Timing Calibration of a GNSS Receiver

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Timing Calibration of a GNSS Receiver

GNSS is well-known for its ability to provide a position with sub-meter accuracy. However, it is less well-known that GNSS provides a very convenient way of obtaining nanosecond (or even sub-nanosecond) timing accuracy via a GNSS receiver. Indeed, in addition to the three spatial dimensions, GNSS enables the user to compute the clock bias and the drift of the receiver’s clock with respect to the atomic clock of the GNSS constellations. To perform this properly, it is necessary to first calibrate the GNSS receiver and the RF setup from the antenna to the receiver.

Precisely measuring the accuracy of the 1-PPS signal of a GNSS receiver can be challenging, especially as we are dealing with nanosecond uncertainties. The variability (atmospheric conditions, multipath, etc.) and unpredictability of live-sky signals prevent the manufacturer or the end user from calibrating equipment using these signals. RF circuitry and signal processing algorithms are also very sensitive to each signal’s frequency and modulation. Delays can vary up to several nanoseconds between each GNSS signal, which explains why the time synchronization needs to be assessed for each signal.

As a result, the best way to correctly measure the accuracy of a GNSS receiver is to use a well-calibrated GNSS simulator as a reference. A GNSS simulator allows the user to control every type of atmospheric effect and to reproduce a deterministic and repetitive signal. The simulator can also provide a 1-PPS signal for use as a reference for the device under test (DUT).

However, in this case the challenge is to measure and certify the accuracy of the GNSS simulator. The classical approach to generating simulated signals is to use real-time hardware (such as FPGA) to synthesize each satellite signal (usually described as channels) in intermediate frequency (IF). The drawback of this approach is that each FPGA can only handle a limited number of channels, which therefore requires independently calibrating each cluster of satellites. This calibration process is laborious and a major source of errors.

One of the key advantages of the Orolia’s Skydel GNSS simulator is its ability to use the power of the GPU to generate digitally and in baseband each and every satellite signal (as well as multipath or interferences). With Skydel, all satellite signals on the same frequency band are synthesized together with the same hardware components from baseband to RF signal. Consequently, the Skydel simulator needs to be calibrated only once for the two GNSS bands, and the delay between each satellite signal on the same carrier is perfectly equal to zero.

Finally, the Skydel GNSS simulator has been designed from the start to be synchronized with an external reference clock and to easily synchronize an unlimited number of Skydel instances among themselves (for instance, synchronizing multiple antennae or multiple receivers).

This application note gives an overview of the typical timing configurations provided by the Skydel simulator and explains how the end user can accurately calibrate the simulator with its specific laboratory setup (RF cables, LNA, splitters, etc.).

Timing configurations

GPSDO Reference clock

The simplest way to use the Skydel GNSS simulator to calibrate a timing receiver is to set up a basic configuration that uses an Ettus X300 SDR equipped with a GPSDO clock inside. In this case, the GPSDO serves as both a 10 MHz and a 1 PPS reference clock.

For this configuration, we must select GPSDO as a reference clock in the X300 output settings.

With this configuration, the RF signal is synchronized with the 1 PPS output of the X300 radio.

External reference clock – single Skydel session

If the user wants to use an external reference clock for the GNSS simulator, it is also possible to synchronize the SDR (or multiple SDRs) with external 10 MHz and 1 PPS references. In this case, connect the 1 PPS input and reference input of each of the X300 SDRs to the corresponding outputs of the external clock. It is important to use strictly identical cables for each of these connections.

For this configuration, we must select External as a reference clock in the X300 settings, doing so for each SDR.

In the Global→ Synchronize simulators settings, we must configure the Skydel simulator as Master.

With this configuration, the RF signal is synchronized with the 1 PPS output of the reference clock. Note that, in this case, the 1 PPS outputs of your SDRs are deactivated as they are not synchronized with any signal.

External reference clock – multiple Skydel sessions

Finally, multiple Skydel sessions can be synchronized with one or more SDRs active in each session. The principle is the same as with a single Skydel session—we need to use an external reference clock to synchronize each of the SDR.

