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Synchronising time in survey spreads
Even with something like an EIVA TimeBox timestamping data telegrams, networked PCs will still need accurate time, provided by NTP (Network Time Protocol). There are simple solutions that set PC clocks using NTP, but they have drawbacks:
Only one NTP service can run on a single PC at any time. Simple solutions such as Time-Sync and AboutTime are adequate where clocks are only required to synchronise occasionally. Where millisecond accuracy and stability are paramount, a solution based upon the reference code published by the NTP Project is indispensable.
Most NTP services operate as both servers and clients at the same time. A stratum 2 server is merely a machine that was configured as a client of a stratum 1 server. Servers may operate in a hierarchy as many as sixteen deep, but to do this without good reason could introduce points of failure.
Configuring NTP throughout a network
At its most basic, all that is necessary to configure NTP on a machine is the IP addresses (or DNS names) of one or more reachable NTP servers. In simpler clients these are usually entered in a Windows dialogue, while full implementations rely on an
You need at least one time server on your network before you can configure lower-stratum machines to use it. You may well already have stratum 1 servers if you are using network-connected GPS receivers like the Fugro StarPack, which provides NTP service by default, but if you cannot identify any existing servers, you will create one using a PC – probably your main Nav PC.
Once you have found your NTP servers, or created one, install the NTP service of your choice on all PCs to be synchronised, configuring each one to use the server(s) as their source(s). If installing a full NTP implementation, you would simply find the server lines in the default
Creating a top-stratum NTP server
If you have no existing time server, select a PC to be your master server. It will need to have a reference clock, and you have three options:
NTP has been running across the Internet since before the World Wide Web. It propagates time down a hierachy of strata, using sophisticated algorithms refined over more than thirty years to determine the accurate time, and discipline local clocks to follow it. After the initial setting, it performs any adjustment by slewing the clock gradually and will never cause it to step more than a limited amount, or step backwards.
At the highest stratum, 0, are the authoritative reference clocks, which are mostly atomic clocks or GPS receivers. Directly connected to the reference clocks are the stratum 1 NTP servers, and below these are servers that do not have directly connected reference clocks. Servers listen on UDP port 123 (the traditional number of the speaking clock). Servers below stratum 1 refer if possible to three or more servers at higher strata, comparing times and using the algorithms to select the most stable source.
A server that selects a preferred source at stratum 1 becomes a stratum 2 server, and so on down the line. On the Internet, there are now many publicly accessible NTP servers at stratum 1 and stratum 2, so NTP clients which rely on the Internet are rarely below stratum 3. Nevertheless, NTP allows for strata from 0 to 16.
NTP is the clock synchronisation solution that Harrods would sell you – except that it is free. But there are some hurdles to negotiate in getting the highest accuracy on Windows machines.
Microsoft first provided a basic native system using SNTP (Simple NTP) with the introduction of the Windows Time Service (W32time) in Windows 2000. W32time has developed until, in Windows Server 2016 and Windows 10, Microsoft claims full accuracy but discloses that “Windows Time service is not an exact implementation of the Network Time Protocol”.
W32time is configured by default to check the clock at fixed intervals, using the Internet source, time.microsoft.com. Finding clear and correct documentation on altering the configuration to obtain highly accurate time from a different source is challenging.
Under earlier versions of Windows, users needing fully-featured NTP had to install implementations of the NTP reference source specially packaged for Windows, with the preferred packages coming first from Trimble and, more recently, from Meinberg. The inadequacy of current documentation on configuring Windows Time Service remains an incentive to prefer Meinberg’s implementation.