RADAR General Description
 
The Cassini Radar (RADAR) uses the five-beam Ku-band antenna feed
assembly associated with the spacecraft high gain antenna to direct
radar transmissions toward targets, and to capture blackbody radiation
and reflected radar signals from targets.
 
 
RADAR Scientific Objectives
 
* To determine whether oceans exist on Titan, and, if so, to determine
  their distribution.
* To investigate the geologic features and topography of the solid
  surface of Titan.
* To acquire data on non-Titan targets (rings, icy satellites) as
  conditions permit.
 
 
RADAR Sensing Instruments
 
* Synthetic Aperture Radar Imager [SAR] (13.78 GHz Ku-band; 0.35 to
  1.7 km resolution; 0.425 MHz and 0.85 MHz bandwidth)
* Altimeter (13.78 GHz Ku-band; 24 to 27 km horizontal, 90 to 150 m
  vertical resolution; 4.25 MHz bandwidth)
* Scatterometer (13.78 GHz Ku-band; 0.1 MHz bandwidth)
* Radiometer (13.78 GHz passive Ku-band; 7 to 310 km resolution;
  135 MHz bandwidth)
 
 
RADAR Instrument Characteristics
 
* Mass = 41.43 kg
* Peak Operating Power = 86 W
* Data Rates
  Radiometer only:  1 kilobit/sec
  Altimeter and Scatterometer / Radiometer:  30 kilobits/sec
  SAR Imaging / Radiometer:  364.800 kilobits/sec
 
 
Number of Nominal Operational Periods
 
One per selected flyby of Titan + icy satellite, distant Titan, and
Saturn observations, or approximately 25 Titan flybys and 120 other
observations for the primary mission.
 
 
Duration of Nominal Operational Period
 
From 300 minutes before to 300 minutes after closest approach to Titan
for prime operation.
 
 
Team Leader
 
The Team Leader for the RADAR instrument is Dr. Charles Elachi.
 
 
Instrument Modes
 
The Cassini Radar (RADAR) will be used to investigate the surface of
Saturn's moon Titan by taking four types of observations:  imaging,
altimetry, backscatter, and radiometry.  In the imaging mode of
operation, the RADAR instrument will bounce pulses of microwave
energy off the surface of Titan from different incidence angles and
record the time it takes the pulses to return to the spacecraft.  These
measurements, when converted to distances (by dividing by the speed
of light), will allow the construction of visual images of the target
surface.  Radar will be used to image Titan because the moon's surface
is hidden from optical view by a thick, cloud-infested atmosphere:
radar can 'see' through such cloud cover.
 
Radar altimetry similarly involves bouncing microwave pulses off the
surface of the target body and measuring the time it takes the 'echo'
to return to the spacecraft.  In this case, however, the goal will not
be to create visual images but rather to obtain numerical data on the
precise altitude of the surface features of Titan.  In the backscatter
mode of operation, the RADAR will act as a scatterometer.  That is, it
will bounce pulses off Titan's surface and then measure the intensity
of the energy returning.  This returning energy or backscatter, is always
less than the original pulse, because surface features inevitably reflect
the pulse in more than one direction.  From the backscatter measurements,
scientists can infer the composition of the surface of Titan.
 
Finally, in the radiometry mode, the RADAR will operate as a passive
instrument, simply recording the energy emanating from the surface of
Titan.  This information will tell scientists the amount of latent heat
(i.e., moisture) in the moon's atmosphere, a factor that has an impact
on the precision of the other measurements taken by the instrument.
 
During imaging, altimetry, and backscatter operations, the RADAR
instrument will transmit linear frequency-modulated Ku-band pulsed
signals toward the surface of Titan using the high-gain antenna
(HGA).  These signals, after reflection from the surface, will be
captured by the same antenna and detected by the RADAR Radio Frequency
Electronics Subsystem.  During radiometry operations, the instrument
will not transmit any radar signals, but the HGA will again be used for
radiometric observations.
 
To improve the surface coverage by radar imaging, a switched, multiple
Ku-band antenna feed array structure is part of the HGA and permits the
formation of five antenna beam patterns.  Each of these beams will have a
different pointing angle relative to the antenna reflector's focal axis.
 
