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= AltOS Telemetry
Keith Packard <keithp@keithp.com>; Bdale Garbee <bdale@gag.com>
:revnumber: v{version}
:revdate: 01 Jan 1970
:copyright: Bdale Garbee and Keith Packard 2021
:stylesheet: am.css
:linkcss:
:doctype: article
:toc:
:numbered:
:pdf-themesdir: .
:pdf-theme: altusmetrum
:pdf-fontsdir: fonts
include::header.adoc[]
== 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 and newer 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 and newer have higher resolution barometric data than
TeleMetrum v1, and so the constant calibration data is
split out into a separate packet.
TeleMetrum v2 and newer 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 and newer Calibration Data packets are always transmitted once per second.
.TeleMetrum v2 and newer 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 and newer 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 v1
|CC1120
|35mW transceiver with SW FEC
|TeleMetrum v2, TeleMega v1
|CC1200
|35mW transceiver with HW FEC
|TeleMetrum v3, TeleMega v2, TeleDongle v3.0, TeleMini v3, TeleBT v3.0, TeleGPS v2
|CC115L
|14mW transmitter with SW FEC
|TeleGPS v1
|====
=== 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.