EXPERIMENTAL PROTOTYPE FOR MSPSR BASED ON OPTICAL FIBER CONNECTED PASSIVE PSR

Masato Watanabe, Junichi Honda, Takuya OtsuyamaElectronic Navigation Research Institute (ENRI) National Institute of Maritime, Port and Aviation

Technology, Chofu, Tokyo

Abstract Multi-static primary surveillance radars

(MSPSR) are being actively studied to deliver surveillance technology for civil aviation. By using multiple receivers, the performance of PSR detection can be improved, as the reflection characteristics, which change with aircraft position, can be suitably synthesized. In this paper, we report experimental results from our proposed optical-fiber-connected passive PSR system with transmit signal installed at the Sendai Airport in Japan. The proposed system is capable of detecting moving aircraft, as demonstrated by a comparison of the experimental results with real surveillance data.

IntroductionIn recent years, surveillance techniques such as multilateration and wide-area multilateration have been adopted, which provide higher updating rates for the SSR response signals [1],[2]. In fact, these techniques have been applied for various surveillance applications, but their performance depends on the target transponder. In the worst-case scenario, a transponder failure disables the SSR operation. Therefore, surveillance systems independent from aircraft equipment, such as PSRs, are still necessary to ensure proper surveillance and safety in air traffic control. Currently, the coverage of a PSR is approximately 60 NM, and its required detection rate is above 70%. Consequently, large antennas and transmitters are required for the PSR operation, and the resulting cost is very high with respect to the operating frequency. To overcome these problems, multi-static PSR (MSPSR) have been proposed as an alternative to the conventional PSRs [3]. As

illustrated in Figure1, one interesting property is the selection of some signal sources, e.g., present radar signals. digital terrestrial television broadcasts, mobile communication (e.g. 3G and LTE), global navigation satellite system, and so on. Specifically, an MSPSR is a passive radar system composed of multiple transmitting and receiving stations, and uses data fusion to merge multiple sources for aircraft surveillance. Such multi-static radars have been mainly used for commercial purposes in Europe [4]–[6]. As a result, MSPSR is being considered as a promising PSR system in civil aviation.

At the Electronic Navigation Research Institute (ENRI), We previously proposed an optical fiber-connected passive PSR (OFC-PPSR) that corresponds to an MSPSR [7],[8]. The proposed system relies on a PSR signal, and its transmitter rotates at constant cycles with signal transmission as low pulse repetition frequency. By adopting radar instead of broadcast or any other type of signal, the principles of radar technology, such as positioning and unnecessary signal suppression, can be applied to this system. Moreover, the proposed system employs radio over fiber (RoF), which enables radiofrequency signals to be transmitted to distant receivers through an optical fiber. Furthermore, the OFC-PPSR architecture is simpler than that of the conventional bistatic radar [9]. Specifically, bistatic radars usually require two receiving systems that acquire direct and scattered waves. In contrast, the RoF technology allows the OFC-PPSR to share transmission signals corresponding to direct waves between the transmitter and receiver. Hence, it is possible to omit the direct wave observation system from the two receivers when using a multi-static strategy.

In this paper, we present the principles of OFC-PPSR and the experimental system, which was deployed at the Sendai airport in Japan. Moreover, we report experimental results from the OFC-PPSR system with PSR transmit signal installed at the Sendai Airport in Japan. The experimental data related to a moving target are analyzed through the signal-to-noise ratio during detection. Then, to estimate the direction of arrival from the delay among scattered waves reflected by the target and

obtained from multiple receivers, we performed experiments considering two stations, one of them being a simplified receiver. In addition, we only used the RoF technology to transmit the scattered wave (i.e., intermediate frequency signal) from a remote receiver. Then, by performing time correction of parameters such as the transmission delay of RoF, we found that the delay of the reflected signal from a stationary target remains within an expected range.

Figure 1. Principles of MSPSR

OFC-PPSRA general PSR has a transmitter unit combined with

a receiver unit. As the receiver unit receives the information of transmitted signals (transmitted timing, rotating antenna angle, etc), estimating the

target position is easy. However, the detection rate depends on the radar cross section and antenna beam width, and consequently, its value is lower than that of SSR. We employ the principle of passive bistatic radar to use scattered waves that do not return to the

PSR site, thus resulting in the improvement of detection rate. This is achieved by the RoF technology that can transmit radar information over a long distance.

Figure 2 illustrates a basic concept of the OFC-PPSR experimental system. An OFC-PPSR receiver unit is located at a distance from the PSR site, and they are connected by an optical fiber. RoF transmitter (Tx) is located at the PSR site, and it collects the transmitted RF signals (short and long pulses) and information of the control unit such as trigger and direction of antenna. The RoF technology enables conversion from RF signals to light and transmits them over an optical fiber. Therefore, RF signals can be transmitted over long distances. Following is the procedure for signal processing. Adjusting signal delays corresponding to the optical fiber length. This is important to process the same operation as conventional PSR.

Collecting RF signals (short and long pulses) transmitted by the rotating PSR antenna

Collecting radar information such as trigger and azimuth

Transmitting information collected by RoF Tx to the OFC-PPSR receiver side

There are two characteristics of the OFC-PPSR system. One is to use the scattered waves that do not return to the PSR site. On combining with the

conventional PSR, the coverage area would expand. The other is that directed waves are not required. Even if the directed waves are interrupted by the large obstacles, the proposed system always operates. As the radiated waves from a rotated antenna are directly transmitted to receiver side by RoF, the signal to noise ratio is also much better than that when using directed waves. Moreover, if the isotropic antenna is selected as a transmitter antenna, the update rate is expected to improve. Following is a summary of the characteristics of OFC-PPSR. The receiver unit can easily obtain the radar information even if the receivers are separated from the transmitter unit

Transmitting radar information in the long distance by RoF

Operating as the conventional PSR

No necessity of a directed wave

Ease of synchronizing several receivers

Can be located together with the other surveillance receivers such as MLAT

Improving the conventional PSR performance

Expanding coverage area of PSR

-Expected improvement in update rate if an isotropic antenna is employed

Figure 2. Principles of OFC-PPSR

Experiments systemWe developed an experimental system of OFC-

PPSR and deployed it at Sendai Airport. Sendai Airport is a mid-sized airport in Japan, which is located in Miyagi prefecture, in the northern part of Main Island. There are branches of Civil Aviation College, Aeronautical Safety College (ASC) training center, Coast Guard school, etc., and they train at the airport. ENRI also have Iwanuma branch for the

evaluation and development of new technologies (SSR, MLAT, MSPSR, FOD, GBAS, etc.).

Figure 3 shows the experimental environment. In the experiment, ASC’s training radar is used, and it is located in the southern part of the airport, where the RoF Tx unit is also installed. The OFC-PPSR receiver unit is located in the western part of the airport. The distance from PSR to the OFC-PPSR receiver unit is approximately 1,800m.

In addition, the transmitted signal was sent to the receiver through RoF. Figure. 4 shows the system configuration, which consists of two receiving stations, where station Rx1 acquires the transmitted signal through RoF from the training radar instead of the direct wave antenna. Specifically, the transmitted signal corresponds to the output of the monitor terminal directly under the antenna and is converted into an optical signal sent to Rx1 through the optical fiber. At Rx1, the optical signal is

converted into an electrical signal. In addition, an antenna installed at Rx1 receives the scattered wave, which is converted into an intermediate frequency instead of a radiofrequency signal for RoF transmission. The other receiving station, Rx2, is distant from Rx1, and consists only of the receiving system to acquire scattered waves, which are transmitted to Rx1 through RoF. The signals are registered using analog-to-digital converters and treated using signal processing units. At these units,

Figure 3. Experiment environment

Rx1 Rx2

ASR

Receiver（Rx2)

ASR

OFC-PPSR system

Antenna(Scatter wave)

Receiver（Rx1)

RoF receiver

Signal processor

Optical Fiber(RoF Transmission)

Optical Fiber(RoF Transmission)

Antenna(Scatter wave)

the intermediate frequency signal acquired by the receiver is sampled at 25 MS/s and transmitted to a recording device. A central control unit triggers the analog-to-digital conversion in the signal processing unit and ultimately processes the signal stored at the recording device. The signal transmission source (i.e., training radar) is located to the south of the airport and surrounded by buildings. Therefore, most of the airport ground, including parking lots and runways, does not have a direct line of sight. The RoF transmitter is installed in the radar station building, and its signal is transmitted to Rx1, which is located on the west side, using the optical fiber infrastructure from the airport. In addition, the receiving station uses a standard horn antenna pointing to the east, where several aircrafts land, to receive scattered waves. To verify the operation of a two-receiver system, we designated the shelter near the center of the airport as Rx2, and used it to transmit signals to Rx1 through RoF. The horn antenna and its setup for Rx2 are the same as those for Rx1. The signal transmission source transmits and receives short (1 μs) and long pulses (chirped pulses of 80 μs) at intervals of 102 μs to sweep in 4 s cycles. The PSR then converts the signal obtained from the two pulses into a composite video. We applied signal processing to the baseband signal, which was previously decimated to 1 MHz, by considering the long-pulse observation interval and width.

Figure 4. Configuration of OFC-PPSR

Experiment results

1 Moving target detectionWe obtained the target range by applying

the pulse compression illustrated in Figure. 5 to the experimental data acquired at station Rx1. Figure. 6 shows the moving target after applying a detection process to Figure. 6. In Figures. 5 and 6, the delay profiles of the respective sweeps are arranged with respect to the transmitter orientation. The vertical axis represents the discretized delay (1 range bin = 1 μs), and the horizontal axis represents the transmitter orientation with respect to the North. In Figure. 6, the moving target indication adopts a double delay line canceller, whereas the multi-Doppler frequency filter (MDF) has six channels with a sinc filter shape. In the Figures, the azimuth beam width of the transmitter is 1.3°. Hence, the average pulse repetition interval is 1400 μs with an antenna rotation cycle of 4 s, and the number of input pulses to the MDF is set to 6 sweeps (i.e., 6 pulses) considering the beam width. The LOG/CFAR receiver employs 16 samples for both the guard and reference cells, which correspond to 16 times the pulse width after compression.

PSR Building

RF Signal RoFTx

Rx1AMP

Rx2AMP

RoFRx

RoFTx

RoFRx

SP CP

* 1 Antenna (for scattered wave observation)* 2 Synchronization signal* 3 IF signal* 4 Observation control command

AMP: signal amplifierRx: ReceiverSP: Signal processing unitCP: central control unit

* 1

* 1

* 2

* 2

* 3

Optical Fiber

Optical Fiber

* 3

* 4

Radar info

Moving object

Moving object

Figure. 7 shows the MDF results corresponding to the highlighted peak in Figure. 7, where it is expected that the signals having the Doppler frequency based on the relative speed generated with the receiver exhibit peaks in channels 2 to 6. From the figure, it can be determined that the signal corresponds to a reflected signal from a moving target, whose complete delay profile is shown in Figure. 8. The horizontal axis represents the

discretized delay, and the vertical axis represents the amplitude normalized with respect to the peak. The peak corresponding to the moving object was improved in 7.4 dB of the signal-to-noise ratio. This result can be considered appropriate even under the loss from the MDF (sinc function) process if compared to the theoretical value of 7.8 dB. Therefore, in this experiment we confirmed that the proposed OFC-PPSR system with only one receiving station can suitably acquire the reflected signals from a moving target showing delays of approximately 168 μs around the airport. Furthermore, we experimentally determined that a signal-to-noise ratio of 39 dB can be obtained by applying moving target detection. The reflected signal from this moving target was compared to the automatic dependent surveillance-broadcast in [8], and it corresponds to a typical reflected signal from an aircraft.

Figure 7. MDF outputs

Figure 8. Comparison of Signal to noise ratio

In addition, we introduce the results of applying the moving target detection process to other data. This is data of 5 scans (20 seconds). As shown in Figure. 9, threshold processing of about 13 dB was

Figure 5. Experimental result (1scan)

Figure 6. Experimental result after application of signal processing(1scan)

performed on noise after applying Moving target detect process. By overlapping the threshold processing results by 5 scans, two moving bodies moving on the sea were detected in the figure. Incidentally, what appears to be circular in the figure is due to the interference of the transmission signal, and in this result, the suppression processing for these is not applied. The other detected objects are thought to be due to reflection from a stationary object such as a mountain. Our future work will consider the detection performance and accuracy of moving targets.

Figure 9. Experimental result after application of signal processing and detection (5scan)

2 Verification in the case of two receiversWe obtained the delay profile to verify the two-

receiver OFC-PPSR operation by applying the process shown in Figure. 10 to the intermediate frequency signals acquired by receiving stations Rx1 and Rx2. The signals from Rx2 were transmitted to Rx1 through RoF for consolidating a single data set. The RoF transmission delay from the transmitter to Rx1 was 10.2 μs, and that from Rx2 to Rx1 was 7.8 μs. Hence, we considered the delay from the PSR transmission signal and used both Rx1 and Rx2, which have delay correction for

the RoF transmission, as the time reference to determine the delay profile.

Figure 10. Signal processing configuration diagram of two receivers

Figure. 11 shows the applied signal processing to one scan, where the delay profiles of the respective sweeps are arranged with reference to the transmitter orientation. The vertical axis represents the discretized delay, and the horizontal axis represents the transmitter orientation with respect to the North. Figure. 12 shows the delay profile from a sweep in which the PSR was oriented northeast, where the signal corresponded to a scattered wave from Mount Kinka (see Figure. 11). We used this signal as adjustment target by the PSR for the signals received at Rx1 and Rx2. It can be seen that the signals at both stations show almost the same strength at around 400 bins. In this experiment, the delay among the reflected signals from Mount Kinka at receiving stations Rx1 and Rx2 was 6 μs. Figure. 13 shows a map with the ellipse obtained from the delay based on the latitude and longitude of the transmitting station and receiving stations Rx1 and Rx2. By performing time correction, the delay among reflected signals from the stationary target (i.e., Mount Kinka) remains within the expected range. Hence, we consider that the proposed system suitably estimates the azimuth by using the arrival time difference at the two receivers.

Moving objects

PC MTI MDF LOGCFAR

DDC

IF BB

PSR(RoF)A/D

Rx1, Rx2(RoF)Scattered wave

Direct wave

Delay profile(Rx1/Rx2)

Timecorrection

(a) Rx1

(b) Rx2

Figure 11. Experimental results (1scan)

(a) Rx1

(b) Rx2

Figure 12. Delay profile (Azimuth sample=680)

Figure 13. Estimated target position

ConclusionThis paper presents experimental results

from our previously proposed OFC-PPSR system using PSR transmit signals generated in a scenario at the Sendai Airport. The signal-to-noise ratio for detection of a moving target verifies the suitability of the system for this task. Likewise, we experimentally verify the OFC-PPSR operation with two receiving stations without direct wave observation. By performing time correction, we verify that the delay among reflected signals from a stationary target remains within the expected range. Hence, the system suitably provides azimuth estimation using the arrival time difference of the signal at two receiving stations. In upcoming developments, we will have integrated processing of multiple receivers to enhance the multi-static radar performance.

References[1] M. C. Stevens, Secondary Surveillance Radar, Norwood, MA: Artech House, 1988.

[2] ICAO, “ICAO Doc 9924, Aeronautical Surveillance Manual,” 2012.

[3] Steffen Marquard, “Suitability of Multi-Static Surveillance System for Aeronautical Use (Passive Radar),” International Civil Aviation Organization Working Paper, Montreal, WP ASP12-12, March 2012.

[4] M. Edrich, A. Schroeder and F. Meyer, "Design and performance evaluation of a mature FM/DAB/DVB-T multi-illuminator passive radar system," in IET Radar, Sonar & Navigation, vol. 8, no. 2, pp. 114-122, February 2014.

[5] M. Contu et al., "Passive Multifrequency Forward-Scatter Radar Measurements of Airborne Targets Using Broadcasting Signals," in IEEE Transactions on Aerospace and Electronic Systems, vol. 53, no. 3, pp. 1067-1087, June 2017.

[6] Y. Liu, X. Wan, H. Tang, J. Yi, Y. Cheng and X. Zhang, "Digital television based passive bistatic radar system for drone detection," 2017 IEEE Radar Conference (RadarConf), Seattle, WA, 2017, pp. 1493-1497.

[7] J. Honda, M. Watanabe and T. Otsuyama, "Optical fiber connected passive primary surveillance radar using two receiver units," 2017 International Symposium on Antennas and Propagation (ISAP), Phuket, 2017, pp. 1-2.

[8] Junichi Honda, Takuya Otsuyama, Optical-Fiber-Connected Passive Primary Surveillance Radar for Aeronautical Surveillance, IEICE Communications Express, December 20, 2017

[9] M. Cherniakov, ed., Bistatic Radar: Emerging Technology, Wiley, 2008.

AcknowledgementsWe would like to express our gratitude to the

Ministry of Land, Infrastructure, Transport and Tourism, the East Japan Civil Aviation Bureau, the Aeronautical Safety College, and Sendai Airport for their support.

Email Addresses Masato Watanabe

m-watanabe@mpat.go.jp

2018 Integrated Communications Navigation and Surveillance (ICNS) Conference

April 10-12, 2018