Mission
Diwata-2 will carry optical payloads that will support the following objectives:
- Determining the extent of damages from disasters
- Monitoring natural and cultural heritage sites
- Monitoring changes in vegetation
- Observing cloud patterns and weather disturbances
In addition to these optical payloads, Diwata-2 will also carry an amateur radio unit. This payload is intended to promote awareness and interest in amateur radios and satellite technology in the country. It is also intends to provide an alternative means of communication at times of disasters and emergencies.
Diwata-2 Structure and Subsystems
Structure
Diwata-2 is a 50x50x50 cm microsatellite with estimated mass of 50kg. It features two deployable systems - solar array panels and amateur radio antennas. Figures 1 and 2 shows the structure of the satellite with the solar array panels and amateur radio antennas in stowed and deployed configuration.
The placement of the amateur radio antennas was carefully considered in the structure due to its size and the need for an omnidirectional pattern. There are a total of two amateur radio antennas - one for VHF and one for UHF band. Figure 3 shows the placement on the antennas in the satellite panels.
The VHF antenna weighs 8.13g and measures 425mm. It has an induction coil 50mm in length and 12mm in outer diameter, and is used to reduce the antenna’s physical length and be accommodated in the satellite. The UHF antenna weighs 1.6g and measures 135mm. Each antenna rod is made up of Al 6061 material with a diameter of 2.4 mm. It is coated with chromium anodic finish to regulate the temperature fluctuations of the rod in orbit, while the termination is coated with a gold plated finish to facilitate soldering. Figure 4 shows the prototype of the VHF and UHF antennas.
Payload
Diwata-2 carries the following payloads to support its mission objectives:
- High Precision Telescope (SMI)
- Spaceborne Multispectral Imager (SMI) with Liquid Crystal Tunable Filter (LCTF)
- Wide Field Camera (WFC)
- Middle Field Camera (MFC)
- Enhanced Resolution Camera (ERC)
- Amateur Radio Unit (ARU)
Communication
This subsystem is responsible for sending and receiving data to and from the ground station. Diwata-2 carries a UHF-band receiver (URX), a S-band receiver (SRX), a S-band transmitter (STX) and a X-band transmitter (XTX). URX and SRX are used for receiving commands from the ground station. The SRX is used for transmitting housekeeping data and as a backup link for sending images to ground. The XRX is dedicated for sending images. Below shows the location of the antennas for these communication modules, along with the GPS and ARU antennas.
Power
This subsystem is responsible for providing power to different modules of Diwata-2. It consists of the power control unit, solar cells and battery. DIWATA-2 will contain deployable solar array panels (DSAP) to increase the power generation potential of the satellite. This improvement paves the way for the additional amateur radio communications unit, which is projected to consume an additional 3.96 W.
Command and Data Handling
DIWATA-2 will contain two main On-Board Computers (OBCs). Firstly, the Satellite Central Unit (SCU) will handle incoming satellite commands as well as outgoing satellite status telemetry. The other OBC is the Attitude Control Unit (ACU), which will calculate the required satellite control maneuver in order to satisfy the required satellite orientation to accomplish its mission.
Aside from these two main OBCs, it is planned to add a peripheral ACU Support (ACUS) unit to delegate some of the time-critical calculations from the ACU. From this, it is hoped that a better overall ADCS performance will be achieved.
Attitude Determination and Control (ADCS)
To accomplish its remote-sensing missions, Diwata-2 should be capable of capturing images of target locations with high accuracy. This is enabled through this subsystem. When Diwata-2 is tasked to capture an image, the ADCS points Diwata-2 to the target location.
The ADCS for DIWATA-2 will include magnetic torquers (MTQ) for coarse attitude control and reaction wheels (RW) for fine attitude control. These controllers are used in conjunction with attitude sensors, which include sun aspect sensors (SAS), geomagnetic aspect sensor (GAS), and star trackers (STT). In addition, a fiber-optic gyroscope (FOG) is put in place to track the movement of the satellite and serve as a secondary source of attitude when the need arises. Additional high-accuracy sun aspect sensors and the aforementioned ACUS aims to improve the accuracy of attitude control maneuvers to error angle of below 1 degree.
Power Budget
To sustain the lifetime of Diwata-2, a stable power system is required. Table 1 lists the energy storage of Diwata-2 and its power consuming modules. Table 2 shows the power budget calculations of Diwata-2.