The major functional components of the RADAR Subsystem are the Radio
Frequency Electronics Subsystem, the Digital Subsystem, and the Energy
Storage Subsystem.
 
 
Radio Frequency Electronics Subsystem (RFES)
 
The Radio Frequency Electronics Subsystem (RFES) has three principal
functions: the transmission of high-power frequency-modulated and
unmodulated pulses, the reception of both reflected energy from the
target and passive radiometric data, and the routing of calibration
signals.  The RFES has a fully enclosed structural housing and Faraday
cage (i.e., an electrostatic shield).  The RFES electronics units
are individually enclosed and are mounted to the RFES housing wall
opposite the wall that mounts to the spacecraft.  For thermal control,
heat flows conductively from the units to the housing wall and is then
radiated away from the RFES.
 
The RFES consists of the following components: a frequency generator, a
digital chirp generator, a chirp up-converter and amplifier, a high-power
amplifier, front-end electronics, a microwave receiver unit, and an RFES
power supply.
 
* The frequency generator (FG) contains an ultra-stable oscillator that
  is the system timing source for the RADAR instrument.
 
* The digital chirp generator (DCG) generates the low-power, baseband
  frequency, modulated pulse upon request from the RADAR Digital
  Subsystem.  Both the bandwidth and the pulse width of this pulse can
  be varied in accordance with the parameters received from the Digital
  Subsystem.
 
* The chirp up-converter and amplifier (CUCA) converts the baseband chirp
  pulse to Ku band and provides the up-converted pulse to the high-power
  amplifier.
 
* The high-power amplifier (HPA) receives a low-power Ku-band chirp pulse
  from the CUCA and amplifies that pulse to the required power level for
  transmission.
 
* The purpose of the front-end electronics (FEE) is to route the
  high-power transmission pulses, the returning low-energy echoes and
  radiometric signals, and the calibration signals.  The FEE receives
  the high-power pulse from the HPA and routes the signal to one of five
  different antenna ports on the RFES via an antenna switch module.  The
  echo returns and radiometric signals are routed from one of the five
  antenna ports to the RFES microwave receiver.  The FEE also steers the
  selected calibration signal to the microwave receiver during periods of
  calibration mode operation.
 
* The microwave receiver (MR) receives signals at Ku band and
  down-converts these to baseband so that they can be properly sampled.
  The sources of these signals are the echo returns, radiometric signals,
  and calibration signals routed through the FEE.  The MR receives the
  re-routed chirp calibration signal from the CUCA and passes that signal
  to the FEE for proper routing.  The MR is also the source of the noise
  diode calibration signal that is provided to the FEE for routing.  MR
  gain and bandwidth information is provided to the MR from the DSS.
 
* The RFES power supply converts the (approximately) 30-volt d.c. input
  from the Power and Pyrotechnic Subsystem to the required voltages for
  the RFES.
 
 
Digital Subsystem (DSS)
 
The RADAR Digital Subsystem (DSS) performs three principal functions:
reception and depacketization of RADAR commands from the Command
and Data Subsystem (CDS), configuration control and timing signal
generation for RADAR, and the packetization of RADAR housekeeping
(i.e., hardware status) data and science data for transfer to the CDS.
 
DSS subassemblies are contained within a spacecraft bay and will be
supported in shear by shear plates and the top and bottom rings of the
Cassini spacecraft bus.  Electronic harnesses, which face inboard on
the spacecraft and be supported by the inboard shear plate, are used
to provide interconnections between the RADAR subassemblies and the
spacecraft.
 
The DSS uses two primary modes of heat transfer in its design.  These
are (1) the conduction of heat from the electronic components to the
subchassis and the outboard shear plate, and (2) the radiation of heat
from the outboard shear plate to the space environment.  High-power
heat dissipation components are mounted on a special heatsink bracket,
which is bolted directly to the outboard shear plate to optimize heat
transfer.  Thermal compounds were applied between the components and the
heatsink to minimize contact thermal resistance.
 
