 1998 under the name Geosta-

tionary Synthetic Aperture Microwave Sounder (GEO/SAMS) in response to a NASA Research Announcement

soliciting innovative measurement concepts suitable for

geostationary applications and intended for demonstration

on the NASA New Millennium EO-3 mission [15]. Similar

to a concept then being studied in Europe called the

Microwave Imaging Radiometer by Aperture Synthesis

(MIRAS) [20], which has now been implemented by the

European Space Agency for the Soil Moisture and OceanSalinity (SMOS) mission [33], GEO/SAMS (and its suc-

cessor GeoSTAR) synthesizes a large aperture to measure

the atmospheric parameters at microwave sounding fre-

quencies (i.e., 50 and 183 GHz) with high spatial reso-

lution from GEO without requiring the very large and

massive dish antenna of a real-aperture system. Although

the MIRAS instrument operates at a widely different

wavelength (L-band, near 1.4 GHz) and is intended tomeasure surface parameters from a LEO orbit, it was

realized at JPL that the MIRAS concept can be scaled: the

ratio between GEO and LEO orbit altitudes (about 40) is

nearly identical to the ratio between the MIRAS measure-

ment frequency and MW temperature sounding frequen-

cies. An analogous approach can therefore be used to

obtain high spatial resolution from GEO at 50 GHz, just as

MIRAS is used to obtain high spatial resolution from LEOat 1.4 GHz.

This insight enabled a great step forward in the

feasibility of implementing a GEO MW sounder. Consider

the AMSU-A sounder: its antenna aperture is about 15 cm

(6 in) in diameter, which yields a spatial resolution of

about 50 km from an orbit altitude of about 850 km. To

maintain the same resolution at higher orbit altitudes, the

aperture diameter needs to be scaled up proportionally.For the GEO altitude of about 37 000 km, the AMSU

antenna scales to about 6.5 m (about 21 ft). Although it is

possible to reduce that somewhat, by degrading the Bbeam

quality,[ the practical barriers against implementing such

a system have been prohibitive and are expected to remain

so for the foreseeable future. The aperture synthesis

approach overcomes those difficulties and therefore makes

a geosynchronous microwave sounder finally possible,after many years of searching for a solution.

GEO/SAMS was one of four concepts chosen for EO-3

Phase-A studies but was ultimately not selected for imple-

mentation. After a hiatus, this concept was again proposed

in response to a 2002 NASA Research Announcement, this

time focusing on pre-mission technology development

sponsored by the NASA Instrument Incubator Program

(IIP), a technology development program under the EarthScience Technology Office (ESTO). GeoSTAR was select-

ed, and an effort got under way at JPL in 2003 to develop

the required technology and demonstrate the feasibility of

the synthetic aperture approach with a small ground based

proof-of-concept prototype. This was done jointly with

collaborators at the NASA Goddard Space Flight Center

and the University of Michigan and in consultation with

Fig. 3. Simulated brightness temperature images: window channel

(left) and opaque channel (right).



personnel from the NOAA National EnvironmentalSatellite Data and Information Service (NESDIS) Office

of System Development as well as NASA and was first

described in 2004 [16]. The first phase of this work was

completed in 2006 with the successful testing of the

GeoSTAR proof-of-concept prototype. Additional risk

reduction is now under way through a second IIP effort.

Progress has been so rapid that a space-based GeoSTAR/

PATH program can soon be initiated.GeoSTAR is a spatial interferometer that essentially

measures the upwelling radiation emitted by the atmo-

sphere in the spatial Fourier domain. This is accomplished

by deploying a large number of individual microwave re-

ceivers arranged in a sparsely filled 2-D array. In the basic

configuration there are three linear arrays arranged in a

BY[ shape, and the effective aperture is then essentially

defined by the circle that circumscribes the BY.[ (Otherconfigurations are possible, such as a rectangle or a ring,

but the Y-configuration is one of the most efficient for a

circular target such as the Earth.) This array configuration

is illustrated in Fig. 4. All of the antennas are pointed in

the same direction, toward Earth. A digital subsystem

computes the complex cross-correlations between all

receiver pairs in the array simultaneously. In the small-

scale example of Fig. 4 there are 24 receivers, 276 complexcorrelations, and 384 unique so-called uv-samples, which

means that the spatial imaging field can be resolved into

384 Bpixels.[ Each receiver pair forms an interferometer,

which measures a particular spatial harmonic of the

brightness temperature image across the field of view.

