Performance Comparison between Monostatic and Bistatic Staring Spotlight Modes Based on Several Scenarios

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J. Electromagn. Eng. Sci. 2025;25(4):318-327

Publication date (electronic) : 2025 July 31

doi : https://doi.org/10.26866/jees.2025.4.r.302

Seong Joo Maeng¹, Suk-Jin Kim², Jung-Hwan Lim³, Jae Wook Lee^3,, Taek-Kyung Lee³, Woo-Kyung Lee³

¹Radar R&D Center, LIG NEX1, Yongin, Korea

²Advanced Defense Science and Technology Research Institute-Satellite PMO, Agency for Defense Development (ADD), Daejeon, Korea

³School of Electronics and Information Engineering, Korea Aerospace University, Goyang, Korea

^*Corresponding Author: Jae Wook Lee (e-mail: jwlee1@kau.ac.kr)

Received 2024 January 23; Revised 2024 June 29; Accepted 2024 December 12.

Abstract

I. Introduction

A synthetic aperture radar (SAR) system is capable of acquiring images in all kinds of weather by overcoming the shortcomings of optical technology that are affected by changes in climate. As a result, it is widely employed as a representative reconnaissance and surveillance mission system. SAR systems can observe a large area, capture high-resolution images depending on the operation mode, and can also be used on various platforms, such as satellites, aircraft, and drones. Consequently, SAR system design has been actively researched in recent times [1].

Among the SAR systems that are currently being actively studied, satellite SAR systems are more expensive to operate than drone and aircraft SAR systems. In view of this expense, it is necessary to analyze the required image quality for a given scenario before a design is implemented so as to determine whether the system is suitably prepared to meet mission performance requirements.

A SAR system should be designed by accounting for the correlation among its performance variables in specific scenarios. Typically, trade-off relationships between the incidence angle, antenna size, and pulse repetition frequency (PRF) have been observed [2]. For instance, the selection of a low PRF tends to improve the range ambiguity-to-signal ratio (RASR) but reduces the azimuth ambiguity-to-signal ratio (AASR). Meanwhile, the selection of a high PRF results in considerable improvements in AASR but reduces RASR. Therefore, considering these trade-off relationships, the performance analysis of a SAR system must factor in the effects of its design variables.

Multiple design studies on monostatic SAR systems that use only one satellite have been conducted in the past. Although the use of a single satellite has allowed for some reconnaissance and surveillance missions to be carried out, such a setup cannot cope with geometric problems in a flexible manner [3], and there are limitations to its performance. As a result, research on designing bistatic/multistatic SAR systems that can operate two or more satellites has recently gained momentum [4]. Bistatic SAR systems have been proven to not only achieve better performance than monostatic SAR systems, but also exhibit stronger resistance to jamming attacks and can flexibly cope with terrain conditions [5, 6].

Considering its many advantages over monostatic SAR systems, an in-depth analysis of the system design for a bistatic SAR system is necessary. In particular, since the transmitter and receiver are the same in a monostatic SAR system, there are fewer variables to consider when implementing platform fluctuation and signal processing algorithms. In contrast, the transmitter and receiver in a bistatic/multistatic SAR system are separate, and they are designed by considering various conditions, such as time/frequency/phase synchronization [7]. This implies that the effects of this separation of the transmitter and receiver must be accounted for when conducting image quality analysis. Therefore, in this paper, a performance analysis is conducted to account for the variable changes caused by the bistatic geometry in the design of a bistatic staring spotlight mode SAR system in which the receiver is vertically separated from the satellite movement but is located at the same altitude.

This paper is structured as follows. In Section II, a brief description of the theory behind the performance variables involved in the monostatic staring spotlight mode is provided [8]. Subsequently, a simulation is conducted based on the theory, and the results are confirmed. In Section III, the performance variables of the bistatic staring spotlight mode are examined based on the bistatic geometry and simulation results. Next, a simulation of the performance analysis is conducted using the performance variables of the bistatic staring spotlight mode. Notably, the bistatic SAR system scenario used in the simulation was constructed based on TerraSAR-X’s monostatic SAR system scenario. Finally, in Section IV, a performance evaluation of the bistatic staring spotlight mode is conducted, and the results are analyzed.

II. Monostatic Staring Spotlight SAR System

Fig. 1 displays the geometric difference between the operation of the staring spotlight mode, which is investigated in this study, and the stripmap and sliding spotlight modes, which are commonly employed in SAR systems. Notably, the stripmap mode works by setting the beam rotation center at an infinite distance. In contrast, the staring spotlight mode uses azimuth beam steering to match the beam rotation center with the target, thereby obtaining higher resolution images than the other SAR operating modes [9]. Owing to these advantages, the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt [DLR]) has been conducting research using the staring spotlight mode with TerraSAR-X [10, 11].