For this configuration, we must also select External as a reference clock in the X300 output settings for each SDR. In the Global→ Synchronize simulators settings, we must configure one of the Skydel simulator sessions as Master.

All of the remaining sessions must be configured as Slaves.



Similar to the configuration with a single Skydel instance, the RF signals are synchronized with the 1 PPS output of the reference clock.

Calibration procedure

Configuration Setup

The Skydel simulator is designed to provide a consistent PPS signal with an accuracy equal or better than 5 ns. This calibration is performed for each configuration described in this document and for each sampling rate selected on the SDR output.

However, the user may have a custom installation with RF cables, LNA, attenuators, and splitters between the RF output and the receiver under test. Each of these components adds a supplemental delay to the RF signal propagation that the user may need to evaluate. Furthermore, with good instrumentation, it is possible to achieve far better delay measurement accuracy (e.g., lower than 1 ns).

The procedure required to evaluate supplemental delays with the Skydel simulator with a high degree of precision is as follows:

First, the measurement setup requires an oscilloscope connected to both the 1 PPS reference and the RF signal where we need to assess the delay (for instance at the input of the receiver). While the following figure illustrates a configuration with an internal reference clock (GPSDO), it is applicable for the other configurations described in this document (i.e., the 1 PPS reference becomes the 1 PPS output of the external clock).

To measure the delay between the RF signal and the 1 PPS, it is then necessary to create a specific scenario on the Skydel simulator. The simplest way to measure the timing of the RF signal is to broadcast a single GPS C/A satellite signal and to observe the transition between the last chip and the first chip of the modulation code. Thanks to the specific design of the Skydel simulator, each of the other GNSS signals will now be perfectly aligned with the C/A code.

Scenario description

Create a new scenario within Skydel and configure a new radio broadcasting-only GPS C/A signal on the output to be measured. In the Settings panel, select the output bandwidth that will be used to evaluate the timing receiver.

In the GPS→ General tab, uncheck the signal propagation delay option. Skydel will then simulate pseudoranges with a zero delay for each of the satellites, enabling it to accurately align the C/A code with the 1 PPS signal.

In the Message Modification→ NAV tab, add a new message modification on satellite #10. Set each of the bits to 0 (including parity bits) on all of the subframe as well as the word. With this modification, we are sure to have a 0/1 chip transition at the end of the modulation code (every ms).

In GPS→ Signals, unselect the RF signal for all satellite signals except PRN 10. (PRN10 is visible in the default configuration of Skydel and, as the last chip of the spreading code, it has the opposite sign of the first chip.)

In GPS→ Signal level, set the global signal power and GPS C/A code to the maximum (10 dB each); this should ensure that the RF signal is displayed on the oscilloscope.

Run the simulation and adjust the oscilloscope to display both the 1 PPS signal and the RF signal. We can now accurately measure the delay between the rising edge of the 1 PPS and the phase inversion of the RF signal. This helps us determine the delay for which to compensate on all future measurements with the same laboratory setup.

Note: Due to a limitation with the oscilloscope used here, the 1 PPS signal is not drawn. However, the 50% rising edge is aligned with the vertical dashed line on the figure. The plain line is synchronized with the phase inversion of the RF signal. In this example, we measure a fixed offset of 520 +/- 100 ps between 1 PPS and RF signals.

Conclusion

While GNSS has shown itself to be an indispensable system for positioning and navigation, it is also critical for a number of timing applications such as banking or energy generation and transmission. For these types of applications, an accurate characterization of the timing receiver is essential; consequently, the use of a GNSS simulator is key to achieving such accuracy.

The power of Orolia’s Skydel GNSS simulator is its ability to synthesize all GNSS signals in baseband, which means that all satellites signals on the same frequency band are perfectly synchronized among themselves. As a result, the system timing calibration—a complicated and expensive operation on other systems—is highly simplified on the Skydel simulator.

