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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 time within each
minute that APRS transmits by changing the APRS offset
value. When the GPS signal is locked and knows the
current time, the APRS offset selects the time with
each minute for the first APRS transmission;
subsequent transmissions occur each APRS interval
seconds thereafter. You can configure the APRS
interval and APRS offset using {application}; that
process is described in <<{configure_section}>>.
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 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
ifdef::altusmetrum[]
|4
|A3.7
|Apogee Igniter Voltage
|5
|M3.7
|Main Igniter Voltage
|6
|1286
|Device Serial Number
endif::altusmetrum[]
ifdef::telegps[]
|4
|1286
|Device Serial Number
endif::telegps[]
|====
ifdef::altusmetrum[]
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
....
endif::altusmetrum[]
ifdef::telegps[]
Here's an example of an APRS comment showing GPS lock with 6
satellites in view and a primary battery at 4.0V from device 1876.
....
L6 B4.0 1876
....
endif::telegps[]
Make sure your primary battery is above 3.8V
ifdef::altusmetrum[]
any connected igniters are above 3.5V
endif::altusmetrum[]
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.
|