Table Of ContentDevelopmentofanExperimentalPhasedArrayFeedSystem
andAlgorithmsforRadioAstronomy
JonathanC.Landon
Adissertationsubmittedtothefacultyof
BrighamYoungUniversity
inpartialfulfillmentoftherequirementsforthedegreeof
DoctorofPhilosophy
BrianD.Jeffs,Chair
KarlF.Warnick
DavidG.Long
MichaelD.Rice
WynnC.Stirling
DepartmentofElectricalandComputerEngineering
BrighamYoungUniversity
August2011
Copyright©2011JonathanC.Landon
AllRightsReserved
ABSTRACT
DevelopmentofanExperimentalPhasedArrayFeedSystem
andAlgorithmsforRadioAstronomy
JonathanC.Landon
DepartmentofElectricalandComputerEngineering,BYU
DoctorofPhilosophy
Phased array feeds (PAFs) are a promising new technology for astronomical radio tele-
scopes. While PAFs have been used in other fields, the demanding sensitivity and calibration
requirements in astronomy present unique new challenges. This dissertation presents some of the
first astronomical PAF results demonstrating the lowest noise temperature and highest sensitivity
at the time (66 Kelvin and 3.3m2/K, respectively), obtained using a narrowband (425 kHz band-
width)prototypearrayof19linearco-polarizedL-banddipolesmountedatthefocusoftheGreen
Bank 20 Meter Telescope at the National Radio Astronomy Observatory (NRAO) in Green Bank,
WestVirginia. Resultsincludespectrallinedetectionofhydroxyl(OH)sourcesW49NandW3OH,
and some of the first radio camera images made using a PAF,including an image of the Cygnus X
region. A novel array Y-factor technique for measuring the isotropic noise response of the array
is shown along with experimental measurements for this PAF. Statistically optimal beamformers
(Maximum SNR and MVDR) are used throughout the work. Radio-frequency interference (RFI)
mitigationisdemonstratedexperimentallyusingspatialcancelationwiththePAF.
Improved RFI mitigation is achieved in the challenging cases of low interference-to-noise
ratio (INR) and moving interference by combining subspace projection (SP) beamforming with a
polynomial model to track a rank 1 subspace. Limiting factors in SP are investigated including
sample estimation error, subspace smearing, noise bias, and spectral scooping; each of these fac-
torsisovercomewiththepolynomialmodelandprewhitening. Numericaloptimizationleadstothe
polynomial subspace projection (PSP) method, and least-squares fitting to the series of dominant
eigenvectors over a series of short term integrations (STIs) leads to the eigenvector polynomial
subspaceprojection(EPSP)method. Expressionsforthegradient,Hessian,andJacobianaregiven
for use in numerical optimization. Results are given for simulated and experimental data, demon-
stratingdeeperbeampatternnullsby6to30dB.
To increase the system bandwidth toward the hundreds of MHz bandwidth required by
astronomers for a fully science-ready instrument, an FPGA digital backend is introduced using a
64-input analog-to-digital converter running at 50 Msamp/sec and the ROACH processing board
developed at the University of California, Berkeley. International efforts to develop digital back
ends for large antenna arrays are considered, and a road map is proposed for development of
a hardware correlator/beamformer at BYU using three ROACH boards communicating over 10
gigabitEthernet.
Keywords: phased array feeds, radio astronomy, isotropic noise response, array Y-factor method,
interference canceling, adaptive array processing, adaptive beamforming, subspace projection,
subspacetracking,covarianceestimation,CASPER,ROACH,FPGA,astronomicalbackends
ACKNOWLEDGMENTS
I am deeply grateful for the generous support of Professors Brian Jeffs and Karl Warnick
who gave me the opportunity to do exciting research in exciting locations. More than just guiding
myresearch,theyencouragedmetobelieveinmyself. FortheseexperimentalresultsIamindebted
tostudents,faculty,andstaffatBrighamYoungUniversity(BYU),theNationalRadioAstronomy
Observatory in Green Bank, West Virginia (NRAO-GB), and the University of California, Berke-
ley.
At BYU, Drs. Jeffs and Warnick directed the research. James Nagel laid the foundation of
this work, designing the original downconverter boxes and 7-element array feed. As a student at
theUniversityofUtah,Jamesgrantedgenerousaccesstotheanechoicchambercriticallyimportant
to this work. My thanks to Micah Lillrose for the software correlator and digital back end; Jacob
Waldron for array construction, hot/cold ground screen construction, and early planning for the
GreenBank20MeterTelescopetests;JiyoungSonforhelpwithexperiments,DavidJonesforhis
mutual coupling work, Alan Stemmons who stayed up through the night to prepare for the initial
FEB mounting, Michael Elmer for his dish tipping work and experimental help, Vikas Asthana
for his help during the anechoic chamber tests and CASPER development, David Carter who
correlatedmuchofthe2008data,andTaylorWebbforhisinsights.
At NRAO-GB, I am grateful for Roger Norrod’s mentoring through my time as a summer
student and for his involvement in all of our experiments at the observatory. Drs. Rick Fisher and
RichardBradleydidthefirstPAFworkatNRAOinthemid-1990’s,openingthewayforthiswork.
Dr. Fisher was also extremely helpful during the planning and execution of these experiments,
providing valuable insights from an astronomer’s perspective as well as pointing of the telescope
itself. Thanks to technicians Bill Shank, Bob Simon, Ken Ward, and Dave Woody for their many
hours preparing for and helping with experiments; Pete Chestnut, Bob Anderson, Mike Hedrick,
andGaryAndersonfortheirhelpinconstructingpartsoftheexperimentalplatformandpreparing
fortheoutdoorantennarangetest.
At UC Berkeley, I am grateful to the late Professor Don Backer for sponsoring me for a
six month visiting studentship and for his kindness and belief in me; Dr. Aaron Parsons and Jason
ManleyfortheirmanyhoursteachingmeabouttheCASPERtoolsandbasicLinuxusage;Dr.Dan
Wertheimer,MattDexter,andDaveMacMahonfortheirfriendshipandassistanceinmanagingthe
PAPER correlator. Jack Hickish at Oxford has provided tremendous assistance getting the x64adc
working, providing several example designs and the yellow-block interface itself (with help from
DaveGeorgeattheKarooArrayTelescope).
This dissertation is for my dad who decided that instead of finishing his dissertation he
would move to Salt Lake City, get another Masters degree, and work late supporting his family.
He has kept my car running so I could focus on school. And it is for my mom who has been
very supportive of the adventures along the way – research trips to Green Bank, West Virginia;
Berkeley,California;TheNetherlands;andPuertoRico. Thankyou!
Description:Bank 20 Meter Telescope at the National Radio Astronomy Observatory (NRAO) in is shown along with experimental measurements for this PAF.
