During the next two months, we will discuss the differences

Transcription

1 EW 101 ES vs. SIGINT By Dave Adamy 42 The Journal of Electronic Defense January 2011 During the next two months, we will discuss the differences between Electronic Support (ES) systems and Signals Intelligence (SIGINT) systems, both of which are designed to receive hostile signals. The differences between SIGINT and ES have to do with the reasons those signals are received, as summarized in Table 1. There are also some technical differences between the typical environments in which these systems work that dictate differences in system design approach and system hardware and software. SIGINT SIGINT is the development of militarily significant information from received signals. It is commonly divided into Communications Intelligence (COMINT) and Electronic Intelligence (ELINT) as shown in Figure 1. Each of these subfields is somewhat related to ES as shown in Figure 2. ES is commonly divided into Communications ES and Radar ES as shown in Figure 3. The nature of communication and radar signals dictates differences in mission between these two subfields. The following paragraphs will focus on systems handling each type of signal, differentiating the intelligence and ES roles. COMINT and Communications ES Figure 3 is a flow diagram showing the relationship between COMINT and Communications ES systems. The dictionary definition of COMINT is gathering of intelligence by intercept of wire or radio communications. Basically, this is listening to what an enemy says to determine his capability, his force structure and his intentions. This implies that a COMINT system deals with the internals (i.e., the information carried in the modulation) of transmitted enemy Table 1: SIGINT vs. ES Mission SIGINT Systems COMINT: Intercept enemy communications and determine enemy capabilities and intentions from information carried on signals. ELINT: Find and identify new threat types. Figure 1: SIGINT comprises COMINT and ELINT to develop intelligence from enemy communications and non-communications signals. signals. Because of the nature of military communication, important signals can be expected to be encrypted and, of course, in the enemy s language. Decryption and translation of signals can be expected to delay the availability of the information recovered. Thus, COMINT can be considered more valuable to strategic and high-level tactical considerations than to determination of appropriate immediate tactical response. Communications ES focuses on the externals of communications signals: the type and level of modulation and the location of the transmitters. It supports tactical responses to current situations by determining the types and locations of enemy emitters. By modeling all of the types of emitters against the type of emitters used by various enemy organizations, estimates of the enemy force structure can be made. The location and location history of the observed emitters can be used to ES Systems Comms ES: Identify and locate enemy communications emitters to allow development of EOB and to support comms jamming. Radar ES: Identify and locate enemy radars to allow threat warning and to support radar countermeasures. Timing Data Collected Timeliness of outputs is not too critical. Gather all possible data on received signals to support detailed analysis. Timeliness of information is central to mission. Gather only enough data to determine threat type, operating mode and location.

2 EW101 Figure 2: ES comprises Comms ES and Radar ES. Both provide information about enemy emitters currently operating in support of EA and weapon engagement. indicate the location and movement of the enemy s forces. The total laydown of transmitters is called the Electronic Order of Battle (EOB), and can be analyzed to determine the enemy s capabilities and even his intentions. In summary, COMINT determines the enemy s capabilities and intentions by listening to what is said (i.e., signal internals), while Communications ES determines the enemy s capabilities and intentions by analysis of signal externals. ELINT and Radar ES ELINT involves the intercept and analysis of non-communications signals, primarily from radars. The purpose of ELINT Figure 3: COMINT classically deals with signal internals to support strategic actions; Comms ES deals with signal externals to support immediate tactical decision making. Figure 4: ELINT systems gather threat data to support the development of ES systems and subsystems for threat warning and countermeasure selection. is to determine the capabilities and vulnerabilities of newly encountered enemy radars. As shown in Figure 4, the ELINT system gathers enough data to support detailed analysis. The first task when a new radar signal type is received is to determine whether the received signal is, in fact, a new threat. Two other possibilities exist: it may be an old threat radar that is malfunctioning or there may have been something wrong with the intercept system. If the received signal is, in fact, a new type of radar or a new operating mode, the detailed analysis will allow modification of ES systems so that they will be able to recognize this new threat type. Radar ES systems also receive hostile radar signals, but their purpose is to quickly determine which of the enemy s known weapons is being deployed against a target at the moment. After threat type and mode identification is complete, this information is displayed to operators along with the location of the threat emitter and/or passed to other electronic warfare systems or subsystems to support countermeasure initiation. If a signal of an unfamiliar type is received, it is considered an unknown. In some ES systems, the operator is merely notified that an unknown threat has been received. However, in other systems, an attempt is made to guess the threat type. In some ES systems, unknown threats are recorded for later analysis. In summary, ELINT determines what capabilities the enemy has, while Radar ES determines which of the enemy s radars is being used at the moment and where the emitter (hence the weapon it controls) is located. What s Next Next month, we will continue our discussion of the differences between ES and SIGINT, covering the technical differences between ES and SIGINT systems which are driven by their respective missions. For your comments and suggestions, Dave Adamy can be reached at dave@lynxpub.com. a The Journal of Electronic Defense January

