Bistatic VHF and UHF SAR for urban environments 2007 03 30. - … · 2009-07-01 · Bistatic synthetic aperture radar (SAR) ... therefore vary between different imaging configurations

SAR techniques, Radar Sensor Technology XI SPIE Defence & Security Symposium 9-13 April, Orlando, Florida, U.S.A

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Bistatic VHF and UHF SAR for urban environments

Johan R. Rasmusson*, Martin Blom, Björn Flood, Per-Olov Frölind, Anders Gustavsson, Tommy Jonsson, Björn Larsson, Gunnar Stenström, and Lars M.H. Ulander

Department of Radar Systems FOI – Swedish Defence Research Agency

P.O. Box 1165, SE-581 11 Linköping, Sweden

ABSTRACT Bistatic synthetic aperture radar (SAR) enables new defense as well as environmental applications where the characteristics of the bistatic reflectivity can be exploited. Experimental results obtained with microwave systems have been reported but not much is published using lower frequencies (< 1GHz). FOI has been active in this part of the electromagnetic spectrum for many years with the development and operation of two airborne SAR sensors, i.e. CARABAS-II (20-90 MHz) and more recently LORA (200-800 MHz). During 2006 experimental work was initiated to investigate the challenges of implementing a bistatic low frequency SAR system. Various synchronization tests were made in the lab as a preparation for the first bistatic VHF SAR data registrations. An area in the vicinity of Linköping city was illuminated using CARABAS-II as the airborne transmitter and the LORA radar electronics as a stationary roof-top mounted receiver unit. The latter was reconfigured to be able to handle the frequency interval 20-90 MHz. The approximately 4.1 km by 4.1 km large common radar scene contains urban environments, open areas and forested parts. The CARABAS-II sensor simultaneously registered monostatic SAR data to facilitate the image interpretation by comparisons although the incidence angle on receive differs considerably. Keywords: bistatic, imaging, mapping, SAR, synthetic aperture radar, urban, VHF, UHF

1. INTRODUCTION The imaging technique referred to as synthetic aperture radar (SAR) is established as a core instrument for defence as well as environmental applications. Commercial SAR systems are found onboard platforms such as satellites, aircraft, helicopters and UAVs. The fielded sensors typically operate in the microwave band, i.e. with wavelengths in the order of centimetres to tens of centimetres. An emerging technique is, however, to realize systems operating at lower frequency bands. The main benefits of using longer wavelengths are: • Countermeasures against stealth designed targets • A capability to map large areas with a high target-to-background contrast • A proven performance to detect targets also hidden in foliage • A potential to penetrate into the ground to obtain sub-surface detections FOI has been active in low frequency SAR technology for many years and developed two airborne imaging sensors, i.e. CARABAS-II and LORA. CARABAS-II is a SAR system operating in the 20-90 MHz (VHF) regime with horizontal polarization and a resolution of 2.5 m [1-2]. Extensive data collections have been undertaken with the system since 1996. The main military focus has been investigations on detection of stationary ground targets found both in the open and hidden under trees. A developed methodology based on change detection techniques has shown to be robust and provide good performance figures in terms of high probability of detection and low false-alarm rate [3]. In this work it has been important to get an understanding of the main backscattering mechanisms from forests when imaged by low radar frequencies. Models describing this have been derived and tested against gathered CARABAS-II data over forest stands exhibiting different characteristics. It has been found that the competing forest clutter, to be suppressed in the military applications, contains retrievable information about some parameters of interest to forest monitoring and

