TIME STAMP OF SHOT EVENTS

⚠ (:toc:) <:vspace> <:vspace> [[<<]] !GPS UNIT ---- We are using a GPS unit that also includes input for time stamping purposes. <:vspace> Attach:ashtech.jpg"Ashtech GG24 GPS + GLONASS" ''Ashtech GG24 GPS + GLONASS Receiver'' <:vspace> * Manual: [[http://www2.geo.uib.no/equipment/misc/gps/ashtech_gg24/documentation/gg24.pdf | gg24.pdf]], dated 08.06.01. This is a local copy; downloaded from [[ftp://ftp.ashtech.com/pub/Reference%20Manuals/GG24%20Eurocard/ | ftp://ftp.ashtech.com/pub/Reference Manuals/GG24 Eurocard/]]. <:vspace> <:vspace> !!Firmware version 6 August 2009 Query of receiver ID, firmware version, and installed options: <pre class='escaped'> $PASHQ,RID $PASHR,RID,G2,GJ00,55OPU-ILEGM-CS*6F </pre> <:vspace> G2 means GG25, GJ00 is the firmware version, and 55OPU-ILEGM-CS represents receiver options. See page 4 of the GG24 manual for description of Receiver Options. <:vspace> <pre class='escaped'> $PASHQ,RIO $PASHR,RIO,GG24C,GJ00,,55OPU-ILEGM-CS,GE01095_____*34 </pre> <:vspace> See manual page 101. [[<<]][[<<]] !DIFFERENCE BETWEEN GPS AND UTC TIME ---- The timestamp telegram provides time in GPS format. GPS time differs from UTC due to insertion of leap seconds. <:vspace> '''As of 26 May 2009, GPS is ahead of UTC by 15 seconds.''' <:vspace> <:vspace> <:vspace> [[<<]][[<<]] !CONFIGURATION ---- The unit does not keep it's configuration. New settings must be entered in serial terminal program at start of each survey. <:vspace> Use PORT C. Remember to let your terminal program issue both ASCII <CR> and <LF> when you press <ENTER>. <:vspace> The following table shows setup that was used in the OBS-2009 survey. <:vspace> (:table border=1 cellpadding=3 cellspacing=0:) (:cell align=center bgcolor=#dddddd:) '''No.''' (:cell align=center bgcolor=#dddddd:) '''Command''' (:cell align=center bgcolor=#dddddd:) '''Description''' (:cellnr align=center:) 1 (:cell align=right:)$PASHS,RST (:cell:) Reset with default settings (screenshot taken after this command was issued). (:cellnr align=center:)2 (:cell align=right:)$PASHS,SYS,MIX (:cell:)Set receiver in mixed GPS/GLONASS mode (:cellnr align=center:)3 (:cell align=right:)$PASHS,PEM,5 (:cell:)Set elevation mask to 5 degrees (:cellnr align=center:)4 (:cell align=right:)$PASHS,PHE,F (:cell:)Set event marker signal to falling edge (:cellnr align=center:)5 (:cell align=right:)$PASHS,NME,TTT,C,ON (:cell:)Output event marker message on PORT C (:cellnr align=center:)6 (:cell align=right:)$PASHS,TSC,GPS (:cell:)Output event marker time in GPS system time (:cellnr align=center:)7 (:cell align=right:)$PASHS,SAV,Y (:cell:)Save settings in battery-backed memory (:tableend:) <:vspace> <:vspace> The terminal program screenshot below shows these commands (and their confirmation). <:vspace> Attach:ashtech-gps-setup-24May2009.jpg"Terminal program screenshot, showing setup of Ashtech GG24 GPS + GLONASS" ''Terminal program screenshot, showing setup of Ashtech GG24 GPS + GLONASS receiver.'' <:vspace> !!Future considerations - improved QC To improve QC, aditional NMEA telegrams should be enabled. Data in these telegrams should be logged on each shotpoint. Suggested additional commands: <:vspace> <:vspace> %box%<pre class='escaped'> Set NMEA "send message interval" to 1.0 s ......: $PASHS,NME,PER,1.0 Output "Signal-to-noise ratio" (SNR) in dB*Hz ..: $PASHS,SNR,DBH POS: Position Message ..........................: $PASHS,NME,POS,C,ON SAT: Satellite Status Message ..................: $PASHS,NME,SAT,C,ON Save settings in battery-backed memory .........: $PASHS,SAV,Y </pre> <:vspace> <:vspace> Query the receiver for current configuration, to confirm settings. <:vspace> Example: <:vspace> <pre class='escaped'> $PASHQ,PAR SPDA:5 SPDB:5 SPDC:5 SPDD:5 GPS:YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY GLO:YYYYYYYYYYYYYYYYYYYYYYYY SYS:MIX DTM:W84 GTM:0 GTF:0 DTG:+000000.0000 TDP:04 GTP:Y PMD:1 FIX:0 ALT:+00000.00 PDP:40 HDP:04 VDP:04 ERM:600,1200,30,60,10,20 PEM:05 UNH:N ION:N SAV:Y RTC:OFF PRT:A NMEA: LTN AIM POS GLL GXP GGA VTG GSN MSG GSA GSV SAT GRS RRE TTT ZDA TCM RMC GST VEC CRT PRTA: OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF PRTB: OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF PRTC: OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF ON OFF OFF OFF OFF OFF OFF PRTD: OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF OFF PER:001.00 </pre> <:vspace> Table 6.13 on page 92 of the GG24 manual explains the response. <:vspace> <:vspace> <:vspace> <:vspace> !!POS: Position Message $PASHS,NME,POS,C,ON <:vspace> This response message is in the form: <:vspace> $PASHR,POS,d1,d2,m1,m2,c1,m3,c2,f1,f2,f3,f4,f5,f6,f7,f8,f9,s*cc <:vspace> Attach:nmea-pos-a.png Attach:nmea-pos-b.png <:vspace> Example: <:vspace> $PASHR,POS,0,06,122910.00,6023.066334,N,00519.784303,E,+00093.916,,277.49,000.27,-000.06,05.3,04.7,02.6,03.4,GJ00*0E <:vspace> <:vspace> !!SAT: Satellite Status Message $PASHS,NME,SAT,C,ON <:vspace> The response is a message in the form: <:vspace> $PASHR,SAT,d1,n(d2,d3,d4,d5,c1)*cc <:vspace> where n is equal to the number of satellites locked. <:vspace> Attach:nmea-sat-a.png <:vspace> Example: <:vspace> $PASHR,SAT,06,27,129,66,52,U,15,168,12,45,U,17,061,36,44,U,09,158,79,45,U,53,143,35,52,U,52,066,49,45,U*1D <:vspace> [[<<]][[<<]] !ANTENNA POSITION ON RV "HÅKON MOSBY" ---- <:vspace> [NOTE: S.C. from UiO is good at this!] <:vspace> Attach:ash1.jpg ''Fasten antenna securely to rail.'' <:vspace> Attach:ash2.jpg ''Use tie-wraps to fasten the cable.'' <:vspace> Attach:ash3.jpg ''Make sure cable is clear of the door.'' <:vspace> <:vspace> [[<<]][[<<]] !TEST OF ASHTECH "GG24 GPS+GLONASS" RECEIVER ---- As quality control of timestamp data, a GEOMAR seismic recorder was operated in parallell with the Ashtech "GG24 GPS+GLONASS" receiver. <:vspace> This has also been done on previous surveys without sign of any anomalies. <:vspace> This time, however, processing of timestamp data indicated a possible fault in the timestamping instruments. Errors reported by E.F., IFM-GEOMAR: <:vspace> <:vspace> %box%<pre class='escaped'>When comparing your trigger times to our trigger time, we notice that 1) about every hour there is a major jump between 0.05 to 0.6 sec between yours and our trigger times. 2) during the approximately one hour there seems to be a linear trend of increasing difference between your and our shot times 3) in addition there is some dither on your shot times compared to ours of a few msecs. </pre> <:vspace> <:vspace> !!Trigger on negative edge? Before we started the investigation, IFG-GEOMAT was asked to state which edge of the trigger signal the GEOMAR recorder uses as detection of trigger time. <:vspace> Trigger signal is from the airgun control system (GUNCO). The signal is normally high TTL-level. The negative edge is the firing moment. Width of the trigger signal is nominally 10 milliseconds, but this can vary, as the falling edge is of importance. Our GPS timing equipment records the falling edge of the signal. <:vspace> Attach:gunco-ftb.gif"GUNCO 'FTB Scope' trigger signal" ''GUNCO 'FTB Scope' trigger signal'' <:vspace> It was confirmed from GEOMAR that their shot times were based on the negative edge of the GUNCO trigger signal. <:vspace> !!Timestamp data from GEOMAR %box%The first one shows a 25 Hz disturbance and a trigger at about 1.52 seconds (arbitrary time scale). The second one is an enlargement. In addition, every 30 Minutes or so there is a positive spike, but we ignored that. I think that you have a problem, because using "Our" trigger timess, the data was okay (water wave was straight), but when using "Your" trigger times, water wave was not straight. <:vspace> <:vspace> [[Attach:trigger-moere_basin.pdf | Attach:trigger-waveform-a.png]] <:vspace> Zooming in: <:vspace> [[Attach:trigger-moere_basin-zoom.pdf | Attach:trigger-waveform-b.png]] !!GG24 susceptible to jamming from Inmarsat? On the internet, there are reports that the GG24 receiver could be susceptible to jamming from Inmarsat. The matter is of relevance as RV Håkon Musby uses Inmarsat communnication. <:vspace> From http://www.gps-gis.net/services/downloads/gg24FAQ.html: -> Currently, if a ship transmits on Inmarsat, and the GG antenna is within about 50 feet of the Inmarsat antenna, there is a very good chance that both GPS and GLONASS GG24 will be jammed. We (advise) all shipboard integrators that the GG24 may be jammed by Inmarsat and other signals in that frequency range (1625MHz, +/- tens of Mhz). <:vspace> <:vspace> <:vspace>