Fig. 1

Geometry of SAR operating mode.

The staring spotlight mode functions by continuously observing the area of interest, which enables it to obtain higher resolution images than the other modes. To put it simply, its main difference from the stripmap mode is the use of electronic beam steering to capture high-resolution images. However, since electronic beam steering leads to variations in antenna patterns based on the steering angle, an analysis of azimuth performance variables must account for these effects. Hence, in the following sections, the performance analysis conducted for the staring spotlight mode is briefly described while also taking these effects into consideration.

1. Theory

Before analyzing the performance variables of SAR, it is necessary to select the appropriate PRF and incidence angle in view of the limitations pertaining to receiving timing [12, 13]. Fig. 2 depicts the two main constraint variables: blind range and nadir return. Blind range refers to a situation where the transmission timing and receiving timing of a signal overlap, while nadir return refers to a sidelobe signal that is reflected from the nearest ground. A timing diagram that takes these two limitations into account can aid in the selection of the accurate PRF and incidence angle. Fig. 3 shows the simulation results obtained for the timing diagram considering a satellite orbit altitude of 514 km, indicating the points at which the PRF and incidence angle should be set to avoid blind range and nadir return After determining the appropriate PRF and incidence angle based on the timing diagram, the performance variables of the SAR system were analyzed. The first performance variable to be analyzed was the noise equivalent sigma zero (NESZ), which refers to the scattering coefficient when the amplitude of the signal is the same as the noise. This implies that the NESZ is the scattering coefficient (σ⁰) that makes the signal-to-noise ratio derived from the radar equation to be 1, as expressed in Eqs. (1) and (2). In other words, it refers to the amount of desired signal that can be obtained compared to noise. The variables used to analyze NESZ are slant range (R_s), flight velocity (V_s), incidence angle (θ_inc), Boltzmann constant (k), noise temperature (T₀), noise figure (NF), chirp bandwidth (BW), total system loss (L_t) wavelength (λ), light speed (c), antenna gain (G_t), pulse width (τ) and maximum transmit power (P_t) [14].

Fig. 2

Geometry of receiving timing limitations.

Fig. 3

Simulation results of the timing diagram (satellite orbit altitude = 514 km).

(1) SNRmo=λ3cGt2τσ0Pt(PRF)4(4π)3Rs3Vssin (θinc)(kT0)(NF)(BW)Lt

(2) NESZmo=4(4π)3Rs3Vssin (θinc)(kT0)(NF)(BW)Ltλ3cGt2τPt(PRF).

Resolution is an indicator that helps distinguish adjacent targets. Such targets can be identified by analyzing the resolution of the images acquired in ideal circumstances [15]. As formulated in Eqs. (3) and (4), resolution can typically be classified into azimuth resolution (ρ_a) and range resolution (ρ_r) based on direction of the signal. Notably, range resolution can be converted into ground range resolution by accounting for the effects of the incidence angle (θ_inc). After conversion, the resolution can be calculated from the chirp bandwidth (BW). In stripmap mode, the azimuth resolution is calculated from the beam width and wavelength (λ), and it can be approximated to half the width of the antenna. In contrast, since the target observation time is proportional to the azimuth steering angle range in the staring spotlight mode, the resolution in this mode needs to be calculated based on the range of the steering angle (Δθ_span) rather than the beam width:

(3) ρr.mo=c21BW1sinθinc

(4) ρa.mo=121Δθspan.

As shown in Fig. 4, the ambiguity-to-signal ratio (ASR) represents the extent to which ghost images are generated by the influence of sidelobe signals. To factor in the influence of the sidelobe, the main variables that should be considered are the antenna pattern (G_Rg,G_Az) and the general cosine window (W(x), which calculates the ASR based on the PRF and the slant range [8, 16].

Fig. 4

Concept of the ambiguity-to-signal ratio.

Azimuth ambiguity is generated by periodic sampling of the main signal’s Doppler frequency. This implies that the sidelobe affected by the PRF generates an ambiguity signal. Considering these effects, the AASR calculates the desired signal and the ambiguity signal in each beam steering angle (θ_n(m); n = azimuth pulse index, m = azimuth ambiguity index, 0 = desired signal index) using Eq. (5) to account for variations in the pattern of the antenna based on the beam steering angle. Furthermore, range ambiguity appears due to the main signal overlapping with the sidelobe signal based on the PRF. As formulated in Eq. (7), RASR can be calculated from the desired signal and the range ambiguity signal (θ_Rg(m_Rg); m_Rg = Range ambiguity index), in accordance with the antenna pattern:

(5) AASRmo=Σn=1NΣm GAz(θn(m))*W2(n)Σn=1N GAz(θn(m))*W2(n)

(6) W(x)=α+(1-α)*cos(2π*xX);α=0.6,         x∈[-X2,+X2]|X=Δθspan

(7) RASRmo=ΣmRg≠0GRg(θRg(mRg))Rs3sin(θinc(mRg))GRg(θRg(0))Rs3sin(θinc(0)).