The spatial harmonic depends on the spacing between the

antennas and the wavelength of the radiation being mea-

sured. The complex cross-correlation measured by an in-

terferometer, called the visibility function (a term coinedby radio astronomers, who have used the BSTAR[ ap-

proach for many years, such as in the Very Large Array

operated by the National Radio Astronomy Observatory in

New Mexico), is essentially the 2-D Fourier transform of

the 2-D brightness temperature field. By sampling it over a

range of spacings and azimuth directions one can

reconstruct, or Bsynthesize,[ an image by discrete Fourier

transform.In Fig. 4, the left panel shows the distribution of re-

ceivers in the instrument’s aperture plane, and the right

panel shows the resulting sampling points in spatial

Fourier space (the so-called uv-plane), i.e., in terms of

spatial harmonics. The measurements made at these Buv[sampling points essentially determine the coefficients of a

2-D Fourier series expansion of the radiometric field, and

the field can then be recovered through an inverse Fouriertransform. It may be noted that there is no sampling point

at the origin of the uv-plane. That point corresponds to the

constant term in a Fourier series. It will be determined

separately, by measuring the mean brightness temperature

of the Earth-disc with a dedicated receiver that is operated

as a conventional radiometer but is otherwise identical to

the array receivers. Other methods can also be used to

determine this value.The smallest spacing of the sample grid in Fig. 4

determines the unambiguous field of view, which for the

basic configuration is larger than the earth disk diameter of

17.5� when viewed from GEO. This sets both the antenna

spacing and diameter at about 3.5 wavelengths, or 2.1 cm

at 50 GHz, for example. The longest baseline determines

the smallest spatial scale that can be resolved, which for

the array in Fig. 4 is about 0.9� (i.e., 17:5�=p

384). To

Fig. 4. Left: Antenna arrayVeach circle represents a receiver; right: resulting ‘‘uv’’ sampling patternVeach dot represents a pair of receivers in

the left panel; the example is for the 50-GHz band, where the wavelength is 0.6 cm and the smallest element spacing is 3.5 wavelengths (2.1 cm).



achieve a 50-km spatial resolution at 50 GHz, a baseline of

about 4 m is required. This corresponds to approximately

100 receiving elements per array arm, or a total of about

300 elements. This in turn results in about 30 000 unique

baselines, 60 000 uv sampling points (given conjugate

symmetry), and therefore 60 000 independent pixels in

the reconstructed brightness temperature image, eachwith an effective diameter of about 0.07�Vabout 45 km at

nadir from GEO.

Fig. 5 shows the hexagonal imaging region (left panel),

resulting from this star-shaped uv-sampling pattern

imposed on the Fourier transform of the Earth brightness

temperature field (right panel). As in all interferometric

systems, there are Bghost[ imaging hexagons adjacent to

the primary oneVthis is illustrated in Fig. 6Vand radia-tion originating from those areas is aliased into the primary

area. In the illustrated example, this is not a problem since

the space beyond the Earth disc is featureless and at a

uniform 2.7 K temperatureVthe effective temperature of

the cosmic background radiation. The sun and the moon

will periodically be aliased into the imaging area, but those

occurrences will be used to help calibrate the system. As

we discuss below, it may be desirable to reduce theprimary imaging area somewhat, to reduce the number of

receiving elements needed to attain the required spatial

resolution or to improve the radiometric sensitivity, and

portions of the observations near the limb would then be

contaminated by aliasing. Some of that can be tolerated,

however, since accurate sounding is limited to relatively

low zenith angles. Alternative antenna designs are being

developed to mitigate aliasing.