*[email protected]; phone +46 13 378419; fax +46 13 378100


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management, e.g. stem volume [4-6]. Another potential demonstrated is extraction of the bald earth topography in regions of foliated terrain by interferometric processing of VHF SAR data registered along two or several parallel and well separated flight lines [7-8]. LORA is an airborne SAR system originally designed to operate in the frequency band 200-800 MHz (UHF) using horizontal polarization [9-11]. The first radar data in flight were gathered in late 2002. The two-way foliage attenuation is higher compared to the VHF-band but manageable to make search for concealed targets meaningful. Processing of the full LORA bandwidth will generate radar images with a resolution of 0.3 m which gives a potential for target recognition, in particular for deployments in the open. The congested radio frequency spectrum found in Sweden above 475 MHz has, however, prohibited any significant contributions to the image formation process from this part and consequently limited the achieved resolution figures to about 0.5 m. It is anticipated that more sophisticated algorithms for the radio frequency interference (RFI) filtering must be implemented to circumvent this obstacle. The LORA antenna system consists of two different element types developed for the low and high part of the full frequency interval [12]. Using the same mechanical design to encompass the antenna arrangement as in CARABAS-II it means that five identical LORA elements of each model can be mounted. This opens for a capability to collect GMTI (Ground Moving Target Indication) data to find and accurately position vehicles in motion by multi-aperture processing techniques. The low frequencies used in LORA will thus also allow detection of vehicles driving along narrow forest roads, most often obscured behind a curtain of trees from the radar line-of-sight. The GMTI realization can be recognized as a bi- or multistatic measuring geometry but with the transmitting and receiving units separated on the same platform to accomplish sufficient baselines to get the required sensitivity for the expected range of ground target velocities [13-14]. The time synchronization problems as well as the need to take into account more than one flight trajectory to retrieve the signal phase history are not present in this case with the full equipment installed onboard the same carrier. Another application with access to several apertures is suppression of jamming signals. Both LORA and CARABAS-II are using the same Sabreliner platform shown in Fig. 1 and can not be installed simultaneously.

Fig. 1. The CARABAS-II and LORA radar sensors are installed and operated onboard a Sabreliner aircraft. The antenna

system is in both cases integrated in an aerodynamically and mechanically certified structure consisting of two eight metres long push booms, with the outer five metres acting as the electrically active part. In the VHF-band, the same antenna design is found inside the booms. For the UHF-band implementation, however, the full bandwidth is realized with 200-400 MHz transmitted and received by the left boom and 400-800 MHz by the right one. The shorter wavelengths make it possible to mount five identical antenna elements in each boom and thus provide a GMTI-capability based on multi-aperture processing techniques. (Photo: © Pia Ericson, FMV).

CARABAS-II is a quite unique SAR sensor in the international arena. The only known competitor is the FOPEN system developed in the US [15]. It adopts also a horizontal polarization but has a narrower VHF bandwidth of 25-52 MHz. The instrument installed onboard an Army RC-12 aircraft has also a fully polarimetric mode operating between 235 and 445 MHz that can co-register UHF SAR data in parallel. This ability is thus similar to LORA but with a polarimetric capability and a narrower bandwidth. Other SAR systems working in the UHF-band are found both in the US and Europe, e.g. EarthData, Intermap Technologies, QinetiQ (UK), DLR (Germany) and ONERA (France). The LORA radar electronics is in fact designed to be able to handle the frequency band 20-90 MHz as well. Only a few parts are then necessary to change when shifting between the two frequency modes. It is primarily the antenna booms, the


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antenna switching network and the power amplifier. The more modern LORA system will therefore gradually replace the older CARABAS-II radar electronics within the next few years as the monostatic SAR asset at FOI. In a bistatic SAR configuration the radar transmitter and receiver units are separated and hence not found on the same moving platform. The technique has been demonstrated in the microwave band with different combinations of spaceborne, airborne and groundbased locations for the two radar devices. From a military point of view, the concept can be seen as a potential means to reduce the vulnerability against various electronic countermeasures as well as a physical attack of the platform by positioning the transmitter at large distance. The latter means that the passive receiver can be operated covertly closer to the area of interest which lessens the requirements on both the coherent integration time and the output power needed for the radar signal. Registrations of the scattered field in other directions may also provide a possibility to improve the detection performance of low observable targets. In contrast to monostatic SAR, the spatial resolution obtained will also depend on the actual bistatic geometry used for the data acquisition and may therefore vary between different imaging configurations. The increased system complexity obtained by the equipment separation is mainly the need to establish a common time reference during the measurements and to incorporate the impact from two flight trajectories in the signal processing. Bistatic SAR has been studied in the US for many years with several experiment results reported in the open literature. One example is summarized in [16] with the radar transmitter spaceborne and the receiver mounted onboard an aircraft. Several European institutes have recently presented efforts made in this field where the two radar units are airborne, i.e. FGAN (Germany) [17], QinetiQ (UK) [18] and a joint Franco-German campaign with DLR and ONERA involved [19]. FOI has also started to look into this research topic but with the focus on using lower frequencies. Simulation results obtained indicate that an appropriate selection of the bistatic SAR geometry may significantly suppress the background clutter in difficult environments, e.g. ground targets under foliage or targets present in urban areas [20]. Modifications in the LORA and CARABAS-II hardware has been made to enable bistatic SAR data collection at the VHF-band with the two units separated and used as receiver and transmitter, respectively. The paper will give an overview of the illuminator and receiver characteristics, the principles developed to attain a common time reference, the first VHF SAR experiment layout and finally some preliminary imagery results with a brief summary of the main processing steps.

