Just a quick reminder what to do, if you are trying to write a kaitai struct and want to use a “user process” to modify the bytestream before parsing:
- write your kaitai struct
- implement you user process in your preferred language
- implement the same functionality in Javascript (JS)
I ran through this process with the ELFIN decoder and implemented the following struct:
meta:
id: elfin
endian: be
seq:
- id: ax25_header
type: ax25_hdr
doc-ref: 'https://www.tapr.org/pub_ax25.html'
size: 16
- id: ax25_info
process: elfin_pp
size-eos: true
type:
switch-on: (_io.size > 78)
cases:
true: elfin_tlm_data
false: elfin_hkp_data
types:
ax25_hdr:
seq:
- id: dest_callsign
type: dest_callsign
- id: dest_ssid
type: u1
- id: src_callsign
type: src_callsign
- id: src_ssid
type: u1
- id: ctl
type: u1
- id: pid
type: u1
dest_callsign:
seq:
- id: dest_callsign
process: ror(1)
size: 6
src_callsign:
seq:
- id: src_callsign
process: ror(1)
size: 6
elfin_tlm_data:
doc-ref: 'https://elfin.igpp.ucla.edu/s/Beacon-Format_v2.xlsx'
seq:
- id: elfin_frame_start
type: u1
doc: |
0x94 marks the framestart
Any data byte following after this framestart must not contain:
0x27, 0x5e or 0x93, as 0x93 is the framestart and 0x5e is the
frameend! If a value reaches 0x93 or 0x5e, this byte is escaped by
0x27. This also throws an exception on values containing 0x27 itself.
An additional escape character (0x27) is added to escape the escape
character.
Substitutions:
b'\x2727' -> b'\x27'
b'\x275e' -> b'\x5e'
b'\x2793' -> b'\x93'
With some values containing 0x27 the maximum size of an ax.25 UI
frame is exceeded (254 bytes).
The 'preprocessing' to remove the escapoe sequences is done by an
external process called 'elfin_pp' and is implemented in a separate
file.
- id: elfin_beacon_setting
type: u1
- id: elfin_status_1_safe_mode
type: b1
- id: elfin_status_1_reserved
type: b3
- id: elfin_status_1_early_orbit
type: b4
doc: 'Safe mode (first bit), early orbit flags (last 4 bits)'
- id: elfin_status_2_payload_power
type: b1
- id: elfin_status_2_9v_boost
type: b1
- id: elfin_status_2_bat_htr_allow
type: b1
- id: elfin_status_2_htr_force
type: b1
- id: elfin_status_2_htr_alert
type: b1
- id: elfin_status_2_reserved
type: b3
doc: |
Bits 7 to 3 (in order):
Payload Power, 9V Boost, battery heater allow, heater force,
heater alert
- id: elfin_reserved
type: u1
- id: elfin_hskp_pwr1_rtcc_year
type: u1
- id: elfin_hskp_pwr1_rtcc_month
type: u1
- id: elfin_hskp_pwr1_rtcc_day
type: u1
- id: elfin_hskp_pwr1_rtcc_hour
type: u1
- id: elfin_hskp_pwr1_rtcc_minute
type: u1
- id: elfin_hskp_pwr1_rtcc_second
type: u1
- id: elfin_hskp_pwr1_adc_data_adc_sa_volt_12
type: u2
- id: elfin_hskp_pwr1_adc_data_adc_sa_volt_34
type: u2
