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How Leap Seconds Can Interfere with GNSS Receivers
A new alert from the US Coast Guard Navigation Center (NAVCEN) has highlighted that some GPS receivers will have problems handling an upcoming leap second timing correction. We examine leap seconds, what they mean for mission-critical GNSS receivers, and how organizations can better prepare their systems for leap second adjustments.

GPS GNSS Receivers


On 2nd February, NAVCEN sent out an alert that an upcoming leap second could be mishandled by some GPS receivers, throwing their timing and positioning accuracy off-course.

Since 21st January this year, GPS signals have begun including adjustment messages for a leap second that will come into effect at the end of June 2015. But several models of GPS receiver are mishandling this adjustment data and applying the leap second now, much too early.

Mishandled leap second adjustments could harm GPS-reliant systems

A second may seem fairly insignificant by human standards, but for systems that rely on positioning and timing data from global navigation satellite systems (GNSS) like GPS, a single-second deficit can have a significant impact.

A one-second offset would render the GNSS navigation device unusable. Whether an organization is tracking a delivery vehicle or using GNSS to assist aircraft landings, this kind of error could have serious ramifications for safety, profitability and compliance.

Financial trading systems, communications networks and power grids, for example, all rely on nanosecond-level timestamps to operate – and increasingly, they are receiving their timing information from positioning satellites. If that timing data is out – by as much as one second - it could result in these systems not working effectively, or even shutting down completely.

It's not inconceivable that a timing error could cause a 'flash crash' in financial trading systems, for example, potentially wiping out hundreds of thousands of trades and having a major knock-on effect on stock prices.

How does a leap second affect GNSS receivers?

The leap second is an irregular timing adjustment similar to the leap year. A single solar day that encompasses a complete rotation of the earth is ever so slightly longer than 24 hours. To make sure Co-ordinated Universal Time (UTC) doesn't drift away from solar time over a long period, one-second adjustments are made at irregular intervals to maintain parity between solar movements and the atomic time we rely on day-to-day.

This update is pushed out to GNSS receivers to ensure that systems relying on GNSS timing information remain up to date on the latest UTC time. It's down to the GNSS receiver's system software to correctly interpret this adjustment data and apply the leap second at the correct time. In the case of this year, that's 23:59:60 UTC on 30 June.

But some makes and models of receiver, according to NAVCEN, have applied the correction too early – an error that must be fixed if the kinds of issues outlined above are to be avoided.

How to protect your systems against leap second irregularities

NAVCEN recommends that users experiencing leap second irregularities should contact their GPS receiver vendor for news on the latest firmware updates to solve the issue. But the big problem with these leap second adjustments is that they are applied automatically, so many end users may not know their systems are being affected until synchronization and timing errors start to occur.

Clearly, prevention is better than cure when it comes to GPS issues like this, but how can you protect your GPS receivers from such an irregular physical phenomena?

Week Day Number

A further complication is the definition of week day numbers (DN) which can be different for each constellation, for example, GPS and Galileo use 1-7 and Beidou use 0-6. If Beidou is important for your applications and this issue has not been checked during development phase, the leap second could happen 24 hours too early. Spirent simulators offer the ability to modify the relevant navigation data bits to define a leap second on any day of the week for enhanced tests.

Reducing the risk of leap second adjustment mistakes

As this is a firmware problem, it's up to GNSS receiver manufacturers to ensure their products are able to handle leap second correction effectively. That can be done by using a GNSS signal simulator and control software to mimic the leap second correction message and ensure the firmware applies it correctly. Better to identify and fix any problems upfront than risk reputational damage by having to issue a software patch or firmware upgrade once receivers are in the hands of users.

Learn more about protecting systems against GNSS anomalies

If you would like to know more about protecting GNSS-reliant systems against leap second errors, spoofing, jamming, or any other sources of interference, get in touch or join our GNSS Vulnerabilities LinkedIn Group.
By: Rahul Gupta - 5/14/2015 6:57:32 PM
Tags: GNSS, GPS

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