NASA's Search for Extraterrestrial Intelligence (SETI) project utilizes two complementary search strategies: a sky survey and a targeted search. The SETI team at the Jet Propulsion Laboratory (JPL) in Pasadena, California, has primary responsibility to develop and carry out the sky survey part. Described here is progress that has been made developing the major elements of the survey including a 2-million channel wideband spectrum analyzer system that is being designed and constructed by JPL for the Deep Space Network (DSN). The system will be a multiuser instrument; it will serve as a prototype for the SETI sky survey processor. This prototype system will be used to test the signal detection and observational strategies on DSN antennas in the near future

Gulkis, S.

Klein, M. J.

Olsen, E. T.

Renzetti, N. A.

English

NASA Technical Reports Server

=ZS
TDA Progress Report 42-98
N90-12804
August 15, 1989
The NASA SETI Sky Survey: Recent Developments
M. J. Klein, S. Gulkis, and E. T. Olsen
SpacePhysicsand AstrophysicsSection
N. A. Renzetti
TDA ScienceOffice
NASA's Search for ExtraterrestriM Intelligence (SETI) project utilizes two
complementary search strategies: a Sky Survey and a Targeted Search. The SETI
team at the Jet Propulsion Laboratory (JPL) in Pasadena, California, has primary
responsibility to develop and carry out the Sky Survey part. This article describes
progress that has been made developing the major elements of the survey including
a 2-million channel wideband spectrum analyzer system that is being designed and
constructed by JPL for the Deep Space Network (DSN). The new system will be a
multiuser instrument; it will serve as a prototype for the SETI Sky Survey processor.
This prototype system will be used to test the signal detection and observational
strategies on DSN antennas in the near future.
I. Introduction
NASA's Search for Extraterrestrial Intelligence
(SETI) microwave observing project is preparing to carry
out two complementary search strategies: a Sky Survey
and a Targeted Search. The NASA Ames Research Cen-
ter has primary responsibility for developing and carrying
out the Targeted Search. Similarly, the Jet Propulsion
Laboratory (JPL) has primary responsibility to develop
and carry out the Sky Survey. The two approaches are
complementary in that the Targeted Search stresses sen-
sitivity to detect either pulsed or continuous signals, and
the Sky Survey gives up sensitivity in order to survey the
99 percent of the sky that is not covered by the Targeted
Search.
The objective of the Sky Survey is to search the entire
sky over the primary frequency range from 1.0 to 10.0 GHz
for evidence of narrowband signals of extraterrestrial ori-
gin. Frequency resolutions as narrow as 20 Hz will be used.
218
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217
Table 1 is a summary of the objectives of the survey. A
significant technical challenge is to design an observational
system that will meet maximum sensitivity requirements,
use existing radio telescopes, and allow completion of the
survey in a "reasonable" time span of 5 to 7 years. The
basic strategy for conducting the survey is introduced in
[1]. This article describes progress that has been made and
gives a high-level description of the hardware and software
of a prototype signal processing system that is currently
under construction.
II. Antenna Scan Strategy
Several scan strategies were considered for the Sky
Survey and parametric studies were conducted to help in
the selection process. These studies clearly showed that
the antenna beam must be actively scanned across the sky
at a high rate if the survey is to be completed in the allot-
ted time. The NASA SETI plan calls for the Sky Survey to
use the 34-m DSN antennas. Because these antennas will
not be dedicated to SETI, the scan strategy is quantized
so that blocks of time as small as 6 hours can be scheduled
and efficiently used.
To accommodate the strategy, the celestial sphere is
divided into 900 sky frames that are 60 deg wide by one deg
high. The shape and size of the sky frame were selected to
maximize the time spent scanning along the width of the
sky frame and to minimize the time lost turning around
at the edges. At the lower frequencies, where the antenna
beamwidths are larger, it will be possible to observe a stack
of several sky frames at one time; the resultant observed
area of sky might be 10 deg high, for example. Careful
scheduling will permit each sky frame to be observed when
the width is nearly parallel with the horizon so that the