- id: elfin_hskp_pwr1_adc_data_adc_sa_volt_56
type: u2
- id: elfin_hskp_pwr1_adc_data_sa_short_circuit_current
type: u2
- id: elfin_hskp_pwr1_adc_data_bat_2_volt
type: u2
- id: elfin_hskp_pwr1_adc_data_bat_1_volt
type: u2
- id: elfin_hskp_pwr1_adc_data_reg_sa_volt_1
type: u2
- id: elfin_hskp_pwr1_adc_data_reg_sa_volt_2
type: u2
- id: elfin_hskp_pwr1_adc_data_reg_sa_volt_3
type: u2
- id: elfin_hskp_pwr1_adc_data_power_bus_current_1
type: u2
- id: elfin_hskp_pwr1_adc_data_power_bus_current_2
type: u2
- id: elfin_hskp_pwr1_bat_mon_1_avg_cur_reg
type: s2
- id: elfin_hskp_pwr1_bat_mon_1_temperature_register
type: s2
- id: elfin_hskp_pwr1_bat_mon_1_volt_reg
type: s2
- id: elfin_hskp_pwr1_bat_mon_1_cur_reg
type: s2
- id: elfin_hskp_pwr1_bat_mon_1_acc_curr_reg
type: s2
- id: elfin_hskp_pwr1_bat_mon_2_avg_cur_reg
type: s2
- id: elfin_hskp_pwr1_bat_mon_2_temperature_register
type: s2
- id: elfin_hskp_pwr1_bat_mon_2_volt_reg
type: s2
- id: elfin_hskp_pwr1_bat_mon_2_cur_reg
type: s2
- id: elfin_hskp_pwr1_bat_mon_2_acc_curr_reg
type: s2
- id: elfin_hskp_pwr1_bv_mon
type: u2
- id: elfin_hskp_pwr1_tmps_tmp1
type: s2
- id: elfin_hskp_pwr1_tmps_tmp2
type: s2
- id: elfin_hskp_pwr1_tmps_tmp3
type: s2
- id: elfin_hskp_pwr1_tmps_tmp4
type: s2
- id: elfin_hskp_pwr1_accumulated_curr_bat1_rarc
type: u1
- id: elfin_hskp_pwr1_accumulated_curr_bat1_rsrc
type: u1
- id: elfin_hskp_pwr1_accumulated_curr_bat2_rarc
type: u1
- id: elfin_hskp_pwr1_accumulated_curr_bat2_rsrc
type: u1
- id: elfin_hskp_pwr2_rtcc_year
type: u1
- id: elfin_hskp_pwr2_rtcc_month
type: u1
- id: elfin_hskp_pwr2_rtcc_day
type: u1
- id: elfin_hskp_pwr2_rtcc_hour
type: u1
- id: elfin_hskp_pwr2_rtcc_minute
type: u1
- id: elfin_hskp_pwr2_rtcc_second
type: u1
- id: elfin_hskp_pwr2_adc_data_adc_sa_volt_12
type: u2
- id: elfin_hskp_pwr2_adc_data_adc_sa_volt_34
type: u2
- id: elfin_hskp_pwr2_adc_data_adc_sa_volt_56
type: u2
- id: elfin_hskp_pwr2_adc_data_sa_short_circuit_current
type: u2
- id: elfin_hskp_pwr2_adc_data_bat_2_volt
type: u2
- id: elfin_hskp_pwr2_adc_data_bat_1_volt
type: u2
- id: elfin_hskp_pwr2_adc_data_reg_sa_volt_1
type: u2
- id: elfin_hskp_pwr2_adc_data_reg_sa_volt_2
type: u2
- id: elfin_hskp_pwr2_adc_data_reg_sa_volt_3
type: u2
- id: elfin_hskp_pwr2_adc_data_power_bus_current_1
type: u2
- id: elfin_hskp_pwr2_adc_data_power_bus_current_2
type: u2
- id: elfin_hskp_pwr2_bat_mon_1_avg_cur_reg
type: s2
- id: elfin_hskp_pwr2_bat_mon_1_temperature_register
type: s2
- id: elfin_hskp_pwr2_bat_mon_1_volt_reg
type: s2
- id: elfin_hskp_pwr2_bat_mon_1_cur_reg
type: s2
- id: elfin_hskp_pwr2_bat_mon_1_acc_curr_reg
type: s2
- id: elfin_hskp_pwr2_bat_mon_2_avg_cur_reg
type: s2
- id: elfin_hskp_pwr2_bat_mon_2_temperature_register
type: s2
