Planetary Radar 741
FIGURE 4 Block diagram of a planetary radar
system. RF LO and IF LO denote radio
frequency and intermediate frequency local
oscillators, and ADC denotes analog-to-digital
converter.constitute what is called a cascade amplifier), and about
half of the input DC power is converted to RF power and
sent out through a waveguide to the antenna feed system
and radiated toward the target. The other half of the input
power is waste heat and must be transported away from the
klystron by cooling water. The impact of the electrons on
the collector anode generates dangerous X-rays that must be
contained by heavy metal shielding surrounding the tube,
a requirement that further boosts the weight, complexity,
and hence cost of a high-power transmitter.
In most single-antenna observations, one transmits for
a duration near the roundtrip propagation time to the tar-
get (i.e., until the echo from the beginning of the trans-
mission is about to arrive) and then receives for a similar
duration. In the “front end” of the receiving system, the
echo signal is amplified by a cooled, low-noise amplifier and
converted from RF down to intermediate frequencies (IF,
e.g., 30 MHz), for which transmission line losses are small
and passed from the proximity of the antenna feed to a
remote control room containing additional stages of signal-
processing equipment, computers, and digital recorders.
The signal is filtered, amplified, and converted to frequen-
cies low enough for analog voltage samples to be digitized
and recorded. The frequency down-conversion can be done
in several stages using analog devices called superhetero-
dyne mixers, but in recent years it has become possible to do
this digitally, at increasingly higher frequencies. The nature
of the final processing prior to recording of data on a hard
disk or magnetic tape depends on the nature of the radar ex-
periment and particularly on the time/frequency structure
of the transmitted waveform. Each year, systems for reduc-
ing and displaying echoes in “real time” and techniques for
processing recorded data are becoming more ambitious as
computers get faster.2.3 Echo Time Delay and Doppler Frequency
The time between transmission of a radar signal and recep-
tion of the echo is called the echo’s roundtriptime delay,
τ, and is of order 2R/c, wherecis the speed of light, by defi-
nition equal to 299,792,458 m sec−^1. Because planetary tar-
gets are not points, even an infinitesimally short transmitted