Packet Definitions Keith Packard 2011 Keith Packard This document is released under the terms of the Creative Commons ShareAlike 3.0 license. 0.1 01 July 2011 Initial content

AltOS telemetry data is split into multiple different packets, all the same size, but each includs an identifier so that the ground station can distinguish among different types. A single flight board will transmit multiple packet types, each type on a different schedule. The ground software need look for only a single packet size, and then decode the information within the packet and merge data from multiple packets to construct the full flight computer state. Each AltOS packet is 32 bytes long. This size was chosen based on the known telemetry data requirements. The power of two size allows them to be stored easily in flash memory without having them split across blocks or leaving gaps at the end. All packet types start with a five byte header which encodes the device serial number, device clock value and the packet type. The remaining 27 bytes encode type-specific data.

This section first defines the packet header common to all packets and then the per-packet data layout.

Offset Data Type Name Description 0 uint16_t serial Device serial Number 2 uint16_t tick Device time in 100ths of a second 4 uint8_t type Packet type 5 Each packet starts with these five bytes which serve to identify which device has transmitted the packet, when it was transmitted and what the rest of the packet contains.

Type Description 0x01 TeleMetrum v1.x Sensor Data 0x02 TeleMini Sensor Data 0x03 TeleNano Sensor Data TeleMetrum v1.x, TeleMini and TeleNano share this same packet format for sensor data. Each uses a distinct packet type so that the receiver knows which data values are valid and which are undefined. Sensor Data packets are transmitted once per second on the ground, 10 times per second during ascent and once per second during descent and landing Offset Data Type Name Description 5uint8_tstateFlight state 6int16_taccelaccelerometer (TM only) 8int16_tprespressure sensor 10int16_ttemptemperature sensor 12int16_tv_battbattery voltage 14int16_tsense_ddrogue continuity sense (TM/Tm) 16int16_tsense_mmain continuity sense (TM/Tm) 18int16_taccelerationm/s² * 16 20int16_tspeedm/s * 16 22int16_theightm 24int16_tground_presAverage barometer reading on ground 26int16_tground_accelTM 28int16_taccel_plus_gTM 30int16_taccel_minus_gTM 32

Type Description 0x08 TeleMega IMU Sensor Data 0x09 TeleMega Kalman and Voltage Data TeleMega has a lot of sensors, and so it splits the sensor data into two packets. The raw IMU data are sent more often; the voltage values don't change very fast, and the Kalman values can be reconstructed from the IMU data. IMU Sensor Data packets are transmitted once per second on the ground, 10 times per second during ascent and once per second during descent and landing Kalman and Voltage Data packets are transmitted once per second on the ground, 5 times per second during ascent and once per second during descent and landing The high-g accelerometer is reported separately from the data for the 9-axis IMU (accel/gyro/mag). The 9-axis IMU is mounted so that the X axis is "across" the board (along the short axis0, the Y axis is "along" the board (along the long axis, with the high-g accelerometer) and the Z axis is "through" the board (perpendicular to the board). Rotation measurements are around the respective axis, so Y rotation measures the spin rate of the rocket while X and Z rotation measure the tilt rate. The overall tilt angle of the rocket is computed by first measuring the orientation of the rocket on the pad using the 3 axis accelerometer, and then integrating the overall tilt rate from the 3 axis gyroscope to compute the total orientation change of the airframe since liftoff. Offset Data Type Name Description 5uint8_torientAngle from vertical in degrees 6int16_taccelHigh G accelerometer 8int32_tprespressure (Pa * 10) 12int16_ttemptemperature (°C * 100) 14int16_taccel_xX axis acceleration (across) 16int16_taccel_yY axis acceleration (along) 18int16_taccel_zZ axis acceleration (through) 20int16_tgyro_xX axis rotation (across) 22int16_tgyro_yY axis rotation (along) 24int16_tgyro_zZ axis rotation (through) 26int16_tmag_xX field strength (across) 28int16_tmag_yY field strength (along) 30int16_tmag_zZ field strength (through) 32 Offset Data Type Name Description 5uint8_tstateFlight state 6int16_tv_battbattery voltage 8int16_tv_pyropyro battery voltage 10int8_t[6]sensepyro continuity sense 16int32_tground_presAverage barometer reading on ground 20int16_tground_accelAverage accelerometer reading on ground 22int16_taccel_plus_gAccel calibration at +1g 24int16_taccel_minus_gAccel calibration at -1g 26int16_taccelerationm/s² * 16 28int16_tspeedm/s * 16 30int16_theightm 32