antenna elevation angle will be nearly constant during each
scan line. This is done to improve the uniformity of the
system sensitivity within the sky frame.
A scan pattern has been selected which can be de-
scribed as the "racetrack." The scan direction is oriented
along a contour of nearly constant elevation and the order
of progression of successive scans is chosen to minimize el-
evation change as time progresses. Thus, the sky frame
is observed from top to bottom as it rises in the east; the
same sky frame is observed bottom to top as it sets in
the west. A schematic diagram of the oval ("racetrack")
pattern is shown in Fig. 1.
Details of this scan pattern have been derived so as to
utilize the time efficiently, to maintain maximum system
sensitivity with uniformity across the sky frame, and to
facilitate the signal detection process. Finally, certain fea-
tures of this pattern can be used to discriminate against
some classes of radio frequency interference (RFI). This
concept is explained in the following section.
III. Receivers and Signal Processing
The SETI Sky Survey requires an end-to-end sys-
tem that includes large antennas equipped with microwave
feeds, polarizers, amplifiers, and receivers that will span
the entire frequency band from 1.0 to 10.0 GHz. Pre-
liminary designs for these subsystems are being studied.
However, the most challenging components of the sys-
tem are the spectrum-analyzer and signal-detection sub-
systems. Much of the challenge of the signal processor
involves the impact of RFI, which is expected to be prob-
lematic, but which is poorly characterized at this time.
The final design of the Sky Survey processor system will
benefit if a prototype system is tested in an observational
environment that includes RFI. For this reason the SETI
Team at JPL decided to build and test a prototype system.
SETI is collaborating with the DSN Advanced Sys-
tems Program to design and construct an engineering de-
velopment model (EDM) of a wideband spectrum analyzer
system to serve as a prototype for the SETI Sky Survey
processor. A functional block diagram of the EDM system
is shown in Fig 2.
The principal features of the EDM system are as fol-
lows:
(1) The Wide Band Spectrum Analyzer (WBSA) is a
22x channel FFT (Fast Fourier Transform) spectrum
analyzer which covers an instantaneous bandpass of
40 MHz. The WBSA will operate on the 40-MHz sig-
nal at selectable resolutions of 20 Hz, 40 Hz, 80 Hz,
160 Hz, and 320 Hz. Clock-down modes will be avail-
able to increase each of these resolutions by factors
of 8 or 64. Arbitrary time-domain windowing func-
tions can be implemented in the hardware. Two inde-
pendent accumulators are available so the system can
function in the Dicke-switched mode commonly used
for radio astronomy. Two 20-MHz wide channels of
opposite polarization can also be implemented.
(2) The Background Estimation Convolution and Thresh-
old (BECAT) assembly is a hardware module that ap-
plies the "along scan" detection algorithm described
below. It uses convolution and order statistics to
determine the detected power baseline in segments
219
across the passband and to apply a signal threshold
that reduces the data rate by a factor of 10a or 104.
(3) The EDM software system manager is a network-
based C language environment based on the ANSI C
standard. It provides observatory control, data acqui-
sition, and analysis and it designed to be portable to
any system running Berkeley UNIX.
The WBSA has been fully designed and is presently
under construction. It operates at 4.5 GOPS (Giga oper-
ations per sec;ond) and uses 20-MHz IEEE standard arith-
metic units. The system contains 76 Mb of dynamic ran-
dom access memory (DRAM). It is contained in a single
electronics equipment rack and is controlled by a separate
Masscomp 5600 computer. Specifications for the BECAT
module are approaching completion; design work will be-
gin in the near future.
The EDM system hardware consists of 37 large-size
(40 x 40 cm) circuit boards capable of holding 600 16-pin
dual-in-fine-packages (DIPs). Twenty boards are unique
designs and 17 are copies. To date, 5 boards have been
completely designed, fabricated and tested; 8 other boards
are currently in process. A completed fixed-point Fast
Fourier Transform (FFT) board is shown in Fig. 3.
The EDM software system has been designed and a
framework program is currently operating at JPL. The
first of three successive software "builds" is scheduled for
completion by the end of 1989. The functionality of the
system is increased as each "build" is completed.
