APRS transmitter sending last known GPS

It’s hard to direction find a transmitter that won’t transmit

One of the features of the FlexTrack(My Fork) library is that it will only transmit an APRS packet if it can see at least 4 satellites.

The problem is that you can’t put a large antenna on a High Altitude Balloon (HAB). So at low altitudes, it is unlikely to reach a repeater or IGate. You’ll end up going to the last known coordinates, but from there you might find yourself poking around the woods and hoping for a visual.

Many HABs solve this by adding a secondary system that turns on a buzzer at low altitude. That may be a feature we want in the future, but a simpler solution for now is to alter the arduino code to remember the last known GPS, and transmit that. Even if the location has deviated greatly, having it continue to transmit means we’ll be able to direction find the exact landing location after arriving in the area during the chase.

Changes to aprs.ino

41
42
43
44

// Declare variables outside of core loop
float LastLat=0;
float LastLon=0;
...

73
74
75
76
77
78
79
80
81
82

// Update variables when we have good GPS reception
if (GPS.Satellites >= 4)
{
  LastLat = GPS.Latitude;
  LastLon = GPS.Longitude;
}

if ((millis() >= NextAPRS) && (LastLat != 0) && (LastLon != 0) && (_txlen == 0))
  // use the cached variables during payload creation
  ...

In addition. . .

We’ve begun building the final payload. The tracker is mounted with an on/off switch and we’ve begun carving a window in the styrofoam cooler for the GoPro.