[问题]Release signature does not verify意味着什么？

[fmying]

我用debmirror维护Ubuntu源，但是每Getting Release 后总会出现
Release signature does not verify.
请问出现这个语句意味着什么？是不是说我的源不会被更新？
我的命令：
/usr/bin/debmirror --method=http -v --progress --ignore-release-gpg --skippackages --ignore-missing-release --section=$SECTION --host=$HOST --dist=$DISTS --arch=$ARCHS $OPTIONS -r $MIRROR_ROOT $LOCAL

其中的$开头的都是一些变量

输出信息如下：
Processing standard using ubuntu.cn99.com
Ubuntu mirror Started update at Thu Jan 4 20:35:30 HKT 2007
Mirroring to /var/ftp/ubuntu from http://anonymous:ubuntu.cn99.com//ubuntu/
Arches: i386,amd64
Dists: dapper,dapper-backports,dapper-proposed,dapper-security,dapper-updates,edgy,edgy-backports,edgy-proposed,edgy-security,edgy-updates
Sections: main,multiverse,restricted,universe
Including source.
Attempting to get lock, this might take 2 minutes before it fails.
Get Release files.
[0%] Getting: dists/dapper/Release... ok
[0%] Getting: dists/dapper/Release.gpg... ok
Can't exec "gpg": No such file or directory at /usr/bin/debmirror line 586.
Release signature does not verify.
[0%] Getting: dists/dapper-backports/Release... ok
[0%] Getting: dists/dapper-backports/Release.gpg... ok
Can't exec "gpg": No such file or directory at /usr/bin/debmirror line 586.
Release signature does not verify.
[0%] Getting: dists/dapper-proposed/Release... ok
[0%] Getting: dists/dapper-proposed/Release.gpg... ok
Can't exec "gpg": No such file or directory at /usr/bin/debmirror line 586.
Release signature does not verify.
[0%] Getting: dists/dapper-security/Release... ok
[0%] Getting: dists/dapper-security/Release.gpg... ok
Can't exec "gpg": No such file or directory at /usr/bin/debmirror line 586.
Release signature does not verify.
[0%] Getting: dists/dapper-updates/Release... ok
[0%] Getting: dists/dapper-updates/Release.gpg... ok
Can't exec "gpg": No such file or directory at /usr/bin/debmirror line 586.
Release signature does not verify.
[0%] Getting: dists/edgy/Release... ok
[0%] Getting: dists/edgy/Release.gpg... ok
Can't exec "gpg": No such file or directory at /usr/bin/debmirror line 586.
Release signature does not verify.
[0%] Getting: dists/edgy-backports/Release... ok
[0%] Getting: dists/edgy-backports/Release.gpg... ok
Can't exec "gpg": No such file or directory at /usr/bin/debmirror line 586.
Release signature does not verify.
[0%] Getting: dists/edgy-proposed/Release... ok
[0%] Getting: dists/edgy-proposed/Release.gpg... ok
Can't exec "gpg": No such file or directory at /usr/bin/debmirror line 586.
Release signature does not verify.
[0%] Getting: dists/edgy-security/Release... ok
[0%] Getting: dists/edgy-security/Release.gpg... ok
Can't exec "gpg": No such file or directory at /usr/bin/debmirror line 586.
Release signature does not verify.
[0%] Getting: dists/edgy-updates/Release... ok
[0%] Getting: dists/edgy-updates/Release.gpg... ok
Can't exec "gpg": No such file or directory at /usr/bin/debmirror line 586.
Release signature does not verify.
Get Packages and Sources files and other miscellany.
Parse Packages and Sources files and add to the file list everything therein.
Cleanup mirror.
Download all files that we need to get (140 MiB).
Downloaded 140 MiB in 917s at 155.82 kiB/s
Errors:
Release signature does not verify.
Release signature does not verify.
Release signature does not verify.
Release signature does not verify.
Release signature does not verify.
Release signature does not verify.
Release signature does not verify.
Release signature does not verify.
Release signature does not verify.
Release signature does not verify.
Everything OK. Moving meta files.
All done.
Ended at: Thu Jan 4 20:52:30 HKT 2007

谢谢～