飞行实验中往往存在很大风险，遥测视频可以使得地面工作人员准确且直观地观察到目标飞行器的状态和位置，对保障飞行实验安全以及降低研发成本有着重要意义。当飞行器发生故障时，为了对最后采集的几帧图像数据进行事后分析，确认故障产生原因，遥测系统对视频编码传输提出了低延时和高鲁棒的要求。传统视频编码算法虽然提高了视频的压缩效率，但难以满足遥测视频的应用需求。

由于视频内容的多样性以及视频编码采用不同预测方式的机制，导致视频帧间比特波动剧烈且码流对误码非常敏感，需要较大的缓冲区来平滑输出码流以匹配遥测信道。而缓冲区的增大将会导致编码延时和需要的存储空间也随之增加，同时在误码率高的无线信道中传输码流，将导致接收到的视频质量下降甚至完全丢失。

针对上述应用限制，本文从码率控制和差错控制方面提出了两种算法并将其融合，相比传统视频编码算法，本文算法降低了编码延时并提高了码流的差错恢复能力，实现了在近一帧缓冲区大小限制下码流平稳输出，更适合航天遥测领域的应用。本文具体的创新点和工作内容如下：

（1）研究了HEVC编码的原理、编码关键技术以及码率控制技术，并针对航天遥测领域进行了适用性分析，确定了遥测视频的编码结构，根据视频编解码延时模型确定了编码延时较高的主要原因。在此基础上，分析了HEVC标准码率控制算法未考虑遥测图像内容特征和缓冲区状态对编码的影响，导致比特分配精度较差且带宽利用率低的问题。

（2）提出了一种基于遥测视频内容的低延时码率控制算法。采用前向图像内容和后向缓冲区联合的码率控制方式调整目标比特分配，前向内容控制利用梯度算子检测视频时空复杂度并结合遥测图像位置特征，对帧层和CTU层比重分配进行调整。后向缓冲区控制通过建立漏桶流量模型来实时计算缓冲区充满度，以此为依据负反馈调节目标比特。

（3）提出了一种基于帧内刷新的低比特波动差错控制方式。采用周期性帧内刷新技术分摊I帧的峰值码率并平滑帧间比特波动，同时截断误差扩散与累积，使得码流具有从中间任意帧开始解码的能力。分析了参考帧数量对帧内刷新差错恢复的影响，调整参考帧数量以提高差错恢复能力。最后结合低延时码率控制算法对I帧和CTU层目标比特重新调整，提高码率控制精度和视频质量。

（4）将低延时码率控制算法和周期性帧内刷新技术融合，与HEVC标准算法进行实验对比。实验结果表明，本文算法在缓冲区大小近一帧的限制下，平滑了帧间比特波动且缓冲区充满度保持在稳定状态，提高了带宽利用率并降低了发送缓冲区延时。在视频码流部分丢失的情况下解码重建视频的PSNR平均提高了1.5dB，提高了差错恢复能力。同时算法的计算复杂度平均下降了25%，降低了编码器延时，进一步提高了视频编码的实时性和鲁棒性。

本文的研究探索了一种遥测视频低延时编码的新方法，使得在遥测低带宽限制下，多路高清遥测视频实时编码传输成为可能，可以应用于各飞行实验、靶场试验和无人机侦察等实际应用中。

There are often great risks in flight experiments, telemetry video can enable ground staff to accurately and intuitively observe the state and position of the target aircraft, which is of great significance for ensuring the safety of flight experiments and reducing research and development costs. When the aircraft fails, in order to analyze the last few frames of image data collected and confirm the cause of the failure, the telemetry system puts forward low-delay and high-robust requirements for video coding transmission. Although the traditional video coding algorithm improves video compression efficiency, it is difficult to meet the application requirements of telemetry video.

Due to the diversity of video contents and the mechanism of different prediction methods for video coding, the video inter-frame bits fluctuate strongly and the bitstream is very sensitive to the error codes, which requires a large buffer to smooth the output bitstream to match the telemetry channel. The increase in the buffer will lead to the increase in coding delay and storage space required, and the transmission of the stream in the wireless channel with a high bit error rate will lead to the degradation of the received video quality or even complete loss.

In view of the above application limitations, this paper proposes two algorithms from the aspects of rate control and error control and integrates them. Compared with the traditional video coding algorithm, the proposed algorithm reduces the coding delay and improves the error recovery ability of the bitstream, and achieves a smooth output of the code stream under the limitation of the buffer size of nearly one frame, which is more suitable for the application of aerospace telemetry. The specific innovation points and work in this paper are as follows:

(1) The principle of HEVC coding is studied, and the applicability analysis is carried out in the field of aerospace telemetry. The coding structure of telemetry video is determined, and the main reason for the high coding delay is identified according to the video coding delay model. On this basis, the influence of the content characteristics and buffer state of the telemetry image on the encoding of the HEVC standard bitrate control algorithm is analyzed, resulting in poor bit allocation accuracy and low bandwidth utilization.

(2) A low-delay rate control algorithm based on telemetry video content is proposed. The target bit allocation is adjusted by the combination of forward image content and backward buffer rate control method. The forward content control uses the gradient operator to detect the space-time complexity of the video and combines the position characteristics of the telemetry image to allocate the proportion of the frame layer and the CTU layer. The backward buffer control calculates the fullness of the buffer in real-time by building a leaky bucket flow model as a basis for negative feedback to adjust the target bits.

(3) A low-bit fluctuation error control method based on intra-frame refresh is proposed. The periodic intra-frame refresh technique is used to share the peak rate of I-frame and smooth the inter-frame bit fluctuation. The error diffusion and accumulation are truncated and the bitstream can be decoded from any frame .The influence of the number of reference frames on the error recovery of intra-frame refresh is analyzed, and the number of reference frames is adjusted to improve the error recovery capability. Finally, combined with the low-delay rate control algorithm, the target bits of the Intra frame and CTU layer are readjusted to improve the rate control accuracy and video quality.

(4) The low-delay rate control algorithm and the intra-frame refresh technique are fused and compared with the HEVC standard algorithm. The experimental results show that the proposed algorithm reduces the inter-frame bit fluctuation and keeps the buffer fullness in a stable state under the limit of the buffer size of nearly one frame, which improves the bandwidth utilization and reduces the buffer delay. In the case of partially lost bitstream, the PSNR of the decoded video is improved by 1.5 dB on average, which improves the error recovery capability. The complexity of the algorithm is reduced by 25% on average, which reduces the encoder delay and improves the real-time and robustness of video coding.

The research in this paper explores a new method for low delay encoding of telemetry video, which makes it possible to transmit multi-channel high-definition telemetry video in real-time under the limitation of low bandwidth, which can be applied to practical applications such as flight experiments, shooting range tests and UAV reconnaissance.