国内外厂商制造的高精度时钟服务器实际都是基于x86服务器，加装铷钟和GPS接收模块构成。树莓派流行起来后，网上也有配套的模块，让大家可以制造一个小巧的时钟服务器。实际这淘宝上有很多USB接口的GPS接收模块，产品特性：GPS+GLONASS双模定位内置FLASH，USB供电自适应1200-921600波特率1Hz输出，NMEA-0183协议输出。NTPD是业内对各种GPS接受器支持的最好的，绝大部分GPS接受模块实际都是使用标准的NMEA协议从/dev/gps输出。

测试的配置如下：

    # For more information about this file, see the man pages
    # ntp.conf(5), ntp_acc(5), ntp_auth(5), ntp_clock(5), ntp_misc(5), ntp_mon(5).

logfile /var/log/ntp.log driftfile /var/lib/ntp/drift

Permit time synchronization with our time source, but do not

permit the source to query or modify the service on this system.

restrict default nomodify notrap nopeer noquery

Permit all access over the loopback interface.  This could

be tightened as well, but to do so would effect some of

the administrative functions.

restrict 127.0.0.1 restrict ::1

Hosts on local network are less restricted.

#restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap

    # Use public servers from the pool.ntp.org project.

Please consider joining the pool (http://www.pool.ntp.org/join.html).

server 127.127.20.1 mode 17 minpoll 4 maxpoll 4 fudge  127.127.20.1 stratum 0 refid GPS  flag1 1 flag2 0 flag3 1 server 127.127.1.1 minpoll 4 maxpoll 4 fudge  127.127.1.1 stratum 1 refid LCK

Enable public key cryptography.

#crypto

includefile /etc/ntp/crypto/pw

Key file containing the keys and key identifiers used when operating

with symmetric key cryptography.

keys /etc/ntp/keys

Specify the key identifiers which are trusted.

#trustedkey 4 8 42

Specify the key identifier to use with the ntpdc utility.

#requestkey 8

Specify the key identifier to use with the ntpq utility.

#controlkey 8

Enable writing of statistics records.

#statistics clockstats cryptostats loopstats peerstats

Disable the monitoring facility to prevent amplification attacks using ntpdc

monlist command when default restrict does not include the noquery flag. See

CVE-2013-5211 for more details.

    # Note: Monitoring will not be disabled with the limited restriction flag.

disable monitor