Communication specification

Technical specification

Hardware

KRAKsat is equipped with two hardware redundant UHF transceivers - parts of SR-NANO-BUS platform.

Frequency

KRAKsat was assigned coordinated frequency for up- and downlink 435.500 MHz with emission designator 25K0D1D and EIRP 32 dBm. Downlink modulation is AFSK 1200bps and uplink is 2GFSK 9600bps. Satellite callsign is SR9KRA.

Downlink

KRAKsat downlink communication can be one of three types:

• beacon - emitted every 15 minutes

• commands responses - specific to uplink type

• payload logs transmission - on request, encoded in Base91

SR-NANO-BUS downlink can be distinguished by their frames format. Data transceived by UHF1 is decoded as APRS message (with a hardcoded callsign), while data from UHF2 contains frame in APRS telemetry format (without a callsign).

Beacon

Beacon is emitted every 15 minutes by SR-NANO-BUS platform OBC. It consists of 8 frames, sent one by one from both radios. Beacon frames are ASCII strings, terminated by \0, containing information about the status of crucial components.

Beacon frame

	Frame name	Frame value
1	Satellite status	M1;STS;[User Flags (32-bit value)]
2	Master status	M1;LOG;[Timestamp];[Boot nr];[CPU voltage];[CPU temperature]
3	UHF1 Status	U1;RL;[Timestamp],[CPU voltage];[Battery voltage];[CPU temperature];[Amplifier temperature]
4	UHF1 Modem Status	U1;MS;[Timestamp];[Current RSSI];[Latch RSSI];[Afc frequency offset]
5	UHF2 Status	U2;RL;[Timestamp],[CPU voltage];[Battery voltage];[CPU temperature];[Amplifier temperature]
6	UHF2 Modem Status	U2;MS;[Timestamp];[Current RSSI];[Latch RSSI];[Afc frequency offset]
7	ADCS Flags Status	A1;FLAGS;[Fault] [Flags1] [Flags2] [Accelerometer temperature] [Magnetometer temperature]
8	Payload status	PL;STATUS;[Payload status as decimal number]

Data from frames 1-7 of the beacon status are calculated in SR-NANO-BUS platform OBC. Frame 8 with payload status is prepared in the payload software and updated every minute.

Beacon's first line, master status (M1;STS) is not properly implemented in SR-NANO-BUS platform, so data from 32-bit flags aren't generated correctly.

Payload status

Parameter	Bits	Comment
MODE	0, 1, 2
NEXT MODE	3, 4, 5
µC VOLTAGE	6, 7	0 - correct 1 - too big > 3.7V 2 - too small < 3.0V
SATELLITE VOLTAGE	8, 9	0 - correct 1 - too big > 9.5V 2 - too small < 9.5V
GYROSCOPE	10	0 - correct 1 - error
MAGNETOMETER	11	0 - correct 1 - error
IMU TEMPERATURE	12	0 - correct 1 - error
EXPERIMENT TEMPERATURE	13	0 - correct 1 - error
UNUSED (2 zeros)	14, 15

Mode and Next mode values in beacon payload status were encoded as 3-bit numbers while having 9 possible values. This software error caused inability to properly distinguish the current payload state.

Commands responses

Every uplink command has a defined command response, based on which the operator knows if the request was successfully received.

