Technical Topics



Do you have several personal computers and would like them all to have accurate clocks? Think that synchronizing your basic home network with a Z3801A receiver means a steep learning curve while you wade through many pages of NTP fundamentals that only refer to yet more pages? Been searching for 'NTP in a box' ?

This article describes a relatively simple approach that eliminates the need for complicated overhead, and instead substitutes common sense techniques.

The beauty of GPSCon, SNTP, and the Z3801 lies in the simplicity of implementation with confidence in the accuracy. If you would like to implement practical time keeping for the PC clocks in your network, read on.

SNTP Fundamentals

SNTP stands for "Simple Network Time Protocol". This is a simplified version of NTP: a standard protocol used to synchronize system clocks on computer systems. SNTP can be installed as a service on Windows computers and will keep your computer's clock correct up to fractions of a second. The service will be constantly updating the system's clock, and can be used as a master time source for other systems on your network. SNTP Servers operating in the Unicast mode determine the correct time, and send standardized packets of time and date information across a network to a specific Client that initiates a request. SNTP Clients listen for server time information across the network and adjust the time and date information on a specific computer. Typically there is one Server and a number of Clients on a given network.

The NTP theoretical accuracy is approximately a few microseconds, but in practice is usually limited to 1-50 milliseconds in a symmetric multi-peer implementation of NTP.

SNTP, a simplified version of the full NTP specification suitable for PC clocks set over the network, typically achieves assurances of 50-250 milliseconds.

The main difference between the two is complexity and resource overhead. NTP can produce more accurate time, but at a price. SNTP is designed to satisfy less demanding applications, while still delivering adequate performance consistent with simple requirements.

GPSCon as an SNTP Server

GPSCon Pro software has the ability to act as an SNTP Server. This facility is available in software versions 1.028 and higher. From the main screen, select Options, then select the Time Services folder. The Clock Correction Interval is the time interval in seconds that GPSCon updates the local computer. The SNTP server facility may be enabled, and the specific SNTP port may be selected. The Poll Interval setting is how often GPSCon checks for a time request from the client. The Delay compensation setting allows you the ability to fine tune any fixed error.

GPSCon Time Accuracy

Practical measurement results:

Server time error (the time difference from UTC to the GPSCon server system time)
< +/- 50ms (< +/- 20ms for 95% of corrections)

Server to client time error:(time difference from server to network client)
< +/- 10ms

In comparison, correcting the server from an Internet time source other than the GPSCon approach (NIST was the best) equaled +/- 250ms.

A Practical SNTP Client

One SNTP Client is called AboutTime. As a time client, it will acquire time from the SNTP Server with great accuracy. AboutTime uses advanced signal-processing techniques to correct for network delays, making high accuracy possible. In a local network with a copy of AboutTime installed on each machine, one can achieve + - 50 millisecond typical synchronization accuracy. AboutTime can be instructed to perform its tasks automatically, at startup or at chosen time intervals. It is a small program that can be run in the background without requiring many resources. AboutTime is compatible with all current Windows versions. "AboutTime is CareWare -- that means no money, now or ever. You just have to care." You may download the software from the AboutTime Home Page.

These AboutTime screen pictures illustrate the typical settings to function with GPSCon Pro software when using the Z3801A GPS receiver. Your network site names and Host name / address will be different, so don't use these exact IP address values.

Another practical network
time client

SymmTime™ from Symmetricom is a convenient multi-zone desktop time utility. SymmTime™ automatically synchronizes your system clock to any NTP (Network Time Protocol) server accessible from your computer. This program is available without cost and may be downloaded from the Symmetricom website.

Program setup is relatively easy. Just use the built-in help functions and make GPSCon your nominated time server. Remove all other internet servers to avoid confusion.

Real World Practical Examples

Brian McCarthy, NX9O reported his experience using GPSCon Pro SNTP Server and AboutTime SNTP Client:

I have used AboutTime on all my PC's at home and all of them can sync to the GPSCon server. Operating Systems include Windows XP, 98 and NT 4.0 SR6. All of them are able to use SNTP port 123 for time requests.

We used a similar configuration on a remote mountain top for the ARRL VHF contest for our limited mulitop entry. We had a PC for each band position of 50, 144, 222 & 432 MHz running Writelog. We also had a "server" PC with Writelog, GPSCon, and the Z3801A. All the log time entries match. There is no question of what happened when. This is a great help when I submit the logs after the contest. We will also be counting on this functionality to provide the time reference for things like WSJT and other timing sensitive applications.

Using AboutTime, GPSCon Pro, and the Z3801A is simple and effective. I think most anyone should be able to put it together.

