# In-Orbit Results and Attitude Analysis of the SNUGLITE CubeSatellite
## 3 In-Orbit Results
### Satellite Status
#### 3.1.1 GPS Receiver Results  
For the performance analysis of the attitude estimation, the satellite positions were required to generate the reference sun-pointing vector, as shown in Equation (22). This section describes the indicators verifying that the SNUGLITE GPS receivers in orbit are performing normally.  
The SNUGLITE CubeSat broadcasts its current position, as calculated by the onboard GPS receiver, every 10 s. Given that the SNUGLITE CubeSat possesses an amateur telecommunication band and uses amateur protocol, the data can be received by any ground stations equipped with conventional amateur equipment. From the launch date until the end of June 2019, a total of 26,400 valid beacons, including GPS position, velocity, and time, have been received. All received GPS positions in latitude and longitude are described in Figure 4. The positioning and velocity results from the SNUGLITE CubeSat were based on the World Geodetic System-84.  
It has been confirmed that the SNUGLITE ground station in the Republic of Korea as well as amateur operators in Europe, North and South America, Oceania, and Asia receive signals from the SNUGLITE CubeSat.  
One of the methods for evaluating the GPS positioning results is to compare them with the two line element set (TLE) data uploaded from NORAD. The positioning results of the SNUGLITE GPS receiver continuously fit with the orbit generated from TLE parameters up to the km level [18], which is reasonable, because TLE provides rough orbital parameters with km-level accuracy [24]. In addition, positioning accuracy analyses, which use an orbit determination technique that considers orbit dynamics with perturbations, have shown that the root mean square error (RMS) error of the position is less than 5 m [18]. These results are indicators that the GPS receiver in the SNUGLITE has been performing normally.  
Altitude results calculated from the GPS receivers in the SNUGLITE CubeSat depict the tendency as described in Figure 5. In Figure 5, 'SNUGLITE' indicates the data received at the SNUGLITE CubeSat ground station, and 'SatNOGS' indicates the data uploaded to the SatNOGS website [13]. 'North-direction' and'south-direction' denote the movement of the satellite from south to north and north to south, respectively. The altitude data are the result obtained when the satellite is located only over the ground station, as described in Figure 4. The altitude varies from 574 to 607 km with a definite period over these regions. The period is caused by the secular motion of perigee, which is also called an apsidal rotation. This secular motion is one of the main effects caused by the first zonal harmonic, which is a J2 by Earth's oblateness. Furthermore, the fundamental period from zonal harmonics has a large magnitude [25].  
Figure 4: Broadcast GPS positions through beacons (blue: received at SNUGLITE ground station; red: received in other ground stations).  
The period is theoretically calculated following [25]. Using NORAD's TLEs, the mean theoretical orbital period was calculated to be 107.92 days, and it is displayed in Figure 5 using red dashed lines.  
The period of altitude variation in the figure coincided with the calculated period. The period of altitude variation can be regarded as another indicator to support the healthy operation of SNUGLITE's GPS receiver.