GPS/LEO无线电掩星开环反演技术

Processing of GPS/LEO radio occultation recorded in open-loop mode

在低对流层,传统的闭环模式容易造成较大的信号追踪误差.开环模式通过内部或外部方式消除导航资料调制(navigation data modulation,NDM)对大气反演的影响,能够准确地恢复信号的相位.内部和外部方式的区别是,前者通过信号相邻采样的内在联系移除NDM,后者利用卫星导航位元数据进行处理.通过内部和外部两种方式处理COSMIC(constellation observing system for meteorology,ionosphere and climate)掩星数据,利用全谱反演方法获得弯曲角,通过Abel积分变换计算折射率.COSMIC掩星个案分析表明,在低对流层,内部方式可能会导致信号的半周跳现象,从而造成反演的折射率出现误差.分别用上述两种方式对2007年第71天至73天共约3130个COSMIC掩星开环数据进行处理.将折射率反演结果与ECMWF(european centre for medium-range weather forecasts)分析场资料进行统计比较,结果显示:外部处理方式的探测深度比内部方式低100—200 m.在热带的3 km以下,内部方式具有比外部方式稍大的折射率负偏差.因此,外部方式能够得到比内部方式略优的反演结果.

In the moist lower troposphere, traditional phase-locked loop tracking of GPS （Global Position System） signal will introduce errors in the retrieved results. Both internal and external method can be used to remove navigation data modulation （NDM） and accurately recover the phase of GPS signals recorded in open-loop mode. The internal method removes the NDM using relationship between adjacent samples, while the external method removes the NDM using the externally recorded NDM bit data. Both above- mentioned methods are used to process the constellation observing system for meteorology, ionosphere and climate （COSMIC） radio occultation data. COSMIC case study shows that internal method may include half-cycle slip in the recovered phase, which will lead to error in the retrieved refractivity. About 3130 COSMIC radio occultation soundings recorded in open-loop mode are processed by the internal and external method, and statistical comparison results of retrieved refractivity with that from the European Centre for medium-range weather forecasts （ECMWF） analyses show that the external method has a better penetration ability than the internal method in the lower moist troposphere. The mean of fractional difference in refractivity retrieved by external method is smaller than that by internal method below about 3 km.