The FLEX-8000 series of software defined radios come equipped with a GNSS receiver. For more information about GNSS receivers, refer to the HelpDesk article What is GNSS and How Is It Different from GPS?
When the GNSS is locked, indicated by the GNSS status in the Radio Settings -> GPS tab, the GNSS receiver provides two radio functions.
- The GNSS, functioning as a GNSS-DO, can be utilized as the 10 MHz Reference Oscillator which is used to discipline the master clock in the radio. The GNSS 10 MHz reference source is more stable and accurate than the internal TCXO.
- It will perform the functions of a stratum 1 NTP (Network Time Protocol) Server.
Since the FLEX-8000 provides the functions of a stratum 1 NTP server, this article will describe NTP and how to utilize it as an accurate time source for network-connected devices.
What is NTP?
Wikipedia reports:
The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks, such as the Internet. In operation since before 1985, NTP is one of the oldest Internet protocols in current use. NTP was designed by David L. Mills of the University of Delaware.
NTP is intended to synchronize participating computers within a few milliseconds of Coordinated Universal Time (UTC).[1]: 3 It uses the intersection algorithm, a modified version of Marzullo's algorithm, to select accurate time servers and is designed to mitigate the effects of variable network latency. NTP can usually maintain time to within tens of milliseconds over the public Internet and can achieve better than one-millisecond accuracy in local area networks under ideal conditions. Asymmetric routes and network congestion can cause errors of 100 ms or more.
Why Is Time Synchronization Important?
In the modern world, precise timing has become essential. At a basic level, utilizing an NTP server for better time accuracy is self-evident. It matches all your device’s times and keeps them spot-on. However, you can understand this point’s significance better by diving into more detail about it. Devices must communicate with public time servers to receive the correct time, but this unavoidably involves a delay, as information is sent and received over the Internet. The slight lag may seem minute, but it can be enough to unsynchronize your network-attached device's time. Using internal clocks is even more difficult, since each device may count seconds faster or slower than the next, so much so that they diverge greatly.
How is NTP Used in My Network?
All or most of the devices connected to your home network are already using NTP to adjust their internal clocks.
Either the PC or device has "hard coded" several external NTP sources to use or your router is providing NTP server information via DHCP. In this case, the network router usually provides DHCP services. By configuring DHCP to instruct a device's NTP client to use the FLEX-8000 as a network time source, all of your device's real-time clocks have greater accuracy and precision.
NTP Server Accuracy
NTP servers must have an accurate time source. The most accurate is an atomic cesium clock, which is very expensive. A relatively inexpensive but highly accurate option uses synchronized time signals from the GNSS/GPS constellation of satellites. When an atomic clock or GPS is used as the reference clock time source for a device configured as an NTP server, it is called a Stratum 0 host. The NTP Stratum model is a hierarchal representation of time servers in an NTP network, where the Stratum level (0-15) indicates the device's distance to the reference clock. The higher the Stratum number, the more the timing accuracy and stability degrades.
Concerning the FLEX-8000 GNSS receiver:
- Stratum 0 means a device (the FLEX-8000 hardware) is directly connected to a GNSS receiver (other accurate hardware time source).
- Stratum 0 devices cannot distribute time over a network directly. They must be linked to a Stratum 1 time server (the compute module running Linux in the FLEX-8000) that distributes time to Stratum 2 servers or clients, and so on over the network.
How To Use the FLEX-8000 as an NTP Server on My Network?
First, the same IP address must be assigned to the FLEX-8000 so it never changes. The recommended way to do this is to use what is known as a DHCP Reservation based on the radio MAC address that always starts with 00:1c:2d (the FlexRadio, Inc. OUI). Instructions on creating a DHCP Reservation for your FLEX-8000 can be found in your router's user guide.
Second, the IP address of the FLEX-8000 needs to be distributed to devices on your network. Again, your network router is used to do this. The network router is either an NTP server itself or has the facilities to distribute IP addresses of NTP servers to devices on your network using DHCP. This is done automatically and normally does not require special configuration tasks other than assigning the FLEX-8000's IP address as the network NTP server Instructions for assigning the FLEX-8000's IP address as an NTP server on your router can be found in your router's user guide.
There is one additional point to make if your router (or other stand-alone device) provides NTP services to your network-attached devices, rather than allocating NTP server IP addresses.
A router providing NTP services is normally reserved for professional-grade firewalls, such as pfSense rather than consumer-grade firewalls. Using the FLEX-8000 as one of its time sources, since it is a Stratum 1 time server, will always configure your router to be a Stratum 2 NTP server. You cannot be guaranteed that your network NTP server will be a Stratum 2 time source when exclusively using Internet-based time sources.
It is preferable, but not required, to configure your network devices to communicate with the Stratum 2 NTP server rather than communicate directly to the FLEX-8000, This is done to reduce network traffic loads on the radio.
Lastly, the FLEX-8000 must be running and the GNSS in a locked state. When a GNSS antenna is connected to the radio's GNSS/GPS antenna port, once it acquires enough satellites to track at least four of them, the GNSS is in a locked state. The greater number of tracked satellites will improve the frequency stability and accuracy of the GNSS-DO. It is recommended to use the powered GNSS antenna and locate it so it has a clear view of the horizon, such as in a window for optimal results.
A SmartSDR client, such as a Maestro or SmartSDR for Windows running on a PC need not be connected to the radio to provide NTP services over the network.