In Orbit Testing

The PEASSS satellite was successfully launched into space on Wednesday 15 February 2017 at 3:58 UTC. Contact was already established a few hours after launch during the first orbit over the ground station located in Delft (the Netherlands). From that moment on data has been gathered onboard the satellite and downloaded to the ground station. Due to the orbit around the earth communication with the ground station located in Delft is only possible three times a day during a period of a about 10 minutes.



Figure 11, location of the PEASSS satellite during experiment number 14, also showing the location of the ground station in Delft (the Netherlands). It is visible that the satellite is at the end of the sunlit period just before entering eclipse. 

Before payload experiments could be started, first the correct operation of the satellite had to be established during LEOP (Launch and Early Operations Phase), followed by Platform commissioning and Payload commissioning. A critical aspect of Payload Commissioning was the release of the launch-lock of the Optical Bench. This launch-lock prevented damaging of the delicate mechanical parts of the Optical Bench resulting from the violent vibrations during launch. The launch lock consists of a Shape Memory Alloy (SMA) beam that returns to its ‘programmed’ shape after heating. Comparing measurements from a tilting experiment of the Optical Bench, performed before and after opening the lock, showed that the lock had opened as planned and that it had protected the delicate mechanics during transport and launch.

The short time between satellite launch and the end of the PEASSS project meant that only a limited number of experiments could be performed and analyzed. The main limitation is the relative low effective download bandwidth resulting from only 3 overhead passes of about 10 minutes each day. Communication is only possible during these 3 passes. Of course the PEASSS satellite will stay operational also after the end of the PEASSS project and it is expected that regularly data will be downloaded and analyzed to observe performance over time.

Interrogator and Fiber Bragg Gratings (FBG’s)

The limited effective download bandwidth was mainly a problem for downloading the data from the interrogator which was used for reading out the Fiber Bragg Gratings (FBG) used for strain- and temperature measurement. Due to the experimental state of the system it was decided not to integrate the algorithms that perform this analysis into the satellite, but to download the raw data and perform this analysis on the ground. This provides the possibility to tune the algorithms but the drawback is that relative large amounts of data have to be transferred from the satellite to the ground. The limited effective bandwidth available in combination with only a limited time per cycle that the satellite is in view of the ground station makes that downloading the complete data set of an experiment with multiple tilt angles takes several days. The result is that in the short time between the launch and the end of the project the data of only a limited number experiments could be downloaded and analysed.

Piezoelectric Actuation Mechanism

An experiment consists of changing the attitude of a sun sensor mounted on a gimballed structure that can be tilted by means of two orthogonally mounted piezo bimorph actuators. A sun sensor measures the attitude of the sun with respect to the body of the sun sensor. By applying different voltage levels to the bimorphs, different tilting angles can be obtained. The bimorphs can be actuated independently which makes it possible to generate a 2D array of tilting angles. In the figure an example pattern with discrete voltage level combinations is presented that is used in an experiment to direct the sun sensor to different pointing angles. Even this limited array consist already of 41 different tilt angle combinations. During the limited time available data from 2 experiments with 5 tilting angle steps was downloaded and from 1 full experiment with 41 tilt angle combinations.


Figure 12, example of 2D array of possible actuation voltages that result in tilting angles of the sun sensor mounted on the gimballed structure

It was demonstrated that a sun sensor could be tilted in two orthogonal directions by means of two piezo bimorph actuators. Feedback of the tilting angle is possible by means of strain gauges glued on top of these bimorph actuators. Strain was measured by means of classic resistive strain gauges but also by means of Fibre Bragg Gratings (FBG’s). The sensitivity of FBG’s to temperature can be cancelled out by mounting an FBG both on the top- and on the bottom side of the bimorph beam. The use of strain measurement on both top- and bottom sides of the bimorph also proved to be necessary to obtain a good measure for the bending of the bimorph and with that for the tilting angle.



Figure 13, for 1 rotation axis from top to bottom the applied voltage to the piezo bimorph actuator during the tilting steps, the strain measured by the differential FBG strain gauge and the resistive strain gauge (SG) and the tilting angle measured by means of the sun sensors.

Pyroelectric Power Generator

For analyses of the Pyroelectric power generation payload experiment only data with a relatively low sample rate is needed to observe the effect of temperature changes. These temperature changes are the result of the satellite entering the shadow of the earth (eclipse) each orbit of about 1.5 hours. A faster temperature change of the piezo power generator may result from the tumbling of the satellite. The temperature change of the Pyroelectric Power Generator is however also related to the attitude of the satellite. An example of the measured temperature change of the Pyroelectric power generator and the resulting output voltage is presented in the next figure. The voltage is measured over a 200 kohm load resistor and therefore represents generated power as well. The power output level is relatively low which is the result of a rather slow temperature change. It is expected that a higher power output level can also be obtained by further optimization of the design. As far as known this is however the first time that pyroelectric power generation is demonstrated in space.

 
Figure 14, temperature of the pyroelectric power generator resulting from the eclipse cycle and the resulting generated output voltage measured over a load resistor.

Thermal analysis

The temperature measurements performed in space were used for thermal analysis. The measurements were compared with thermal models. However due to the limited effective download bandwidth the data available for thermal analysis comparison purposes allowed only a comparison for external solar panels and a limited number of internal subsystems of the satellite (battery, transceiver and iMTQ). The pictures show both the simulated solar panel temperatures for cold case 01 and the measured solar panel temperatures. The temperatures’ profile on the panels is comparable with the analysis outcome. In the cold parts of the orbit (eclipse) the temperatures of the panels tend to equilibrate since no different radiative loads from external sources are present on the different sides of the satellite. The behaviour of each side is different once the satellite is exposed to the Sun. This is due to the different spinning rate of the satellite around each axis.



Figure 15, simulated solar panel temperatures for the Cold case 01.


Figure 16, solar panel temperatures measured in space.