HR: 0800h
AN: G21A-1259
TI: An evaluation of atmospheric path delay correction in differential VLBI experiments for spacecraft tracking
AU: * ICHIKAWA, R
EM: richi(AT)nict.go.jp
AF: Kashima Space Research Center, National Institute of Information and Communications Technology, 893-1 Hirai, Kashima, 3148501 Japan
AU: SEKIDO, M
EM: sekido(AT)nict.go.jp
AF: Kashima Space Research Center, National Institute of Information and Communications Technology, 893-1 Hirai, Kashima, 3148501 Japan
AU: KOYAMA, Y
EM: koyama(AT)nict.go.jp
AF: Kashima Space Research Center, National Institute of Information and Communications Technology, 893-1 Hirai, Kashima, 3148501 Japan
AU: KONDO, T
EM: kondo(AT)nict.go.jp
AF: Kashima Space Research Center, National Institute of Information and Communications Technology, 893-1 Hirai, Kashima, 3148501 Japan
AB: We performed differential VLBI (Δ VLBI) experiments for tracking of the interplanetary spacecraft. Our main goal is to obtain the precise and quasi-realtime navigation technique of the spacecraft using Δ VLBI technique. With VLBI time delay measurements, differenced between the spacecraft and angularly nearby quasars to cancel common measurement errors such as the propagation delays due to the ionosphere and the neutral atmosphere. However, we can't always observe desirable quasars. Unfortunately, sometimes we have no choice but to use quasars which are angularly far from the spacecraft. Then, we tried to evaluate the reduction effect by subtracting the group delays of the reference radio source from those of the spacecraft. Two HAYABUSA Δ VLBI experiments were carried out in order to evaluate reducing propagation delays on October, 2004. The spacecraft HAYABUSA has been flying steadily towards an asteroid named ``Itokawa'' and it will orbit the asteroid as of September 2005, land on it, and bring back a sample from its surface. The HAYABUSA spacecraft and an angularly nearby quasar ``2126-158'' were observed sequentially, not simultaneously, during each period with various time intervals of data acquisition. The maximum angular separations of the spacecraft from the quasar are less than 3 degrees. We estimated the zenith path delay due to the water vapor (ZWD: Zenith Wet Delay) using the data sets of the GPS stations which are adjacent to each VLBI antenna. A principle observable feature of VLBI is the difference in arrival times of radio signals between two stations. Then, we calculated difference between the slant path delays which are values as a ZWDs at each station multiplied by a mapping function. We defined this ``differential wet delay''. If the angular separation is sufficiently small, the differential wet delays for both radio sources are almost equal. Then, these are canceled out by the difference procedure. However, if these are different, the differences between them is added directly to the observables as an error source. According to our analysis, a large difference value of up to 10 cm of the differential wet delay for the Kashima-Uchinoura baseline was estimated in spite of a small separation angle between the HAYABUSA spacecraft and quasar. Such large value was mainly caused by the humid condition around Uchinoura due to the typhoon approaching. Moreover, the east-west direction of the baseline vector and the large difference in elevation angle of radio source between both stations helped to enlarge the difference value.
DE: 1220 Atmosphere monitoring with geodetic techniques (6952)
DE: 1294 Instruments and techniques
DE: 6924 Interferometry (1207, 1209, 1242)
DE: 6964 Radio wave propagation
DE: 6994 Instruments and techniques (1241)
SC: Geodesy [G]
MN: Fall Meeting 2005