Table 1: Diwata-2’s energy storage and power consuming modules
| Energy Storage | ||
| Number | Type | Total capcity (Wh) |
| 18 | NiMH Battery | 79.92 |
| Power Consumption | ||
| Subsystem | Name | Power (W) |
| Mission | SHU | 5 |
| HPT | 3.72 | |
| WFC | 0.98 | |
| MFC | 0.98 | |
| SMI | 1.86 | |
| ARU | 3.96 | |
| ADCS | 40.28 | |
| Power | 1.15 | |
| Comms. | 14.9 | |
| C&DH | 7.6 | |
Table 2: Diwata-2 Power Budget
| Mode | Comm | Comm with X-band | Obs with Comm |
| Power Consumption | 15.78 | 27.92 W | 56.28 W |
| Discharging Current | 1.461 A | 2.585 A | 5.211 A |
| Duration | 22 hours / day | 1 hour / day | 1 hour / day |
| Discharging Capability | 32,142 mAh / day | 2,585 mAh / day | 5,221 mAh / day |
| Net Discharging Capability | 39,938 mAh / day | ||
| Total # of cells | 170 |
| Average number of cells exposed to sun | 42.93 |
| Average generated power of solar panel system @ 80C | 38.72 W |
| Charging capability | 46,269 mAh / day |
Diwata-2’s Amateur Radio Unit
Diwata-2’s Amateur Radio Unit (ARU) operates in two possible modes: FM voice repeater (FMVR) mode, and APRS-based digital repeater (digipeater) mode. The uplink and downlink of these modes are implemented at 437.7MHz and 145.9MHz respectively. The following lists some information that may be useful when utilizing the radio’s services.
Telemetry Link Budget
| UPLINK | ||
| Specification | Value | Unit |
| Mean Orbital Altitude | 600 | [km] |
| Min UL Elevation Angle | 10 | [deg] |
| UL Center Frequency | 437.7 | [MHz] |
| UL Bandwidth | 20 | [KHz] |
| UL Free Space Path Loss | 151 | [dB] |
| GS Tx Output Power (in W) | 5 | [W] |
| GS Tx Transmission Line (Cable) Losses | 0.35 | [dB] |
| GS Tx Filters+Connectors+In-Line Switch Losses | 0 | [dB] |
| GS Tx Antenna Mismatch Loss | 0.52 | [dB] |
| GS Tx Total Line Losses | 0.87 | [dB] |
| GS Tx Antenna Gain | 9.7 | [dBi] |
| GS Tx Power Delivered to Antenna (in dBm) | 36.12 | [dBm] |
| GS Tx EIRP (in dBm) | 45.82 | [dBm] |
| GS Antenna Pointing Loss | 1.1 | [dB] |
| GS-to-SC Antenna Polarization Loss | 3 | [dB] |
| Free Space Path Loss | 151 | [dB] |
| Atmospheric Losses | 1.1 | [dB] |
| Ionospheric Losses | 0.7 | [dB] |
| Rain and Cloud Losses | 0 | [dB] |
| Shadowing Losses | 1 | [dB] |
| Other Losses | 3 | [dB] |
| Isotropic Signal Level at SC Antenna (in dBm) | -115.08 | [dBm] |
| SC Antenna Pointing Loss | 4.7 | [dB] |
| SC Rx Antenna Gain | 4 | [dBi] |
| SC Rx Transmission Line (Cable) Losses | 0.35 | [dB] |
| SC Rx Filters+Connectors+In-Line Switch Losses | 0.3 | [dB] |
| SC Rx Total Line Losses | 0.65 | [dB] |
| SC Rx Transmission Line Coefficient | 0.861 | |
| SC Rx Antenna Sky Temperature | 1000 | K |
| SC Rx Transmission Line Temperature | 280 | K |
| SC Rx Effective System Noise Temperature | 1015 | K |
| SC Rx Figure of Merit | -26.72 | dB/K |
| SC Rx Sensitivity (for 12dB SINAD) | -123 | [dBm] |
| SC Rx Desired Minimum SNR | 10 | dB |
| Signal Power at SC Rx Input Port (in dBm) | -116.43 | [dBm] |
| Noise Power at SC Rx Input Port (in dBm) | -125.53 | [dBm] |
