altos: Perform time comparisons using 16-bit arithmetic to handle wrap

[fw/altos] / doc / telemetry.txt

= AltOS Telemetry
:doctype: article
:toc:
:numbered:

== 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.

== 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
                [options="border",cols="2,3,3,9"]
                |====
                |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.

        === TeleMetrum v1.x, TeleMini v1.0 and TeleNano Sensor Data

                .Sensor Packet Type
                [options="border",cols="1,3"]
                |====
                |Type   |Description
                |0x01   |TeleMetrum v1.x Sensor Data
                |0x02   |TeleMini v1.0 Sensor Data
                |0x03   |TeleNano Sensor Data
                |====

                TeleMetrum v1.x, TeleMini v1.0 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
                [options="border",cols="2,3,3,9"]
                |====
                |Offset |Data Type      |Name           |Description
                |5      |uint8_t        |state          |Flight state
                |6      |int16_t        |accel          |accelerometer (TM only)
                |8      |int16_t        |pres           |pressure sensor
                |10     |int16_t        |temp           |temperature sensor
                |12     |int16_t        |v_batt         |battery voltage
                |14     |int16_t        |sense_d        |drogue continuity sense (TM/Tm)
                |16     |int16_t        |sense_m        |main continuity sense (TM/Tm)
                |18     |int16_t        |acceleration   |m/s² * 16
                |20     |int16_t        |speed          |m/s * 16
                |22     |int16_t        |height         |m
                |24     |int16_t        |ground_pres    |Average barometer reading on ground
                |26     |int16_t        |ground_accel   |TM
                |28     |int16_t        |accel_plus_g   |TM
                |30     |int16_t        |accel_minus_g  |TM
                |32
                |====

        === TeleMega Sensor Data

                .TeleMega Packet Type
                [options="border",cols="1,3"]
                |====
                |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.

                .TeleMega IMU Sensor Packet Contents
                [options="border",cols="2,3,3,9"]
                |====
                |Offset |Data Type      |Name           |Description
                |5      |uint8_t        |orient         |Angle from vertical in degrees
                |6      |int16_t        |accel          |High G accelerometer
                |8      |int32_t        |pres           |pressure (Pa * 10)
                |12     |int16_t        |temp           |temperature (°C * 100)
                |14     |int16_t        |accel_x        |X axis acceleration (across)
                |16     |int16_t        |accel_y        |Y axis acceleration (along)
                |18     |int16_t        |accel_z        |Z axis acceleration (through)
                |20     |int16_t        |gyro_x         |X axis rotation (across)
                |22     |int16_t        |gyro_y         |Y axis rotation (along)
                |24     |int16_t        |gyro_z         |Z axis rotation (through)
                |26     |int16_t        |mag_x          |X field strength (across)
                |28     |int16_t        |mag_y          |Y field strength (along)
                |30     |int16_t        |mag_z          |Z field strength (through)
                |32
                |====

                .TeleMega Kalman and Voltage Data Packet Contents
                [options="border",cols="2,3,3,9"]
                |====
                |Offset |Data Type      |Name           |Description
                |5      |uint8_t        |state          |Flight state
                |6      |int16_t        |v_batt         |battery voltage
                |8      |int16_t        |v_pyro         |pyro battery voltage
                |10     |int8_t[6]      |sense          |pyro continuity sense
                |16     |int32_t        |ground_pres    |Average barometer reading on ground
                |20     |int16_t        |ground_accel   |Average accelerometer reading on ground
                |22     |int16_t        |accel_plus_g   |Accel calibration at +1g
                |24     |int16_t        |accel_minus_g  |Accel calibration at -1g
                |26     |int16_t        |acceleration   |m/s² * 16
                |28     |int16_t        |speed          |m/s * 16
                |30     |int16_t        |height         |m
                |32
                |====

        === TeleMetrum v2 Sensor Data

                .TeleMetrum v2 Packet Type
                [options="border",cols="1,3"]
                |====
                |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.

                .TeleMetrum v2 Sensor Packet Contents
                [options="border",cols="2,3,3,9"]
                |====
                |Offset |Data Type      |Name           |Description
                |5      |uint8_t        |state          |Flight state
                |6      |int16_t        |accel          |accelerometer
                |8      |int32_t        |pres           |pressure sensor (Pa * 10)
                |12     |int16_t        |temp           |temperature sensor (°C * 100)
                |14     |int16_t        |acceleration   |m/s² * 16
                |16     |int16_t        |speed          |m/s * 16
                |18     |int16_t        |height         |m
                |20     |int16_t        |v_batt         |battery voltage
                |22     |int16_t        |sense_d        |drogue continuity sense
                |24     |int16_t        |sense_m        |main continuity sense
                |26     |pad[6]         |pad bytes      |
                |32
                |====

                .TeleMetrum v2 Calibration Data Packet Contents
                [options="border",cols="2,3,3,9"]
                |====
                |Offset |Data Type      |Name           |Description
                |5      |pad[3]         |pad bytes      |
                |8      |int32_t        |ground_pres    |Average barometer reading on ground
                |12     |int16_t        |ground_accel   |Average accelerometer reading on ground
                |14     |int16_t        |accel_plus_g   |Accel calibration at +1g
                |16     |int16_t        |accel_minus_g  |Accel calibration at -1g
                |18     |pad[14]        |pad bytes      |
                |32
                |====

        === TeleMini v3.0 Sensor Data
        
                .Sensor Packet Type
                [options="border",cols="1,3"]
                |====
                |Type   |Description
                |0x11   |TeleMini v3.0 Sensor Data
                |====

                TeleMini v3.0 uses this
                packet format for sensor data.

