Mountain-based GPS Occultation Observation Experiment at Mt Wuling

Mountain-based GPS occultation technique is referred to as receiving the radio signals of GPS satellites with very low elevations and negative elevations using a GPS receiver at the top of high mountain and retrieving the lower atmospheric refractivity profiles. A mountain-based GPS occultation observation experiment is performed at Mt Wuling (40.60°N, 117.48°E, 2118 m) in Hebei Province during August 1—29, 2005. The campaign is organized by China Meteorological Administration, a few other organizations participate in the experiment. Totally 576-hour raw observation data are collected by JAVAD two-frequency GPS receiver provided by Center for Space Science and Applied Research, Chinese Academy of Sciences, and 1136 occultation events are recorded. Out of the total occultation events, 621 are rising occultation events and 515 are setting occultation ones. There are about 2 occultation events observed in one hour on average. Detailed statistics and analyses are made to show features of all the observed occultation events, such as the distribution of the occurring time, the duration, minimum elevations and azimuth. The results are as follows. The distribution of the occurring time of the observed mountain-based occultation events is nearly random uniform, and it seems that there are more observed occultation events for two periods of time, one is from 20:00 (local time) to 22:00, and the other is from 04:00 to 06:00. The duration of most occultation events is from 15 minutes to 20 minutes, and about 18 minutes on average. The range of azimuth of observed occultation events is between 110° and 290°, the peak of the distribution of azimuth is between 180° and 195°. These features are related to some important factors, such as the distribution of GPS satellites' orbits, the location of GPS receiver and the direction which the antenna points to. The minimum elevations of most occultation events are between -3° and -2.5°, the lowest negative elevations of all the events is -4.994° from south direction, which is possibly resulted from the landform around the observation station. The minimum elevations of setting occultation events are lower than that of rising occultation events obviously. It shows that the ability to track rising occultation events of commercial GPS receiver is weak. If the same GPS satellite is occulted, it is occulted mostly from the same azimuth, and their occurring time is close (their differences are usually less than two hour). This is determined by distribution of GPS satellites' orbits, cycle and features of movement. A new effective method is provided by mountain-based occultation observations for monitoring lower atmospheric environment. This emerging technique has potential applications. Above are the first statistic and analytic results from observed data by which reference can be provided for mountain-based occultation observation's operation application.