V. GeoSTAR PROTOTYPE

A small-scale prototype implementing the array shown in

Fig. 4 and operating at 4 AMSU-A channels between 50

and 54 GHz, was built to address the major technical

challenges facing GeoSTAR. Fig. 7 shows a photo of the

prototype, and Fig. 8 shows its configuration schema-

tically. The challenges with the aperture synthesis ap-

proach are centered on the issues of system design and

calibration. Power consumption has also been a majorconcern, but the continuing miniaturization of integrated

circuit technology has demonstrated that this is no longer a

major issue. Synthesis arrays are new and untested in

atmospheric remote sensing applications, and the calibra-

tion poses many new challenges, such as stabilizing and

characterizing the phase and amplitude response of the

antenna elements and of the receivers and correlators. To

address these challenges, the prototype was built withsimilar receiver technology, antenna design, calibration

Fig. 5. Brightness temperature image of Earth (left) and its

Fourier transform (right); the six-pointed star outline in the right panel

is the same as the uv-sampling outline in Fig. 4 (but rotated), which

results in the hexagonal imaging area outlined in the left panel.

Fig. 6. Aliasing regionsVpart of an infinite series; the central hexagon

is the same as the one outlined in Fig. 5 (left).

Fig. 7. GeoSTAR proof-of-concept prototype (early stage);

each receiver feedhorn has a diameter of 2.1 cm, and the effective

aperture diameter is about 35 cm.



circuitry, and signal processing schemes as are required for

the spaceborne system. Only the number of antenna ele-

ments differs significantly. The overarching objective was

to prove the concept and not to develop a space system.

A number of tests were first done in a laboratory set-ting, with very encouraging resultsVno serious problems

were identified, and the system was working exactly as

expected, Bout of the box[Va notable achievement. Fol-

lowing initial laboratory tests, the system was moved out-

doors to observe the sky and the sun. The array was

pointed into the path of the sun, and the sun was allowed

to pass through the center of the field of view at an

elevation of 45�, as shown in Fig. 9. The observations werethen processed and inverted to form brightness temper-

ature images of the sun. The right panel of Fig. 9 shows

the spatial response function derived from the solar

observationsVa nearly perfect 2-D hexagonally-symmetric

Bsinc[ function, as expected. It may be noted that this is

excellent newsVthere are well-established image proces-sing techniques that can be used to eliminate the inter-

ferometric imaging artifacts and reduce the effective

Bsidelobes[ well below what is possible with real-aperture

systems and thus achieve the high beam efficiency re-

quired for accurate soundings. An example is the BROF[algorithm [31], which can be modified to effect

Bdeblurring[ when the blurring kernel is known (as is

the case with GeoSTAR). It may also be possible to applyBsuper resolution[ techniques [24], which take advantage

of repeated observations of the same scene to increase the

effective spatial resolution. The elemental antenna

patterns have been measured at a compact antenna range

at the Goddard Space Flight Center, shown in Fig. 10, and

further confirmed the excellent performance of this

system. This is the first successful demonstration of 2-D

aperture synthesis at these frequencies.Fig. 11 shows an outdoor targetVa hillside scene at

JPL. The left panel is a photograph and the right panel is

the reconstructed brightness temperature image. The hex-

agonal Bwindow[ corresponds to the central imaging

region of GeoSTARVa result of the hexagonal symmetry

imposed by the Y-configuration of the receiver array, as

discussed above. It is surrounded by a set of closely packed

hexagonal windows, as illustrated in Fig. 6, which aliasinto the central window. Thus, the brightness temperature

image is a composite of the scene within the central

window and surrounding scenes that are aliased into the

central window. For example, the Bwarm[ hillside outside

the imaging area in the upper-right is aliased into the

lower-left portion of the central imaging area. Fig. 12

shows an indoor target, again with a photograph (left

panel) and a reconstructed brightness temperature image(right panel). The most notable feature here is the coffee

cup, which appears warm because the plastic is nearly

Fig. 8. Simplified GeoSTAR block diagram.

Fig. 9. First-light observations of solar transit: instrument configuration (left) and derived spatial response function (right).



Fig. 10. Antenna test chamber (left) and measured antenna pattern (right).

Fig. 11. Imaging of outdoor target in the far field at 50.3 GHz, with an integration time of about 3 min.

Fig. 12. Imaging of indoor target in the near field at 50.3 GHz and an integration time of 3 min; the brightness temperature image has been

adjusted for near-field distortions as discussed in the text.



transparent at 50.3 GHz. Near-field imaging, such asshown in Fig. 12, is made possible with a correction algo-

rithm that compensates for image distortion that otherwise

dominates [37]. These images have not been corrected for

the BGibbs ringing[ sidelobe-artifacts represented in

Fig. 9. Several methods are available to reduce those ef-

fects, however, and will be implemented for the opera-

tional system.

