Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/JPLNEWS1/0338.PR

Text File | 1993-04-21 | 10KB | 174 lines

OFFICE OF PUBLIC EDUCATION AND INFORMATION
JET PROPULSION LABORATORY, CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA.  TELEPHONE [phone]

FOR RELEASE SUNDAY, JUNE 20, 1965

          For 204 days the Mariner IV spacecraft has been charting
the vast expanse between the planets Earth and Mars on the
longest deep space mission in history.
          Since Mariner IV was launched last November 28, four
American astronauts have orbited the Earth, two Ranger spacecraft
have photographed the moon and dozens of other missions have begun
and ended.
          Mariner must fly yet another four weeks before it passes
within 6000 miles of Mars for close-up photography of the planet's
surface and as many weeks beyond Mars for transmission of the
pictures and other scientific data back to Earth.
          Although the spacecraft is now more than 113 million
miles away, it remains in constant touch with its home base, the
Jet Propulsion Laboratory in Pasadena, California, through a
remarkable space communications link.
          In a sense, Mariner IV is at the end of a long, taut,
invisible kite string, reeling itself out hundreds of thousands
of miles each day.  It's along this straight line that Mariner's
radio signal travels to Earth.
          For about nine hours each day, engineers and technicians
at the Goldstone Space Communication Station in Southern Califor-
nia have a firm grip on the imaginary length of string.  One of
Goldstone's 85-foot dish antennas is trained on Mariner, gathering

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in its faint broadcast and stands ready to transmit commands to
the spacecraft should they be required.
          As the Earth turns under Mariner, the antenna moves
slowly from east to west, finally losing radio contact on the
horizon.  Before the spacecraft gets out of earshot, another
station near Canberra, Australia, picks up the string for the
next nine-hour stint.  From Canberra, control is handed over to
Johannesburg, South Africa, and again back to Goldstone.
          Goldstone, Canberra and Johannesburg are stations of
NASA's Deep Space Network, which handles communications for
America's unmanned exploration of the moon, planets and inter-
planetary space.  The stations are located approximately 120
degrees apart around the glode so that at least one antenna is in
Mariner's line of sight at all times.
          Control center for the Mariner mission is headquartered
in the Space Flight Operations Facility at JPL in Pasadena.  It
is linked to the DSN stations by a ground communications system.
          The control center has been operating on a 24-hour
basis since launch day, the Friday following Thanksgiving Day,
1964.  It is here that engineers and scientists have almost
immediate access to Mariner's constant flow of messages--about
100,000 each day--that include measurements made by the
scientific instruments and those indicating the current condition
and performance of the spacecraft.  It is here also that the data
is analyzed and decisions are made to send commands to Mariner.
          Latest engineering event monitored in near real-time
was a command issued by the Mariner's on-board central computer

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and sequencer to the Canopus tracker last Monday morning to
compensate for the changing relationship between the spacecraft,
the sun and the star Canopus.  The command electronically changed
the "look angle" of the tracker so that the star will stay in
view through the encounter sequence next month and beyond.
          The Johannesburg station was tracking Mariner when the
"update" occurred right on time at 11:40 a.m. EDT.  It was about
10 minutes later that Mariner's keepers in Pasadena learned that
the command was issued and acted upon properly.  Because of the
communications distance on that day, Mariner's radio signal needed
10 minutes to reach the big antenna in Johannesburg.
          As Mariner IV gets closer to Mars and to the critical
encounter sequence, activity will heighten in the Space Flight
Operations Facility and at the tracking stations.  During the
week prior to Mariner's July 14 planet fly-by, the three stations
now tracking Mariner will follow it from horizon to horizon,
increasing the viewing time of each to about 12 hours a day and
hence the overlapping coverage.  Three additional stations of the
DSN will be "on line"--at Woomera, Australia, Madrid, Spain, and
a second station at Goldstone.
          Power level of Mariner's radio signal, if measured at
the spacecraft antenna, is about 10 watts.  On July 14, at the
point of Mariner's closest approach to Mars where the distance to
Earth will be more than 134 million miles, power of the signal
received at Goldstone will be, in engineering language, 10 ?-19\
watt, or .0000000000000000001 of one watt.  This means that the

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signal strength will dwindle from 10 watts at Mars to one-bil-
lionth-of-one-billionth-of-one-watt at Earth.
          Super-sensitive receivers, coupled with the big 85-foot
antennas at the DSN stations are able to home in on this signal,
as faint as it is, and amplify it for telemetry processing,
recording and relay via the ground communications system to the
control center in Pasadena.
          Before the technical personnel at any one of the
tracking stations can get hold of Mariner's long kite string by
"locking up" the signal with the station receiver, they must know
where to point the antenna when their turn comes around.  This
information is supplied by the radio signal itself.  As the
tracking data is received at one of the stations and is relayed
to the Space Flight Operations Facility, it reveals to trajectory
experts its exact location in space and its velocities relative
to the sun, Earth and Mars.  With the aid of computers of the
data processing center in the SFOF, predictions are made and
transmitted to the station waiting to lock up the signal.  These
predictions tell the station personnel where to expect Mariner to
appear in the sky as it clears the horizon and at what radio
frequency to tune the receiver.
          Accurate tracking of Mariner IV is based on the Doppler
shift of its radio signal, or the apparent change in frequency of
the signal as the spacecraft moves farther away from Earth.  Two-
way Doppler, used by the DSN for tracking lunar and planetary
spacecraft, utilizes a signal transmitted from the station to the
spacecraft receiver-transmitter where it is converted to a new

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frequency in an exact ratio with the ground frequency and then
retransmitted to Earth.  Since the frequency of the signal sent
from the ground can be determined with great precision, the
resulting Doppler information and velocity calculations are very
accurate.
          Transmitter power at each of the stations is 10,000
watts.  The transmitter is used both for two-way Doppler tracking
and for sending commands to Mariner IV.  The spacecraft has re-
ceived and acted upon 42 commands frm Earth since launch.  Others
may be transmitted during the encounter phase of the mission.
          The two-way Doppler technique is the key to one of
Mariner IV's planetary investigations--the occultation experiment.
About an hour after the point of closest approach, the spacecraft
will pass behind Mars as viewed from Earth.  The Doppler effect
upon the radio signal as it penetrates the Martian atmosphere
will permit scientists to determine the density and scale height
of the atmosphere.
          If Mariner IV is still operating at Mars, its
television camera system will take and record as many as 21
black-and-white still pictures of the planet's surface for later
playback to Earth beginning about 10 hours after the fly-by.
          Because of the data rate possible at the Earth-Mars
distance--8 1/3 bits per second--it will take more than eight
hours to transmit one picture.  Each picture contains about
250,000 bits of information.  It is planned to play back each
picture twice, requiring nearly three weeks for return of all
picture data.

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          Since the picture data will be received in binary form--
ones and zeros which form values representing light intensity
from white to black--it is possible to receive part of a picture
at Johannesburg or Madrid and another part at Goldstone, losing
nothing in the transfer.  The time-coded digital information can
be matched at JPL where it will be converted into a photograph of
the surface of Mars.  The conversion process, involving computer
programs and specialized equipment, may take several days.
          On April 29, 1965, Mariner IV established a new space
communications distance record of 66 million miles.  The mark
will more than double at encounter.
          Mariner project officials do not anticipate a break in
the long communications thread connecting Earth and the spacecraft
for several months.  It is probable that Mariner IV will continue
broadcasting for a long time as it orbits the sun, but out of
range of the Earth.

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