##### Document Text Contents

Page 245

AOA of the signal. Emitter location is determined from one of the techniques (such as

triangulation) discussed earlier.

We will begin by discussing single baseline interferometers and then will cover

correlative and multiple baseline interferometers.

6.6.14 Single Baseline Interferometer

Although virtually all interferometer systems employ multiple baselines, the single

baseline interferometer uses one baseline at a time. The presence of multiple baselines

allows for the resolution of ambiguities. It also allows multiple, independent

measurements to be averaged to reduce the impact of multipath and other equipment-

based sources of error.

Figure 6.58 is a basic block diagram of an interferometric DF system. Signals from

two antennas are compared in phase, and the DOA of the signal is determined from the

measured phase difference. Remember that we characterize the transmitted signal as a sine

wave traveling at the speed of light. One cycle (360 phase degrees) of the traveling sine

wave is called the wavelength. The relation between the frequency of the transmitted

signal and its wavelength is defined by the formula:

c = λf

where c is the speed of light (3 × 108 m/s), λ is the wavelength (in meters), and f is the

frequency in cycles per second (units are 1/sec).

The interferometric principle is best explained by consideration of the interferometric

triangle as shown in Figure 6.59. The two antennas from Figure 6.58 form a baseline. It is

assumed that the distance between the two antennas and their precise location are known

precisely. The wavefront is a line perpendicular to the direction from which the signal is

arriving at the direction finding station. This is a line of constant phase for the arriving

signal. The signal expands spherically from the transmitting antenna, so the wavefront is

actually a circular segment. However, since the baseline can be assumed to be much

shorter than the distance from the transmitter, it is very reasonable to show the wavefront

as a straight line in this drawing. The precise location of the station is taken to be the

center of the baseline. Because the signal has the same phase along the wavefront, the

phases at point A and point B are equal. Hence, the phase difference between the signals at

the two antennas (i.e., points A and C) is equal to the phase difference between the signal

at points B and C.

Page 246

Figure 6.58 The interferometer compares the phase of a signal at two antennas and uses the phase difference to

calculate the angle of arrival.

Figure 6.59 The operation of an interferometer is best understood through consideration of the interferometric

triangle.

The length of line BC is known from the formula:

BC = ΔΦ(λ/360°)

where ΔΦ is the phase difference and λ is the signal wavelength.

The angle at point B in the diagram is 90° by definition, so the angle at point A (call it

angle A) is defined by:

A = arcsin(BC/AC)

where AC is the length of the baseline.

The AOA of the signal is reported out relative to the perpendicular to the baseline at its

center point, because the interferometer provides maximum accuracy at that angle. Note

that the ratio of phase degrees to angular degrees is maximum here. By construction, you

can see that angle D is equal to angle A.

Page 490

resolution cell, 405

See also Decoys

Trackers

crossed linear array, 350

effect of jammer on, 372–73

imaging, 350–51, 366–70

rosette, 349–50

temperature-sensing, 359–60

Tracking rate

angular, 155–56

link bandwidth versus, 156

Tracking reticles, 346–51

Track-while scan (TWS) radar, 73

Transmission security

on links from higher value assets, 27

message security versus, 25–30

requirement, 16

spread spectrum (SS), 43

transmission bandwidth versus, 29

Transmitted bit stream

parity and EDC, 137

required bandwidth, 136–37

signals, 133–34

synchronization, 134–35

transmitted bit rate versus information bit rate and, 134

Transmitter power, 55

Triangulation

illustrated, 206

in location of communications transmitters, 205–8

moving DF system, 207

sites, 207

Trojan horses, 31

Page 491

Two-color sensors, 354–55

Two-ray propagation

decibel formula, 179–80

defined, 178

dominant loss effect, 179

minimum antenna height for, 180–81

Ultralow side lobes

ES system detection and, 127

EW impact, 89–91

gain pattern, 88, 89

J/S, 90

Uplink jamming, cell phone

from air, 295–96

from ground, 293–95

Velocity gate pull-off (VGPO), 109

Video compression, 165–66

Viruses, 31

Voice communication, 10

Wagon wheel reticle, 346–47

Watson-Watt direction finding technique, 219–20

Wavelet compression, 165

Weak signal intercept, in strong signal

environment, 193–94

Weather, 156–58

Wideband DRFM

defined, 300

frequency conversion, 301

jammer system, 300–301

sampling generation approaches, 301–2

See also Digital RF memory (DRFM)

AOA of the signal. Emitter location is determined from one of the techniques (such as

triangulation) discussed earlier.

