Improved Minimum Detectable Velocity in Bistatic Space-Based Radar

TSINGHUA SCIENCE AND TECHNOLOGY ISSN 1007-0214 05/19 pp30-34 Volume 13, Number 1, February 2008

Improved Minimum Detectable Velocity in Bistatic Space-Based Radar LI Hua (李 华), TANG Jun (汤 俊), PENG Yingning (彭应宁)** Department of Electronic Engineering, Tsinghua University, Beijing 100084, China Abstract: Single orbit bistatic space-based radar (SBR) is composed of two radars in the same orbit. The characteristics of the clutter Doppler-angle spectrum of a single orbit bistatic SBR show that the slope of the mainbeam clutter spectrum is highly sensitive to the cone angles. Therefore, the minimum detectable velocity of the bistatic system is dependent on the cone angle. Then a new combined working mode of singleorbit bistatic SBR system was developed in which one radar will act as the transmitter and another as the receiver to improve detection performance for all angles. Simulation results by space-time adaptive processing verify the improved detection performance. The new design also reduces the average power of each radar system and the size and weight of the on-board solar array-battery system. Key words: bistatic space based radar; minimum detectable velocity; clutter; space-time adaptive processing

Introduction Recently, there has been increasing research interest in monostatic/bistatic space-based radar (SBR) systems which provide continuous access to important tactical areas as well as rapid surveillance of large regions on the ground which are unreachable by airborne platforms[1-4]. The proper bistatic angle to the target in a bistatic SBR may increase the radar cross section of the target, which can help improve detection performance for a stealthy target[5-7]. This analysis focuses on a single-orbit bistatic system, in which the transmitter and receiver are placed on two separate satellites in the same orbit whose relative positions are fixed. Various space-time adaptive processing (STAP) methods have been developed to suppress clutter in these downlooking radar systems[8-13]. However, due to the high platform velocity of the low earth orbit satellite, the mainlobe clutter Doppler spread greatly affects the Received: 2006-09-25; revised: 2006-11-05

﹡﹡To whom correspondence should be addressed. E-mail: [email protected]; Tel: 86-10-62781375

performance of ground moving target indication. Clutter Doppler-angle spectrum are useful in analyzing the clutter characteristic in airborne radar systems[14-16]. In this paper, the clutter Doppler-angle spectrum is analyzed in a single-orbit bistatic SBR system, then the minimum detectable velocity (MDV)[8] performance of the STAP method is analyzed. The results show that the MDV performance of a single-orbit bistatic SBR system varies with the mainbeam cone angle. A combined working mode was developed to improve the MDV performance of a single-orbit bistatic SBR system.

1

Doppler Frequency for Bistatic Radar

In a single-orbit bistatic SBR, the transmitter (T) and the receiver (R) are separated by a central angle α. For convenience, assume that the transmitter and receiver radar parameters are identical, as listed in Table 1. In both the transmitter and receiver, the antennas are uniform linear array antenna (ULA) placed along the velocity direction of the satellite. The ULAs work in

LI Hua (李 华) et al：Improved Minimum Detectable Velocity in Bistatic …

side-looking mode. The clutter Doppler-angle spectrum is defined as the clutter Doppler frequency as a function of cosine cone angles φR s for specified bistatic range, where φR is the cone angle of the clutter patch with respect to the receiver array axis.

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a large portion of the Doppler space, which will greatly affect the MDV performance of the STAP method. Therefore, in bistatic SBR, the STAP performance is highly sensitive to the detection angle, which is quite different from that in the monostatic SBR case.

Table 1 Radar parameters Symbol

Description

Value

N

Element number

80

M

Pulses per CPI

8

d

Azimuth channel spacing (m)

0.15

λ

Wavelength (m)

0.3

fr

Pulse repetition frequency (Hz)

2469.6

H

Platform altitude (km)

850

V

Platform velocity (m/s)

7408

In a monostatic radar, i.e. α = 0, the normalized clut2V cos φR ter Doppler frequency is , the clutter Dopλ fr pler-angle spectrum is a straight line with slope β ,

β=

2V λ fr

(1)

which shows that β is independent of the cone angles φR . For bistatic SBR, the motion and geometry of both the transmit and receive platforms contribute to the observed clutter Doppler frequency, the normalized Doppler frequency, fd, is then V cos φR VT cos φT + (2) fd = R λ fr λ fr

Fig. 1 Clutter Doppler-angle spectrum for monostatic and bistatic SBR

2

Analysis of the MDV for Bistatic SBR

The clutter Doppler-angle trace for a specified bistatic range shown in Fig. 1 is for a transmitter flying ahead of the receiver with α of 10°, as shown in Fig. 2. The trace varies sharply in directions where cos φR is large. The slope, β , is plotted versus cos φR in Fig. 3 for three bistatic ranges where L denotes the bistatic range. As expected, β increases as cos φR increases.

