PH8 PHS8 GPS leap seconds | Telit Cinterion IoT Developer Community
July 3, 2015 - 5:22pm, 3199 views
Hi,
I have a device that includes the PHS8 GPS. I am using this for an application that requires time synchronisation between several units. Therefore I would like to know exactly how the PHS8 handles leap seconds.
Does the PHS8 unit store 'last known' leap seconds for the next cold-start?
I understand that leap second correction is only seond once every 20mins (appox) and I cannot afford for there to be a significant jump in time when the first leap second correction message is received.
I hope this question makes sense, and look forward to an answer as soon as possible!
Thanks.
James.
Hello,
As I understand the leap second is a one-second adjustment that is occasionally applied to UTC in order to keep its time of day close to the mean solar time. And since this system of correction was implemented in 1972, 26 leap seconds have been inserted, the most recent on June 30, 2015 at 23:59:60 UTC. This frequency is far from what you have describe. So we probably think about different things.
Could you write more details about what you mean by leap second?
Regards,
Bartłomiej
Hi,
Thanks for your reply. You are right, leap seconds only occur about every 18months (currently 17). BUT, GPS systems use GPS Time (fixed since 1980) and then the satellites distribute a 'leap second correction' so that the GPS can correct from GPS time to 'current' UTC. The leap second correction message is distributed by the satellite every 20 mins (approx).
Within the NMEA protocol the client of the GPS does not really know whether they are recieving the time with or without leap seconds - so with some GPS devices when cold started the time will jump significantly within the first 20mins.
Typically there are three ways that a GPS device handles leap seconds:
- Use 0 until updated by the satellite (therfore very inaccurate for a while after cold start)
- Store value in FW (usually the leap seconds when manufactured e.g. 15, 16). Therefore time will jump a small amount sometime after cold start
- Store 'last known leap seconds' in storage on the device. Update that value each time the leap seconds change (every 18 months or so).
I am hoping the PH8 does the last option - but need to know this with confidence.
Does that make sense?
Thanks
James.
Hi,
Thank you for the detailed response. Now everything is perfectly clear.
However I'm not able to answer yet. The GPS is integrated into the communication chip which is provided by the external company. We will probably need to ask them for this information.
Similar or same question has been also just reported to the support line. So it will be handled together.
Please be patient. I'll publish this information as soon as we will have the response.
Best regards,
Bartłomiej
Hi again,
I already have the answer for you.
The PHS8-P Rel3.001 ARN 01.001.07 uses the fixed leap seconds from the read only memory (15sec) until the current values from GPS satellite is available.
The NMEA shall always give the UTC, but the module only receives the GPS time from the satellite. Therefore it corrects the time first with the 15sec and as soon it has received the leap seconds from GPS satellite (currently 17sec) it corrects with this value (since GPS-time started in 1980, there were 17 corrections of UTC so far).
So it seems that is is neither the worst case nor the best but the middle one. But actually much closer to the best then the worst.
Best regards,
Bartłomiej
Thanks for the answer - which is clear. It's not quite what I expected after my own tests (which seemed to show no jump in time within 20 mins of cold start), but at least we know exactly what it is doing.
James.