Abstract
Abstract The same beam VLBI method (SBV) is newly applied to the multi-frequency VLBI method in the VRAD mission of SELENE (KAGUYA). By simultaneously observing two nearby spacecraft with one antenna, the error sources of VLBI measurement common in two propagation paths can be almost canceled out. In this paper, error estimation and simulation analysis are carried out for a feasibility study to apply the SBV method to the VRAD mission. Differential phase delay can be estimated without cycle ambiguity even if tropospheric fluctuation is large and/or traveling ionospheric disturbance occurs. The sensitivity of the differential phase delay with respect to the average elevation angle and the elongation of two spacecraft is also investigated. Moreover, a method is developed for estimating differential phase delay in switching VLBI observations using the cycle ambiguity derived from SBV observations. This method can be performed in more than 90% of the VRAD mission’s total paths. Precise positioning with SBV contributes to accurate estimation of the low degree coefficients of lunar gravity fields by more than one order of magnitude than previous results.
Highlights
The total electron content (TEC) error condition of 0.23 TECU can be satisfied when Elave is larger than 17 degrees even if traveling ionospheric disturbance (TID) occurs in the ionosphere above the VLBI station
Bands When the climate condition is rainy, as discussed in Section 4.2.2 and/or TID occurs in the ionosphere above the VLBI station, as discussed in Section 4.3.2, satisfying the conditions of differential phase delay estimation by multi-frequency VLBI (MFV) method is impossible
The evaluation for error sources of same beam VLBI observations are carried out, and it is shown that the conditions of the MFV method can be satisfied even if tropospheric fluctuation is large and/or TID occurs in the ionosphere above the VLBI station
Summary
The VLBI technique has been applied to spacecraft tracking since the 1960s (e.g., Border et al, 1992; Sagdeyev et al, 1992). VLBI observations of spacecraft have been used for deep space missions of NASA and ESA, for example, the orbit determination of Mars Odyssey during its interplanetary cruise (Antreasian et al, 2002; Thornton and Border, 2003). The accuracy of group delay is not sufficient for precise lunar gravity field estimation. In order to estimate the low degree coefficients of lunar gravity fields by more than one order of magnitude than previous results, phase delay estimation whose accuracy is expected to be several ps is needed (Hanada et al, 2002)
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