3.1

Experimental setups

Figure 3 illustrates the experimental setup of ground-to-satellite optical link experiment by using the LUCAS with 14 centimeters optical antenna. Ground-to-satellite optical links, which implements downlink and uplink, can be established between the LUCAS and the OGS installed in NICT Okinawa by a series of acquisition and tracking sequences. The LUCAS exists within the range of the elevation angle between 35 degrees and 42 degrees from the corresponding OGS. The LUCAS outputs about 60 Mbps downlink optical signal to the OGS. The downlink optical signals are received by the check-out equipment via 1 meter telescope in the OGS. The data rate of 60 Mbps includes user data length and forward error correction (FEC) code length. The check-out equipment outputs 2.5Gbps uplink optical signal from the OGS to the LUCAS. The uplink optical signals are received by LUCAS. The data rate of 2.5 Gbps also includes user data length and FEC code length. In our BER performance evaluation, LUCAS forwards the PN (Pseudo random Noise) code to the OGS instead of using FEC code.

Figure 3.

Experimental setup of ground-to-satellite optical link experiment by using LUCAS and OGS installed in NICT Okinawa

Figure 4(a) describes the experimental setup around 1 meter telescope inside the OGS installed in NICT Okinawa, and Figure 4(b) shows the block diagram of the check-out equipment. The telescope condenses the downlink optical signals onto the check-out equipment and transmits the uplink optical signals generated from the check-out equipment. The downlink optical signal is received by the ATP function and optical receiver of the check-out equipment. The uplink optical signal is transmitted by ATP function and optical transmitter of the check-out equipment. An IR (Infrared) camera is implemented on the right side of 1 meter telescope. The IR camera captures the images of the downlink and uplink optical signals. A custom photo detector module with 5 centimeters is implemented on the left side of 1 meter telescope. The photo detector module measures the received optical power of the downlink optical signals by using 5 centimeters telescope. Differential image motion monitor (DIMM) device is implemented on the lower side of 1 meter telescope. The DIMM device measures many images of received downlink optical signals to analyze Fried parameter that is one of parameters for atmospheric turbulences. The analytical results of Fried parameter will be discussed in another paper [11].

Figure 4.

(a) Experimental setup around 1 meter telescope inside the OGS installed in NICT Okinawa and (b) the block diagram of the check-out equipment

3.2

Experimental conditions

We conducted the trial campaign of acquisition and tracking of 83 times at the night in November 2021. We successfully demonstrated the acquisition and tracking trials of 35 times in the campaign. In each unsuccessful case, the acquisition and tracking has mainly not been completed by climates and clouds. In other cases, we have not confirmed technical issues for LUCAS. In this paper, we select three cases including (a) stable, (b) moderate and (c) unstable cases for the downlink among the successful cases of acquisition and tracking from the viewpoint of received optical power and fade duration. We also select two cases for the uplink in the successful cases of acquisition and tracking. The measurement duration of each case is 20 seconds. Table 2 shows experimental environmental conditions of each case in the experimental demonstration campaign. The environmental data has been captured by Observation system of the patch of Blue Sky for Optical Communication (OBSOC) [12].

Table 2.

Experimental environmental conditions in the experimental demonstration campaign.

4.1

Received optical power and scintillation index in the downlink and the uplink

Figure 5 shows received optical power and scintillation index (SI) of the downlink, measured by fast pointing sensor (FPS) of the ATP function of the check-out equipment with 1 meter telescope in three cases including (a) stable, (b) moderate and (c) unstable cases. Figure 6 also shows received optical power and SI of the downlink, measured by the custom photo detector module with 5 centimeters telescope in three cases including (a) stable, (b) moderate and (c) unstable cases. Received optical power has been measured at a sampling frequency of a few kHz. It is normalized by using received optical power averaged in the measurement span and is shown as the blue line. SI, which is calculated at the time interval of 0.1 second by using the equation expressed in [13], is also shown as the green line.

Figure 5.

Received optical power and SI of the downlink, measured by the check-out equipment with 1 meter telescope in three cases including (a) stable (b) moderate and (c) unstable cases

Figure 6.

Received optical power and SI of the downlink measured by the custom photo detector module with 5 centimeters telescope in three cases including (a) stable (b) moderate and (c) unstable cases

From Figure 5(a), it can be seen that received optical power keeps stable at the center of average optical power and SI is also suppressed to the lowest value. As shown in Figure 5(b), it appears that SI of (b) moderate case is higher than that of (a) stable case and instantaneous fades of received optical power occasionally happen. Figure 5(c) presents the long fade duration caused by fades of received optical power in (c) unstable case. Thus, we can say that received optical power and SI have different characteristics in each case.

Furthermore, we compare received optical power and SI in each case, by using experimental environmental conditions listed in Table 2. It can be inferred that the wind speed may cause the strong power fluctuations, according to the results of (a) stable case and (b) moderate case. The results also predict that the humidity may cause the strong power fluctuations. However, little still may not be clear about the relationship between received optical power fluctuations and environmental conditions from these results only. In order to clarify such relationship, we need to evaluate and analyze experimental data continuously.