Two problems need to be solved in any time-related application:

StableNet Network Management Solutions 5


Master Clock

  1. Which clock is used as the reference for all other clocks
  2. How to transfer the time from the reference clock to all other clocks

The solution is to use a master clock as your reference. Master clock systems are used in a wide variety of applications and industries including aerospace and defence, broadcast, radio and telecom, network systems, financial services, emergency operations, call centers, and healthcare — essentially anywhere reliability of data and signals are paramount.

What is a master clock?  

A master clock takes one or more precise timing reference signals as inputs, and then converts and distributes those timing references to other devices. The method by which the accuracy of the master clock is transferred to other secondary clocks is known as synchronization. Typically, GPS satellite signals are utilized for synchronization to ensure accurate time, but other references may be used such as local atomic clocks or other time standards.

A core feature of all master clock systems is that they accept precise timing reference signals as input. It is a rare case for a master clock to be free-running and not continuously synchronized, or at least compared against an external reference. Orolia’s SecureSync modular time and frequency synchronization system can accept over 14 different signal types to discipline its local clock. This system can then generate a similar number of signal types to synchronize other devices. In case of loss of the external reference (or any redundant references), the local clock maintains timing accuracy using a local clock oscillator until the reference(s) can be restored. Several different clock oscillators are offered depending on the accuracy required during the “hold over” period.

Network master clocks can distribute their timing references over local or wide area networks. Master clocks with wireless transmitters enable synchronization of devices like display clocks without having to run wires between them for the synchronization signal. There are also highly accurate master clock solutions that utilize copper or fibre connections for precise analog and digital signal distribution, such as IRIG timecode signals.

Orolia offers a variety of master clock systems to meet the requirements for your application of accurate time. Learn more about flexible SecureSync Master Clocks

What is a Leap Second and How Does It Affect Me?

By David Sohn, Solution Architect

A leap second is a discontinuity in the world’s official timescale and is a risk for those developing and maintaining GPS/GNSS systems and/or managing a time synchronization deployment. The last leap second occurred on December 31, 2016.

This blog is to help you understand the leap second vulnerability and plan ahead for the next leap second so that your applications will continue to operate smoothly. The ITU-R issued a statement about leap seconds that the WRC decided not to change the characteristics of radio broadcasts of UTC so leap seconds will continue at least until 2023.

What Is a Leap Second?

Since the official definition of time is based on atomic standards, a leap second is inserted in the UTC time scale to keep it in step with the solar day — much like a leap day is used to keep the calendar in step with the seasons. A leap second can be added or removed, although historically leap seconds have only been added. A leap second typically occurs at the end of the day (UTC) on December 31 or June 30 and usually announced approximately six months in advance.

How Does a Leap Second Affect GPS/GNSS?

You probably know that time synchronization is fundamental to how GPS/GNSS works. The developers of GNSS knew that the system could not tolerate a discontinuity, so GPS/GNSS time is not affected by the leap second. But, since GPS/GNSS is used ubiquitously for time transfer, the message includes leap second information that all GNSS devices need to decode and properly handle. Orolia simulators can test GPS/GNSS devices compliance to the GPS interface specification for proper leap second handling and identify any detrimental effects of the leap second on the application. They also can perform similar testing on leap second handling in other GNSS systems. It is important not only to test the handling of leap seconds by the GPS/GNSS system, but also to test how that time propagates through a time synchronization deployment to the end system.

How Does a Leap Second Affect Time Synchronization?

Most time synchronization messages transmit time-of-day information in UTC, the official timescale, which includes leap seconds. The most popular network time synchronization protocols, NTP and PTP, have a mechanism to alert that a leap second is pending — but it is up to the computer operating system to manage it properly. Other timing signal protocols like IRIG, HAVE QUICK, and ASCII serial protocols may also transmit leap seconds or be affected by the occurrence of leap seconds further upstream. We have prepared a short video to help understand the issue of leap second handling in high precision computing applications.

