[time-nuts] Why is holdover LED on HP 58503A not lit, when GPS lock is unlit too?

[Tim Shoppa]

A common misconception, is that holdover is the opposite of GPS lock.
Sometimes we might even talk about the two as if we're in one or the other.
But really the power-on state, is that we're in neither holdover or GPS
lock.

Holdover means the smartclock previously had GPS lock and had used it to
characterize the aging of the OCXO, and is maintaining the EFC
extrapolation, such that it believes it can still produce accurate time and
frequency through the GPS loss.

The state of your clock at the moment, is that it has never had GPS lock
since regaining power, it does not know what time it is, and it has not
characterized or extrapolated the aging of the OCXO. So it is definitely
not in holdover.

Tim N3QE

On Sun, Dec 18, 2016 at 8:01 AM, Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby at kirkbymicrowave.co.uk> wrote:

> My 58503 GPS time/frequency reference had its power disconnected a few
> times yesterday as I moved things in the lab. At the modem the status of
> the LEDs are
>
> * Power green
> * GPS lock - not lit
> * Holdover - not lit
> * Alarm - not lit.
>
> That is what the manual says will happen  when power is first applied, but
> I'm puzzled why.
>
> The SYSTEM:STATUS? shows the information at the end of this email. What I
> can't understand is why the holdover light is not lit, when clearly its not
> tracking any satellites. It seems logical to me that holdover should be on,
> when GPS lock is not, and visa versa. But that does not seem to be the
> case.
>
> Looking at the data below, the date/time is obviously wrong (01 Jan 1998),
> the height at -14.04 m  (MSL) is wrong, but latitude and longitude look
> about right, although they are certainly not exactly agreeing with Google
> maps, which show 51°39'04.1"N+0°46'36.4"E.
>
> I must be misunderstanding the purpose of these lights.
>
> I'm also a bit puzzled it has been on about 1.5 hours, and can't seem to
> find a single satellite. The antenna is fairly clear of anything else, and
> it was certainly working yesterday, with the power and GPS lock LEDs both
> lit.
>
> Any ideas what's going on?
>
> scpi > SYSTEM:STATUS?
> ------------------------------- Receiver Status
> -------------------------------
>
> SYNCHRONIZATION ........................................... [ Outputs
> Invalid ]
> SmartClock Mode ___________________________   Reference Outputs
> _______________
>    Locked                                     TFOM     9
> FFOM     3
>    Recovery                                   1PPS TI  --
>    Holdover                                   HOLD THR 1.000 us
> >> Power-up: GPS acquisition                  Holdover Uncertainty
> ____________
>                                               Predict  --
>
> ACQUISITION .............................................. [ GPS 1PPS
> Invalid ]
> Tracking: 0 ____   Not Tracking: 1 ________   Time _____ +1 leap second
> pending
>                    PRN  El  Az                UTC      12:59:16 [?] 01 Jan
> 1998
>                    *32  -- ---                GPS 1PPS Invalid: not
> tracking
>                                               ANT DLY  0 ns
>                                               Position
> ________________________
>                                               MODE     Survey:      0%
> complete
>                                                        Suspended: track <4
> sats
>                                               INIT LAT N  51:39:03.825
>                                               INIT LON E   0:46:36.278
> ELEV MASK 10 deg   *attempting to track       INIT HGT          -14.04 m
> (MSL)
> HEALTH MONITOR ......................................................... [
> OK ]
> Self Test: OK    Int Pwr: OK   Oven Pwr: OK   OCXO: OK   EFC: OK   GPS Rcv:
> OK
> scpi >
>
>
>
> Dr. David Kirkby Ph.D CEng MIET
> Kirkby Microwave Ltd
> Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
> UK.
> Registered in England and Wales, company number 08914892.
> http://www.kirkbymicrowave.co.uk/
> Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
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