Conference 7.286::space

    As you noted there's a lot of stuff about the DSN scattered in
    throughout this conference.  Note 532.206 has a little about the DSN in
    relationship to the Voyager project.
    
    A little background on the DSN might be helpful:
    
    The DSN was an outgrowth of the Deep Space Instrumentation Facility's
    (DSIF) requirements to track and acquire data from lunar and planetary
    missions.   Compared to tracking of orbital satellites, there are some
    fundamental differences because of the lower powers and greater
    pointing accuracies needed.   DSN also has to respond both normal and
    emergency conditions of probes (which vary the data rates and powers
    that are needed to communicate with them).
    
    The DSN was started in the late 50's/early 60's under the guidance of
    Ed Buckley (NASA HQ), Eb Rechtin (JPL), and Walter Victor (JPL). 
    Victor was the principal system designer for the DSN.
    
    Early sites were Camp Irwin (Mojave Desert, California), a dry lake bed
    near the Woomera test range in East Central Australian, and in a
    shallow valley near Johannesburg, South Africa.   These sites were
    chosen based on there geography (approx. 120 degrees apart on the
    globe), freedom from manmade radio interference, and political
    feasibility.   That final criterion caused an additional site in
    Madrid, Spain to be constructed in 1965 (prompted by political unrest
    in South Africa and U.S./South African relations).
    
    Unlike existing tracking stations at the time which use Elevation-
    Azimuth bearings, the DSN used the equatorial bearings used by radio
    astronomers.  Equatorial bearings allow very precise pointing of the
    antenna taking advantage of the earth's rotation (instead of constantly
    fighting it).   The first dishes were 85 feet in diameter, weighed
    hundreds of thousands of pounds, precisely balanced, and capable of the
    fine pointing required to obtain the weak signals from probes yet still
    survive winds, rain, etc.
    
    Unlike radio astronomy dishes, the DSN dishes transmitted and received
    at the same time.  Diplexers were used for this capability.  Automatic
    "closed-loop" facilities were also added to improve tracking.
    
    Doppler tracking, coherent communications, etc. have been discussed in
    this conference as well.
    
    The DSN had to deal with changing frequencies over the years.  Early
    lunar mission used 890-960Mhz frequencies, but air traffic forced NASA
    to use S-band (2110-2300MHz) - which required major modifications at
    the sites, but eventually yielded higher bandwidths, narrower beams,
    etc.  Larger dishes (64-meter/210-foot) were added to the DSN sites to
    augment the (26 meter/85-foot) dishes.
    
    During the early years the DSN used tube amplifiers, but their high
    operating temperatures added noised to the weak signals.  The early
    60's brought solid-state parametric amplifiers which provided an order
    of magnitude improvement in the signal-to-noise ratio.   Eventually
    (and I believe the current system) employs masers whose amplication
    elements are cooled at 4 degrees kelvin (by liquid helium).
    
    The maser amplifier uses a synthetic ruby crystal (immersed in the
    liquid helium) which is "pumped" with microwave energy.  The weak
    signal received is then used to modulate the generated microwave
    radiation.    In the early days, because the maser is located right at
    the dish (to minimize noise in cables, etc.) the liquid helium dewars
    had to be refilled by hand about every 10 hours by someone in a cherry
    picker crane.   Eventually a self-contained refrigeration system was
    developed.
    
    The maser amplifiers border on the ultimate in signal-to-noise
    capabilies theoretically possible.  Apparently if they located a dish
    on the far side of the moon, they might get a 2x improvement - not a
    lot more considering the difficulty involved in putting a site there.
    Weather, etc. of course also affect the ability of the dishes at any
    one time.
    
    I'm not really sure how they get the data from the 3 DSN sites to the
    JPL, but I'll bet it's a network of leased lines, satellites, etc.
    
    
    I'll see what else I can dig up.  Perhaps someone could work up a set
    of cross-references that already exist in this conference.
    