The prototype was subsequently integrated in a com-pact temperature controlled package suitable for precise

measurements in an outdoor environment, and a target

that emulates the Earth as seen from GEO, illustrated in

Fig. 13, was constructed and used to characterize the sys-

tem performance quantitatively. It incorporates two tem-

perature controlled areas and a central Bbeacon,[ and

Fig. 13 also shows examples of the measurements obtained

with this facility. A detailed discussion of the prototypesystem and the test results is provided by Tanner [36].

In order to have a practical design that can be imple-

mented within reasonable satellite resource constraints

and costs, it has been desirable to optimize the GeoSTAR

design and develop the key technologies further. A result

of this effort has been a significant advance in millimeter-

wave receiver and monolithic microwave integrated

circuits (MMIC) technology. Thus, the GeoSTAR systemwill use receivers that have a radiometric sensitivity nearly

an order of magnitude better than previous MW sounders.

Another area being pursued is the antenna array design,which is focused on maximizing the effective antenna

efficiency (i.e., minimizing Bwasted[ energy picked up

outside the desired field of view) and maximizing

radiometric sensitivity. This is the focus of the second

IIP effort.

In summary, the GeoSTAR prototype has been very

successfully completed and tested. Results are excellent,

and this development can be characterized as proof of the2-D aperture synthesis concept. This constitutes a major

breakthrough in remote sensing capabilities. Further tech-

nology development is under way, both as risk reduction

and to enhance the measurement capabilities of the

GeoSTAR/PATH system. At the same time, efforts are also

proceeding to identify sponsorship and secure funding for

a space demonstration mission in the 2015–2018 time

frame.

VI. MISSION DESIGN

A preliminary PATH mission study was conducted for

NASA in 2007, which concluded that all requirements

discussed in the NRC Decadal Survey can be met with the

GeoSTAR design, and mission cost is projected to be

within 15% of NRC’s estimate. The study also showed thata PATH/GeoSTAR mission is feasible in terms of mass and

power. Power consumption has been a matter of concern,

Fig. 13. Outdoor near-field calibration facility: insert shows response to central beacon (near-field uncorrectedVtop and correctedVbottom);

right panels show observations (top) and temperatures of three target regions (bottom).



since a full-size instrument will have nearly 1000 receivers

and close to 1 million correlatorsVdigital multipliers, each

operating at 100 million multiplications per second or

more. The state of integrated-circuit technology is now

such that this can be done with comfortable margins, and

the mass and power requirements for the entire instru-

ment are currently estimated at less than 250 kg and350 W, respectively. These numbers are expected to de-

cline as the technology matures further.

The baseline GeoSTAR design consists of two collinear

arrays, one with �300 receivers in the 50-GHz band and

one with �600 receivers in the 183-GHz band. Each

Barm[ of the 50-GHz array is approximately 2 m long. That

results in a spatial resolution of better than 50 km for

temperature sounding and 25 km for water vaporsoundingVthis compares with 50 and 15 km, respectively,

for the LEO AMSU system. Future versions are envisioned

to have significantly higher spatial resolution, which can

be achieved by adding receivers and thus extending the

lengths of the array arms. Fig. 14 shows the array layout,

which features provisions for redundancy near the center

of the array, i.e., for the critical larger spatial scales

(which represent most of the radiative energy). In gen-eral, an aperture synthesis system like this is quite fault

tolerant: the loss of a receiver or correlator will create a

gap in the uv sampling pattern shown in Fig. 1, but the

consequence of that is usually merely a degradation of

image quality. The redundant receivers and correlators

reduce the chance of such degradation. The visibility

spectrum of the Earth is such that most of the energy is in

the shorter baselines (i.e., larger spatial scales), and theloss of a receiver near the center of the array is therefore

the most serious. This design protects against the most

serious type of loss.

The design illustrated in Fig. 14 uses two receiverspacings; the spacing in the outer portion of the array is

twice that in the inner portion, and the receiver horns are

scaled correspondingly. This Bdual-gain[ design innova-

tion results in a more focused imaging area, shown in

Fig. 15, and increases the effective antenna efficiency [38].

