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- - AltOS Telemetry - 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 - - - -
- Packet Format Design - - 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. - -
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- Packet Formats - This section first defines the packet header common to all packets - and then the per-packet data layout. -
- Packet Header - - Telemetry Packet Header - - - - - - - - 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. - -
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- Sensor Data - - - - - - - Type - Description - - - - - 0x01 - TeleMetrum Sensor Data - - - 0x02 - TeleMini Sensor Data - - - 0x03 - TeleNano Sensor Data - - - - - - TeleMetrum, 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 - - - Sensor Packet Contents - - - - - - - - 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_taccelm/s² * 16 - - - 20int16_tspeedm/s * 16 - - - 22int16_theightm - - - 24int16_tground_accelTM - - - 26int16_tground_presAverage barometer reading on ground - - - 28int16_taccel_plus_gTM - - - 30int16_taccel_minus_gTM - - - 32 - - - -
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- Configuration Data - - - - - - - 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 - - - Sensor Packet Contents - - - - - - - - Offset - Data Type - Name - Description - - - - - 5uint8_ttypeDevice type - - - 6uint16_tflightFlight number - - - 8uint8_tconfig_majorConfig major version - - - 9uint8_tconfig_minorConfig minor version - - - 10uint16_tmain_deployMain deploy alt in meters - - - 12uint32_tflight_log_maxMaximum flight log size (B) - - - 16charcallsign[8]Radio operator identifier - - - 24charversion[8]Software version identifier - - - 32 - - - -
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- GPS Location - - - - - - - Type - Description - - - - - 0x05 - GPS Location - - - - - - This packet provides all of the information available from the - Venus SkyTraq GPS receiver—position, time, speed and precision - estimates. - - - GPS Location packets are transmitted once per second during - all phases of the flight - - - GPS Location Packet Contents - - - - - - - - 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 - - - 28uint8_tcourse/ 2 - - - 29uint8_tunused[3] - - - 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. - - - GPS Flags - - - - - - - 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. - - - GPS Mode - - - - - - - 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 - - - -
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- GPS Satellite Data - - - - - - - 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. - - - GPS Satellite Data Contents - - - - - - - - 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 - - - -
- - GPS Per-Satellite data (sat_info_t) - - - - - - - - Offset - Data Type - Name - Description - - - - - 0uint8_tsvid - Space Vehicle Identifier - - - 1uint8_tc_n_1 - C/N1 signal quality indicator - - - 2 - - - -
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- Data Transmission - - Altus Metrum devices use the Texas Instruments CC1111 - microcontroller which includes an integrated sub-GHz digital - transceiver. This transceiver is used to both transmit and - receive the telemetry packets. This section discusses what - modulation scheme is used and how this device is configured. - -
- Modulation Scheme - - Texas Instruments provides a tool for computing modulation - parameters given a desired modulation format and basic bit - rate. For AltOS, the basic bit rate was specified as 38 kBaud, - resulting in the following signal parmeters: - - - - - - - - - Parameter - Value - Description - - - - - Modulation - GFSK - Gaussian Frequency Shift Keying - - - Deviation - 20.507812 kHz - Frequency modulation - - - Data rate - 38.360596 kBaud - Raw bit rate - - - RX Filter Bandwidth - 93.75 kHz - Receiver Band pass filter bandwidth - - - IF Frequency - 140.62 kHz - Receiver intermediate frequency - - - -
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- - The cc1111 provides forward error correction in hardware, - which AltOS uses to improve reception of weak signals. The - overall effect of this is to halve the available bandwidth for - data from 38 kBaud to 19 kBaud. - - Error Correction - - - - - - - - Parameter - Value - Description - - - - - Error Correction - Convolutional coding FEC - 1/2 code, 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 - - - -
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- History and Motivation - - 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. - -
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