1 <html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>AltOS Telemetry</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="article"><div class="titlepage"><div><div><h2 class="title"><a name="idp22409744"></a>AltOS Telemetry</h2></div><div><h3 class="subtitle"><i>Packet Definitions</i></h3></div><div><div class="author"><h3 class="author"><span class="firstname">Keith</span> <span class="surname">Packard</span></h3></div></div><div><p class="copyright">Copyright © 2011 Keith Packard</p></div><div><div class="legalnotice"><a name="idp48922224"></a><p>
2 This document is released under the terms of the
3 <a class="ulink" href="http://creativecommons.org/licenses/by-sa/3.0/" target="_top">
4 Creative Commons ShareAlike 3.0
7 </p></div></div><div><div class="revhistory"><table style="border-style:solid; width:100%;" summary="Revision History"><tr><th align="left" valign="top" colspan="2"><b>Revision History</b></th></tr><tr><td align="left">Revision 0.1</td><td align="left">01 July 2011</td></tr><tr><td align="left" colspan="2">Initial content</td></tr></table></div></div></div><hr></div><div class="toc"><p><b>Table of Contents</b></p><dl class="toc"><dt><span class="section"><a href="#idp49629568">1. Packet Format Design</a></span></dt><dt><span class="section"><a href="#idp49632560">2. Packet Formats</a></span></dt><dd><dl><dt><span class="section"><a href="#idp49633760">2.1. Packet Header</a></span></dt><dt><span class="section"><a href="#idp48868944">2.2. Sensor Data</a></span></dt><dt><span class="section"><a href="#idp54256176">2.3. Configuration Data</a></span></dt><dt><span class="section"><a href="#idp54292032">2.4. GPS Location</a></span></dt><dt><span class="section"><a href="#idp54365584">2.5. GPS Satellite Data</a></span></dt></dl></dd><dt><span class="section"><a href="#idp54404304">3. Data Transmission</a></span></dt><dd><dl><dt><span class="section"><a href="#idp54405680">3.1. Modulation Scheme</a></span></dt><dt><span class="section"><a href="#idp54422112">3.2. Error Correction</a></span></dt></dl></dd><dt><span class="section"><a href="#idp54435584">4. TeleDongle packet format</a></span></dt><dt><span class="section"><a href="#idp54457360">5. History and Motivation</a></span></dt></dl></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idp49629568"></a>1. Packet Format Design</h2></div></div></div><p>
8 AltOS telemetry data is split into multiple different packets,
9 all the same size, but each includs an identifier so that the
10 ground station can distinguish among different types. A single
11 flight board will transmit multiple packet types, each type on a
12 different schedule. The ground software need look for only a
13 single packet size, and then decode the information within the
14 packet and merge data from multiple packets to construct the
15 full flight computer state.
17 Each AltOS packet is 32 bytes long. This size was chosen based
18 on the known telemetry data requirements. The power of two size
19 allows them to be stored easily in flash memory without having
20 them split across blocks or leaving gaps at the end.
22 All packet types start with a five byte header which encodes the
23 device serial number, device clock value and the packet
24 type. The remaining 27 bytes encode type-specific data.
25 </p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idp49632560"></a>2. Packet Formats</h2></div></div></div><p>
26 This section first defines the packet header common to all packets
27 and then the per-packet data layout.
28 </p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idp49633760"></a>2.1. Packet Header</h3></div></div></div><div class="table"><a name="idp49634432"></a><p class="title"><b>Table 1. Telemetry Packet Header</b></p><div class="table-contents"><table summary="Telemetry Packet Header" border="1"><colgroup><col align="center" class="Offset"><col align="center" class="Data Type"><col align="left" class="Name"><col align="left" class="Description"></colgroup><thead><tr><th align="center">Offset</th><th align="center">Data Type</th><th align="center">Name</th><th align="center">Description</th></tr></thead><tbody><tr><td align="center">0</td><td align="center">uint16_t</td><td align="left">serial</td><td align="left">Device serial Number</td></tr><tr><td align="center">2</td><td align="center">uint16_t</td><td align="left">tick</td><td align="left">Device time in 100ths of a second</td></tr><tr><td align="center">4</td><td align="center">uint8_t</td><td align="left">type</td><td align="left">Packet type</td></tr><tr><td align="center">5</td><td class="auto-generated"> </td><td class="auto-generated"> </td><td class="auto-generated"> </td></tr></tbody></table></div></div><br class="table-break"><p>
29 Each packet starts with these five bytes which serve to identify
30 which device has transmitted the packet, when it was transmitted
31 and what the rest of the packet contains.