Command	Action	Response
ADCS_LOGS_ASCII_GET	Get ADCS FLAGS	A1;FLAGS;[fault] [flags1] [flags2] [acc temperature] [mag temperature]
AOCS_ADCS_CONTROL_FLAGS_SET	Set ADCS control flags
BROADCAST_TIMESTAMP_SET	Set the current timestamp on the satellite modules	M1; TS; [number of platform's boot]; [timestamp set]; [set value]
BROADCAST_DEV_RESET	Reboot the satellite
MASTER_CLEAN_PLAN	Clear the planner	M1;TSK;CLEAN_PLAN
MASTER_CMD_DOWNLINK_S	Enable/disable downlink for chosen radio	M1;DWN;154;0;1.
MASTER_FLASH_BLOCK_ERASE	Erase sector (64kB) of FLASH data from the given address (independent of buffers)	FLASH BLOCK ERASE STATUS;[address];[status]
MASTER_FLASH_SECTOR_ERASE	Erase the sector (4kB) of FLASH data from the given address (independent of buffers)	FLASH SECTOR ERASE STATUS;[address];[status]
MASTER_GET_NR_OF_TASKS	Return the current number of tasks in the planner	M1;TSK;LEN;[amount of tasks]
MASTER_GET_PLAN_G	Return information about the Mission Planner status	M1; TSK; [task number]; [time of planned execution]; [destination module]; [command]
MASTER_LOG_G	Get timestamp, boot number, CPU voltage and temperature	M1;LOG;[TS];[Boot nr];[CPU voltage];[CPU temperature]
MASTER_LOGGER_INFO	Information about the normal buffer	FL; [full buffer]; [amount of used space]; [start of data address]; [end of data address]
MASTER_LOGGER_POP	Remove a given amount of data from the normal buffer, along with the offset of the data origin address	FL; [full buffer]; [amount of used space]; [start of data address]; [end of data address]
MASTER_PAYLOAD_STATUS	Return the last received payload status	PL;STATUS;[payload status]
MASTER_PL_ADCS_COIL_X_VAL	Set PWM duty for X magnetorquer in manual mode
MASTER_PL_ADCS_COIL_Y_VAL	Set PWM duty for Y magnetorquer in manual mode
MASTER_PL_ADCS_COIL_Z_VAL	Set PWM duty for Z magnetorquer in manual mode
MASTER_PL_ADCS_COILS_ALL	Set PWM duty for all magnetorquers in manual mode
MASTER_PL_ADCS_CONFIG_K_X	Set the gain for detumbling the X axis
MASTER_PL_ADCS_CONFIG_K_Y	Set the gain for detumbling the Y axis
MASTER_PL_ADCS_CONFIG_K_Z	Set the gain for detumbling the Z axis
MASTER_PL_ADCS_MAX_DUTY_ALL	Set max PWM duty for all magnetorquers
MASTER_PL_ADCS_MAX_DUTY_X	Set max PWM duty for X magnetorquer
MASTER_PL_ADCS_MAX_DUTY_Y	Set max PWM duty for Y magnetorquer
MASTER_PL_ADCS_MAX_DUTY_Z	Set max PWM duty for Z magnetorquer
MASTER_PL_CALIBRATION_CONFIG	Set configuration param - config param ID - config param
MASTER_PL_DISABLE	Turn off the payload power supply
MASTER_PL_ENABLE	Turn on the payload power supply
MASTER_PL_EXP_ACC_TIME	Set experiment acceleration time
MASTER_PL_EXP_END_PERIOD	Set experiment end period
MASTER_PL_EXP_MODE	Set experiment parameters: -experiment mode -experiment duty
MASTER_PL_EXP_PARAMS1	Set experiment parameters: - Experiment mode - Experiment duty - Experiment period - Experiment PWM frequency - Experiment PWM duty
MASTER_PL_EXP_PARAMS2	Set experiment parameters: - Experiment starting period - Experiment end period - Experiment acceleration time
MASTER_PL_EXP_PERIOD	Set experiment period
MASTER_PL_EXP_PWM	Set experiment parameters: - experiment PWN frequency -experiment PWM duty
MASTER_PL_EXP_START_PERIOD	Set experiment starting period
MASTER_PL_FRW_LIST_ARG	Set experiment add list element
MASTER_PL_FRW_MAX_DUTY	Set the max PWM duty for FRW electromagnets
MASTER_PL_GO_IDLE	Set payload to IDLE mode
MASTER_PL_LOG1MIN_CONFIG _ADD_REG	Provide address of registers that are assigned to the log with a period of 1 min (add)
