[IGSMAIL-3071]: Z12 receivers & clock steering

Wed Oct 25 05:37:18 PDT 2000

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IGS Electronic Mail      25 Oct 05:37:20 PDT 2000      Message Number 3071
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Author: Jim Ray

ATTN: all operators of Ashtech Z12 receivers using external
      frequency standards

Please note the following forwarded message from Mikhail Kogan concerning
the proper operating settings for an Ashtech Z12 receiver equipped with
an external frequency standard -- in particular, the CLOCK STEERING option.

In order to interpret your clock results, I ask that each Ashtech station
operator:

1) report to IGS Mail your setting of the CLOCK STEERING option and any
changes;

2) update your IGS Site Log to include this information in section 6 on
frequency standard.  This can be done under "Notes", for example, by

6.1  Standard Type            : H-MASER
       Frequency              : 5 MHz
       Effective Dates        : 24-APR-1997
       Notes                  : CLOCK STEERING set NO on 24-APR-1997

Many thanks to Dr. Kogan for his detective work and for your cooperation,
--Jim

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Forwarded message:

From: kogan at ldeo.columbia.edu
Date: Sun, 22 Oct 2000 11:37:10 -0400 (EDT)
Subject: Z12 Receiver with External Freq

Dear colleagues,

This is a message for all those who use a configuration:
Ashtech Z12 recever (ordinary, not the T-, not the Metronome versions)
with the external frequency standard (cesium or h-maser).

Several such configurations in the IGS network show a very poor performance 
of the receiver clock despite the "slaving" by the external standard: 
erratic rapid excursions to tens of nanosec are common (see IGS clock files in 
http://maia.usno.navy.mil/gpsclocks/finals). This is NOT what you expected
when investing tens or hundreds of thousands of $$ in the frequency standard!

If this is your case, most certainly you have the improper setting of the 
CLOCK STEERING option in your Z12 (Screen 4, RCVR CTRL Submenu). It must by set
to "N" (NO). If you left it set to "Y" (YES), which is the factory default, 
the external frequency does NOT slave the internal oscillator. Rather, your
internal oscillator is controlled ("steered") by the satellite GPS time which
removes the linear drift but does NOT improve the internal crystal quality.

We performed numerous tests here at the Lamont-Doherty Earth Observatory (LDEO)
with an ordinary Z12 with the cesium clock. With the proper setting of CLOCK
STEERING (i.e., to "N"), the clock file showed 1-sigma of +-0.6 nanosec with
deviations never exceeding 2 nanosec. The reference chosen was a combination
of four h-masers at IGS stations FAIR, YELL, WES2, and USNO, all equipped with
ACT receivers. With intentional improper setting of CLOCK STEERING, we exactly 
reproduced the poor performance observed at many IGS sites.

A less probable blunder is that the AUTO-SWITCH field in the EXTERNAL FREQUENCY
Submenu was set to "N". In this case, the external frequency will not be locked
after the occasional power cycle of the receiver. It must be set to "Y".

Do not forget to press "E" button after settings of CLOCK STEERING and AUTO-SWITCH
in order to save changes made to these fields.

If you have the remote (serial port) access to your receivers, yuo can check
(and correct) the setting of CLOCK STEERING by using the CGREMOTE utility supplied
by Ashtech. 

I made all tests with an ordinary Z12 under firmware CD00 which is now commonly
used at the IGS stations and freely available at the Ashtech web page.

Immediately after you connect the external frequency to the Z12, there is a transient, 
exponentially decaying clock drift with an amplitude to about 5-7 nanosec, 
which disappears in about 2 hours. After that, the receiver reproduces faithfully,
in a stable manner, the external frequency quality.

*** Please let me know if our experiment helped you. If it did not, I will believe
in miracles.

In performing these tests, I benefited from a collaboration with following people: 
Dave Stowers, Jim Zumberge (both at JPL), Jim Ray (USNO), Brian Tallman (NIMA),
Bob King, Tom Herring (both at MIT), Richard Phelan, and Art Sauer (both at 
Ashtech/Magellan).

You will find more details on our experiment at the web site: 
http://maia.usno.navy.mil/gpst, mail list maintained by Jim Ray.

Regards,
- Mikhail Kogan

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_/                                                                        _/
_/ Mikhail G. Kogan                                                       _/
_/ Lamont-Doherty Earth Observatory    E-mail   : kogan at ldeo.columbia.edu _/
_/ P.O. Box 1000 / 61, Route 9W        Telephone: 914-365-8882            _/
_/ Palisades, NY 10964 USA             Fax      : 914-365-8150            _/
_/                                                                        _/
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