Why Use GPS As An Accurate Timing Reference?

Why Use GPS As An Accurate Timing Reference?
What is GPS?

GPS is increasingly being used as a standard hardware clock reference for NTP servers and other computer timing systems. GPS is an US military system that consists of an array of 24 satellites arranged in a constellation that covers the entire globe. Each satellite contains a high precision atomic clock to provide accurate time and positioning information to Earth based systems.

Very accurate time is a fundamental component of the GPS system, which makes it an ideal reference for computer timing equipment, such as time servers.

Advantages Of GPS Over Other Hardware Clock References

There are a number of advantages associated with GPS timing systems over other hardware clocks. Firstly, GPS is a global system, it can be utilised anywhere in the world. Radio timing systems by their very nature are localised, generally by national boundaries. Also, radio timing signals vary from location to location, they have different frequencies and carrier variations. GPS signals are the same whether you’re in the USA or Australia – identical equipment can be used for signal reception. Also, at 60 nanoseconds, GPS is also much more accurate than any radio time and frequency broadcast .

Antenna Location

A GPS antenna generally needs to have a good view of the sky in order to receive a consistently good signal. Often, this can mean locating an antenna on a roof-top with good all-round view. Long cable runs can often be required to achieve this. Low-loss coax cable allows very long cable runs of up to 120m and more. Cable runs can be further extended using amplifiers or fibre optic systems.

 

 

 

 

 
What advantages are associated with GPS timing solutions ?
Using GPS for timing applications has a number of advantages over radio time and frequency services.

GPS is a global system, it consists of a number of orbiting satellites. Provided that you can site a GPS antenna with a good view of the sky, you can obtain a GPS signal lock anywhere in the world. By contrast, radio time and frequency services are regional with a finite reception range.

The GPS system provides a highly accurate timing reference, providing accuracies in the nanosecond range. Radio time and frequency transmissions generally provide accuracies of a few milliseconds.

Provided a GPS antenna has a reasonably good view of the sky, a GPS system can provide a continuously accurate signal that is less prone to interference and maintenance periods.

NTP Time Server – GPS Accuracy

GPS is a much more accurate timing signal than radio based time transmissions. The GPS timing signal is typically accurate to 10 nanoseconds. However, most GPS receivers lose timing accuracy in the interpretation of the signal. A typical GPS receiver with a pulse per second output can provide an accuracy of 100 nanoseconds to 1 microsecond.

TimeTools GPS T1000 and T2000 time servers units synchronise to 100 nanoseconds.

Do I need a GPS Amplifier ?

A GPS amplifier may be required for longer cable runs from the GPS receiver to the GPS antenna. The GPS antenna sits in-line on the coax cable and boosts the signal from the GPS antenna to extend the cable run to the receiver. The amplifier is powered from the GPS receiver and requires no additional power source.

TimeTools GPS amplifier provides an extra 20dB of gain. This allows cable runs of over 100m with low-loss coax cable. Still longer cable runs can be accommodated by installing multiple amplifiers.

A single GPS antenna can be shared between multiple GPS NTP servers by using a GPS splitter. A GPS splitter allows a single GPS antenna to be shared between multiple GPS receivers.

TimeTools GPS splitters are available as 2 way, 4 way and 8 way devices, which allow 2, 4 or 8 GPS NTP servers to share a single antenna. The GPS splitters are active devices, which means they have no insertion signal loss. The devices are supplied with TNC connectors to the antenna and to the NTP server.

GPS Splitter

The GPS splitter takes power from one of the GPS receivers, which means no additional power supply is required. Additionally, they can be used with pole or patch type antennas and lightning suppressors.

Models:

2-way active GPS splitter: model GPS-SP2AN
4-way active GPS splitter: model GPS-SP4AN
8-way active GPS splitter: model GPS-SP8AN

What is the maximum cable length between a GPS receiver and antenna ?

The maximum cable length that can be utilised by a GPS NTP time server antenna is dependant on the gain of the antenna and the quality of the coax cable utilised. TimeTools provides two types of antenna, a patch type GPS antenna and a pole-mounting type GPS antenna.

TimeTools NTP s5100 pole-mounting antenna has a higher gain than the patch type antenna, so can utilise a longer cable length. The gain of the pole-mounting antenna is 35dB, while the gain of the patch type antenna is 27dB.

The maximum cable distance for the patch type antenna are as follows:

Coax Cable Type Maximum Cable Distance that can be utilised
Standard RG58 Coax 20m (25m overall)
TimeTools Low-Loss Coax 25m (30m) overall
The maximum cable distance for the pole-mounting antenna are as follows:

Cable Type Maximum Cable Distance that can be utilised
Standard RG58 Coax 30m
TimeTools Low-Loss Coax 50m
TimeTools Ultra-Low Loss Coax 100m

What is the ideal location for a GPS antenna ?

A GPS antenna needs to have a good view of the sky in order to obtain a signal lock. Ideally, a full 360 degree of the sky is required. However, in practice GPS antennas can obtain a signal lock with a partially obscured view. Often a good signal lock can be obtained by sitting the antenna on a window sill or ledge provided that the horizon is not too obscured.

As a general guide, the better the view of the sky provided the antenna, the better the chance of a good continuous signal lock.

 

What area does the GPS Signal cover ?

The Global Positioning System (GPS) provides accurate time information anywhere in the world. A GPS antenna requires a good view of the sky in order to receive information from the GPS satellites. Provided the antenna can view a large enough area of the sky, a signal lock can be achieved anywhere on the face of the planet.

Issues Obtaining a GPS Signal Lock.

GPS time synchronisation is quite straight-forward, providing the GPS antenna has an adequate view of the sky.

As a guide, the better the view of the sky that can be provided to the antenna, the better the chance of a good consistent signal lock.

A signal lock should take about 5 minutes with a good view of the sky, longer with an obscured view.

Coax cable distances to the antenna can be critical, antenna gain and cable losses should be taken into account. Provided the supplied coax cables have been used, this should not be an issue.

 

What is the Galileo GPS system ?

When operational, Galileo will be the European equivalent to the US GPS global satellite navigation and timing system.

However, unlike the US GPS System and the Russian GLONASS systems, which are military based, Galileo will be tailored to civil markets. Galileo will utilise 30 earth orbiting satellites and will be as accurate as the current US military GPS satellite navigation system and will guarantee continuous transmission.

Using NMEA as a GPS Timing Reference

Many GPS devices use the NMEA protocol, which has been adopted as a standard protocol for GPS devices. The NMEA protocol is ideal for providing continuous positioning information along with a timestamp indicating when information was generated. However, often NMEA timing sentences are not generated at times consistent with the supplied timestamp. This basically means that it is difficult to obtain an accurate consistent time from an NMEA GPS device.

Many NMEA GPS devices therefore only provide an accuracy of about +/- 1 second, which is just not accurate enough for network timing purposes.

Many GPS devices also provide a pulse per second (PPS) output to provide an accurate timing reference. However, the NMEA GPS timing sentence is not synchronised to the pulse output providing poor accuracy. The pulse output is provided at a TTL (5V) signal level rather than a RS232 level that can be recognized by PC equipment. This makes it hard to integrate the GPS device with a computer system.

TimeTools T1000 and T2000 GPS Clock devices utilise an accurate GPS timing receiver with a binary protocol synchronised with a highly accurate timing pulse. This allows for a much more stable and highly accurate timing solution to be implemented.