Both Figure 5 and Figure 6 show that received optical power and SI have the same trends in both telescopes. However, it can be seen that received optical power and SI with 5 centimeters telescope fluctuates more strongly than with 1 meter telescope. The results show that the aperture averaging effect caused by 1 meter telescope enable decreasing SI. Consequently, we clarified that a large-aperture size telescope can contribute to the suppression of received optical power fluctuations.

Figure 7 shows received optical power and scintillation index (SI) of the uplink, measured by the FPS of the ATP function of the LUCAS with 14 centimeters optical antenna in two cases including both (a) case and (b) case. Received optical power was measured at a sampling frequency of about 100 Hz. It is normalized by using received optical power averaged in the measurement span and is shown as the blue line. SI, which is calculated at the time interval of 0.1 second, is also shown as the green line.

Figure 7.

Received optical power and SI of the uplink, measured by the LUCAS with 14 centimeters optical antenna in two cases including both (a) case and (b) case

According to Figure 5 and Figure 7, received optical power and SI with 1 meter telescope of OGS fluctuate more strongly than those with 14 centimeters telescope of LUCAS. From these results, since the optical aperture of LUCAS is smaller than that of OGS, it is considered that aperture averaging effect is decreased to the low value. Furthermore, according to Figure 6 and Figure 7, despite the optical aperture of LUCAS is larger than that of the custom photo detector module with 5 centimeters telescope, received optical power and SI in the uplink fluctuates more strongly than in the downlink. These results predict that the strength of atmospheric turbulence in the uplink is stronger than in the downlink. It needs to be investigated in more detail.

4.2

Acquisition and tracking performance in the downlink

Figure 8 shows tracking error of the downlink in both axes (X, Y), measured by the FPS of the ATP function of the check-out equipment with 1 meter telescope in three cases including (a) stable (b) moderate and (c) unstable cases. Tracking error has been measured as the acquisition and tracking performance in the unit of urad at a sampling frequency of a few kHz. It is shown as the blue line. Tracking errors, which are averaged at the time interval of 0.1 second in both axes, are also shown as the orange line. Means of absolute value of tracking error are displayed in each graph.

Figure 8.

Tracking error of the downlink, measured by the check-out equipment with 1 meter telescope in three cases including (a) stable (b) moderate and (c) unstable cases

According to Figure 8(a), tracking error keeps stable at the center of 0 urad in both axes and mean of absolute value of tracking error is also suppressed to the lower value. It can also be confirmed that the distribution of tracking error becomes broadened at the center of 0 urad in both axes in Figure 8(b). Both Figure 8(a) and Figure 8(b) indicates that mean of absolute value of tracking error in (a) stable case is higher than in (b) moderate case. As depicted in Figure 8(c), we can find that the center value of tracking error increases in X axis and mean of absolute value of tracking error also reaches to the highest value in each case. Compared with received optical power described in Figure 5(c), tracking error largely varies at the timing of fades. Therefore, it is clearly found that received optical power and tracking error have the mutual relationship.

4.3

Bit error rate performance in the downlink

Figure 9 shows the minimum received optical power of the downlink, measured by the FPS of the ATP function of the check-out equipment with 1 meter telescope in three cases including (a) stable (b) moderate and (c) unstable cases. Figure 10 shows the bit error rate (BER) performance of the downlink, measured by the check-out equipment. We have defined minimum value of received optical power sampled at a few of kHz frequency at the time interval of about 100 milli-seconds as minimum received optical power. It is normalized by using received optical power averaged in the measurement span and is shown as the green line. BER has been measured at the time interval of about 0.1 second in each case.

Figure 9.

Minimum received optical power of the downlink, measured by the check-out equipment with 1 meter telescope in three cases including (a) stable (b) moderate and (c) unstable cases

Figure 10.

BER performance of the downlink, measured by the check-out equipment with 1 meter telescope in three cases including (a) stable (b) moderate and (c) unstable cases

Figure 10(a) indicates that BER reaches to error free. The main reason is the low fluctuations of minimum power illustrated in Figure 9(a). Despite minimum received optical power illustrated in Figure 9(b) is relatively stable, BER occurs at several times. Moreover, when normalized power is lower than -10dB in Figure 9(b), BER becomes higher than 10^-2. It can be confirmed that BER instantaneously increases due to the rapid change of received optical power. Furthermore, as shown in Figure 10(c), some continuous fades of received optical power described in Figure 9(c) cause to the burst error.

Consequently, we can clarify that there is a relationship between minimum received optical power described in Figure 9 and BER described in Figure 10. These results present that atmospheric conditions can be estimated at some level as BER is monitored. However, it can be found from the log-scale graph depicted in Figure 10 that BER sometimes increases to 10^-3 at the timing point when minimum received optical power slightly change. It can be inferred that BER needs to be measured at higher sampling frequency that is higher than about 10 Hz in order to monitor accurate atmospheric conditions from BER. Thus, the fast sampled BER monitoring function needs to be considered in the development of next-generation ground-to-satellite optical communication systems.