The Akebono satellite observed the Australian Omega signals when it passed about 1000km over the Omega station. In this paper, we compare the observed Omega signal intensities with the values obtained using a full wave calculation and we discuss a mechanism of modulation of the signals. The relative spatial variations of the calculated Omega intensities are quite consistent with those observed, but the absolute calculated intensities themselves are several dB larger than the observed intensities. This difference in intensity may be due to the horizontal inhomogeneity of the D region, which is not modeled in the full wave calculation, or to an incorrect assumption about radiation characteristics of the Omega antenna. It is found that modulation of the observed signals is caused by the interference between the waves with different k vectors.

Voyager Neptune radio science data were collected using three antennas on Earth on August 25, 1989. A parabolic antenna at Canberra, Australia, of 70 meter diameter received 2.3GHz and 8.4GHz carriers. The 64 meter parabolic antennas at Parkes. Australia and Usuda, Japan, received only the 8.4GHz and only the 2.3GHz carriers, respectively. It is necessary to reduce the frequency variation in the received signal carrier to extract accurate information on physically interesting objects such as Neptune's atmosphere, ionosphere, or the rings. After the frequency stabilization process, the frequency drift was reduced from 180Hz down to a maximum of 5Hz, making it possible to reduce the data bandwidth and, consequently, the data volume, by a factor of 30. The uncertainty of the signal frequency estimates were also reduced from 5 down to 510-3Hz/sec above the atmosphere, from 5 down to 0.5Hz/sec in the atmosphere, and from 50 down to 3Hz/sec at the beginning and the end of the atmospheric occultation. Much of the remaining uncertainty is due to scintillations in Neptune's atmosphere and cannot be reduced further. The estimates are thus meaningfully accurate and suitable for scientific analysis and coherent arraying of data from different antennas. Two results based on these estimates are shown: a preliminary temperature-pressure (T-p) profile of Neptune's atmosphere down to a pressure level of 2 bar computed using the Usuda 2.3GHz data, and a multipath phenomenon in the atmosphere seen in Canberra 8.4GHz data. Our T-p profile shows good agreement with the results presented by Lindal et al. within 1K below 100mbar pressure level, even though our result is based on an independent data set and processing. A comparison of the multipath phenomena at Neptune with that at Uranus implies that it was created by a cloud layer with a smaller scale height than the atmosphere above and below it. The processing methods described were developed in part with the interest to coherently array Canberra, Parkes and Usuda data. In this sense, while this paper does not extend any science results, the observations and results are derived independently from other published results, and in the case of Usuda, are completely new.