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=== 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.
|