Type Description 0x0A TeleMetrum v2 Sensor Data 0x0B TeleMetrum v2 Calibration Data TeleMetrum v2 has higher resolution barometric data than TeleMetrum v1, and so the constant calibration data is split out into a separate packet. TeleMetrum v2 Sensor Data packets are transmitted once per second on the ground, 10 times per second during ascent and once per second during descent and landing TeleMetrum v2 Calibration Data packets are always transmitted once per second. Offset Data Type Name Description 5uint8_tstateFlight state 6int16_taccelaccelerometer 8int32_tprespressure sensor (Pa * 10) 12int16_ttemptemperature sensor (°C * 100) 14int16_taccelerationm/s² * 16 16int16_tspeedm/s * 16 18int16_theightm 20int16_tv_battbattery voltage 22int16_tsense_ddrogue continuity sense 24int16_tsense_mmain continuity sense 26pad[6]pad bytes 32 Offset Data Type Name Description 5pad[3]pad bytes 8int32_tground_presAverage barometer reading on ground 12int16_tground_accelAverage accelerometer reading on ground 14int16_taccel_plus_gAccel calibration at +1g 16int16_taccel_minus_gAccel calibration at -1g 18pad[14]pad bytes 32

Type Description 0x04 Configuration Data This provides a description of the software installed on the flight computer as well as any user-specified configuration data. Configuration data packets are transmitted once per second during all phases of the flight Offset Data Type Name Description 5uint8_ttypeDevice type 6uint16_tflightFlight number 8uint8_tconfig_majorConfig major version 9uint8_tconfig_minorConfig minor version 10uint16_tapogee_delay Apogee deploy delay in seconds 12uint16_tmain_deployMain deploy alt in meters 14uint16_tflight_log_max Maximum flight log size (kB) 16charcallsign[8]Radio operator identifier 24charversion[8]Software version identifier 32

Type Description 0x05 GPS Location This packet provides all of the information available from the GPS receiver—position, time, speed and precision estimates. GPS Location packets are transmitted once per second during all phases of the flight Offset Data Type Name Description 5uint8_tflags See GPS Flags table below 6int16_taltitudem 8int32_tlatitudedegrees * 107 12int32_tlongitudedegrees * 107 16uint8_tyear 17uint8_tmonth 18uint8_tday 19uint8_thour 20uint8_tminute 21uint8_tsecond 22uint8_tpdop* 5 23uint8_thdop* 5 24uint8_tvdop* 5 25uint8_tmode See GPS Mode table below 26uint16_tground_speedcm/s 28int16_tclimb_ratecm/s 30uint8_tcourse/ 2 31uint8_tunused[1] 32 Packed into a one byte field are status flags and the count of satellites used to compute the position fix. Note that this number may be lower than the number of satellites being tracked; the receiver will not use information from satellites with weak signals or which are close enough to the horizon to have significantly degraded position accuracy. Bits Name Description 0-3 nsats Number of satellites in solution 4 valid GPS solution is valid 5 running GPS receiver is operational 6 date_valid Reported date is valid 7 course_valid ground speed, course and climb rates are valid Here are all of the valid GPS operational modes. Altus Metrum products will only ever report 'N' (not valid), 'A' (Autonomous) modes or 'E' (Estimated). The remaining modes are either testing modes or require additional data. Mode Name Decsription N Not Valid All data are invalid A Autonomous mode Data are derived from satellite data D Differential Mode Data are augmented with differential data from a known ground station. The SkyTraq unit in TeleMetrum does not support this mode E Estimated Data are estimated using dead reckoning from the last known data M Manual Data were entered manually S Simulated GPS receiver testing mode

Type Description 0x06 GPS Satellite Data This packet provides space vehicle identifiers and signal quality information in the form of a C/N1 number for up to 12 satellites. The order of the svids is not specified. GPS Satellite data are transmitted once per second during all phases of the flight. Offset Data Type Name Description 5uint8_tchannels Number of reported satellite information 6sat_info_tsats[12] See Per-Satellite data table below 30uint8_tunused[2] 32 Offset Data Type Name Description 0uint8_tsvid Space Vehicle Identifier 1uint8_tc_n_1 C/N1 signal quality indicator 2

