Comparative Analysis: Sensors and Radar Equipment
Keynotes
An introduction to the workings of radar technology: A thorough examination of the technical workings of radar systems that will provide the method for analysis
A comparison of different radar systems on a cost/benefit basis: A comparative study into three distinct radar systems for the purposes of building our understanding through case analysis
A historical description of radar technology: A description of the historical context in which these radar systems saw use to deepen our understanding of the comparative advantages of their us
Introduction
Sensor technology is a key component of military advantage comprising communications, detection, and targeting equipment. Sensor equipment operates at frequencies across the electromagnetic spectrum but for our purposes, we will focus on radar equipment. Radar equipment operates by sending out radio energy from a broadcast array and detecting a target by measuring variations in the energy frequency returned to a receiving device. Target detection is inherently probabilistic due to a wide variety of factors beyond the scope of this essay. The design of a specific radar system can boost the probability that it detects its target, the range at which it operates, the resolution of the signal detected, and the field of view to accomplish its function. It accomplishes this by changing the average power of the system, the gain of the system, how fast the system cycles through its pulse repetition interval, how it chooses to filter out noise/clutter, and the tracking and scanning system used(or not used).
A given radar system cannot accomplish all of these things simultaneously. For a radar system to gain more range it has to give up the probability of detection or resolution. For it to gain probability of detection it has to give up range or field of view. For it to gain resolution it has to give up range. These trade-offs may be mitigated somewhat by throwing money at the problem, or technological development.
The systems compared here will be the AN/SPY-1, the Notional AAM, and the Fan Song E. The AN/SPY-1 is the electronically scanned fixed-array radar used by the Navy’s Aegis Fleet Defense System [1]. The Notional AAM is a theoretical example of a typical terminal guidance system for radar-guided air-to-air missiles. The Fan Song E is a missile control radar system. To determine the comparative strengths and weaknesses of these sensor systems it is necessary to view the specifications chosen by the designer across the compared systems [2].
Power Comparison
The average power affects the Probability of Detection and the Range of operation. The more power you give the radar system the stronger the signal broadcast and since radar waves become less energetic over time the stronger you can make those waves at the beginning the farther those waves will reach and have a meaningful chance of returning a valid detection to a receiver. You can gain average power by increasing the peak power of a radar system or by decreasing the Pulse Repetition Interval since average power is a function of the strength of the signal at peak power and the length of time the system spends not at peak power. The average power of the AN/SPY-1 is 58000 watts [1], the AAM is 3.5 Watts, and the Fan Song E has an average power between 600 and 1800 watts, this range reflects the fact the maximum and minimum values are derived through performing math on two different pulse width specs listed in the spec sheet. These values are partly reflections of their use cases. The AAM is trying to identify precise locations of targets in direct proximity and thus has no need to broadcast a signal at the kinds of magnitudes that allow the AN/SPY-1 to detect targets at ranges measured in kilometers. The Fan Song due to its use as a missile guidance system only has to be able to detect its targets at ranges within the range of the missile system with which it is used. These values also reflect the power output of their platforms. Air-to-air missile guidance systems must necessarily have a weaker average power than those radar systems mounted on naval vessels because the power output of an individual missile is not remotely comparable to the power output of a nuclear-fueled engine [2].
Beamwidth
Beamwidth is a function of the gain of the radar system and the aperture of the antenna and affects both the probability of detecting a target and resolution. This is because in order for a radar system to discover the location of a threat target that system must return a negative signal following a positive signal. For multiple targets operating in close proximity, detecting that negative space between the targets without detecting either target is crucial for determining the number of targets you are detecting. The beamwidth of the Fan Song E is 7.5 degrees wide in the “fan” and 1.5 degrees wide in the scanning dimension. The Fan Song is equipped with a Lewis Scanner system and thus turns along the scanning axis. The beamwidth of the AAM is 12 degrees. The beamwidth of the Fan Song is much thinner than the AAM despite its wavelength being smaller due to the fact the diameter of its dish is much bigger than the AAM. This is likely because the Fan Song is expected to detect and identify with precision hostile targets at range, the AAM has as its use case identifying precise target locations at short range. The AAM therefore does not have to differentiate between targets that exist, only know their precise location. The Fan Song likely needs to better differentiate between multiple targets. The Fan Song is also equipped with a scanning system, which reduces one of the downsides of narrow beam width, which is the narrow field of view. The Fan Song can rotate and thus increase its field of view without giving up the target differentiation capabilities granted by narrow beam width.[2] The beam width of the AN/SPY-1 is 1.7 degrees. Since the AN/SPY is a phase array system, this beam rotates across the full arc of its capability within milliseconds. The beam width is therefore incredibly thin as a result of the phase array technology offering full field of view while incurring little to no cost in the beam width and the power supply being a ship engine [1].
Frequency
The AN/SPY1 operates at a frequency of 3.3 GHz, or in the E/F band [1]. This is contrasted with the Nominal AAM system operating at a frequency of 12 Ghz, or in the J band, and the Fan Song E operating at a frequency of around 5 Ghz, or in the G band. These different operational frequencies speak to the different use cases for each system. The Nominal AAM uses a higher frequency than the other two systems due to its use case of assisting in guiding relatively small airborne targets toward other relatively small airborne targets in the critical moments before impact requiring the greater resolution supplied by higher frequencies. The Fan Song operates at a lower frequency than the AAM but a greater frequency than the AN/SPY due to the Fan Song requiring the resolution to detect the approximate location of a missile in flight but accepting a reduction in resolution possibly because it is not trying to guide the missile towards another object that is also in flight within tight spatial constraints [2]. The AN/SPY-1 on the other hand is a radar system designed to provide omnidirectional detection capability to naval platforms, and thus it can operate at a lower frequency due to its function being general detection, not specific targeting or guidance [1].