2. Simulation

Table 1 presents the system parameters of TerraSAR-X [17], based on which the simulation was conducted. Notably, simulation results for the ASR were verified based on previous research [8], since the simulation conducted for the staring spotlight mode in this study it is a novel approach, unlike simulations for the stripmap mode, which can be conducted in the laboratory [18]. The antenna pattern employed in this study, as shown in Fig. 5, was created using a sinc function [15]. Although the results exhibit a slight difference from those obtained by the phased array antenna used in TerraSAR-X (null at −5 degrees in the element factor), it is not large enough to affect the performance analysis.

Table 1

TerraSAR-X system parameters

Fig. 5

Antenna pattern used in the simulation.

To obtain the high quality images from the satellite SAR system, NESZ and ASR were generally designed to be less than − 17 dB. As illustrated in Figs. 6–9, which present the simulation results for the TerraSAR-X scenario, NESZ and ASR are considerably lower than the −17 dB. From these results, it can be theoretically confirmed that the design parameters of TerraSAR- X are designed to acquire high-quality images.

Fig. 6

NESZ simulation results for the monostatic SAR system.

Fig. 7

Simulation results of the resolution achieved by the monostatic SAR system: (a) azimuth and (b) ground range.

Fig. 8

Comparison between results obtained for AASR through the simulation and in the reference study [8].

Fig. 9

Comparison of results obtained for RASR by the simulation and the reference study [8] for the monostatic SAR system.

III. Bistatic Staring Spotlight SAR System

Section II presented a description of the performance analysis conducted for the monostatic staring spotlight mode, following which simulations were performed and verified. Expanding on this, the performance of the bistatic staring spotlight mode is analyzed in this section. As shown in Figs. 10 and 11, the bistatic geometry adopted in this study comprises a receiver, which was positioned perpendicular to the flight direction of the satellite, at the same altitude in the vicinity of the target. Table 2 presents the geometric differences between the variables of the bistatic and monostatic SAR systems. The variables investigated for this performance analysis were the slant range between each sensor and the target, the antenna pattern, and the incidence angle [4, 19].

Fig. 10

A 3D geometry of the bistatic staring spotlight mode.

Fig. 11

A 2D geometry of the bistatic staring spotlight mode.

Table 2

Parameter difference between monostatic and bistatic SAR systems

In Section II, a timing diagram was created to confirm the limitations pertaining to receiving timing before conducting performance analysis for the monostatic SAR system. In this section, the theory of a timing diagram for a bistatic SAR system is described. Bistatic SAR systems feature an extra limitation, in addition to the two constraints noted for the monostatic SAR system. This extra limitation is a direct signal, which is a limitation that must be addressed through the reception of a signal generated from the sidelobe of the transmit antenna, which travels by line of sight to the receiver.

Fig. 12 displays the timing diagram for a bistatic system, with the baseline set to 30 km in the TerraSAR-X scenario, featuring the parameters presented in Table 1. While the timing diagram of the monostatic SAR system comprises two types of lines, another set of lines is added in the bistatic SAR system. Although the bistatic SAR system offers many advantages, the PRF and incidence angle in the system must be determined in more detail.

Fig. 12

Timing diagram of the bistatic SAR system.

The performance analysis was conducted after checking the timing diagram of the bistatic SAR system. Table 2 presents the differences between the performance variables—the slant range, incidence angle, and each antenna pattern difference—observed when applying bistatic geometry. First, in the case of the bistatic NESZ, all changes caused by the bistatic geometry were considered and analyzed using Eq. (8):

The azimuth resolution was calculated through approximation using Eq. (10) by accounting for the antenna illumination taper [4]. Since the slant range between the target and each transmitter and receiver was the same in the monostatic SAR system, it is expressed as ½ in Eq. (4). However, in the case of the bistatic SAR system, it is calculated by considering the slant range between the target and each transmitter and receiver, as expressed in Eq. (10). The range resolution was converted into ground range resolution using the sine function. Furthermore, since the transceiver and receiver were separated, the bistatic angle was converted into ground range resolution to account for each incidence angle, formulated using Eq. (11):

The AASR was also analyzed by accounting for the effects described above. First, the antenna pattern of each transceiver and receiver was considered, following which the ambiguity signal was determined by accounting for the antenna illumination taper when analyzing the azimuth resolution. Furthermore, as in the case of NESZ, RASR was analyzed by considering each antenna pattern and incidence angle. AASR and RASR can be expressed as Eqs. (12) and (13), respectively:

Simulations for the bistatic SAR system performance analysis were conducted using the above formula. A comparison of the results obtained for the proposed structure and those attained for the monostatic SAR system are depicted in Figs. 13–16.

