The effects of power transmission from the solar power satellite and of radiation from the ground based rectennas on radio astronomy, radar astronomy, and optical astronomy are discussed. Interference sources, acceptable signal and noise levels, and conflicting site requirements for observatories and rectennas are considered

Thompson, A. R.

English

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THE EFFECTS OF A SATELLITE POWERSYSTEM ON GROUND-BASED ASTRONOMY
A. R. Thompson
National Radio Astronomy Observatory*, P. O. Box_,b,_c_ro,2, NM 87801_2 7_9
Bands allocated to radio astronomy occur at roughly octave intervals across the
radio spectrum. Threshold levels for harmful interference in these bands are
specified in CCIR Report Number 224-4, and are based upon observations with a
single antenna which are the most sensitive observations to radio interference.
They also assume reception of an unwanted signal in sidelobes of 0 dBigain.
These levels for the principal radio astronomy bands are shown in Figure 1.
In considering the effects of the SPS on radio astronomy the following compo-
nents of the SPS spectrum must be included.
The Power Signal The typical level at an observatory not nearer than 100 km to
a rectenna is estimated to be 0.01Wm -2, increasing by 5 dB if phase lock is
lost at a satellite. This leve_, received in 0 dBi sidelobes of radio tele-
scope, produces a signal of I0- W which is 35 dB above the overload threshold
for a typical parametric amplifier, or I0 dB above the overload threshold for an
FET amplifier. At present most radio astronomy receivers contain little or no
filtering between the antenna and the first amplifier stage, to avoid the noise
resulting from loss in ambient temperature filters. Cryogenically cooled fil-
ters, when developed, will prevent overload from the SPS power signal with
little loss in sensitivity for most bands other than 2.69-2.70 GHz and 4.99-5.0
GHz. In these last two bands, which are close to the power signal and its
second harmonic, impaired performance is likely to result.
Harmonics of the Power Signal Of several harmonics that fall close to radio
astronomy bands the second presents a serious •problem since it is likely to
cause overloading when antennas are pointed close to the satellites.
Transmitter-Generated Noise Bands of noise generated by the power transmitting
system will be centered on 2.45 GHz and low-order harmonics. For klystrons,
Arndt and Leopold (1978) estimate that the noise should be less than the CCIR-
224-4 level in the 2.69-2.70 GHz band. Crossed-field tubes and transistors are
also being considered as power generating devices and may have significantly
different noise properties.
Thermal Noise From The Collector Arrays The collector array on each satellite
subtends a maximum solid angle of 0.5 sq arcmin at the earth and operates at a
temperature of approximately 360 K. The thermal emission from 60 satellites
assuming unit emssivity is shown in Figure I for an observer's local midnight
when the 60 satellites appear most nearly broadside-on. The actual emissivity
of the cell arrays is not known, but probably results in a flux density level 3
to I0 dB lower than shown in Figure I, i.e., very close to the CCIR 224-4 lev-
els. Thus, for pointing angles closer than the separation of the 0 dBi sidelobe
level from the main beam of the radio astronomy antenna, interference can occur.
Intermodulation Products Intermodulation products are to be expected from
interaction of the power signal in nonlinear elements such as corroded joints in
towers and fences, receiving systems and possibly the ionosphere. Widely dis-
tributed signals such as television broadcast signals are most likely to be
*The National Radio Astronomy Observatory is operated by Associated Universi-
ties, Incorporated, under contract with the National Science Foundation.
419
https://ntrs.nasa.gov/search.jsp?R=19820014875 2020-03-21T16:26:33+00:00Z
involved.
Failure Related Signals With 6 x 106 tubes in orbit, a tube lifetime of 220,000
hours results in 26 failures per hour. Some failures may be associated with
increased noise, development of parasitic oscillations or phase-lock failure.
Klystrons are believed to be much less likely to produce such unwanted emissions
than other types of microwave-generating devices. However, since so many units
are involved, relatively rare failure modes will occur. Life testing of a large
number of units is required to evaluate this effect.
Rectenna Radiation Some incident power will be reflected from the rectennas
