The installations that do not use an user-exit program similar to the UH system but generate NMEA-formatted GPS les or use the RDI program navsoft to store these in the user buer structure can use the programnmea_gpsto convert from that format to a text le with columns suitable for use with the succeeding steps. If the RDI navsoft program was used, the program scanping can be used to extract the user buer ASCII messages to a text le, which can then be run through the nmea_gpsprogram to produce columnar output of GPS xes.
See
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on how to use the scanpingprogram.Thenmea_gpsprogram uses a control le (Appendix A.18) that requires editing as follows. TheYMD_BASE:parameter speci es the year, month, and day of the rst observation in the time range requested for output. It is used to convert the NMEA le times to decimal days. The TIME_RANGE:speci es the time range over which to extract data. Setting it toALLconverts the entire input le(s). For navigation purposes, the OUTPUT_ASCII_FILE: must be speci ed (root only).
This output le will have the extension.gpsand can be used for the remainder of the navigation calculations. The INPUT_GPS_FILES:parameter precedes the list of NMEA GPS input les to be read from.
The program is then run by typing
nmea_gpsnmea gps.cnt
A few lines from a sample output le are displayed in Figure 21. The rst three columns provide the required decimal day, longitude and latitude information.
The remaining columns provide quality indications: number of satellites, qual-ity, horizontal dilution of precision (hdop), north component dilution of preci-sion (ndop), east component dilution of precipreci-sion (edop), vertical component dilution of precision (vdop), and altitude. This ASCII le can then be used in place of the ubprint .gps, .ags, or.trs les.
Other cases.
If the options above are not applicable for your source of ship position data, you will need to write a program to reformat position xes from whatever source to text les, where the rst three columns of each line provide the x time in decimal days, and longitude and latitude in decimal degrees. If quality parameters are available, they could be recorded following the formats describe above for GPS or Transit Summary output les so they can be used with theedfixediting program in the next step. Otherwise, they could also be hand edited directly, and used with the post-editing steps described below.58
==========> variable: avg_GPS_summary (*.ags) <=====================
DD LO LA NS Q HD ALT 338.7837616 -149.5695583 -17.5372167 3 1 4 16.00 338.7872222 -149.5694000 -17.5369250 4 1 1 16.10
. .
==========> variable: GPS_summary (*.gps) <========================
DD LO LA NS Q HD ND ED VD ALT 190.7958333 -157.8863133 21.3161917 5 1 1 1 1 1 150 190.7975694 -157.8864850 21.3160817 4 1 1 1 1 1 150
. .
==========> variable: TRANSIT_summary (*.trs) <===================
DD LO LA EL NI DR F DT 190.8604630 -157.7488861 21.1823694 43 2 0.57 1 18 190.9175579 -157.5449083 21.0662833 52 2 1.00 1 18
. .
==========> NMEA_GPS output <======================================
DD LO LA NS Q HD ND ED VD ALT 191.7958333 -159.8863133 31.3161917 6 1 1 1 1 1 150 191.7975694 -159.8864850 31.3160817 4 1 1 1 1 1 150
. .
Key to columns:
DD: decimal day LO: longitude LA: latitude
NS: number of satellites
Q: quality flag as part of the GPGGA message, 0 = GPS not available 1 = GPS available
HD: HDOP, horizontal dilution of precision ALT: altitude
ND: NDOP, North dilution of precision ED: EDOP, East dilution of precision VD: VDOP, vertical dilution of precision EL: elevation of satellite
NI: number of iterations
F: flag (1 if fix accepted by navigator, 0 otherwise) DT: time difference in sec between PC and navigator
Figure 21: Sample ship position x les (abridged).
59 The next step is to edit out bad xes from the x le(s). The preferred way of implementing this is by inserting a %character in the rst column of the bad x entry (thereby commenting out the entry as opposed to deleting the line). One may also wish to ll in data gaps from a particular x source with data from an alternative source.
As mentioned previously, the
rst
gross check on the x quality is done by pro-ducing the cruise track. Thesecond
method is to use the program edfix to auto-matically edit out bad xes. This method is limited due to the constraints on the input format its applicability ends with the 1320-user buer as it presumes certain columns. The 720/2240 user-buer does not provide the same columns that ed x expects. However, the quality of the GPS data from these latter user buer types is better, and in many cases does not require program edfix, and can be assessed in the following. Thethird
method involves calculating and plotting absolute currents.7.1.2 Editing the Fix le(s) with Edx
The program edfixis used to automatically edit out bad xes based on the quality criteria. The edfix.cnt control le is used to specify what type of editing is to be performed and whether or not the .trs le is to be merged into the .gps le (Appendix A.9).
One must rst edit the edfix.cnt control le. The output: parameter es the output lename. The transit_file: parameter speci es the name of the Transit x le (.trs) or can be set to none if the user does not want the Transit xes merged into the .gps le. The next ve lines de ne the editing criteria for Transit xes (or are ignored if the user speci es none for the transit_file: op-tion). The min_elevation=and max_elevation=parameters de ne the acceptable range of satellite elevation (we use 7 to 10 ). The max_iterations= speci es the maximum number of iterations in the x calculations (we use 3). Themax_dr= spec-i es the maxspec-imum dspec-istance of the x posspec-itspec-ion to the dead reckonspec-ing posspec-itspec-ion. The
time_since_gpscriterion edits out Transit xes that occur too close to a Transit x (we use .006 decimal days or under nine minutes roughly).
The gps_file:parameter speci es the name of the input.gps le. It is followed by the editing criteria for GPS xes. The max_hdop= criterion speci es the maxi-mum acceptable horizontal dilution of precision (we use an upper limit of 6). The
time_since_3 criterion speci es the maximum time elapsed since a three-satellite x occurred and is calculated from the number of satellites and time columns.
This is useful in the case where the GPS receiver has a timing device such that it can use two satellites and the clock to calculate a x. We usually start with a value of .25 decimal day, meaning that a two- satellite x will be edited out if the last three-satellite x occurred less than 6 hours before.
After setting up the control le, the program is run by typing
edfixedx.cnt
60
The output le will have an extension *.edf.
Regardless of whether edfix has been run or not, it is a good idea to visually scan the nal x le for remaining outliers. It should be noted that edfix edits based on the quality indices alone, and does not examine the x values themselves for reasonableness. One may also choose to override some ofedfix's decisions if they result in too large a gap where data look reasonable enough. That is one reason why bad xes are merely commented out rather than completely omitted from the output le.
7.1.3 Editing the Fix File Using Plots of Absolute Currents
Another useful way to assess the quality of the x data is by calculating and plotting absolute currents for a reference layer. The steps involved are the same steps used in the Navigation stage of ADCP data processing, which is explained in detail in Chapter 8. For now, an explanation is given on how absolute currents can be used to gauge the x quality and briey on how to perform the steps, with the majority of the description of these routines postponed until later.
In a nutshell, the ADCP measures relative velocities between the ADCP trans-ducer mounted on the hull of the ship and scatterers that are advected by the currents.
In order to obtain absolute currents, one must subtract out the velocities of the ship relative to the earth coordinate system, which are determined by the xes. Thus, assuming the ADCP data editing has been satisfactory, spurious features in absolute currents are attributed to erroneous x values.
The general steps involved are as follows: