[USRP-users] time_spec_t GPS Time or UTC?

Peter Witkowski pwitkowski at gmail.com
Wed Dec 10 16:37:52 EST 2014


Hi Marcus,

I've looked at the documentation (prior to sending my original mailing list
message) and am still lost.  Let me rephrase my question.  Since
time_spec_t just holds a seconds count, when did the count start (assuming
the USRP is synched to UTC via the GPSDO)?  Restated once more, if I get
back 0, what time is it?

I'm assuming that once synched, the USRP is just using seconds after Unix
epoch (this would be logical), but would like to confirm this.

On Wed, Dec 10, 2014 at 4:11 PM, Marcus D. Leech via USRP-users <
usrp-users at lists.ettus.com> wrote:

>  On 12/10/2014 04:02 PM, Peter Witkowski via USRP-users wrote:
>
>  Thanks for the info thus far.  That explains a lot and at least I know I
> don't have to worry about conversions from UTC to GPS time.
>
>  So now that it looks like the USRP is using UTC, when I get a time_spec_t
> structure back from it (or pass it one as a start time in
> issue_stream_cmd()), what does the time_spec_t structure hold?  UTC seconds
> after Unix epoch or something else entirely?
>
>  Note all this assumes that the GPSDO is working properly and locked.
>
>
> http://files.ettus.com/manual/classuhd_1_1time__spec__t.html#a2bd6c4c8ad7209d219c3ca5ae238fb6e
>
>
>
>    On Wed, Dec 10, 2014 at 3:32 PM, Perelman, Nathan via USRP-users <
> usrp-users at lists.ettus.com> wrote:
>
>>  The time returned via get_mboard_sensor(“gps_time”) will be UTC already
>> (assuming the GPSDO is locked). The GPSDO corrects for leap seconds. If you
>> look up the GPRMC string, you can see that the time/date given are UTC.
>>
>> -Nathan
>>
>>
>>
>> *From:* USRP-users [mailto:usrp-users-bounces at lists.ettus.com] *On
>> Behalf Of *Marcus D. Leech via USRP-users
>> *Sent:* Wednesday, December 10, 2014 3:18 PM
>> *To:* usrp-users at lists.ettus.com
>> *Subject:* Re: [USRP-users] time_spec_t GPS Time or UTC?
>>
>>
>>
>> On 12/10/2014 03:10 PM, Peter Witkowski via USRP-users wrote:
>>
>> Hello,
>>
>> I have a question regarding the time reference used by the USRP (namely
>> the x300).
>>
>> If I call the following UHD API functions (see list below), what time
>> reference is used (GPS or UTC)?  Does the USRP itself handle leap seconds
>> and the delta between Unix time and the time it's getting from it's GPS
>> receiver (assuming a GPSDO is installed and locked)?
>>
>> 1.  When I specify the time_spec_t value in issue_stream_cmd(), what time
>> reference is the USRP expecting?  For example, do I pass it seconds after
>> epoch (Unix time) or something else?
>>
>> 2. When I get time via get_time_last_pps(), what is the time reference?
>>
>> 3. Same question as above except for retrieving time via
>> get_mboard_sensor("gps_time").
>>
>> 4.  When I get samples from the sensor using recv(), what time reference
>> is used for the time_spec_t timestamp in the rx_metadata_t structure?  I
>> understand that is the timestamp of the first sample, but is the timestamp
>> seconds after epoch (i.e. Unix time) or based on something else?
>>
>> Just wondering if I need to worry about translating between GPS seconds
>> and Unix seconds or something similar.
>>
>> Thanks in advance for the help.
>>
>>
>> --
>>
>> Peter Witkowski
>> pwitkowski at gmail.com
>>
>>
>>
>>
>>
>> _______________________________________________
>>
>> USRP-users mailing list
>>
>> USRP-users at lists.ettus.com
>>
>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>
>> The USRP doesn't actually care, or know, about epoch-based time
>> systems.   The TOD clock in USRPs is just a wide counter that increments at
>>   the master clock rate.
>>
>> The only requirement in most cases is that everybody in a collection of
>> USRPs agrees on the interpretation of time.  The gps_time returned in the
>>   sensor is just parsed from the NMEA data, as far as I know, without any
>> further manipulation to make it consistent with (for example) Unix epoch
>>   time or any other time system.
>>
>> In many cases, applications that don't care about
>> time-as-it-relates-to-the-rest-of-the-world, they load the TOD registers in
>> a cluster of USRPs
>>   with 0, so that the TOD clock becomes kind of a "mission elapsed timer".
>>
>>
>>
>>
>>  --
>>
>> Marcus Leech
>>
>> Principal Investigator
>>
>> Shirleys Bay Radio Astronomy Consortium
>>
>> http://www.sbrac.org
>>
>>
>> _______________________________________________
>> USRP-users mailing list
>> USRP-users at lists.ettus.com
>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>
>>
>
>
> --
> Peter Witkowski
> pwitkowski at gmail.com
>
>
> _______________________________________________
> USRP-users mailing listUSRP-users at lists.ettus.comhttp://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>
>
>
> _______________________________________________
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> USRP-users at lists.ettus.com
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>
>


-- 
Peter Witkowski
pwitkowski at gmail.com
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