This approach results in higher radiometric sensitivity in

the central region of interest. The basic element spacing is

4 wavelengths in this design, which results in an alias-freefield of view that is slightly smaller than the Earth disc but

also results in regions near the Earth limb that are affected

by aliasing, as shown in Fig. 4. This is of minor conse-

quence, however, since atmospheric profiles cannot be

reliably Bretrieved[ at such large zenith angles. Other de-

signs, such as using four parallel rows of receivers and

modified feedhorns, are also being explored.

The array shown in Fig. 14 has a total of 312 50-GHzreceivers and 576 183-GHz receivers. Each receiver

consumes 100–150 mW. The 50-GHz array requires about

150 000 correlator cells, each of which is a multiplier-

accumulator operating at about 200 MHz. The 183-GHz

array requires about 460 000 such cells. Implemented

as a 1-bit multiplier-with-accumulator on a 90-nm

application-specific integrated circuit (ASIC), each cell

will consume less than 10 �W. Power consumption of theentire 183-GHz correlator will then be less than 5 W.

(These estimates are based on computing 4 visibility com-

ponents per baseline, each producing two pairs of real and

imaginary readings; this also results in a significant mea-

sure of redundancy.) This is expected to decrease as newer

IC technologies become available, which will continue to

Fig. 14. Dual-band dual-gain array layout to narrow the imaging

region, increase sensitivity, and provide redundancy.

Fig. 15. Central region of highest sensitivity resulting from the

array shown in Fig. 14.



drive down overall GeoSTAR power consumption andmake it possible to increase radiometric performance by

operating several correlator systems in parallel and thus

extending the per-channel integration time. (In the

current design, spectral channels are sampled sequentially

interleaved, and the integration time for a full sounding is

the sum of per-channel integration times.)

With the design described above, but using broadband

1.5-bit correlators implemented with 65- or 90-nm ASICsnow under development and using recently developed

MMIC technology that yields receiver noise temperatures

near 300 K [12], we estimate a radiometric sensitivity in

the 183-GHz band of about 1/3 K, similar to AMSU per-

formance, for all channels every 15 min, which is adequate

to meet or exceed all relevant measurement requirements.

Table 1 lists the nominal set of spectral channels. There are

6 channels in the 50-GHz band, which match the tropo-spheric temperature sounding channels of AMSU-A, and

there are 4 channels near 183 GHz, which largely match

the water vapor sounding channels of AMSU-B. AMSU-B has

a Bwindow[ channel at 89 GHz, which will not be imple-

mented in the first GeoSTAR, since it would likely require a

third antenna array. However, the possibility of covering a

broad frequency range, such as 50–90 GHz or 90–180 GHz,

with one of the two baseline arrays is being explored. TheGeoSTAR 166-GHz channel will provide the same function-

ality as the AMSU-B 150-GHz channel, an approach that is

also used with the next-generation MW sounder for POES

satellites, the Advanced Technology Microwave Sounder

(ATMS) [23]. We note that lower-frequency channels, such

as those used in so-called microwave imaging systems (i.e.,

19–37 GHz and lower) as well as window channels (such as

89 GHz) do not play an important role in atmosphericsounding but are important for precipitation.

VII. APPLICATIONS

Table 2 summarizes the PATH measurements and data

products and Table 3 lists some of the applications these

observations will enable. With the GeoSTAR system it will

be possible to produce temperature soundings within the

troposphere for most of the visible Earth disc (out to azenith angle of 60� or more but possibly limited to a

smaller region, as shown in Fig. 15) with a �2 km vertical

resolution, and humidity soundings with a vertical reso-

lution of�3 km, every 15–20 min under nearly all weather

conditions. (Stratospheric temperature sounding can also

be added, but with a longer refresh cycle to match the

slower temporal evolution of stratospheric temperature

fields.) GeoSTAR is a nonscanning 2-D imaging system.Soundings are obtained everywhere at the same time, i.e.,

there is no time lag between different portions of the scene

as there is in a mechanically scanned system. That also

makes this system ideal for derivation of wind profiles

through tracking of water vapor features, using methods

developed for the GOES sounders and the Moderate

Resolution Imaging Spectroradiometer (MODIS) flying on

the NASA Terra and Aqua satellites [13], [27]. Although thevertical resolution is limited, the potential for estimating

wind vectors in and below clouds makes this a useful ca-

pability, since almost complete spatial and temporal cov-

erage will be achieved. Assimilating such wind vector fields

into numerical weather prediction models is expected to

have a significant impact on prediction accuracy and range.

