Aiming at the new CNAV/CNAV-2 navigation message format in the GPS modernization process, the current GPS navigation message format and the new format in data expression were compared. To evaluate the navigation and positioning influence of different data truncated ways, the concept of truncation er ror was brought up to analysis the problem. The actual RINEX navigation data was utilized for generation, pro cessing two message structures; range errors caused by ephemeris and clock parameters truncation were com pared between NAV and CNAV/CNAV-2 message. Results show that CNAV/CNAV-2 range error caused by parameters truncation is smaller than NAV message. NAV message is more sensitive to truncation errors. Meanwhile, to CNAV/CNAV-2 message ephemeris parameters truncation can improve range error up to a cen timeter level, clock parameters truncation can improve range error up to a decimeter level. Thus, clock repre sentation accuracy should be improved with priority in limited bandwidth circumstance. More significant im provements of the accuracy can be achieved by adjusting the representation of the satellite clock parameters.%针对在GPS现代化进程中将出现的新的CNAV／CNAV．2导航电文格式，比较了目前发播的GPSNAV导航电文与新电文在数据表达方式上的差别．为了衡量不同的数据截断方式对于导航定位的影响，引用导航数据截断测距误差的概念进行分析．利用实际的导航数据对于两种电文结构进行了仿真生成、处理，比较星历／星钟参数截断带来的测距误差影响．结果表明：相比于NAV电文，CNAV／CNAV-2由于截断所造成的测距误差更小，NAV电文对截断误差更敏感．其中星历参数截断的测距误差改善可达厘米级，星钟参数截断的测距误差改善可达分米级．因此，在带宽有限的条件下应优先考虑提高星钟参数的表示精度．改进星钟参数的表达形式可望获得更高的精度提升．
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