SB007 Alta X – GPS Dropouts

Date Released: June 12, 2023

Subject: GPS dropouts affecting Alta X functionality

Effectivity: Effective immediately

Required Action: Update F9P GPS configuration to robustify the system.

 

INTRODUCTION

This Service Bulletin is issued to inform all Alta X operators of temporary GPS dropouts experienced in 2023, affecting the flight operations of multiple aircraft. These dropouts caused the aircraft to switch to altitude mode unexpectedly. The incident started with GPS lock at 30+ satellites, then suddenly dropped to zero satellites and no fix for a few seconds, then jumped back to full lock, with no indication of signal degradation prior to the dropout.

 

BACKGROUND

In 2023, we received several reports of GPS dropouts causing unreliable positioning of Alta X. Investigation revealed that the issues were likely caused by environmental and geomagnetic factors impacting the F9P GPS module, triggering a potential firmware bug in the F9P module itself (this is speculative based on our early intel) that resulted in sudden dropout of positioning during drone operation. The issue appears to be related to the differential positioning mode of the GPS, as a strong correlation was found between dropouts and this mode.  Similar issues have been seen on Freefly Astro, which uses the same GPS module.

 

DISCUSSION

Engineering analysis of the logs shows that the dropouts occur while F9P GPS is in differential correction mode (SBAS), or during transition into this mode (fix_mode = 4 in flight logs). 

A bug in the F9P firmware is likely contributing to the issue:

• Reviewing the F9P firmware release notes, it appears that the F9P v1.13 firmware used by Alta X might have overloaded the ublox F9P CPU with the new SBAS feature enabled. The newer v1.30 and v1.32  firmwares reduced the max output solution rate from 10 Hz to 9 Hz when using all 4 constellations to mitigate this, as per the updated datasheet.  This could explain why the issues only occurred in differential correction mode.
• Reconstructed flight path using onboard recorded observation files and nearby CORS network data showed clean results, indicating that the raw output of the GPS module was fine while the processed position fix data was corrupted. This shows that the issue originates from the F9P GPS module, and it is not an issue on the flight controller side.
• Additionally, the same symptoms were observed in 3 different Freefly Systems Astros back in February 2023. Log reviews showed that issues for each Astro happened in the same 2-hour time window on the same day, despite being in different geographic locations. This shows possible relation to the current peak of the solar cycle and its impact on GPS.

We have discovered some changes that increase the robustness of the system:

• Configuring the F9P to disable Beidou and SBAS ensures that the GPS will not transition into the differential correction mode, which should reduce the CPU load on the GPS module to mitigate the GPS dropouts problem discussed in this service bulletin. 
• Engineering team has tested this configuration, and we suggest this change as one of the recommendations below.
• Additionally, Freefly will continue to investigate this issue and may recommend a GPS firmware update in the future.

 

RECOMMENDATIONS

1. As a precaution, configure the Aircraft GPS module to disable Beidou and SBAS to reduce CPU overload. Please follow the instructions in the following link -> https://freeflysystems.com/sb007-alta-x-updater-tool
2. Ensure proficiency in flying in altitude and manual modes, and switch to it in case of GPS faults. GPS is not required in these modes. https://freefly.gitbook.io/freefly-public/products/alta-x/untitled-7#flight-modes
   • When the GPS dropouts occur, the drone switches to altitude mode and may switch back and forth between modes. The corrective action for the pilot is to switch to altitude mode and stabilize the vehicle. 
3. Never leave aircraft unattended during automated flights, including RTL and mission waypoints. Non-automated modes, such as altitude or manual modes, are not affected by the specific GPS output issues explained above.

 

APPENDIX

Updated ZED-F9P datasheet showing that the PVT solution frequency has been reduced to 9 hz with all satellite constellations in use. With BDS disabled, it is still capable of 10hz. 

ZED-F9P new FW release notes indicating that the output rate has been lowered. It does call out that this change is compared to previous firmware.