2.1

RISESAT

RISESAT is Rapid International Scientific Experiment Satellite, and an overview of RISESAT is shown in Table 1^[3]. RISESAT has eight scientific missions and eight technology demonstration missions. One of them is an optical communication experiment using VSOTA^[4]. RISESAT is a satellite with a size of 50 cm cube and equipped with a GPS receiver. In this experiment, the experiment was carried out using this GPS data.

Table 1.

Overview of RISESAT

2.2

Optical ground station (OGS)

Figure 1 shows an overview of the optical ground station. The OGS is a Cassegrain reflecting telescope with a 1m aperture. Table 2 shows the performance of OGS. The tracking performance of the LEO satellite is less than 10 seconds accurate. This telescope system has one Cassegrain focus, two Nathmys focal points, and two Coude focal points. The azimuth and elevation tracking logs of the telescope are recorded at 10Hz.

Figure 1.

Overview of RISESAT

Table 2.

Overview of OGS[5]

Table 2.

Camera performance

Table 3 shows the specifications of the cameras used in the experiment. In this experiment, this camera was installed at the Cassegrain focal point of the optical ground station, and the image of the satellite reflected by sunlight was acquired by the camera. The experiment was carried out with the integration time of the camera set to 1 second. The telescope and camera are calibrated using stars. However, even if star-calibration is performed, all errors cannot be removed, so the residual error is approximately 0.005 degrees.

Table 3.

Order of Image processing

4.1

Processing of camera data

The satellite position in the camera is calculated from the camera acquisition data from the brightness center of gravity. However, the amount of light received by the camera fluctuates significantly due to the tilt of the satellite, the influence of clouds, background noise, etc., in the reflection of sunlight on the satellite. Therefore, if the threshold value is easily obtained and the brightness center of gravity is obtained, an error increase. Therefore, we performed image processing according to the procedure in Table 3. The result of the image processing is shown in Figure 4.

Figure 4.

An example of Image filter application result (Enlarged view)

This image processing obtained the brightness center of gravity, and the tracking error of the telescope was corrected from the camera viewing angle and the number of pixels in Table 2. Figure 5 shows the amount of correction obtained from the image. The amount of correction in this experiment was in the range of -0.03 degrees to +0.03 degrees.

Figure 5.

Telescope offset angle obtained from camera images

4.2

Comparison of predicted and telescope tracking data

The telescope tracking data corrected from the camera image, satellite GPS data, and TLE information were compared. We used the following TLEs, NORAD TLE epoch time about 8 hours before the experiment (NORAD-TLE (before 8h)), NORAD TLE about 0.5 hours after the experiment (NORAD-TLE (after 0.5h)), and TLE generated from GPS data of RISESAT. The TLE generated from the GPS data of RISESAT was generated using GPS data sampled every 20 seconds from about 9 hours to 14 hours before the experiment time^[2].

Figure 6 shows the results of comparing each data based on the telescope tracking data corrected from the camera image. Figure 7 shows the total error amount of azimuth and elevation angle using Equation 1. o_az and o_el is azimuth and elevation angle of telescope tracking, t_az and t_el is of azimuth and elevation angle of each data to be compared.

Figure 6.

Compare each data based on telescope tracking data corrected from camera images

Figure 7.

Total error of azimuth and elevation angle

Tracking errors of GPS data sometimes show a value that is largely deviated due to GPS data errors. The error of the TLE generated from the RISESAT GPS data was the smallest in the elevation tracking error, and the NORAD-TLE (before 8h) was the smallest in the azimuth tracking error. The sizeable initial fluctuation in the azimuth angle is due to the large offset of the telescope in the azimuth angle direction to capture the satellite at the center of the camera.

The best result was shown when tracking with the NORAD-TLE (before 8h) in the evaluation of the elevation angle and the azimuth angle in figure 7. It was found that the fluctuation of the elevation angle error was small as a whole, and the fluctuation of the azimuth angle error was larger in the analysis of this experiment.

According to the results of this experiment, it is considered that an uplink beacon with a divergence angle of 1 milliradian can be captured at a low elevation angle. As the elevation angle increases, the azimuth error fluctuates largely. Therefore, it is necessary to prepare an uplink beacon with a wider divergence angle or move the beam such as a spiral scan, when considering the entire experiment time. Since the satellite’s position error tends to be primarily deviated by the azimuth angle, it seems good to give an offset in the azimuth angle direction to adjust the uplink beacon.