remote sensing

Article Design and Implementation of a Multi-Band Active Calibrator for SAR

Liang Li 1,2,3,*, Gukun Liu 1,2,3, Jun Hong 1,2,3, Feng Ming 1,2 and Yu Wang 1,2

1 Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China; [email protected] (G.L.); [email protected] (J.H.); [email protected] (F.M.); [email protected] (Y.W.) 2 National Key Laboratory of Sciences and Technology on Imaging, Beijing 100190, China 3 University of Chinese Academy of Sciences, Beijing 100049, China * Correspondence: [email protected]; Tel.: +86-10-58887665

 Received: 29 April 2019; Accepted: 23 May 2019; Published: 1 June 2019 

Abstract: Over the past decade, IECAS (Institute of Electronics, Chinese Academy of Sciences) has developed a set of L-, S-, C-, and X-band active radar calibrators that are deployed during the calibration campaigns for HJ1C synthetic aperture radar (SAR), Gaofen-3 SAR, and so on. In the near future, P-band and Ka-band spaceborne SARs will be launched. We found that it is not convenient to develop special active radar calibrators (ARCs) for a specific SAR or a specific band SAR, and the acquired experience could help in the design and development of a multi-band ARC. This paper describes the design and implementation of a multi-band active radar calibrator which can operate in the L-, C-, X-, and Ka-bands. Moreover, laboratory measurements are performed to characterize the performance of the multi-band ARC, paying particular attention to the gain stability, the system transfer function, the gain flatness, and the linearity of the ARC receiver. Three such ARCs are developed, and to our knowledge, the multi-band ARC is the first of its kind in China or even in the world, and it can be used to implement the calibration campaigns of the Chinese Gaofen-3 SAR, Shenzhen-1 SAR, Luojia-2 SAR, and so on.

Keywords: multi-band; transponder; active radar calibrator; calibration; SAR

1. Introduction Synthetic aperture (SARs) are an important remote sensing tool for the acquisition of quantitative information about the Earth’s environment. Nowadays, there are L-, S-, C-, and X-band spaceborne SARs in orbit. Additionally, lower P-band and higher Ka-band spaceborne SARs will be launched in the near future. For quantitative application, external calibration should be executed, and the active radar calibrator (ARC) plays an important role in the external calibration of SAR instruments due to its adjustable radar cross section (RCS), capability of changing its internal delay, high RCS, and small size. So, ARCs have been developed for many famous spaceborne SARs such as Radarsat-1/2, TerraSAR, Sentinel, and Gaofen-3 [1–7]. Most of the existing ARCs operate in a single band. For example, the ARCs developed for Radarsat-1/2, Sentinel, ERS-1, and Gaofen-3 operate in the C-band [1,3,4,6,8], the ARCs developed for TerraSAR operate in the X-band [2], the ARCs developed for ALOS/PALSAR operate in the L-band [9], and the ARC developed for Chinese HJ1C operates in the S-band [10]. Even for multi-band SAR systems, the ARCs are also developed as single-band instruments. For instance, the NASA/JPL DC-8 aircraft SAR operated at three (L-, C-, and P-bands), but the ARCs used for calibration operated at a single frequency (three L-band ARCs and three C-band ARCs were used) [11]. Another

Remote Sens. 2019, 11, 1312; doi:10.3390/rs11111312 www.mdpi.com/journal/remotesensing Remote Sens. 2019, 11, 1312 2 of 15

SIR-C/X-SAR operated in the L-, C- and X-bands; however, two L-band ARCs, two X-band ARCs, and three C-band ARCs were used for the calibration of the SIR-C/X-SAR [12]. Single-band ARCs have been developed for SAR systems, even for multi-band SAR instruments. However, with the increase of spaceborne SARs, it is not convenient to develop special ARCs for a specific SAR or a specific frequency band SAR. Therefore, the concept of a new multi-band ARC for spaceborne radar calibration is proposed in this study. Of course, it is very difficult to develop such an ARC, especially when it has many operational frequency bands. Many key technologies such as multi-band antenna, multi-band gain stability, and a large receiver dynamic range must be implemented for the development of the multi-band ARC. We took many measures and overcame these key technologies to design and develop three multi-band ARCs, which work in the L-, C-, X- and Ka-, operational bands. The specifications such as RCS stability, RCS ripper, linearity, and dynamic range were confirmed by the measurements. The three multi-band ARCs will be used to implement the calibration campaigns of Chinese Gaofen-3 SAR, Shenzhen-1 SAR, Luojia-2 SAR, and so on.

2. Materials and Methods

2.1. Principles of ARC The RCS of an ARC is given by Equation (1) [11]:

λ2 σ = GaG G (1) 4π T R where GT and GR are the gain of the transmitting and receiving antennas of ARC, respectively; Ga is the gain of the (RF) channels inserted between the receiving and transmitting antennas to increase the RCS; and λ is the radar . It can be seen from Equation (1) that the RCS of ARC depends on the gain of the RF channel and the TX and RX antenna gain. Actually, the gains of the receiving antenna and the transmitting antenna are the same. The RCS can be high enough in theory if either the RF channel gain or the antenna gain, or both, is large enough. However, they are restricted by each other. The higher the antenna gain is, the bigger its size is. It is very hard to keep the ARC at a reasonable size, and it is very heavy due to the bigger structure of the antennas, which then have pointing uncertainty and deployment issues. If the RF channel gain is designed to be very high, on the one hand, the gain stability requirements cannot be satisfied, and on the other hand, the isolation between the receiving and transmitting antennas is difficult to satisfy [13,14]. Thus, many factors should be considered such as the value of RCS, the size and weight of the ARC, the stability of RCS, and so on. The level of the receiving signal is also a precondition for the design of an ARC. For a certain RCS, the higher the signal level reaching the ARC is, the higher the output level of the ARC is. Therefore, the demands such as saturated output power, a 1 dB compression point, and linearity for the final amplifier are very high. The received signal level of the ARC can be given according to the radar equation [15,16]: λ2 Pr = PEIRP GR (2) (4πR)2 · · where PEIRP is effective the isotropic radiation power (EIRP) of SAR, and R is the distance between SAR and ARC. The distance is decided by the orbit and the incident angle of SAR. The multi-band ARC was designed for several SARs working in the L-, C-, X-, and Ka-bands. The received signal level will be much different even for SARs in the same band, because the EIRP, orbit, and incident angle will be different. Therefore, the ARC is required to have a large dynamic range. So, we should calculate the received signal level first and then deduce the output power of ARC according to the RCS. The final amplifier of ARC can be decided based on the output power, and the ARC can be designed, including the antenna size, the gain of RF channels, and so on. Remote Sens. 2019, 11, x FOR PEER REVIEW 3 of 15

In the framework of future SAR ground calibration campaigns, three equal ARCs will be deployed, covering the four operational bands (L\C\X\Ka) of the SAR sensors. Figure 1 shows the block diagram of the ARC. It consists of an antenna and servo subsystem, a radio frequency (RF) subsystem, a control and data collection subsystem, and a remote-controlled subsystem. The antenna and servo subsystem comprises two similar multi-band antennas used for reception and transmission, and the servo allows the ARC to move mechanically in the elevation and azimuth directions. The RF subsystem includes a wideband RF receiver, a Ka-band receiver, an Remoteoptical Sens. fiber2019 delayer,, 11, 1312 a gain adjustor, a wideband RF amplifier, a Ka-band amplifier, an internal3 of 15 calibration module, and a switched filter module (SFM). It amplifies the signal level that complies 2.2.with The the multi-band required RCS ARC value and selects the work frequency, delay, and so on. The control and data collection subsystem is used to control the whole ARC and collect the SAR signal from the radio frequencyIn the frameworksubsystem. ofThe future remote-controlled SAR ground calibration subsystem campaigns, is in charge three of equalmonitoring, ARCs willcontrolling, be deployed, and covering the four operational bands (L C X Ka) of the SAR sensors. Figure1 shows the block diagram scheduling the ARC. This subsystem \is located\ \ away from the ARC, and its communication method ofis the 4G ARC. communication It consists of an network. antenna In and the servo following subsystem, text, we a radioemphasize frequency the design (RF) subsystem, of the antenna a control and andradio data frequency collection system. subsystem, and a remote-controlled subsystem.