The MODIS winds, which can only be derived in polar

regions (where the revisit time of the two satellites isfrequent enough) and do not cover cloudy scenes, have had

a positive impact even on global forecasts [5], [18].

Each channel requires an integration time of 3–4 min-

utes to reach a noise level of about 1/3 K. With the two

bands being operated simultaneously in parallel, it then

takes 15–20 min to form a complete temperature and

water vapor sounding of the portion of the Earth disc that

is within the region of high sensitivity (see Fig. 15). Thesystem cycles sequentially through the channels in a

matter of seconds. There is therefore no discernible time

lag between channels, and the observations are in effect

simultaneous. Intermediate results are transmitted to the

ground several times per minute for further aggregation

Table 1 GeoSTAR Channel Set

Table 2 PATH Data Products



there. Thus the 15–20 min sounding Bwindow[ is a sliding

window that advances at sub-minute intervals. This high

degree of temporal oversampling will make it possible to

recover dynamic features with a shorter time scale than

15–20 min, using common signal processing techniques.This can be exploited when rapidly evolving processes

need to be resolved more precisely and quickly. The re-

trieval of vertical profiles of temperature, water vapor

density and liquid density from spectrally sampled bright-

ness temperatures is well established [11], and such pro-

files are routinely derived from AMSU observations. These

methods will also be used with PATH.

The GeoSTAR system allows for a high degree offlexibility. In some applications, it may not be necessary to

achieve a noise level of 1/3 K, and the user could decide to

aggregate the observations in 5-min windows instead. For

example, if the application is to track rapidly evolving deep

convection, where the amplitude of the scattering signal

could be in the 10–100 K range, an adequate signal to noise

ratio can be reached in a very short time. It is also possible

to optimize the temporal sampling by either omitting cer-tain channels or by adjusting their duty cycle, since this is

determined entirely through the on-board flight software

that controls the local oscillator, i.e., the local oscillator is

stepped from channel to channel, usually in a fixed pattern

with equal time allotted to each channel sequentially. The

system can be commanded from the ground to execute a

different pattern. Additionally, if a frequency synthesizer

were used to drive the local oscillator, GeoSTAR could beoperated in a number of Bresearch modes.[ For example, it

would then be feasible to explore hyperspectral sounding,

which could make it possible to achieve exceptionally high

vertical resolution in the boundary layer.

In addition to atmospheric profiles, PATH will also be

used to measure precipitation. Several methods, all

depending on scattering, have been developed for this.

An example is the method developed by Ferraro and Grody

[10], which uses window channels at 89 and 150 GHz to

measure the scattering caused by ice particles formed in

and above rain cells. This method may be adapted to use a50-GHz window channel in lieu of 89 GHz. Although

there are limitations with these methods (some stratiform

and Bwarm rain[ conditions are problematic), PATH offers

the advantage of continuous full-disc coverage and can

therefore be used to fill in the gaps between the narrow

swaths of LEO-based systems. In addition, frozen precip-

itation (snow), which is difficult to detect by conventional

means, can also be inferred with these methods [32], andalgorithms are being developed for the Global Precipita-

tion Measurement (GPM) mission. These capabilities will

be used to complement the observations obtained from

GPM, by using data fusion methods to merge LEO and

GEO observations and in effect use the GEO observations

to fill in spatial and temporal gaps in the LEO observations.

GeoSTAR will not implement low-frequency channels

commonly used to measure surface emissivity and surfacetemperature; however, it will be possible to use the mode-

rately transparent 50.3-GHz channel for that purpose. In a

typical atmosphere, this channel has a transmittance of

about 0.7, i.e., about 70% of surface radiation reaches the

radiometer. The surface radiation is the product of two

unknownsVsurface emissivity and surface temperature.

The temperature has a pronounced diurnal cycle but is

otherwise reasonably stable, and the emissivity evolvesnormally very slowly. In both cases, weather events may

cause temporary anomalies. Nevertheless, due to the sta-

tionary nature of the scene visible from GEO, it will be

possible to build up a high-fidelity emissivity map that can

be used as the Ba priori[ in an optimal estimation system.