2. TRANSMITTER AND RECEIVER USED IN THE BISTATIC VHF SAR The first bistatic VHF SAR experiment was conducted during the spring 2006. CARABAS-II operated as the transmitter onboard the Sabreliner whereas LORA was used in a receiver mode only and stationary deployed on ground. A similar approach with a combination of an airborne and a groundbased device is described in [21] for an experiment conducted at 35 GHz. The LORA radar electronics was connected to a log-periodic antenna mounted on an 8 m high mast at the roof-deck of one of the FOI buildings. The antenna was adjusted to point optimally towards the illuminated ground scene. The specification of the frequency range and gain for this antenna was 27-87 MHz and 5 dBi, respectively.

2.1. Transmitter The main blocks of the CARABAS-II transmitter/receiver layout are given in Fig. 2 and consist of:

• Radar control unit (RCU) • VHF power amplifier (PA/VHF) • Switch for receive/transmit and lobe shaping (AS/VHF) • Local oscillator (LO) locked to 10 MHz, synchronized via GPS • GPS antenna, splitter and GPS receiver (EPSILON) • Integrated GPS/IMU positioning and navigation system (APPLANIX) • Steinbrecher receiver for the left and right VHF antenna in the booms (DTR and DTL) • Data storage unit (DS)

2.2. Receiver The main blocks of the LORA receiver layout configured for the VHF-band are given in Fig. 3 and consist of:

• Data control unit (DCU) • Unit for detection of the signal 1 PPS (SYNC) • Oscillator unit for LORA/VHF (LOVHF3)


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• Interface for control of LOVHF3 from DCU (LOCI) • Watkins-Johnson receiver (DT) • Data storage device (LORADS) • GPS antenna, splitter and GPS receiver (EPSILON) • GPS receiver to notify the start and stop time of the bistatic registration (ASHTECH) • A log-periodic antenna from Rohde & Schwarz (HL 026)

Fig. 2. Block diagram of the airborne CARABAS-II transmitter/receiver during the bistatic VHF SAR experiment.

Fig. 3. Block diagram of the groundbased LORA receiver set-up during the bistatic VHF SAR experiment. The photo

shows the log-periodic antenna deployed at the roof-deck of one of the FOI buildings.


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3. SYNCHRONIZATION A prerequisite for successful bistatic SAR imaging is good synchronization between the separated transmitter and receiver during a full flight mission. The used approach to achieve this can be described in two different steps:

• The synchronization of the transmitter and receiver using two external GPS disciplined 10 MHz oscillators • Controlled operations of the receiver and transmitter at a selected pulse repetition frequency (PRF)

3.1. Synchronization of transmitter and receiver via external GPS Synchronization in a monostatic SAR system can rather easily be made, e.g. using an external 10 MHz oscillator as one common reference signal. This reference may or may not be stabilized via GPS. In a bistatic SAR configuration, however, the synchronization must be maintained over long time periods (hours) with the two parts physically and electrically separated. One possibility to fulfil this requirement is use two external 10 MHz oscillators as reference signals stabilized via GPS. The two situations are conceptually illustrated in Fig. 4. In the modifications of the CARABAS-II and LORA for the bistatic VHF SAR measurements a GPS disciplined 10 MHz quartz oscillator has been integrated in both systems and connected to existing GPS antennas through a splitter as shown in Fig. 2 and 3. The implemented solution is based on a hardware manufactured by TEMEX and shown in Fig. 5 [22].

Fig. 4. A schematic drawing of the difference between a monostatic (left) and bistatic (right) SAR system with respect to

the synchronization problem. In the bistatic case the transmitter and receiver are separated physically and electrically.