    - dave

    Cheers for the information, the technology behind just to recieve radio
    from these spacecraft looks amazing, I now start to understand some
    of the problems they get into as reported in this notes file....
    
    Chris

    The Voyager Neptune encounter marked 'the first operational space use
    of High Electron Mobility Transistor amplifiers', solid state preamps
    that offer very low noise figures. They were used for X-band (8.5GHz)
    reception.
    
    Actually, HEMT amplifiers have been in operational use for 4Ghz and
    12GHz ground stations for some time. I guess commercial use doesn't
    count in the research world :-)
    
    ref IEEE Communications magazine, Sept 1990, Voyager Mission
    Telecommunication Firsts.
    
    gary

Interesting; I had not put all this together in my mind till reading this note.
How many independently-steerable dishes does each site have?  I'm thinking,
for example, of how Magellan needs to have a regular tracking pass lasting
several hours several (?) times per day, and not in sync with the Earth's
rotation.  This means that if there is only one dish, all the other events
have to be scheduled around these fairly fixed passes.  Not only that, but
for the really distant probes, you have to schedule a transmission and a
reception pass at a fixed distance in time from each other (and probably on
different DSN sites) and squeeze these in around Magellan as well.

Burns

[From 532.206]

The current DSN capabilities are:
    
    Goldstone:
    
       DSS 12 - 34-meter, transmit/receive
       DSS 14 - 70-meter, transmit/receive
       DSS 15 - 34-meter, receive only
       All three can be arrayed to improve signal strength.
    
    Canberra (Australia - New South Wales at Tidbinbilla)
       DSS 42, DSS 43, and DSS 45 - Same as their counterparts at
       Goldstone.
    
    Madrid (Spain - Robledo actually)
       DSS 61, DSS 63, and DSS 65 - Same setup

Recent breakthroughs in superconductive microwave front end
components will increase the sensitivity of this equipment by
several orders of magnitude.  The new filters (and amps are not
too far behind) have insertion losses 3 to 6 db less than the
conventional circuits.

Looks like a longer life for our deep space voyagers!

Re: .6

Hmmm...  While the sensitivity of the receivers can be improved, there are
real noise sources that, if my understanding of all this stuff is anywhere
near right, would essentially swamp the system.   Perhaps new filters will
take care of this.  [I know zip about RF engineering.]

Heat from buildings, existing RF emissions, and noise from the dish itself
already make up a good chunk of the noise the system has to deal with now.
[I have some figures at home -- I'll try to dig them up.]

Are these components planned for the DSN?  Do they apply to the wavelengths
that the probes are transmitting at?


Like I said, the ground systems are sometimes more impressive than the
flight hardware....


- dave

According to the superconducting news sources on the network
(check the superconducting conference), most of this stuff is
military, either Star Wars, or normal military applications.  But
it doesn't take a rocket scientist to figure that this will have
deep space applications.

The breakthroughs I wrote about came in the December '90-Jan '91
time frame, still pretty new stuff.

P.S. Here's another enticing tidbit: researchers have also
recently succeeded in approaching the high power magnetic fields
needed for fusion reactor containment.  Not on a practical level
yet, but it'll be soon.  Wonder how far away is nuclear power for
aircraft, such as a space plane?

Re .1

>    Apparently if they located a dish
>    on the far side of the moon, they might get a 2x improvement - not a
>    lot more considering the difficulty involved in putting a site there.
>    Weather, etc. of course also affect the ability of the dishes at any
>    one time.

Curious I thought atmospheric absorption was quite high at these frequencies ?

Would not the lower level of man made RF improve matters ?

How about the lower temperature of the surrounding area when the moon is on the 
far side of the earth from the sun (lower noise temperature, lower ambient RF).


Mark.

    Re: .9, .others
    
    O.K., first a clarification.  The 2x improvment was in regard to noise
    temperature -- not with other sources of noise (e.g., RF emissions,
    etc.).
    