Figure 1.1. The block diagram of the activeactive radar calibrator (ARC).

3. ResultsThe antenna and servo subsystem comprises two similar multi-band antennas used for reception and transmission, and the servo allows the ARC to move mechanically in the elevation and azimuth directions.3.1. Design of The the RF ARC subsystem includes a wideband RF receiver, a Ka-band receiver, an optical fiber delayer, a gain adjustor, a wideband RF amplifier, a Ka-band amplifier, an internal calibration module, and3.1.1. a Antenna switched Design filter module (SFM). It amplifies the signal level that complies with the required RCS valueThe and form selects and the performance work frequency, of the delay, antenna and dete so on.rmine The the control whole and structure data collection and performance subsystem of is usedthe ARC to control to some the wholeextent. ARCConsidering and collect the the operatio SAR signaln at a from multi-band the radio frequency frequency of subsystem. one ARC, Thethe remote-controlledantennas must be subsystemable to work is in at charge a frequency of monitoring, range from controlling, the L-band and schedulingto Ka-band. the It ARC. is almost This subsystemimpossible isto locateddesign an away ultra-wideband from the ARC, antenna and its that communication works from the method L-band is to the the 4G Ka-band communication with the network. In the following text, we emphasize the design of the antenna and radio frequency system.

Remote Sens. 2019, 11, 1312 4 of 15

3. Results

3.1. Design of the ARC

3.1.1. Antenna Design The form and performance of the antenna determine the whole structure and performance of the RemoteARC Sens. to some 2019, 11 extent., x FOR Considering PEER REVIEW the operation at a multi-band frequency of one ARC, the antennas4 of 15 must be able to work at a frequency range from the L-band to Ka-band. It is almost impossible to specifications of gain, a 3 dB beam width, an antenna pattern, and isolation between receiving and design an ultra-wideband antenna that works from the L-band to the Ka-band with the specifications transmitting antenna to satisfy the requirements for the ARC at all four bands. So, a multi-band of gain, a 3 dB beam width, an antenna pattern, and isolation between receiving and transmitting antenna needed to be designed for the ARC. We designed a combined antenna consisting of an antenna to satisfy the requirements for the ARC at all four bands. So, a multi-band antenna needed L/C-band common aperture horn antenna [17,18], an X-band dual-polarimetric corrugated horn to be designed for the ARC. We designed a combined antenna consisting of an L/C-band common antenna, and a Ka-band corrugated horn antenna for the multi-band ARC. A schematic illustration aperture horn antenna [17,18], an X-band dual-polarimetric corrugated horn antenna, and a Ka-band of the combined antenna is shown in Figure 2. The types of polarization used are H-pol for the corrugated horn antenna for the multi-band ARC. A schematic illustration of the combined antenna is L-band, H- and V-pol for the C- and X-bands, and V-pol for the Ka-band. shown in Figure2. The types of polarization used are H-pol for the L-band, H- and V-pol for the C- and X-bands, and V-pol for the Ka-band.

FigureFigure 2. 2. SchematicSchematic illustration illustration of of the the combined combined antenna. antenna.

ThereThere are are many many key key technologies technologies and didifficultiesfficulties inin thethe designdesign of of the the multi-band multi-band antenna antenna such such as asL /L/C-bandC-band common common aperture aperture design, design, C/X-band C/X-band dual-polarimetric dual-polarimetric and highand polarizationhigh polarization isolation isolation design, design,L-band L-band miniaturization miniaturization design, design, and so on.and Theso on. dual-polarimetric The dual-polarimetric and high and polarization high polarization isolation isolationdesign technology design technology has been has introduced been introduced in existing in literatureexisting literature [6,19]. We [6, emphasize 19]. We emphasize the key technology the key technologyof L/C-band of commonL/C-band aperture common design. aperture A design. schematic A schematic illustration illustration of the L/ C-bandof the L/C-band common common aperture apertureantenna antenna is shown is in shown Figure in3. Figure 3.

Figure 3. Schematic illustration of the L/C-band common aperture antenna. OMT: ortho-mode transducer.

Remote Sens. 2019, 11, x FOR PEER REVIEW 4 of 15 specifications of gain, a 3 dB beam width, an antenna pattern, and isolation between receiving and transmitting antenna to satisfy the requirements for the ARC at all four bands. So, a multi-band antenna needed to be designed for the ARC. We designed a combined antenna consisting of an L/C-band common aperture horn antenna [17,18], an X-band dual-polarimetric corrugated horn antenna, and a Ka-band corrugated horn antenna for the multi-band ARC. A schematic illustration of the combined antenna is shown in Figure 2. The types of polarization used are H-pol for the L-band, H- and V-pol for the C- and X-bands, and V-pol for the Ka-band.

Figure 2. Schematic illustration of the combined antenna.

There are many key technologies and difficulties in the design of the multi-band antenna such as L/C-band common aperture design, C/X-band dual-polarimetric and high polarization isolation design, L-band miniaturization design, and so on. The dual-polarimetric and high polarization isolation design technology has been introduced in existing literature [6, 19]. We emphasize the key technology of L/C-band common aperture design. A schematic illustration of the L/C-band common apertureRemote Sens. antenna2019, 11, is 1312 shown in Figure 3. 5 of 15

Figure 3. Schematic illustration of the L/C-band common aperture antenna. OMT: ortho-mode transducer. Figure 3. Schematic illustration of the L/C-band common aperture antenna. OMT: ortho-mode transducer. The L/C-band common aperture antenna consists of a common radiation section, an L-band feed waveguide, a C-band transmission section, and an ortho-mode transducer (OMT). The amplitude and phase distribution of the aperture field will affect the antenna’s radiation performance, so the structure design of the common radiation section is very important. Generally speaking, the feed port of an antenna is located at the transmission section. However, it can be seen from Figure3 that the L-band feed waveguide is placed at the common radiation section, which reduces the length of the antenna significantly. In addition, the demand for isolation between the receiving antenna and transmitting antenna is very high in order to ensure precision in the RCS of the ARC. In general, the isolation should be at least 25 dB better than the gain of the radio frequency circuit [14]. In order to satisfy the demand, on the one hand, the level at 90 degree from the boresight should be designed to be as low as possible, and on the other hand, choke slots were placed on the sidewall of the radome. Based on the measures above, the isolation reaches 79 dB for the L-band, 80 dB for the C-band, 82 dB for the X-band, and 84 dB for the Ka-band, based on measurement in an open space. The isolation is very high and it is hard to measure. First we chose an open space and the Tx and Rx antennas pointed up to the sky in the measurement, and then the IF band of vector network analyzer was set to 100Hz. The gains designed on respective center frequencies are 12 dB for the L-band, 22 dB for the C-band, 23 dB for the X-band and 27 dB for the Ka-band, with a typical sidelobe level of 17 dB for the four − bands, which minimizes multipath effects and background noise.