Fig. 13

NESZ simulation results of the bistatic SAR system.

Fig. 14

Simulation results for the resolution of the bistatic SAR system: (a) azimuth and (b) ground range.

Fig. 15

AASR simulation results of the bistatic SAR system.

Fig. 16

RASR simulation results of the bistatic SAR system.

In the case of NESZ, a slight improvement was observed for the bistatic SAR system (Fig. 13), resulting from a decrease in the slant range due to the receiver’s position being close to the target. The azimuth variable also improved for the same reason. An improvement in azimuth resolution owing to the antenna illumination taper is confirmed (Fig. 14). Drawing on the same reason, it is possible to confirm that AASR tends to improve due to the lowering of the sidelobe signal. However, it was also observed that it worsened according to the PRF because it was affected by the grating lobe (Fig. 15). As the receiver approached the target, the slant range decreased but the range difference of the ambiguity signal did not change. Consequently, the angle of receiving the ambiguity signal was relatively far away compared to the transmitter. Hence, the tendency of improvement in RASR in a bistatic SAR system is confirmed.

On the other hand, as depicted in Fig. 14(b), the ground range resolution deteriorated. As the total slant range from the target decreased, the incidence angle had to be lowered to observe the same target, resulting in deterioration in the ground range resolution. Through this performance analysis of the bistatic SAR system, the quality of the images it is capable of producing in specific scenarios was verified before moving on to modeling the system.

These simulations confirm that most performance variables tend to improve with an increase in the baseline (with the position of the receiver approaching the target), except for ground range resolution. Based on this finding, an ASR simulation was conducted to verify whether the same trend persists as the baseline increases. Fig. 17 displays the ASR results when the baseline is set to 10 km, 30 km, 50 km, 70 km, and 100 km.

Fig. 17

ASR simulation results according to the baseline: (a) RASR and (b) AASR.

The figures above show that the ASR improved with an increase in the baseline, consistent with our analysis results. However, the RASR in Fig. 17(a) shows a different tendency when the PRF is lower than 4,500 Hz. This observation can be attributed to variations in the angle at which the ambiguity signal is received, depending on the PRF. The ambiguity signal received in the given scenario was near the null of the antenna pattern. If the angle at which the ambiguity signal is received corresponds to the null of the antenna pattern, the RASR improves. In contrast, if it corresponds to the peak of the upper lip, the RASR worsens. As a result, the influence of the baseline was analyzed using the PRF band (4,500–5,000 Hz), since it is less affected by the antenna pattern.

IV. Conclusion

In this study, a performance analysis of the bistatic staring spotlight mode was conducted. This study lays the theoretical foundations for designing a bistatic SAR system that involves sophisticated design variables. To ensure precision, theorization, simulation, and verification of the monostatic staring spotlight mode were first conducted, with regard to previous studies in the area. Subsequently, a simulation of the monostatic staring spotlight mode was carried out, and its results were compared to those attained by previous studies in the literature. After verifying the simulation results for the monostatic staring spotlight mode, performance analysis of the bistatic SAR system was conducted, especially accounting for its geometry. The performance variables were analyzed based on the bistatic geometry, and a performance analysis formula for the bistatic staring spotlight mode was devised. Using this formula, we analyzed a bistatic SAR system scenario considering a 30 km baseline. Notably, this scenario was created by expanding the TerraSAR- X scenario—a monostatic SAR system consisting of previously acquired images. The performance comparison results are presented in Figs. 13–16. The NESZ, ASR, and azimuth resolution all tended to improve, while the ground range resolution tended to deteriorate. This finding can be attributed to the incidence angle of the receiver being lower than that of the transceiver, since the receiver was positioned close to the target. Notably, before conducting the bistatic simulation, the results of the monostatic SAR system were confirmed to be consistent with the expected results, since the lower the incident angle, the similar the trend when the slant range is the same.

In this study, the performance of the bistatic staring spotlight mode was analyzed using a receiver at the same altitude and azimuth coordinates as the transceiver. The results of this study, in conjunction with those of previous research [20] conducted on the bistatic stripmap mode, offer basic knowledge and lay the foundation for further study on various bistatic scenarios in the future.

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