where VR and VT are the receiver and transmitter velocities, respectively; φT is the cone angle with respect to the transmitted array axis. Obviously, in a specified bistatic range gate, the clutter Doppler-angle spectrum will be a complex curve instead of a straight line. An example of this curve is shown in Fig. 1. The unfolded normalized Doppler frequency is plotted in this figure because the main focus is the slope of the traces. The trace slope β is equal for all directions with monostatic SBR, therefore, the mainlobe clutter spread, ∆f d , is equal in all directions. However, β varies with direction in bistatic SBR as shown in Eq. (2) when β is dependent on cos φR . If cos φR corresponds to a large β , the mainlobe clutter will occupy

Fig. 2 Single-orbit bistatic SBR working mode

Tsinghua Science and Technology, February 2008, 13(1): 30-34

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Figure 6 shows that MDV performance deteriorates as β increases. It compares the SINR loss (Lsinr) when detecting targets in different directions with cos φR equal to 0.64, 0, and –0.42 for bistatic SBR

Fig. 3

Value of β in a bistatic radar

However, if the receiver is in front of the transmitter with the same size α , i.e. positions R′ and T ′ in Fig. 2, then φR′ is in fact equal to φT , where φR′ is the cone angle with respect to the array axis of R′ . The Doppler and β traces are plotted in Figs. 4 and 5 for the same three specified bistatic range mentioned above. The traces vary sharply in directions where cos φR is small, with β decreasing as cos φR increases.

Fig. 4

with the optimum STAP under the same condition mentioned above. Here, for bistatic SBR, the transmitter is flying ahead of the receiver and α is 10°. All the central frequencies of the curves were normalized to 0 Hz to compare the width of the notches. Assume the target velocity corresponding to Lsinr= −5 dB of the mainlobe clutter notch as the MDV. The narrower the notch is, the better the MDV is. In the monostatic case, since β is identical for all directions, the three curves overlap and the MDV performances are the same, so they are not plotted in Fig. 7. For the bistatic range of 3.3×106 m, as cos φR increases, the notch becomes wider, which verifies the conclusion that the MDV performance of larger cosine angles is worse than that of smaller cosine angles. At large cos φR , the MDV performance is badly degraded.

Fig. 6 MDV performance for different angles for bistatic SBR

Doppler traces for bistatic radar

However, as illustrated in Fig. 5, if the transmitter is behind the receiver, β decreases as cos φR′ increases, which means better MDV performance at larger cosine angles.

3

Fig. 5 β traces for bistatic radar

New Combined Bistatic SBR Mode

Since both radar systems are in the same orbit and their relative positions are fixed, if they can switch their working mode in turn, the MDV performance can be improved for all directions. For example, if the system seeks to detect an area corresponding to a small cos φR , then the lead radar should be the transmitter

LI Hua (李 华) et al：Improved Minimum Detectable Velocity in Bistatic …

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and the latter radar should be the receiver. If the system seeks to detect an area corresponding to a large cos φR , then the lead radar should be the receiver and the latter should be the transmitter. The transmitter and the receiver will exchange their functions to maintain a small β for all directions. For instance, suppose the system wants to detect the direction of point P in Fig. 2, if the original configuration is used where the transmitter is flying ahead with cos φR = 0.64 , then the results in Fig. 3 show that β is 28.9. However, if the configuration is reversed with the receiver flying ahead with cos φR′ = 0.1 , from Fig. 5 β is reduced to 15.5. The mainlobe clutter Doppler spread will then be greatly reduced, which will lead to better MDV performance. Figure 7 compares the MDV performance for two configurations. The MDV performance is remarkably improved with the combined working mode where the two-radar systems cooperate to cover all directions. An angle threshold, cos φthre , can be calculated to determine when the two-radar systems should exchange functions. For a specified bistatic range, a plot of β versus cos φR for a specified bistatic range for the two modes will be like that in Fig. 8. cos φthre is the intersection of the two lines. When cos φR > cos φthre , the system should work with the transmitter behind the receiver, which will always ensure β less than 20, which will give good MDV performance for the bistatic SBR. cos φthre derived for various bistatic ranges in this way will decrease slightly as the bistatic range increases.

Fig. 8

cosφthre for a specified bistatic range

Besides the improved MDV performance, this combined working mode may offer other advantages. Power is one of the most important issues in spacebased systems. The two radars transmiting signals alternatively will save energy greatly and therefore may reduce the size and weight of the solar array-battery system. In addition, this mode can maintain the MDV performance for nearly all cone angles without adding additional antenna channels, which will reduce the launch vehicle costs and the complexity of the onboard processing system.

4

Conclusions

This paper describes the characteristics of the clutter spectrum of a single-orbit bistatic space based radar system where the slope of the mainbeam clutter spectrum is highly sensitive to the cone angle. Therefore, the minimum detectable velocity of the bistatic SBR system is dependent on cone angle. A combined working mode of a single-orbit bistatic SBR system that maintains the MDV performance in all directions was developed. Simulation results verify the effectiveness of this new method. References [1]

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