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   1 <?xml version="1.0" encoding="utf-8" ?>
   2 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3 "/usr/share/xml/docbook/schema/dtd/4.5/docbookx.dtd">
   4
   5 <article>
   6   <articleinfo>
   7     <title>AltOS Telemetry</title>
   8     <subtitle>Packet Definitions</subtitle>
   9     <author>
  10       <firstname>Keith</firstname>
  11       <surname>Packard</surname>
  12     </author>
  13     <copyright>
  14       <year>2011</year>
  15       <holder>Keith Packard</holder>
  16     </copyright>
  17     <legalnotice>
  18       <para>
  19         This document is released under the terms of the
  20         <ulink url="http://creativecommons.org/licenses/by-sa/3.0/">
  21           Creative Commons ShareAlike 3.0
  22         </ulink>
  23         license.
  24       </para>
  25     </legalnotice>
  26     <revhistory>
  27       <revision>
  28         <revnumber>0.1</revnumber>
  29         <date>01 July 2011</date>
  30         <revremark>Initial content</revremark>
  31       </revision>
  32     </revhistory>
  33   </articleinfo>
  34   <section>
  35     <title>Packet Format Design</title>
  36     <para>
  37       AltOS telemetry data is split into multiple different packets,
  38       all the same size, but each includs an identifier so that the
  39       ground station can distinguish among different types. A single
  40       flight board will transmit multiple packet types, each type on a
  41       different schedule. The ground software need look for only a
  42       single packet size, and then decode the information within the
  43       packet and merge data from multiple packets to construct the
  44       full flight computer state.
  45     </para>
  46     <para>
  47       Each AltOS packet is 32 bytes long. This size was chosen based
  48       on the known telemetry data requirements. The power of two size
  49       allows them to be stored easily in flash memory without having
  50       them split across blocks or leaving gaps at the end.
  51     </para>
  52     <para>
  53       All packet types start with a five byte header which encodes the
  54       device serial number, device clock value and the packet
  55       type. The remaining 27 bytes encode type-specific data.
  56     </para>
  57   </section>
  58   <section>
  59     <title>Packet Formats</title>
  60     This section first defines the packet header common to all packets
  61     and then the per-packet data layout.
  62     <section>
  63       <title>Packet Header</title>
  64       <table frame='all'>
  65         <title>Telemetry Packet Header</title>
  66         <tgroup cols='4' align='center' colsep='1' rowsep='1'>
  67           <colspec align='center' colwidth='*' colname='Offset'/>
  68           <colspec align='center' colwidth='3*' colname='Data Type'/>
  69           <colspec align='left' colwidth='3*' colname='Name'/>
  70           <colspec align='left' colwidth='9*' colname='Description'/>
  71           <thead>
  72             <row>
  73               <entry align='center'>Offset</entry>
  74               <entry align='center'>Data Type</entry>
  75               <entry align='center'>Name</entry>
  76               <entry align='center'>Description</entry>
  77             </row>
  78           </thead>
  79           <tbody>
  80             <row>
  81               <entry>0</entry>
  82               <entry>uint16_t</entry>
  83               <entry>serial</entry>
  84               <entry>Device serial Number</entry>
  85             </row>
  86             <row>
  87               <entry>2</entry>
  88               <entry>uint16_t</entry>
  89               <entry>tick</entry>
  90               <entry>Device time in 100ths of a second</entry>
  91             </row>
  92             <row>
  93               <entry>4</entry>
  94               <entry>uint8_t</entry>
  95               <entry>type</entry>
  96               <entry>Packet type</entry>
  97             </row>
  98             <row>
  99               <entry>5</entry>
 100             </row>
 101           </tbody>
 102         </tgroup>
 103       </table>
 104       <para>
 105       Each packet starts with these five bytes which serve to identify
 106       which device has transmitted the packet, when it was transmitted
 107       and what the rest of the packet contains.
