Page 115 - Radar Technology Encyclopedia

data encoding DECIBEL 105

Data encoding is the process by which information is trans- store (VLDS), high-density digital recording (HDDR), mag-
formed to a sequence of standard signals, so that the informa- netic disks (fixed and removable), and optical disks.
tion may be transmitted between a processing system and a SAL
digital computer. This coding is generally performed using a Ref.: Currie (1989), pp. 189–201.
binary code in which the only symbols are 0 and 1.
data sampling (see SAMPLING).
Ref.: Sloka (1970), p. 140; Jordan (1985), Ch. 24.
data smoothing (see SMOOTHING).
Data fusion is the combining of information obtained from
several sources relating to the same target. It is employed in Data transmission permits data to be combined and pro-
radar networks, multistatic radar systems and multiple-sensor cessed at points other than the radar site. In addition to radar
systems (e.g., those that combine radar and infrared sensors). data, the transmission channel is used to transmit data neces-
The general methods of data fusion include combining radio sary for the effective operation of the radar system as a whole:
or intermediate frequency signals in the linear portion of the control commands, synchronization, and reference signals.
receiver, combining video signals, combining detections and The transmission lines may be either digital and analog (e.g.,
individual reports, and combining trajectories. When radar cable, fiber-optic, and radio lines). The composition of the
signals from different sensors (e.g., different sites in multi- transmitted radar data and the requirements placed on the
static radar) are combined within the processing system, all transmission network are determined by the extent to which
the information about the target and interference sources is data must be combined within the radar system.
transmitted, requiring wideband communications links and The transmission of individual reports and trajectory data
special data-compression methods. The advantage to such a is distinguished from the transmission of video and radio fre-
system is that it permits coherent processing in which infor- quency signals obtained from the various radar receivers. The
mation about the spatial structure of the electromagnetic former may usually be transmitted over common telephone
waves is used. channels (cable and radio-relay links) using modems, while
When the processing system combines trajectory data, transmission of video and radio signals requires wideband
the data processed at each site are transmitted using more channels.
simple transmission techniques. The greater the level of data An important problem is that of transmitting synchroni-
collection at the processing site, the lower the information zation signals with the accuracy required for multistatic radar
loss at the individual radars and the greater the capabilities of systems, for which special navigational systems and intersite
the system as a whole. The primary problems associated with communications links are used, including those based on the
combining individual reports and trajectories are the extrac- reception of a direct signal from the transmitting position.
tion and association of data as belonging to a given target, and The availability of accurate timing and frequency references
the calculation of smoothed estimates of the parameters through the Global Positioning System (GPS) offers a new set
describing the trajectory and the reflection characteristics. of solutions to these problems.
The conditions necessary to combine radar data at the RF IAM
signal level are defined by the ability to control and synchro- Ref.: Barton (1964), pp. 413–415; Salah, J. E., and Morriello, J. E., IEEE Int.
Conf. Radar-80, p. 88; Retzer, G., IEEE Int. Conf. Radar-80, p., 288.
nize the various radar sites, and to form reference signals with
sufficient stability to permit coherent processing to be per- DECEPTION, radar. Radar deception is the term used as an
formed at each site. abridged variant of deception electronic countermeasures (see
Data fusion from multiple radars is used in air traffic con- ELECTRONIC COUNTERMEASURES). Typically,
trol, antimissile defense, sea-surface surveillance, and other angle, range, and velocity deception are distinguished (see
systems. JAMMING, deception). Other categories of deception are
IAM manipulative (the introduction of radiations into enemy chan-
Ref.: Chernyak (1993), p. 9; Milne, K., IEE Int. Conf. Radar-77, p. 46; Buch- nel that imitates its own emissions) and imitative (the alter-
ner, M. R., IEE Int. Conf. Radar-77, p. 72; Hovanessian (1988), pp. 260– ation and simulation of friendly electromagnetic radiations to
267; Antony (1995); Hall (1992); Waltz (1990); Tech Reach (1996).
accomplish deception). A radar deceiver is ECM equipment
data quantization (see QUANTIZATION). that attempts to deceive or mislead radar by emitting a pulse-
like signal similar to the radar signal.
Data recording is the process of registration and storage of
SAL
information acquired during radar operation. From the point
of view of performance the recording systems can be classi- Ref.: Johnston (1979), pp. 57, 61, 63, 65; Schleher (1986), pp. 9, 138.
fied as real-time and non-real-time systems, and from the DECIBEL. The decibel (dB) is one-tenth of a bel. The num-
point of view of practical implementation as analog and digi- ber of decibels denotes the ratio of the two power or voltage
tal. The main analog recording medium that is still widely levels:
used is frequency modulation (FM) recording. Modern P U
recording techniques are primarily digital, and the main digi- dB = 10log ------ = 20log ------ 0
U
P
0
tal recording media are 9-track digital tape, very large data
where P is a power level to be related to the reference level
P , and U is a voltage level to be related to the reference level
0