With most of the atmospheric information coming form

Table 3 PATH Applications



other channels, it will then be possible to solve for thesurface temperature. We estimate that it will be possible to

reach an accuracy of less than 1 K for sea surface

temperature (SST) by averaging over 1–2 hours in the

temporal domain and 100 km in the spatial domain. This

promising approach is currently under study.

VIII . SPACE MISSION OPPORTUNITIES

Since the inception of the IIP prototyping project, the

GeoSTAR team has worked closely with NASA Headquar-

ters, which has provided programmatic and scientific

oversight and with representatives from the NOAA

NESDIS. This has resulted in the GeoSTAR design being

closely aligned with Bcustomer[ needs. The measurements

that PATH will provide are needed by NASA for research

use and are needed as well by NOAA for operational use.NOAA now considers a GEO MW sounder to be one of

its highest-priority unmet needs and carries GeoSTAR near

the top of their list of instruments required for GOES-R

Pre-Planned Product Improvements ðP3IÞ, i.e., unmet

needs (http://www.osd.noaa.gov/rpsi/tech_ident.htm).

Normally, as soon as funding for such a payload has been

identified and the technological maturity is sufficient for

an operational mission, the payload is elevated to baselinestatus. The GeoSTAR prototype has retired some of the

more challenging technology risk. However, it is still diffi-

cult for NOAA to sponsor the first space mission of a new

instrument, and NOAA generally looks to NASA to do so.

On the NASA side, science research missions tend to have

higher priority than pre-operational missions (as a NOAA/

GeoSTAR space demonstration might be viewed as), and

although NASA intends to implement all of the recom-mended Bdecadal-survey[ missions, it has been difficult for

NASA to commit early funding for PATH. A promising

alternative avenue is through the Bresearch-to-operations[path, which has been mandated by the U.S. Congress and is

acknowledged by both agencies as worthwhile. Here,

NASA first develops the necessary technology, followed by

a space Bresearch[ mission that demonstrates the capabil-

ities and elevates the maturity to operational statusVatwhich point the mission is handed over to NOAA for ope-

rational use. NOAA would subsequently procure additional

copies of the payload to populate future platforms. A

GeoSTAR/PATH space demonstration mission as a hosted

Binstrument of opportunity[ payload on a NOAA satellite

(e.g., GOES-R or GOES-S) is being discussed, and there

is a reasonable probability that such a mission will pro-

ceed. The first launch in the GOES-R series (i.e., ofGOES-R) is now scheduled for 2015, followed by the

second satellite about 2 years later. A space version of

GeoSTAR can be built in 3–4 years, and that makes a

launch in the 2015–2017 time frame possible. Thus,

GeoSTAR could become part of the GOES payloadVsubject to the availability of funds. The cost savings of a

joint NASA-NOAA mission would be substantial for both

agenciesVNOAA would get an Badvanced sounder[ for thecost of integration and mission management, and NASA

would implement a Bdecadal-survey[ mission for less than

half of nominal cost.

IX. SUMMARY

The GeoSTAR concept and the related technology have

been maturing rapidly, and this makes a PATH missionpossible in the near future. The continuing efforts to dev-

elop the technology further will enhance the system’s per-

formance as well as retire technology risk, and mature the

concept well enough that a space mission can be imple-

mented in the 2015–2017 time frame. The only major ob-

stacles remaining will be of a programmatic and budgetary

nature. It is probable that these will be overcome, making a

PATH mission likely within the next 10 years. This will addsignificantly to the nation’s remote sensing capabilities:

GeoSTAR/PATH observations will have a significant im-

pact on weather forecast accuracy and will greatly benefit

research related to the hydrologic cycle. In particular, these

observations will add much to our ability to observe,

understand and predict hurricanes and other severe storms.

The advantages of a synthetic aperture system over a

real aperture system are significant. Error budget calcula-tions based on simulation studies indicate that a synthetic

aperture system can be greatly expanded in size without

unduly stressing the all-important phase stability require-

ments. It is therefore well suited to meet future needs, as

the spatial resolution of atmospheric models increase and

the need for matching observations grow. Further, the

GeoSTAR system does not require platform-disturbing

mechanical scanning, nor is there a time lag betweendifferent portions of the images, as there is in mechanically

scanned real-aperture systems (where there could be a

time lag of as much as an hour between the start-of-scan at

the northern limit of the Earth disk and the end-of-scan at

the southern limit). GeoSTAR thus produces true synoptic

soundings; no other sounder, whether infrared or micro-

wave, has that capability. Moreover, the GeoSTAR concept

has the advantage of significant fault tolerance. TheBgraceful degradation[ in GeoSTAR is in sharp contrast

with the catastrophic failure modes of a conventional

system, where the loss of one receiver or detector can

cause the loss of an entire sounding band.

