Frequently Asked Questions (Time Synchronization)

GNSS time synchronization is used in mobile base stations and seismometers, as well as critical infrastructure such as train radio, disaster prevention radio, and firefighting radio for commercial use, securities trading, and television broadcasting.
The accuracy required varies depending on the application, for example, on the order of nanoseconds.
For more detailed information, please refer to the Telecommunications magazine article in the related link.

Related Links:

• Telecommunication article: Basics of Network Time Synchronization

Yes, there are differences between the times provided based on the different Coordinated Universal Time (UTC) standards.
For more information, please refer to the following National Institute of Information and Communications Technology (NICT) page linked here.

Related Links:

• Q&A [NICT Website]

Yes, there is an offset (time difference) between GPS time and UTC (USNO, United States Naval Observatory) time.
There are two types of offset: an integer offset and a fractional offset.
The integer offset is based on the "leap second" difference between GPS time and UTC (USNO) time.
The fractional offset is an adjustment from the difference between GPS time and UTC (USNO) time, which should be less than 1 microsecond.
In practice (actual results from operations), this is a difference of within several nanoseconds (in the single digits).
GPS satellites broadcast UTC parameters for the difference between GPS Time and UTC (USNO) in the almanac data, so the Furuno GNSS receiver synchronizes to UTC (USNO) by default after acquiring these UTC parameters.
Depending on the user's configurations, synchronization with different reference times is also possible such as GPS Time, UTC (EU), and other organizations' UTC.

The NMEA data (sentence) of NMEA indicates the time of the output timing of the following 1PPS.
Position and status information are generated based on the positioning results from 1 second ago.
For details such as relationship diagrams, please refer to the "Relationship between 1PPS and sentence output timing" in the protocol specification.

Related Links:

• Downloads (Documents, Software)

From the PPS setting command, please change the PPS type from LEGACY to GCLK.
The default LEGACY PPS setting offers the advantage of a shorter time of 1PPS stabilization from power on, but it does not phase-align precisely with the GCLK frequency output.
For more detailed information, please refer to the protocol specifications linked below.

Related Links:

• Downloads (Documents, Software)

With the "fixed position" configuration, it is possible to achieve the time accuracy shown in the specifications even when receiving a single GNSS signal from just one satellite.
In the past, when the GPS satellite was the only system to support global positioning, only a limited number of satellites were available at certain times of the day.
Today, there are always dozens of GNSS satellites accessible in the skies above, so it is rare to have access to signals from only one satellite.
However, there are increasing numbers of situations in which GNSS receivers are used in environments not suited to effective signal reception (under elevated road/railway structures, next to windows inside buildings, etc.), in which cases it may be possible to receive signals from only one satellite.

Yes, these products may be used in a mobile application such as moving vehicles. Please make sure to change the positioning mode according to your usage.
If your application is in a continually moving object, NAV (Navigation) Mode is recommended, which calculates position information every second.
Details of each position mode are provided in the terminology explanation at the beginning of the protocol specifications (NAV mode, TO mode, SS mode, CSS mode).
Also, please note that the specification values of "time accuracy 40 ns, time precision 4.5 ns (1σ)" are specification values at a fixed point (Stationary application).
For more information, please check the protocol specifications linked below.

Related Links:

• Downloads (Documents, Software)

The World Geodetic System 1984 (WGS-84).

Use the standard elevation above sea level for Altitude.

NAV (Navigation) Mode is a mode wherein latitude, longitude, altitude, speed, direction and time are calculated on a per-second basis. This mode updates the position, speed and direction every second, making it optimal for use when in a moving vehicle. Positioning operations in NAV Mode require signals from at least four satellites (except in the case of SBAS).
SS (Self Survey) Mode is used to calculate the fixed position for use in TO (Time Only) Mode. SS Mode calculates latitude, longitude, altitude and time on a per-second basis, and enables high-precision calculation of fixed position based on positioning information acquired over a set time period. Following the completion of calculations in SS Mode, the device automatically switches to TO Mode.
Fixed-position calculation in SS Mode requires signals from at least four satellites (except in the case of SBAS). However, even if signals from fewer than four satellites are available, as long as at least one satellite signal is available, fixed positioning information calculated thus far can be used to processed in the same manner as with TO Mode and update the time accordingly, thus maintaining accurate 1PPS and frequency.
The protocol specifications contain, as part of the terminology explanations near the beginning, details on each positioning mode (NAV Mode, TO Mode, SS Mode, and CSS Mode).
Follow the "Downloads (Documents, Software)" link below to download the protocol specifications, or follow the "GPS/GNSS Glossary" link to access relevant information.

Related Links:

• Downloads (Documents, Software)
• GPS/GNSS Glossary

Use the FLASHBACKUP command to save the following commands' settings to flash ROM.

・FREQ command
・DEFLS command
・TIMEALIGN command
・FIXMASK command
・GNSS command
・ECLK command
・NLOSMASK command
・SURVEY command
・TIMEZONE command

For more information on the FLASHBACKUP command, refer to the protocol specifications linked below.

Related Links:

• Downloads (Documents, Software)

An explanation of Dynamic Satellite Selection™ algorithm is provided in the March 2019 issue of NTT Technical Review; we encourage you to have a read for yourself.
Note: Dynamic Satellite Selection™ is algorithm-based multipath mitigation technology developed by NTT.

Related Links:

• NTT Technical review [NTT Website]

Based on the default configuration, a "NMEA sentence," or set of time data, indicates time for the following 1PPS signal. However, positioning-related information and status information are generated based on positioning results from 1 second prior.
For relationship diagrams and other detailed information, please refer to "Output Timing of 1PPS and Serial Data" in the protocol specifications linked below.

Related Links:

• Downloads (Documents, Software)

With the "fixed position" configuration, it is possible to achieve the time accuracy shown in the specifications even when receiving a single GNSS signal from just one satellite.
In the past, when the GPS satellite was the only system to support global positioning, only a limited number of satellites were available at certain times of the day.
Today, there are always dozens of GNSS satellites accessible in the skies above, so it is rare to have access to signals from only one satellite.
However, there are increasing numbers of situations in which GNSS receivers are used in environments not suited to effective signal reception (under elevated road/railway structures, next to windows inside buildings, etc.), in which cases it may be possible to receive signals from only one satellite.

Yes, these products may be used in a mobile application such as moving vehicles. Please make sure to change the positioning mode according to your usage.
If your application is in a continually moving object, NAV (Navigation) Mode is recommended, which calculates position information every second.
Details of each position mode are provided in the terminology explanation at the beginning of the protocol specifications (NAV mode, TO mode, SS mode, CSS mode).
Also, please note that the specification values of "time accuracy 40 ns, time stability 4.5 ns (1σ)" are specification values at a fixed point (Stationary application).
For more information, please check the protocol specifications linked below.

Related Links:

• Downloads (Documents, Software)

Even if you store position information (latitude, longitude, etc.), a certain amount of time is required to make frequency adjustments. Therefore, no major change to setting time will result.

Relevant data is included in GF-88 series user's manuals. Please follow the link below.

Related Links:

• Downloads (Documents, Software)

As long as the GNSS receiver is receiving signals from GNSS satellites, the time information output by the receiver will not be affected by ambient temperature changes. Only issues such as lightning strikes to the antenna, which prevent the receipt of satellite signals, will have adverse effects on time precision. In such a situation, FURUNO ELECTRIC GF-88 series utilizes the holdover function, wherein the built-in crystal oscillator's frequency is temporarily used to maintain timekeeping.
Ambient temperatures generally do affect crystal oscillator frequency, so this situation may result in reduced time accuracy in a GNSS receiver without built-in design countermeasures. However, the GF-88 series is controlled using crystal oscillators designed after considering the possibility of temperature changes, so declines in accuracy are kept to a minimum.
When comparing products' holdover specifications, it is important to confirm whether or not such systems have been designed considering the temperature changes.
The following white paper provides detailed information on the holdover function, as well as the conditions for achieving optimal performance in holdover.

Related Links:

• White Paper: "Holdover function" as measures against GNSS reception interruptions

This can be confirmed via "Freq Mode" in the CRZ (TPS4) sentence.
Refer to the protocol specifications linked below for more information.

Related Links:

• Downloads (Documents, Software)

Even in the holdover state, the device will continue to output time information via the NMEA sentence such as GGA, etc.

Yes, the mode transition to holdover is possible as long as the device has been powered on for at least 7 days, regardless of whether COARSE LOCK, FINE LOCK or any other frequency mode is used.

Yes, it will be reset. The learning time in the CRZ sentence will be returned to zero.

"Oscillator Fault" alarm occurs when the built-in oscillator's frequency (10 MHz) of GF-880x is stopped.
"Oscillator Control Fault" alarm occurs if GF-880x encounters difficulty in controlling the built-in oscillator's frequency— for example, when major discrepancies in oscillator frequency occur, when control is no longer possible, when the oscillator has reached the end of its product life, or when the oscillator has broken down.

If an antenna "short" alarm occurs and the power output stops, would it be possible to recover the output power again once the antenna "short" alarm has been cleared?

The World Geodetic System 1984 (WGS-84).

Use the standard elevation above sea level for Altitude.

