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IAC-07-A4.2.04 QUANTIFYING PAST…
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Created: Fri Mar 30 22:33:04 2007
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IAC-07-A4.2.04
QUANTIFYING PAST TRANSMISSIONS USING THE SAN MARINO SCALE
Shuch, H. Paul Almár, Iván
The SETI League, Inc. Konkoly Observatory
121 Florence Drive Hungarian Academy of Sciences
Cogan Station PA 17728 USA Budapest, Hungary
paul@setileague.org almar@konkoly.hu
ABSTRACT
To date, at least five experiments which could be classified as Active SETI, or METI
(Messaging to Extra-Terrestrial Intelligence) have been conducted from Planet Earth: the
well-known Arecibo Message of 1974, two Cosmic Call transmissions from Evpatoria,
the Teen-Age Message to the Stars also transmitted from Evpatoria, and the paradigm-
altering Invitation to ETI, being quasi-transmitted continuously via the Internet. In addi-
tion, planetary defense radar transmissions from Earth, radiated for the purpose of detect-
ing potentially hazardous asteroids, can be considered inadvertent METI signals, to the
extent that they can be detected over interstellar distances. Planetary radar transmissions
from both Goldstone and Arecibo are considered. Each of these various emissions is ana-
lyzed in terms of duration, directionality, information content, and transmitter power, and
then each is assigned an integer ordinal value on the proposed San Marino scale for quan-
tifying transmissions from Earth. A comparative analysis of these quantified transmis-
sions underscores the difference in impact of various METI experiments, suggesting the
utility of the San Marino Scale as a valuable analytical tool for making informed policy
decisions.
INTRODUCTION ing the potential impact of such trans-
missions from Earth. Here we apply the
SETI, the well established science San Marino Scale to several historical
involved with Searching for Extra- transmissions, to better assess its utility.
Terrestrial Intelligence, has traditionally
applied high-gain antennas and sensitive SIGNAL INTENSITY REFERENCE
microwave receivers to the challenge of
detecting artificial emissions from dis- An important factor in determining
tant civilizations. The companion activ- the significance of a transmission from
ity becoming known as METI (Messag- Earth is its intensity, which is in term
ing to Extra-Terrestrial Intelligence) in- related to effective isotropic radiated
volves the converse: transmitting from power (EIRP) and spectral dispersion.
Earth signals which extraterrestrial SETI Such characteristics can be readily quan-
scientists could presumably detect. Rec- tified for any transmission, historical or
ognizing that such transmissions are not hypothetical, planned, past, or proposed,
wholly without risk, The San Marino and then expressed by comparison to an
Scale (Almár, 2007) has been introduced established standard.
as an integer ordinal index for quantify-
It has been shown (Shuch, 2006) that vary widely. At minima in the cycle of
the Sun affords us with such a standard, solar activity, the background radiation
when describing the intensity of trans- emitted by the sun is obviously at a
missions from Earth. Solar radiation minimum, hence has the least impact on
varies with frequency in a predictable the signal to noise ratio (SNR) of any
way, which we can readily model. How- terrestrial transmission being received by
ever, we must consider that the sun's purported alien civilizations.
spectral flux density tends to vary widely By quantifying our transmissions
throughout an eleven year solar activity relative to minimum solar flux, we are
cycle (see Figure 1). perhaps overstating the SNR which a
We describe transmission intensity given terrestrial transmission might im-
as a multiple relative to the solar flux. It part on extraterrestrial receivers. This
can be seen in Figure 1 that this value approach ensures that our resulting In-
can vary by perhaps a factor of five be- tensity term, which contributes to the
tween minimum and maximum values. overall San Marino Scale value, is a
Rather than addressing the daunting task best-case number as far as signal detect-
of determining the actual solar flux den- ability is concerned. Since it is the po-
sity which prevailed at the instant of tential negative consequences of trans-
each transmission being analyzed, we mission which we seek to quantify, we
propose to model the quiet sun. Estab- believe this conservative approach,
lishing a minimum noise baseline seems which may slightly overstate signal im-
especially appropriate in the case of pact, is appropriate to the function the
long-duration METI experiments during San Marino Scale was intended to serve.