    4 kelvins are the background noise from space (tough to eliminate :-) )
    3-4 kelvins are due to maser inefficiencies
    6-8 kelvins are due to atmospheric effects (depending on the frequency
    used.
    
    They believe the theoretical minimum for the system is 20 kelvins -
    which the above numbers have to fit into.  The dish will always radiate
    some heat - no matter where you put it.
    
    X-band frequencies (8500MHz) are affected by weather, but apparently not
    as much as lower frequencies (I'm talking about noise here).
    
    
    I'm not qualified to answer (or even speculate) on your other
    questions.
    
    - dave

From:	DECWRL::"usenet-space-news-request@ames.arc.nasa.gov" 29-JUN-1992 
        12:48:57.14
To:	usenet-space-news@ames.arc.nasa.gov
CC:	
Subj:	DSN Earthquake Report - 06/29/92

                              DSN Status Report
                              Earthquake Damage
                                June 29, 1992

     A major earthquake occurred in southern California on June 28 at
4:58AM (PDT).  Building 230 at JPL was evacuated.  No major damages
were sustained at JPL.  Caltech reported a magnitude 7.4 on the
richter scale with the epicenter at Yucca Valley.  Goldstone was down
for structural integrity check for all antennas.  Goldstone lost power
for 4 minutes and was on generator power. 

     A second earthquake occured at 8:04 AM (PDT).  Building 230 was
evacuated again, personnel returned at 8:26 AM.  Caltech reported a
magnitude 6.5 with the epicenter near Big Bear Lake.  At 11AM,
Goldstone reported that the 70 meter antenna (DSS-14) was declared red
due to subreflector assembly being damaged.  It was torn loose by the
earthquake as the antenna was pointed towards the horizon waiting for
Pioneer 10 to rise.  Current estimates are one week down-time.  Supports 
scheduled for DSS-14 on June 28-29 moved to other resources wherever 
possible. 

     ___    _____     ___
    /_ /|  /____/ \  /_ /|     Ron Baalke         | baalke@kelvin.jpl.nasa.gov
    | | | |  __ \ /| | | |     Jet Propulsion Lab |
 ___| | | | |__) |/  | | |__   M/S 525-3684 Telos | Pound for pound,
/___| | | |  ___/    | |/__ /| Pasadena, CA 91109 | grasshoppers are 3 times as
|_____|/  |_|/       |_____|/                     | nutritious as beef.

Article: 24764
From: baalke@kelvin.jpl.nasa.gov (Ron Baalke)
Newsgroups: sci.space,sci.astro,ca.earthquakes
Subject: DSN Update - 07/20/92
Date: 21 Jul 92 00:08:13 GMT
Sender: news@elroy.jpl.nasa.gov (Usenet)
Organization: Jet Propulsion Laboratory
 
                        Deep Space Network Status Report
                               July 20, 1992
 
     On July 19 at 9:47PM PDT, a 4.6 earthquake centered 5 miles
northeast of Barstow caused DSS-12 (Goldstone 34 meter antenna) to
halt their Voyager 1 support and DSS-15 (Goldstone's other 34 meter
antenna) to halt their Voyager 2 support.   DSS-15 took the antenna to
stow to check for structural damage, and DSS-12 also checked for
structural damage but did not go to stow.  The station reported that
there was no structural damage to any of the antennas. 
 
     The Goldstone 70 meter antenna which was damaged by the two
earthquakes on June 28 is still having its subreflector repaired, and
is due to be back up on July 22. 

     ___    _____     ___
    /_ /|  /____/ \  /_ /|     Ron Baalke         | baalke@kelvin.jpl.nasa.gov
    | | | |  __ \ /| | | |     Jet Propulsion Lab |
 ___| | | | |__) |/  | | |__   M/S 525-3684 Telos | Most of the things you 
/___| | | |  ___/    | |/__ /| Pasadena, CA 91109 | worry about will never
|_____|/  |_|/       |_____|/                     | happen.