Watch the Video:

Products Related to Leap Seconds

Orolia time synchronization systems are fully compliant to best practices for leap second handling. The SecureSync system can even be used in a leap second test mode to easily test propagation through time synchronization deployments ahead of these scheduled events. For testing the capability of GPS/GNSS devices and systems to manage the leap second, any Orolia simulator can perform the most realistic test to identify any potential problem in advance of a leap second event.

Let us help you understand your risk for the leap second vulnerability.

CONTACT US

About David Sohn

David Sohn is a Solution Architect at Orolia, designing and developing solutions leveraging the organization’s precision timing solution portfolio, including their flagship SecureSync and VersaSync products, and contributing to its entire portfolio of resilient PNT solutions. He has more than 10 years of experience designing, developing, and managing precision timing solutions and holds a BS in computer engineering from The Pennsylvania State University.

Customization Nation with Sapling Digital Clocks

No matter the product, everyone has different tastes and styles they prefer. Because of this, people really enjoy the ability to customize the items they purchase to meet these preferences. Giving customers the option to personalize their product or service has benefited many different companies in many different industries.

Let’s take the shoe industry as an example. Nike has been wildly successful with the Nikeid option on their website. This option gives their patron the option to customize any type of shoe they want with any combination of colors. The car industry has also jumped on the customization bandwagon. Almost every major car company has an option on their website for their customers to customize the make, model, color, accessories and so much more.

The Sapling Company understands the importance of customization and as the manufacturer of synchronized time systems; Sapling has an array of options to satisfy the broadest of needs. We offer four different synchronized time system options, including: Wired, Wireless, Talkback, and IP. These systems include a master clock at the center of the network and multiple secondary clocks that display the accurate time. The master clock is updated with the accurate time from NTP of GPA, and then sends a signal to the secondary clocks. More specifically within a wireless clock system, the secondary clocks both receive and transmit the signal, until all of the clocks are properly updated.

Within the four systems is the option of what type of clock you would want: analog or digital. If you chose the round analog clocks, then you would get the option of the 12” or 16”clock. Sapling also offers a 9” or 12” square clock for more variety within the analog family. Both the round and square clocks have the additional options of customizable hands and dials!

If you chose the digital clocks, then you would be hit with the brand new color customization display options. While red is the standard color option, you will now have the choice between green, white, amber and blue.

Thanks to Sapling for the article.

The Benefits of Using 2-Wire Digital Master Clock System

If you are considering a wired clock system for your facility, the 2-wired Digital Master Clock System option with Sapling may be the best option for you. Take a look at the unique advantages of this system below. In addition to the written description, check out our video at the bottom to see a visual depiction of how the 2-Wire Digital Clock System works.

Power/Data on the Same Line

Most wired clock systems require three or four wires. With Sapling’s 2-Wire Digital Communication System, the converter box supplies the power and amplifies the data, so that power and data are integrated on the same line. Fewer wires mean a cleaner, less cumbersome and more efficient system.

Instant Correction

As with all of Sapling’s clock systems, our goal is to provide synchronized, accurate time to keep your education, healthcare or business facility operating at its best. The 2-Wire Digital Communication System provides time updates to all of the clocks as often as once per second. With such frequent corrections, your clocks are guaranteed to show the accurate time, all the time. Another auto correction feature is the 5 minute synchronization after a power outage. If power is lost, you won’t have to worry about resetting the clocks or waiting a few hours for them to be re-synchronized. Within five minutes of getting power back, the master clock will send a signal to reset all of the clocks to the accurate time. Even if a power outage causes some temporary chaos in other areas, clock malfunctions or time inaccuracies will not be issues to add to the mix. Sapling takes care of that part for you.

Effortless Installation

The installation of the 2-Wired System is simple and straightforward for a few reasons. First, the low voltage requirement means that you do not need a certified electrician to install the system in most countries. Having two wires going from the master clock to each individual clock instead of four also makes setup quicker and easier. Even if a mistake is made with the two wires, our cutting-edge reverse polarity detection technology will recognize the error and autocorrect it. What could be easier than that?

Hopefully, the only thing easier is making the decision to install Sapling’s 2-Wire Digital Master Clock System for its advanced technological capabilities, ease, accuracy and the superior quality and service that you can expect from Sapling.

