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DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
1-16 SECTION 1 - GENERAL SYSTEM DESCRIPTION
Coded Pulses to Facilitate Pulse Compression Radar Techniques
1:29. Another general technique to improve on the simple pulse sounder is to stretch out the pulse by a factor
of N, thus increasing the duty cycle so the pulse contains more energy without requiring a higher power trans-
mitter (power x time = energy). However, lengthening the pulse deteriorates its range resolution properties. To
maintain the higher range resolution of the simple short pulse, the long pulse can be phase modulated with a
code to enable the receiver to create a synthetic pulse with the original (i.e., that of the short pulse) range reso-
lution. Bi-phase, or phase reversal modulation was implemented in a network of sounders operated by the U.S.
Navy in the 1960’s using a 13-bit Barker Code.
1:30. The critical factor in the use of pulse compression waveforms for any radar type measurement is the
correlation properties of the internal phase code. Phase codes proposed and experimented with included the
Barker Code [Barker, 1953], Huffman Sequences [Huffman 1962], Convoluted Codes [Coll, 1961], Maximal
Length Sequence Shift Register Codes (M-codes) [Sarwate and Pursley, 1980], or Golay’s Complementary Se-
quences [Golay, 1961], which have been implemented in the VHF mesospheric sounding radar at Ohio State
University [Schmidt et al., 1979] and in the DPS.
1:31. The Digisonde-4D is able to be miniaturized by lengthening the transmitted pulse beyond the pulse
width required to achieve the desired range resolution where the radar range resolution is defined as,
ΔR = c / 2β, where β is the system bandwidth of 30
kHz, or
= cT / 2 for a simple rectangular pulse
waveform, with T being the width
of a rectangular pulse of 33.333 µs
The longer pulse allows a low voltage solid state amplifier to transmit an amount of energy equal to that trans-
mitted by a high power pulse transmitter (energy = power x time, and power = V
2
/R) without having to provide
components to handle the high voltages required for tens of kilowatt power levels. The time resolution of the
short pulse is provided by intrapulse phase modulation using programmable phase codes (user selectable and
firmware expandable), the Complementary Codes, and M-codes are standard. The use of a Complementary
Code pulse compression technique is described in this chapter, which shows that at 300 W of transmitter power
the expected measurement quality is the same as that of a conventional sounder of about 500 kW peak pulse
power.
1:32. The transmitted spread spectrum signal s(t) is a biphase (180° phase reversal) modulated pulse. As il-
lustrated in Figure 1-9, bi-phase modulation is a linear multiplication of the binary spreading code p(t) (a.k.a. a
chipping sequence, where each code bit is a “chip”) with a carrier signal sin(2πf
0
t) or in complex form,
exp[j2πf
0
t], to create a transmitted signal,
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