ES System Technologies and Implementation

ES systems are typically comprised of antenna, receiver, and processing sub-systems. Early ES systems were often improvisations based on civilian equipment. For example, receivers developed for radio amateurs had relatively good sensitivity and frequency coverage and were widely used by the Allies during WW2. The National HRO, which had excellent frequency resolution, was used to intercept communication signals in the medium and high frequency bands.

The Hallicrafters S-27, which provided contiguous coverage in the lower portion of the very high frequency (VHF) band, was widely used to receive signals associated with German VHF radar, air-to-air communication, and bombing navigation systems. These receivers, although useful, had significant limitations. Their frequency coverage was limited, and their effectiveness was heavily dependent on the training and skill of the operators.

The continued evolution of the technologies used by communication and radar systems has contributed to the development of specialized ES receivers. A fundamental issue concerns the differences in the waveforms used by communication and radar signals.

Most communication systems transmit a continuous or near-continuous narrow bandwidth signal during the transmission of a message. A primary goal is to make efficient use of bandwidth to transmit information, which thereby allows the available radio frequency bands to be divided between many users.

Communication signals have continued to evolve:
1. The bandwidth and channel spacing associated with conventional narrowband signals has decreased because of developments in more efficient modulation formats and accurate frequency references and synthesizers;

2. Digital modulation techniques are being increasingly used to transmit information in the form of binary data;

3. Time division multiplexing access techniques are being used by some systems, such as those based on the GSM cell phone standard, to provide a way of time sharing bandwidth between multiple users;

4. Classes of spread-spectrum techniques are being used in some military and civilian communication systems. Frequency hopping (FH) systems superpose periodic changes on the center frequency of a transmitted signal following a predetermined sequence. These changes typically occur at rates that are tens or hundreds of times per second. The portion of a transmission that corresponds to a dwell at a single frequency is often referred to as a hop.

To minimize interference between FH communication systems, careful coordination is needed in the assignment of hop frequencies and/ or the codes that define the hop sequences. Direct sequence spread spectrum (DSSS) uses a different approach. In the basic form, a pseudo-random number (PRN) sequence is used by the transmitter to spread the narrowband information content over a much larger bandwidth.

The receiver uses the same PRN sequence to recover the information. Multiple systems can share the same bandwidth without seriously interfering with each other if they are assigned different PRN sequences. Code division multiple access (CDMA) cell phone systems are a major application of DSSS techniques. Because the detection of spread- spectrum signals often requires special techniques (17), these signals are sometimes referred to as low probability of intercept signals.

5. Mobile communication systems and networks have proliferated and are widely used. These systems are based on the idea of dividing a geographical area into cells. Each cell has a base station that performs the functions of relaying messages between the short- range handset radios within the cell and a communication network interface to other carriers, such as the public telephone system network. Cellular telephone systems usually operate in the ultra high frequency band.

The classic pulsed-radar concept, however, involves the transmission of short duration pulses with relatively large time intervals between successive pulses. This method sidesteps the difficult problem of detecting the relatively weak signals reflected from the target during the simultaneous transmission of a high power signal. Requirements for range resolution often dictate the use of pulse widths on the order of a microsecond or less, which thereby results in relatively large bandwidths on the order of MHz.

The waveforms used by advanced radars have increased in sophistication:
1. Coherent radars transmit signals whose waveforms are precisely defined;
2. Frequency or phase modulation may be used to increase range resolution;

3. The time intervals between successive pulses (pulse repetition interval) may be varied in a periodic or random sequence (pulse repetition interval stagger);
4. Multifunction radars select between different waveforms depending on the functionality that is required.

Application requirements for high angular resolution and compact antenna dimensions have motivated the extensive use of frequencies above 8 GHz.

The differences between radar and communication signals have motivated the development of specialized ES equipment:

1. Communication ES receivers feature extended frequency coverage to reduce the need to use different receivers, selective filters for separating signals that are closely spaced in frequency, comprehensive capabilities for demodulating the signal message content, and provisions for the measurement of signal parameters;

2. Radar ES receivers emphasize microwave frequency coverage and are optimized for the reception of pulse signals;

3. Specialized radar ES receivers have been developed for strategic and tactical applications. For example, electronic intelligence receivers are designed for the precision measurement of signal parameters, whereas radar warning receivers are designed to provide warnings of threat signals, be simple to use, and satisfy size and cost constraints;

4. Multichannel receivers have been developed to process multiple signals from antenna arrays with the accurate phase and amplitude matching needed for applications such as direction finding.

General trends in all systems include the use of precision frequency references and synthesizers to permit accurate and repeatable tuning, progressive reductions in size, and the use of form factors that permit the convenient installation of multiple receivers in standardized rack configurations.