Many design choices play a role in emphasizing or deemphasizing field of view. The AAM as shown in the image has no obvious TWS technology nor a phased array system in place. The Fan Song, due to its use of the Lewis Scanner as well as two different antennas, one each for target search and target tracking, has a TWS system which greatly increases the field of view over the AAM while losing some certainty in targeting due to its mechanical rotation [2]. The AN/SPY-1 is a phased array system with individual arrays mounted such that coverage is provided in every direction. This gives the same advantage as TWS systems but since its functionality is provided by electronic charge differences among dipole antennas that can direct the beam across its full arc of rotation within milliseconds it dodges the main weakness of mechanical TWS systems which is losing sight of a target as the system rotates [1].
Comparative Analysis
The AN/SPY-1 has the best range and probability of detection of the three systems. This likely indicates that its primary function is defensive detection of enemy hostiles headed towards a large naval vessel. The system needs both range and a high probability of detection due to its centrality in the mission of protecting hundreds of lives and billions of dollars in equipment. It accepts a reduction in resolution in comparison to the other two systems as it does not function as a targeting or guidance system. It has an omnidirectional field of view due to its implementation of phase array technology. Lastly, it costs nearly 100x more than the AAM and is mounted onto a nuclear engine with an energy output orders of magnitude larger than the AAM, thus we should expect this system to outperform the AAM in nearly all categories if it wishes [1]. The fact it does not outperform the AAM in one specific spec is testament to the fact that even in a different class of budget, a piece of equipment cannot completely dodge these trade-off relationships [2].
The AAM, due to its size and power limitations, cannot have the aperture or power supply of either of the other two systems and thus spends the entirety of its 250,000 dollar design budget on improved resolution through its high pulse repetition frequency to the exclusion of virtually everything else. It is seemingly precision-designed for resolution at close range. That is evident through its massive pulse repetition frequency granting it increased resolution. The AAM is designed for one function and due to the design precision that focus allows it outperforms both other systems in the one spec it is seemingly built to optimize despite being on a much tighter budget.
Implications
The Fan Song is more general purpose than either of the more extreme systems we have looked at and this is reflected through its position consistently between the two other systems in every spec. It seems to have as its mission objective the detection of aircraft entering its airspace and missile guidance toward that target. It may be considered a complimentary system to the AAM in that it performs target detection and missile guidance up to the range the more focused AAM can take over.
The AN/SPY-1 accepted its underperformance in the sub-virtue of resolution due to that being the price of getting high specs everywhere else. This points to the nature of its mission being the general detection of faraway objects that may pose a threat to the vessel. Assumedly, a hit on the AN/SPY-1 would trigger more precise radar to identify the specifics of the threat. Identifying the reasons its strengths were not emphasized requires some guesswork. It may be that getting more performance out of the system required more power than the vessels could give without eating into other functions. It may be that increasing range beyond that at which it operates has diminishing returns, or detection at that distance has too much clutter to filter out and is thus unfeasible. It may be that the pulse array technology represents the best available technology and any further improvement in beam width or field of view would have an outsize cost for marginal improvement. It may be that you cannot increase the field of view beyond the 360-degree hemispheric umbrella already projected.
The AAM accepts its underperformance in virtually every spec to have incredibly accurate resolution due to its incredibly specific mission requirements of guiding a missile during the critical moments right before impact, a situation when being off by an inch could be the difference between a hit and a miss. It cannot have even more resolution likely due to its small budget and power allotment, but it may also be the case that increasing pulse repetition frequency beyond this point leads to too much loss to atmospheric forces and thus depressed function compared to this frequency.
The Fan Song performs in the middle of the two more extreme systems on virtually every spec. It accepts this because its smaller power supply than the AN/SPY-1 is operable on land and appears to be mounted on a mobile vehicle of some sort. That additional operational theater and mobility would allow it to be more versatile than either the hulking ocean-bound AN/SPY or the small and precise AAM. If the AAM is an F1 race car, the AN/SPY a tank, the Fan Song is an SUV. The specifications of the Fan Song reflect this theory of its general purpose use case.
The use case of the Fan Song as part of the North Vietnamese SAM system during the Vietnam war demonstrates the back and forth nature of these systems. While initially effective against American air power, America began fielding jamming technology and using Shrike missiles that would force the Fan Song to either shut off and lose guidance of its missile or be blown up by the incoming Shrike. This worked likely because the Fan Song was ill-equipped for the kind of fidelity necessitated by air-to-air confrontation. Following this new strategy, Fan Song operators began using battery storage to get the Fan Song to full power quickly. This, in concert with newly fielded Mig-21s gave them the advantage in the air until the American pilots began kitting their fighters out with jamming radar to lure the Mig’s into combat whereupon they would be torn to bits by the superior American fighter jet. This example serves to highlight the quality of counter and counter-counter these systems can encourage (Price, 2001. 5-14).
Managing the design trade-offs for a given radar system should be a function of its use case, its cost, and the technology available at the time of design. It is a complex engineering process with many variables beyond even those discussed in this essay. A radar design, even if optimized for a given mission parameter, may become obsolete based on technology or strategy employed by the enemy. Understanding the advantages sensor technology can grant and the limitations of that tactical edge are the most important factors in the success of the technology on the battlefield.
References
Northrop Grumman Electronic Systems and Sensor Center. “Joint Stars AN/APY-3 Joint Surveillance Attack Radar System.” In Jane’s Avionics, 20th ed. London: Jane’s Information Group, 2002.
Grant, Rebecca. The Radar Game. Arlington, VA: Mitchell Institute Press, 2010.