Article: 24900
Newsgroups: sci.space,sci.astro
From: baalke@kelvin.jpl.nasa.gov (Ron Baalke)
Subject: DSN Update - 07/23/92
Sender: news@elroy.jpl.nasa.gov (Usenet)
Organization: Jet Propulsion Laboratory
Date: Thu, 23 Jul 1992 23:33:54 GMT
 
                         Deep Space Network Status Report
                                July 23, 1992
 
     The 70-meter antenna at Goldstone (near Barstow in California)
returned to operations early this morning, supporting the Galileo
mission with no problems.  The antenna has been down since June 28
when the two large earthquakes damaged its subreflector. 

     ___    _____     ___
    /_ /|  /____/ \  /_ /|     Ron Baalke         | baalke@kelvin.jpl.nasa.gov
    | | | |  __ \ /| | | |     Jet Propulsion Lab |
 ___| | | | |__) |/  | | |__   M/S 525-3684 Telos | Most of the things you 
/___| | | |  ___/    | |/__ /| Pasadena, CA 91109 | worry about will never
|_____|/  |_|/       |_____|/                     | happen.

There is an article in the Jul/Aug issue of Air&Space on DSN operations.

I've only skimmed it (I parcel out my A&S reading time), but it looked
interesting.

- dave

Article: 25326
Newsgroups: sci.space,sci.astro
From: baalke@kelvin.jpl.nasa.gov (Ron Baalke)
Subject: DSN Update - 08/06/92
Sender: news@elroy.jpl.nasa.gov (Usenet)
Organization: Jet Propulsion Laboratory
Date: Fri, 7 Aug 1992 00:56:43 GMT
 
                         DEEP SPACE NETWORK STATUS REPORT
                                August 6, 1992
 
     The Deep Space Network at Goldstone, California experienced an
earthquake on August 5 at 2215Z.  All of the antennas were stowed to
be checked out for any damage.  All antennas were found to be in good
shape.  The Pioneer project was in the process of sending the downlink
off command.  DSS-42 (Canberra 34 meter antenna) was brought up at
2200Z to support Pioneer Venus 1. 
 
     On August 5 at 1710Z, SPC60 (DSN's Signal Processing Center in
Madrid, Spain) experienced a severe electrical storm causing all
antenna's to go to brake and all data lines to JPL was lost.  DSS-61
(34 meter antenna) and DSS-66 (26 meter antenna) suffered damage to
their encoders, and DSS-61 also had subreflector problems.  Because of
the antennas being down, support for Nimbus-7 and Ulysses were missed,
and a SAMPEX support is in doubt. 
 
     The Nimbus project delcared a spacecraft emergency on August 5 at
2220Z, but due to the DSS-66 being down, DSS-46 (Canberra 26 meter
antenna) was brought in to support the emergency at 0003Z on August 6.
The spacecraft emergency was lifted at 0049Z. 

     ___    _____     ___
    /_ /|  /____/ \  /_ /|     Ron Baalke         | baalke@kelvin.jpl.nasa.gov
    | | | |  __ \ /| | | |     Jet Propulsion Lab |
 ___| | | | |__) |/  | | |__   M/S 525-3684 Telos | You can't hide broccoli in
/___| | | |  ___/    | |/__ /| Pasadena, CA 91109 | a glass of milk - 
|_____|/  |_|/       |_____|/                     | anonymous 7-year old.
 

Article: 1401
From: baalke@kelvin.jpl.nasa.gov (Ron Baalke)
Newsgroups: alt.sci.planetary,sci.space
Subject: Re: DSN Usage
Date: 2 Jun 1993 16:34 UT
Organization: Jet Propulsion Laboratory
 
In article <1ugf58INNhac@rave.larc.nasa.gov>, s.d.derry@larc.nasa.gov writes...
>How busy is the DSN?  
 
