In this paper, we propose a solution of designing a topside broadcast ionospheric model to enable the future low earth orbit (LEO) navigation augmentation (LEO-NA) services. Considering the lack of global station observations to develop the LEO-NA ionosphere model, we utilize abundant global navigation satellite system (GNSS) data from LEO satellites to determine the topside global broadcast ionospheric delay. This delay can be combined with existing GNSS broadcast ionospheric delay correction models to determine LEO-NA ionospheric delay. First, the performance of the different-order spherical harmonic (SH) model is evaluated in generating a global topside ionospheric map. The results indicate that by increasing the order from 1 to 2, the internal and external accuracy of the model improves significantly. However, increasing the order from 2 to 8 leads to a decrease in accuracy of 0.10 and 0.11 TECU (total electron content unit) for the internal and external root mean square error. Taking into account compatibility with the Beidou global ionospheric delay correction model, limited data capacity in the navigation message, ionospheric model accuracy, and computational efficiency, we select the second-order SH model as the topside ionosphere broadcast model and outline the strategy for calculating broadcast coefficients. Finally, the accuracy of the topside global broadcast ionospheric delay correction model is evaluated during periods of high and low solar activity. The mean values of root mean square in 2009 and 2014 are 1.49 and 1.88 TECU, respectively. The model in 2009 and 2014 can correct for 67.30% and 72.49% of the ionospheric delay, respectively.

在本文中，我们提出了一种设计上部广播电离层模型的解决方案，以实现未来的低地球轨道（LEO）导航增强（LEO-NA）服务。考虑到缺乏全球站观测来开发 LEO-NA 电离层模型，我们利用来自 LEO 卫星的丰富的全球导航卫星系统 (GNSS) 数据来确定上部全球广播电离层延迟。该延迟可以与现有的 GNSS 广播电离层延迟校正模型相结合，以确定 LEO-NA 电离层延迟。首先，评估不同阶球谐（SH）模型在生成全球上部电离层图时的性能。结果表明，通过将阶数从 1 增加到 2，模型的内部和外部精度显着提高。然而，将阶数从 2 增加到 8 会导致内部和外部均方根误差的精度降低 0.10 和 0.11 TECU（总电子含量单位）。考虑到与北斗全球电离层延迟修正模型的兼容性、导航电文数据容量有限、电离层模型精度和计算效率等因素，选择二阶SH模型作为上部电离层广播模型，并概述了广播计算策略系数。最后，在太阳活动高和低的时期评估上部全球广播电离层延迟校正模型的准确性。 2009年和2014年均方根平均值分别为1.49和1.88 TECU。 2009年和2014年的模型可以分别校正67.30%和72.49%的电离层延迟。