The DSS consists of the following components: a bus interface unit,
a flight computer unit, a control and timing unit, a signal conditioner
unit, and a DSS power supply.
 
* The bus interface unit (BUI) is the interface between RADAR and the
  CDS.  On the RADAR side, the BIU interfaces to the flight computer unit
  for command, software, and data transfers.
 
* The flight computer unit (FCU) receives commands and software
  from the CDS and sends data and status to CDS by way of the BIU.
  It depacketizes the commands and provides the RADAR configuration and
  timing information to the control and timing unit.  It also receives
  housekeeping values in a predetermined order from the low-speed A/D
  converter and packetizes the housekeeping and science data to be
  passed to the CDS by way of the BIU.  In addition, the FCU receives
  spacecraft time broadcasts and RADAR software uploads from CDS by way
  of the BIU.  The FCU is built around an engineering flight computer
  (EFC) with additional banks of ROM and internal interface circuitry.
 
* The purpose of the control and timing unit (CTU) is to control
  the hardware configuration and the timing of control signals
  within RADAR.  The parameters for determining RADAR configuration
  and timing are passed to the CTU from the FCU.  The CTU provides
  the configuration and timing control signals to the RFES and to
  other portions of the DSS.  In addition, the CTU is responsible
  for updating to millisecond resolution the spacecraft time received
  from the CDS.
 
* The signal conditioner unit (SCU) consists of a science data
  buffer and high- and low-speed analog-to-digital (A/D) converters.
  The science data buffer (SDB) is the digital data rate buffer for
  RADAR.  The sole purpose of the SDB is to receive and store the
  high-rate digital science data from the high-speed A/D converter
  during the proper receive window period (as determined by the
  CTU) and then to provide this data upon request to the FCU at a
  slower rate.  The high-speed A/D converter digitizes the imaging
  data output from the RFES microwave receiver and provides the data
  to the SDB for buffering.  The low-speed A/D converter performs
  two tasks.  It digitizes the analog housekeeping telemetry values
  from throughout RADAR at predetermined times and provides these
  digitized values to the FCU upon request.  It also digitizes the
  radiometer output from the RFES microwave receiver and provides
  those values to the FCU upon request.
 
* The DSS power supply converts the (approximately) 30-volt d.c. input
  from the Power and Pyrotechnic Subsystem to the voltages required for
  the DSS.
 
 
Energy Storage Subsystem (ESS)
 
The RADAR Energy Storage Subsystem (ESS) converts the (approximately)
30-volt d.c. input from the PPS to a higher voltage, stores energy
in a capacitor bank, and provides a regulated voltage to the
high-power amplifier (HPA) of the RFES.  The ESS subassemblies are
contained within a spacecraft bay and are supported in shear by shear
plates and the top and bottom rings of the Cassini spacecraft bus.
High-strength fasteners will be used to tie the electronics assemblies
to the spacecraft.  Electronic harnesses, which face inboard on the
spacecraft and be supported by the inboard shear plate, are used
to provide interconnections between the RADAR subassemblies and the
spacecraft.
 
The ESS uses two primary modes of heat transfer in its design.  These
are (1) the conduction of heat from the electronic components to the
subchassis and the outboard shear plate, and (2) the radiation of heat
from the outboard shear plate to the space environment.  High-power heat
dissipation components will be mounted on a special heatsink bracket,
which will be bolted directly to the outboard shear plate to optimize
heat transfer.  Thermal compounds will be applied between the components
and the heatsink to minimize contact thermal resistance.
 
The ESS consists of boost circuitry, the capacitor bank, and a buck
regulator.
 
* The boost circuitry increases the (approximately) 30-volt
  d.c. input power to approximately 85 volts d.c. for more efficient
  energy storage by the capacitor bank.  Soft-start circuitry limits
  the current draw from the power source, and an input voltage filter
  prevents electromagnetic interference (EMI) from being conducted
  back into the source.
 
* The capacitor bank stores energy to supply to the buck regulator
  (and the HPA) during RADAR pulse bursts.  The capacitor bank voltage
  drops during each burst but returns to normal before the next burst.
 
* The buck regulator regulates the varying capacitor bank voltage for
  the HPA.