Hiroo Kunimori

Polarization measurements through space-to-ground atmospheric propagation paths by using a highly polarized laser source in space

The polarization characteristics of an artificial laser source in space were measured through space-to-ground atmospheric transmission paths. An existing Japanese laser communication satellite and optical ground station were used to measure Stokes parameters and the degree of polarization of the laser beam transmitted from the satellite. As a result, the polarization was preserved within an rms error of 1.6 degrees, and the degree of polarization was 99.4+/-4.4% through the space-to-ground atmosphere. These results contribute to the link estimation for quantum key distribution via space and provide the potential for enhancements in quantum cryptography worldwide in the future.

Ground-to-satellite laser communication experiments

Ground-to-satellite laser communication experiments between the optical ground station located in Koganei, central Tokyo, and a low earth orbit (LEO) satellite were jointly performed by the Japan Aerospace Exploration Agency and the National Institute of Information and Communications Technology. In 18 trials during three non-consecutive months, the satellite was acquired and tracked 61 % of the time, when clear or partly-clear conditions were predominant. The optical link was maintained for about 6 minutes when the satellite was visible in spite of the high angular velocity of the satellite. In 3 of the 18 trials, the link was not interrupted due to clouds during the six-minute transit. The failures (39% of the time) occurred when cloudy or rainy conditions were predominant. Fluctuation in the uplink received signal power was minimized by using multiple laser beam transmissions. The measured uplink and downlink bit error ratios were 10-7-10-4. These results demonstrate the applicability of free-space laser communication for not only geostationary earth orbit-LEO optical links but also ground-to-LEO optical links.

Overview of the inter-orbit and the orbit-to-ground laser communication demonstration by OICETS

The experiment results on the inter-orbit laser communications between OICETS and a geostationary satellite and the results of two kinds of orbit-to-ground laser communications between OICETS and ground stations are summarized. The geostationary satellite for the inter-orbit demonstrations is the European Space Agencys geostationary satellite, ARTEMIS, and the ground stations for the orbit-to-ground demonstrations are of the National Institute of Information, and Communications Technology (NICT) in Japan and the German Aerospace Center (DLR), respectively. The descriptions of those experiments contain some statistically analyzed results as well as data samples measured during the demonstrations. The authors present the overview of these demonstration progresses and discuss on the results.

Spin motion of the AJISAI satellite derived from spectral analysis of laser ranging data

The spin rate of the geodetic satellite AJISAI was derived through a spectral analysis of laser range data obtained by precise high-return-rate tracking stations. This new analysis method utilizes the directional dependence of the target optical depth resulting from the sparse distribution of corner cube reflectors on the satellite surface. The third and sixth harmonics of the spin rate were found to be dominant in the pass-by-pass spectral analysis of the full-rate range residuals from multiphoton laser ranging systems. Accumulating hundreds of passes, the analysis results gave the slowdown of AJISAIs spinning from 1997 to 1998. Its trend was almost identical to the conventional flash observation. Although the spin rate derived from this method was not as precise as the conventional one, it can provide global spatial coverage and continuous time coverage to monitor the spin motion. This is a new approach to research satellite orientation and orbit and is applicable to other satellites.

Data analysis results from the KODEN experiments

The first bi-directional laser communications demonstration between the optical ground station developed by the National Institute of Information and Communications Technology (NICT) located in Koganei, Tokyo and the Optical Inter-orbit Communication Engineering Test Satellite (OICETS) was successfully conducted in March, May, and September, 2006. The Kirari Optical communication Demonstration Experiments with the NICT optical ground station (KODEN) were jointly conducted by the Japan Aerospace Exploration Agency (JAXA) and NICT. Data from the uplink and downlink optical communication links were analyzed. For the downlink, the scintillation index agreed well with the theoretical results calculated based on the strong fluctuation theory. The aperture averaging effect was the dominant factor in reducing the variation of the downlink signals. The probability density functions as a function of elevation angles were measured and compared with the theoretical model, showing good agreement. For the uplink, the scintillation index disagreed with the calculated results based on the strong fluctuation theory. The multiple beam effect of the uplink transmission with large beams will have an additional reduction factor, which will help to establish ground-to-satellite laser communication links in the future. Four laser beams transmitted from the optical ground station to the OICETS satellite also helped to reduce the optical signals intensity fluctuation due to atmospheric turbulence.