                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
                [options="border",cols="2,3,3,9"]
                |====
                |Offset |Data Type      |Name           |Description
                |5      |uint8_t        |state          |Flight state
                |6      |int16_t        |v_batt         |battery voltage
                |8      |int16_t        |sense_a        |apogee continuity sense
                |10     |int16_t        |sense_m        |main continuity sense
                |12     |int32_t        |pres           |pressure sensor (Pa * 10)
                |16     |int16_t        |temp           |temperature sensor (°C * 100)
                |18     |int16_t        |acceleration   |m/s² * 16
                |20     |int16_t        |speed          |m/s * 16
                |22     |int16_t        |height         |m
                |24     |int16_t        |ground_pres    |Average barometer reading on ground
                |28     |pad[4]         |pad bytes      |
                |32
                |====


        === Configuration Data

                .Configuration Packet Type
                [options="border",cols="1,3"]
                |====
                |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

                .Configuration Packet Contents
                [options="border",cols="2,3,3,9"]
                |====
                |Offset |Data Type      |Name           |Description
                |5      |uint8_t        |type           |Device type
                |6      |uint16_t       |flight         |Flight number
                |8      |uint8_t        |config_major   |Config major version
                |9      |uint8_t        |config_minor   |Config minor version
                |10     |uint16_t       |apogee_delay   |Apogee deploy delay in seconds
                |12     |uint16_t       |main_deploy    |Main deploy alt in meters
                |14     |uint16_t       |flight_log_max |Maximum flight log size (kB)
                |16     |char           |callsign[8]    |Radio operator identifier
                |24     |char           |version[8]     |Software version identifier
                |32
                |====

        === GPS Location

                .GPS Packet Type
                [options="border",cols="1,3"]
                |====
                |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

                .GPS Location Packet Contents
                [options="border",cols="2,3,3,9"]
                |====
                |Offset |Data Type      |Name           |Description
                |5      |uint8_t        |flags          |See GPS Flags table below
                |6      |int16_t        |altitude       |m
                |8      |int32_t        |latitude       |degrees * 107
                |12     |int32_t        |longitude      |degrees * 107
                |16     |uint8_t        |year           |
                |17     |uint8_t        |month          |
                |18     |uint8_t        |day            |
                |19     |uint8_t        |hour           |
                |20     |uint8_t        |minute         |
                |21     |uint8_t        |second         |
                |22     |uint8_t        |pdop           |* 5
                |23     |uint8_t        |hdop           |* 5
                |24     |uint8_t        |vdop           |* 5
                |25     |uint8_t        |mode           |See GPS Mode table below
                |26     |uint16_t       |ground_speed   |cm/s
                |28     |int16_t        |climb_rate     |cm/s
                |30     |uint8_t        |course         |/ 2
                |31     |uint8_t        |unused[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.

                .GPS Flags
                [options="border",cols="1,2,7"]
                |====
                |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
                [options="border",cols="1,3,7"]
                |====
                |Mode           |Name                   |Description
                |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

                |====

        === GPS Satellite Data

                .GPS Satellite Data Packet Type
                [options="border",cols="1,3"]
                |====
                |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
                [options="border",cols="2,3,3,9"]
                |====
                |Offset |Data Type      |Name           |Description
                |5      |uint8_t        |channels       |Number of reported satellite information
                |6      |sat_info_t     |sats[12]       |See Per-Satellite data table below
                |30     |uint8_t        |unused[2]      |
                |32
                |====

                .GPS Per-Satellite data (sat_info_t)
                [options="border",cols="2,3,3,9"]
                |====
                |Offset |Data Type      |Name           |Description
                |0      |uint8_t        |svid           |Space Vehicle Identifier
                |1      |uint8_t        |c_n_1          |C/N1 signal quality indicator
                |2
                |====

        === Companion Data

                .Companion Data Packet Type
                [options="border",cols="1,3"]
                |====
                |Type   |Description
                |0x07   |Companion 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.

                .Companion Data Contents
                [options="border",cols="2,3,3,9"]
                |====
                |Offset |Data Type      |Name           |Description
                |5      |uint8_t        |board_id       |Type of companion board attached
                |6      |uint8_t        |update_period  |How often telemetry is sent, in 1/100ths of a second
                |7      |uint8_t        |channels       |Number of data channels supplied
                |8      |uint16_t[12]   |companion_data |Up to 12 channels of 16-bit companion data
                |32
                |====

== Data Transmission

        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.

        .Altus Metrum Radio Parts
        [options="border",cols="1,4,4"]
        |====
        |Part Number    |Description    |Used in

        |CC1111
        |10mW transceiver with integrated SoC
        |TeleDongle v0.2, TeleBT v1.0, TeleMetrum v1.x, TeleMini

        |CC1120
        |35mW transceiver with SW FEC
        |TeleMetrum v2, TeleMega

        |CC1200
        |35mW transceiver with HW FEC
        |TeleDongle v3.0, TeleBT v3.0

        |CC115L
        |14mW transmitter with SW FEC
        |TeleGPS

        |====

        === Modulation Scheme

                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:

                .Modulation Scheme
                [options="border",cols="1,1,1"]
                |====
                |Rate           |Deviation      |Receiver Bandwidth
                |38.4kBaud      |20.5kHz        |100kHz
                |9.6kBaud       |5.125kHz       |25kHz
                |2.4kBaud       |1.5kHz         |5kHz
                |====

        === Error Correction

                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.

                .Error Correction
                [options="border",cols="1,1,1"]
                |====
                |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 serial packet format

        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.

        .TeleDongle serial Packet Format

        [options="border",cols="2,1,1,5"]
        |====
        |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

        |====

== 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.