- id: elfin_hskp_pwr2_bat_mon_2_volt_reg
type: s2
- id: elfin_hskp_pwr2_bat_mon_2_cur_reg
type: s2
- id: elfin_hskp_pwr2_bat_mon_2_acc_curr_reg
type: s2
- id: elfin_hskp_pwr2_bv_mon
type: s2
- id: elfin_hskp_pwr2_tmps_tmp1
type: s2
- id: elfin_hskp_pwr2_tmps_tmp2
type: s2
- id: elfin_hskp_pwr2_tmps_tmp3
type: s2
- id: elfin_hskp_pwr2_tmps_tmp4
type: s2
- id: elfin_hskp_pwr2_accumulated_curr_bat1_rarc
type: u1
- id: elfin_hskp_pwr2_accumulated_curr_bat1_rsrc
type: u1
- id: elfin_hskp_pwr2_accumulated_curr_bat2_rarc
type: u1
- id: elfin_hskp_pwr2_accumulated_curr_bat2_rsrc
type: u1
- id: elfin_acb_pc_data1_rtcc_year
type: u1
- id: elfin_acb_pc_data1_rtcc_month
type: u1
- id: elfin_acb_pc_data1_rtcc_day
type: u1
- id: elfin_acb_pc_data1_rtcc_hour
type: u1
- id: elfin_acb_pc_data1_rtcc_minute
type: u1
- id: elfin_acb_pc_data1_rtcc_second
type: u1
- id: elfin_acb_pc_data1_acb_mrm_x
type: s2
- id: elfin_acb_pc_data1_acb_mrm_y
type: s2
- id: elfin_acb_pc_data1_acb_mrm_z
type: s2
- id: elfin_acb_pc_data1_ipdu_mrm_x
type: s2
- id: elfin_acb_pc_data1_ipdu_mrm_y
type: s2
- id: elfin_acb_pc_data1_ipdu_mrm_z
type: s2
- id: elfin_acb_pc_data1_tmps_tmp1
type: u2
- id: elfin_acb_pc_data1_tmps_tmp2
type: u2
- id: elfin_acb_pc_data1_tmps_tmp3
type: u2
- id: elfin_acb_pc_data1_tmps_tmp4
type: u2
- id: elfin_acb_pc_data2_rtcc_year
type: u1
- id: elfin_acb_pc_data2_rtcc_month
type: u1
- id: elfin_acb_pc_data2_rtcc_day
type: u1
- id: elfin_acb_pc_data2_rtcc_hour
type: u1
- id: elfin_acb_pc_data2_rtcc_minute
type: u1
- id: elfin_acb_pc_data2_rtcc_second
type: u1
- id: elfin_acb_pc_data2_acb_mrm_x
type: s2
- id: elfin_acb_pc_data2_acb_mrm_y
type: s2
- id: elfin_acb_pc_data2_acb_mrm_z
type: s2
- id: elfin_acb_pc_data2_ipdu_mrm_x
type: s2
- id: elfin_acb_pc_data2_ipdu_mrm_y
type: s2
- id: elfin_acb_pc_data2_ipdu_mrm_z
type: s2
- id: elfin_acb_pc_data2_tmps_tmp1
type: u2
- id: elfin_acb_pc_data2_tmps_tmp2
type: u2
- id: elfin_acb_pc_data2_tmps_tmp3
type: u2
- id: elfin_acb_pc_data2_tmps_tmp4
type: u2
- id: elfin_acb_sense_adc_data_current
type: u2le
- id: elfin_acb_sense_adc_data_voltage
type: u2le
- id: elfin_fc_counters_cmds_recv
type: u1
- id: elfin_fc_counters_badcmds_recv
type: u1
- id: elfin_fc_counters_badpkts_fm_radio
type: u1
- id: elfin_fc_counters_fcpkts_fm_radio
type: u1
- id: elfin_fc_counters_errors
type: u1
- id: elfin_fc_counters_reboots
type: u1
- id: elfin_fc_counters_intrnl_wdttmout
type: u1
- id: elfin_fc_counters_brwnouts
type: u1
- id: elfin_fc_counters_wdpicrst
type: u1
- id: elfin_fc_counters_porst
type: u1
- id: elfin_fc_counters_uart1_recvpkts
type: u1
- id: elfin_fc_counters_uart1_parseerrs
type: u1
- id: elfin_fc_counters_sips_ovcur_evts
type: u1
- id: elfin_fc_counters_vu1_on
type: u1
- id: elfin_fc_counters_vu1_off
type: u1
- id: elfin_fc_counters_vu2_on
type: u1
- id: elfin_fc_counters_vu2_off
type: u1
- id: elfin_radio_tlm_rssi
type: u1
- id: elfin_radio_tlm_bytes_rx
type: u4
- id: elfin_radio_tlm_bytes_tx
type: u4
- id: elfin_radio_cfg_read_radio_palvl
type: u1
- id: elfin_errors_error1_day
type: u1
- id: elfin_errors_error1_hour
type: u1
- id: elfin_errors_error1_minute
type: u1
- id: elfin_errors_error1_second
type: u1
- id: elfin_errors_error1_error
type: u1
- id: elfin_errors_error2_day
type: u1
- id: elfin_errors_error2_hour
type: u1
- id: elfin_errors_error2_minute
type: u1
- id: elfin_errors_error2_second
type: u1
- id: elfin_errors_error2_error
type: u1
- id: elfin_errors_error3_day
type: u1
- id: elfin_errors_error3_hour
type: u1
- id: elfin_errors_error3_minute
type: u1
- id: elfin_errors_error3_second
type: u1
- id: elfin_errors_error3_error
type: u1
- id: elfin_errors_error4_day
type: u1
- id: elfin_errors_error4_hour
type: u1
- id: elfin_errors_error4_minute
type: u1
- id: elfin_errors_error4_second
type: u1
- id: elfin_errors_error4_error
type: u1
- id: elfin_errors_error5_day
type: u1
- id: elfin_errors_error5_hour
type: u1
- id: elfin_errors_error5_minute
type: u1
- id: elfin_errors_error5_second
type: u1
- id: elfin_errors_error5_error
type: u1
- id: elfin_errors_error6_day
type: u1
- id: elfin_errors_error6_hour
type: u1
- id: elfin_errors_error6_minute
type: u1
- id: elfin_errors_error6_second
type: u1
- id: elfin_errors_error6_error
type: u1
- id: elfin_errors_error7_day
type: u1
- id: elfin_errors_error7_hour
type: u1
- id: elfin_errors_error7_minute
type: u1
- id: elfin_errors_error7_second
type: u1
- id: elfin_errors_error7_error
type: u1
- id: elfin_fc_salt
size: 4
- id: elfin_fc_crc
type: u1
- id: elfin_frame_end
type: u1
doc: '0x5e marks the end of a frame'
elfin_hkp_data:
seq:
- id: elfin_hskp_pwr1_rtcc_year
type: u1
- id: elfin_hskp_pwr1_rtcc_month
type: u1
- id: elfin_hskp_pwr1_rtcc_day
type: u1
- id: elfin_hskp_pwr1_rtcc_hour
type: u1
- id: elfin_hskp_pwr1_rtcc_minute
type: u1
- id: elfin_hskp_pwr1_rtcc_second
type: u1
- id: elfin_hskp_pwr1_pwr_board_id
type: u1
- id: elfin_hskp_pwr1_adc_data_adc_sa_volt_12
type: u2
- id: elfin_hskp_pwr1_adc_data_adc_sa_volt_34
type: u2
- id: elfin_hskp_pwr1_adc_data_adc_sa_volt_56
type: u2
- id: elfin_hskp_pwr1_adc_data_sa_short_circuit_current
type: u2
- id: elfin_hskp_pwr1_adc_data_bat_2_volt
type: u2
- id: elfin_hskp_pwr1_adc_data_bat_1_volt
type: u2
- id: elfin_hskp_pwr1_adc_data_reg_sa_volt_1
type: u2
- id: elfin_hskp_pwr1_adc_data_reg_sa_volt_2
type: u2
- id: elfin_hskp_pwr1_adc_data_reg_sa_volt_3
type: u2
- id: elfin_hskp_pwr1_adc_data_power_bus_current_1
type: u2
