Dawn Technology Limited (di-gps.com) makes a great GPS tracker for DSLRs that don’t have GPS tracking built in. The one I bought for my D810 was the di-GPS Eco ProFessional PF1-M. It worked great and I wanted to get another one for my D850. I bought Dawn Tech’s newer version, the di-GPS ECO PF-2DC.
The earlier version needed the camera to be turned on, and the camera’s “Standby Timer” set to “No Limit” (so the meter and the di-GPS would stay powered-on). This unit draws a small amount of power from the camera’s battery so it can lock-in to satellite signals. When the “Standby Timer” shuts the meter off, the di-GPS powers off as well, but uses the last GPS coordinates in memory for the next shot… until it turns on and locks-on to a satellite again.
The newer di-GPS ECO PF-2DC unit has an optional “Alway On” button that is supposed to keep the unit powered up so it doesn’t loose the GPS signal when the camera’s “Standby Timer” shuts off. There are two LEDs on the unit - steady Green (satellite locked-in) and the red “Always On” button. However, there is a small discrepancy or misunderstanding with the di-GPS unit. I don’t know if it’s just a faulty unit or this is the way it actually works with the D850.
The user manual states when the “Aways On” button is clicked on, and the Green LED is steady (satellite acquired), it doesn’t matter if the camera’s “Standby Timer” shuts off, or you turn the camera off, the GPS signal will always be locked in until you turn off the “Always on” button.
Here’s the problem. I noticed that when I set the “Standby Timer” on my D850 to 4 or 6 or 10 seconds for example, then turn on the di-GPS’s red LED switch, the actual shutoff time becomes sporadic - increasing to 20 or 30 seconds, in one case 50 seconds. Also, the GPS co-ordinates are sometimes not recorded with a photo even though the green LED is steady. It seems that the ECO PF-2DC is interfering with the D850s “Standby Timer” function or the timer is interfering with the ECO PF-2DC’s ability to record GPS information on some of the photos.
The solution was to set the “Standby Timer” to “No Limit” (never shut off), then press the “Always-on Mode” button on the ECO PF-2DC. This keeps the unit on all the time even if I shut the camera off. Once the Green LED is steady, the GPS info is recorded with every photo.