FloripaSat's Communication Info

Receiving/Transmiting and Decoding/Coding the Packets

FloripaSat-I is capable to communicate through two different bands: VHF and UHF. The first one is used to transmit a beacon signal and the second is the main communication band of satellite (Downlink and Uplink). The main information about them will be described as follows.

Orbit Parameters

The orbit configuration of the FloripaSat-I is not completly defined yet, and are still under analysis.

RF Parameters

Beacon

  • Frequency: 145,9 MHz
  • Modulation: GFSK
  • Deviation: 2,4 kHz
  • Baudrate: 1200 bps
  • Output Power: 30 dBm

Downlink and Uplink

  • Frequency: 437,9 MHz
  • Modulation: GFSK
  • Deviation: 5 kHz
  • Baudrate: 2400 bps
  • Output Power: 30 dBm (downlink only)

NOTE: The both frequencies will probably change during the operation license acquisition process.

Protocols

FloripaSat uses two protocols for communications between the satellite and the Earth: NGHam and AX.25.

Those protocols are described in the sections below.

NGHam

The figure below describes the NGHam protocol fields of each packet.

NGHAM v4

This is the main protocol of satellite and all communication links use it.

The only customized parameters from the protocol changed in FloripaSat, are the preamble and the sync. word:

  • Preamble: 0xAA, 0xAA, 0xAA, 0xAA
  • Sync. Word: 0x5D, 0xE6, 0x2A, 0x7E

More information about the NGHam protocol can be found here.

AX.25

The beacon packets are also transmitted using the AX.25 protocol (the same packet with both protocols).

Field Length [bytes] Content
Preamble 4 0xAA, 0xAA, 0xAA, 0xAA
Sync. word 4 0x5D, 0xE6, 0x2A, 0x7E
Flag 1 0x7E
Destination Address 7 Ground Station Callsign and SSID
Source Address 7 Satellite Callsign and SSID
Control 1 0x03
Protocol Identifier 1 0xF0
Data 60, 41 or 10 "FLORIPASAT" + Data
Frame Check Sequence 2 Variable
Flag 1 0x7E

NOTE: The callsign and SSID of both satellite and ground station, will be defined accordingly to the obtained licenses.

More information about the AX.25 protocol can be found here.

Packets Payload

Beacon

The Beacon of FloripaSat transmits packets periodically, with 5 possible periods (accordingly to the overall energy level of the satellite):

Level Beacon Period [s]
L1 10
L2 10
L3 20
L4 30
L5 30

In normal satellite operation, the beacon packets contains the following data:

Field Offset Length [bytes] Content
Satellite ID 0 10 "FLORIPASAT"
Batteries voltages 10 4 Variable
Batteries tempearatures 14 6 Variable
Total charge of batteries 20 2 Variable
Solar panels currents 22 12 Variable
Solar panels voltages 34 6 Variable
Overall status of the satellite 40 2 Variable
Accelerometer and gyroscope 42 12 Variable
Time since boot 54 4 Variable
Number of OBDH module resets since launch 58 2 Variable

If a fault on the OBDH module occurs (if the TTC module stops receiving data from it), only the EPS data are transmitted:

Field Offset Length [bytes] Content
Satellite ID 0 10 "FLORIPASAT"
Batteries voltages 10 4 Variable
Batteries tempearatures 14 6 Variable
Total charge of batteries 20 2 Variable
Solar panels currents 22 12 Variable
Solar panels voltages 34 6 Variable
Energy level 40 1 Variable

If a fault occurs in the OBDH and EPS modules, only the satellite ID is transmitted:

Field Offset Length [bytes] Content
Satellite ID 0 10 "FLORIPASAT"

Downlink

The Downlink of FloripaSat also transmits packets periodically (it contains a lot more data than the beacon packets), accordingly to the energy level of the satellite:

Level Downlink Period [s]
L1 30
L2 30
L3 30
L4 30
L5 30

The content of each periodic downlink packet is described in the table below:

Field Offset Length [bytes] Content
Flags 0 2 Variable
OBDH Status 2 6 Variable
IMU Accelerometer 8 12 Variable
IMU Gyroscope 20 12 Variable
OBDH Misc. 32 6 Variable
OBDH Uptime 38 4 Variable
Solar Panel Sensors 42 12 Variable
Main Radio 54 19 Variable
Solar Panels Data 73 18 Variable
EPS Misc. 91 8 Variable
Battery Monitor 99 21 Variable
Temperatures 120 21 Variable
Energy Level 141 1 Variable
RUSH Data 142 40 Variable
Payload X 182 7 Variable

The downlink also transmits sporadic packets with the answers of the uplink telecommands.

Uplink

There are four types of uplink packets: Data request, ping, message broadcast and hibernation. Each one is described in the table below:

Command Length [bytes] Content
Data request 8 "dr" + flags
Ping 8 "pg" + requester callsign
Message broadcast Up to 28 "br" + requester callsign + message
Hibernation Not public Not public
  • Data request: To request data of the satellite use this command. Selecting the flags, the user can download a set of past data of sensors or modules.
  • Ping: This command can be used to test the communication with satellite. When received, the satellite transmits back a ping packet.
  • Message broadcast: When received, the satellite transmits back the same content (an string with up to 20 characters) with the callsign of the source of command. This command can be used for communication between two or more ground stations.
  • Hibernation: Stops all the RF transmissions of the satellite for a desired period.

NOTE: The hibernation telecommand is not public and it is only available to partner ground stations.

Decoding the Packets

To decode the packets, there is a custom made sofware developed by the group. This software is capable of handling SDR devices, receive/transmit the signals and code/decode packets.

More information about this software can be found here. To obtain the program, go to this page and download the latest release.