which the solar flux can be expected to
Figure 1
Solar intensity plotted over five full solar cycles (from Tapping, 2001)
QUANTIFYING THE QUIET SUN wide range of frequencies in the micro-
wave spectrum. The unit of measure,
Figure 1 above, which records daily SFU, equates to Watts x 10-22 per Hz of
average solar flux densities in Solar Flux bandwidth per square meter of area at
Units (SFU) over more than half a cen- the Earth's surface. Since one Jansky
tury of observations at a wavelength of (Jy) equals 10-26 W m-2 Hz-1, it follows
10.7 cm (corresponding to a frequency that 1 SFU equals 104 Jy. Quiet sun val-
of 2.8 GHz), derives from data taken ues across the entire microwave spec-
first near Ottawa ON, Canada, and later trum are plotted in Figure 2.
at the Dominion Radio Astrophysical Nonlinear regression analysis shows
Observatory in Penticton BC, Canada. 1 that Tapping's data fit closely a cubic
Solar flux density at the minimum of equation, the respective coefficients for
the solar cycle (with no sunspots or other which are depicted in Figure 2. We use
geomagnetic activity in evidence) has this cubic model to interpolate flux den-
been quantified (Tapping, 2001) over a sity values for the quiet Sun at any fre-
1
quency for which we are analyzing the
Sunspots or geomagnetic storms can of course impact of a terrestrial transmission.
occur even during a solar cycle minimum, as
suggested in Figure 1.
Figure 2
Flux density of the quiet Sun, as a function of microwave frequency
(from data in Tapping, 2001)
OUTPUT OF THE SOLAR SPHERE
It is common practice, when analyz- Remember that flux density refers to
ing terrestrial transmission intensity, to radiation intensity over one square meter
multiply transmitter power (typically in at the Earth's surface. Multiplying the
Watts) by antenna gain relative to an iso- solar flux density (at any given fre-
tropic radiator. An isotropic source quency) by the area of the sphere de-
would radiate uniformly across all 4 fined by the Earth's orbit, we find the
steradians of space.2 total Solar spectral density centered on
The isotropic reference is a mathe- the selected frequency, in Watts per Hz.
matical convenience, since physical, For example, at 15 GHz, Figure 2
truly isotropic radiators do not exist (you suggests a quiet sun flux density of 500
cannot build one, buy one, or find one in SFU. Multiplying 500 SFU by 10-22 W
nature). 3 Nevertheless, practical anten- m-2 Hz-1 (our definition of the SFU) by
nas are calibrated in decibels relative to 2.83 x 1023 m2 (the surface area of our
isotropic (dBi). sphere of 1 AU radius), we see that the
Similarly, although we have quanti- quiet Sun delivers about 1.4 x 104 W/Hz
fied solar flux density relative to one isotropically at 15 GHz. By applying the
square meter of area on the Earth's sur- cubic model for solar flux derived in
face, the Sun radiates (almost) isotropi- Figure 2, we can arrive at similar values
cally, 4 Thus, to determine the total So- for solar spectral density, centered on
lar radiation output, we calculate the sur- any frequency in the microwave spec-
face area of a sphere of radius 1 Astro- trum.
nomical Unit (AU), the mean distance All that remains now is to similarly
between Earth and Sun. This radius is a express the spectral strength of any
distance of 1.5 x 1011 meters, hence the transmission from Earth in Watts per Hz
relevant surface area is 4 steradians (isotropic). To do so, we need to know
multiplied by 1 AU, or 2.83 x 1023 m2. transmitter power, antenna gain, and
modulation bandwidth. We then com-
2
In other words, an isotropic antenna works pare the resulting value to the corre-
equally poorly in all directions! sponding solar spectral density, as calcu-
lated previously, to determine the `I' (in-
3
The cosmic microwave background radiation tensity) term of the San Marino Scale.
is very nearly isotropic, but (as evidenced by A Microsoft Excel® template for
WMAP satellite measurements) even that radia- calculating the `I' term for any terrestrial
tion demonstrates some irregularities. transmission is shown as Figure 3. This
template is available for download from
4
We say "almost" because the Sun, like all ro- the website of the IAA SETI Permanent
tating bodies, is slightly oblate, hence its radia- Study Group, at this URL:
tion pattern cannot be truly isotropic. In addi-
tion, coronal holes and active regions further
disturb its isotropy.
San Marino Scale Intensity Calculator
The Signal: Quiet Sun:
= 5.99 cm = 5.99 cm
= 5.01 GHz = 5.01 GHz
P xmtr = 1.50E+05 W = 103.7 SFU
G ant = 69.4 dBi = 1.04E+06 Jy
EIRP = 1.21E+02 dBW = 1.04E-20 W/Hz*m^2
= 1.31E+12 W = 1.50E+11 m
f = 2.40E+04 Hz ^2
4 = 2.83E+23 m^2
Data Rate = 2000 Baud P sun = 2.93E+03 W/Hz Isotropic
BW = 5.00E+04 Hz
Duty Cycle = 100 %
P sig = 2.61E+07 W/Hz Isotropic
SNR max =
P sig / P sun = 8.92E+03
San Marino 'I' = 4
Calculator rev. 19 Dec 2006 by H. Paul Shuch
Figure 3
San Marino Scale `I' term calculator template
See http://iaaseti.org/SMI_I.xls
CONSIDERING THE `C' TERM is also important, as far as the signal's
overall impact is concerned.
In calculating San Marino Scale val- The Characteristic term `C' in the
ues (as currently defined), the intensity San Marino scale is a categorical, ordi-
term `I' takes on integer values between nal integer between 1 and 5, related to
0 and 5. Since the overall San Marino information content.
Scale yields an integer value between 1 At the lower extreme, it is argued
and 10, it is clear that intensity tells only that either a radar pulse or a steady, un-
half the story. The character of the in- modulated CW carrier imparts only lim-
formation content contained in the signal
ited useful information, hence its associ- driving a 70 meter diameter fully steer-
ated `C' term is 1.5 able parabolic antenna providing an es-
At the opposite extreme, any trans- timated gain of +69.4 dBi, for a maxi-
mission from Earth in response to a con- mum EIRP of 1.3 x 1012 Watts.
firmed SETI detection would be infor- Evpatoria has been the uplink facility
mation-rich, 6 suggesting its associated for three METI experiments to date, the
`C' term would have to take on a maxi- Cosmic Call transmissions of 1999 and
mum value of 5. Other degrees of in- 2003, and the Teen Age Message to the
formation content will of course take on Stars in 2001 (Zaitsev, 2006). Although
intermediate values for `C'. all three of these transmissions included
different information content, they
SAN MARINO SCALE shared similar modulation characteris-
tics, encoding their messages with fre-
As described in Shuch (2006) and quency shift keying at +24 kHz peak de-
Almar (2007), the San Marino Scale viation, at a maximum data rate of 2
combines a parametric term quantifying kBits per second. The corresponding
intensity with a categorical one indicat- modulation bandwidth is 50 kHz, for a
ing message content. The resulting sum spectral signal density of 2.6 x 107 Watts
of two integers, one on a scale of 0 to 5, per Hz.
the other varying between 1 and 5, takes At the 5.99 cm wavelength corre-
on an intuitive (and appealing) range of sponding to the frequency of Evpatoria's
values: 1 to 10. transmitter, the minimum isotropic solar
Next, we shall determine `I' and `C' flux equates to 2.9 x 103 Watts per Hz.
values for a variety of transmissions Thus, the signal amplitude of the Evpa-
from Earth, and combine them into de- toria METI experiments exceeds that of
scriptive San Marino Scale indices the quiet sun by roughly four orders of
unique to each example. magnitude. By design, the `I' term of the
San Marino scale is the integer common
EXAMPLE: EVPATORIA logarithm of this ratio, which in this case
is a value of 4 out of a possible 5.
