138 lines
4.0 KiB
HTML
138 lines
4.0 KiB
HTML
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<HTML>
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<HEAD>
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<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
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<META NAME="GENERATOR" CONTENT="Mozilla/4.01 [en] (Win95; I) [Netscape]">
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<TITLE>Trak 8820 GPS Receiver
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</TITLE>
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</HEAD>
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<BODY>
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<H3>
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Trak 8820 GPS Receiver</H3>
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<HR>
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<H4>
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Synopsis</H4>
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Address: 127.127.2.<I>u</I>
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<BR>Reference ID: <TT>GPS</TT>
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<BR>Driver ID: <TT>GPS_TRAK</TT>
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<BR>Serial Port: <TT>/dev/trak<I>u</I></TT>; 9600 baud, 8-bits, no parity
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<BR>Features: <TT>tty_clk</TT>
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<H4>
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Description</H4>
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This driver supports the Trak 8820 GPS Station Clock. The claimed accuracy
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at the 1-PPS output is 200-300 ns relative to the broadcast signal; however,
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in most cases the actual accuracy is limited by the precision of the timecode
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and the latencies of the serial interface and operating system.
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<P>For best accuracy, this radio requires the <TT>tty_clk</TT> line discipline,
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which captures a timestamp at the <TT>*</TT> on-time character of the timecode.
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Using this discipline the jitter is in the order of 1 ms and systematic
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error about 0.5 ms. If unavailable, the buffer timestamp is used, which
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is captured at the <TT>\r</TT> ending the timecode message. This introduces
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a systematic error of 23 character times, or about 24 ms at 9600 bps, together
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with a jitter well over 8 ms on Sun IPC-class machines.
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<P>Using the menus, the radio should be set for 9600 bps, one stop bit
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and no parity. It should be set to operate in computer (no echo) mode.
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The timecode format includes neither the year nor leap-second warning.
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<P>In operation, this driver sends a <TT>RQTS\r</TT> request to the radio
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at initialization in order to put it in continuous time output mode. The
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radio then sends the following message once each second:
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<PRE>*RQTS U,ddd:hh:mm:ss.0,q<cr><lf>
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on-time = '*'
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ddd = day of year
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hh:mm:ss = hours, minutes, seconds
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q = quality indicator (phase error), 0-6:
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0 > 20 us
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6 > 10 us
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5 > 1 us
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4 > 100 ns
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3 > 10 ns
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2 < 10 ns</PRE>
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The alarm condition is indicated by <TT>0</TT> at <TT>Q</TT>, which means
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the radio has a phase error greater than 20 us relative to the broadcast
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time. The absence of year, DST and leap-second warning in this format is
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also alarmed.
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<P>The continuous time mode is disabled using the <TT>RQTX\r</TT> request,
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following which the radio sends a <TT>RQTX DONE<cr><lf></TT> response.
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In the normal mode, other control and status requests are effective, including
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the leap-second status request <TT>RQLS<cr></TT>. The radio responds
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with <TT>RQLS yy,mm,dd<cr><lf></TT>, where <TT>yy,mm,dd</TT> are
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the year, month and day. Presumably, this gives the epoch of the next leap
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second, <TT>RQLS 00,00,00</TT> if none is specified in the GPS message.
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Specified in this form, the information is generally useless and is ignored
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by the driver.
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<H4>
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Monitor Data</H4>
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When enabled by the <TT>flag4</TT> fudge flag, every received timecode
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is written as-is to the <TT>clockstats</TT> file.
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<H4>
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Fudge Factors</H4>
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<DL>
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<DT>
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<TT>time1 <I>time</I></TT></DT>
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<DD>
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Specifies the time offset calibration factor, in seconds and fraction,
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with default 0.0.</DD>
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<DT>
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<TT>time2 <I>time</I></TT></DT>
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<DD>
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Not used by this driver.</DD>
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<DT>
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<TT>stratum <I>number</I></TT></DT>
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<DD>
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Specifies the driver stratum, in decimal from 0 to 15, with default 0.</DD>
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<DT>
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<TT>refid <I>string</I></TT></DT>
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<DD>
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Specifies the driver reference identifier, an ASCII string from one to
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four characters, with default <TT>GPS</TT>.</DD>
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<DT>
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<TT>flag1 0 | 1</TT></DT>
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<DD>
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Not used by this driver.</DD>
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<DT>
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<TT>flag2 0 | 1</TT></DT>
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<DD>
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Not used by this driver.</DD>
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<DT>
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<TT>flag3 0 | 1</TT></DT>
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<DD>
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Not used by this driver.</DD>
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<DT>
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<TT>flag4 0 | 1</TT></DT>
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<DD>
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Not used by this driver.</DD>
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<P>Additional Information
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<P><A HREF="refclock.htm">Reference Clock Drivers</A></DL>
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<HR>
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<ADDRESS>
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David L. Mills (mills@udel.edu)</ADDRESS>
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</BODY>
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</HTML>
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