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ULG file taking 25 Go

I would like to know what are ULG file on Ubuntu. After trying to find what was using some space, I find a repertory using 25 Go on 28 Go total, and this repertory have got 785 ULG file.

Note that I'm working on an Intel Aero RTF, using Ubuntu 16.04, and it can store 28 Go.

Output of du command (essential part):

root@intel-aero:/var/lib/mavlink-router# du -h /var/lib/mavlink-router/
25G /var/lib/mavlink-router/

Number of file :

root@intel-aero:/var/lib/mavlink-router# find /var/lib/mavlink-router/ -type f | wc -l
785

Part of result of ls:

root@intel-aero:/var/lib/mavlink-router# ls -lh
total 25G
-rw-r--r-- 1 root root   73M May 22 05:45 00000-2018-05-22_05-14-52.ulg
-rw-r--r-- 1 root root   36M May 22 06:00 00001-2018-05-22_05-45-23.ulg
-rw-r--r-- 1 root root  9.0M May 22 05:00 00002-2018-05-22_04-57-05.ulg
-rw-r--r-- 1 root root   11M May 22 05:01 00003-2018-05-22_04-57-05.ulg
-rw-r--r-- 1 root root  117M May 22 05:46 00004-2018-05-22_04-57-05.ulg
-rw-r--r-- 1 root root  220M May 22  2018 00005-2018-05-22_04-57-05.ulg
-rw-r--r-- 1 root root     0 May 22  2018 00006-2018-05-22_06-31-13.ulg
-rw-r--r-- 1 root root     0 May 22  2018 00007-2018-05-22_06-31-14.ulg

df result :

root@intel-aero:/var/lib/mavlink-router# df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/root        28G   28G     0 100% /
devtmpfs        1.9G     0  1.9G   0% /dev
tmpfs           1.9G     0  1.9G   0% /dev/shm
tmpfs           1.9G   76M  1.8G   4% /run
tmpfs           1.9G     0  1.9G   0% /sys/fs/cgroup
tmpfs           1.9G   48K  1.9G   1% /var/volatile
tmpfs           1.9G  4.0K  1.9G   1% /tmp
/dev/mmcblk0p1   28M  7.7M   21M  28% /boot
tmpfs           382M     0  382M   0% /run/user/0

So the question is : what type of file is ULG file? Is it a save, config, ... ? Can I remove some of them (or all) without any issue later?