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1.Creating A Work Area
2.Downloading GPS Data
3.Downloading an Ephemeris
4.Selecting Sites To Hold Fixed
5.Creating The POM Files
6.Running run_survey
7.Manually Modifying The Processing

Description

Surprisingly little knowledge about a GPS site is needed to process that site's data.  Less surprising is the fact that the more one knows about a site, the greater the likelihood of successfully processing that site's data.  Four basic pieces of information about a site are needed:
  1. In truth, the site's location isn't needed.  Determining this is probably why the data were taken in the first place.  However, determining the position to high accuracy starting with no knowledge of where the site was located would require several iterations (some of which will be discussed in these tutorials).  By providing an initial guess for the position, potentially, some of the iterations can be eliminated and the desired accuracy reached more quickly.  The better the initial guess, the faster the process.  Broadly speaking, when starting with no knowledge of a site's location, three iterations are required:
    1. The receiver itself probably determines its position "good" to a few tens of meters and records its position in the data stream.
    2. The program tdsolve is capable of improving site coordinates from uncertainties of hundreds of meters to a few tens of centimeters.
    3. Finally, the pages program can improve site coordinates from a few tens of meters to a few tens of millimeters.
    It bears repeating that if a priori knowledge of a site's coordinates surpasses the accuracy of one of the steps listed here, that step can be omitted.
  2. Accepting that knowing approximate site coordinates is not needed but advantageous, there will almost certainly be site specific characterizing about how the antenna was mounted and positioned relative to a survey mark, benchmark or other monument that can not be determined from the data and must still be provided by the user.
  3. The type of equipment used to take the observations affects those observations.  Although manufacturers make every effort to minimize these equipment dependencies, residual corrections are necessary, particularly for variations in the characteristics of the antenna.  Thus the type of antenna used to take data is required information for data processing.  The hardware is recorded in the RINEX header this information but should be verify by the person processing the data if that person did not participate in taking the data.  This is particularly true for historical data more than a few years old.
  4. Effectively, the environmental characteristics of a site, i.e. multipath, obstructions, interference and atmospheric effects, are unique to each site. Many common effects which change the path length of the GPS signal as it traverses the atmosphere can be corrected but some, multipath for instance, are still the subject of research.  Ionospheric effects can be largely eliminated by using dual frequency data or, with single frequency data, by keeping baseline lengths short or using ionospheric models.  Path length delays caused by the neutral atmosphere, the tropo in the GPS vernacular, can be modeled and removed with varying degrees of success from surface meteorological data or, more commonly and as is done the pages program, a correction to the tropo model can be estimated from the GPS data itself.  Much like site coordinates, the better the starting met values, the better the final correction.  The run_survey suite can make use of a global seasonal surface met model but can also use in situ measurements.
In summary, an approximate site position, antenna offset and type, and meteorological information are the minimal requirements to process data.  Initially, this information can be entered through the POM (Position / Offset / Met) file. The values in this file do not need to be "perfect" but the more accurate this information is, the better. The POM files are simple ASCII text files; therefore, they can be created, reviewed and modified with an editor. Later in the processing, other more accurate or complete sources for these values can be substituted.  POM file contains basic site information on four lines. They are, in this order:
  1. the Earth centered, fixed (XYZ) coordinates of the monument or reference point at a site in meters,
  2. the north / east / up offsets from the reference point given on line 1 to the antenna's L1 phase center plus the L1-L2 offset in height, all in meters,
  3. surface temperature, pressure and humidity, in degrees C, millibar and percent saturation respectively, and
  4. the antenna type using a conventional antenna ID.
One POM file must exist for each site.  The POM file for a site is named with the four character site ID plus the extension .pom. All POM files are stored in the "files/" directory and, because the info slowly or rarely changes, can be reused for other processing.

Tutorial

At the end of the last tutorial, the current directory was "~/tutorial/files/".