IV. Sky Survey Signal Processing
The instantaneous bandpass of the EDM is 40 MHz.
Because the system gain profile will not be perfectly flat
over the passband, the first step in the background estima-
tion is to normalize the spectral data across the band. This
is done by multiplying with an inverse gain matrix that is
regularly updated during the calibrations performed as the
antenna is turning from one scan to the next.
The resulting normalized spectrum is then divided
into subspectra whose bandwidths are optimized for "spec-
tral flatness." Each subspectrum is then examined to de-
termine a pair of order statistics. The rlth smallest order
statistic is used to determine the mean noise temperature
of the subspectrum and the r2th smallest is a test of the
validity of the assumed probability distribution function.
The algorithm leads to an excellent estimate of the noise
power in each subspectrum. The estimate is also robust
against strong RFI because it avoids using outliers of the
probability distribution function.
For the next stages of the EDM detection process, it
is convenient to describe the steps as they are applied to a
single frequency bin. These steps are schematically shown
in Fig. 4. As the antenna beam scans across the sky at a
rapid rate, the WBSA produces a complete 221 point spec-
trum every 50 msec. For each frequency bin in the spec-
trum, the WBSA hardware accumulates several spectra
and passes the accumulated data on to the BECAT mod-
ule (Step 2). The number of accumulated spectra and the
antenna scan rate are chosen so that five independent accu-
mulations are acquired during the time that the beam has
moved one half-power beam width (HPBW) on the sky. In
Step 3, the accumulated data samples for each frequency
bin are convolved with a 5-point filter that matches the
shape of the point-source response function (output power
versus time) for the scanning antenna beam. The output is
compared to the mean noise power (described above) and
signals that exceed a preselected threshold are compared
with RFI frequency "maps" resident in the system. Those
which do not match frequency patterns of previously de-
tected RFI are stored on disk and passed on to the signal
detection software (Step 4).
Next, one considers the inter-scan tests applied in the
EDM software. The algorithm is based on the underly-
ing assumption that putative extraterrestrial signals will
be originating so far from earth that they will be spa-
tially fixed on the sky; that is, they will match the side-
real motion of the stars. Therefore, a signal with sufficient
strength will appear on two or more adjacent scan lines,
and the lines will appear at the same place along the scan
line because the sky frames are produced after the scan
line data are compensated for sidereal motion. This fea-
ture makes interstellar signals distinguishable from RFI
and random noise, which will generally not move with
sidereal rate and direction. A schematic description of
this process is shown in Fig. 5.
Another useful discriminator against RFI is the ratio
test, which also utilizes the features of adjacent scan lines
as well as knowledge of the beam shape in the direction
orthogonal to the scan line. For this test we identify a
candidate signal, then look for a companion signal on the
adjacent scan lines. If the candidate signal is strong, the
companion signal(s) should appear with the proper ratio
unless the signal is RFI or has disappeared in the time
interval between scans. The effectiveness of the ratio test
diminishes as candidate signals become weak relative to
the system noise.
220
The BECAT module is designed to provide up to 8192
reports during each spectrum accumulation. This corre-
sponds to a data rate of 800 Kbytes per sec, which can be
compared with the 84,000 Kbytes per second data stream
from the WBSA accumulator. Each report consists of four
numbers, as follows:
(1) The first frequency of a contiguous run of bins that
exceeds threshold
(2) The number of bins in the contiguous run
(3) The integrated (convolved) power contained within
the run
(4) The maximum (unconvolved) power in a single bin of
the run
The first three quantities characterize the signal that
appeared above threshold. The fourth quantity is used to
determine if the detection was due to a signal that was
present throughout the five accumulations that match the
convolutional filter, or if a single accumulation contained a
burst with sufficient intensity to exceed threshold even af-
ter being diluted by the four other accumulations. Events
