3960 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 46, NO. 12, DECEMBER 2008 Improved Space-Based Moving Target Indication via Alternate Transmission and Receiver Switching Joachim H. G. Ender, Senior Member, IEEE, Christoph H. Gierull, Senior Member, IEEE, and Delphine Cerutti-Maori Abstract—Ground moving target indication (GMTI) by space- based radar systems requires special antenna and data acquisi- tion concepts to overcome the problem of discriminating target signals from clutter returns. Owing to the high satellite speed, the clutter contains a broad mixture of radial velocities within the antenna beam, leading to a large Doppler spread. Effective clutter suppression can solely be achieved by multiple aperture or phase center antennas. For space-based systems, however, the number of receiver channels connected to subapertures is very limited due to their weight, volume, and high data rates (current systems such as TerraSAR-X and RADARSAT-2 possess only two). This classical along-track interferometry architecture, in which the antenna is split into two halves, achieves only suboptimum GMTI performance. This paper presents and statistically analyzes an innovative approach to create additional independent phase centers to improve the GMTI performance considerably. The ex- tra degrees of freedom are created by cyclical phase and amplitude switchings of the transmit/receive modules for transmitter and receiver between pulses, hence trading Doppler bandwidth for extra spatial diversity. In the first part of this paper, different strategies of spatial–temporal diversity are introduced and ana- lyzed for realistic system parameters with respect to ambiguities and detection performance. The second part is concerned with the elaborate theoretical analysis of the relocation improvement for the spatial diversity approach. Index Terms—Electronic switching systems, radar detection, radar signal analysis, radar velocity measurement, spaceborne radar, synthetic aperture radar, traffic information systems. I. I NTRODUCTION I N THE synthetic aperture radar (SAR) community, the measurement of motion within a scene is generally related to along-track interferometry (ATI) [1], which is based on the complex conjugated product of two focused SAR images from a pair of along-track subapertures. The motion of scatterers expresses itself as an interferometric phase difference, whereas the phase of the stationary background ideally cancels out. Manuscript received December 3, 2007; revised April 11, 2008 and June 24, 2008. Current version published November 26, 2008. This work was supported in part by the German Federal Ministry of Defence (BMVg) and in part by the Federal Office of Defense Technology and Procurement (BWB). J. H. G. Ender and D. Cerutti-Maori are with the Research Institute for High Frequency Physics and Radar Techniques, Research Establishment for Applied Science, 53343 Wachtberg, Germany. C. H. Gierull is with the Defence Research and Development Canada– Ottawa, Ottawa, ON K1A 0Z4, Canada, and also with the Research Institute for High Frequency Physics and Radar Techniques, Research Establishment for Applied Science, 53343 Wachtberg, Germany. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TGRS.2008.2002266 Recently, the detection of ground moving targets (GMTs) has generated interest for civilian traffic monitoring, e.g., [2], [23], and [30], as well as for military surveillance and reconnais- sance, e.g. [16], [25], and [32]. The problem of GMT indica- tion (GMTI) is twofold: first, the detection of moving targets within severe ground clutter and, second, the estimation of their parameters, such as velocity and exact location. Owing to the ambiguity between radial velocity and azimuth position, the SAR processor images the moving object at an incorrect position. Repositioning can be performed by the exploitation of the ATI phase; however, if the clutter contribution from the affected resolution cell is not negligible compared to the signal power, a severe estimation error will result [17]. This error may be intolerable, particularly for space-based systems with their extreme ranges to the ground. Without or with only small clutter interference, ATI offers a basis for the simultaneous estimation of radial velocity and azimuth position. The statistics of interferograms has been analyzed in depth in [16] and [18], and some aspects concerning space-based MTI for TerraSAR-X can been found in [23]. For medium or strong clutter interference, it is necessary to directly use raw data rather than the final image; sufficient performance can only be achieved with more than two parallel receiver channels or phase centers. It has been shown that a larger number of subapertures significantly improves the performance by exploiting the full space-time adaptive processing (STAP) in conjunction with SAR [10], [16]. However, a larger number of parallel receiver channels are not attractive for space-based systems due to severe weight, power consumption, and data rate restrictions. This paper is concerned with monostatic space-based SAR/ GMTI systems, such as the recently launched RADARSAT-2 [16], TerraSAR-X [30], and Cosmo-Skymed [27], which each having an experimental two-channel GMTI mode onboard. These satellites employ phased array technology which offers the opportunity to vary the transmit/receive (T/R) modules’ phases and amplitudes of transmitter and receiver indepen- dently from pulse to pulse. Whereas alternating transmission from different parts of the antenna was, in the past, some- times denoted as toggling, e.g., [14], we will refer to the receiver equivalent as switching. This capability facilitates the application of innovative spatial–temporal diversity concepts to virtually increase the number of independent phase centers by trading off unambiguous bandwidth of the system [9], [12]. Hence, additional degrees of freedom can be introduced, increasing the GMTI performance considerably, such that this 0196-2892/$25.00 © 2008 IEEE