=== 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}>>. 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.