 108       </para>
 109     </section>
 110     <section>
 111       <title>Sensor Data</title>
 112       <informaltable frame='none' label='' tocentry='0'>
 113         <tgroup cols='2' align='center' colsep='1' rowsep='1'>
 114           <colspec align='center' colwidth='*' colname='Offset'/>
 115           <colspec align='left' colwidth='3*' colname='Description'/>
 116           <thead>
 117             <row>
 118               <entry>Type</entry>
 119               <entry>Description</entry>
 120             </row>
 121           </thead>
 122           <tbody>
 123             <row>
 124               <entry>0x01</entry>
 125               <entry>TeleMetrum Sensor Data</entry>
 126             </row>
 127             <row>
 128               <entry>0x02</entry>
 129               <entry>TeleMini Sensor Data</entry>
 130             </row>
 131             <row>
 132               <entry>0x03</entry>
 133               <entry>TeleNano Sensor Data</entry>
 134             </row>
 135           </tbody>
 136         </tgroup>
 137       </informaltable>
 138       <para>
 139         TeleMetrum, TeleMini and TeleNano share this same packet
 140         format for sensor data. Each uses a distinct packet type so
 141         that the receiver knows which data values are valid and which
 142         are undefined.
 143       </para>
 144       <para>
 145         Sensor Data packets are transmitted once per second on the
 146         ground, 10 times per second during ascent and once per second
 147         during descent and landing
 148       </para>
 149       <table frame='all'>
 150         <title>Sensor Packet Contents</title>
 151         <tgroup cols='4' align='center' colsep='1' rowsep='1'>
 152           <colspec align='center' colwidth='*' colname='Offset'/>
 153           <colspec align='center' colwidth='3*' colname='Data Type'/>
 154           <colspec align='left' colwidth='3*' colname='Name'/>
 155           <colspec align='left' colwidth='9*' colname='Description'/>
 156           <thead>
 157             <row>
 158               <entry align='center'>Offset</entry>
 159               <entry align='center'>Data Type</entry>
 160               <entry align='center'>Name</entry>
 161               <entry align='center'>Description</entry>
 162             </row>
 163           </thead>
 164           <tbody>
 165             <row>
 166               <entry>5</entry><entry>uint8_t</entry><entry>state</entry><entry>Flight state</entry>
 167             </row>
 168             <row>
 169               <entry>6</entry><entry>int16_t</entry><entry>accel</entry><entry>accelerometer (TM only)</entry>
 170             </row>
 171             <row>
 172               <entry>8</entry><entry>int16_t</entry><entry>pres</entry><entry>pressure sensor</entry>
 173             </row>
 174             <row>
 175               <entry>10</entry><entry>int16_t</entry><entry>temp</entry><entry>temperature sensor</entry>
 176             </row>
 177             <row>
 178               <entry>12</entry><entry>int16_t</entry><entry>v_batt</entry><entry>battery voltage</entry>
 179             </row>
 180             <row>
 181               <entry>14</entry><entry>int16_t</entry><entry>sense_d</entry><entry>drogue continuity sense (TM/Tm)</entry>
 182             </row>
 183             <row>
 184               <entry>16</entry><entry>int16_t</entry><entry>sense_m</entry><entry>main continuity sense (TM/Tm)</entry>
 185             </row>
 186             <row>
 187               <entry>18</entry><entry>int16_t</entry><entry>acceleration</entry><entry>m/s² * 16</entry>
 188             </row>
 189             <row>
 190               <entry>20</entry><entry>int16_t</entry><entry>speed</entry><entry>m/s * 16</entry>
 191             </row>
 192             <row>
 193               <entry>22</entry><entry>int16_t</entry><entry>height</entry><entry>m</entry>
 194             </row>
 195             <row>
 196               <entry>24</entry><entry>int16_t</entry><entry>ground_pres</entry><entry>Average barometer reading on ground</entry>
 197             </row>
 198             <row>
 199               <entry>26</entry><entry>int16_t</entry><entry>ground_accel</entry><entry>TM</entry>
 200             </row>
 201             <row>
 202               <entry>28</entry><entry>int16_t</entry><entry>accel_plus_g</entry><entry>TM</entry>
 203             </row>
 204             <row>
 205               <entry>30</entry><entry>int16_t</entry><entry>accel_minus_g</entry><entry>TM</entry>
 206             </row>
 207             <row>
 208               <entry>32</entry>
 209             </row>
 210           </tbody>
 211         </tgroup>
 212       </table>
 213     </section>
 214     <section>
 215       <title>Configuration Data</title>
 216       <informaltable frame='none' label='' tocentry='0'>
 217         <tgroup cols='2' align='center' colsep='1' rowsep='1'>
 218           <colspec align='center' colwidth='*' colname='Offset'/>
 219           <colspec align='left' colwidth='3*' colname='Description'/>
 220           <thead>
 221             <row>
 222               <entry>Type</entry>
 223               <entry>Description</entry>
 224             </row>
 225           </thead>
 226           <tbody>
 227             <row>
 228               <entry>0x04</entry>
 229               <entry>Configuration Data</entry>
 230             </row>
 231           </tbody>
 232         </tgroup>
 233       </informaltable>
 234       <para>
 235         This provides a description of the software installed on the
 236         flight computer as well as any user-specified configuration data.