It is very busy.  Here's a list of the spacecraft we've supported in the
past 24 hours:  Voyager 1, Voyager 2, Mars Observer, Magellan, Ulysses, 
Pioneer 11, Solar-A , Nimbus-7 , Hippocarus, Astro-D, Geotail, Rosat.
Also, some VLBI work was done.  
 
>What are the most heavily loaded DSN resources?  
 
I would say usage of the the 70 meter antenna.  Each station only has
one of these. 
 
>Which missions are using DSN, either on a regular or irregular basis?  
 
I count at least 71 spacecraft.  Don't ask me to list them all.  All of
the deep space spacecraft are supported along with several Earth orbiters.
Interesting enough, the DSN still officially supports Pioneer Venus.  It
usually takes a year or two after a spacecraft has died before it is
removed from the list.
 
>How much time does each get?  How far in advance is DSN time scheduled?  
 
Varies widely, depends on what the project requests.  Can range from
minutes to hours.  The DSN time is usually scheduled at least three
months in advance.  This coming August will be particularly busy, since
two major events are happening only 4 days apart.  Mars Observer will
be going into orbit around Mars on August 24, and Galileo will be
flying by Ida on August 28.  Both spacecraft will be roughly in the
same part of the sky as viewed from Earth.  Both projects have carefully 
scheduled their DSN resources to not to interfere with each other. 
 
>How are "spacecraft emergencies" handled? 
 
The are handled as they happen.  Any idle antennas are pulled in service,
and usually a project or two is asked if they will give up their antenna time 
to support the emergency.
 
>Are things going to get better or worse?
> 
Probably worse. I see more spacecraft being launched than dying out.
     ___    _____     ___
    /_ /|  /____/ \  /_ /|     Ron Baalke         | baalke@kelvin.jpl.nasa.gov
    | | | |  __ \ /| | | |     Jet Propulsion Lab |
 ___| | | | |__) |/  | | |__   M/S 525-3684 Telos | The tuatara, a lizard-like
/___| | | |  ___/    | |/__ /| Pasadena, CA 91109 | reptile from New Zealand,
|_____|/  |_|/       |_____|/                     | has three eyes.
 

Article: 83576
From: Admin@ccmail.Jpl.Nasa.Gov (JPL Public Information)
Newsgroups: sci.space
Subject: JPL/Deep Space Network fact sheet
Date: Mon, 28 Feb 1994 18:47:04 GMT
Organization: Jet Propulsion Laboratory
 
FACT SHEET:            DEEP SPACE NETWORK
 
     The Deep Space Network (DSN), with stations strategically
placed on three continents, is the largest and most sensitive
scientific telecommunications system in the world.  It is the
Earth-based communications terminal for all of NASA's
interplanetary spacecraft.
 
     Additionally, the DSN performs radio and radar astronomy
observations for the exploration of the solar system and the
universe.  The network is a facility of the NASA Office of Space
Communications and is managed and operated for NASA by the Jet
Propulsion Laboratory.
 
     The DSN currently consists of three deep-space
communications facilities placed approximately 120 degrees apart
around the world: at Goldstone in California's Mojave Desert;
near Madrid, Spain; and near Canberra, Australia.  That
configuration permits constant observation of spacecraft as Earth
rotates.  Each complex contains four deep-space stations equipped
with large parabolic reflector antennas.  The operations control
center for all three facilities is located at JPL in Pasadena.
 
     The predecessor to the DSN was established in January 1958
when JPL, then under contract to the U.S. Army, deployed portable
radio tracking stations in Nigeria, Singapore and California to
receive telemetry and plot the orbit of Explorer 1, the first
successful U.S. satellite.
 
     On December 3, 1958, JPL was transferred from the Army to
the newly created NASA and given responsibility for the design
and execution of automated lunar and planetary exploration
programs.  The DSN also supports some Earth-orbiting missions,
including emergency support for orbiting space shuttles.
 
