Sapling uses a HopeRF UHF LoRa radio module. LoRa is a relatively new low power, long range, low bitrate radio modulation technique. It uses a chirp spread spectrum modulation scheme and is especially useful because signals can be demodulated even below the noise floor.
Sapling possesses a deployable omnidirectional tape spring v-dipole antenna. The antennas are made of standard tape measures, which are both soldered and screwed to the Z+ face of the satellite. The radiation patterns are shown in figures below
Horizontal Radiation Pattern
Vertical Radiation Pattern
Sapling will interface with the ever-growing TinyGS network, an open network of over 1200 active amateur ground stations worldwide, for telemetry. Those interested in listening for Sapling, including students at partner institutions, are encouraged to build their own ground station and join the TinyGS community at tinygs.com. Sapling will be commanded via the Durand ground station on Stanford’s Campus.
The beacon packet which Sapling Giganteum will transmit every 60 seconds while in idle mode or every 60 minutes while in low power mode is around 50 bytes and contains the following:
<aside> đź“» callsign
state_machine.current_state (boolean)
radio.tx_power
solar_charging (boolean)
enable_rf.value (boolean), whether or not the LDO for the radio is enabled
gc.mem_free (amount of free memory in the SAMD chip)
boot count
batt_voltage,
system_voltage,
system_current,
batt_temp
radio.rssi (on last message received)
radio.snr (on last message received)
ambient light sensor values packet as 12 bit ints, ranging from 0-40k lux, from all 6 faces
16bit magnetomer readings in 3 axes
16bit gyroscope readings in 3 axis
imu temperature
16 bit int lat
16 bit int lon
16 bit int altitude
GPS time as seconds from epoch (configured as march 1st)
</aside>
Communications Requirements
| Requirement | Parameter | Description | Parent(s) |
|---|---|---|---|
| Defining Radio Requirements | |||
| COM-101 | F = 437.4 MHz, BW=125kHz, CR=5, BW=8 | The spacecraft shall communicate over FCC and IARU coordinated frequencies and abide by all communications regulations. | SYS-101, SYS-102 |
| COM-102 | F = 437.4 MHz, BW=125kHz, CR=5, BW=8 | The radio shall use fixed Frequency, BW, CR, SF LoRa chirp spread spectrum signals. | SYS-101, COM-101 |
| COM-201 | The spacecraft shall transmit via UHF radio. | SYS-101, SYS-102 | |
| COM-202 | 1/2 Wavelength | The antenna shall be omnidirectional. | SYS-401 |
| Beacon Requirements | |||
| COM-301 | KN6HCC | The spacecraft transmits the HAM radio license call sign. | COM-101 |
| Compliance with federal regulation. | |||
| COM-302 | 1 min | The beacon shall be periodically autonomously transmitted. | SYS-501, SYS-102 |
| COM-303 | 1W | The spacecraft shall always transmit at its highest power. | SYS-101 |
| COM-401 | The spacecraft shall receive commands via radio | SYS-101 | |
| Margin and System Strength | |||
| COM-501 | 10 dB, 1000km | The link budget shall have adequate margin on downlink. | SYS-102 |
| COM-502 | 10 dB, 1000km | The link budget shall have adequate margin on uplink. | SYS-102 |
| COM-503 | 1,000km | The spacecraft shall be capable of communication with the ground at its maximum distance from the Durand ground station while within line of sight on orbit. | SYS-101 |
| Other | |||
| COM-601 | The spacecraft shall inhibit RF emissions before launch | SYS-201 | |
| COM-701 | The spacecraft shall interface with the TinyGS LoRa network. | COM-102 | |
| TinyGS is composed of 1200+ LoRa ground stations around the world, including several owned and operated by SSI satellites team members and parters. |