Fig. 5. A photo of the Epsilon OEM II board (70 mm x 250 mm) from TEMEX with a GPS disciplined 10 MHz quartz

oscillator using the 1 PPS (pulse per second) signal. The type used in CARABAS-II and LORA is equipped with the optional double oven for the 10 MHz oscillator with a 24 h accuracy of < 1* 10-12 when connected and locked to the GPS [22].


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3.2. Controlled operations of the receiver and transmitter The inherent design of CARABAS-II is based on a stepped-frequency technique to transmit a larger bandwidth. To exploit the full band of 20-90 MHz in the monostatic SAR mode 37 frequency sub-bands are used in total, separated by 1.875 MHz and having a bandwidth of 2.5 MHz each [1]. In the first bistatic experiment the CARABAS-II transmitter used 24 sub-bands to sweep the selected bandwidth of 29-74 MHz. Because of timing restrictions imposed on the existing system by the introduced 1 PPS (pulse per second) signal the frequency band 74-90 MHz could not be used. The shrinkage in the lower end of the frequency band was primarily made in accordance with the receiving log-periodic antenna characteristics. The timing sequence for the transmitter and receiver operations is schematically outlined in Fig. 6. The receiver uses 25 time and frequency slots, each with an opening time of 200 µs and bandwidths in steps of 10 MHz to receive the emitted signals. The operation of the receiver and transmitter was computer-controlled at a selected pulse repetition frequency (PRF). Exactly 200 full sequences were repeated of the receiver and transmitter every second. For further control and to obtain a synchronized operation of the receiver and transmitter, the starts of these sequences were triggered by the 1 PPS signal.

Fig. 6. The figure shows a typical start sequence in the bistatic VHF SAR experiment. The red rectangles represent time

and frequency of the emitted pulses from the airborne CARABAS-II transmitter. The green rectangles indicate when in time and where in frequency the groundbased LORA receiver is active. The time allocation of the 25th sub-band is present in the receiver part (green) but no energy could be transmitted (red) as seen due to limitations in the control hardware.

3.3. Measurements of the phase drift between two GPS disciplined 10 MHz oscillators Various tests have been conducted in the lab with the two purchased Epsilon II OEM boards to get a better understanding of their characteristics and the corresponding implications on the bistatic VHF SAR performance. One crucial parameter is the long term drift in the reference signal to be retrieved in parallel by the radar transmitter and receiver and thus assumed to be similar in the synchronization process. An experimental set-up to examine this kind of stability behavior is shown in Fig. 7. One of the two units is considered to define the signal reference and the other is the device under test (DUT). The 1 PPS signal from the reference is used as the external trigger of the oscilloscope found in Fig. 8. The 10 MHz sinusoidal signal from the reference is connected to channel 1, the 10 MHz sinusoidal signal from the DUT to channel 2 and the 1 PPS signal from the DUT to channel 3. These three signals are marked in yellow, violet and blue, respectively, in Fig. 7. In a test with the duration of 16 hours the yellow and violet traces on the oscilloscope in Fig. 8 were logged and the phase difference between them was calculated and stored every second. The differences


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for the total registration period are found in the graph at the top in Fig. 9. As an example, during an observation of 23 minutes a phase drift of 270° (corresponding to 75 ns time lag) was found between the two 10 MHz signals.

Fig. 7. Experimental set-up during the evaluation of the phase or time drift between the two Epsilon OEM II boards with

GPS disciplined 10 MHz oscillators in laboratory conditions. One of the oscillators was used as the 10 MHz signal reference and the other was considered to be the device under test (DUT).

Fig. 8. Oscilloscope (LeCroy Wavepro 950, 4 Gsamples/s per channel) measurements during laboratory evaluation of the

phase drift (time drift) between the two Epsilon OEM II boards with GPS disciplined 10 MHz oscillators.