| SNR at SC Rx Input Port | 9.1 | dB |
| UPLINK Signal Power Margin | 6.57 | [dB] |
| UPLINK SNR Margin | -0.9 | [dB] |
| DOWNLINK | ||
| Specification | Value | Unit |
| Mean Orbital Altitude | 600 | [km] |
| Min DL Elevation Angle | 10 | [deg] |
| DL Center Frequency | 145.9 | [MHz] |
| DL Bandwidth | 20 | [KHz] |
| DL Free Space Path Loss | 141.45 | [dB] |
| SC Tx Output Power (in W) | 0.8 | [W] |
| SC Tx Transmission Line (Cable) Losses | 0.35 | [dB] |
| SC Tx Filters+Connectors+In-Line Switch Losses | 0 | [dB] |
| SC Tx Antenna Mismatch Loss | 0.52 | [dB] |
| SC Tx Total Line Losses | 0.87 | [dB] |
| SC Tx Antenna Gain | 4 | [dBi] |
| SC Tx Power Delivered to Antenna (in dBm) | 28.17 | [dBm] |
| SC Tx EIRP (in dBm) | 32.17 | [dBm] |
| SC Antenna Pointing Loss | 4.7 | [dB] |
| GS-to-SC Antenna Polarization Loss | 3 | [dB] |
| Free Space Path Loss | 141.45 | [dB] |
| Atmospheric Losses | 1.1 | [dB] |
| Ionospheric Losses | 0.4 | [dB] |
| Rain and Cloud Losses | 0 | [dB] |
| Shadowing Losses | 1 | [dB] |
| Other Losses | 3 | [dB] |
| Isotropic Signal Level at SC Antenna (in dBm) | -122.48 | [dBm] |
| GS Antenna Pointing Loss | 2.6 | [dB] |
| GS Rx Antenna Gain | 13.4 | [dBi] |
| GS Rx Transmission Line (Cable) Losses | 0.35 | [dB] |
| GS Rx Filters+Connectors+In-Line Switch Losses | 0 | [dB] |
| GS Rx Total Line Losses | 0.35 | [dB] |
| GS Rx Transmission Line Coefficient | 0.923 | |
| GS Rx Antenna Sky Temperature | 270 | K |
| GS Rx Transmission Line Temperature | 290 | K |
| GS Rx Effective System Noise Temperature | 454 | K |
| GS Rx Figure of Merit | -13.53 | dB/K |
| GS Rx Sensitivity (for 12dB SINAD) | -121 | [dBm] |
| GS Rx Desired Minimum SNR | 10 | dB |
| Signal Power at GS Rx Input Port (in dBm) | -112.03 | [dBm] |
| Noise Power at GS Rx Input Port (in dBm) | -129.03 | [dBm] |
| SNR at GS Rx Input Port | 17 | dB |
| DOWNLINK Signal Power Margin | 8.97 | [dB] |
| DOWNLINK SNR Margin | 7 | [dB] |
Telemetry Formats
The Diwata-2 amateur radio unit (ARU) provides telemetry data in two forms depending on its operational mode: CW (enabled in ARU voice repeater mode), and FM (enabled in ARU digipeater mode. As the two operational modes are scheduled according to the day of the week, the available telemetry differ in format and content correspondingly. This section explains the data format and specifications for the ARU telemetry.
Continuous-wave telemetry (CW)
The CW telemetry is available when the satellite is in voice repeater mode. The telemetry messages expected from the ARU will be sent every 1 minute, when the ARU is idle, i.e. there are no ongoing voice operations. This said, the telemetry will not be available when the ARU is busy, and likewise, the telemetry transmission will be abruptly aborted when a voice repeating operation starts while in the midst of telemetry downlink. The data can be received in Morse code format of 120 WPM, at a frequency of 145.9 MHz. The pertinent data format is shown below, and further explained in the table that follows.
