With the following steps you can reset you iPhone memory. This has a possitive effect on the incoming GPS information. With this reset all of your internal memory will be deleted. So it's absolutely necessary to back up your iPhone (see point 1)

  1. Follow the instructions of the Apple support site to back up:
    https://support.apple.com/en-us/ht203977 

  2. Put your iPhone on the charger.

  3. Go to Settings > General > Reset.

  4. Tap then on Erase All Content and Settings and accept the deleting process.

  5. The deleting process can last up to two hours depending on the size of the memory.

  6. After the deleting process connect your iPhone with iTunes or the iCloud.

  7. Now play your backup to your iPhone.