Amateur radio services

The satellite is equipped with two UHF transceivers operating in half-duplex mode with cold standby redundancy. Both transceivers have HAM radio digipeaters which operate constantly but can be interrupted by telemetry, scientific data transmission or communication with the ground station.

• Callsign OK0CVR
• Frequency 436.025 MHz
• Speed 9600 bps
• Modulation GFSK (James Miller G3RUH compatible)
• HDLC Framing and scrambling
• NRZI Coding

Used protocols:
Morse code beacon (once a day only)
AX.25 (basic radio telemetry, downlink and uplink, TX)
CSP (general data transfer, downlink and uplink, TX OBC diagnostics)

Digipeater and DNxD

Digipeater allows immediate repetition of messages upon receipt.
DNxD allows repeating a message with a time delay that we specify in the message.

Message format for digipeater
Without a specific recipient (e.g., from station XY1Z):
XY1Z>CQ: your message
The satellite then repeats:
XY1Z>CQ,OK0CVR: your message

With a specified recipient (e.g., XY1A):
XY1Z>XY1A: your message
The satellite then repeats:
XY1Z>XY1A,OK0CVR: your message

DNxD (delayed repeater)
It works the same as the digipeater, but we add a transmission delay.
The delay is defined using @xxx, where xxx is the number of minutes (e.g., @060 = 60 minutes).
A space must follow @xxx.

Example message format:
XY1Z>CQ:@060 your message
Satellite confirmation:
The satellite confirms that the message will be sent in 60 minutes.
After the time elapses, the message is sent 3 times over the target destination in the format:
XY1Z>CQ,OK0CVR:@060 your message

Important note
The satellite can store only one DNxD message at a time.
If you attempt to upload another message while memory is occupied, the satellite will respond that it is busy and recommend trying again in x minutes.

You can find a detailed description of using the equal digipeater on the GRBAlpha satellite provided by ji1izr in his blog

Data message format

There are several messages the satellite OBC can provide, all of them are encoded using Cubesat Space Protocol (CSP)

Headers

All the messages from the satellite control computer expect to have the same CSP header
{0x31, 0x30, 0x00, 0x00}
Next byte after the header determines the type of the message

0x6A Telemetry message

Telemetry message expected to be broadcasted every 90 seconds, it contains basic information regarding the satellite state

Vbus

To convert Vbus to voltage value should be divided by 2048

Vsens

To convert Vsens to current value should be divided by 8192

Timestamp

This is Unix timestamp in seconds if highest bit is set to 1 this information is considered invalid

Data index

Data index shows the ring buffer index currently used to save scientific data

Temperature

Temperature in C

0x6C Acknowledge

This message is a response to the Ground station command with a single byte of data equal to the command type

0x66 Message from computer A

The whole message contains ASCII characters of the console output from the computer A. Length is variable and not transmitted in the message.

0x67 Message from computer B

The whole message contains ASCII characters of the console output from the computer A. Length is variable and not transmitted in the message.

0x6E Geiger data

This message contains an offset for the whole dataset (and the first set of data) and 52 data entries with individual offsets from the first one.

Offset

Offset is offset of the specific entry from the first one in 0.1s increments, it is expected to have offset 0 or 1 for the first event in the packet.
For the event N the real UTC timestamp is
Common timestamp + Offset / 10

Value

Value can hold one of two types of information: it's either the amount of the geiger counter events in 10s predating the entry timestamp or the doserate over 60 seconds predating the entry timestamp.
The highest bit of the value determines the value type, 1 means its amount of events since the last entry, 0 means it’s per hour doserate in 10nanoSievert increments.
To calculate the doserate in nanoSievert/h the value should be multiplied by 10.

AX.25 Transceiver telemetry

The data you receive are in CSV, comma separated, text file. All numbers there are decimal, so no need to convert from HEX, BIN or other formats. The comma separator between parts is always followed by part identifier (R, V, I2C1, etc.). Packet does not have fixed byte length, it always depends on what is inside. However it will always contain all parts that are mentioned in the table below.

Conversion table for the NTC reading: