Copyright © 2011 Keith Packard
This document is released under the terms of the Creative Commons ShareAlike 3.0 license.
|Revision 0.1||01 July 2011|
Table of Contents
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.
Table 1. Telemetry Packet Header
|0||uint16_t||serial||Device serial Number|
|2||uint16_t||tick||Device time in 100ths of a second|
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.
|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
Table 2. Sensor Packet Contents
|6||int16_t||accel||accelerometer (TM only)|
|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|
|24||int16_t||ground_pres||Average barometer reading on ground|
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
Table 3. Sensor Packet Contents
|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||Radio operator identifier|
|24||char||version||Software version identifier|
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
Table 4. GPS Location Packet Contents
|5||uint8_t||flags||See GPS Flags table below|
|8||int32_t||latitude||degrees * 107|
|12||int32_t||longitude||degrees * 107|
|25||uint8_t||mode||See GPS Mode table below|
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.
Table 5. GPS Flags
|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.
Table 6. GPS Mode
|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|
|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.
Table 7. GPS Satellite Data Contents
|5||uint8_t||channels||Number of reported satellite information|
|6||sat_info_t||sats||See Per-Satellite data table below|
Table 8. GPS Per-Satellite data (sat_info_t)
|0||uint8_t||svid||Space Vehicle Identifier|
|1||uint8_t||c_n_1||C/N1 signal quality indicator|
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.
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:
|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|
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||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|
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.
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.
|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.