 237       </para>
 238       <para>
 239         Configuration data packets are transmitted once per second
 240         during all phases of the flight
 241       </para>
 242       <table frame='all'>
 243         <title>Sensor Packet Contents</title>
 244         <tgroup cols='4' align='center' colsep='1' rowsep='1'>
 245           <colspec align='center' colwidth='*' colname='Offset'/>
 246           <colspec align='center' colwidth='3*' colname='Data Type'/>
 247           <colspec align='left' colwidth='3*' colname='Name'/>
 248           <colspec align='left' colwidth='9*' colname='Description'/>
 249           <thead>
 250             <row>
 251               <entry align='center'>Offset</entry>
 252               <entry align='center'>Data Type</entry>
 253               <entry align='center'>Name</entry>
 254               <entry align='center'>Description</entry>
 255             </row>
 256           </thead>
 257           <tbody>
 258             <row>
 259               <entry>5</entry><entry>uint8_t</entry><entry>type</entry><entry>Device type</entry>
 260             </row>
 261             <row>
 262               <entry>6</entry><entry>uint16_t</entry><entry>flight</entry><entry>Flight number</entry>
 263             </row>
 264             <row>
 265               <entry>8</entry><entry>uint8_t</entry><entry>config_major</entry><entry>Config major version</entry>
 266             </row>
 267             <row>
 268               <entry>9</entry><entry>uint8_t</entry><entry>config_minor</entry><entry>Config minor version</entry>
 269             </row>
 270             <row>
 271               <entry>10</entry><entry>uint16_t</entry><entry>main_deploy</entry><entry>Main deploy alt in meters</entry>
 272             </row>
 273             <row>
 274               <entry>12</entry><entry>uint32_t</entry><entry>flight_log_max</entry><entry>Maximum flight log size (B)</entry>
 275             </row>
 276             <row>
 277               <entry>16</entry><entry>char</entry><entry>callsign[8]</entry><entry>Radio operator identifier</entry>
 278             </row>
 279             <row>
 280               <entry>24</entry><entry>char</entry><entry>version[8]</entry><entry>Software version identifier</entry>
 281             </row>
 282             <row>
 283               <entry>32</entry>
 284             </row>
 285           </tbody>
 286         </tgroup>
 287       </table>
 288     </section>
 289     <section>
 290       <title>GPS Location</title>
 291       <informaltable frame='none' label='' tocentry='0'>
 292         <tgroup cols='2' align='center' colsep='1' rowsep='1'>
 293           <colspec align='center' colwidth='*' colname='Offset'/>
 294           <colspec align='left' colwidth='3*' colname='Description'/>
 295           <thead>
 296             <row>
 297               <entry>Type</entry>
 298               <entry>Description</entry>
 299             </row>
 300           </thead>
 301           <tbody>
 302             <row>
 303               <entry>0x05</entry>
 304               <entry>GPS Location</entry>
 305             </row>
 306           </tbody>
 307         </tgroup>
 308       </informaltable>
 309       <para>
 310         This packet provides all of the information available from the
 311         Venus SkyTraq GPS receiver—position, time, speed and precision
 312         estimates.