MASTER_PL_LOG1MIN_CONFIG _DELETE_REG	Provide address of registers that are no more assigned to the log with a period of 1 min (delete)
MASTER_PL_LOG1S_CONFIG _ADD_REG	Provide address of registers that are assigned to log with a period of 1000 ms (add)
MASTER_PL_LOG1S_CONFIG _DELETE_REG	Provide address of registers that are no more assigned to the log with a period of 1000 ms (delete)
MASTER_PL_LOG200MS_CONFIG _ADD_REG	Provide address of registers that are assigned to the log with a period of 200 ms (add)
MASTER_PL_LOG200MS_CONFIG _DELETE_REG	Provide address of registers that are no more assigned to the log with a period of 200ms (delete)
MASTER_PL_MODE_CONFIG	Set payload mode with parameters: -current mode -next mode -time for mode
MASTER_PL_WAKE_UP	Wake up payload from IDLE mode
MASTER_SPECIAL_LOGGER_INFO	Information about the special buffer	FSL; [full buffer]; [amount of used space]; [start of data address]; [end of data address]
MASTER_SPECIAL_LOGGER_POP	Remove a given amount of data from the special buffer, along with the offset of the data origin address	FSL; [full buffer]; [amount of used space]; [start of data address]; [end of data address]
MASTER_TIMESTAMP_GET	Get current timestamp from the satellite	M1;TG;[platforms boot amount]; [timestamp get]
MASTER_TRANS_END_FAIL	End of transaction - failure (no indicator shift)	PL;FINISH;sent = [address of the indicator];transaction FAIL
MASTER_TRANS_END_SUCCESS	End of transaction - success (indicator moved by the amount of data transferred)	PL; FINISH; sent = [address of the indicator], transaction SUCCESS
MASTER_TRANS_GET_CURRENT_ADDR	Information about the address of the indicator for the current transaction	PL; CURRENT ADDR = [address of the indicator]
MASTER_TRANS_SEND_1KB_LOG	Launch of a 1kB downlink	PL; [number 1kB part]; [chunk number]; [number of chunks]; [data] PL; INFO; WAITING FOR RETRANSMISSION REQUESTS PL; INFO; NO TRANSACTION IN PROGRESS
MASTER_TRANS_SEND_1KB_RETRY	Start the retransmission of a given data range within a part (1kB)	PL; R; [part number]; [chunk number]; [data] PL; RET; [part number]; [number of chunks]; 1kB retransmission done PL; INFO; NO TRANSACTION IN PROGRESS
MASTER_TRANS_SEND_NUM_PARTS	Initialization of a new transaction	PL;NOP;[timestamp];[number of parts in the transaction]
MASTER_TRANS_SET_ADDR	Set the address of the indicator for the current transaction	PL; TRANS START ADDR = [address of the indicator]
MASTER_TRANS_SET_CHUNKSIZE	Set the size of a single data chunk in the current transaction
MASTER_TRANS_SET_DATASIZE	Set the amount of data to be sent in the current transaction
MASTER_TRANS_SET_DEFAULT	Set the default transaction parameters
MASTER_TRANS_SET_NORMAL_DATA _START_ADDR	Set the normal transaction rate to the beginning address of the data in the normal buffer
MASTER_TRANS_SET_PARTSIZE	Set the size of a single piece of data in the current transaction
MASTER_TRANS_SET_SPECIAL_DATA _START_ADDR	Set the special transaction rate to the beginning address of the data in the special buffer
UHF_CHANNEL_RX_G	Get current frequency	U1;CH;[timestamp];[current channel]
UHF_CHANNEL_TX_G	Get current frequency	U1;CH;[timestamp];[current channel]
UHF_FEC_G	Read FEC flag	U1;FEC;[fec flag]
UHF_FEC_S	Set FEC flag (valid only for binary data sent through UHF)	U1;FEC;[fec flag]
UHF_LOG_G	Get battery voltage	[U1;RL;[timestamp],[3V3 bus voltage];[12V bus voltage];[RF amp temp];[MCU temp] ]
UHF_MODEM_STAT_G	Get radio data	[U1;MS;[timestamp];[current RSSI];[latched RSSI];[automatic frequency control offset] ]
UHF1_ECHO	Send an echo	48byte ASCII STRING
UHF2_ECHO	Send an echo	48byte ASCII STRING