     NASA's scientific investigations of the solar system are
accomplished mainly through the use of uncrewed robotic
spacecraft.  The DSN provides the two-way communications link
that guides and controls the spacecraft and brings back the
images and other scientific data they collect.  All DSN antennas
are steerable, high-gain parabolic reflector antennas.
 
     Each of the DSN stations has a 70-meter-diameter (230-foot)
antenna.  These are the largest and most sensitive DSN antennas,
and are capable of tracking spacecraft traveling more than 16
billion kilometers (10 billion miles) from Earth.  The surface of
the 70-meter reflector must remain accurate within a fraction of
the signal wavelength, meaning that the precision across the
3,850-square-meter (4,600-square-yard) surface is maintained
within 1 centimeter (0.4 inch).  The dish reflector and its mount
-- which move in the azimuth, or horizontally, as well as in
elevation -- weigh nearly 2.7 million kilograms (8,000 U.S. tons).
 
     Each station also has a 34-meter (111-foot) standard antenna
which was originally constructed as a 26-meter (85-foot) antenna
and later extended to 34 meters in preparation for outer planet
missions.  The mechanical design of the standard antenna is
nearly identical to that of radio astronomy antennas developed in
the 1950s, in that the mount and pointing system is designed to
track at Earth's rotation rate (0.004 degree per second).
 
     That rate allows tracking of planetary spacecraft which
appear in the sky much like any celestial object, rising in the
east and setting in the west in about 8 to 12 hours.
 
     There are also 34-meter high-efficiency antennas at each
station.  These antennas incorporate more recent advances in
antenna design and mechanics.  The mount is an azimuth-elevation
type and operates in both axes at up to 0.40 degrees per second. 
The reflector surface is precision-shaped for maximum signal-
gathering capability.
 
     The other antennas at the DSN sites are 26 meters (85 feet)
in diameter.  They are used for tracking Earth-orbiting
satellites, most of which are in orbits 160 to 1,000 kilometers
(100 to 620 miles) above Earth.  The two-axis mount allows the
antenna to point low on the horizon to pick up the fast-moving
Earth orbiters as soon as they come into view.  The maximum
tracking speed is 3 degrees per second.
 
     The 26-meter antennas were originally built to support the
crewed Apollo missions to the moon between 1967 and 1975.
 
     In addition, there is one 13-centimeter-diameter (5-inch)
omnidirectional antenna at each complex that receives signals
from Navstar satellites in the Defense Department's Global
Positioning System (GPS).  The DSN's navigation activities use
GPS signals to measure Earth platform characteristics needed for
generating deep-space navigation data and determining precise
near-Earth satellite orbits.
 
     At Goldstone there is an additional 9-meter-diameter (30-
foot) antenna designed and primarily used for communicating with
Earth-orbiting satellites, which have receiving and tracking
requirements that are basically different from deep-space
missions.  Earth-orbiters have relatively short periods in view
of a given ground station, lasting on average about 35 minutes
and in some cases as short as about 10 minutes.  Satellite
signals are relatively strong and do not require the larger-
diameter antennas and ultrasensitive low-noise receivers used for
deep-space missions.
 
     Because of those differences, the communications link for
the majority of U.S. scientific Earth orbiters, including the
Hubble Space Telescope and space shuttle flights, is provided by
NASA's spaceborne Tracking and Data Relay Satellite System
(TDRSS) which is managed by NASA's Goddard Space Flight Center in
Greenbelt, Maryland.
 
     The TDRS system consists of several communications
satellites in geostationary orbits 36,000 kilometers (22,300
miles) above Earth.  At that altitude, they remain in position
over the same point on Earth at all times.  The principal ground
station is at White Sands, New Mexico.  Instead of short ground-
station view periods, the relay satellites have a near-continuous
view of orbiting spacecraft beneath them.
 
     Some scientific satellites are in very high Earth orbits
that extend to 1.7 million kilometers (1.05 million miles),
beyond the range of the TDRS system.  Those high orbiters and a
selected group of low scientific orbiters are tracked by the Deep
Space Network's 26-meter antennas, which are designed to move at
up to 3 degrees per second.
 
     The control center at the Jet Propulsion Laboratory is the
operations hub for the network.  Control center staff directs and
monitors operations, transmits commands and is responsible for
the quality of spacecraft telemetry and navigation data delivered
to network users.
 
     The Ground Communications Facility provides and controls the
communications that link the three complexes to the control
center at JPL and to flight control centers in the United States
and overseas.  Voice and data traffic between those locations is
sent via land lines, submarine cable, microwave links and
communications satellites.  The circuits are leased from common
carriers by the NASA Communications Network and provided to the
Ground Communications Facility as needed.
 
     DSN staff in the United States consists of JPL engineering
and technical personnel, assisted by contract engineers and
technicians.  They are primarily responsible for operating and
maintaining the Goldstone communications complex and the
operations control center and ground communications center at
JPL.  The Madrid and Canberra complexes are staffed and operated
by agencies of the Spanish and Australian governments and their
contractors.  The total international network staff currently
numbers more than 1,600 people.
 
     The Deep Space Network's radio link to spacecraft is
basically the same as other point-to-point microwave
communications systems except for the very long distances
involved, and the very low spacecraft signal strength.  The total
signal power arriving at a network antenna from a spacecraft
encounter among the outer planets can be 20 billion times weaker
than the power level in a modern digital wristwatch battery.
 
     The extreme weakness of the signal results from restrictions
placed on the size, weight and power supply of the spacecraft by
the cargo area and weight-lifting limitations of the launch
vehicle.  Consequently, the design of the radio link is the
result of engineering trade-offs between spacecraft transmitter
power and antenna diameter, and the sensitivity that can be built
into the ground receiving system.
 
     Typically a spacecraft signal is limited to 20 watts, or
about the same power required to light a refrigerator bulb.  When
the signal arrives at Earth from outer space -- say, from the
neighborhood of Saturn -- it is spread over an area with a
diameter equal to about 1,000 Earth diameters.  As a result, the
ground antenna is able to receive only a very small part of the
signal power, which is degraded by background radio noise, or static.
 
     Noise is radiated naturally from nearly all objects in the
universe, including Earth and the sun.  Since there will always
be noise amplified with the signal, the ability of the ground
receiving system to separate noise from the signal is critical. 
Low-noise receivers and telemetry coding techniques are used, along 
with state-of-the-art sensitivity and efficiency of the network.
 
     An additional technique used to acquire more of the signal
from very distant spacecraft is a technique called "arraying." 
That technique involves the temporary augmentation of the DSN's
own antennas with other suitable radio astronomy facilities. 
Studies were begun in 1981 to prepare for Voyager 2's Uranus
encounter in 1986 and its encounter with Neptune in 1989.
 
     Arrangements were made to use the Parkes Radio Telescope in
Australia, operated by the Commonwealth Scientific and Industrial
Research Organization (CSIRO).  Parkes was arrayed with the DSN
70-meter antenna at Canberra.  Data from Parkes were transmitted
over a new microwave link to Canberra.  The arrangement resulted in 
an increase of approximately 50 percent in reception of Uranus data.
 
     For the Neptune encounter, it was determined that the best
array configuration would be to combine Goldstone with the Very
Large Array (VLA) at Socorro, New Mexico.  The VLA is operated by
the National Radio Astronomy Observatory and sponsored by the
National Science Foundation.  It consists of 27 antennas, each 25
meters (82 feet) in diameter configured in a "Y" arrangement on
railroad tracks over a 20-kilometer (12.5-mile) area.  Arraying
Goldstone with the VLA resulted in more than doubling Goldstone's
capability on its own.
 
