GPS授时服务器DIY

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