3 EW 101 ES vs. SIGINT Part 2 Antenna and Range Considerations By Dave Adamy 50 The Journal of Electronic Defense February 2011 This month, we will deal with some technical differences between ES and SIGINT systems dictated by mission and environment considerations. These differences have to do with the anticipated intercept geometry, the different types of information taken from intercepted hostile signals, and time criticality of intercepts. ANTENNA ISSUES Antennas can be characterized as directional or nondirectional. This is, of course, a great over-simplification. Antennas, such as whips and dipoles, are sometimes (incorrectly) described as omni-directional. This is not true, as both antenna types have nulls in their coverage. However, both types, if vertically oriented, provide 360 degrees of azimuthal coverage. There are also circular arrays of directional antennas that provide full azimuthal coverage. Directional antennas (including but not limited to parabolic dishes, phased arrays or log periodic antennas) restrict their coverage to a reduced angular sector. Angular coverage has a significant impact on the probability of intercepting a hostile signal at an unknown direction of arrival. As shown in Figure 1, a 360-degree coverage antenna (or array of antennas) looks in all directions all of Figure 2: The percentage of angular space within the antenna beam varies inversely with the beamwidth, as does the dwell time at the signal s angle of arrival. the time, so it will input any new signal to a receiver as soon as it occurs. The directional antenna, on the other hand, must be scanned to the direction of arrival of a new signal before it can be received. If a hostile signal is present for a limited time, the probability of intercept is a function of the antenna beam width and the scan rate of the antenna. For an intercept to occur, the antenna must be moved to place the signal s direction of arrival into the antenna beam Figure 1: A 360-degree antenna, like a dipole or whip, provides 100 percent coverage of all azimuths of arrival, while a narrow-beam antenna must be scanned to the correct direction of arrival. coverage area. As shown in Figure 2, the beamwidth determines the percentage of possible angles of arrival covered by the antenna. To use this part of the figure, draw a line straight up from the beamwidth to the black line, then draw right to the black ordinate value. This considers only one search dimension (e.g., azimuthal search); a two-dimensional search is significantly more difficult. In the same figure, the amount of time that a scanning antenna will dwell on the signal s angle of arrival (also in azimuth only) is shown as a function of beam width for various circular scan periods. To use this part of the figure, draw straight up from the beam width to the green line

4 Figure 3: The gain of a narrow-beam antenna varies inversely with its beamwidth. for the selected scan period, then draw left to the green ordinate value. It should be noted that a frequency search must be made during the time the antenna is pointed at each possible angle of arrival. The narrower the antenna beam, the slower the receiving antenna must be scanned to allow for frequency search. Thus, the longer it will take to find a signal of interest at unknown frequency and angle of arrival. Frequency search will be discussed next month. Normally, SIGINT intercepts are less time critical. Thus, a delay in intercept caused by scanning a narrow beam antenna is likely to be acceptable. However, because ES systems must typically intercept a hostile signal within a small number of seconds, a wide coverage antenna or array of antennas is usually required. As shown in Figure 3, there is a trade-off of the half power (3 db) beamwidth of an antenna and the antenna gain. This figure is for a 55-percent efficient parabolic dish antenna, but this trade-off applies to all types of narrow beam antennas. The receiving antenna gain is an important consideration in the range at which a hostile signal can be intercepted, as discussed below. This means that wide-coverage (hence low-gain) antennas are almost always required for ES systems, while narrowbeam (hence high-gain) antennas may be the best solution for SIGINT systems. Figure 4: The range at which a receiving system can intercept a hostile emitter signal is a function of antenna gain and receiver system sensitivity. EW101 INTERCEPT RANGE CONSIDERATIONS Figure 4 shows an intercept situation for either an ES or SIGINT system. Note that the range at which a receiving system can intercept a hostile signal depends on the effective radiated power of the target signal, the applicable propagation mode, the receiving antenna gain in the direction of the emitter, and the sensitivity of the receiving system. Propagation modes are discussed in detail in the EW 101 columns in the July, August and September 2007 editions of JED. Radar and data link signals typically propagate in the line-of-sight mode. In this mode, the intercept range is given by the formula: RI = Anti-log {[ERPT log(f) + GR S] / 20} Where: RI = the intercept range in km ERPT = the effective radiated power of the target emitter in dbm F = the frequency of the transmitted signal GR = the gain of the receiving antenna in the direction of the target emitter S = the sensitivity of the receiver system in dbm Communication signals will propagate in the line-of-sight or 2-ray modes, depending on the link distance, antenna heights and frequency. If propagation is in the 2-ray mode, the intercept range is given by the formula: RI = Anti-log {[ERPT log(ht) + 20 log (hr) + GR S] / 40} Where: RI = the intercept range in km ERPT = the effective radiated power of the target emitter in dbm ht = the height of the transmitting antenna in meters hr = the height of the receiving antenna in meters GR = the gain of the receiving antenna in the direction of the target emitter S = the sensitivity of the receiver system in dbm As you can see from these formulas, the intercept range is always impacted by the receiving antenna gain and the sensitivity of the receiving system. Note that the sensitivity is the required signal strength for a successful intercept. The more sensitivity the receiving system has, the lower this number will be. For example, a high-sensitivity receiver might have a sensitivity of -120 dbm, while a low-sensitivity receiver could have a sensitivity of -50 dbm. The effective radiated power (ERP) of the target emitter is the amount of power it transmits in the direction of the intercepting receiver. Tactical communication threats will usually have 360-degree antennas with fairly constant gain vs. azimuth; the ERP is the sum of the transmitter power (in dbm) and the The Journal of Electronic Defense February