 313       </para>
 314       <para>
 315         GPS Location packets are transmitted once per second during
 316         all phases of the flight
 317       </para>
 318       <table frame='all'>
 319         <title>GPS Location Packet Contents</title>
 320         <tgroup cols='4' align='center' colsep='1' rowsep='1'>
 321           <colspec align='center' colwidth='*' colname='Offset'/>
 322           <colspec align='center' colwidth='3*' colname='Data Type'/>
 323           <colspec align='left' colwidth='3*' colname='Name'/>
 324           <colspec align='left' colwidth='9*' colname='Description'/>
 325           <thead>
 326             <row>
 327               <entry align='center'>Offset</entry>
 328               <entry align='center'>Data Type</entry>
 329               <entry align='center'>Name</entry>
 330               <entry align='center'>Description</entry>
 331             </row>
 332           </thead>
 333           <tbody>
 334             <row>
 335               <entry>5</entry><entry>uint8_t</entry><entry>flags</entry>
 336               <entry>See GPS Flags table below</entry>
 337             </row>
 338             <row>
 339               <entry>6</entry><entry>int16_t</entry><entry>altitude</entry><entry>m</entry>
 340             </row>
 341             <row>
 342               <entry>8</entry><entry>int32_t</entry><entry>latitude</entry><entry>degrees * 10<superscript>7</superscript></entry>
 343             </row>
 344             <row>
 345               <entry>12</entry><entry>int32_t</entry><entry>longitude</entry><entry>degrees * 10<superscript>7</superscript></entry>
 346             </row>
 347             <row>
 348               <entry>16</entry><entry>uint8_t</entry><entry>year</entry>
 349             </row>
 350             <row>
 351               <entry>17</entry><entry>uint8_t</entry><entry>month</entry>
 352             </row>
 353             <row>
 354               <entry>18</entry><entry>uint8_t</entry><entry>day</entry>
 355             </row>
 356             <row>
 357               <entry>19</entry><entry>uint8_t</entry><entry>hour</entry>
 358             </row>
 359             <row>
 360               <entry>20</entry><entry>uint8_t</entry><entry>minute</entry>
 361             </row>
 362             <row>
 363               <entry>21</entry><entry>uint8_t</entry><entry>second</entry>
 364             </row>
 365             <row>
 366               <entry>22</entry><entry>uint8_t</entry><entry>pdop</entry><entry>* 5</entry>
 367             </row>
 368             <row>
 369               <entry>23</entry><entry>uint8_t</entry><entry>hdop</entry><entry>* 5</entry>
 370             </row>
 371             <row>
 372               <entry>24</entry><entry>uint8_t</entry><entry>vdop</entry><entry>* 5</entry>
 373             </row>
 374             <row>
 375               <entry>25</entry><entry>uint8_t</entry><entry>mode</entry>
 376               <entry>See GPS Mode table below</entry>
 377             </row>
 378             <row>
 379               <entry>26</entry><entry>uint16_t</entry><entry>ground_speed</entry><entry>cm/s</entry>
 380             </row>
 381             <row>
 382               <entry>28</entry><entry>uint8_t</entry><entry>course</entry><entry>/ 2</entry>
 383             </row>
 384             <row>
 385               <entry>29</entry><entry>uint8_t</entry><entry>unused[3]</entry>
 386             </row>
 387             <row>
 388               <entry>32</entry>
 389             </row>
 390           </tbody>
 391         </tgroup>
 392       </table>
 393       <para>
 394         Packed into a one byte field are status flags and the count of
 395         satellites used to compute the position fix. Note that this
 396         number may be lower than the number of satellites being
 397         tracked; the receiver will not use information from satellites
 398         with weak signals or which are close enough to the horizon to
 399         have significantly degraded position accuracy.