4. BISTATIC GEOMETRY In the first bistatic VHF SAR experiment, carried out in late March 2006, CARABAS-II illuminated the ground from an altitude of 5 km. Six imaging passes were made with an heading of 20° (or 200° opposite) on the western side of the FOI building where the log-periodic antenna was deployed. The flight tracks were parallel but shifted to get a variation in incidence angle at the groundbased receiving antenna in steps of 5° and ranging from 40° to 65° as is illustrated in Fig. 10. The radar transmission as well as the registration of monostatic CARABAS-II data continued along a distance of 28 km in each pass with the roof-deck as the radar aim point. The centre point for the common radar scene in the generated monostatic and bistatic VHF SAR images presented in this paper is found approximately 2 km on the eastern side from the FOI building. The difference in altitude between the bistatic receiving antenna and this point is only about 10 m. Within the whole mapped area, the ground elevation varies between 50 and 80 m compared to the 84 m given for the receiving antenna in Fig. 9. Of course this is far from an ideal bistatic geometry situation, with the extreme grazing angle causing severe shadowing effects and other possible propagation disturbances. It is known from monostatic


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CARABAS-II VHF SAR measurements that the radar cross section (RCS) for vehicle-sized objects starts to drop rapidly for incidence angles larger than 70°-75°.

Fig. 9. The plot at the top shows the time (phase) difference over 16 hours between the two Epsilon OEM II boards with

GPS disciplined 10 MHz oscillators. The graph in the middle presents the calculated first difference of the data in the time series at the top, i.e. the frequency deviation. The square root of the Allan variance as a function of averaging time is shown in the bottom plot. This is an established measure of the fractional frequency stability [23].

Fig. 10. The figure shows the geometry used during the first bistatic VHF SAR experiment. r corresponds to the direct

signal between the airborne transmitter and the roof-top mounted receiver. u is the backscattered signal from the illuminated ground scene.

5. BISTATIC SAR SIGNAL PROCESSING The bistatic SAR signal processing begins with pulse compression, filtering and spectral normalization before the image formation is carried out using back-projection techniques. In the latter step the information from a digital elevation model is taken into account which means that the output image is generated directly in a geocoded format.

5.1. Pulse compression The full high resolution signal bandwidth (approximately 45 MHz) is reconstructed by concatenation of the low resolution (approximately 2 MHz) responses from the 24 different frequency sub-bands transmitted by CARABAS-II.


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For each sub-band the received and digitized signal is first correlated with a reference signal, i.e. the direct signal r in Fig. 10 which travels the shortest distance between the airborne transmitter and the LORA receiver. In the next step, the reference signal is excluded from the received signal u and thus not present in the subsequent signal processing. The different parts of the received signal along the synthetic aperture are shown in Fig. 11 for one of the 24 frequency sub-bands used. The low resolution spectral response Uik is calculated as:

)()(

ik

ikik rfft

ufftU = , i = 0...23 and k = 0...32767,

where uik is the backscattered signal from the target area and rik the direct signal at frequency band i and flight (aperture) position k. The wideband spectrum Uk is then constructed by concatenation of the central spectral samples from each sub-band spectrum Uik as indicated in Fig 12.

Fig. 11. The figure at the top (upper left) shows the received radar echo from one of the 24 frequency sub-bands transmitted during the full bistatic VHF SAR registration period used of 220 s. The x-axis represents the sample numbers and the y-axis gives the time in seconds along the flown track. The red line corresponds to the slant range distance between the transmitter and receiver. It is accurately known and retrieved from the GPS/IMU data registered onboard the aircraft and calculated relative the position of the groundbased receiving VHF antenna. Between the green and blue border lines in the figure below (middle left) is the direct signal (r) located and extracted from. Finally, beyond the blue line in the figure at the bottom (lower left) is the much weaker echoed signal (u) from the target area found. The direct signal has here been removed. In the diagrams to the right, the y-axis shows the amplitude variations for one of the flight positions present in the corresponding figures to the left.

5.2. Normalization and back-projection As a pre-processing step to the image formation, the reconstructed wideband spectrum Uk is block wise first normalized with the mean spectral amplitude and then spurious signals generated internally or externally are identified and truncated. For each k, a correct phase centre of the transmitter is calculated based on the lever arm in combination with the aircraft position and attitude. The fixed phase centre of the stationary receiver is calculated once by a lever arm and a GPS measured position of the log-periodic antenna. Each pixel in the final bistatic SAR image corresponds to a grid point equidistantly spaced (1 m) in easting (y) and northing (x) according to the Swedish datum RT90 (2.5 gon V 0:-15). The associated up-coordinate (z) is derived from a digital elevation model of the area. For each k, the high resolution time response uk is calculated as the inverse Fourier transform of the pre-processed data vector Uk and then for each grid point the sum of distances to the transmitter and receiver phase centres are used to update the corresponding pixel in the image from the propagation time given by uk. After a 20 km long synthetic aperture with the CARABAS-II transmitter, the bistatic VHF SAR image resolution


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becomes approximately 4 m. The dynamic range is in the order of 32 dB. The processing scheme used in this image formation is a bistatic extension of the existing fast recursive back-projection method described in [24].