which fail this test cannot be summarily dismissed as RFI,
however, because they could be caused by a pulsed signal.
These events will be identified for follow-up observations.
The one remaining free parameter is the angular scan
rate of the antenna. The tradeoff is between sensitivity and
survey duration; slow scan rates improve the sensitivity
and extend the time required to complete the survey. The
JPL studies show that a reasonable compromise is to scan
at 0.2 deg per sec and to schedule about 30 sec to complete
the turnaround between scans. With these rates, one can
compute the time required to scan the sky for each of the
31 frequency bands that span the 1-10 GHz range. The
result is shown in Fig. 6. Note that the scan time increases
almost a factor of 10 from the lowest to highest frequency
surveys.
The right-hand ordinate in Fig. 6 displays the cumu-
lative scan time as successively higher frequency surveys
are included in the ensemble. For our scan rate of 0.2
deg per sec, the complete survey will require 725 days,
or about 2 years of continuous round-the-clock observa-
tions. Of course, this means that the real survey will take
3 or 4 times as long when one considers that the antenna
may only be scheduled one-third of the time for the sur-
vey and that even scheduled time must be allocated for
maintenance, weather, and follow-up observations of "in-
teresting" signals. Thus the observing schedule must be
adhered to in order to complete the survey in the allot-
ted time. The alternative would likely be to reduce the
frequency range of the survey.
V. Duration of the Sky Survey
The time required to complete the Sky Survey de-
pends upon several factors that affect the scan rate and
the number of scans that must be made. Some of these
parameters have already been selected: the survey will
span the frequency range from 1 to 10 GHz, and it will
be carried out with 34-m parabolic antennas. Preliminary
designs of microwave receiving systems indicate that the
instantaneous bandwidth for the 6 lowest frequency bands
will be 160 MHz, and the bandwidth for the remaining 25
higher frequency bands will be 320 Mttz.
The spacing of the adjacent scan tracks in the sky
frame affects the spatial uniformity of the survey sensi-
tivity. The nominal spacing is 1/2 HPBW. As the HPBW
decreases with frequency, the number of scan lines for each
sky frame will increase linearly with frequency.
VI. Conclusions
The EDM system is scheduled to undergo validation
tests at Goldstone in 1990. These include the following:
(1) Testing and refinement of the signal detection algo-
rithms
(2) Testing of alternative scan strategies and observatory
control functions
(3) Testing of RFI rejection techniques
Results from these tests need to be fed back into the
design of the full-scale Sky Survey processor, which will be
built in the early 1990s.
221
Reference
[1] S. Gulkis and E. T. Olsen, "The Search for Extraterrestrial Intelligence," in
Proceedings of the NRAO Workshop Number 11, K. I. Kellerman and G. A.
Seielstad, eds., pp. 161-162, 1986.
222
Table 1. Objectives of the NASA SETI sky survey
Parameters Objectives
Spatial coverage
Frequency range
Duration
Frequency resolution
Instantaneous bandpass
Sensitivity
Polarization
Signals
Entire celestial sphere
1.0 to 10.0 GHz inclusive and
higher spot bands
6 years
_, 30 Hz
300 MHz
5 10-23v/_GHz) Wm-2
Simultaneous dual circular
Primarily CW with natural radio
astronomy fallout
223
Fig. 1. Sky survey oval scan pattern. 
HARDWARE 
PROTOTYPE (EDM) SOFTWARE SYSTEM 
I - - - - - - - - - - - - -  1 SUBSYSTEMS 
Fig. 2. Prototype system functional diagram. 
224 ORIGINAL PAGE 
BLACK AND WHITE PHOTOGRAPH 
Fig. 3. Engineering Development Model (EDM) of survey processor: 
a fixed-point FFT board. 
ORIGINAL PAGE 
BLACK AND WHITE PHOTOGRAPH 
225 
SKY SURVEY SCAN LINES 
DIRECTION OF TELESCOPE MOTION 
c 
(a) 
SCAN 
NO. 
SKY SURVEY SCAN LINES 
DIRECTION OF TELESCOPE MOTION 
0 = ACCUMULATION POINT ABOVE BECATTHRESHOLD 
Fig. 5. Signal processing in prototype hardware. The 
detection software compares candidate points aligned 
on adjacent spans. 
0 = ACCUMULATION POINT 
Fig. 4. Signal processing in prototype hardware: (a) high- 
speed sampling: (b) several spectra are co-added (ac- 
cumulated) by the WBSA: (c) a 5-point convolutional 
filter is applied by the BECAT: (d) data above BECAT 
threshold are passed t o  the signal-detection software. 
01 I I I I I I I I J 
1 2  3 4 5 6 7 8 9 10 
FREOUENCY, GHz 
Fig. 6. Time required t o  scan the sky in 31 adjacent 
frequency bands. 
226 


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