GeoSTAR, with its capabilities and advantages, repre-

sents a breakthrough in remote sensing, and PATH, with

its focus on hurricanes and other pressing national needs,

is a mission of great importance.

Acknowledgment

The authors acknowledge the support of the NASA

ESTO Program (G. Komar) and the Atmospheric Dynamics

Program (R. Kakar) as well as the valuable contributions of

W. Wilson, now retired from JPL.



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ABOUT T HE AUTHO RS

Bjorn Lambrigtsen joined the NASA Jet Pro-

pulsion Laboratory, California Institute of Tech-

nology, Pasadena, in 1982. He specializes in

atmospheric remote sensing and related research.

He is the GeoSTAR Principal Investigator and leads

a number of other efforts as well, including

hurricane-related research. He is a member of

the NPOESS Preparatory Project science team, is

the Microwave Instrument Scientist for the Atmo-

spheric Infrared Sounder project, and leads the

AIRS Atmospheric Science Group at JPL.

Shannon T. Brown received the B.S degree in

meteorology from the Pennsylvania State Univer-

sity, University Park, and the M.S. degree from the

University of Michigan, Ann Arbor. In 2005, he

received the Ph.D. degree from the University of

Michigan.

He joined the NASA Jet Propulsion Laboratory,

California Institute of Technology, Pasadena, in

2005 as a Member of the Engineering Staff in the

Microwave Advanced Systems section. His re-

search interests include microwave radiometer calibration, geophysical

algorithm development for both passive and active sensors, and cloud

and precipitation science.

Todd C. Gaier received the Ph.D. degree in

physics from University of California, Santa

Barbara, in 1993.

He is the Supervisor of the Microwave Astro-

physics and Earth Science Systems Group, Jet

Propulsion Laboratory, California Institute of

Technology, Pasadena. His research interests

include millimeter wave electronics for applica-

tions in astrophysics and Earth remote sensing.

His group develops technologies and instruments

using monolithic microwave integrated circuit (MMIC) components

operating at frequencies of 10–250 GHz. Active projects in the group

include the Planck-LFI mission to study the anisotropy and polarization

of the cosmic microwave background (CMB); the Q/U Imaging Experiment

(QUIET) exploring the polarization of the CMB; GeoSTAR, an interfero-

metric synthetic aperture imager for Earth atmospheric sounding from

geostationary orbit and the advanced microwave radiometers for the

Jason-II Mission mapping small variations in sea level across the globe

monitoring conditions such as El-Nino.

Linda Herrell received the B.A. degree in math/computer science/

languages from the University of Texas and the M.S.M.E. degree in fluids

and heat transfer from the City College of New York.

In addition to analytical work in computer science and thermal and

structural analysis, she has worked as both a payload (instrument) and

spacecraft systems engineer on Earthorbiting [Hubble Space Telescope,

Earth Observing System (EOS)] and deep space (Cassini) NASA missions,

and as Proposal Manager for several NASA science missions. She

currently serves as the Program Architect for NASA’s New Millennium

Program.

Pekka P. Kangaslahti received the M.Sc. and

Ph.D. degrees in electrical engineering from

Helsinki University of Technology, Espoo, Finland,

in 1992 and 1999, respectively.

Since 1999, he has been with the Jet Propulsion

Laboratory, California Institute of Technology,

Pasadena, first as a Visiting Engineer and currently

as a Senior Engineer. His main research interests

include monolithic millimeter wave integrated

circuits (MMICs) and MMIC module technology.

Alan B. Tanner received the B.S. and Ph.D.

degrees from University of Massachusetts,

Amherst, in 1984 and 1989, respectively.

He is a Microwave Systems Engineer with the

Jet Propulsion Laboratory (JPL), California Insti-

tute of Technology, Pasadena. His work with JPL

has focused on the design and calibration of

radiometers and radar scatterometers for remote

sensing.



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