- id: elfin_hskp_pwr1_adc_data_power_bus_current_2
type: u2
- id: elfin_hskp_pwr1_bat_mon_1_avg_cur_reg
type: s2
- id: elfin_hskp_pwr1_bat_mon_1_temperature_register
type: s2
- id: elfin_hskp_pwr1_bat_mon_1_volt_reg
type: s2
- id: elfin_hskp_pwr1_bat_mon_1_cur_reg
type: s2
- id: elfin_hskp_pwr1_bat_mon_1_acc_curr_reg
type: s2
- id: elfin_hskp_pwr1_bat_mon_2_avg_cur_reg
type: s2
- id: elfin_hskp_pwr1_bat_mon_2_temperature_register
type: s2
- id: elfin_hskp_pwr1_bat_mon_2_volt_reg
type: s2
- id: elfin_hskp_pwr1_bat_mon_2_cur_reg
type: s2
- id: elfin_hskp_pwr1_bat_mon_2_acc_curr_reg
type: s2
- id: elfin_hskp_pwr1_bv_mon
type: u2
- id: elfin_hskp_pwr1_tmps_tmp1
type: s2
- id: elfin_hskp_pwr1_tmps_tmp2
type: s2
- id: elfin_hskp_pwr1_tmps_tmp3
type: s2
- id: elfin_hskp_pwr1_tmps_tmp4
type: s2
- id: elfin_hskp_pwr1_accumulated_curr_bat1_rsrc
type: u1
- id: elfin_hskp_pwr1_accumulated_curr_bat1_rarc
type: u1
- id: elfin_fc_status_safe_mode
type: b1
- id: elfin_fc_status_reserved
type: b3
- id: elfin_fc_status_early_orbit
type: b4
doc: 'Safe mode (first bit), early orbit flags (last 4 bits)'
type: u1
Then I got some help on implementing the “pre-processor” to remove the escape sequences, the flight computer adds if some special values are transmitted in the AX.25 data frame:
import binascii
class ElfinPp:
def decode(self, bindata):
i = 0
binlen = len(bindata)
out = b''
while i < binlen:
ch = ord(bindata[i])
if ch == 0x27 and i + 1 < binlen:
next_ch = ord(bindata[i + 1])
if next_ch == 0x27 or next_ch == 0x5e or next_ch == 0x9e:
i += 1
out += bindata[i]
i += 1
return out
…and tried to implement the same functionality to test the kaitai struct in the kaitai WebIDE, which is the “devel” Version which supports this functionality.
class ElfinPp {
constructor(_key) {
this.key = _key;
}
_read() {
this._io = {};
this._debug = {};
}
decode(bindata)
{
var binlen = bindata.length;
var ch, next_ch, i = 0;
var out = [];
while (i < binlen) {
ch = bindata[i];
if ((ch == 0x27) && ((i + 1) < binlen))
next_ch = bindata[i + 1];
if ((next_ch == 0x27) || (next_ch == 0x5e) || (next_ch == 0x9e))
i += 1;
out = out.concat(bindata[i]);
ch = 0, next_ch = 0;
i += 1;
}
var rbuf = new Uint8Array(out.length);
i = 0;
while (i < out.length)
{
rbuf[i] = out[i];
i += 1;
}
return rbuf;
}
} this.ElfinPp = ElfinPp;
This might not be the most elegant implementation in JS…
Now you have to “load” this code into the IDE following the instructions here.
In short:
- open kaitai WebIDE devel
- inside your browser open the java console (CTRL+SHIFT+i)
- paste your script enclosed in
localStorage.setItem("userTypes", `<your_code>`); - edit this code in the “JS code (debug)” tab as you need
- reload the input data file to see it working