The Evpatoria Planetary Radar Tele- In terms of their message content,
scope on the Crimea Peninsula in each of the three Evpatoria METI ex-
Ukraine boasts a 150 kW C-band trans- periments was a "special signal targeting
mitter, normally operated at 5.01 GHz, a specific star or stars, at a preselected
5
time, in order to draw the attention of
Some argue that such a signal contains no data, ETI astronomers." As described in Al-
hence the corresponding `C' integer should actu-
ally be zero. In fact, even an unmodulated car-
már (2007), this corresponds to a `C'
rier can be considered a one-bit digital message, term of 3, also out of a possible 5. Thus,
clearly conveying information: "Here I am!" the total San Marino Scale value for
Additionally, an unmodulated carrier onto which these particular messages is 7, a level to
Doppler shift has been imparted can convey a which we have assigned a significance
wealth of astronomical data.
descriptor of "high" (Shuch 2006).
6
Implicit in a reply to a confirmed SETI detec-
tion, even absent any message of our own, is the
information "We have heard you," which con-
veys to our communications partners our (admit-
tedly limited) technological prowess.
EXAMPLE: ARECIBO MESSAGE very nature makes them potentially de-
tectable over interstellar distances.
The world's largest radar telescope, Thus, we consider such radar leakage to
the 305 meter diameter spherical dish at serve as a de-facto METI signal, and will
Arecibo, Puerto Rico, was home to the analyze them accordingly.
first METI experiment on record, in No- Existing NEO radar transmitters em-
vember 1974. A 950 kW transmitter ploy high power klystrons and high gain
driving the +72.4 dBi antenna produced antennas, operating at S-band (Arecibo)
an effective isotropic radiated power and X-band (Goldstone). Modulation is
(EIRP) of 1.65 x 1013 Watts at a wave- either continuous wave (CW) for detec-
length of 12.6 cm (LaLonde, 1974). tion or binary phase-coded CW for range
Narrow frequency shift keying at a 10 resolution (Ostro, 2006).
bit per second data rate produced a In the case of the Arecibo transmis-
modulation bandwidth on the order of 20 sions, hardware is quite similar to that
kHz. Consequently, the isotropic spec- used for the 1974 Arecibo Message
tral density of the radiated signal was on transmission, thus, the EIRP is similar to
the order of 8.2 x 108 Watts per Hz. that in the prior example. However, sig-
At the 2380 MHz transmission fre- nificantly reduced modulation band-
quency of the Arecibo Message, a quiet width is observed for either CW or bi-
Sun would produce an isotropic flux of nary phase-coded CW. Thus, the signal
1.68 x 103 Watts per Hz. Thus, the Are- density exceeds that of the Sun by more
cibo Message can be seen to have out- than five orders of magnitude, yielding a
shone the Sun by a factor of 105, for a San Marino Scale `I' term of 5.
San Marino Scale `I' term of 5. The X-band signals from Goldstone
Directed as it was toward M13, a star consist of a similarly modulated trans-
cluster some 25,100 LY from Earth, the mitter, this time at a wavelength of 3.5
Arecibo Message (like the Evpatoria cm (frequency of 8.56 GHz), driving a
transmissions discussed in the foregoing dish with +74.4 dBi gain. The resulting
section) was indeed a "special signal tar- EIRP is 1.24 x 1013 W, similar to that at
geting a specific star or stars, at a prese- Arecibo, also producing a spectral den-
lected time, in order to draw the atten- sity more than five orders of magnitude
tion of ETI astronomers." This corre- greater than the quiet Sun. Hence, the
sponds to a San Marino `C' term of 3. `I' term from Goldstone also equals 5.
The overall San Marino score for the The signal characteristic for both of
Arecibo message is thus 8, which char- these NEO detection transmissions is
acterizes its significance as "far- best described as "A beacon without any
reaching." message content (e.g., planetary radar),"
for a corresponding `C' value of 1. The
EXAMPLE: NEO RADAR resulting San Marino Scale index for
NEO radars thus totals 6, for an overall
Planetary protection radars at Are- significance descriptor of "noteworthy."