We will begin by discussing single baseline interferometers and then will cover

correlative and multiple baseline interferometers.

6.6.14 Single Baseline Interferometer

Although virtually all interferometer systems employ multiple baselines, the single

baseline interferometer uses one baseline at a time. The presence of multiple baselines

allows for the resolution of ambiguities. It also allows multiple, independent

measurements to be averaged to reduce the impact of multipath and other equipment-

based sources of error.

Figure 6.58 is a basic block diagram of an interferometric DF system. Signals from

two antennas are compared in phase, and the DOA of the signal is determined from the

measured phase difference. Remember that we characterize the transmitted signal as a sine

wave traveling at the speed of light. One cycle (360 phase degrees) of the traveling sine

wave is called the wavelength. The relation between the frequency of the transmitted

signal and its wavelength is defined by the formula:

c = λf

where c is the speed of light (3 × 108 m/s), λ is the wavelength (in meters), and f is the

frequency in cycles per second (units are 1/sec).

The interferometric principle is best explained by consideration of the interferometric

triangle as shown in Figure 6.59. The two antennas from Figure 6.58 form a baseline. It is

assumed that the distance between the two antennas and their precise location are known

precisely. The wavefront is a line perpendicular to the direction from which the signal is

arriving at the direction finding station. This is a line of constant phase for the arriving

signal. The signal expands spherically from the transmitting antenna, so the wavefront is

actually a circular segment. However, since the baseline can be assumed to be much

shorter than the distance from the transmitter, it is very reasonable to show the wavefront

as a straight line in this drawing. The precise location of the station is taken to be the

center of the baseline. Because the signal has the same phase along the wavefront, the

phases at point A and point B are equal. Hence, the phase difference between the signals at

the two antennas (i.e., points A and C) is equal to the phase difference between the signal

at points B and C.

Page 246

Figure 6.58 The interferometer compares the phase of a signal at two antennas and uses the phase difference to

calculate the angle of arrival.

Figure 6.59 The operation of an interferometer is best understood through consideration of the interferometric

triangle.

The length of line BC is known from the formula:

BC = ΔΦ(λ/360°)

where ΔΦ is the phase difference and λ is the signal wavelength.

The angle at point B in the diagram is 90° by definition, so the angle at point A (call it

angle A) is defined by:

A = arcsin(BC/AC)

where AC is the length of the baseline.

The AOA of the signal is reported out relative to the perpendicular to the baseline at its

center point, because the interferometer provides maximum accuracy at that angle. Note

that the ratio of phase degrees to angular degrees is maximum here. By construction, you

can see that angle D is equal to angle A.

Page 490

resolution cell, 405

See also Decoys

Trackers

crossed linear array, 350

effect of jammer on, 372–73

imaging, 350–51, 366–70

rosette, 349–50

temperature-sensing, 359–60

Tracking rate

angular, 155–56

link bandwidth versus, 156

Tracking reticles, 346–51

Track-while scan (TWS) radar, 73

Transmission security

on links from higher value assets, 27

message security versus, 25–30

requirement, 16

spread spectrum (SS), 43

transmission bandwidth versus, 29

Transmitted bit stream

parity and EDC, 137

required bandwidth, 136–37

signals, 133–34

synchronization, 134–35

transmitted bit rate versus information bit rate and, 134

Transmitter power, 55

Triangulation

illustrated, 206

in location of communications transmitters, 205–8

moving DF system, 207

sites, 207

Trojan horses, 31

Page 491

Two-color sensors, 354–55

Two-ray propagation

decibel formula, 179–80

defined, 178

dominant loss effect, 179

minimum antenna height for, 180–81

Ultralow side lobes

ES system detection and, 127

EW impact, 89–91

gain pattern, 88, 89

J/S, 90

Uplink jamming, cell phone

from air, 295–96

from ground, 293–95

Velocity gate pull-off (VGPO), 109

Video compression, 165–66

Viruses, 31

Voice communication, 10

Wagon wheel reticle, 346–47

Watson-Watt direction finding technique, 219–20

Wavelet compression, 165

Weak signal intercept, in strong signal

environment, 193–94

Weather, 156–58

Wideband DRFM

defined, 300

frequency conversion, 301

jammer system, 300–301

sampling generation approaches, 301–2

See also Digital RF memory (DRFM)