+ === APRS
+
+ {aprsdevices} can send APRS if desired, and the
+ interval between APRS packets can be configured. As each APRS
+ packet takes a full second to transmit, we recommend an
+ interval of at least 5 seconds to avoid consuming too much
+ battery power or radio channel bandwidth. You can configure
+ the APRS interval using {application}; that process is described in
+ <<{configure_section}>> in the {application} chapter.
+
+ AltOS supports both compressed and uncompressed APRS
+ position report data formats. The compressed format
+ provides for higher position precision and shorter
+ packets than the uncompressed APRS format. We've found
+ some older APRS receivers that do not handle the
+ compressed format. The Kenwood TH-72A requires the use
+ of uncompressed format to display altitude information
+ correctly. The Yaesu FT1D requires the use of
+ compressed format to display altitude information.
+
+ APRS packets include an SSID (Secondary Station Identifier)
+ field that allows one operator to have multiple
+ transmitters. AltOS allows you to set this to a single digit
+ from 0 to 9, allowing you to fly multiple transmitters at the
+ same time while keeping the identify of each one separate in
+ the receiver. By default, the SSID is set to the last digit of
+ the device serial number.
+
+ The APRS packet format includes a comment field that can have
+ arbitrary text in it. AltOS uses this to send status
+ information about the flight computer. It sends four fields as
+ shown in the following table.
+
+ .Altus Metrum APRS Comments
+ [options="header",cols="1,1,3"]
+ |====
+ |Field |Example |Description
+
+ |1
+ |L
+ |GPS Status U for unlocked, L for locked
+
+ |2
+ |6
+ |Number of Satellites in View
+
+ |3
+ |B4.0
+ |Altimeter Battery Voltage
+
+ |4
+ |A3.7
+ |Apogee Igniter Voltage
+
+ |5
+ |M3.7
+ |Main Igniter Voltage
+
+ |6
+ |1286
+ |Device Serial Number
+ |====
+
+ Here's an example of an APRS comment showing GPS lock with 6
+ satellites in view, a primary battery at 4.0V, and
+ apogee and main igniters both at 3.7V from device 1286.
+
+ ....
+ L6 B4.0 A3.7 M3.7 1286
+ ....
+
+ Make sure your primary battery is above 3.8V, any
+ connected igniters are above 3.5V and GPS is locked
+ with at least 5 or 6 satellites in view before
+ flying. If GPS is switching between L and U regularly,
+ then it doesn't have a good lock and you should wait
+ until it becomes stable.
+
+ If the GPS receiver loses lock, the APRS data
+ transmitted will contain the last position for which
+ GPS lock was available. You can tell that this has
+ happened by noticing that the GPS status character
+ switches from 'L' to 'U'. Before GPS has locked, APRS
+ will transmit zero for latitude, longitude and
+ altitude.