NTP Hardware Reference Clocks: GPS, Multi-GNSS or Radio?

There are a number of sources of accurate time available to timing applications. They divide into satellite navigation systems and national time and frequency radio broadcasts.

TimeTools T-Series NTP servers use either GPS or Multi-GNSS timing references. Below is a list the associated advantages and disadvantages of each.

GPS (Global Positioning System)

GPS (Global Positioning System) is a constellation of 24 orbiting satellites owned and maintained by the US government. The system provides free to air, subscription free, navigation and timing services.

GPS Advantages

GPS signals can be received worldwide.
Very accurate – GPS accurate to less than 15 nanoseconds.
Generally unaffected by local topography.
T-Series GPS receivers can operate down to a single satellite in view allowing a window or sill located antenna.
TimeTools high sensitivity GPS receivers can often receive signals with an indoor located antenna.

GPS Disadvantages

Ideal location for a GPS antenna is on a roof top with a full view of the sky.
Antenna installation in some instances can be costly.

Multi-GNSS

Multi-GNSS (Global Navigation Satellite System) is a standard generic term for satellite navigation systems. The term is used to refer to a combination of GPS, GLONASS, Beidou and Galileo satellite constellations. All the services are free to air and do not require any subscription.

GPS (Global Positioning System) is a constellation of satellites owned and maintained by the US government. GLONASS is Russia’s version of GPS (Global Positioning System). The BeiDou Navigation Satellite System is a Chinese satellite navigation system. While Galileo is the civilian global navigation satellite system that is currently being constructed by the European Union and the European Space Agency.

Multi-GNSS Advantages

Worldwide reception of GPS, GLONASS and Galileo satellite constellations.
Multiple satellite constellations improves reliability and redundancy.
T-Series GPS receivers can operate down to a single satellite in view allowing a window or sill located antenna.
TimeTools high sensitivity GPS receivers can often receive signals with an indoor located antenna.
Though satellite systems don’t often fail, if one fails Multi-GNSS receivers can pick up signals from other systems.
If line of sight is obstructed, having access to many more satellites is a benefit.
Very accurate – GPS accurate to less than 15 nanoseconds.
Unaffected by local topography.
Widely accepted as the best solution for timing applications.

Multi-GNSS Disadvantages

Ideal location for a GNSS antenna is on a roof top with a full view of the sky.
Antenna installation in some instances can be costly.

Radio Time References

There a number of national radio time and frequency radio broadcasts, each referred to by a call sign. The most well-known being MSF in the UK, DCF-77 in Germany and WWVB in the USA.

Radio Advantages

Lower cost antennas and receiving equipment.

Radio Disadvantages

Only covers national areas. Availability restricted to a few thousand kilometers of the transmitter.
Single transmitter – can go offline for maintenance.
Poor signal reception in many areas due to local topography.
Far less accurate than GPS and GNSS systems – < 10 milliseconds.
Not recommended as a sole source of time for NTP servers.

Summary

TimeTools manufactured radio NTP servers and timing equipment for many years. However, due to frequent signal reception and availability issues, GPS and Multi-GNSS satellite based systems are far superior.

GPS provides an excellent, reliable and highly accurate, source of time. Multi-GNSS improves on this by adding additional reliability and redundancy.