An ideal target for geodetic very long baseline interferometry (VLBI) is a strong and point-like radio source. In reality, most celestial sources used in geodetic VLBI have spatial structure. This is as a major source of error in VLBI Global Observing System (VGOS) and also affects legacy S/X observations. Source structure causes a systematic delay, which can affect the geodetic estimates if not modelled or otherwise accounted for. In this work, we aim to mitigate its impact by extending the stochastic model used in the least-squares fitting of the VLBI group delays. We have developed a weighting scheme to re-weight the observations by parameterizing the source structure component in terms of closure delays and jet orientation relative to the observing baseline. It was implemented in the Vienna VLBI Software. To assess the performance of the extended stochastic model, we analysed the CONT17 legacy sessions and generated suitable reference solutions for comparison. The effects of re-weighting were evaluated with respect to the session fit statistics, source-wise residuals, and geodetic parameters. We find that this relatively simple noise model consistently improves the session fit by about 5% with moderate variation from session to session. The geodetic estimates are not affected to a significant level by this new weighting method. Source-wise we see improved post-fit residuals for 63 out of a total of 91 sources observed.

大地测量甚长基线干涉测量 (VLBI) 的理想目标是强点状射电源。实际上，大地测量 VLBI 中使用的大多数天体源都具有空间结构。这是 VLBI 全球观测系统 (VGOS) 的主要误差源，也会影响传统的 S/X 观测。源结构会导致系统延迟，如果不进行建模或以其他方式进行解释，可能会影响大地测量估计。在这项工作中，我们的目标是通过扩展 VLBI 群延迟最小二乘拟合中使用的随机模型来减轻其影响。我们开发了一种加权方案，通过根据闭合延迟和相对于观测基线的喷流方向参数化源结构组件来重新加权观测。它是在维也纳 VLBI 软件中实现的。为了评估扩展随机模型的性能，我们分析了 CONT17 遗留会话并生成了合适的参考解决方案进行比较。根据会话拟合统计、源残差和大地测量参数评估重新加权的效果。我们发现，这种相对简单的噪声模型始终将会话拟合度提高了约 5%，并且会话之间存在适度的变化。这种新的加权方法不会对大地测量估计产生显着影响。从源角度来看，我们发现在观察到的 91 个源中，有 63 个源的拟合后残差得到了改善。