 400       </para>
 401       <table frame='all'>
 402         <title>GPS Flags</title>
 403         <tgroup cols='3' colsep='1' rowsep='1'>
 404           <colspec align='center' colwidth='*' colname='bits'/>
 405           <colspec align='left' colwidth='2*' colname='name'/>
 406           <colspec align='left' colwidth='7*' colname='description'/>
 407           <thead>
 408             <row>
 409               <entry align='center'>Bits</entry>
 410               <entry align='center'>Name</entry>
 411               <entry align='center'>Description</entry>
 412             </row>
 413           </thead>
 414           <tbody>
 415             <row>
 416               <entry>0-3</entry>
 417               <entry>nsats</entry>
 418               <entry>Number of satellites in solution</entry>
 419             </row>
 420             <row>
 421               <entry>4</entry>
 422               <entry>valid</entry>
 423               <entry>GPS solution is valid</entry>
 424             </row>
 425             <row>
 426               <entry>5</entry>
 427               <entry>running</entry>
 428               <entry>GPS receiver is operational</entry>
 429             </row>
 430             <row>
 431               <entry>6</entry>
 432               <entry>date_valid</entry>
 433               <entry>Reported date is valid</entry>
 434             </row>
 435             <row>
 436               <entry>7</entry>
 437               <entry>course_valid</entry>
 438               <entry>ground speed, course and climb rates are valid</entry>
 439             </row>
 440           </tbody>
 441         </tgroup>
 442       </table>
 443       <para>
 444         Here are all of the valid GPS operational modes. Altus Metrum
 445         products will only ever report 'N' (not valid), 'A'
 446         (Autonomous) modes or 'E' (Estimated). The remaining modes
 447         are either testing modes or require additional data.
 448       </para>
 449       <table frame='all'>
 450         <title>GPS Mode</title>
 451         <tgroup cols='3' colsep='1' rowsep='1'>
 452           <colspec align='center' colwidth='*' colname='value'/>
 453           <colspec align='center' colwidth='3*' colname='name'/>
 454           <colspec align='left' colwidth='7*' colname='description'/>
 455           <thead>
 456             <row>
 457               <entry align='center'>Mode</entry>
 458               <entry align='center'>Name</entry>
 459               <entry align='center'>Decsription</entry>
 460             </row>
 461           </thead>
 462           <tbody>
 463             <row>
 464               <entry>N</entry>
 465               <entry>Not Valid</entry>
 466               <entry>All data are invalid</entry>
 467             </row>
 468             <row>
 469               <entry>A</entry>
 470               <entry>Autonomous mode</entry>
 471               <entry>Data are derived from satellite data</entry>
 472             </row>
 473             <row>
 474               <entry>D</entry>
 475               <entry>Differential Mode</entry>
 476               <entry>
 477                   Data are augmented with differential data from a
 478                   known ground station. The SkyTraq unit in TeleMetrum
 479                   does not support this mode
 480                 </entry>
 481             </row>
 482             <row>
 483               <entry>E</entry>
 484               <entry>Estimated</entry>
 485               <entry>
 486                   Data are estimated using dead reckoning from the
 487                   last known data
 488                 </entry>
 489             </row>
 490             <row>
 491               <entry>M</entry>
 492               <entry>Manual</entry>
 493               <entry>Data were entered manually</entry>
 494             </row>
 495             <row>
 496               <entry>S</entry>
 497               <entry>Simulated</entry>
 498               <entry>GPS receiver testing mode</entry>
 499             </row>
 500           </tbody>
 501         </tgroup>
 502       </table>
 503     </section>
 504     <section>
 505       <title>GPS Satellite Data</title>
 506       <informaltable frame='none' label='' tocentry='0'>
 507         <tgroup cols='2' align='center' colsep='1' rowsep='1'>
 508           <colspec align='center' colwidth='*' colname='Offset'/>
 509           <colspec align='left' colwidth='3*' colname='Description'/>
 510           <thead>
 511             <row>
 512               <entry>Type</entry>
 513               <entry>Description</entry>
 514             </row>
 515           </thead>
 516           <tbody>
 517             <row>
 518               <entry>0x06</entry>
 519               <entry>GPS Satellite Data</entry>
 520             </row>
 521           </tbody>
 522         </tgroup>
 523       </informaltable>
 524       <para>
 525         This packet provides space vehicle identifiers and signal
 526         quality information in the form of a C/N1 number for up to 12
 527         satellites. The order of the svids is not specified.