3.1.2. RF Subsystem Design The RF subsystem is very important, and many functions and performance aspects of ARC are realized based on the RF subsystem. The scheme of the RF subsystem is depicted in Figure1. It consists of a receiver, optical fiber delay, a gain adjustor, an amplifier, an internal calibration module, an SFM, and so on. Based on the demands for the RCS of the ARC, the gains are about 54 dB for the L-band, 47 dB for the C-band, 50 dB for the X-band, and 59 dB for the Ka-band at maximum, and this can be reduced by 15 dB in 5 dB steps. The bandwidth is 200 MHz for the L-band, 240 MHz for the C-band, 1200 MHz for the X-band and 1200 MHz for the Ka-band. The multi-band ARC demands that the RF subsystem should be able to work from the L-band to the Ka-band. Furthermore, specifications such as gain stabilization and gain flatness are very high. RF channel • Remote Sens. 2019, 11, 1312 6 of 15

Based on the value of RCS and the gain of the antenna, it is can be deduced from Equation (1) that the gain of the RF channel should be about 54 dB for the L-band, 47 dB for the C-band, 50 dB for the X-band, and 59 dB for the Ka-band at maximum. As is commonly known, many attenuating devices such as attenuators used for impedance matching, filters used to filter the useless signals and so on are situated in the RF channel, so the total gain of amplifiers will be far higher than the gain of the RF channel. Questions of stabilization and gain flatness will arise subsequently. Technology using a multistage and adjustable equalization filter was utilized to ensure the gain flatness. The cavity filter shown in Figure4 with some tuning rods was used somewhere in the RF channel. The tuning rods were tuned until the gain flatness satisfied the requirement, and then they Remotewere fixed.Sens. 2019, 11, x FOR PEER REVIEW 6 of 15

Figure 4. Example of the cavity filter with some tuning rods. Figure 4. Example of the cavity filter with some tuning rods. Moreover, two SFMs were used to attenuate out-of-band spurious signals. For example, if the ARCMoreover, was used totwo calibrate SFMs were the SAR used operating to attenuate at C-band, out-of-band the SFMs spurious would signals. be switched For example, to the C-band if the ARCfilter. was This used can to attenuate calibrate out-of-band the SAR operating spurious at C- signalsband, andthe SFMs reduce would the thermal be switched noise, to and the thenC-band the filter.signal-to-noise This can attenuate ratio can beout-of-band improved. spurious signals and reduce the thermal noise, and then the signal-to-noise ratio can be improved. Fiber optic delay module • • Fiber optic delay module There many methods that can be used to achieve tunable delay, such as optical fiber delay, bulk acousticThere wave many (BAW), methods surface that acoustic can be waveused to (SAW) achiev delay,e tunable digital delay, delay, such and soas on.optical An opticalfiber delay, fiber bulkdelay acoustic unit was wave chosen (BAW), because surface of itsacoustic advantages wave (SAW) of no frequencydelay, digital translation delay, and being so on. required, An optical low fiberloss, anddelay a wideunit was bandwidth. chosen because However, of theits opticaladvantages fiber of is temperature-sensitive,no frequency translation and being its consequent required, lowamplitude loss, and instability a wide should bandwidth. be paid suHowever,fficient attention. the optical A temperature fiber is temperature-sensitive, control measure was takenand its in consequentthe optical fiberamplitude delay unit, instability and a special should design be paid for the sufficient structure attention. of the optical A fibertemperature was determined. control measureThe was fiber taken optic in delay the optical module fiber can delay move unit the, pointand a target special response design for in thethe finalstructure image of tothe a opticallow-backscattering fiber was determined. background. For example, when the ARC is placed near water surfaces, through fiberThe optic fiber delay, optic the echodelay return module can can be delayedmove the as ifpoin thet ARCtarget was response located in on the the final water image where to the a low-backscatteringbackground noise is background. low. This greatly For example, reduces thewhen demands the ARC for is the placed calibration near water site. Furthermore,surfaces, through fiber fiberoptic optic delay delay, can mean the echo that thereturn point can target be delayed response as in if the the final ARC image was located is not at on the the location water ofwhere the ARC, the backgroundwhich reduces noise the re-radiationis low. This egreatlyffects of reduces the structure the demands itself; therefore, for the acalibration higher signal-to-clutter site. Furthermore, ratio fiberand betteroptic calibrationdelay can mean accuracy that can the be point achieved target [20 response]. Furthermore, in the final distortion image due is not to antennaat the location leakage of is thenon-coherent ARC, which after fiberreduces optic the delay re-radiation and can be effects suppressed of the with structure range compression itself; therefore, [21]. Three a higher optical signal-to-clutterfibers with the length ratio ofand 0.5us, better 0.5us calibration and 1us wereaccuracy used can between be achieved laser and [20]. detector, Furthermore, and the distortion delay was duetuned to throughantenna theleakage optical is switches.non-coherent after fiber optic delay and can be suppressed with range compressionGain calibration [21]. Three circuit optical fibers with the length of 0.5us, 0.5us and 1us were used between laser• and detector, and the delay was tuned through the optical switches. The gain stability is achieved by the gain calibration circuit, as depicted in Figure5. A series • of calibrationGain calibration pulses circuit are fed to the circuit near its input through CP1. These go around the main RF circuit, with a 1 or 2 µs delay, and they are then detected at detector C. The original pulse without The gain stability is achieved by the gain calibration circuit, as depicted in Figure 5. A series of calibration pulses are fed to the circuit near its input through CP1. These go around the main RF circuit, with a 1 or 2 μs delay, and they are then detected at detector C. The original pulse without the delay is also detected at the same detector in the gain calibration module. The amplitudes of the delayed and direct pulses are compared, and any error nulled using the voltage control attenuator in the main RF circuit. The RF unit gain is therefore stabilized. Gain calibration module, the couplers CP1 and CP2 are housed in a weatherproof enclosure, which is maintained at 55 ± 2 °C. Thus, the gain stability is independent of the surrounding temperature. In this way, the gain of the ARC is tied to the gain of the calibration module, which only includes highly-stable passive components and is housed in a constant-temperature box. So, the gain stability can be rather good.

Remote Sens. 2019, 11, 1312 7 of 15 the delay is also detected at the same detector in the gain calibration module. The amplitudes of the delayed and direct pulses are compared, and any error nulled using the voltage control attenuator in the main RF circuit. The RF unit gain is therefore stabilized. Gain calibration module, the couplers CP1 and CP2 are housed in a weatherproof enclosure, which is maintained at 55 2 C. Thus, the gain ± ◦ stability is independent of the surrounding temperature. In this way, the gain of the ARC is tied to the gain of the calibration module, which only includes highly-stable passive components and is housed Remotein a constant-temperature Sens. 2019, 11, x FOR PEER box. REVIEW So, the gain stability can be rather good. 7 of 15

FigureFigure 5. 5. GainGain calibration calibration circuit circuit scheme. scheme.