cibo and Goldstone are routinely used to Although planetary defense radar signals
manage the risk of impact from near are among the most powerful artificial
earth objects (NEOs) such as asteroids, emissions emanating from planet Earth,
comets, and meteors. Although not in- earning the highest possible `I' value,
tended as interstellar transmissions, their their information content is minimal, rat-
ing the lowest possible value of the `C' Analyzing the link budget of a wide
term. Thus, it is not surprising that they variety of telecommunications satellite
score near the middle of the San Marino uplinks, for either geosynchronous
Scale. (Clarke orbit) or low Earth orbit (LEO)
satellite constellations, we find that even
EXAMPLE: IETI the most powerful uplink signal emitted
is still many orders of magnitude weaker
Unlike other METI experiments, the than the solar flux. Thus, even if pages
Invitation to ETI initiative does not rely from the Invitation to ETI website
upon interstellar transmissions in order (http://ieti.org) happen to be accessed by
to establish contact with our cosmic an internet user via satellite link, the cor-
companions. This experiment contem- responding San Marino Scale `I' term
plates the existence of ETI civilizations will be zero.
so advanced that they possess the tech- The Invitation to ETI itself, on the
nology to monitor the terrestrial internet. other hand, is extremely information-
Implicit in this assumption is the exis- rich. The website's many pages provide
tence of ETI probes somewhere in the a wealth of information about human
vicinity of our solar system (Tough, technology, culture, and society, as the
2000). ultimate goal of the experiment is to en-
As a rule, our terrestrial internet is courage dialog. Thus, the content of the
just that a network of terrestrial signal `transmission' is best described as a
distribution links, employing coaxial ca- "continuous, broadband transmission of
ble, fiber optics, low-power wireless re- a message to ETI," which corresponds to
lay, and the occasional microwave a San Marino Scale `C' term of 4. The
communications satellite. Of these, only resulting overall San Marino Scale value
the communications satellite uplink is of is 4, which corresponds to a significance
interest for the purpose of the present described as "moderate."
analysis, because it is susceptible to in- For the foregoing examples, the rele-
tercept by any alien probes purported vant San Marino Scale terms, score, and
monitoring our planet's electromagnetic significance descriptor, are summarized
environment. in Table 1.
San Marino Scale
METI Experiment "I" "C" Overall Significance
Arecibo Message 5 3 8 Far-Reaching
Evpatoria 4 3 7 High
NEO Radar 5 1 6 Noteworthy
Invitation to ETI 0 4 4 Moderate
Table 1
Analysis of Example METI Transmissions on San Marino Scale
CONCLUSIONS REFERENCES
By applying the San Marino Scale to Almár, Iván, and H. Paul Shuch, "The
a variety of historical METI experi- San Marino Scale: a new analytical tool
ments, both willful and de-facto, we for assessing transmission risk." Acta
have demonstrated its utility in compar- Astronautica 60, 57 59, 2007
ing the impact of disparate transmissions
into space from planet Earth. It is clear LaLonde, L.M., "Upgrading the Arecibo
from this analysis that blanket policy Observatory." Science Vol 186 no 4160,
decisions, either restricting or sanction- pp. 213 218, 18 October 1974
ing transmissions from Earth on an
across-the-board basis, are inappropriate. Shuch, H. Paul, and Iván Almár, "Updat-
Instead, we feel we have demonstrated ing the San Marino Scale." Paper IAC-
the importance of quantifying each 06-A4.1.01, 35th Symposium on SETI,
transmission on its individual merits, 57th International Astronautical Con-
using an objective scale, and taking this gress, Valencia Spain, October 2006.
quantification into consideration in the
policy-making arena. Ostro, Steve, NASA/JPL, email corre-
Following a process of review by spondence with the first author, Decem-
and feedback from our colleagues, we ber 2006.
recommend that the San Mario Scale
proposed herein be considered, and pos- Tapping, Ken, "Antenna calibration Us-
sibly adopted, by the SETI Permanent ing the 10.7 cm solar flux." 30th Ameri-
Study Group of the International Acad- can Meteorological Society Conference
emy of Astronautics. on Radar Metrology, Munich Germany,
We emphasize that the San Marino July 2001.
Scale remains a work in progress, having
been neither endorsed nor adopted by the Tough, Allen, "How to Achieve Contact:
International Academy of Astronautics, Five Promising Strategies." Section V,
or any other body. Nevertheless, we Paper 6 in When SETI Succeeds:
consider it a useful tool for assessing the The Impact of High-Information Con-
potential impact of transmissions from tact, 115-126, Foundation for the Future,
Earth. 2000.
Zaitsev, Alexander L., "Messaging to
extra-terrestrial intelligence." arXiv,
,
2006.