Thanks to Sapling for the article.

Sapling’s Master Clock – Leader of the Pack – Part 2

In continuing with our post explaining how Sapling’s master clocks can receive time, the second option a user has to receive time is through a GPS Receiver.

Receiving time from a GPS satellite is not only extremely accurate, it is also very secure. With the GPS Receiver, the facility does not have to go outside the facility’s established firewall and use a time source via the Internet.

A GPS receiver is built-in to the master clock and sends out the master clock’s exact location to satellites around the world. From the satellite, the master clock receives the accurate UTC (Coordinated Universal Time), then corrects to the local time based on the user’s location.

The GPS Receiver option can be used in conjunction with the NTP Server option, which you can learn about here. A user has the ability to choose which option will be the main time source and which will be the backup time source. By utilizing both options, a user will have redundancy within their system, ensuring accuracy. Receiving time via GPS is optional with a Sapling’s master clock.

Stay tuned next week for the third way Sapling’s master clocks can receive time! If you are interested in learning more about our master clocks in the meantime, visit our website for more info or check out our YouTube video explaining our master clocks more in depth.

Thanks to Sapling for the article.

Unique Features of Sapling’s IP Clock System

Sapling’s IP Clock System is unlike your typical clock system. This synchronized clock system is powered by (PoE) or Power-over-Ethernet, receiving power and data through a CAT5 cable, there is no need for an additional outlet. This system allows for easy set up and operation for any user and has a number of unique features that make it stand out among the rest.

One such feature is that each clock has its own built-in web interface. The web interface allows a user to adjust many different settings or enable certain features on the clock. For example, a user can choose to set the time to display in 12 or 24 hour mode (digital clocks only), automatically update for Daylight Saving Time, both domestically and internationally and even give that particular clock a name, for example its location in a facility.

Features such as the brightness option can help companies and organizations become more energy efficient. For example, if you work in a school, it’s not a 24/7 operation. A school’s peak hours are typically between 7 a.m and 5 p.m., that’s a ten hour time frame in which the school building has a constant flow of traffic, with dozens of eye balls gazing at the clocks at one point or another. In those peak hours, the brightness is typically set to the high setting for maximum visibility. After school lets out for the day, the clocks aren’t being viewed as much. To conserve energy and money, Sapling gives a user the ability to adjust the brightness to medium, low or off (digital clocks only) after peak hours. This helps a school or any other type of facility save money over the life of the clock system. A user can also establish a brightness schedule for all the digital clocks within a facility.

Another feature that the IP clock system has is the ability to send email alerts on any changes or disruptions that occur. The email notification is sent right to the operator of the clock system if any power failures, major time changes, NTP/SNTP server synchronization issues, display faults or mechanical failures happen.

Sapling prides itself on being the forefront of technology, and our IP clock system, with its web interface allows custom ability of many settings (Brightness Settings, automatic Daylight Saving Time updates, email alerts, etc.) to help your company propel into the future. For more information, do not hesitate to contact us today!

Thanks to Sapling for the article.

Ready, Set, Go! Time Synchronization with Sapling Clocks

For high school students, the short break in between classes is no relaxing matter. They have only a few minutes to get from one classroom to the next. If a student needs to pick up their books for the next class and must stop at their locker, this makes the trip even more challenging. Those few minutes in between classes can get hectic with hundreds of students flooding the hallways sharing the common goal of reaching their next destination on time. Most students are even unaware of how much time they have to reach their next class due to the discrepancy between the times their watch or cell phones display and the time the school clocks display.

A synchronized time keeping system can make the trip between classes more efficient for both teachers and students. If the time displayed on the school’s clocks is extremely accurate, then this time discrepancy will have less of an impact of a high school’s overall schedule. Upon eliminating this time dissimilarity, the amount of students late for class can go down and the overall amount of students who are penalized for nocuous activity can go down as well.

