Suzy B Software 2/Suzy B Software CD-ROM 2 (1994).iso/nasa/mglnradr/mglnradr.txt

Text File | 1995-05-02 | 6KB | 122 lines

FACT SHEET: MAGELLAN SYNTHETIC APERTURE RADAR

Astronomical imaging is traditionally done with optical instruments. There have
been spectacular results using optical scanning instruments on spacecraft such
as Voyager. The traditional approach is useless at Venus, however, where thick
clouds obscure the surface.

The longer wavelengths of radar are needed for the Magellan mission for
successful imaging and a new approach, specifically a synthetic aperture radar,
is needed to acquire fine resolution images.

Synthetic aperture radar (SAR) is sometimes called imaging radar, or
side-looking radar because it looks at its target to one side of the radar
track. It is a technique that uses many radar echoes gathered over an extended
interval to sharpen, or increase, the resolution. SAR relies heavily on
ground-based computers and otherwise differs significantly from real aperture
radar in which each echo is processed by itself.

SAR technique involves mounting the radar on a moving platform, an aircraft or
spacecraft, and directing the radar energy in the form of short pulses with a
highly directional antenna to the side.

The direction along the track of motion is called azimuth and the direction
across the track of motion is called range.

The radar energy is sent out in short pulses and the echoes are recorded in the
dead time between transmissions. Resolution is improved in the azimuth
direction because of the movement of the radar antenna. While one echo looks
very much the same as the next if observed on an oscilloscope, the echoes have
subtle differences due to the motion of the antenna between echoes.

This motion causes an effect similar to a train whistle pitch change as it
passes by. This pitch or frequency change is called the Doppler frequency and
is used to sharpen the resolution in azimuth.

The resolution the Magellan SAR will achieve at Venus will range from about 120
meters to 300 meters. The best resolution, down to 120 meters, will be at and
near periapsis, about 10 degrees north of the equator.

SAR sends out several thousand pulses of radio energy each second at the speed
of light (186,200 miles per second) as it moves along its path. The Magellan
SAR is designed to illuminate a target swath between 17 and 28 kilometers (10
and 17 miles) wide as Venus rotates slowly on its axis below it.

Venus rotates in a retrograde direction (that is opposite from the rotation of
the Earth and most of the other planets) and one Venus rotation takes 243 Earth
days.

The swaths will slightly overlap as Magellan orbits Venus. The imaging swaths
will be made at an altitude of 250 kilometers (155 miles) at its closest
approach, or periapsis, out to 2,100 kilometers (1,302 miles) at the ends of
each swath.

Magellan will look at each target area a minimum of four times from which will
be constructed a two-dimensional radar image from three characteristics of each
radar pulse:

-- The time the signal takes to make the round trip between the transmitter and
   the target.

-- The Doppler shift, a measurement of relative motion that is similar to a
   change in pitch, measured as the radar and target pass each other.

-- The brightness, or reflectivity, of each component, added to a geometric
   grid to complete the image.

These factors are brought together and defined by a computer.

There is another way to look at it. The SAR takes advantage of the spacecraft's
motion to create a synthetic aperture many times its actual size. It collects
many echoes as it moves along.

As it moves along its path, it looks at the target at an angle toward the side
which extends the size of the beam, or footprint.

At the same time, the altimeter looks straight down with a separate antenna to
determine the elevation of ground features.

The large antenna used for SAR has one more function. It also acts as a
radiometer. In this passive mode it observes the natural thermal emissions of
the surface. This will aid scientists in determining the composition of surface
material.

The altimeter antenna is fixed to the spacecraft separate from, but adjacent
to, the large SAR antenna. The horn-like antenna generates a fan beam about 10
by 30 degrees along the spacecraft's ground track. It is offset 25 degrees from
the SAR antenna and a portion of the beam always looks straight down despite
variations in the SAR look-angle which ranges from 13 degrees to 44 degrees.

The altimeter resolution will be about 30 meters. The altimeter measures the
echo time, and therefore the distance, between the radar and the surface below
the radar. Many altimeter systems send out a signal and wait for the echo.
Because of the orbital altitude and need to improve the signal strength, the
Magellan radar altimeter sends 17 pulses and listens for their return.

The altimeter footprint is usually very broad, 5 to 20 kilometers (3 to 12
miles). The data from the altimeter are combined with the spacecraft's position
to produce a topography map which represents the height above the mean
planetary surface.

The SAR generates large volumes of data compared with almost any other space
data system. The data are buffered and stored in the radar and the spacecraft
for playback to Earth later in the orbit.

The radar data comes into the digital portion of the radar electronics in
bursts of 36 million bits per second (Mbps) and are buffered down to a constant
rate of 800 thousand bits per second (kbps). These data are recorded and played
back to Earth at 270 kbps.

The SAR data are stored on two multi-track digital recorders for playback. The
data storage capacity of the two digital tape recorders is about 1.8 billion
bits.

In one orbit, Magellan will look at a swath 160 degrees from north to south,
acquiring 1.7 billion bits of data. Magellan will make a total of 1,852 orbits
during its 243 Earth days of primary mission.

Magellan will send back more data than has been acquired in all previous space
missions.

3-31-89 JJD