«STATE-OF-THE-ART AND TRENDS IN PHASED ARRAY RADAR P. VAN GENDEREN Delft University of Technology Faculty of Information Technology and Systems PO Box ...»
STATE-OF-THE-ART AND TRENDS IN PHASED ARRAY RADAR
P. VAN GENDEREN
Delft University of Technology
Faculty of Information Technology and Systems
PO Box 5031, 2600 GA Delft, The Netherlands
Also at: Hollandse Signaalapparaten BV
PO Box 42, 7550 GD Hengelo, The Netherlands
The paper first introduces phased array configurations for radar and astronomy applications. In
radar various trends can be observed, on the one hand driven by cost issues and on the other hand by increasing performance demands. The trends discussed, are the increasing bandwidth, the improvement of the Power Added Efficiency of transmit/receive modules, the phase shifter technology, alternative radiator designs, substitution of analog technology by digital technology and the introduction of sparse arrays. The paper concludes by observing that in radar applications phased arrays are now commercially viable, but that due to their complexity the cost will maintain to be high.
1 Introduction Principles of phased arrays have been applied in radar since World War II. Most of the advances in theory and technology were achieved in the fifties and the sixties. However the phased arrays came into operational use late in the sixties and the seventies. Two components were driving the development at that time: fast phase shifters and computertechnology for phased array control. The number of economies capable of bearing the cost of such radars was very low. While the technology required for phased array radars was maturing in the eighties, in the current decade several companies are capable of developing and manufacturing them. The paper discusses the trends in phased array radar technology, combined with occasional side steps to the phased arrays for astronomy applications.
2 Phased array configurations Phased Array Radars appear in very different configurations.
Fig. 1 shows an example of one face of a phased array radar having four of these, installed in a large anechoic chamber. It is the antenna part, including all transmit and receive electronics.
Figure 1: One face of the Active Phased Array Radar APAR (Courtesy Hollandse Signaalapparaten) A.B. Smolders and M.P. van Haarlem (eds.) Perspectives on Radio Astronomy – Technologies for Large Antenna Arrays Netherlands Foundation for Research in Astronomy - 1999 The antenna consists of an array of active radiators. “Active” here means that the radiated power is generated in the radiator module. The receive function is integrated with this design. Of course the full radar has other system components for signal processing and management. These are not shown on the figure. The radar, called APAR, operates in the X-band (so 10 GHz). The size of the antenna is approximately 1 x 1 m2.  Although many phased array radars have an appearance like this one, not all the phased array radars look like the APAR. Other examples are the impressive phased array radars that served the Ballistic Missile Early Warning system from the United States, with their equipment building as high as a 10 floor apartment building. Or the large Russian Over the Horizon mattress antennas, or the long Jindalee Over The Horizon radar in Australia.
An extraordinary concept is the French RIAS (Radar a Impulsion et Antenne Synthetique), set up by ONERA and built by Thomson-CSF/Airsys. Fig. 2 shows a picture of the antenna system. Note the two circular arrays, sparsely populated. The array elements are mounted on the white vertical masts.
This radar operates in the HF/VHF band (50 MHz). The diameter of the outer annulus is approximately 500 m. [2-3]
From the antenna point f view, the arrays as used in radio astronomy may look like arrays in radar, as illustrated by the artist impression of NFRA’s Square Kilometer Array, shown in Fig. 3.
There are several reasons for selection of the phased array technique in radar.
Of course, to start with, radar is an active device (although we have seen some proposals for radars using the solar noise as the emitted signal. So the radar itself then would be passive). “Active device” here means that the radar transmits a signal and by matching the received signal to the emitted one, analyses on the presence of echoes of objects and their properties. The associated signal processing may either use or not use phase and amplitude of the received signal. Phase processing here serves compression of the time domain signal to achieve improved resolution or some Doppler-related signal property. Radar signal processing has evolved that far that identification routines are currently required in al novel systems. These routines should support identification of objects, in order not to start off inappropriate military action.
The emphasis in radar is on localization and identification in range, bearing and sometimes elevation and speed. Angular resolution is of the order of 1 to 2 degrees. Range accuracy is of course depending on the application, but typically for surveillance functions in the order of tens of meters.
High range and cross-range resolution profiles are required for the identification function. One should think of less than 50 cm (bandwidth 300 MHz). Anti-stealth waveforms may go further in bandwidth.
Ground penetrating radar for anti-personnel landmine detection extends up to 3 GHz of bandwidth.
This is all in sharp contrast with the needs in radio astronomy, where the main quest is to achieve a spatial resolution of 1 arc minute or even less than one milli arcsecond at a certain sensitivity.
Now the question may be posed: why a phased array solution.
The responses to this question in het communities of radio astronomy and radar are of course very different, because the applications are wide apart.
The main reason in radio astronomy is that very large apertures can be created, whilst the angular resolution is established by synthesizing the direction of arrival from this -maybe sparsely populatedaperture.
The main reason in radar is, that with the phased array technique, the beam of the antenna can be steered in a certain new direction which is very much differing from its current position without delay due to mechanical inertia. This might be called “beam agility”. This beam agility is important when the radar has multiple functions to be performed simultaneously. The multifunction capability of phased array radar is illustrated in Fig. 4. The grey level in a beam represents a particular waveform.
As the scenario evolves over time, the radar will spend its resources accordingly.
Figure 4: Phased Array Radars are preferred for their multifunction capability.