Sapling’s master clock can receive accurate time from any NTP server or GPS satellite. Another feature that Sapling’s master clock comes with is the ability to display a countdown in between classes for the roaming students. While the classes of a high school are switching, the time on the clocks will display a countdown on the display instead of the time. This will let student know exactly how long they have to get from one class to another.

Punctual students make it easier for the teachers of the high school to get through their entire lesson plan. They can start their lessons without being interrupted and they do not have to punish the student(s) for being late. With the assistance of The Sapling Company and the addition of their synchronized clock systems, both teachers and students will have a less hectic day.

Thanks to Sapling Clocks for the article.

Manufacturing Made Easy with Saplings Synchronized Clock Systems

The production department in any company typically runs hand in hand with the company’s overall success. If the manufactures cannot produce goods fast enough then the company’s customer satisfaction will plummet if demand is not met in a timely fashion. The speed in which these manufacturers can produce a product relies heavily on their time management skills. If a manufacturer is not efficient in his or her time management then they will simply fall behind schedule. Emerging technologies has provided new growth for the efficiency of manufacturing plants. With the help of a synchronized time keeping system, a manufacturing plant can become more efficient than ever.

A hold up in production is a recurring problem faced by manufacturers today. A bottleneck in the line can backup each subdivision of the plant. This can be detrimental to the quota that must be met for any particular day. In order to help avoid backup during production and missed quotas, a synchronized time keeping system should be installed. These integrated time keeping systems will help ensure that each stage of production is on time, and every part of the operation is moving forward as desired.

The Sapling Company specializes in synchronized clock systems. The synchronization of time and the ability to easily manage the time system can be an enormous benefactor to any manufacturing plant. Even though these production plants are usually in big warehouses, Sapling’s synchronized wireless clock system can be easily installed to accommodate a building of any size or number of floors.

Within a Sapling wireless system, a master clock is first installed into the establishment. After configuring the master clock to your desired settings, it will send the time signal to the clocks in the plant. The clocks each contain a built-in repeater which allows them to receive the signal and then repeat it to neighboring clocks. This feature permits the master clock to be less expensive than our competitors whose clocks need a lot more power to reach all of the clocks within a facility.

Sapling’s wireless clock system can assist a production manager record the time the spent on assembly by employees and an accurate read of when products are going to be shipped. This is essential when management is deciding where and when to allocate their resources. Sapling prides itself on the reliability and innovation of its synchronized clock systems. In regards to questions or any additional information, please visit our website or contact us.

Thanks to Sapling for the article.

Timing Behind the Scenes – Hospital Transportation Department

Hospital examination rooms, the ER and the various operating rooms are always filled with patients looking to recover from their ailments or diseases, recovering all the way to full health. Hospitals and their employees provide the place and care for these patients to become full operational people. The need to go to several different areas of the hospital facility throughout the day, to get examined or x-rays performed, is necessary in order to heal the ailments. Most of these patients, however, won’t be able to maneuver from certain areas of the hospital to others. They need assistance to get around; this is where the hospital’s transportation department comes into play.

As you know, a hospital’s employees are pivotal to the daily operations of a hospital. Doctors and nurses are involved with performing surgeries, administering medications and prescription drugs, or providing the best overall care to patients that they can. These are obviously large responsibilities, but they couldn’t be done without the help of dozens of other employees behind the scenes. The transportation crew is definitely one of these outstanding resources. These people use wheel chairs, push gurneys or movable beds to pound the tile floor for miles a shift, transporting the patients to where they need to be on time.

Furthermore, their personal interaction with these patients cannot be understated. Many times, providing a comforting hand helps eases the stress and nerves contained by the patient. This will also benefit the doctors and nurses, making their job a little bit easier.

Something that makes every hospital employee’s jobs a bit better is a Sapling synchronized clock system. Providing the most accurate time that’s in sync with every other room in the hospital is quite beneficial: medicine can be administered at the proper time, accurate record keeping can occur and the hospital transportation crew will limit, if not eliminate miscommunication, and always bring the patient to the room they need to be in at the correct time.

Thanks to Sapling for the article.