Diwata-2

Diwata-2 is the Philippines’ second microsatellite designed and developed by Filipino scientists and engineers under the PHL-Microsat Program.


Mission

Diwata-2 will carry optical payloads that will support the following objectives:

  1. Determining the extent of damages from disasters
  2. Monitoring natural and cultural heritage sites
  3. Monitoring changes in vegetation
  4. 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.

Lights
Figure 4: Figure 4: VHF and UHF antenna prototype

Payload

Diwata-2 carries the following payloads to support its mission objectives:

  1. High Precision Telescope (SMI)
  2. Spaceborne Multispectral Imager (SMI) with Liquid Crystal Tunable Filter (LCTF)
  3. Wide Field Camera (WFC)
  4. Middle Field Camera (MFC)
  5. Enhanced Resolution Camera (ERC)
  6. 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.

Lights
Figure 5

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
NumberTypeTotal capcity (Wh)
18NiMH Battery79.92
Power Consumption
SubsystemNamePower (W)
MissionSHU5
HPT3.72
WFC0.98
MFC0.98
SMI1.86
ARU3.96
ADCS 40.28
Power 1.15
Comms. 14.9
C&DH 7.6

Table 2: Diwata-2 Power Budget

ModeCommComm with X-bandObs with Comm
Power Consumption15.7827.92 W56.28 W
Discharging Current1.461 A2.585 A5.211 A
Duration22 hours / day1 hour / day1 hour / day
Discharging Capability32,142 mAh / day2,585 mAh / day5,221 mAh / day
Net Discharging Capability39,938 mAh / day
Total # of cells170
Average number of cells exposed to sun42.93
Average generated power of solar panel system @ 80C38.72 W
Charging capability46,269 mAh / day
* All Comm assumes ARU in active mode

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 Altitude600[km]
Min UL Elevation Angle10[deg]
UL Center Frequency437.7[MHz]
UL Bandwidth20[KHz]
UL Free Space Path Loss151[dB]
GS Tx Output Power (in W)5[W]
GS Tx Transmission Line (Cable) Losses0.35[dB]
GS Tx Filters+Connectors+In-Line Switch Losses0[dB]
GS Tx Antenna Mismatch Loss0.52[dB]
GS Tx Total Line Losses0.87[dB]
GS Tx Antenna Gain9.7[dBi]
GS Tx Power Delivered to Antenna (in dBm)36.12[dBm]
GS Tx EIRP (in dBm)45.82[dBm]
GS Antenna Pointing Loss1.1[dB]
GS-to-SC Antenna Polarization Loss3[dB]
Free Space Path Loss151[dB]
Atmospheric Losses1.1[dB]
Ionospheric Losses0.7[dB]
Rain and Cloud Losses0[dB]
Shadowing Losses1[dB]
Other Losses3[dB]
Isotropic Signal Level at SC Antenna (in dBm)-115.08[dBm]
SC Antenna Pointing Loss4.7[dB]
SC Rx Antenna Gain4[dBi]
SC Rx Transmission Line (Cable) Losses0.35[dB]
SC Rx Filters+Connectors+In-Line Switch Losses0.3[dB]
SC Rx Total Line Losses0.65[dB]
SC Rx Transmission Line Coefficient0.861
SC Rx Antenna Sky Temperature1000K
SC Rx Transmission Line Temperature280K
SC Rx Effective System Noise Temperature1015K
SC Rx Figure of Merit-26.72dB/K
SC Rx Sensitivity (for 12dB SINAD)-123[dBm]
SC Rx Desired Minimum SNR10dB
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 Port9.1dB
UPLINK Signal Power Margin6.57[dB]
UPLINK SNR Margin-0.9[dB]
DOWNLINK
Specification Value Unit
Mean Orbital Altitude600[km]
Min DL Elevation Angle10[deg]
DL Center Frequency145.9[MHz]
DL Bandwidth20[KHz]
DL Free Space Path Loss141.45[dB]
SC Tx Output Power (in W)0.8[W]
SC Tx Transmission Line (Cable) Losses0.35[dB]
SC Tx Filters+Connectors+In-Line Switch Losses0[dB]
SC Tx Antenna Mismatch Loss0.52[dB]
SC Tx Total Line Losses0.87[dB]
SC Tx Antenna Gain4[dBi]
SC Tx Power Delivered to Antenna (in dBm)28.17[dBm]
SC Tx EIRP (in dBm)32.17[dBm]
SC Antenna Pointing Loss4.7[dB]
GS-to-SC Antenna Polarization Loss3[dB]
Free Space Path Loss141.45[dB]
Atmospheric Losses1.1[dB]
Ionospheric Losses0.4[dB]
Rain and Cloud Losses0[dB]
Shadowing Losses1[dB]
Other Losses3[dB]
Isotropic Signal Level at SC Antenna (in dBm)-122.48[dBm]
GS Antenna Pointing Loss2.6[dB]
GS Rx Antenna Gain13.4[dBi]
GS Rx Transmission Line (Cable) Losses0.35[dB]
GS Rx Filters+Connectors+In-Line Switch Losses0[dB]
GS Rx Total Line Losses0.35[dB]
GS Rx Transmission Line Coefficient0.923
GS Rx Antenna Sky Temperature270K
GS Rx Transmission Line Temperature290K
GS Rx Effective System Noise Temperature454K
GS Rx Figure of Merit-13.53dB/K
GS Rx Sensitivity (for 12dB SINAD)-121[dBm]
GS Rx Desired Minimum SNR10dB
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 Port17dB
DOWNLINK Signal Power Margin8.97[dB]
DOWNLINK SNR Margin7[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