Frequently Asked Questions

Question - My Windows system clock display (the one with the analog clock with a second hand) doesn't seem to agree very closely with either the audible time ticks of WWV or a portable WWVB clock. The time is within one second but it doesn't maintain a steady relationship to WWV. I hoped I could do better.

The Windows 98 (maybe others) default time display is not intended for more than a one second accuracy. During a one minute interval, the displayed system time as compared to WWV time ticks or a WWVB LCD clock may either be correct, it may be one second slow or fast, or anywhere in between. It will throw you off completely if you are trying to estimate your system clock accuracy.

This is simply a display problem using the default Windows display. The fundamental system time may be within expected tolerance, but it is sometimes difficult to get a steady time display. The GPSCon time display screen is also intended as a basic time display, and is not suitable as a more critical indicator of actual system time.

A good measuring tool for system time is called ClickTime. This utility is courtesy of Graham Baxter, G8OAD. ClickTime will generate a negative edge on the DTR line of the nominated COM port at the turn of the second of the system clock. In addition, it can generate a pretrigger pulse on the same line 50 ms before the main pulse. The DTR lines idle high, and drop low for a few ms. The falling edge is the datum.

These functions allow easy triggering of test equipment. Also, an audible click is generated by the sound system. However the time accuracy of the sound is relatively poor when compared with the DTR pulses. The audible clicks are available for aural comparisons, but they drift around by >100ms. ClickTime is designed so that if it can't do the beep, click or pulse within 50ms of the correct time it skips it. This has been noticed on Win98 systems.

ClickTime and all related documentation is Copyright © 2003 by Graham Baxter. ClickTime is freeware and no fee or license is required for you to use it. The copyright is still in effect. You are granted the right to make copies of ClickTime and its documentation for personal backup purposes only.

You may not copy ClickTime, sell it, distribute it using the web, FTP, CD, or other media, or otherwise reproduce it. If you need additional copies, please get them here. This allows some level of version control and helps make certain that the latest software is being distributed. This product is offered on an as is basis, and no further warranty or guarantee is implied.

Download ClickTime Version 1.5 now.

Question - I don't have a network now, but I have three computers I would like to hook together. What hardware do I need to get started?

Practical networks require a network hub or port switch. Computers on the network have a connection to this electronic switch. Each PC will also have a Network Interface Card (NIC). Sometimes this NIC is built into the PC motherboard. Typically each NIC connects to the port switch using CAT5 cables, although wireless options are available that make it easier to hook up.

Port Switch

Port switches are commonly available, and often have a high speed cable or DSL router included. These are useful to minimize interconnect wiring and allows Internet access for each PC. If you have three computers in your proposed network, a four-port switch, or a DSL router with four-port switch is what you need, and you will have one spare port. A typical wired router with internal four-port switch is the Linksys model BEFSR41, although D-Link, Net Gear and others make capable products too.

Network Configuration

This drawing illustrates the connections required for a router and a port switch. If an integrated router with port switch is used, just eliminate the DSL router wiring and hook the cable/DSL modem directly to the network hub or switch.

Network Troubleshooting Checklist

Does AboutTime report "Timed out waiting for GPSCon"?

- Is GPSCon running on the server machine?
- Has the GPSCon time server been enabled? If not, select the check-box in GPSCon.
- Are the network cards IP addresses static or dynamic?
- If dynamic are you sure of the GPSCon machine IP?
- Can the GPSCon machine be "pinged" from another machine?
  Start - Run - Command - ping <your GPSCon PC server's name>
- Don't use the IP address in the typical screen displays as outlined above.
- Is About Time disabled for all except SNTP? The GPSCon machine should be
  the only server in the list.
- Is the GPSCon port number set to the same as the SNTP port on About Time?  

AboutTime's host set-up must have the IP address of the GPSCon machine or it won't work. Although the host set-up will accept a host name, without a DNS server on your system it has no way of converting that name to an IP address. Windows can access other computers on the network by name because of the Windows computer browsing service; AboutTime has no knowledge of this.

If you haven't configured your network cards then there is a DHCP server on the system doing it for you. This will almost certainly be the router.

On the GPSCon machine (assuming Win98) go to Start - Run and type winipcfg in the box, press return and you should see the machine IP configuration. In the adapter selector, select the network card (if not already selected) and the current card IP should be shown. This is the host address that AboutTime requires.

In Win2K or WinXP, run a command prompt; type ipconfig /all.


Thanks to Brian McCarthy -NX9O, Steve Smith -G8LMX, and Graham Baxter -G8OAD.