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diff --git a/contrib/ntp/html/Oncore-SHMEM.htm b/contrib/ntp/html/Oncore-SHMEM.htm new file mode 100644 index 0000000..3148f6a --- /dev/null +++ b/contrib/ntp/html/Oncore-SHMEM.htm @@ -0,0 +1,257 @@ +<HTML> +<HEAD> + <TITLE> ONCORE - SHMEM </TITLE> +</HEAD> +<BODY> +<H3> +Motorola ONCORE - The Shared Memory Interface +</H3> +<HR> + +<H4> +Introduction +</H4> + +<P> +In NMEA mode, the Oncore GPS receiver provides the user with the same information as +other GPS receivers. +In BINARY mode, it can provide a lot of additional information. +<P> +In particular, you can ask for satellite positions, satellite health, signal levels, +the ephemeris and the almanac, and you can set many operational parameters. +In the case of the VP, +you can get the pseudorange corrections necessary to act as a DGPS base station, and you can see +the raw satellite data messages themselves. +<P> +When using the Oncore GPS receiver with NTP, this additional information is usually +not available since the receiver is only talking to the oncore driver in NTPD. +To make this information available for use in other programs, +(say graphic displays of satellites positions, plots of SA, etc.), a shared memory interface +(SHMEM) has been added to the refclock_oncore driver on those operating systems that support +shared memory. +<P> +To make use of this information you will need an Oncore Reference Manual for the +Oncore GPS receiver that you have. The Manual for the VP only exists as a paper +document, the UT manuals are available as a pdf document online. +<P> +This interface was written by Poul-Henning Kamp (phk@FreeBSD.org), and modified by +Reg Clemens (reg@dwf.com). +The interface is known to work in FreeBSD, Linux, and Solaris. +<H4> +Activating the Interface +</H4> +Although the Shared Memory Interface will be compiled into the Oncore driver +on those systems where Shared Memory is supported, to activate this interface you must +include a <B>STATUS</B> line in the <tt>/etc/ntp.oncore</tt> data file that looks like +<PRE> + STATUS < file_name > +</PRE> +Thus a line like +<PRE> + STATUS /var/adm/ntpstats/ONCORE +</PRE> +would be acceptable. +This file name will be used to access the Shared Memory. +<P> +In addition, one the two keywords <B>Posn2D</B> and <B>Posn3D</B> can be added to +see @@Ea records containing the 2D or 3D position of the station (see below). +Thus to activate the interface, and see 3D positions, something like +<PRE> + STATUS /var/adm/ntpstats/ONCORE + Posn3D +</PRE> +would be required. +<H4> +Storage of Messages in Shared Memory +</H4> +With the shared memory interface, the oncore driver (refclock_oncore) allocates space +for all of the messages that it is configured to receive, and then puts each message +in the appropriate slot in shared memory as it arrives from the receiver. +Since there is no easy way for a client program to know when the shared memory has +been updated, +a sequence number is associated with each message, and is incremented when a new message +arrives. +With the sequence number it is easy to check through the shared memory segment for messages that +have changed. +<P> +The Oncore binary messages are kept in their full length, as described in the Reference +manual, that is everything from the @@ prefix thru the <checksum><CR><LF>. +<P> +The data starts at location ONE of SHMEM (NOT location ZERO). +<P> +The messages are stacked in a series of variable length structures, that look like +<PRE> + struct message { + u_int length; + u_char sequence; + u_char message[length]; + } +</PRE> +<P> +if something like that were legal. +That is, there are two bytes (caution, these may NOT be aligned with word boundaries, so +the field needs to be treated as a pair of u_char), that contains the length of the next +message. +This is followed by a u_char sequence number, that is incremented whenever a new message of +this type is received. +This is followed by 'length' characters of the actual message. +<P> +The next structure starts immediately following the last char of the previous message (no alignment). +Thus, each structure starts a distance of 'length+3' from the previous structure. +<P> +Following the last structure, is a u_int containing a zero length to indicate the end +of the data. +<P> +The messages are recognized by reading the headers in the data itself, viz @@Ea or whatever. +<P> +There are two special cases. +<P> +(1) The almanac