32 </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idp48868944"></a>2.2. Sensor Data</h3></div></div></div><div class="informaltable"><table border="0"><colgroup><col align="center" class="Offset"><col align="left" class="Description"></colgroup><thead><tr><th align="center">Type</th><th align="left">Description</th></tr></thead><tbody><tr><td align="center">0x01</td><td align="left">TeleMetrum Sensor Data</td></tr><tr><td align="center">0x02</td><td align="left">TeleMini Sensor Data</td></tr><tr><td align="center">0x03</td><td align="left">TeleNano Sensor Data</td></tr></tbody></table></div><p>
33 TeleMetrum, TeleMini and TeleNano share this same packet
34 format for sensor data. Each uses a distinct packet type so
35 that the receiver knows which data values are valid and which
38 Sensor Data packets are transmitted once per second on the
39 ground, 10 times per second during ascent and once per second
40 during descent and landing
41 </p><div class="table"><a name="idp48748512"></a><p class="title"><b>Table 2. Sensor Packet Contents</b></p><div class="table-contents"><table summary="Sensor Packet Contents" border="1"><colgroup><col align="center" class="Offset"><col align="center" class="Data Type"><col align="left" class="Name"><col align="left" class="Description"></colgroup><thead><tr><th align="center">Offset</th><th align="center">Data Type</th><th align="center">Name</th><th align="center">Description</th></tr></thead><tbody><tr><td align="center">5</td><td align="center">uint8_t</td><td align="left">state</td><td align="left">Flight state</td></tr><tr><td align="center">6</td><td align="center">int16_t</td><td align="left">accel</td><td align="left">accelerometer (TM only)</td></tr><tr><td align="center">8</td><td align="center">int16_t</td><td align="left">pres</td><td align="left">pressure sensor</td></tr><tr><td align="center">10</td><td align="center">int16_t</td><td align="left">temp</td><td align="left">temperature sensor</td></tr><tr><td align="center">12</td><td align="center">int16_t</td><td align="left">v_batt</td><td align="left">battery voltage</td></tr><tr><td align="center">14</td><td align="center">int16_t</td><td align="left">sense_d</td><td align="left">drogue continuity sense (TM/Tm)</td></tr><tr><td align="center">16</td><td align="center">int16_t</td><td align="left">sense_m</td><td align="left">main continuity sense (TM/Tm)</td></tr><tr><td align="center">18</td><td align="center">int16_t</td><td align="left">acceleration</td><td align="left">m/s² * 16</td></tr><tr><td align="center">20</td><td align="center">int16_t</td><td align="left">speed</td><td align="left">m/s * 16</td></tr><tr><td align="center">22</td><td align="center">int16_t</td><td align="left">height</td><td align="left">m</td></tr><tr><td align="center">24</td><td align="center">int16_t</td><td align="left">ground_pres</td><td align="left">Average barometer reading on ground</td></tr><tr><td align="center">26</td><td align="center">int16_t</td><td align="left">ground_accel</td><td align="left">TM</td></tr><tr><td align="center">28</td><td align="center">int16_t</td><td align="left">accel_plus_g</td><td align="left">TM</td></tr><tr><td align="center">30</td><td align="center">int16_t</td><td align="left">accel_minus_g</td><td align="left">TM</td></tr><tr><td align="center">32</td><td class="auto-generated"> </td><td class="auto-generated"> </td><td class="auto-generated"> </td></tr></tbody></table></div></div><br class="table-break"></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idp54256176"></a>2.3. Configuration Data</h3></div></div></div><div class="informaltable"><table border="0"><colgroup><col align="center" class="Offset"><col align="left" class="Description"></colgroup><thead><tr><th align="center">Type</th><th align="left">Description</th></tr></thead><tbody><tr><td align="center">0x04</td><td align="left">Configuration Data</td></tr></tbody></table></div><p>
42 This provides a description of the software installed on the
43 flight computer as well as any user-specified configuration data.
45 Configuration data packets are transmitted once per second
46 during all phases of the flight
47 </p><div class="table"><a name="idp54265584"></a><p class="title"><b>Table 3. Sensor Packet Contents</b></p><div class="table-contents"><table summary="Sensor Packet Contents" border="1"><colgroup><col align="center" class="Offset"><col align="center" class="Data Type"><col align="left" class="Name"><col align="left" class="Description"></colgroup><thead><tr><th align="center">Offset</th><th align="center">Data Type</th><th align="center">Name</th><th align="center">Description</th></tr></thead><tbody><tr><td align="center">5</td><td align="center">uint8_t</td><td align="left">type</td><td align="left">Device type</td></tr><tr><td align="center">6</td><td align="center">uint16_t</td><td align="left">flight</td><td align="left">Flight number</td></tr><tr><td align="center">8</td><td align="center">uint8_t</td><td align="left">config_major</td><td align="left">Config major version</td></tr><tr><td align="center">9</td><td align="center">uint8_t</td><td align="left">config_minor</td><td align="left">Config minor version</td></tr><tr><td align="center">10</td><td align="center">uint16_t</td><td align="left">apogee_delay</td><td align="left">Apogee deploy delay in seconds</td></tr><tr><td align="center">12</td><td align="center">uint16_t</td><td align="left">main_deploy</td><td align="left">Main deploy alt in meters</td></tr><tr><td align="center">14</td><td align="center">uint16_t</td><td align="left">flight_log_max</td><td align="left">Maximum flight log size (kB)</td></tr><tr><td align="center">16</td><td align="center">char</td><td align="left">callsign[8]</td><td align="left">Radio operator identifier</td></tr><tr><td align="center">24</td><td align="center">char</td><td align="left">version[8]</td><td align="left">Software version identifier</td></tr><tr><td align="center">32</td><td class="auto-generated"> </td><td class="auto-generated"> </td><td class="auto-generated"> </td></tr></tbody></table></div></div><br class="table-break"></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idp54292032"></a>2.4. GPS Location</h3></div></div></div><div class="informaltable"><table border="0"><colgroup><col align="center" class="Offset"><col align="left" class="Description"></colgroup><thead><tr><th align="center">Type</th><th align="left">Description</th></tr></thead><tbody><tr><td align="center">0x05</td><td align="left">GPS Location</td></tr></tbody></table></div><p>
48 This packet provides all of the information available from the
49 Venus SkyTraq GPS receiver—position, time, speed and precision
52 GPS Location packets are transmitted once per second during
53 all phases of the flight
54 </p><div class="table"><a name="idp54301696"></a><p class="title"><b>Table 4. GPS Location Packet Contents</b></p><div class="table-contents"><table summary="GPS Location Packet Contents" border="1"><colgroup><col align="center" class="Offset"><col align="center" class="Data Type"><col align="left" class="Name"><col align="left" class="Description"></colgroup><thead><tr><th align="center">Offset</th><th align="center">Data Type</th><th align="center">Name</th><th align="center">Description</th></tr></thead><tbody><tr><td align="center">5</td><td align="center">uint8_t</td><td align="left">flags</td><td align="left">See GPS Flags table below</td></tr><tr><td align="center">6</td><td align="center">int16_t</td><td align="left">altitude</td><td align="left">m</td></tr><tr><td align="center">8</td><td align="center">int32_t</td><td align="left">latitude</td><td align="left">degrees * 10<sup>7</sup></td></tr><tr><td align="center">12</td><td align="center">int32_t</td><td align="left">longitude</td><td align="left">degrees * 10<sup>7</sup></td></tr><tr><td align="center">16</td><td align="center">uint8_t</td><td align="left">year</td><td class="auto-generated"> </td></tr><tr><td align="center">17</td><td align="center">uint8_t</td><td align="left">month</td><td class="auto-generated"> </td></tr><tr><td align="center">18</td><td align="center">uint8_t</td><td align="left">day</td><td class="auto-generated"> </td></tr><tr><td align="center">19</td><td align="center">uint8_t</td><td align="left">hour</td><td class="auto-generated"> </td></tr><tr><td align="center">20</td><td align="center">uint8_t</td><td align="left">minute</td><td class="auto-generated"> </td></tr><tr><td align="center">21</td><td align="center">uint8_t</td><td align="left">second</td><td class="auto-generated"> </td></tr><tr><td align="center">22</td><td align="center">uint8_t</td><td align="left">pdop</td><td align="left">* 5</td></tr><tr><td align="center">23</td><td align="center">uint8_t</td><td align="left">hdop</td><td align="left">* 5</td></tr><tr><td align="center">24</td><td align="center">uint8_t</td><td align="left">vdop</td><td align="left">* 5</td></tr><tr><td align="center">25</td><td align="center">uint8_t</td><td align="left">mode</td><td align="left">See GPS Mode table below</td></tr><tr><td align="center">26</td><td align="center">uint16_t</td><td align="left">ground_speed</td><td align="left">cm/s</td></tr><tr><td align="center">28</td><td align="center">int16_t</td><td align="left">climb_rate</td><td align="left">cm/s</td></tr><tr><td align="center">30</td><td align="center">uint8_t</td><td align="left">course</td><td align="left">/ 2</td></tr><tr><td align="center">31</td><td align="center">uint8_t</td><td align="left">unused[1]</td><td class="auto-generated"> </td></tr><tr><td align="center">32</td><td class="auto-generated"> </td><td class="auto-generated"> </td><td class="auto-generated"> </td></tr></tbody></table></div></div><br class="table-break"><p>
55 Packed into a one byte field are status flags and the count of
56 satellites used to compute the position fix. Note that this
57 number may be lower than the number of satellites being
58 tracked; the receiver will not use information from satellites
59 with weak signals or which are close enough to the horizon to
60 have significantly degraded position accuracy.
61 </p><div class="table"><a name="idp51294064"></a><p class="title"><b>Table 5. GPS Flags</b></p><div class="table-contents"><table summary="GPS Flags" border="1"><colgroup><col align="center" class="bits"><col align="left" class="name"><col align="left" class="description"></colgroup><thead><tr><th align="center">Bits</th><th align="center">Name</th><th align="center">Description</th></tr></thead><tbody><tr><td align="center">0-3</td><td align="left">nsats</td><td align="left">Number of satellites in solution</td></tr><tr><td align="center">4</td><td align="left">valid</td><td align="left">GPS solution is valid</td></tr><tr><td align="center">5</td><td align="left">running</td><td align="left">GPS receiver is operational</td></tr><tr><td align="center">6</td><td align="left">date_valid</td><td align="left">Reported date is valid</td></tr><tr><td align="center">7</td><td align="left">course_valid</td><td align="left">ground speed, course and climb rates are valid</td></tr></tbody></table></div></div><br class="table-break"><p>
62 Here are all of the valid GPS operational modes. Altus Metrum
63 products will only ever report 'N' (not valid), 'A'
64 (Autonomous) modes or 'E' (Estimated). The remaining modes
65 are either testing modes or require additional data.
66 </p><div class="table"><a name="idp54348016"></a><p class="title"><b>Table 6. GPS Mode</b></p><div class="table-contents"><table summary="GPS Mode" border="1"><colgroup><col align="center" class="value"><col align="center" class="name"><col align="left" class="description"></colgroup><thead><tr><th align="center">Mode</th><th align="center">Name</th><th align="center">Decsription</th></tr></thead><tbody><tr><td align="center">N</td><td align="center">Not Valid</td><td align="left">All data are invalid</td></tr><tr><td align="center">A</td><td align="center">Autonomous mode</td><td align="left">Data are derived from satellite data</td></tr><tr><td align="center">D</td><td align="center">Differential Mode</td><td align="left">
67 Data are augmented with differential data from a
68 known ground station. The SkyTraq unit in TeleMetrum
69 does not support this mode
70 </td></tr><tr><td align="center">E</td><td align="center">Estimated</td><td align="left">
71 Data are estimated using dead reckoning from the
73 </td></tr><tr><td align="center">M</td><td align="center">Manual</td><td align="left">Data were entered manually</td></tr><tr><td align="center">S</td><td align="center">Simulated</td><td align="left">GPS receiver testing mode</td></tr></tbody></table></div></div><br class="table-break"></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idp54365584"></a>2.5. GPS Satellite Data</h3></div></div></div><div class="informaltable"><table border="0"><colgroup><col align="center" class="Offset"><col align="left" class="Description"></colgroup><thead><tr><th align="center">Type</th><th align="left">Description</th></tr></thead><tbody><tr><td align="center">0x06</td><td align="left">GPS Satellite Data</td></tr></tbody></table></div><p>
74 This packet provides space vehicle identifiers and signal
75 quality information in the form of a C/N1 number for up to 12
76 satellites. The order of the svids is not specified.