3.1.3.3.1.3. Design Design Specifications Specifications BasedBased on on the the design design above, above, the the multi-band multi-band ARC ARC can can hold hold for for the the following following specifications specifications shown shown inin Table Table 1.1. Table 1. Specifications of the multi-band ARC. Table 1. Specifications of the multi-band ARC. Parameter Value Parameter Value Frequency L/C/X/Ka-bands FrequencyBand 200 MHz (L-band), 240L/C/X/Ka-bands MHz (C-band), 1200 MHz (X/Ka-bands) Radar cross section (RCS) 35~55 dBsm BandRange delay 200 MHz (L-band), 240 MHz 0.5 µ(C-band),s, 1 µs, 2 µs, 1200 MHz (X/Ka-bands) Radar cross sectionRCS step (RCS) 35~55 5 dBsm dB Polarization Isolation More than 40 dB RangeRCS delay stability 0.2 dB (L 0.5/C /X-bands),μs, 1 μs, 0.32 μ dBs, (Ka-band) Gain flatness 1 dBp_p (L/C/X-band), 3 dBp_p (Ka-band) RCS step L: approx. 15.9 dBm 5 dB/m 2 to 69.9 dBm/m2 − − C: approx. 3 dBm/m2 to 68 dBm/m2 Dynamic range − − Polarization Isolation X: approx. More 1.5 dBm than/m 240to dB58.5 dBm/m2 − Ka: approx. 9.25 dBm/m2 to 43.25 dBm/m2 RCS stability 0.2 dB (L/C/X-bands),− 0.3 dB− (Ka-band) Polarization H-pol (L-band), H and V-pol (C/X-bands), V-pol (Ka-band) AntennaGain flatness pointing precision 1 dBp_p (L/C/X-band), Better than 3 dBp_p 0.2◦ (Ka-band) L: approx.–15.9dBm/m2 to –69.9 dBm/m2 Except for the fairly good specifications,C: approx. the multi-band –3 dBm/m ARC2 to –68 is also dBm/m designed2 for several operationalDynamic modes range such as the transponder mode, receiver mode, standby mode, self-calibration mode, X: approx. 1.5 dBm/m2 to –58.5 dBm/m2 and RCS calibration mode. In the transponder mode, the active RCSKa: approx. setting is –9.25 supported dBm/m by2 to a –43.25 5 dB step dBm/m attenuator,2 and the accuracy ofPolarization RCS is monitored and guaranteed H-pol (L-band), by the H gain and calibrationV-pol (C/X-bands), circuit. Moreover, V-pol (Ka-band) tunable delay is possibleAntenna inpointing the range, precision which means that the point target Better response than 0.2° in the final image is not at the location of the ARC, which reduces the re-radiation effects of the structure of itself. Except for the fairly good specifications, the multi-band ARC is also designed for several In receiver mode, the azimuth pattern of the SAR can be recorded by one ARC, and the elevation operational modes such as the transponder mode, receiver mode, standby mode, self-calibration pattern of the SAR can also be acquired if some ARCs are placed in dedicated areas. The characteristics mode, and RCS calibration mode. of the pulse transmitted by the SAR such as the amplitude flatness, the orthogonality, and the imbalance In the transponder mode, the active RCS setting is supported by a 5 dB step attenuator, and the between the I and Q signals, the pulse width, and so on can also be measured and monitored in accuracy of RCS is monitored and guaranteed by the gain calibration circuit. Moreover, tunable receiver mode. delay is possible in the range, which means that the point target response in the final image is not at the location of the ARC, which reduces the re-radiation effects of the structure of itself. In receiver mode, the azimuth pattern of the SAR can be recorded by one ARC, and the elevation pattern of the SAR can also be acquired if some ARCs are placed in dedicated areas. The characteristics of the pulse transmitted by the SAR such as the amplitude flatness, the orthogonality, and the imbalance between the I and Q signals, the pulse width, and so on can also be measured and monitored in receiver mode. In standby mode, the control unit waits for the satellite to pass by. The control and data collection subsystem will be booted up at a pre-programmed wake-up time to power the whole ARC and set the proper operational mode.In self-calibration mode, the ARC will check the status itself

Remote Sens. 2019, 11, 1312 8 of 15

In standby mode, the control unit waits for the satellite to pass by. The control and data collection subsystem will be booted up at a pre-programmed wake-up time to power the whole ARC and set the proper operational mode.In self-calibration mode, the ARC will check the status itself and record the statusRemote information. Sens. 2019, 11,Also, x FOR PEER thegain REVIEW will be calibrated to a standard to ensure the accuracy of8 of RCS 15 and Remote Sens. 2019, 11, x FOR PEER REVIEW 8 of 15 the stability of the receiver. and record the status information. Also, the gain will be calibrated to a standard to ensure the In RCS calibration mode, an outside test range and a metal plate with a calculated radar cross accuracyand record of RCSthe statusand the information. stability of the Also, receiver. the gain will be calibrated to a standard to ensure the sectionaccuracy areIn used.RCS of RCScalibration A calibrationand the mode, stability pulse an ofoutside isthe transmitted receiver. test range fromand a the metal ARC, plate reflected with a calculated by the plate, radar received cross by the ARC,sectionIn and areRCS retransmittedused. calibration A calibration mode, after pulsean a 2outsideµ iss transmitted delay test untilrange fr the omand pulsethe a metal ARC, is attenuatedplate reflected with by a to calculatedthe a plate, small received enoughradar cross by size. A seriesthesection of ARC, decaying are and used. retransmitted pulses A calibration are monitored after pulse a 2 μis ands transmitted delay recorded until thefrom within pulse the ARC, theis attenuated RF reflected loop fromto by a thesmall which plate, enough the received absolute size. byA RCS of theseriesthe ARC ARC, of can decaying and be retransmitted extracted. pulses are [22 after monitored,23] a 2 μs delayand record until theed withinpulse is the attenuated RF loop fromto a small which enough the absolute size. A RCSseries of of the decaying ARC can pulses be extracted. are monitored [22,23] and recorded within the RF loop from which the absolute 3.2. ManufactureRCS of the ARC and can Integration be extracted. of the [22,23] ARC 3.2. Manufacture and Integration of the ARC Using3.2. Manufacture the aforementioned and Integration design of the methodsARC and specifications, we manufactured three multi-band ARCs to calibrateUsing the the aforementioned Gapfen-3 SAR, design Shenzhen-1 methods SAR, and Luojia-2 specifications, SAR, and we so manufactured on. Figure6 isthree an outer multi-bandUsing ARCsthe aforementioned to calibrate the Gapfen-3 design SAR,methods Shenzhen-1 and specifications, SAR, Luojia-2 weSAR, manufactured and so on. Figure three 6 view of the ARC. We can see from Figure6 that the receiving and transmitting antennas, the radio ismulti-band an outer view ARCs of to the calibrate ARC. We the can Gapfen-3 see from SAR, Figu Shreenzhen-1 6 that the SAR, receiving Luojia-2 and SAR, transmitting and so on. antennas, Figure 6 frequency box (RF box), and the digital control box (DC box) were installed on the same flatform; theis an radio outer frequency view of the box ARC. (RF box),We can and see the from digital Figu corentrol 6 that box the (DC receiving box) were and transmittinginstalled on antennas,the same therefore,flatform;the radio there therefore, frequency is no relative there box (RFis motion no box), relative among and motionthe the digital antenna, among control the RF boxantenna, box, (DC and box) RF DC box, were box and wheninstalled DC the box on ARC when the works.same the So, the errorARCflatform; dueworks. totherefore, cablesSo, the connectederrorthere dueis no to to relativecables the antenna,connected motion among RF to box,the theantenna, and antenna, DC RF box box,RF is box, and decreased andDC boxDC a isbox lot. decreased when Furthermore, the a 4G antennaslot.ARC Furthermore, works. for remoteSo, the 4G error communicationantennas due to for cables remote andconnected communication a GPS to/Beidou the antenna, and antenna a GPS/BeidouRF box, for acquiringand antennaDC box time isfor decreased wereacquiring installed a on thetimelot. ARC. Furthermore, were installed 4G on antennas the ARC. for remote communication and a GPS/Beidou antenna for acquiring time were installed on the ARC.