The center-of-mass correction of the geodetic satellite AJISAI for single-photon laser ranging

Detection timing of laser pulse returned from the satellite AJISAI is modeled for a single-photon laser ranging system. The average distribution of ten thousand return pulses simulated under their model agrees well with the distribution of residuals of full-rate range data taken from Herstmonceux station that uses the single-photon detection concept. Their model indicates that the center-of-mass correction for AJISAI will differ by 28 mm between multiphoton detection and single-photon detection. The ranging data of AJISAI and LAGEOS satellites from 1995-1996 was analyzed to estimate the range bias for each laser station, and Herstmonceux station was found to have a 26 mm difference between AJISAI and LAGEOS. The other multiphoton stations have smaller differences. The results show that the difference between the multiphoton system and the single-photon system is detected and close to that predicted by our AJISAI model.

Displacement measuring technique for satellite-to-satellite laser interferometer to determine Earth's gravity field

We present a new displacement measuring technique with simplicity, robustness, high sensitivity and wide measurement range. A set of a frequency shifter and a voltage–frequency converter is used to lock a homodyne interferometer on the half-bright fringe by eliminating the Doppler fringe resulting from mirror motion. The mirror displacement is directly retrieved from the feedback signal of a fringe control loop. By developing a table-top interferometer, we successfully demonstrated signal recovery without significant degradation. The achieved displacement sensitivity and measurement range of the interferometer were 24 nm Hz−1/2 and 1.3 mm at a Fourier frequency of 0.1 Hz, respectively. This technique was found to have a potential for application to precise displacement measurements. It is, in particular, suitable for a satellite-to-satellite laser interferometer to determine Earths gravity field.

In-orbit measurements of spacecraft microvibrations for satellite laser communication links

Angular microvibrations of platform jitter on the optical inter-orbit communications engineering test satellite are measured in space during ground-to-satellite laser communication links. The microaccelerations are measured by the onboard accelerometers at a sampling rate of 2048 Hz. The angular microvibrations are estimated from the measured microaccelerations using the tracking characteristics of the laser communications terminal and the conversion factor on the basis of microvibration data obtained from ground-based tests. The power spectral density (PSD) of the satellite microvibrations is analyzed by using the fast Fourier transform analysis and the data length is examined according to the frequency resolution of the PSD. The in-orbit measurements of the PSDs are compared with those obtained from the ground test. The angular microvibrational base motion is estimated and a PSD up to 1024 Hz is additionally provided as a database of the real measurement results with previously obtained in-orbit measurements. The measured results will contribute to the angular jitter estimation and the design of a tracking control loop for space laser communication systems in the future.

Tracking and pointing characteristics of OICETS optical terminal in communication demonstrations with ground stations

The tracking and pointing performance of the Laser Utilizing Communications Equipment (LUCE) equipped on the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) is presented. The operation characteristics of LUCE observed in the ground-satellite communication demonstrations are focused on. Since the angular movement of LUCEs optical antenna required in the ground-satellite trials exceeds the specification demanded in the inter-satellite communications, the marginal performance for proper tracking and pointing can be observed. During the ground-satellite communication trials, the ground stations found periodical discontinuances in the optical link. By looking at the LUCEs telemetries, the cause of the repeated breaks is revealed.

Results from Phase-1, Phase-2 and Phase-3 Kirari Optical Communication Demonstration Experiments with the NICT optical ground station (KODEN)

[Abstract] Bi-directional ground-to-satellite laser communication experiments between the optical ground station developed by the National Institute of Information and Communications Technology (NICT) located in Koganei of downtown Tokyo and a low earth orbit (LEO) satellite, the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) called “Kirari”, were successfully performed in March, May and September, 2006. The Kirari Optical Communication Demonstration Experiments with the NICT optical ground station (KODEN) was conducted in cooperation between the Japan Aerospace Exploration Agency (JAXA) and NICT. The ground-to-OICETS laser communication experiment is the first in-orbit demonstration with respect to the LEO satellite. Phase-1 laser communication experiment was conducted in March, 2006 and the downlink bit error ratio (BER) of around 1e-5 was measured. Phase-2 laser communication experiment was done in May, 2006 but there was no significant improvement due to bad weather. The uplink and downlink BERs of 1e-4 ~ 1e-7 were simultaneously measured in Phase-3 in September, 2006 with an uplink of 2 Mbps and a downlink of 50 Mbps, respectively. Eighteen trials over the course of three months were conducted during the test campaign. There were eleven successful passes out of all the trials. Acquisition and tracking of the OICETS satellite were always successful when partly clear skies were predominant, showing the optical link was successfully established by 100 % without bad weather conditions. For the uplink, multi-beam laser transmission was used from the optical ground station to the OICETS satellite in order to reduce the intensity fluctuation of the optical signal due to atmospheric turbulence. The fluctuation of the received signal power was well minimized by using four-beam laser transmission. The applicability of the onboard optical terminal was demonstrated, aiming not only for geostationary earth orbit (GEO)-LEO links but also for ground-to-LEO optical links. This paper presents the results of Phase-1, Phase-2 and Phase-3 KODEN experiments.