SR-NANO-BUS platform contains IMU sensors for ADCS (gyroscope and magnetometer) but because of a software bug, IMU data can't be transmitted. It's connected with the fact that instead of being sent as ASCII, frames with IMU data are binary. '\0' sign that is used in APRS as the end of line can be anywhere in ADCS data, so if it's somewhere at the beginning, all the data behind won't be copied to AX.25 frame and transmitted.

Payload logs transmission on request

Payload logs are saved in the flash memory, part of SR-NANO-BUS platform. There is no special way of handling data e.g. filesystem - every new binary frame is just put into the first free memory address, depending on the payload mode (as described in logging logic).

Flash memory	Special	Normal
INFO	0x0	0x1000
CAPACITY	6MB - 16KB	2MB - 16KB
START	0x 20 00	0x60 00 00
END	0x 5F E0 00	0x 7F C0 00

Payload log frame

Every log frame contains 6 bytes of information - relative timestamp, register address (key) and register value. Its available parameters and logging interval are specific to the current payload mode. All log frames are prepared in the payload software and transmitted to the OBC as described in Logging logic chapter.

Log frame info	Size
Relative timestamp	2B
Register address	2B
Register value	2B

Raw payload log frames

Payload log frames downlink

Log frames downlink is accomplished in form of a transaction. First, the transaction has to be initialized and a chosen address set. After that, 1 KB of logs can be downloaded. A part of data consists of chunks (typically 35-38 of them), numbered and encoded in Base91. Base91 algorithm used by SR-NANO-BUS platform is so-called rlyeh's modification.

Although UHF1 and UHF2 from SR-NANO-BUS are hardware redundant, their software has some differences. The biggest difference manifests itself in the fact that logs transmission can be provided only using UHF1. UHF2 can never transmit log frames.

Example of payload logs (1 kB)