D1W2PH_DIWATA2_VR_A7A6A5A4A3A2A1A0R3R2R1R0_ Y3Y2Y1Y0M1M0D1D0H1H0M1M0S1S0_B2B1B0C2C1C0T2T1T0_V4V3V2V1V0
Data | Format | Characters | Description |
D1W2PH | Alphanumeric | 6 | Callsign of Diwata-2 ARU |
_ | Alphanumeric | 1 | Space character |
DIWATA2 | Alphanumeric | 7 | Satellite name |
_ | Alphanumeric | 1 | Space character |
VR | Alphanumeric | 2 | ARU operational mode (VR) |
_ | Alphanumeric | 1 | Space character |
A7A6A5A4A3A2A1A0 | Hex | 8 | Seconds in continuous operation, incrementing even when ARU switches operational mode. Can be used to track software or hardware resets Operating time is calculated as Top = A7 x 167 + A6 x 166 + A5 x 165 + A4 x 164 + A3 x 163 + A2 x 162 + A1 x 16 + A0 |
R3R2R1R0 | Hex | 4 | Number of reboots/resets: the very first startup upon launch is counted as first reboot Number of reboots = R3 x 163 + R2 x 162 + R1 x 16 + R0 |
_ | Alphanumeric | 1 | Space character |
Y3Y2Y1Y0M1M0D1D0 | Numeric | 8 | Date as registered in the ARU |
H1H0M1M0S1S0 | Numeric | 6 | Time as registered in the ARU |
_ | Alphanumeric | 1 | Space character |
B2B1B0 | Hex | 3 | Battery supply voltage as detected by ARU Vbatt = (B2 x 162 + B1 x 16 + B0) / 1023 * 15.0 |
C2C1C0 | Hex | 3 | ARU supply current coming from battery supply Ibatt = (C2 x 162 + C1 x 16 + C0) / 1023 * 10.0 |
T2T1T0 | Hex | 3 | ARU temperature (ambient temperature inside enclosure) Ibatt = (T2 x 162 + T1 x 16 + T0) / 1023 * 50.0 |
_ | Alphanumeric | 1 | Space character |
V4V3V2V1V0 | Hex | 5 | Voice time: time (s) spent by ARU for the current day to forward voice messages tvoice = V4 x 162 + V3 x 162 + V2 x 162 + V1 x 16 + V0 |
Each character in the CW string is a human-readable letter/number, and the hex-formatted numbers are encoded in the alphanumeric. For example, when the ARU has already operated continuously for 5678 seconds, this translates to 0x162E, hence the CW data can take the human-readable string D1W2PH_DIWATA2_CW_0000162E_1F93.
FM telemetry
Diwata-2 ARU in APRS digipeating mode provides FM telemetry, which is available at 145.9 MHz, 1200 bps AFSK. The data available in the FM telemetry are similar to that of CW telemetry, but also gives information on the EEPROM status. This EEPROM mainly stores messages to be forwarded at a scheduled time and date.
D1W2PH_DIWATA2_DP_A7A6A5A4A3A2A1A0R3R2R1R0_ Y3Y2Y1Y0N1N0D1D0H1H0U1U0S1S0_B2B1B0C2C1C0T2T1T0_M2M1M0E2E1E0
Data | Format | Characters | Description |
D1W2PH | Alphanumeric | 6 | Callsign of Diwata-2 ARU |
_ | Alphanumeric | 1 | Space character |
DIWATA2 | Alphanumeric | 7 | Satellite name |
_ | Alphanumeric | 1 | Space character |
DP | Alphanumeric | 2 | ARU operational mode (Digipeater) |
_ | Alphanumeric | 1 | Space character |
A7A6A5A4A3A2A1A0 | Hex | 8 | Seconds in continuous operation, incrementing even when ARU switches operational mode. Can be used to track software or hardware resets Operating time is calculated as Top = A7 x 167 + A6 x 166 + A5 x 165 + A4 x 164 + A3 x 163 + A2 x 162 + A1 x 16 + A0 |
R3R2R1R0 | Hex | 4 | Number of reboots/resets: the very first startup upon launch is counted as first reboot Number of reboots = R3 x 163 + R2 x 162 + R1 x 16 + R0 |
_ | Alphanumeric | 1 | Space character |
Y3Y2Y1Y0N1N0D1D0 | Numeric | 8 | Date as registered in the ARU |
H1H0U1U0S1S0 | Numeric | 6 | Time as registered in the ARU |
_ | Alphanumeric | 1 | Space character |
B2B1B0 | Hex | 3 | Battery supply voltage as detected by ARU Vbatt = (B2 x 162 + B1 x 16 + B0) / 1023 * 15.0 |
C2C1C0 | Hex | 3 | ARU supply current coming from battery supply Ibatt = (C2 x 162 + C1 x 16 + C0) / 1023 * 10.0 |
T2T1T0 | Hex | 3 | ARU temperature (ambient temperature inside enclosure) Ibatt = (T2 x 162 + T1 x 16 + T0) / 1023 * 50.0 |
_ | Alphanumeric | 1 | Space character |
M2M1M0 | Hex | 3 | Number of stored messages in EEPROM msg_count = C2 x 162 + C1 x 16 + C0 |
E2E1E0 | Hex | 3 | % occupied storage in EEPROM mem_percent = (C2 x 162 + C1 x 16 + C0) / 4095 * 100 |