5 52 EW101 antenna gain (in db). However, radar threats are expected to have narrow-beam antennas. As shown in Figure 5, the narrowbeam antenna has a main lobe and side lobes. The side lobes are shown simplified in that they are all the same strength actual antenna side lobes vary. However, the drawing is realistic in that the nulls between the lobes are much narrower than the lobes. This means that an intercept receiver pointed at the radar threat emitter away from the direction of the main beam can be expected to encounter an ERP at the average side lobe level. This level is usually stated as: S/L = -N db, where N is the number of db that the average side lobe level is below the boresight gain. Although not always true, it is fairly common for an ES system to be specified to receive the main lobe of a radar threat Figure 5: Radar ESM systems are often characterized as receiving signals from the boresight of threat radar antennas, while ELINT systems are often characterized as receiving average side lobe level signals. while an ELINT system would be specified to intercept side lobe transmissions from target radar emitters. This means that an ES system will often require less sensitivity and/or receiving antenna gain than an ELINT system. SIGINT systems are generally assumed to require greater intercept range than ES systems however, as with all generalities, this depends on the specific mission and situation. If we accept that SIGINT systems require greater intercept range, the receiving antenna gain and/or the sensitivity must be greater than required for ES systems. Narrow-beam antennas have higher gain, but provide reduced probability of intercept (in a short time period). Thus, they are more appropriate for SIGINT applications. Full-coverage antennas, while they provide less gain, can provide significantly better probability of intercept in a short time period, so are generally most appropriate for ES systems. What s Next Next month, we will continue our discussion of the technical differences between ES and SIGINT, with coverage of receiver and data collection issues. For your comments and suggestions, Dave Adamy can be reached at dave@lynxpub.com. a The Journal of Electronic Defense February 2011 The Billy Mitchell Chapter Invites You to Attend the 2011 Mountain-Western Region Technical Symposium EW/IO in a Cyber World You are invited to join us April 11th through April 14th, 2011 in San Antonio during our fabulous Fiesta week! H eld in conjunction with the National Collegiate Cyber Defense Competition (NCCDC) (8-10 April), and the Congressional Cyber Security Caucus Summit (9 April), FiestaCrow 2011 will bring together renowned cyber/io policy makers and technical experts, providing you the opportunity to meet and hear senior officials such as Mr. Howard Schmidt, White House Cyber Policy Coordinator; Mr. Robert Butler, Deputy Assistant Secretary of Defense for Space and Cyber Policy; Major General Richard Webber, Commander, 24th Air Force; and Major General Bradley Heithold, Commander, Air Force Intelligence, Surveillance & Reconnaissance Agency. FC11 is co-sponsored by the Air Force ISR Agency and 24th Air Force. For additional Information, go to our website at S A N FIE STACROW A N T O N ASSOC BI LLY A I T I ON 11 T X I O, O M I TCH E LL F OLD CROW CHAPTE R S

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