3 {aprsdevices} can send APRS if desired, and the
4 interval between APRS packets can be configured. As
5 each APRS packet takes a full second to transmit, we
6 recommend an interval of at least 5 seconds to avoid
7 consuming too much battery power or radio channel
8 bandwidth. You can configure the time within each
9 minute that APRS transmits by changing the APRS offset
10 value. When the GPS signal is locked and knows the
11 current time, the APRS offset selects the time with
12 each minute for the first APRS transmission;
13 subsequent transmissions occur each APRS interval
14 seconds thereafter. You can configure the APRS
15 interval and APRS offset using {application}; that
16 process is described in <<{configure_section}>>.
18 AltOS supports both compressed and uncompressed APRS
19 position report data formats. The compressed format
20 provides for higher position precision and shorter
21 packets than the uncompressed APRS format. We've found
22 some older APRS receivers that do not handle the
23 compressed format. The Kenwood TH-72A requires the use
24 of uncompressed format to display altitude information
25 correctly. The Yaesu FT1D requires the use of
26 compressed format to display altitude information.
28 APRS packets include an SSID (Secondary Station Identifier)
29 field that allows one operator to have multiple
30 transmitters. AltOS allows you to set this to a single digit
31 from 0 to 9, allowing you to fly multiple transmitters at the
32 same time while keeping the identify of each one separate in
33 the receiver. By default, the SSID is set to the last digit of
34 the device serial number.
36 The APRS packet format includes a comment field that
37 can have arbitrary text in it. AltOS uses this to send
38 status information as shown in the following table.
40 .Altus Metrum APRS Comments
41 [options="header",cols="1,1,3"]
43 |Field |Example |Description
47 |GPS Status U for unlocked, L for locked
51 |Number of Satellites in View
55 |Altimeter Battery Voltage
61 |Apogee Igniter Voltage
83 Here's an example of an APRS comment showing GPS lock with 6
84 satellites in view, a primary battery at 4.0V, and
85 apogee and main igniters both at 3.7V from device 1286.
88 L6 B4.0 A3.7 M3.7 1286
93 Here's an example of an APRS comment showing GPS lock with 6
94 satellites in view and a primary battery at 4.0V from device 1876.
101 Make sure your primary battery is above 3.8V
103 any connected igniters are above 3.5V
105 and GPS is locked with at least 5 or 6 satellites in
106 view before flying. If GPS is switching between L and
107 U regularly, then it doesn't have a good lock and you
108 should wait until it becomes stable.
110 If the GPS receiver loses lock, the APRS data
111 transmitted will contain the last position for which
112 GPS lock was available. You can tell that this has
113 happened by noticing that the GPS status character
114 switches from 'L' to 'U'. Before GPS has locked, APRS
115 will transmit zero for latitude, longitude and