###### Abstract  
The PIESAT-01 constellation is the world's first multi-baseline distributed synthetic aperture radar (SAR) constellation with a \"Cartwheel\" formation. The \"Cartwheel\" formation is a unique formation in which four satellites fly in companion orbits, ensuring that at any given moment, the main satellite remains at the center, with three auxiliary satellites orbiting around it. Due to this unique configuration of the PIESAT-01 constellation, four images of the same region and six pairs of baselines can be obtained with each shot. So far, there has been no imaging and interference research based on four-satellite constellation measured data, and there is an urgent need to explore algorithms for the \"Cartwheel\" configuration imaging and digital surface model (DSM) production. This paper introduces an improved bistatic SAR imaging algorithm under the four-satellites interferometric mode, which solves the problem of multi-orbit nonparallelism in imaging while ensuring imaging coherence and focusing ability. Subsequently, it presents an interferometric processing method for the six pairs of baselines, weighted fusion based on elevation ambiguity from different baselines, to obtain a high-precision DSM. Finally, this paper selects the Dingxi region of China and other regions with diverse terrains for imaging and DSM production and compares the DSM results with ICESat-2 global geolocated photon data and TanDEM DSM data. The results indicate that the accuracy of PIESAT-01 DSM meets the standards of China's 1:50,000 scale and HRTI-3, demonstrating a high level of precision. Moreover, PIESAT-01 data alleviate the reliance on simulated data for research on multi-baseline imaging and multi-baseline phase unwrapping algorithms and can provide more effective and realistic measured data. PIESAT-01 constellation; \"Cartwheel\" formation; improved bistatic SAR imaging; DSM generation and multi-baseline fusion 2  
all terrestrial regions between 60\({}^{\circ}\) N and 56\({}^{\circ}\) S. The STRM DEM met the standards of high-resolution terrain information-2 (HRTI-2), attesting to the mission's achievement in providing high-resolution terrain information [7; 8]. The Deutsches Zentrum fur Luft- und Raumfahrt (DLR) successively launched the TerraSAR-X satellite and the TanDEM-X satellite in 2007 and 2010, respectively [9]. These two satellites constitute the world's first spaceborne formation-flying distributed InSAR system, conducting over three years of high-precision global land DEM measurement missions, and the TanDEM DEM met the HRTI-3 standard [10]. Compared to the SRTM mission, the TerraSAR-X constellation extends the coverage of terrain mapping globally, thus serving as the new generation global DEM foundational dataset. Subsequently, Subsequently, more countries commenced launching satellites to accomplish their respective mapping missions. For instance, Canada launched RADARSAT-2 in 2007 [11], Italy launched COSMO in 2007 [12], and Japan launched ALOS-2 in 2014 [13]. Imaging is the most crucial task in the ground processing of SAR satellites. In 1998, [PERSON] first discussed the lack of distinct Doppler spreading in the echo signals, which made it difficult to utilize conventional pulse or FM-CW SAR imaging algorithms. [PERSON] proposed time domain correlation (TDC) processing and Fourier-based methods for image formation in WB-CW monostatic and bistatic SAR systems [14]. In 2009, [PERSON] et al. introduced a new algorithm based on complex empirical mode decomposition (ECMD), focusing on the suppression of narrow band interference (NBI) in SAR imaging. By using traditional SAR imaging algorithms to separate target echoes, focused SAR images could be obtained from the suppressed NBI signals [15]. Later, in 2013, [PERSON] et al. proposed a simple method for focusing phase history data in SAR imaging [16]. By 2015, [PERSON] et al. introduced a new SAR imaging algorithm based on compressed sensing (CS), effectively reducing the number of measurements required for SAR data acquisition [17].