NAV (Navigation) Mode is a mode wherein latitude, longitude, altitude, speed, direction and time are calculated on a per-second basis. This mode updates the position, speed and direction every second, making it optimal for use when in a moving vehicle. Positioning operations in NAV Mode require signals from at least four satellites (except in the case of SBAS).
SS (Self Survey) Mode is used to calculate the fixed position for use in TO (Time Only) Mode. SS Mode calculates latitude, longitude, altitude and time on a per-second basis, and enables high-precision calculation of fixed position based on positioning information acquired over a set time period. Following the completion of calculations in SS Mode, the device automatically switches to TO Mode.
Fixed-position calculation in SS Mode requires signals from at least four satellites (except in the case of SBAS). However, even if signals from fewer than four satellites are available, as long as at least one satellite signal is available, fixed positioning information calculated thus far can be used to processed in the same manner as with TO Mode and update the time accordingly, thus maintaining accurate 1PPS and frequency.
The protocol specifications contain, as part of the terminology explanations near the beginning, details on each positioning mode (NAV Mode, TO Mode, SS Mode, and CSS Mode).
Follow the "Downloads (Documents, Software)" link below to download the protocol specifications, or follow the "GPS/GNSS Glossary" link to access relevant information.

Related Links:

• Downloads (Documents, Software)
• GPS/GNSS Glossary

Use the FLASHBACKUP command to save the following commands' settings to flash ROM.
・GCLK command
・DEFLS command
・TIMEALIGN command
・FIXMASK command
・GNSS command
・PPS command
・NLOSMASK command
・SURVEY command
・HOSET command
・MODESET command
・TIMEZONE command

For more information on the FLASHBACKUP command, refer to the protocol specifications linked below.

Related Links:

• Downloads (Documents, Software)

An explanation of Dynamic Satellite Selection™ algorithm is provided in the March 2019 issue of NTT Technical Review; we encourage you to have a read for yourself.
Note: Dynamic Satellite Selection™ is algorithm-based multipath mitigation technology developed by the Nippon Telegraph and Telephone Corporation (NTT).

Related Links:

• NTT Technical review [NTT Website]

Yes, TB-1 supports the holdover function, and the specifications are as follows.
Long-term holdover: < ±1.5 μs / 2 hr., < ±50 μs / 24 hr.
Short-term holdover: < ±3 μs / 1 hr. (TYP)
For your reference, there is a technical white paper that describes how Furuno's GNSS receivers operate internally in the holdover function, as well as the conditions for achieving the holdover performance.
Please take a look at the white paper, which also includes an explanation of long-term and short-term.

Related Links:

• White Paper: "Holdover function" as measures against GNSS reception interruptions

It is strongly recommended that the GNSS antenna should be installed in an unshielded, unobstructed location that is open to a 360° clear sky view.
Refrain from installing it near any device that emits strong radio (RF) signals.

Multiple GNSS antennas should be at least 50 centimeters apart, and ideally 1 meter or more is recommended.

The cable length must be within the specification that allows the required total input gain from the GNSS antenna to the GNSS receiver.
The total input gain is calculated by subtracting the total cable attenuation from the GNSS antenna's output gain.
Refer to the GNSS receiver device's specifications and other documentation for further details.
The total cable attenuation is calculated by multiplying the cable attenuation amount (dB/m) by the cable length (m).
For your information, the cable attenuation varies depending on cable type, material, and thickness.
Please contact the cable manufacturer to get more information on the attenuation specifications for the GNSS frequency bands.
The GNSS Antenna Installation Guide contains information on total gain calculations and more.
Please download the GNSS Antenna Installation Guide from the link below.
In addition, due to the complex calculation formula, we have also prepared an Excel spreadsheet for calculating "total gain" and "total NF" as an additional document.

Related Links:

• GNSS Antenna Installation Guide

Power can be supplied from any of the four distributed terminals (Output ports).
If the power is fed from multiple terminals simultaneously, the highest voltage power will be fed among the terminals.
The four terminals are diode-ORed, so there is no hysteresis or timing. The higher voltage simply takes priority.

TVA-03C should be connected directly near the GNSS antenna to be protected, and TVA-03V directly near the GNSS receiver.

There is no specified connection polarity; either way connection should be ok.

For both TVA-03C and TVA-03V, a 5.5 to 8.0 sq ground wire is recommended.
Any size within this range should be acceptable. The optimal (and recommended) size is the thickest one possible, 8 sq, as it passes lightning surge more readily.

FURUNO TVA-03C and TVA-03V surge protective devices (SPDs) do not have any specific or special grounding requirements.
There are no special stipulations regarding resistance value, grounding type, etc.
However, it is important to ensure that the SPD ground uses a shared, common electric ground with the lightning-countermeasure device.
Equipment to which communication-use SPDs are connected is often 200V or less, so the SPD is often grounded via connection to the protective device's D-type ground.

TVA-03V will break down in response to a short circuit fault, which will cut off GNSS signals.
Therefore, if TVA-03V is damaged, GNSS signals are not able to pass through TVA-03V to a GNSS receiver.
In most cases, TVA-03C will also break down in response to a short circuit fault, which will cut off GNSS signals.
However, in rare cases, TVA-03C may break down even when no electric current is flowing through, meaning it may not be in a short-circuit state upon breakdown.
In this case, GNSS signals may continue to pass through. Following a lightning strike, we recommend carrying out device maintenance using an SPD checker or similar.