78 GPS Satellite data are transmitted once per second during all
80 </p><div class="table"><a name="idp54374768"></a><p class="title"><b>Table 7. GPS Satellite Data Contents</b></p><div class="table-contents"><table summary="GPS Satellite Data Contents" border="1"><colgroup><col align="right" class="Offset"><col align="center" class="Data Type"><col align="left" class="Name"><col align="left" class="Description"></colgroup><thead><tr><th align="center">Offset</th><th align="center">Data Type</th><th align="center">Name</th><th align="center">Description</th></tr></thead><tbody><tr><td align="right">5</td><td align="center">uint8_t</td><td align="left">channels</td><td align="left">Number of reported satellite information</td></tr><tr><td align="right">6</td><td align="center">sat_info_t</td><td align="left">sats[12]</td><td align="left">See Per-Satellite data table below</td></tr><tr><td align="right">30</td><td align="center">uint8_t</td><td align="left">unused[2]</td><td class="auto-generated"> </td></tr><tr><td align="right">32</td><td class="auto-generated"> </td><td class="auto-generated"> </td><td class="auto-generated"> </td></tr></tbody></table></div></div><br class="table-break"><div class="table"><a name="idp54390048"></a><p class="title"><b>Table 8. GPS Per-Satellite data (sat_info_t)</b></p><div class="table-contents"><table summary="GPS Per-Satellite data (sat_info_t)" border="1"><colgroup><col align="right" class="Offset"><col align="center" class="Data Type"><col align="left" class="Name"><col align="left" class="Description"></colgroup><thead><tr><th align="center">Offset</th><th align="center">Data Type</th><th align="center">Name</th><th align="center">Description</th></tr></thead><tbody><tr><td align="right">0</td><td align="center">uint8_t</td><td align="left">svid</td><td align="left">Space Vehicle Identifier</td></tr><tr><td align="right">1</td><td align="center">uint8_t</td><td align="left">c_n_1</td><td align="left">C/N1 signal quality indicator</td></tr><tr><td align="right">2</td><td class="auto-generated"> </td><td class="auto-generated"> </td><td class="auto-generated"> </td></tr></tbody></table></div></div><br class="table-break"></div></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idp54404304"></a>3. Data Transmission</h2></div></div></div><p>
81 Altus Metrum devices use the Texas Instruments CC1111
82 microcontroller which includes an integrated sub-GHz digital
83 transceiver. This transceiver is used to both transmit and
84 receive the telemetry packets. This section discusses what
85 modulation scheme is used and how this device is configured.
86 </p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idp54405680"></a>3.1. Modulation Scheme</h3></div></div></div><p>
87 Texas Instruments provides a tool for computing modulation
88 parameters given a desired modulation format and basic bit
89 rate. For AltOS, the basic bit rate was specified as 38 kBaud,
90 resulting in the following signal parmeters:
91 </p><div class="table"><a name="idp54406960"></a><p class="title"><b>Table 9. Modulation Scheme</b></p><div class="table-contents"><table summary="Modulation Scheme" border="1"><colgroup><col align="center" class="parameter"><col align="center" class="value"><col align="center" class="description"></colgroup><thead><tr><th align="center">Parameter</th><th align="center">Value</th><th align="center">Description</th></tr></thead><tbody><tr><td align="center">Modulation</td><td align="center">GFSK</td><td align="center">Gaussian Frequency Shift Keying</td></tr><tr><td align="center">Deviation</td><td align="center">20.507812 kHz</td><td align="center">Frequency modulation</td></tr><tr><td align="center">Data rate</td><td align="center">38.360596 kBaud</td><td align="center">Raw bit rate</td></tr><tr><td align="center">RX Filter Bandwidth</td><td align="center">93.75 kHz</td><td align="center">Receiver Band pass filter bandwidth</td></tr><tr><td align="center">IF Frequency</td><td align="center">140.62 kHz</td><td align="center">Receiver intermediate frequency</td></tr></tbody></table></div></div><br class="table-break"></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idp54422112"></a>3.2. Error Correction</h3></div></div></div><p>
92 The cc1111 provides forward error correction in hardware,
93 which AltOS uses to improve reception of weak signals. The
94 overall effect of this is to halve the available bandwidth for
95 data from 38 kBaud to 19 kBaud.