(Courtesy Hollandse Signaalapparaten) The functions the radar must be capable of could be
• limited volume search, with waveforms dedicated to classes of objects (e.g. missies, ships, helicopters, …)
• accurate tracking of objects
• identification waveforms, requiring relatively long dwell time in a certain angular sector
• guidance of missiles The order of magnitude of the antenna dwell time in a certain direction may vary –in the same radarfrom less than 1 msec to more than 100 msec.
Phased array radars can be controlled to spend their time and power resources in a user defined optimum way. This is not the case with the common surveillance or track radar, which are optimized for one particular mission only.
Managing the radar so that it complies with its operational objectives, leaving no time and no power unspent is a very complex task. The beam agility is just one aspect of the complexity of the design, introduced by the multifunction capability. It is one of the cost drivers, making that phased array radars cost multiple tens of millions of dollars per system.
Complexity is paid by the manufacturer at design stage.
There is one other cost driver: the antenna. Either it is the power distribution network and phase shifting capability or -in other system approaches- the transmit/receive module which is pushing cost sky high. Current cost of T/R modules at X-band is in the order of $1000 a piece, where the expectation is that it will lower to $500 once mass production can be realized. Note that one phased array is normally populated by 1000 to 3000 of these T/R modules per array.
Given this cost factor, it should be no surprise that the majority of technology actions concerns either reduction of the cost by technology evolution or by technology substitution. The complexity however remains.
This list of phased array systems presented in Table 1 does by no means pretend to be exhaustive. It shows that many industries have the capability to design and manufacture phased array radars. In effect, there are that many industries, that the tremendous investment required for the first of class per system doesn’t pay back soon. So technological advances will not be picked up readily. It might take considerable time before budgets are available to make innovative-and costly- steps forward.
Competition is fierce however, so that if a manufacturer could produce new systems at a considerably reduced level of cost, he may have a decisive technological advantage and gain a larger market share.
National defence strategies of course interfere with this market mechanism.
4. Trends in Technology for Phased Array Radar
Having thus introduced phased array radar, and having stated that certain aspects of the antenna are cost drivers, now the question what technological trends are visible that could either lift the problems of phased array radar designers or that may present solutions to important system performance requirements will be addressed. Such a review can hardly be complete; so no conclusive statement can be presented related to the ultimately achievable objectives in terms of cost or performance.
4.1 Increase of bandwidth
The first trend in all radar applications is, that they should provide a higher bandwidth, either by synthesizing narrow band signals into a wide band or by instantaneous wide or ultra wide band signals. The problem in synthesizing wide band signals based on a set of narrow band signals is that it takes measurement time. Note that time is scarce in phased array radar.
The alternative to synthesized wide band would be instantaneous wide band, eventually even leading to true time delay beam forming in stead of phase shifting. We saw an example in the RIAS radar.
The important fact is, that several of the technologies described in next sections can be optimized for narrow band radar, but not for wide band radar, or only at a very high cost.
4.2 Transmit/receive module
One of the important functions in active phased array radar is the transmit/receive module. In active phased array radar, there may be several hundreds to several thousands (in extreme cases even tens of
thousands) of these. Two aspects of this component are highlighted:
• the power added efficiency, and
• the cost per die.
First the Power Added Efficiency.
Fig. 5 shows the power added efficiency as a function of the RF frequency for the MESFET technology. Power Added Efficiency (PAE) is defined as the power difference between the output and the input of the device compared to the power supplied to it. One may see the PAE is acceptable at Figure 5a PHEMT technology Figure 5b MESFET technology relatively low frequencies (1 GHz). And also that at Ka band (good for tracking accuracy) the efficiency drops to below 20%. AT X-band the PAE is around 40%. This means that in order to obtain 10 kW of radiated power more than 25 kW should be supplied. And thus that 15 kW must be carried off some way or another. For PHEMT technology (Fig. 5a), the efficiency at 10 GHz is higher than in the MESFET technology, but at higher frequencies it is really getting very low. Now these numbers are taken from literature as old as 1989 . But they have been confirmed to be still valid today, at industrial series production level.
The other important aspect is the cost of MMIC’s. Fig. 6a shows the cost per die, as a function of the surface of the MMIC . The far right of the horizontal axis is 100 mm2. The lower band of cost numbers represent the optimum numbers when the yield of the production is very high. But the fact is that the yield decreases significantly with the die size. In fact, in 1986 the yield for the largest size (100 mm2) was as low as 10%.
These numbers have improved since then. State of the art industrial yield is now that the smaller dies, say 2 mm2, can be manufactured at a yield level of 90 - 95 %. Larger dies, say 10 mm2, can be made with, say, 65% of yield.
However, the cost of dies maintains a critical issue. The yield may be considerably improved when tolerances of the performance could be relaxed. One particular example of the criticality of the tolerances is illustrated by Fig. 6b . This figure shows the rms error distribution of the phase of a phase shifter over a number of wafers. So for each wafer, the rms phase error of all shifters on the wafer was measured, resulting in one number per wafer. Then, the statistic was compiled over all wafers, resulting in the indicated graph. Added to this figure is the impact on the root mean squared beam sidelobe level (dB). Knowing that this example concerns a 6 bits phase shifter, where the phase quantization alone would result in -32 dB side lobes below the gain of each element, a considerable number of wafers should have been discarded and considered as unacceptable, because of unbalance between the system side lobes and the contribution of these phase shift dispersion to these side lobes.
The message is: the performance specification is very critical; the size of the dies should be included as system design parameter.
4.3 Phase Shifter Technology 4.3.1 The Micro Electrical Mechanical Switches Looking for alternatives to Gallium Arsenide phase shifters, several technologies emerge. The first one I will address here is the Micro Electrical Mechanical Switch .
It is a compact mechanical device, as illustrated in Fig 7, where a membrane can be switched to either: