Software Defined VHF Surveillance Radar System

Imre Bíró1., Benjamin Babják1., Dr. Bertalan Eged2.

1.Budapest University of Technology and Economics, Dept. of Broadband Communications 1111 Budapest, Goldmann Gy. Tér 3.

2.Sagax Communications, Ltd., Haller u. 11-13. Budapest 1096 Hungary

Introduction In this paper we'll introduce a complete, tested, experimental VHF software radar system, that was

developed in cooperation between Sagax Communications Ltd., the Budapest University of Technology and Economics, Dept. of Broadband Communications and the Technology Agency, Ministry of Defence. During the tests the radar system used the mechanics and antenna of a russian made P-18 radar (NATO reporting

name “Spoon Rest D”). It is important to note that this is not a modernization of the P-18 but a whole new design using modern software radio technology. The system could be used with any antenna that is capable of transmission and reception in the used frequency range.

The goal of this project was to create an experimental transmitter / receiver for demonstration purposes that can prove those theoretical concepts which makes possible to detect unexpected, low altitude

aerial targets with small effective reflective surface. Large scale tests that have been done few years ago confirmed that we reached this goal however further improvement of the system was necessary to become competitive to currently used conventional radars.

First we'll introduce the hardware and software elements of the system, and finally a few words about the results of the 2005 Livorno field trial.

Fig. 1: The P-18 radar

The P-18 radar The P-18 or 1LR13I or by the NATO reporting name “Spoon Rest D” is a 2D VHF early warning radar

developed and operated by the former Soviet Union. It's in operation since 1970 and was superseded by the 1L13 in 1984 but still used in successor states or where it was exported. Hungary currently operates a modernized version.

The P-18 is mounted on two Ural 4320 trucks for mobility. The radar has a single antenna for transmission and reception and it is composed of sixteen yagi elements. The antenna can be rotated 360

degrees in azimuth and from -5 to 15 degrees in elevation. The height can be also changed during operation. Mechanical azimuth scan speed is 10 rpm. As I mentioned before we used only the truck chassis, the antenna and the rotating mechanics.

Hardware elements of the new radar system

Some elements like the 3.2kW solid state transmission power amplifier was developed directly for this system and some of them, especially signal processing components are from the active product line of Sagax Communciations Ltd.

As you can see on Fig. 2. the space consumption of the whole system is very small. The components from left to right are the following:

• Power distribution unit: 3x400V or 3x240V AC in, controlled power for rotating motors, and 240V AC power for system components.

• Rotation controller computer • Rotation synchronizer (under the desk) • Signal processing computer

• 3.2kW solid state power amplifier

Fig. 2: Hardware components

Fig. 3: System diagram

Figure 3. shows the simplified diagram of the system hardware. The signal processing computer generates

impulses for transmission and processes the reflected signal. It is a general purpose x86 based system with a Sagax DCU-304 wideband two-way converter card in it.

The transmission is synchronized to the antenna rotation by the rotation controller computer through a simple RS-232 line. In the appropriate moment pulse signal can be sent out from the prefilled FIFO. The D/A

converter on the output is able to produce IF signal up to 40MHz. In sync to to the transmission the card enables its input FIFO and records reflected signal for processing. The size of the input FIFO is a limiting factor for radar range but in this case it proved to be enough.

The DCU-304 communicates to the host PC via 66MHz 64bit PCI bus. The use of a general purpose CPU in applications like this is evident. The hardware is cheap and widely available. Software development is easy

and well supported. For applications requiring more signal processing the processing power of an x86 CPU might be insufficient, but for this system a dedicated DSP based hardware is not required.

The outgoing and incoming IF signals are shifted to and from VHF band RF by respectively a transmit and a

receive tuner. The same technology used there is now available in the Sagax STR-3000/1200 and STT-1200 tuners. It's worth to mention that true to the software defined concept the exact same hardware introduced

now runs a variety of applications. For example the SRS series of scanning receivers. In the SRS receivers even the firmware of the hardware components are the same, only the processing and operating software on the PC that is different.

Fig. 4: Block diagram of signal processing hardware

Fig. 5: Sagax DCU-304 with optional DRU-304 4/16 channel

digital tuner extension board

The RF output from the tuner is fed into the 3.2kW solid state power amplifier that was developed directly

for this system in cooperation with the Budapest University of Technology and Economics, Dept. of Broadband Communications and Electromagnetic Theory. The amplifier is composed of sixteen 200W modules with outputs combined. This seems negligible

compared to the 260kW power of the P-18 but this is a fully solid state amplifier. Effective range can be improved by pulse compression techniques and if required more modules can be combined to improve total output power. The amplifier is housed in a 12U high 19” rack.

The output of the power amplifier connects to a transmit / receive switch and then to the antenna. After the transmission of the pulse the system toggles the t/r switch. The incoming signal is fed through a low noise

amplifier to the tuner for downconversion. The IF input signal is then routed to the input of the DCU-304 card in the signal processing computer. The computer processes the received signal and displays it on a local or a remote indicator.

Fig. 6: Sagax STR-3000-4 four channel 3GHz

receive tuner

Fig. 7: SRS-3000 Digital scanning receiver

Fig. 8: Solid state 3.2kW transmitt power amplifier

Software components The software stack of the radar system can be seen on Figure 10. The programs run on 32bit Microsoft

Windows operating system. The kernel mode driver and the user mode API are the same for any application using the DCU-304 card.

The purpose of these components is to set up the converter card for operation. Data transfer is done through direct memory access (DMA) by the hardware. Only buffer addresses and synchronization commands have to be sent through the API and driver.

The signal processing stage is an ordinary user mode program working with data buffers that are prefilled or

read by the hardware. The signal flow of the processing can be seen on Figure 11. The digitized signal is first shifted to baseband, filtered and downsampled. The digital baseband

processing is done with coherent IQ separation (not shown on figure). The downsampled signal is fed to a discrete matched filter. This filter together with the transmitted sub-impulse modulated signal is used to achieve pulse compression. Different impulse modulations and corresponding filters can be easily

interchanged. The compressed signal can be displayed on the indicator.

Fig. 10: Software stack

Fig. 9: Power amplifier 3.4kW peak output power

measurement

The compressed signal is connected to the inputs of the moving target indicator (MTI) and constant false alarm rate (CFAR) filter subsystems. The output of the MTI showing possible moving targets can be

displayed. The results combined gives the final video signal containing hits. A decision making block selects targets from hits. Those can be displayed as plots, and can be tracked even when not visible on the video signal by human eyes.

A few of the tested pulse compression methods are shown on figures 12.-15.

Fig. 11: DSP Signal Flow

Fig. 12: Pulse compression - Baker

Fig. 13: Pulse compression - LFM

Fig. 14: Pulse compression - MLS

The original 64µs impulses were compressed to 1µs. The height of the compressed pulse above the whole can be referred to as compression gain. As can be seen on the plots above this can be over 20dB which is like

transmitting 20dB higher power. This makes possible the use of a relatively low power solid state amplifier. Figure 16. shows a comparison of the compressed signal and the signal on the output of the MTI filter with possible moving targets only.

The user interface of the radar software can be seen on Figure 17. and 18. The main window contains many settings that are required only for testing. An A-type indicator can be found on the bottom of the window. A more expressive PP (plain position) type indicator can be displayed in a separate window. The PP indicator

can be run remotely on another workstation. In this case it connects to the processing computer through TCP/IP.

Fig. 15: Pulse compression - P2

Fig. 16: Compressed signal on left, video signal after MTI filter on right

Fig. 17: Plot on PP indicator

Field trial results In September 2005 we had the opportunity to test the radar in Livorno, Italy. The trial was a part of the NATO workgroup project: Multiband Radar for Air Defence System. The test measurements got the name

COSIMA that stands for Collocated Simultaneous Multiband Radar Analysis. On Figure 19. some of the radars can be seen that were deployed on the coastal measurement site. During the test radars operated in the frequency range from 150MHz to 94GHz. To minimize interference the

careful positioning of the devices and limiting the angles of radiation were necessary.

First we detected typical stationary targets. On figure 20. the hit of a mount on the island of Corse 110km from Livorno can be observed. Of course we had the opportunity to test with moving targets also. Those and

their trajectories can be seen on Figure 21. Figure 23. shows the detection of a plane flying on trajectory “A”. On the first few pictures the side

lobes of pulse can be seen. Some sophisticated gain control algorithm has to be implemented to overcome this issue. The small island 36km from the site seen on the indicator as a stationary hit can be taken as a good point of reference. We lost the target 45km away. Plots compared to GPS track data can be seen on figure 24.

Fig. 18: Graphical user interface and A-

scope Fig. 19: PP indicator

Fig. 20: Test site

The trial was a success for us. The device worked error free during the measurements, and we've recorded more than 200GBs of measurement data.

Possible Further Improvements

Unfortunately the active development of the radar has been stopped due to financial reasons. However it's worth to take a look at current developments for other applications because those devices and concepts could be valuable addition to the radar system if implemented with relatively low effort.

There are ongoing research at the university on the topic of multi antenna systems. One of the use of such a system is beamforming. It means that with a static set of antennas we can achieve different direction characteristics by modifying the amplitude and phase of the signal on each

Fig. 21: Detecting a stationary target

Fig. 22: Targets and trajectories

antenna. It is important in a radar system because the mechanical rotation of the antenna requires large, heavy, power consuming machinery especially in this frequency band where the antenna has to be large. Sagax Communications has adequate experience in multi channel radio systems that is required for a

beamforming setup. Most of the hardware components introduced in this paper have multi channel versions. One of our current research topics is the evaluation and compensation of phase and amplitude differences between channels in a multi channel system. See figure 25. for example. We are able to precisely

characterize each channel and digitally pre-distort the transmit signal in the desired way. With this existing technology beamforming can be achieved. Another way of development is the use of direct digital synthesis. High speed DACs are available

today at low cost that are capable of outputting signals directly to the VHF band. On the receive side the 500MHz input bandwidth of the ADCs used in this system are already capable of digitizing VHF RF signal. One important factor in this approach is the elevated noise resulting in worse SNR due to sampling clock

jitter. Since jitter and resulting noise is dependent on the incoming signal frequency rather than sampling frequency digitizing the RF signal directly gives noisier results. Figure 26. shows the phase noise plot of some of our converter cards' clock signal. The red one is from an older board generated by a cheap crystal

oscillator and multiplied by a simple, internal PLL clock divider ic. A clock like this results in approximately

Fig. 23: Detecting a moving target

20dB SNR for a 60MHz input signal. The green line shows the clock from a more expensive SAW oscillator divided by a more sophisticated integrated circuit. Even with this setup the SNR for 170MHz signal used in

this application drops below 50dB. Even though it is worth to consider this solution because the frequency extension stage could be left out what eliminates some distortion and improves flexibility.

Conclusion In this paper we've introduced an experimental software defined radar system. It's remarkable parts are the

3.2kW solid state power amplifier, and the versatile signal processing hardware. The 2005 Livorno field trials show that with some further development this could be a competitive radar system.

References

B. Babják, I. Bíró, A. M. Mágori, B. Eged, "IMPLEMENTATION AND CALIBRATION OF MULTI-CHANNEL RECEIVER PLATFORM FOR PHASED-ARRAY RECEPTION" EMC 2008 19th INTERNATIONAL WROCLAW SYMPOSIUM AND EXHIBITION ON ELECTROMAGNETIC

COMPATIBILITY, WROCLAW, 11 - 13 JUNE, 2008

Fig. 24: Measurement plots (red) and GPS track (blue) of targets

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Fig. 25: Phase difference between two tuner

channels Fig. 26: Phase noise plot of sampling clocks

B. Eged, "TOSZEG VHF-Radar System and Signal Description" SET-121 Specialists Meeting on "Multi-

band Radar for Air Defence" NORTH ATLANTIC TREATY ORGANISATION, RESEARCH AND TECHNOLOGY ORGANISATION, SPECIALISTS' MEETING Multi-band Radar for Air Defence, SET-

121RSM-009/MSE, NURC, Italy 9-10 October, 2007 B. Eged and B. Babják, "Software Defined Radio Components for Multichannel Receiver Applications" 12th

Microcoll Conference, Budapest, 14-18 May, 2007