Article 33733 of sci.geo.satellite-nav: Path: matra.meer.net!news.neumedia.net!uunet!in3.uu.net!204.127.130.5!worldnet.att.net!howland.erols.net!cloudbreak.rs.itd.umich.edu!newsrelay.iastate.edu!news.iastate.edu!not-for-mail From: Sam Wormley Newsgroups: sci.geo.satellite-nav Subject: Re: GPS 1024 Week Rollover Problem Date: 6 Mar 1997 23:30:40 GMT Organization: Iowa State University, Ames, Iowa, USA Lines: 141 Message-ID: <5fnk30$che$1@news.iastate.edu> References: <330DA80D.347C@ms2.hinet.net> NNTP-Posting-Host: paprika.cnde.iastate.edu Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Mailer: Mozilla 1.1IS (X11; I; IRIX 6.2 IP22) X-URL: news:330DA80D.347C@ms2.hinet.net Xref: matra.meer.net sci.geo.satellite-nav:33733 GPS TIME REFERENCES 1. GPS Interface Control Document ICD-GPS-200 - NAVSTAR GPS Space Segment and Navigation User Interfaces 2. Wells D, Guide To GPS Positioning, 2nd ed, Canadian GPS Associates, Frederiction, New Brunswick, Canada 1987 3. Email - Bill Straka GPS MESSAGE CONTENT see: http://www.cnde.iastate.edu/staff/swormley/gps/Wells.et.al.6.06.gif SUBFRAME 1 Flags (L2 code & data; week #; satellite accuracy and health) Age of Data Satellite clock correction coefficients Telemetry Word (TLM) - Each TLM word is 30 bits long, occurs every six seconds in the data frame, and is the first word in each subframe/page. The format shall be as partially shown below. Bit 1 is transmitted first. Each TLM word shall begin with a preamble (10001011) followed by the TLM message, two reserved bits, and six parity bits. The TLM message contains information needed by the authorized user. Handover Word (HOW) - The HOW shall be 30 bits long and shall be the second word in each subframe/page, immediately following the TML word. A HOW occurs every 6 seconds in the data frame. The format and content of the HOW shall be as partially shown below. The MSB is transmitted first. The HOW begins with the 17 MSBs of the time-of-week (TOW) count. (The full TOW count consists of the 19 LSBs of the 29-bit Z-count. These 17 bits correspond to the TOW-count at the X1 epoch which occurs at the start (leading edge of the next following subframe. Word 3 in subframe 1 is the week number (WN), which gives you the date. Combine the week number with the TOW, plus GPS origin date, and you have GPS time. Actually, you need to look a little further on at the SV's clock correction, drift rate and IODC (Issue of Data, Clock - in other words, when the correction parameter was inserted into the message), but this is pretty small. In the present format of the message, there is no provision for giving which 1024 week cycle you are in. There is some discussion of providing a couple extra bits in the reserved parts of the message. For a receiver built today, the way around the 1024 week cycle limitation is to have the user's receiver provide that information (are you between 1980 and 1999? Are you between 1999 and .... etc, or simpler, are you in the 1980's, 1990's, 2000's, or whatever decade? or tie it closer to the year.) It can be done as a user input, just like the time zone/daylight time offset. Or, just use the ROM issue date or software version date. This allows your GPS receiver to sync with GPS time.... Take a look at subframe 4 page 18 for the relationship of GPS time with UTC. |<--------- TLM Word -------->|<----------- HOW ----------->| (word 1) (word 2) (word 3) |1 |31 |61 |91 300 |---------------------|-|-----|----------------|-|--|-|-----|---------|-|---|-----|-|-----|----/ /---| | | | | | | | | | | | | | | | | | TLM |C| P | TOW | | |t| P | WN | | | | | P | | | | | | MS 17-BITS | | | | | 10-BITS | | | | | | | |10001011 | | |MSB LSB| | | | | | | | | | | | |---------------------|-|-----|----------------|-|--|-|-----|---------|-|---|-----|-|-----|----/ /---| SUBFRAMES 2 & 3 Orbit Parameters |<--------- TLM Word -------->|<----------- HOW ----------->| (word 1) (word 2) (word 3) |1 |31 |61 |91 300 |---------------------|-|-----|---------------------|-|-----|-------|---------------|-----|----/ /---| | | | | | | | | | | | | TLM |C| P | HOW |t| P | IODE | C(rs) | P | | | 22 BITS | | | 22 BITS | | |8 BITS | 16 BITS | | | |10001011 | | | | | | | | | | |---------------------|-|-----|---------------------|-|-----|-------|---------------|-----|----/ /---| |1 |31 |61 |91 300 |---------------------|-|-----|---------------------|-|-----|---------------|-------|-----|----/ /---| | | | | | | | | | | | | TLM |C| P | HOW |t| P | C(ic) | | P | | | 22 BITS | | | 22 BITS | | | 16 BITS |8 BITS | | | |10001011 | | | | | | | | | | |---------------------|-|-----|---------------------|-|-----|---------------|-------|-----|----/ /---| SUBFRAME 4 Almanac for satellites 25-32 (pages 2-5, 7-10) Ionospheric model, and UTC data (page 18) partially shown Antispoof flag - 32 satellites (page 25) Satellite configuration - 32 satellites (page 25) Health of satellites 25-32 (page 25) Reserved (pages 1,6,11,12,16,19,20-24) Spares (pages 13-15) Special messages (page 17) |1 |211 219 227 |241 249 257 |271 279 300 |----/ /---|-------|-------|-------|-----|-------|-------|-------|-----|-------|-------------|-|-----| | | | | | | | | | | | | | | | | | t(ot) | WN(t) | P |dt(LS) |WN(LSF)| DN | P |dt(LSF)| SPARE |t| P | | |8 BITS |8 BITS |8 BITS | |8 BITS |8 BITS |8 BITS | |8 BITS | 14 BITS | | | | | | | | | | | | | | | | | |----/ /---|-------|-------|-------|-----|-------|-------|-------|-----|-------|-------------|-|-----| | | | | (word 8) (word 9) (word 10) Universal Coordinated Time (UTC) - Page 18 of subframe 4 includes (1) the parameters needed to relate GPS time to UTC, and (2) notice to the user regarding the scheduled future or recent past (relative to NAV message upload) value of the delta time to to leap seconds dt(LSF), together with the week number WN(LSF) and the day number (DN) at the end of which the leap second become effective. "Day one" is the forst day relative to the end/start of week and the WN(LSF) value consists of eight LSBs of the full week number. The user must account for the truncated nature of this parameter as well as truncation of WN, WN(t), and W(LSF) due to rollover of the full week number. The Command Segment (CS) shall manage these parameters such that the absolute value of the difference between the untruncated WN and WN(LSF) value shall not exceed 127. For more detail see section 20.3.3.5.2.4 of ICD-GPS-200. Note at the time of this writing the difference between UTC and GPS time is exactly 11 seconds. The next leap second for UTC is scheduled for 97.06.30 23:59:59 UTC. SUBFRAME 5 Almanac for satellites 1-24 (pages 1-24) Health of satellites 1-24 (page 25) __________________________________________________ Sam Wormley - http://www.cnde.iastate.edu/gps.html