     Spacecraft telemetry consists of the information produced by
the scientific instruments and the engineering data about the
spacecraft's own systems.  It is transmitted in binary code,
using only the symbols 1 and 0.
 
     The spacecraft organizes and encodes the data for
transmission back to ground stations.  Ground stations are
equipped with equipment that detect the individual bits, decode
the data stream and format the information for transmission to
the data user.
 
     Noise from various sources interferes with the decoding
process.  If there is a high signal-to-noise ratio, the number of
decoding errors will be low.  If bit errors are excessive, the
data transmission rate, measured in bits per second, must be reduced 
to give the decoder more time to determine the value of each bit.  
 
     In the process of coding data, additional or redundant data
are fed into the data stream which are used to detect and correct
errors after transmission.  The equations used in this process
are sufficiently detailed to allow individual and multiple errors
to be detected and corrected.  After correction, the redundant
digits are eliminated from the data, leaving a validated sequence
of information to be delivered to the data user.
 
     Error detecting and encoding techniques can increase the
data rate many times over transmissions that are not coded for
error detection.  DSN coding techniques have the capability of
reducing transmission errors in spacecraft science information to
less than one in a million.
 
                              #####
 
2-94 JJD

    I had the opportunity last week to see the DSN site at Tidbinbilla,
    near Canberra, ACT, Australia.  BTW, I'm not 100% convinced it is in
    New South Wales.  I did not see a state boundary line as we drove out
    there, so it may well be in ACT.  But in any case...
    
    There are, in fact, more than 3 antennae there.  However, not all of
    the are operational.  I got the impression that at least one (the one
    that received Neil Armstrong's first words/pictures on the moon) are
    kept for historical reasons.
    
    The 70m dish is awesome to see.  Imagine a round metal football field
    being suspended at weird angles in the sky, and you can get some idea.
    But even more awesome is to look at the antenna and kind of orient your
    face along the direction it is pointing and know that you are looking
    at Galileo, millions of miles away, and that this behemoth next to you
    is in communication with it picking up pictures of the SL9 collision!
    
    At the same time, the two 34-meter units were tracking Magellan and
    Ulysses (there was no one in the visitor center, but the snack bar
    attendant was nice enough to call into the control center for me to ask what
    was being tracked).
    
    Very exciting.  I tried to see if I could see the 70m move, but I could
    not.  We went on to the Tidbinbilla wildlife preserve, and stopped into
    the DSN station again on the way back, and it was pointing in a
    completely different direction (this was Galileo...dont' know where it
    was at first).  Rats!  I wish I could have seen it slew around!
    
    Burns
    

re .18
    
>    I had the opportunity last week to see the DSN site at Tidbinbilla,
>    near Canberra, ACT, Australia.  BTW, I'm not 100% convinced it is in
>    New South Wales.  I did not see a state boundary line as we drove out
>    there, so it may well be in ACT.
    
    I can confirm that the Tidbinbilla DSN station is in the ACT. As the
    crow flies it's actually quite close to "downtown" Canberra but the
    road out there is circuitous.
    
>    Imagine a round metal football field being suspended at weird angles in
>    the sky, and you can get some idea.
    
    Or imagine a 20+ storey building being waved around.
    
    P.S. The visitors' center and exhibition is quite modest and not likely
    to be very informative for readers of this conference.

    Right, I confirm Garson's comment about the visitor center.  The best
    part was the schedule board which was, unfortuantely, a day out of date
    (which is why I had the snack bar folks call in for me).  Too bad there
    was not a glass window into the control building!
    
    Burns
    

The Very Large Array in the western US (New Mexico, I believe) is an interesting
tour. It's set up with recorders and plaques so you just wander around on your
own. I wish there was a schedule of slew times posted somewhere since they did
move while we were on-site but we missed it. They do have a window into the
control room.