takes a total of 34 submessages all starting with @@Cb. <br> +35 slots are allocated in shared memory. +Each @@Cb message is initially placed in the first of these locations, +and then later it is moved to the appropriate location for that submessage. +The submessages can be distinguished by the first two characters following the @@Cb header, +and new data is received only when the almanac changes. +<P> +(2) The @@Ea message contains the calculated location of the antenna, and is received +once per second. +However, when in timekeeping mode, the receiver is normally put in 0D mode, with the +position fixed, to get better accuracy. +In 0D mode no position is calculated. +<P> +When the SHMEM option is active, +and if one of <B>Posn2D</B> or <B>Posn3D</B> is specified, +one @@Ea record is hijacked each 15s, and the receiver +is put back in 2D/3D mode so the the current location can be determined (for position determination, or for +tracking SA). +The timekeeping code is careful NOT to use the time associated with this (less accurate) 2D/3D tick +in its timekeeping functions. +<P> +Following the initial @@Ea message are 3 additional slots for a total of four. +As with the almanac, the first gets filled each time a new record becomes available, +later in the code, the message is distributed to the appropriate slot. +The additional slots are for messages containing 0D, 2D and 3D positions. +These messages can be distinguished by different bit patterns in the last data byte of the record. +<H4> +Opening the Shared Memory File +</H4> +The shared memory segment is accessed through a file name given on a <B>ACCESS</B> card in the +<tt>/etc/ntp.oncore</tt> input file. +The following code could be used to open the Shared Memory Segment: + +<PRE> + char *Buf, *file; + int size, fd; + struct stat statbuf; + + file = "/var/adm/ntpstats/ONCORE"; /* the file name on my ACCESS card */ + if ((fd=open(file, O_RDONLY)) < 0) { + fprintf(stderr, "Cant open %s\n", file); + exit(1); + } + + if (stat(file, &statbuf) < 0) { + fprintf(stderr, "Cant stat %s\n", file); + exit(1); + } + + size = statbuf.st_size; + if ((Buf=mmap(0, size, PROT_READ, MAP_SHARED, fd, (off_t) 0)) < 0) { + fprintf(stderr, "MMAP failed\n"); + exit(1); + } +</PRE> + +<H4> +Accessing the data +</H4> +The following code shows how to get to the individual records. + +<PRE> + void oncore_msg_Ea(), oncore_msg_As(), oncore_msg_Bb(); + + struct Msg { + char c[5]; + unsigned int seq; + void (*go_to)(uchar *); + }; + + struct Msg Hdr[] = { {"@@Bb", 0, &oncore_msg_Bb}, + {"@@Ea", 0, &oncore_msg_Ea}, + {"@@As", 0, &oncore_msg_As}}; + + void + read_data() + { + int i, j, k, n, iseq, jseq; + uchar *cp, *cp1; + + + for(cp=Buf+1; (n = 256*(*cp) + *(cp+1)) != 0; cp+=(n+3)) { + for (k=0; k < sizeof(Hdr)/sizeof(Hdr[0]); k++) { + if (!strncmp(cp+3, Hdr[k].c, 4)) { /* am I interested? */ + iseq = *(cp+2); + jseq = Hdr[k].seq; + Hdr[k].seq = iseq; + if (iseq > jseq) { /* has it changed? */ + /* verify checksum */ + j = 0; + cp1 = cp+3; /* points to start of oncore response */ + for (i=2; i < n-3; i++) + j ^= cp1[i]; + if (j == cp1[n-3]) { /* good checksum */ + Hdr[k].go_to(cp1); + } else { + fprintf(stderr, "Bad Checksum for %s\n", Hdr[k].c); + break; + } + } + } + } + if (!strncmp(cp+3, "@@Ea", 4)) + cp += 3*(n+3); + if (!strncmp(cp+3, "@@Cb", 4)) + cp += 34*(n+3); + } + } + + oncore_msg_Bb(uchar *buf) + { + /* process Bb messages */ + } + + oncore_msg_Ea(uchar *buf) + { + /* process Ea messages */ + } + + oncore_msg_As(uchar *buf) + { + /* process As messages */ + } +</PRE> + +The structure Hdr contains the Identifying string for each of the messages that +we want to examine, and the name of a program to call when a new message of that +type is arrives. +The loop can be run every few seconds to check for new data. +<H4> +Examples +</H4> +There are two complete examples available. +The first plots satellite positions and the station position as affected by SA, and +keeps track of the mean station position, so you can run it for periods of days +to get a better station position. +The second shows the effective horizon by watching satellite tracks. +The examples will be found in the GNU-zipped tar file +<A HREF=ftp://ftp.udel.edu/pub/ntp/software/OncorePlot.tar.gz> +ftp://ftp.udel.edu/pub/ntp/software/OncorePlot.tar.gz</A>. +<P> +Try the new interface, enjoy. +<HR> +<ADDRESS> +Reg.Clemens (reg@dwf.com), +Poul-Henning Kamp (phk@FreeBSD.org) +<ADDRESS> +</BODY> +</HTML> |
