1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
|
<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="idp23713408"></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="idp50134720"></a><p>
This document is released under the terms of the
<a class="ulink" href="http://creativecommons.org/licenses/by-sa/3.0/" target="_top">
Creative Commons ShareAlike 3.0
</a>
license.
</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="#idp50058048">1. Packet Format Design</a></span></dt><dt><span class="section"><a href="#idp50061040">2. Packet Formats</a></span></dt><dd><dl><dt><span class="section"><a href="#idp50062240">2.1. Packet Header</a></span></dt><dt><span class="section"><a href="#idp50230704">2.2. Sensor Data</a></span></dt><dt><span class="section"><a href="#idp55625232">2.3. Configuration Data</a></span></dt><dt><span class="section"><a href="#idp55660688">2.4. GPS Location</a></span></dt><dt><span class="section"><a href="#idp52445616">2.5. GPS Satellite Data</a></span></dt></dl></dd><dt><span class="section"><a href="#idp55771872">3. Data Transmission</a></span></dt><dd><dl><dt><span class="section"><a href="#idp55773248">3.1. Modulation Scheme</a></span></dt><dt><span class="section"><a href="#idp55789680">3.2. Error Correction</a></span></dt></dl></dd><dt><span class="section"><a href="#idp55803152">4. TeleDongle packet format</a></span></dt><dt><span class="section"><a href="#idp55824736">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="idp50058048"></a>1. Packet Format Design</h2></div></div></div><p>
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.
</p><p>
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.
</p><p>
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.
</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idp50061040"></a>2. Packet Formats</h2></div></div></div><p>
This section first defines the packet header common to all packets
and then the per-packet data layout.
</p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idp50062240"></a>2.1. Packet Header</h3></div></div></div><div class="table"><a name="idp50062912"></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>
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.
</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idp50230704"></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>
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.
</p><p>
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
</p><div class="table"><a name="idp50242768"></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="idp55625232"></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>
This provides a description of the software installed on the
flight computer as well as any user-specified configuration data.
</p><p>
Configuration data packets are transmitted once per second
during all phases of the flight
</p><div class="table"><a name="idp55634272"></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="idp55660688"></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>
This packet provides all of the information available from the
Venus SkyTraq GPS receiver—position, time, speed and precision
estimates.
</p><p>
GPS Location packets are transmitted once per second during
all phases of the flight
</p><div class="table"><a name="idp55670352"></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>
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.
</p><div class="table"><a name="idp55710640"></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>
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.
</p><div class="table"><a name="idp55727856"></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">
Data are augmented with differential data from a
known ground station. The SkyTraq unit in TeleMetrum
does not support this mode
</td></tr><tr><td align="center">E</td><td align="center">Estimated</td><td align="left">
Data are estimated using dead reckoning from the
last known data
</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="idp52445616"></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>
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.
</p><p>
GPS Satellite data are transmitted once per second during all
phases of the flight.
</p><div class="table"><a name="idp55742336"></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="idp55757616"></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="idp55771872"></a>3. Data Transmission</h2></div></div></div><p>
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.
</p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idp55773248"></a>3.1. Modulation Scheme</h3></div></div></div><p>
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:
</p><div class="table"><a name="idp55774528"></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="idp55789680"></a>3.2. Error Correction</h3></div></div></div><p>
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.
</p><div class="table"><a name="idp55790944"></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
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="idp55803152"></a>4. TeleDongle packet format</h2></div></div></div><p>
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.
</p><pre class="programlisting">TELEM 224f01080b05765e00701f1a1bbeb8d7b60b070605140c000600000000000000003fa988</pre><p>
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.
</p><div class="table"><a name="idp55805856"></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
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
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="idp55824736"></a>5. History and Motivation</h2></div></div></div><p>
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.
</p><p>
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.
</p><p>
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.
</p></div></div></body></html>
|