 528       </para>
 529       <para>
 530         GPS Satellite data are transmitted once per second during all
 531         phases of the flight.
 532       </para>
 533       <table frame='all'>
 534         <title>GPS Satellite Data Contents</title>
 535         <tgroup cols='4' align='center' colsep='1' rowsep='1'>
 536           <colspec align='right' colwidth='*' colname='Offset'/>
 537           <colspec align='center' colwidth='3*' colname='Data Type'/>
 538           <colspec align='left' colwidth='3*' colname='Name'/>
 539           <colspec align='left' colwidth='9*' colname='Description'/>
 540           <thead>
 541             <row>
 542               <entry align='center'>Offset</entry>
 543               <entry align='center'>Data Type</entry>
 544               <entry align='center'>Name</entry>
 545               <entry align='center'>Description</entry>
 546             </row>
 547           </thead>
 548           <tbody>
 549             <row>
 550               <entry>5</entry><entry>uint8_t</entry><entry>channels</entry>
 551               <entry>Number of reported satellite information</entry>
 552             </row>
 553             <row>
 554               <entry>6</entry><entry>sat_info_t</entry><entry>sats[12]</entry>
 555               <entry>See Per-Satellite data table below</entry>
 556             </row>
 557             <row>
 558               <entry>30</entry><entry>uint8_t</entry><entry>unused[2]</entry>
 559             </row>
 560             <row>
 561               <entry>32</entry>
 562             </row>
 563           </tbody>
 564         </tgroup>
 565       </table>
 566       <table frame='all'>
 567         <title>GPS Per-Satellite data (sat_info_t)</title>
 568         <tgroup cols='4' align='center' colsep='1' rowsep='1'>
 569           <colspec align='right' colwidth='*' colname='Offset'/>
 570           <colspec align='center' colwidth='3*' colname='Data Type'/>
 571           <colspec align='left' colwidth='3*' colname='Name'/>
 572           <colspec align='left' colwidth='9*' colname='Description'/>
 573           <thead>
 574             <row>
 575               <entry align='center'>Offset</entry>
 576               <entry align='center'>Data Type</entry>
 577               <entry align='center'>Name</entry>
 578               <entry align='center'>Description</entry>
 579             </row>
 580           </thead>
 581           <tbody>
 582             <row>
 583               <entry>0</entry><entry>uint8_t</entry><entry>svid</entry>
 584               <entry>Space Vehicle Identifier</entry>
 585             </row>
 586             <row>
 587               <entry>1</entry><entry>uint8_t</entry><entry>c_n_1</entry>
 588               <entry>C/N1 signal quality indicator</entry>
 589             </row>
 590             <row>
 591               <entry>2</entry>
 592             </row>
 593           </tbody>
 594         </tgroup>
 595       </table>
 596     </section>
 597   </section>
 598   <section>
 599     <title>Data Transmission</title>
 600     <para>
 601       Altus Metrum devices use the Texas Instruments CC1111
 602       microcontroller which includes an integrated sub-GHz digital
 603       transceiver. This transceiver is used to both transmit and
 604       receive the telemetry packets. This section discusses what
 605       modulation scheme is used and how this device is configured.
 606     </para>
 607     <section>
 608       <title>Modulation Scheme</title>
 609       <para>
 610         Texas Instruments provides a tool for computing modulation
 611         parameters given a desired modulation format and basic bit
 612         rate. For AltOS, the basic bit rate was specified as 38 kBaud,
 613         resulting in the following signal parmeters:
 614       </para>
 615       <table>
 616         <tgroup cols='3'>
 617           <colspec align="center" colwidth="*" colname="parameter"/>
 618           <colspec align="center" text-align="." colwidth="*" colname="value"/>
 619           <colspec align="center" colwidth="*" colname="description"/>
 620           <thead>
 621             <row>
 622               <entry align='center'>Parameter</entry>
 623               <entry align='center'>Value</entry>
 624               <entry align='center'>Description</entry>
 625             </row>
 626           </thead>
 627           <tbody>
 628             <row>
 629               <entry>Modulation</entry>
 630               <entry>GFSK</entry>
 631               <entry>Gaussian Frequency Shift Keying</entry>
 632             </row>
 633             <row>
 634               <entry>Deviation</entry>
 635               <entry>20.507812 kHz</entry>
 636               <entry>Frequency modulation</entry>
 637             </row>
 638             <row>
 639               <entry>Data rate</entry>
 640               <entry>38.360596 kBaud</entry>
 641               <entry>Raw bit rate</entry>
 642             </row>
 643             <row>
 644               <entry>RX Filter Bandwidth</entry>
 645               <entry>93.75 kHz</entry>
 646               <entry>Receiver Band pass filter bandwidth</entry>
 647             </row>
 648             <row>
 649               <entry>IF Frequency</entry>
 650               <entry>140.62 kHz</entry>
 651               <entry>Receiver intermediate frequency</entry>
 652             </row>
 653           </tbody>
 654         </tgroup>
 655       </table>
 656     </section>
 657     <section>
 658       <para>
 659         The cc1111 provides forward error correction in hardware,
 660         which AltOS uses to improve reception of weak signals. The
 661         overall effect of this is to halve the available bandwidth for
 662         data from 38 kBaud to 19 kBaud.
 663       </para>
 664       <title>Error Correction</title>
 665       <table>
 666         <tgroup cols='3'>
 667           <colspec align="center" colwidth="*" colname="parameter"/>
 668           <colspec align="center" colwidth="*" colname="value"/>
 669           <colspec align="center" colwidth="*" colname="description"/>
 670           <thead>
 671             <row>
 672               <entry align='center'>Parameter</entry>
 673               <entry align='center'>Value</entry>
 674               <entry align='center'>Description</entry>
 675             </row>
 676           </thead>
 677           <tbody>
 678             <row>
 679               <entry>Error Correction</entry>
 680               <entry>Convolutional coding FEC</entry>
 681               <entry>1/2 code, constraint length m=4</entry>
 682             </row>
 683             <row>
 684               <entry>Interleaving</entry>
 685               <entry>4 x 4</entry>
 686               <entry>Reduce effect of noise burst</entry>
 687             </row>
 688             <row>
 689               <entry>Data Whitening</entry>
 690               <entry>XOR with 9-bit PNR</entry>
 691               <entry>Rotate right with bit 8 = bit 0 xor bit 5, initial
 692               value 111111111</entry>
 693             </row>
 694           </tbody>
 695         </tgroup>
 696       </table>
 697     </section>
 698   </section>
 699   <section>
 700     <title>History and Motivation</title>
 701     <para>
 702       The original AltoOS telemetry mechanism encoded everything
 703       available piece of information on the TeleMetrum hardware into a
 704       single unified packet. Initially, the packets contained very
 705       little data—some raw sensor readings along with the current GPS
 706       coordinates when a GPS receiver was connected. Over time, the
 707       amount of data grew to include sensor calibration data, GPS
 708       satellite information and a host of internal state information
 709       designed to help diagnose flight failures in case of a loss of
 710       the on-board flight data.
 711     </para>
 712     <para>
 713       Because every packet contained all of the data, packets were
 714       huge—95 bytes long. Much of the information was also specific to
 715       the TeleMetrum hardware. With the introduction of the TeleMini
 716       flight computer, most of the data contained in the telemetry
 717       packets was unavailable. Initially, a shorter, but still
 718       comprehensive packet was implemented. This required that the
 719       ground station be pre-configured as to which kind of packet to
 720       expect.
 721     </para>
 722     <para>
 723       The development of several companion boards also made the
 724       shortcomings evident—each companion board would want to include
 725       telemetry data in the radio link; with the original design, the
 726       packet would have to hold the new data as well, requiring
 727       additional TeleMetrum and ground station changes.
 728     </para>
 729   </section>
 730 </article>