Type Description 0x07 Companion Data Data When a companion board is attached to TeleMega or TeleMetrum, it can provide telemetry data to be included in the downlink. The companion board can provide up to 12 16-bit data values. The companion board itself specifies the transmission rate. On the ground and during descent, that rate is limited to one packet per second. During ascent, that rate is limited to 10 packets per second. Offset Data Type Name Description 5uint8_tboard_id Type of companion board attached 6uint8_tupdate_period How often telemetry is sent, in 1/100ths of a second 7uint8_tchannels Number of data channels supplied 8uint16_t[12]companion_data Up to 12 channels of 16-bit companion data 32

Altus Metrum devices use Texas Instruments sub-GHz digital radio products. Ground stations use parts with HW FEC while some flight computers perform FEC in software. TeleGPS is transmit-only. Part Number Description Used in CC111110mW transceiver with integrated SoC TeleDongle v0.2, TeleBT v1.0, TeleMetrum v1.x, TeleMini CC112035mW transceiver with SW FEC TeleMetrum v2, TeleMega CC120035mW transceiver with HW FEC TeleDongle v3.0, TeleBT v3.0 CC115L14mW transmitter with SW FEC TeleGPS

Texas Instruments provides a tool for computing modulation parameters given a desired modulation format and basic bit rate. While we might like to use something with better low-signal performance like BPSK, the radios we use don't support that, but do support Gaussian frequency shift keying (GFSK). Regular frequency shift keying (FSK) encodes the signal by switching the carrier between two frequencies. The Gaussian version is essentially the same, but the shift between frequencies gently follows a gaussian curve, rather than switching immediately. This tames the bandwidth of the signal without affecting the ability to transmit data. For AltOS, there are three available bit rates, 38.4kBaud, 9.6kBaud and 2.4kBaud resulting in the following signal parmeters: Rate Deviation Receiver Bandwidth 38.4kBaud 20.5kHz 100kHz 9.6kBaud 5.125kHz 25kHz 2.4kBaud 1.5kHz 5kHz

The cc1111 and cc1200 provide forward error correction in hardware; on the cc1120 and cc115l that's done in software. AltOS uses this to improve reception of weak signals. As it's a rate 1/2 encoding, each bit of data takes two bits when transmitted, so the effective data rate is half of the raw transmitted bit rate. Parameter Value Description Error Correction Convolutional coding 1/2 rate, constraint length m=4 Interleaving 4 x 4 Reduce effect of noise burst Data Whitening XOR with 9-bit PNR Rotate right with bit 8 = bit 0 xor bit 5, initial value 111111111

TeleDongle does not do any interpretation of the packet data, instead it is configured to receive packets of a specified length (32 bytes in this case). For each received packet, TeleDongle produces a single line of text. This line starts with the string "TELEM " and is followed by a list of hexadecimal encoded bytes. TELEM 224f01080b05765e00701f1a1bbeb8d7b60b070605140c000600000000000000003fa988 The hexadecimal encoded string of bytes contains a length byte, the packet data, two bytes added by the cc1111 radio receiver hardware and finally a checksum so that the host software can validate that the line was transmitted without any errors. Offset Name Example Description 0 length 22 Total length of data bytes in the line. Note that this includes the added RSSI and status bytes 1 ·· length-3 packet 4f ·· 00 Bytes of actual packet data length-2 rssi 3f Received signal strength. dBm = rssi / 2 - 74 length-1 lqi a9 Link Quality Indicator and CRC status. Bit 7 is set when the CRC is correct length checksum 88 (0x5a + sum(bytes 1 ·· length-1)) % 256

The original AltoOS telemetry mechanism encoded everything available piece of information on the TeleMetrum hardware into a single unified packet. Initially, the packets contained very little data—some raw sensor readings along with the current GPS coordinates when a GPS receiver was connected. Over time, the amount of data grew to include sensor calibration data, GPS satellite information and a host of internal state information designed to help diagnose flight failures in case of a loss of the on-board flight data. Because every packet contained all of the data, packets were huge—95 bytes long. Much of the information was also specific to the TeleMetrum hardware. With the introduction of the TeleMini flight computer, most of the data contained in the telemetry packets was unavailable. Initially, a shorter, but still comprehensive packet was implemented. This required that the ground station be pre-configured as to which kind of packet to expect. The development of several companion boards also made the shortcomings evident—each companion board would want to include telemetry data in the radio link; with the original design, the packet would have to hold the new data as well, requiring additional TeleMetrum and ground station changes.