= Hewlett Packard GPS Receivers -  HP 58503A and HP Z3801A
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== Synopsis

["verse",subs="normal"]
Name: hpgps
Reference ID: GPS
Driver ID: GPS_HP
Serial Port: /dev/hpgps__u__; 9600 bps 8N1, 19200 bps 7N2 for the HP Z3801A

== Warning

As of September 2017 we have reports that the internal clock on a
Z3801A was observed to roll over to 1998 (see
link:rollover.html[Rollover issues in time sources]). Users should
audit for rollover before deploying any of these devices. One or more "g"
suffixes on your 'time1' option may be useful as a workaround if
your device does not support setting the era date.

== Description

This driver supports the HP 58503A Time and Frequency Reference Receiver
and HP Z3801A GPS Receiver. It will probably work with other Z38xx models.
Two-unit pairs of these are sometimes known as a KS-24361. All of
these product are well past end-of-life.

These receivers are very old designs, dating from around 1990, so
their GPS receiver stages are not very sensitive by modern
standards. On the other hand, lots of them are available cheap,
because they used to be standard kit on cell towers and were surplussed
off in the thousands.

They use HP SmartClock (TM) to implement an Enhanced GPS receiver. The
receiver accuracy when locked to GPS in normal operation is better
than 1 µs. The accuracy when operating in holdover is typically
better than 10 µs per day. The receiver should be operated with
factory default settings. Initial driver operation: expects the
receiver to be already locked to GPS, configured and able to output
timecode format 2 messages.

The driver uses the poll sequence +:PTIME:TCODE?+ to get a response from
the receiver. The receiver responds with a timecode string of ASCII
printing characters, followed by a <cr><lf>, followed by a prompt string
issued by the receiver, in the following format:

-------------------------------------
T#yyyymmddhhmmssMFLRVcc<cr><lf>scpi >
-------------------------------------

The driver processes the response at the <cr> and <lf>, so what the
driver sees is the prompt from the previous poll, followed by this
timecode. The prompt from the current poll is (usually) left unread
until the next poll. So (except on the very first poll) the driver sees
this:

-------------------------------------
scpi >T#yyyymmddhhmmssMFLRVcc<cr><lf>
-------------------------------------

The T is the on-time character, at 980 ms before the next 1PPS edge.
The # is the timecode format type. We look for format 2. Without any of
the CLK or PPS stuff, then, the receiver buffer timestamp at the <cr> is
24 characters later, which is about 25 ms. at 9600 bps, so the first
approximation for the +time1+ option is nominally -0.955 s. This number
probably needs adjusting for each machine / OS type, so far: -0.955000 s
on an HP 9000 Model 712/80 HP-UX 9.05 -0.953175 s on an HP 9000 Model 370
HP-UX 9.10

This driver will probably work with the 58503B and 59551A if they are
set up appropriately.

To use an HP Z3801A, specify +subtype 1+ on the server config line to setup
the right line parameters.

The timekeeping portion of HP's business was sold to Symmetricom; Symmetricom
was later acquired by https://www.microsemi.com/[Microsemi].

== Rollover compensation

Some devices in this product line have a front-panel display of
date-time and controls to manipulate it.  On these, it is possible to
reset the device's hidden base date for your current era. Consult
your documentation for details.

== Driver Options

+unit+ 'number'::
  The driver unit number, defaulting to 0. Used as a distinguishing
  suffix in the driver device name.
+time1+ 'time'::
   Specifies the time offset calibration factor, in seconds and fraction,
   with default 0.0.
+time2+ 'time'::
   Not used by this driver.
+stratum+ 'number'::
   Specifies the driver stratum, in decimal from 0 to 15, with default 0.
+refid+ 'string'::
   Specifies the driver reference identifier, an ASCII string from one to
   four characters, with default +GPS+.
+flag1 {0 | 1}+::
   Not used by this driver.
+flag2 {0 | 1}+::
   Not used by this driver.
+flag3 {0 | 1}+::
   Not used by this driver.
+flag4 {0 | 1}+::
   Not used by this driver.
+subtype+::
   Setting mode 1 configures for the HP Z3801A.
+mode+::
  Synonym for subtype, retained for backward compatibility.
+path+ 'filename'::
  Overrides the default device path.
+ppspath+ 'filename'::
  Not used by this driver.
+baud+ 'number'::
  Overrides the default baud rate.

== Configuration Example

----------------------------------------------------------------------------
refclock hpgps
----------------------------------------------------------------------------

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