Project Cyclops, A Design... - Department of Earth and Planetary ...

mation are common to all antennas in a particular
•subarray. The rest of the information is specific to a
particular antenna. In principle, it is not necessary to
send both position and rate information. However,
unless a closed-loop data system is provided for continuous
monitoring of all position information, it is
impossible to correct the positions by rate signals alone.
If only position information is sent, then in one instance
at least (the local oscillator phase shift), the refresh rate
would have to be several times per second. By sending
both, and by incorporating a modest amount of logic
and arithmetic capability at each antenna, we can greatly
reduce the required refresh rate. In fact, the refresh rate
will probably be set by the delay one can tolerate in
having the array respond to new instructions. For the
discussion to follow, we assume a refresh rate of once a
second.
To address one of a thousand or more antennas we
will need an address code of 10 or more bits. Let us
assume a 12-bit code. We can then divide the array into
natural sectors determined by the tunnel pattern and
serve all the antennas in each sector from a common
coaxial (or shielded pair) cable system. The first three
bits of the antenna address would then determine over
which of up to eight cable systems the remainder of the
addresses and the data for the antennas in a given sector
were sent. Thus, we can address any of 4000 antennas
with only a nine bit transmitted
address.
During each refresh cycle we would then send the
address plus the LO phase and phase rate-a total of 32
bits of information-to each of as many as 512 antennas
in each sector for a total of 16,384 bits.
In addition we propose to send over all cables all the
position information needed for all possible subarrays.
An antenna assignment code, previously transmitted to
each antenna would determine which set of position
data is read by each antenna. Each array requires 68 bits
of information. If we assume that 16 subarrays are
possible at any one time, an additional 1088 bits are
required for a total of 17,472 bits of information per
refresh cycle.
Synchronizing and punctuation information must also
be transmitted. This can be done in several ways, such as
by pulses of a different amplitude or polarity. One rather
attractive method _ uses a set of unique code symbols,
no one of which can occur as a combination of the
others under any time displacement. In addition:
1. The code is self-clocking; a transition occurs each
Nyquist interval.
2. There is zero dc component in each symbol.
3. The code is immediate; each symbol is uniquely
dicipherable
in and of itself.
The method requires a little more than three Nyquist
intervals per symbol on the average, but in our case this
theoretically would mean only 26 kHz of bandwidth. In
practice, 50 kHz of bandwidth would be ample.
The information in the last five items listed in Table
10-4 need only be sent occasionally-when an antenna is
being calibrated, tested, or transferred from one subarray
to another. It is proposed that this information,
when needed, be sent in lieu of the phase shift
information to the antenna or antennas involved, the
latter information being unnecessary at such times. A
special code group after the antenna address would
signal the processer that the data was command, not
tracking, information.
Thus in setting up a subarray, the operator would list
the antennas wanted in the array, the operating frequency,
and the right ascension and declination of the
source. These data would be fed into the central
computer, which would search for the first vacant array
codes. It would then assign the array code to all the
antennas involved and specify the proper receiver and
pump frequency. It would then compute the azimuth,
elevation and phase data, and start sending these in the
proper slots. An operational command code would then
be sent to slew to the new position. While this is
happening position error and alarm signals are disabled,
and no IF signal is transmitted. After all antennas have
been interrogated (see the next section) to assure that
they are in proper position and operating correctly, a
start transmission command would be sent.
The logic and memory required at each antenna to
make the above control system operable is very inexpensive.
It could very likely all be put on one or two LSI
chips, and its cost should not exceed $100 per unit in
quantities of 1000 or mo01. However, the output
circuitry and switching system required to interface the
data with the units involved would raise the cost
considerably. A total cost of $5000 per antenna control
unit seems reasonable.
The cabling involved costs about 25 cents/m and its
length is about equal to the total tunnel length. Probably
no repeaters would be needed; only transformers at the
branch points and bridging amplifiers at the antenna
sites. For a lOOO-element array OSth 300-m spacing the
total cost should not exceed $125,000 or another $125
per antenna.
it thus appears that the cost of the control system is
so small as to be lost in the cost uncertainty of the
expensive parts of the system.
Monitoring and Calibration System
It is not very important in the search mode, if, say,
' S. Walther,private communication.
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