19/07/18:00/12/23:SR9KRA-6:=PL;   0; 0;37;AABtWAAAAAOmUEAAC'WAAAAAOmUE5FAA5A
19/07/18:00/12/24:SR9KRA-6:=PL;   0; 1;37;BAUA^XHnZ4AA,'pB@AoI~+OtCAv|&DDHAA
19/07/18:00/12/25:SR9KRA-6:=PL;   0; 2;37;3+`AKAg@u^McAA`wyB@ArPP&LtCA2OfbDH
19/07/18:00/12/26:SR9KRA-6:=PL;   0; 3;37;uWGXFCoA\20^uWBAyE&MvDAAFAC'CAzDAA
19/07/18:00/12/27:SR9KRA-6:=PL;   0; 4;37;vDuWKAAAKA)_bWMcBtjtrB*B@Q/LRAFAv|
19/07/18:00/12/28:SR9KRA-6:=PL;   0; 5;37;^ADHC'yemAKAR`kpMcC'v~sAoAiay9wWBA
19/07/18:00/12/29:SR9KRA-6:=PL;   0; 6;37;tNiKvDN/YcxAKA?zpVMcBtNuUD*BBtCAC'
19/07/18:00/12/30:SR9KRA-6:=PL;   0; 7;37;CA,BAAvDXLFAC'HAj|1mJVuW#W(A8A5*~t
19/07/18:00/12/31:SR9KRA-6:=PL;   0; 8;37;,\BAk~dBZFBtpJHAPAR`pqSqC'`,QCdBrP
19/07/18:00/12/32:SR9KRA-6:=PL;   0; 9;37;m=LtHA&qEDJVuWGXICdB#TJ:OtHAJJkYJV
19/07/18:00/12/33:SR9KRA-6:=PL;   0;10;37;AAKAAAPALHAAJVBtUAAAoAr?xcuWD'jtrB
19/07/18:00/12/34:SR9KRA-6:=PL;   0;11;37;vD@QcebAKAv|9CMcAAZg#AoAg@^_uWBAr\
19/07/18:00/12/35:SR9KRA-6:=PL;   0;12;37;mA*BrPOVJAFAtx^UGOY4x4GB@Ao.J=BtCA
19/07/18:00/12/36:SR9KRA-6:=PL;   0;13;37;ZCtGDHC'CAAAFA,BAADHXLFAC'MAtZ~nPj
19/07/18:00/12/37:SR9KRA-6:=PL;   0;14;37;uW#W(AJBVx8xyWGA+}\B1RC';7XAZAR`'n
19/07/18:00/12/38:SR9KRA-6:=PL;   0;15;37;e+C'QmRBSCrPwPKtMAgw)RPjuWGXPCSC#T
19/07/18:00/12/39:SR9KRA-6:=PL;   0;16;37;ReBtMAJJZZPjAAKAAAZALHAAPjBtUAAA8A
19/07/18:00/12/40:SR9KRA-6:=PL;   0;17;37;|L$/,\D'jtpBZF@Qg\WAPAv|}BSqAAjyVA
19/07/18:00/12/41:SR9KRA-6:=PL;   0;18;37;8Ag@/]+\BACZKCZF{eFfSAPAV%!fSquWGX
19/07/18:00/12/42:SR9KRA-6:=PL;   0;19;37;OC6C#T-lD'HAyE&MJVAAFAAAPAzDAAJVuW
19/07/18:00/12/43:SR9KRA-6:=PL;   0;20;37;KAAAjA)_FLoICtjtrBOG@Q~(U'RAv|nEVx
19/07/18:00/12/44:SR9KRA-6:=PL;   0;21;37;AAH2.A+Ag@@=@QCAp0WDOG{eFfT'RAs-pT
19/07/18:00/12/45:SR9KRA-6:=PL;   0;22;37;VxuWGXVCHD#TJKKtRAJJ!aVxAAKAAAjALH
19/07/18:00/12/46:SR9KRA-6:=PL;   0;23;37;AAVxBtUAAA@A}4..CtE'jtrBDH@QK0JAUA
19/07/18:00/12/47:SR9KRA-6:=PL;   0;24;37;v||EY4AAtk8A@Ag@1\BtCAGNODDH{eFfRA
19/07/18:00/12/48:SR9KRA-6:=PL;   0;25;37;UA37XsY4uWGXaCvD#T-1EAKAyE3NMcAAFA
19/07/18:00/12/49:SR9KRA-6:=PL;   0;26;37;AAUAzDAAMcuWKAAAtA(n$,0kCtjtrB^H@Q
19/07/18:00/12/50:SR9KRA-6:=PL;   0;27;37;GUD'bAiqInh=uW#W(AmCVx5#2(NA+}bC!m
19/07/18:00/12/51:SR9KRA-6:=PL;   0;28;37;C'd1gA3AR`#o$$D'hD`B-ErP9}JtbA&q8I
19/07/18:00/12/52:SR9KRA-6:=PL;   0;29;37;h=uWGXSC-E#Tq|EtbAJJJZh=AAKAAA3ALH
19/07/18:00/12/53:SR9KRA-6:=PL;   0;30;37;AAh=BtUAAAdB_)FyEtHA,'rByK@QXpCAeA
19/07/18:00/12/54:SR9KRA-6:=PL;   0;31;37;v|&Dk_AAEvgAdB!+z)CtHA=H5KJV[9JRkA
19/07/18:00/12/55:SR9KRA-6:=PL;   0;32;37;8A`ROh+\CtMugEyKjn17CAeAJJUYk_AAKA
19/07/18:00/12/56:SR9KRA-6:=PL;   0;33;37;AA8ALHAAk_BtUAAAnB^X@|aLIA,'rBnL@Q
19/07/18:00/12/57:SR9KRA-6:=PL;   0;34;37;r=L'gAv|yDlBBAX+`AnB!+`!YLIASouD\W
19/07/18:00/12/58:SR9KRA-6:=PL;   0;35;37;[94_iA%A{!^m:CDtMuqEnLjn0WX'gAJJZZ
19/07/18:00/12/59:SR9KRA-6:=PL;   0;36;37;lBBAK