Health Trend Analysis of In Orbit Satellite and Image Quality Monitoring of the Payload

LU Chunling, BAI Zhaoguang, DI Guodong, ZHENG Qinbiao

DFH satellite CO., LTD., Beijing 100094

Abstract: This paper introduces the satellite system running status and key events. This satellite has been observing the Earth for six years after its launch into a 645 km sun-synchronous orbit. Through the health trend analysis of the platform and subsystems, the orbit, power supply, rotating parts status, temperature, fuel consumption and so on are introduced in detail. The cameras' status also are monitored and analyzed.

Key words: health trend analysis, image quality monitoring, satellite

1 INTRODUCTION

This satellite has already been running in orbit for six years.In its 645 km sun-synchronous orbit, every day the spacecraft orbits the Earth about 14.75 times, cycles from high and low temperature variations, charge and discharge, with the solar array driver which has run for about 32,303 circles over six years.It had gone through six summer solstices and winter solstices in the low Earth orbit. Platform and payloads are all running stably in the space environment. Also the condition of all subsystems are very good and have realized perfect capability without any component faults.

2 KEY EVENTS AND STATUS OF THE SATELLITE

1) At the beginning of the mission, the satellite camera's focal plane was adjusted to make images clearer. Then,based on the images' gray histograms, the camera's time delay and integration (TDI) stages and gain were adjusted to widen the greyscale dynamic range distribution. Then, the side-slither radiometric calibration was completed by yawing the satellite 90°[1-3]. Later,temperature and power supply setting was adjusted according to the system demands.

2) The “Early Extended Mission”[4]phase was from 2014 to 2016. The satellite operates orbit maintenance about once every 2 months since April 2014. The focal plane is adjusted every year[5]. It also conducts ground radiometric calibration and synchronous observing. The calibration coefficient can be submitted every year.

3) The “Extended Mission” is from 2017 until now. The spacecraft has not only uploaded the information for three new receiving stations, but also extended imaging time and storage time since May 2017 (Figure 1).

Today all components function well and run stably without any switch to backups.

Figure 1 Key events of the satellite in orbit

3 HEALTH TREND ANALYSIS OF PLATFORM

3.1 Orbit Analysis

The satellite initial altitude was 1.68 km higher than the reference orbit[5]. In order to avoid the fuel consumption there was no active intervention to reduce the altitude. However due to satellite drag the altitude decayed naturally in the first year. In April 2014, the first orbit manoeuvre was conducted to maintain the 645 km reference orbit altitude. After that, orbit maintenance has been conducted once every two or three months in order to increase semi-major axis (SMA) and maintain the target phase point on reference orbit (Figure 2). If the SMA is below the reference orbit, the nadir track will drift in the positive phase direction. The relationship between relative SMA, relative phase and spacecraft drag is shown in Figure 2[6]. Orbit maintenance ensures the nadir track always fluctuate ±10 km that of the reference track, as shown in Figure 3.

Figure 2 Satellite orbit maneuver trajectory

Figure 3 Satellite orbit fluctuate statistics

3.2 Fuel Consumption Analysis

By the end of March 2019, the satellite had conducted orbit maintenance 28 times. Over six years 1.4 kg of fuel was consumed. Fuel consumption matches with the trend of total jet time and the tank pressure telemetry (Figure 4). The average jet time is about 7 s. The temperature of the pressure sensor changes from 22 to 29.7℃. The maximum and minimum temperature match with the winter solstices and summer solstices；hence the residual fuel is about 31 kg which is enough for the 8-year design lifetime.

3.3 Power Supply Analysis

While in orbit, the solar array generates electrical power,supplying 40.4 - 47.78 A of current for the on-board equipment and charging the batteries. The peaks output current for the solar array matches with the six years' maximum sun constant,near winter solstice, and the troughs match with the minimum sun constant, near summer solstice. The trend for the six years'solar array current and the battery voltage shows the power supply capability is extremely stable without any reduction (Figure 5).

3.4 Temperature Analysis

The temperature of the platform and equipment, which is controlled by passive thermal control, is very stable within more than 12℃ margin. The temperature of the steadystate working equipment is higher near winter solstice than near summer solstices. In the first year the temperature trend raised 1.0 - 1.4℃, then slightly increased further to about 2.7℃ over six years. The sun constant affects the temperature with a variation of about 3℃, as shown in Figure 6. All phenomena are related to the degradation of the thermal control coatings.

The temperature of the active thermal control equipment is stable without large fluctuation, as shown in Figure 6.

3.5 Rotating Components Analysis

The rotating components consist of momentum wheels,infrared Earth sensor, solar array driver and gyroscope. Their health trend through currents can be seen in Figure 7. The various current telemetry for rotating components are stable with no abnormal fluctuations.

Figure 4 Total jet time, tank pressure and temperature of pressure sensor statistics (the horizontal axis represents date and time, the vertical axis represents telemetry values)

Figure 7 Currents telemetry of rotating parts (The horizontal axis represents date and time, the vertical axis represents telemetry values)

4 IMAGE AND STATUS MONITERING OF HIGH RESOLUTION CAMERA

4.1 Relative Radiometric Calibration

This satellite adopts an attitude yaw of 90° to realize relative radiometric calibration using a side-slither technique. In this configuration, each detector on the focal plane is positioned parallel to the ground-track direction thereby exposing each detector to the same segment of the ground[7]. This method enables both pixel-to-pixel and chip-to-chip response differences quickly with no need for a large calibration site.

The side-slither calibration method is shown in Figure 8 (b).In order to have a different gray level, a side-slither calibration site with high-middle-low gray is chosen rather than a radiometric flat-field. The side-slither derives detector correction parameters by using the histograms method[8]. More gray stage lines indicate that more a wider distribution histograms for each pixels can be obtained[9-10]. A significant improvement in image correction was achieved.

Each detector's non-uniformity and average band response also can be obtained efficiently by this method.

Figure 8 (a) principle of side-slither calibration by yaw 90° (b) raw side slither image (c) histograms method

Figure 9 Sensor's relative spectral response Figure 10 The gain of absolute radiometric calibration

4.2 Absolute Radiometric Calibration

Two methods are used to determine the absolute radiometric calibration of the sensor. Pre-launch laboratory calibration and after-launch ground site calibration. The former provides initial values, while the latter provides updated values year by year. In order to calculate the radiance of the optical pupil, the sensor's relative spectral response curves used are shown in Figure 9. The gain of the absolute radiometric calibration not only presents the relationship between incident radiance and detector digital output, but also show radiometric resolution by W.m-2.sr-1.μm-1/ DN. 10 bit quantization improves the radiometric resolution much more than 8 bit quantization. All pixels are working without failure pixel.

In September 2016, the sensor's TDI stages and gain were adjusted for increasing average gray level and enlarged the dynamic range of the histograms. Therefore, the gain of ground absolute radiometric calibration can be seen in Figure 10, where it can be seen the curve changed dramatically in 2017.

4.3 TDI Stage and Gain Adjustment

Panchromatic arrays: TDI stages 6, 12, 24, 32, 48；

Multi-spectral arrays: B, G, R, NIR； TDI stages 04, 08, 16,24, 32.

In September 2016, TDI stages and gain were adjusted (Table 1). The effect was very obvious. The gray distribution and dynamic range of imagery were widened. The Ratio of change was about 1.58 times.

4.4 Focus Calibration

Due to the effect of weightlessness, the release of stress and temperature shift, camera focal plane will change. In April 2014, the camera focal plane was adjusted. Through focal plane movement, a better focus position was located and images became more distinct. Six years in orbit, the PMS1 and PMS2 focal plane shift about +88 and +112 steps in total, which is approximately 0.147 mm and 0.173 mm compare to pre-launch (Figure 11).Now the camera focus is adjusted once every one or two years.

Table 1 Adjust TDI stages and gain

Figure 11 Focal plane position shift

5 SUMMARY

Over the six years' health trend analysis of the platform and subsystem in orbit, the satellite works steadily and has excellent performance indicating the superiority of its design. The solar array and battery supply sufficient power without degradation.Fuel consumption in the six years was 1.4 kg, which was only 4.3% of the total 32.5 kg fuel. The focus calibration of the camera operates regularly along with adjustment of the TDI stages and gain to meet the dynamic range of scenery. The image quality of the cameras are stable. All components function well and run stably without any reversion to backups. The mission has exceeded its original 5 years' lifetime and there is full confidence 8 years will be exceeded. It will benefit other missions for the years to come.