Fig. 12. Example of the 24 sub-bands generated by the CARABAS-II transmitter as they appear in the VHF-band

configured LORA receiver after down-conversion in frequency. Only the central parts marked in red of the received signals are used in the reconstruction of the wideband spectrum Uk to be input in the subsequent bistatic processing.

6. BISTATIC AND MONOSTATIC SAR RESULTS The results shown here is based on the radar data acquired with the geometry defined by flight track one in Fig. 10. The monostatic and bistatic VHF SAR imagery is shown in Fig. 13 together with the map information for comparison. The strongest signatures in the bistatic scene correspond mainly from areas with tall building complexes. In the monostatic image the blocks are resolved with the streets in between whereas in the bistatic case this is only seen when the houses are well separated, e.g. the hospital area and the former Army garrison both found in the upper middle part according to the map. In the more dense urban areas, depicted in purple on the map, it is primarily the segments facing the direction of illumination that is highlighted, with the buildings and streets located behind less visible due to shadowing effects. A large area in the northeast corner is more or less lacking in the bistatic image. This part of the city exhibits the lowest ground elevations with a river intersecting. Most of the forested areas can by identified in both radar images with an intermediate intensity level. The FOI building with the receiving log-periodic antenna is located outside the map, about 450 m to the west just above the middle.

7. SUMMARY AND CONCLUSIONS A first bistatic VHF SAR experiment has been performed using CARABAS-II as an airborne transmitter and the LORA radar electronics as a stationary roof-top mounted receiver reconfigured to work in this frequency band. The main observations made from the initial analysis and evaluation of the collected data set can be summarized by the following points: • The results obtained show that the adopted synchronization method for the transmitter and receiver based on

two external 10 MHz oscillators as reference signals and stabilized via GPS works satisfactory • Bistatic VHF SAR images can be generated using a modified version of an existing fast back-projection

algorithm developed for monostatic SAR mapping • Surprisingly good bistatic SAR images were obtained despite the poor bistatic imaging geometry available


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Fig. 13. A generated bistatic SAR image covering approximately 4.1 km x 4.1 km of an area located to the southwest of the central part of Linköping city (upper left). Monostatic SAR data were registered simultaneously by the CARABAS-II illuminator (upper right). The map shows different environment features found in the radar scenes (lower left). To facilitate the identification of the main contributing scatterers, the bistatic SAR image is combined with the map information (lower right). (© Map: Lantmäteriverket Gävle 2007. Ref. no. I 2007/630).

So far, only one of the six data collections conducted during the first flight mission has been analyzed. With the new developed bistatic SAR processing chain it is of interest to investigate how easily the remaining radar scenes can be produced. All of them were acquired later on and this will give an indication of the long term stability (hours) between the two reference signals retrieved separately by the transmitter and receiver. In upcoming bistatic SAR experiments it is of importance to get an opportunity to estimate the image quality by deployment of some type of reference targets in the illuminated ground scene. Another improvement in the hardware configuration is the possibility to activate a second receiving channel available in the LORA system and attach a separate antenna. With two receiving antennas the gain and other critical parameters can be tuned independently for an optimal registration of the direct signal and the weaker backscattered signal.


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The possibility to carry out an experiment in an area with high relief is also given high priority. In a mountainous region, a steeper incidence angle on receive can be reached by carefully taking into account the topography and deploy the receiver on some peak. A future full-fledged experiment with both the transmitter and receiver airborne would be most cost efficient to conduct in collaboration with some international partner operating a low frequency SAR system.

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Conference on Radar Sensor Technology, Orlando, FL, 8-9 April 1996, SPIE Vol. 2747, pp. 48-60, 1996. 2. L.M.H. Ulander and H. Hellsten, “A New Formula for SAR Spatial Resolution,” AEÜ (International Journal of Electronics and

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