In recent years, unmanned aerial vehicles (UAVs) have emerged as a transformative technology that can revolutionize various industries and enable a wide range of versatile aerial applications. To facilitate future generations of computing-intensive applications, we have developed a UAV-based airborne computing platform. However, the existing system only supports Wi-Fi connectivity. Recognizing the critical role of Long-Term Evolution (LTE) in providing robust and reliable wireless communication, as well as its compatibility with current and future cellular systems, we present an improved airborne computing platform that combines UAV technology with software-defined radio (SDR). SDR is programmable and can be dynamically configured to enable different wireless communication technologies. Specifically, by using an appropriate SDR module and open-source software, we enhanced the connectivity of our platform by setting it as an LTE base station, thus enabling seamless communication between UAVs and LTE-enabled ground devices. In this paper, we describe the detailed design and implementation of our UAV-based airborne computing platform, emphasizing the integration of SDR technology to build an LTE base station. We also conduct extensive experiments to evaluate the performance of our platform in both indoor and outdoor scenarios, focusing on latency, throughput, signal-to-noise ratio, and resource consumption. Our findings highlight the impact of the computing platform on system performance and emphasize the importance of optimizing software components for specific hardware configurations. Despite the challenges, our results demonstrate the feasibility and potential of SDR-enabled UAV platforms for enhanced aerial connectivity. This work opens up new possibilities for UAV applications, such as edge computing, real-time object detection, remote sensing, and disaster management, all facilitated by the robust and reliable LTE connectivity provided by our UAV-based LTE base station.