FigureFigure 6. 6.Outer Outer view of of the the multi-band multi-band ARC. ARC. Figure 6. Outer view of the multi-band ARC. FigureFigure7 shows 7 shows an an outer outer view view ofof the manufactured manufactured multi-band multi-band antenna, antenna, and Figure and Figure8 shows8 theshows Figure 7 shows an outer view of the manufactured multi-band antenna, and Figure 8 shows the the patternspatterns for the four bands bands measured measured in inthe the anechoic anechoic chamber chamber of Science of Science and andTechnology Technology on on patterns for the four bands measured in the anechoic chamber of Science and Technology on AntennaAntenna and and Microwave Microwave Laboratory, Laboratory, Xidian Xidian UniversityUniversity of of China. China. We We also also show show the thecross-polarized cross-polarized patternAntenna for and comparison. Microwave We Laboratory, can see that Xidian the cross-poUniversitylarization of China. level We is also more show than the 30 cross-polarizeddB lower than pattern for comparison. We can see that the cross-polarization level is more than 30 dB lower than the thepattern co-polarization for comparison. level forWe the can L-band see that and the Ka-band cross-po andlarization 40 dB levellower is than more the than co-polarization 30 dB lower levelthan co-polarization level for the L-band and Ka-band and 40 dB lower than the co-polarization level for the forthe the co-polarization C-band and X-band.level for the L-band and Ka-band and 40 dB lower than the co-polarization level C-bandfor andthe C-band X-band. and X-band.

Figure 7. Outer view of the multi-band antenna. FigureFigure 7. 7.Outer Outer view view of the multi-band multi-band antenna. antenna.

Remote Sens. 2019, 11, 1312 9 of 15 Remote Sens. 2019, 11, x FOR PEER REVIEW 9 of 15

Remote Sens. 2019, 11, x FOR PEER REVIEW 9 of 15

(a) (b)

(a) (b)

(c) (d) (c) (d) Figure 8.8. Examples of measured patterns: (a) L-band; (b) C-band; (c) X-band; ((d)d) Ka-band. Figure 8. Examples of measured patterns: (a) L-band; (b) C-band; (c) X-band; (d) Ka-band. The integration of ARC was performed after all the subsystems had been fulfilled.fulfilled. Actually, the The integration of ARC was performed after all the subsystems had been fulfilled. Actually, the main step in integration isis thethe integrationintegration ofof thethe RFRF boxbox andand DCDC box.box. Figure9 9 shows shows anan outerouter viewview ofof main step in integration is the integration of the RF box and DC box. Figure 9 shows an outer view of thethe integrationintegrationthe integration of thethe of the RFRF RF boxbox box andand and DCDC DC boxbox box onon on thethethe table.table. During During the the integration integration of the of theRF box RF boxandbox DC andand DCDC box, thebox, standard the standard of the of the receiver receiver gaingain gain andand and transpondertranspon transponderder gain gaingain were werewere decided. decided.decided. After After the theintegration integration of RF of RF box andandbox DC andDC box, DCbox, thebox, the RF the RF box RF box and box DCand and boxDC DC were boxbox installedwere installedinstalled on the on ARC,on the the andARC, ARC, the andRCS and the absolute theRCS RCS absolute calibration absolute ofcalibration ARCcalibration was of done. ARC of ARC was was done. done.

Figure 9. Outer view of the integration of the radio frequency (RF) box and digital control (DC) box Figure 9. Outer view of the integration of the radio frequency (RF) box and digital control (DC) box on the table.on the table. Figure 9. Outer view of the integration of the radio frequency (RF) box and digital control (DC) box on the table.

Remote Sens. 2019, 11, 1312 10 of 15 Remote Sens. 2019, 11, x FOR PEER REVIEW 10 of 15 Remote Sens. 2019, 11, x FOR PEER REVIEW 10 of 15 Figure 10 is an outer view of the RF box (RF subsystem) for the multi-band ARC. We can see Figure 10 is an outer view of the RF box (RF(RF subsystem) for the multi-bandmulti-band ARC. We cancan seesee from Figure 10 that the RF subsystem is very complicated and that a modular design was used. The from FigureFigure 1010 thatthat thethe RFRF subsystemsubsystem isis veryvery complicatedcomplicated andand thatthat aa modularmodular designdesign waswas used.used. The maximum gain is about 67 dB for the L-band, 55 dB for the C-band, 60 dB for the X-band, and 64 dB maximum gaingain isis aboutabout 67 67 dB dB for for the the L-band, L-band, 55 55 dB dB for for the the C-band, C-band, 60 60 dB dB for for the the X-band, X-band, and and 64 dB64 fordB for the Ka-band and these can be reduced by 15 dB in 5 dB steps. The gain can also be reduced by 20 thefor the Ka-band Ka-band and and these these can can be reducedbe reduced by 15by dB 15 indB 5 in dB 5 steps.dB steps. The The gain gain can can also also be reducedbe reduced by 20by dB20 dB using a voltage-tuned attenuator. usingdB using a voltage-tuned a voltage-tuned attenuator. attenuator.

(a) (b) (a) (b) Figure 10. Outer view of the RF box: (a) one side; (b) the other side. Figure 10. Outer view of the RF box: ( a) one side; ( b) the other side. 3.3. Measurement of the ARC 3.3. Measurement of the ARC A set of lab measurements were performed in order to characterize the multi-band ARC at its A set of lablab measurementsmeasurements were performedperformed in order toto characterizecharacterize the multi-band ARC at its operational frequencies. The following measurements were made. operational frequencies. The following measurements were made. 3.3.1. Gain Stability 3.3.1. Gain Stability Based on the gain calibration method mentioned in the previous section, the gain stability is Based onon thethe gain gain calibration calibration method method mentioned mentioned in thein the previous previous section, section, the gainthe gain stability stability is very is very good. We measured the gain of the multi-band ARC for about two weeks from 28 November to good.very good. We measured We measured the gain the gain of the of multi-bandthe multi-ba ARCnd ARC for aboutfor about two two weeks weeks from from 28 November28 November to 14to 14 December 2018. During these two weeks, we measured the gain several times every day using a December14 December 2018. 2018. During During these these two two weeks, weeks, we measured we measured the gain the severalgain several times times every every day using day ausing vector a vector network analyzer. Every measurement was made after the gain had been calibrated using our networkvector network analyzer. analyzer. Every Every measurement measurement was made was made after theafter gain the had gain been had calibratedbeen calibrated using using our gain our gain calibration circuit. Figure 11 shows the gain during the two weeks. It can be seen from Figure 11 calibrationgain calibration circuit. circuit. Figure Figure 11 shows 11 shows the gain the gain during du thering two the weeks.two weeks. It can It becan seen be seen from from Figure Figure 11 that 11 that the gain stability is less than ± 0.2 dB for the L-band, ± 0.22 dB for the C-band, ± 0.21 dB for the thethat gain the gain stability stability is less is than less than0.2 dB± 0.2 for dB the for L-band, the L-band,0.22 dB± 0.22 for thedB for C-band, the C-band,0.21 dB ± for0.21 the dB X-band, for the X-band, and about ± 0.25 ±dB for the Ka-band. The± gain is very stable ±over a long time, thus andX-band, about and0.25 about dB for± 0.25 the Ka-band.dB for the The Ka-band. gain is very The stable gain overis very a long stable time, over thus a guaranteeinglong time, thus the guaranteeing± the RCS stability of the ARC. RCSguaranteeing stability ofthe the RCS ARC. stability of the ARC.

