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/* $Id$ */
/* subframe.c -- interpret satellite subframe data. */
#include <sys/types.h>
#include "gpsd.h"
#if 0
static char sf4map[] = {-1, 57, 25, 26, 27, 28, 57, 29, 30, 31, 32, 57, 62, 52, 53, 54, 57, 55, 56, 58, 59, 57, 60, 61, 62, 63};
static char sf5map[] = {-1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 51};
#endif
/*@ -usedef @*/
void gpsd_interpret_subframe(struct gps_device_t *session,unsigned int words[])
/* extract leap-second from RTCM-104 subframe data */
{
/*
* Heavy black magic begins here!
*
* A description of how to decode these bits is at
* <http://home-2.worldonline.nl/~samsvl/nav2eu.htm>
*
* We're after subframe 4 page 18 word 9, the leap second correction.
* We assume that the chip is presenting clean data that has been
* parity-checked.
*
* To date this code has been tested on iTrax, SiRF and ublox. It's in
* the core because other chipsets reporting only GPS time but with the
* capability to read subframe data may want it.
*/
unsigned int pageid, subframe, data_id, leap;
gpsd_report(LOG_IO,
"50B (raw): %08x %08x %08x %08x %08x %08x %08x %08x %08x %08x\n",
words[0], words[1], words[2], words[3], words[4],
words[5], words[6], words[7], words[8], words[9]);
/*
* It is the responsibility of each driver to mask off the high 2 bits
* and shift out the 6 parity bits or do whatever else is necessary to
* present an array of acceptable words to this decoder. Hopefully parity
* checks are done in hardware, but if not, the driver should do them.
*/
/* The subframe ID is in the Hand Over Word (page 80) */
subframe = ((words[1] >> 2) & 0x07);
/*
* Consult the latest revision of IS-GPS-200 for the mapping
* between magic SVIDs and pages.
*/
pageid = (words[2] & 0x3F0000) >> 16;
data_id = (words[2] >> 22) & 0x3;
gpsd_report(LOG_PROG, "Subframe %d SVID %d data_id %d\n", subframe, pageid, data_id);
/* we're not interested in anything but subframe 4 - for now*/
if (subframe != 4)
return;
/* once we've filtered, we can ignore the TEL and HOW words */
gpsd_report(LOG_PROG, "50B: %06x %06x %06x %06x %06x %06x %06x %06x\n",
words[2], words[3], words[4], words[5],
words[6], words[7], words[8], words[9]);
switch(pageid) {
case 55:
/*
* "The requisite 176 bits shall occupy bits 9 through 24 of word
* TWO, the 24 MSBs of words THREE through EIGHT, plus the 16 MSBs
* of word NINE." (word numbers changed to account for zero-indexing)
*
* Since we've already stripped the low six parity bits, and shifted
* the data to a byte boundary, we can just copy it out. */
{
char str[24];
int j = 0;
/*@ -type @*/
str[j++] = (words[2] >> 8) & 0xff;
str[j++] = (words[2]) & 0xff;
str[j++] = (words[3] >> 16) & 0xff;
str[j++] = (words[3] >> 8) & 0xff;
str[j++] = (words[3]) & 0xff;
str[j++] = (words[4] >> 16) & 0xff;
str[j++] = (words[4] >> 8) & 0xff;
str[j++] = (words[4]) & 0xff;
str[j++] = (words[5] >> 16) & 0xff;
str[j++] = (words[5] >> 8) & 0xff;
str[j++] = (words[5]) & 0xff;
str[j++] = (words[6] >> 16) & 0xff;
str[j++] = (words[6] >> 8) & 0xff;
str[j++] = (words[6]) & 0xff;
str[j++] = (words[7] >> 16) & 0xff;
str[j++] = (words[7] >> 8) & 0xff;
str[j++] = (words[7]) & 0xff;
str[j++] = (words[8] >> 16) & 0xff;
str[j++] = (words[8] >> 8) & 0xff;
str[j++] = (words[8]) & 0xff;
str[j++] = (words[9] >> 16) & 0xff;
str[j++] = (words[9] >> 8) & 0xff;
str[j++] = '\0';
/*@ +type @*/
gpsd_report(LOG_INF, "gps system message is %s\n", str);
}
break;
case 56:
leap = (words[8] & 0xff0000) >> 16;
/*
* On SiRFs, there appears to be some bizarre bug that
* randomly causes this field to come out two's-complemented.
* This could very well be a general problem; work around it.
* At the current expected rate of issuing leap-seconds this
* kluge won't bite until about 2070, by which time the
* vendors had better have fixed their damn firmware...
*
* Carl: ...I am unsure, and suggest you
* experiment. The D30 bit is in bit 30 of the 32-bit
* word (next to MSB), and should signal an inverted
* value when it is one coming over the air. But if
* the bit is set and the word decodes right without
* inversion, then we properly caught it. Cases where
* you see subframe 6 rather than 1 means we should
* have done the inversion but we did not. Some other
* things you can watch for: in any subframe, the
* second word (HOW word) should have last 2 parity
* bits 00 -- there are bits within the rest of the
* word that are set as required to ensure that. The
* same goes for word 10. That means that both words
* 1 and 3 (the words that immediately follow words 10
* and 2, respectively) should always be uninverted.
* In these cases, the D29 and D30 from the previous
* words, found in the two MSBs of the word, should
* show 00 -- if they don't then you may find an
* unintended inversion due to noise on the data link.
*/
if (leap > 128)
leap ^= 0xff;
gpsd_report(LOG_INF, "leap-seconds is %d\n", leap);
session->context->leap_seconds = (int)leap;
session->context->valid |= LEAP_SECOND_VALID;
break;
default:
; /* no op */
}
}
/*@ +usedef @*/
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