96 </p><div class="table"><a name="idp54423376"></a><p class="title"><b>Table 10. Error Correction</b></p><div class="table-contents"><table summary="Error Correction" border="1"><colgroup><col align="center" class="parameter"><col align="center" class="value"><col align="center" class="description"></colgroup><thead><tr><th align="center">Parameter</th><th align="center">Value</th><th align="center">Description</th></tr></thead><tbody><tr><td align="center">Error Correction</td><td align="center">Convolutional coding</td><td align="center">1/2 rate, constraint length m=4</td></tr><tr><td align="center">Interleaving</td><td align="center">4 x 4</td><td align="center">Reduce effect of noise burst</td></tr><tr><td align="center">Data Whitening</td><td align="center">XOR with 9-bit PNR</td><td align="center">Rotate right with bit 8 = bit 0 xor bit 5, initial
97 value 111111111</td></tr></tbody></table></div></div><br class="table-break"></div></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idp54435584"></a>4. TeleDongle packet format</h2></div></div></div><p>
98 TeleDongle does not do any interpretation of the packet data,
99 instead it is configured to receive packets of a specified
100 length (32 bytes in this case). For each received packet,
101 TeleDongle produces a single line of text. This line starts with
102 the string "TELEM " and is followed by a list of hexadecimal
104 </p><pre class="programlisting">TELEM 224f01080b05765e00701f1a1bbeb8d7b60b070605140c000600000000000000003fa988</pre><p>
105 The hexadecimal encoded string of bytes contains a length byte,
106 the packet data, two bytes added by the cc1111 radio receiver
107 hardware and finally a checksum so that the host software can
108 validate that the line was transmitted without any errors.
109 </p><div class="table"><a name="idp54438384"></a><p class="title"><b>Table 11. Packet Format</b></p><div class="table-contents"><table summary="Packet Format" border="1"><colgroup><col align="center" class="offset"><col align="center" class="name"><col align="center" class="value"><col align="center" class="description"></colgroup><thead><tr><th align="center">Offset</th><th align="center">Name</th><th align="center">Example</th><th align="center">Description</th></tr></thead><tbody><tr><td align="center">0</td><td align="center">length</td><td align="center">22</td><td align="center">Total length of data bytes in the line. Note that
110 this includes the added RSSI and status bytes</td></tr><tr><td align="center">1 ·· length-3</td><td align="center">packet</td><td align="center">4f ·· 00</td><td align="center">Bytes of actual packet data</td></tr><tr><td align="center">length-2</td><td align="center">rssi</td><td align="center">3f</td><td align="center">Received signal strength. dBm = rssi / 2 - 74</td></tr><tr><td align="center">length-1</td><td align="center">lqi</td><td align="center">a9</td><td align="center">Link Quality Indicator and CRC status. Bit 7
111 is set when the CRC is correct</td></tr><tr><td align="center">length</td><td align="center">checksum</td><td align="center">88</td><td align="center">(0x5a + sum(bytes 1 ·· length-1)) % 256</td></tr></tbody></table></div></div><br class="table-break"></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idp54457360"></a>5. History and Motivation</h2></div></div></div><p>
112 The original AltoOS telemetry mechanism encoded everything
113 available piece of information on the TeleMetrum hardware into a
114 single unified packet. Initially, the packets contained very
115 little data—some raw sensor readings along with the current GPS
116 coordinates when a GPS receiver was connected. Over time, the
117 amount of data grew to include sensor calibration data, GPS
118 satellite information and a host of internal state information
119 designed to help diagnose flight failures in case of a loss of
120 the on-board flight data.
122 Because every packet contained all of the data, packets were
123 huge—95 bytes long. Much of the information was also specific to
124 the TeleMetrum hardware. With the introduction of the TeleMini
125 flight computer, most of the data contained in the telemetry
126 packets was unavailable. Initially, a shorter, but still
127 comprehensive packet was implemented. This required that the
128 ground station be pre-configured as to which kind of packet to
131 The development of several companion boards also made the
132 shortcomings evident—each companion board would want to include
133 telemetry data in the radio link; with the original design, the
134 packet would have to hold the new data as well, requiring
135 additional TeleMetrum and ground station changes.
136 </p></div></div></body></html>
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