54.7 47.4 Max : 54.58dB 54.7 Max : 47.32dB Min : 54.19dB 47.4 Max△ : : 54.58dB 0.39dB Min : 46.89dB 54.6 Min : 54.19dB 47.3 Max△ : : 47.32dB 0.43dB △ : 0.39dB Min : 46.89dB 54.6 47.3 △ : 0.43dB 54.5 47.2

54.5 47.2

54.4 47.1 Gain(dB) 54.4 47.1Gain(dB) Gain(dB) 54.3 Gain(dB) 47

54.3 47

54.2 46.9

54.2 46.9

46.8 11.28 11.29 11.3012.03 12.04 12.05 12.0612.07 12.10 12.11 12.12 12.13 12.14 11.2611.27 11.28 11.29 11.30 12.03 12.04 12.05 12.06 12.07 12.10 12.11 12.12 12.13 12.14 Data/2018 Data/2018 46.8 11.28 11.29 11.3012.03 12.04 12.05 12.0612.07 12.10 12.11 12.12 12.13 12.14 11.2611.27 11.28 11.29 11.30 12.03 12.04 12.05 12.06 12.07 12.10 12.11 12.12 12.13 12.14 Data/2018 Data/2018 (a) (b) (a) (b) Figure 11. Cont.

RemoteRemote Sens. Sens.2019 2019, ,11 11,, 1312 x FOR PEER REVIEW 1111 ofof 1515 Remote Sens. 2019, 11, x FOR PEER REVIEW 11 of 15

X波波波波波波RCS=55dBsm时时时时时时时 50.9 60 Max : 50.78dB X波波波波波波RCS=55dBsm时时时时时时时 Max : 59.90dB 50.9 Min : 50.37dB Min : 59.39dB △ : 0.41dB 60 △ : 0.51dB 50.8 Max : 50.78dB 59.9 Max : 59.90dB Min : 50.37dB Min : 59.39dB △ : 0.41dB △ : 0.51dB 50.8 59.9 59.8 50.7

59.8 50.7 59.7 dB)

（ 50.6 59.7 dB) Gain Gain(dB) 59.6

（ 50.6

50.5 Gain Gain(dB) 59.6 59.5 50.5

50.4 59.5 59.4

50.4 59.4 50.3 59.3 11.28 11.29 11.30 12.03 12.04 12.05 12.06 12.07 12.10 12.11 12.12 12.13 12.14 11.28 11.29 12.03 12.04 12.05 12.06 12.07 12.10 12.11 12.12 12.13 12.14 Data/2018 Data/2018 50.3 59.3 11.28 11.29 11.30 12.03 12.04 12.05 12.06 12.07 12.10 12.11 12.12 12.13 12.14 11.28 11.29 12.03 12.04 12.05 12.06 12.07 12.10 12.11 12.12 12.13 12.14 Data/2018 Data/2018 (c) (d) (c) (d) Figure 11. Gain during the two week period: (a) L-band; (b) C-band; (c) X-band; (d) Ka-band. FigureFigure 11.11. GainGain duringduring thethe twotwo weekweek period:period: (a)) L-band;L-band; ((b)) C-band;C-band; ((cc)) X-band;X-band; ((dd)) Ka-band.Ka-band. 3.3.2. System Transfer Function 3.3.2.3.3.2. System Transfer FunctionFunction To evaluate the system transfer function, a measurement system consisting of a signal generator To evaluate the system transfer function, a measurement system consisting of a signal generator and aTo spectrum evaluate analyzer the system was transfer set up function,[12]. The atest meas signalurement was sent system to the consisting ARC by of the a signalsignal generatorgenerator and a spectrum analyzer was set up [12]. The test signal was sent to the ARC by the signal generator andand athen spectrum to the analyzerspectrum was analyzer. set up The[12]. amplitude The test signal of the was signal sent spanned to the ARC a range by the from signal –60 generator dBm to – and then to the spectrum analyzer. The amplitude of the signal spanned a range from 60 dBm to and27 dBm then for to thethe L-band,spectrum –60 analyzer. dBm to –17The dBm amplitude for the ofC-band, the signal –60 dBmspanned to –16 a range dBm forfrom the –60 −X-band, dBm toand – 27 dBm for the L-band, 60 dBm to 17 dBm for the C-band, 60 dBm to 16 dBm for the X-band, −27–70 dBm dBm for to the –34 L-band, dBm –60for− dBmthe Ka-band, to –17− dBm according for the C-band, to various –60− dBmparameters to –16 −dBm including for the theX-band, effective and and 70 dBm to 34 dBm for the Ka-band, according to various parameters including the effective –70isotropic −dBm radiatedto –34− dBm power for (EIRP), the Ka-band, the incidence according angle, to and various the orbit parameters of the spaceborne including SAR. the Figureeffective 12 isotropic radiated power (EIRP), the incidence angle, and the orbit of the spaceborne SAR. Figure 12 isotropicgives the radiatedtransfer powerfunctions (EIRP), for the the four incidence bands. angle, From andFigure the 12, orbit we of calculated the spaceborne the linearity, SAR. Figure which 12 is gives the transfer functions for the four bands. From Figure 12, we calculated the linearity, which is givesbetter the than transfer 0.1 dB functions for the L-band, for the 0.15four dBbands. for the From C-band, Figure 0.2 12, dB we for calculated the X-band, the linearity,and 0.5 dB which for the is better than 0.1 dB for the L-band, 0.15 dB for the C-band, 0.2 dB for the X-band, and 0.5 dB for the betterKa-band, than and 0.1 the dB linearityfor the L-band, of the ARC 0.15 isdB rather for the good. C-band, 0.2 dB for the X-band, and 0.5 dB for the Ka-band,Ka-band, andand thethe linearitylinearity ofof thethe ARCARC isis ratherrather good.good.

35 40 35 40 30 30 30 25 30 25 20 20 20 20 15 15 10 10 10 10 Output Power(dBm) 5 Power(dBm) Output 0 Output Power(dBm) 5 Power(dBm) Output 0 0 0 -10 -5 -10 -5 -10 -20 -65 -60 -55 -50 -45 -40 -35 -30 -25 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 Input Power(dBm) -20 Input Power(dBm) -65 -60 -55 -50 -45 -40 -35 -30 -25 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 Input Power(dBm) Input Power(dBm) (a) (b) (a) (b)

40 30 40 30 25 30 25 30 20 20 20 15 20 15 10 10 10 10 5 5 Output Power(dBm) Output Power(dBm) 0 0 Output Power(dBm) Output Power(dBm) 0 0 -5 -10 -5 -10 -10 -10 -20 -15 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -70 -65 -60 -55 -50 -45 -40 -35 -30 -20 Input Power(dBm) -15 Input Power(dBm) -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -70 -65 -60 -55 -50 -45 -40 -35 -30 Input Power(dBm) Input Power(dBm) (c) (d) (c) (d) FigureFigure 12.12. TransferTransfer function:function: ((aa)) L-band;L-band; ((bb)) C-band;C-band; ((cc)) X-band;X-band; ((dd)) Ka-band.Ka-band. Figure 12. Transfer function: (a) L-band; (b) C-band; (c) X-band; (d) Ka-band.

Remote Sens. 2019, 11, 1312 12 of 15 Remote Sens. 2019, 11, x FOR PEER REVIEW 12 of 15

3.3.3.3.3.3. Gain Gain over over Frequency Frequency Bandwidth Bandwidth ThisThis measurement measurement allowed allowed us us to to evaluate evaluate the the working working frequency frequency bandwidth bandwidth of of ARC ARC and and the the relativerelative gain gain flatness. flatness. The The inferior inferior gain gain flatness flatness influences influences the the target target image image quality quality of of the the ARC. ARC. Many Many measuresmeasures were were taken taken to to improve improve the the specifications specifications of of the the ARC. ARC. In In Figure Figure 13 13,, the the gain gain flatness flatness for for the the fourfour bands bands is shown.is shown. It canIt can be seenbe seen from from Figure Figure 13 that 13 the that manufactured the manufactured ARC has ARC gain has flatness gain valuesflatness ofvalues0.5 dBof (L-band),± 0.5 dB (L-band),0.2 dB (C-band),± 0.2 dB (C-band),0.5 dB (X-band), ± 0.5 dB and(X-band),1.5 dBand (Ka-band) ± 1.5 dB over(Ka-band) the available over the ± ± ± ± bandwidthavailable bandwidth of 200 MHz of for 200 the MHz L-band, for the 240 L-band, MHz for 240 the MHz C-band, for the 1200 C-band, MHz 1200 for the MHz X-band, for the and X-band, 1200 MHzand 1200 for the MHz Ka-band, for the respectively. Ka-band, respectively.

58 50

57 49.8

56 49.6

55 49.4

54 49.2

53 49

Gain (dB) 52 48.8

51 48.6

50 48.4

49 48.2

48 48 1.16 1.18 1.2 1.22 1.24 1.26 1.28 1.3 1.32 1.34 1.36 5.3 5.35 5.4 5.45 5.5 Frequency (GHz) Frequency (GHz) (a) (b)

53.5

53

52.5

52

51.5

51

50.5 Gain (dB)

50

49.5

49

48.5 9 9.2 9.4 9.6 9.8 10 10.2 Frequency (GHz) (c) (d)

FigureFigure 13. 13.The The gain gain flatness: flatness: ( a()a) L-band; L-band; ( b(b)) C-band; C-band; ( c()c) X-band; X-band; (d (d) Ka-band.) Ka-band.

3.3.4.3.3.4. Linearity Linearity of of the the ARC ARC Receiver Receiver LinearityLinearity is is one one the the key key features features of of the the ARC ARC receiver receiver [24 [24].]. After After manufacturing manufacturing the the ARC, ARC, the the linearitylinearity was was measured measured using using the the signal signal generator. generator. The The signal signal was was input input into into the the ARC, ARC, and and the the ARC ARC receiverreceiver collected collected the the signal. signal. The The linearity linearity errors errors for for the the full full dynamic dynamic range range were were acquired acquired from from the the collectedcollected data, data, and and they they are are plotted plotted in in Figure Figure 14 14.. It It can can be be seen seen from from Figure Figure 14 14 that that the the linearity linearity errors errors areare mostly mostly below below 0.2 0.2 dB, dB, which which suggests suggests very very precise precise antenna antenna pattern pattern recognition, recognition, as as the the pattern pattern is is mathematicallymathematically derived derived from from the the received received power. power.

Remote Sens. 11 Remote Sens.2019 2019, , 11, 1312, x FOR PEER REVIEW 1313 of of 15 15

0.4 0.4

0.3 0.3

0.2 0.2

0.1 0.1

0 0

-0.1 -0.1

Linearity error (dB) error Linearity -0.2 (dB) error Linearity -0.2

-0.3 -0.3

-0.4 -0.4 -80 -70 -60 -50 -40 -30 -20 -80 -70 -60 -50 -40 -30 -20 -10 Input Power (dBm) Input power (dBm) (a) (b)

0.4 0.4

0.3 0.3

0.2 0.2

0.1 0.1

0 0

-0.1 -0.1

Linearity error (dB) error Linearity -0.2 (dB) error Linearity -0.2

-0.3 -0.3

-0.4 -0.4 -70 -60 -50 -40 -30 -20 -10 -70 -60 -50 -40 -30 Input power (dBm) Input power (dBm)

(c) (d)

FigureFigure 14. 14.Linearity Linearity error error for for the the full full dynamic dynamic range: range: (a ()a L-band;) L-band; (b ()b C-band;) C-band; (c ()c X-band;) X-band; (d ()d Ka-band.) Ka-band.

4.4. Discussion Discussion WeWe developed developed a a multi-band multi-band ARC ARC that that can can calibrate calibrate L-, L-, C-, C-, X- X- and and Ka-band Ka-band spaceborne spaceborne SARs, SARs, evaluateevaluate the the image image quality, quality, and and verify verify the the calibration calibration results. results. Although Although the the multi-band multi-band ARC ARC has has manymany advantages, advantages, the the system, system, especially especially the the RF RF subsystem subsystem and and antenna, antenna, is is very very complex, complex, and and it it is is ratherrather expensive expensive compared compared with with a a single-band single-band ARC. ARC. Moreover, Moreover, any any piece piece of of equipment equipment has has a a given given lifetime.lifetime. Therefore, Therefore, if if the the interval interval between between the the launch launch of of various various band band spaceborne spaceborne SARs SARs is is too too long, long, wewe should should consider consider using using a a single-band single-band ARC ARC for for each each SAR SAR for for the the sake sake of of cost cost and and simplification. simplification. Of Of course,course, if if there there are are already already various various band band SARs SARs in in space, space, the the use use of of the the multi-band multi-band ARC ARC will will be be very very meaningfulmeaningful because because on on the the one one hand, hand, the the cost cost of of a a multi-band multi-band ARC ARC will will be be much much lower lower than than the the total total costcost of of multiple multiple single-band single-band ARCs, ARCs, and and on on the the other other hand, hand, the the number number of ARC of ARC will will be reduced be reduced a lot a by lot usingby using the multi-band the multi-band ARC ARC compared compared with with the single-band the single-band ARC ARC for each for each SAR, SAR, which which will reducewill reduce the workloadthe workload of the of calibration the calibration campaign. campaign. InIn this this paper, paper, we we presented presented the the results results of of the the multi-band multi-band ARC. ARC. We We found found that that the the gain gain stability stability andand the the gain gain flatness flatness of of Ka-band Ka-band were were slightly slightly worse wors thane than the the other other three three bands, bands, which which is attributed is attributed to theto verythe highvery frequencyhigh frequency of the Ka-band,of the Ka-band, and the measurement and the measurement error, which error, will be which large. will Furthermore, be large. thisFurthermore, paper only includesthis paper measurements only includes conducted measur inements the laboratory, conducted and in the the experimental laboratory, results and forthe in-orbitexperimental calibration results will for be in-orbit given after calibration the satellite will isbe launched given after in ourthe satellite future work. is launched in our future work. 5. Conclusions 5. ConclusionsThis paper describes the principles, design, manufacture, and measurement results of a multi-band ARC workingThis paper in the describes L-, C-, X- the and principles, Ka-bands. Multi-banddesign, manufacture, and multi-functional and measurement antennas, forresults example, of a multi-band ARC working in the L-, C-, X- and Ka-bands. Multi-band and multi-functional antennas,

Remote Sens. 2019, 11, 1312 14 of 15 the L/C-band common aperture and the dual-polarized C-band, were used in the ARC for the first time. A multi-band gain calibration circuit with a thermal stability of better than 0.1 dB over the temperature range of 20 to 50 C was designed to ensure a gain stability of about 0.2 dB for a significant period − − ◦ of time for all four operational frequency bands. We manufactured three multi-band ARCs, and the gain stability, system transfer function, gain flatness, and linearity of ARC receiver were measured in the laboratory in order to characterize the multi-band ARC at its operational frequency bands. The results agreed well with the theoretical predictions and proved that the present multi-band ARC has excellent performance. For example, the gain stability is better than 0.2 dB for the L-band, 0.22 dB for the C-band, 0.21 dB for the X-band, ± ± ± and about 0.25 dB for the Ka-band for a significant period of time—about two weeks. These three ± multi-band ARCs will be used as the prime calibration standards during a validation and calibration campaign of the Shenzhen-1 SAR, Luojia-2 SAR, and other SARs after the launch of the satellites.

Author Contributions: L.L. and J.H. conceived and designed the experiments; L.L. and G.L. performed the experiments; L.L., F.M. and G.L. analyzed the data; L.L., F.M. and Y.W. contributed reagents/materials/analysis tools; L.L.wrote the paper. Acknowledgments: This research was funded by the National Natural Science Foundation of China grant number 61571417; The authors thank the anonymous reviewers for their constructive comments and suggestions. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Touzi, R.; Hawkins, R.K.; Côté, S. High-precision assessment and calibration of polarimetric radarsat-2 SAR using transponder measurements. IEEE Trans. Geosci. Remote Sens. 2013, 51, 487–503. [CrossRef] 2. Lenz, R.; Pontes, J.; Wiesbeck, W. A high accuracy calibration and receive instrument for TerraSAR-X ground calibration. In Proceedings of the European Radar Conference, Paris, France, 3–4 October 2005; pp. 427–430. 3. Paul, S.; Evert, A.; Bjoern, R.; Nicolas, F.; Malcolm, D.; Berthyl, D. Transponder development for Sentinel-1. In Proceedings of the 8th European Conference on Synthetic Aperture Radar, Aachen, Germany, 7–10 June 2010; pp. 354–357. 4. Matthias, J.; Björn, D.; Daniel, R.; Sebastian, R.; Marco, S. Development of the highly accurate DLR Kalibri transponder. In Proceedings of the 10th European Conference on Synthetic Aperture Radar, Berlin, Germany, 3–5 June 2014; pp. 1176–1179. 5. Ming, F.; Hong, J.; Li, L. The external calibration system of GF-3 satellite. In Proceedings of the 12th European Conference on Synthetic Aperture Radar, Aachen, Germany, 4–6 June 2018; pp. 417–420. 6. Li, L.; Zhu, Y.T.; Hong, J.; Ming, F.; Wang, Y. Design and implementation of a novel polarimetric active radar calibrator for Gaofen-3 SAR. Sensors 2018, 18, 2620. [CrossRef][PubMed] 7. Liang, W.B.; Jia, Z.Z.; Qiu, X.; Hong, J.; Zhang, Q.; Lei, B.; Zhang, F.; Deng, Z.; Wang, A. Polarimetric calibration of the GaoFen-3 mission using active radar calibrators and the applicable conditions of system model for radar polarimeters. Remote Sens. 2019, 11, 176. [CrossRef] 8. Woode, A.D.; Desnos, Y.L.; Jackson, H. The development and first results from the ESTEC ERS-1 active radar calibration unit. IEEE Trans. Geosci. Remote Sens. 1992, 30, 1122–1130. [CrossRef] 9. Fujita, M. Development of a retrodirective PARC for ALOS/PALSAR calibration. IEEE Trans. Geosci. Remote Sens. 2003, 4, 2177–2186. [CrossRef] 10. Li, L.; Hong, J.; Ming, F. Study on digital coded technology in active radar calibrator of SAR. In Proceedings of the International Geoscience and Remote Sensing Symposium, Munich, Germany, 22–27 July 2012; pp. 4509–4512. 11. Freeman, A.; Shen, Y.; Werner, C.L. Polarimetric SAR calibration experiment using active radar calibrators. IEEE Trans. Geosci. Remote Sens. 1990, 28, 224–240. [CrossRef] 12. Schena, V.D.; Posa, F.; Ponte, S.; De Carolis, G. Development and performance validation of L-, C- and X-band active radar calibrators (ARC) by means of laboratory tests and SIR-C/X-SAR experiments. In Proceedings of the International Geoscience and Remote Sensing Symposium, Firenze, Italy, 10–14 July 1995; pp. 80–82. Remote Sens. 2019, 11, 1312 15 of 15

13. Sarabandi, K.; Kashanianfard, M.; Nashashibi, A.Y.; Pierce, L.E.; Hampton, R. A polarimetric active transponder with extremely large RCS for absolute radiometric calibration of SMAP radar. IEEE Trans. Geosci. Remote Sens. 2018, 56, 1269–1277. [CrossRef] 14. Massimiliano, P.; Lapo, M. Cross-pol transponder with frequency shifter for bistatic ground-based synthetic aperture radar. Remote Sens. 2018, 10, 1364. 15. Zhang, X.X. Unified jamming equation of synthetic aperture radar. J. China Acad. Electron. Inf. Technol. 2006, 1, 107–113. 16. Skolnik, M. Introduction to Radar Systems; McGraw-Hill Education: New York, NY, USA, 2002. 17. Zhang, Q.; Yuan, C.W.; Liu, L. Radiation characteristics of L/C dual-band coaxial horn feed for high power . High Power Part. Beam. 2011, 1, 146–150. (In Chinese) [CrossRef] 18. Lin, W.Q.; Zhang, Z.Y.; Fu, G. Design of a high gain and low cross-polarization tri-band horn antenna. In Proceedings of the International Conference Microwave and Millimeter Wave Technology (ICMMT), Chengdu, China, 6–10 May 2018; pp. 1–3. 19. Li, L.; Hong, J.; Wang, Y.; Yi, M.; Zhu, Y.; Ming, F. In orbit calibration of GF-3 SAR using quad-polarized transponder. Acta Electron. Scin. 2017, 46, 2157–2164. (In Chinese) 20. Park, D.J.; Ahn, S.I.; Chun, Y.S.; Shin, J.M.; Yoon, J.C.; Kim, J.H. Development of active transponder for KOMPSAT-5 mission. In Proceedings of the 3rd International Asia-Pacific Conference on Synthetic Aperture Radar (APSAR), Seoul, Korea, 26–30 September 2011; pp. 794–797. 21. Jackson, H.D.; Alan, W. Development of the ERS-1 active radar calibration Unit. IEEE Trans. Microwave Theory Tech. 1992, 40, 1063–1069. [CrossRef] 22. Rudolf, D.; Raab, S.; Döring, B.J.; Jirousek, M.; Reimann, J.; Schwerdt, M. Absolute radiometric calibration of the novel DLR “Kalibri” transponder. In Proceedings of the Germany Microwave Conference, Nuremberg, Germany, 16–18 March 2015; pp. 323–326. 23. Rudolf, D.; Döring, B.J.; Matthias, J. Absolute radiometric calibration of C-Band transponders with proven plausibility. In Proceedings of the 10th European Conference on Synthetic Aperture Radar, Berlin, Germany, 3–5 June 2014; pp. 1180–1183. 24. Lenz, R.; Pontes, J.; Wiesbeck, W. The TerraSAR-X ground calibration system and pattern estimation software. In Proceedings of the International Geoscience and Remote Sensing Symposium, Seoul, Korea, 25–29 July 2005; pp. 4894–4897.

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).