(Arndt and Leopold 1978) and noise and harmonics will be generated in the recti-
fication process. The mean distance between 60 rectennas within the U.S. will
be about 350 km. Choice of rectenna sites must make use of mountain ranges to
obtain adequate isolation of observatory sites.
Synthesis arrays in which the signals from many antennas are combined in pairs
to produce maps of the sky with high angular resolution are less susceptible to
radio interference than single-antenna radio telescopes by factors that range
from 10 to 40 dB depending upon the frequency, antenna spacing, bandwidth and
other observing parameters (Thompson 1979). Observations using very long base-
line interferometry (VLBI) are the least susceptible of all to radio interfer-
ence (Burke 1979). Thus in Figure 1 the area between the lines marked VLBI and
CCIR 224-4 represents the range of harmful interference thresholds for the
various types of radio astronomy instruments. Unfortunately synthesis arrays
and VLBI systems are not applicable to all types of astronomical investigations.
The principal effects of the SPS on radio astronomy can be summarized as fol-
lows. (1) For any type of radio astronomy system there will be an angular
distance from the satellites within which harmful interference will occur. The
width of this precluded zone depends upon various parameters of the observing
instrument and is estimated to vary from 20° for a single antenna to a few
degrees for a VLBI system. (2) For the 2.69-2.70 GHz and 4.99-5.00 GHz radio
astronomy bands sufficient filtering to prevent overloading by the power signal
or its second harmonic may not be achievable without significant impairment of
sensitivity resulting from filter insertion loss. (3) Conflicts between site
requirements for observatories and rectennas are likely to occur.
The above three effects represent the minimum likely interaction with radio
astronomy, and are sufficient to cause significant restrictions. The effects of
intermodulation products and failure-related signals discussed above could be
much more serious, but should be more amenable to mitigation. The quality of
SPS engineering and maintenance appears crucial to the coexistence of radio
astronomy.
Radar astronomy, like radio astronomy, uses large antennas and highly sensitive
receivers. Interference effects (David 1979) differ from those for radio as-
tronomy chiefly in the following ways. (1) Bandwidths are usually much less
than in radio astronomy, resulting in harmful thresholds 10-40 dB higher than
the CCIR 224-4 levels. (2) 2380 MHz is an important frequency, particularly at
the National Astronomy and Ionosphere Center, Arecibo, Puerto Rico. This is so
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near to the SPSpower frequency that impairment of sensitivity will result• (3)
Radar astronomy targets lie close to the ecliptic, and antenna pointing angles
will generally be within 30° of the geosynchronous orbit.
In optical astronomy the effects of the SPSresult mainly from the increase in
sky brightness caused by diffuse reflection of sunlight from the satellites and
subsequent scattering of the light in the earth's atmosphere. The effects are
discussed in several papers in the report of the Battelle Workshopon Satellite
Power Systems Effects on Optical and Radio Astronomy. They are difficult to
quantify precisely because there is at least a factor of two uncertainty in the
diffuse albedo of the satellites and the atmospheric scattering depends to some
extent upon atmospheric conditions. In a zone 10° to 20° wide centered on the
satellites, light contamination will cause impaired performance that cannot be
compensatedfor by increased observing time. Noticeable effects will be seen
over a band of sky at least 60° wide. The effects on ground-based optical
astronomy are probably more severe than upon radio astronomy.
References
Arndt, G. D. and Leopold, L., i3th Intersociety Energy Conversion Engineering
Conference, San Diego, CA, August, 1978.
Battelle Workshop on Satellite Power Systems Effects on Optical and Radio As-
tronomy, Seattle, WA, May, 1979. Report edited by G. M. Stokes and P. A.
Ekstrom is being published by the U. S. Department of Energy and NASA.
Burke, B. F., 1979, in Battelle Workshop Report.
Davis, M., 1979, in Battelle Workshop Report.
Thompson, A. R., 1979, in Battelle Workshop Report.
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The effects of a satellite power system on ground-based astronomy

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