Creating a POM File the Hard Way

Previously, criteria used in selecting reference sites had been discussed and, in this case, GODE selected as the reference site.  Observations from GODE have been contributed to the ITRF solutions, discussion of which are found at the IERS web page.  Selecting the more recent ITRF97 GPS solution from this source, the coordinates and velocity for GODE are:
  
ITRF97 STATION POSITIONS AT EPOCH 1997.0 AND VELOCITIES
DOMES NB. SITE NAME ID.          X/Vx        Y/Vy         Z/Vz        Sigmas
                           -----------------------m/m/y-------------------------
--------------------------------------------------------------------------------
40451M123 GREENBELT GODE  1130773.874 -4831253.581 3994200.393  .001  .002  .002
40451M123                      -.0144      -.0023        .0009 .0003 .0004 .0004
  The first line gives the X, Y, Z coordinates and formal errors, all in meters, for GODE.  The second line gives the X, Y, Z velocity components and errors all in meters per year.  Note that the epoch of these coordinates is 1997.0.  In other words, this is the computed position of this site at the start of 1997.  For dates before or after that epoch, the velocity must be used to compute the position.

The first task will be to use the velocity information and compute the position of GODE at the epoch of the data, 2001/033.

  1. Compute the number of years since the reference epoch 1997.0.

    2. Using the utility program year, we see that the epoch of the data transformed into a decimal year is

      year 2001 033
      2001.087671

    The difference between the data and reference epochs is

      2001.087671 - 1997.0 = 4.087671 years
     

  2. Multiplying each velocity component by the time span computed above gives

    3.       -.0144 m/yr     -.0023 m/yr     .0009 m/yr
        × 4.088 yr      × 4.088 yr     × 4.088 yr
       --------------   ------------   ------------
          -.0589 m        -.0094 m        .0037 m
     
  3. Now add the change cause by the site velocity to the site coordinates

    4. 1130773.874 m  -4831253.581 m  3994200.393 m
          -.0589m         -.0094m        +.0037m
    -------------  --------------  -------------
    1130773.815 m  -4831253.590 m  3994200.397 m at epoch 2001.088
Next, the antenna type and where the antenna is mounted relative to the GODE's monument must be determined.  Excluding having actually been on site to take the data, the simplest source is the GPS RINEX data file itself.  The header for the GODE RINEX files gives:
  
129                AOAD/M_T        JPLA                   ANT # / TYPE
        0.0614       0.0000        0.0000                 ANTENNA: DELTA H/E/N
  This information is correct but, unfortunately, historically, for some sites, this is not the case.  In the global community, the "ultimate" source for this information is the site log.  Recall that, storage areas for sites logs were noted when downloading data from the data centers.  The IGS central bureau also makes available up-to-date site logs: using a clickable map or an alphabetical list via the WWW , the log files themselves or a summary containing all sites in the SINEX format via anonymous FTP.  The entries from the data file header show that the GODE antenna is a "AOAD/M_T        JPLA".  Less succinctly, the antenna is an Allen Osborne Associate Dorne/Margolin T-type antenna with a JPLA style radome.  The succinct name is the appropriate string and spacing is significant.  A list of IGS sanctioned antenna names can be found at the IGS central bureau or, perhaps more practically, from the ant info file itself.  The offset listed is from the monument to the antenna reference point (ARP).  Therefore the ant info file must be consulted to find the ARP to L1 phase center (PC) offset and the L1-L2 offset.  Showing only the pertinent lines here, the AOAD/M_T listing in the ant_info.003 file gives
  
AOAD/M_T             Dorne Margolin T, chokerings (TurboRogue)NGS (  0) 97/10/27
      0.0       0.0     110.0
  The header of the ant_info.003 file informs the user that these values are north/east/up vector components, in millimeters, for the L1 ARP to PC offset.  Similarly, but not explicitly shown here, the L2 north/east/up vector components are 0.0 / 0.0 / 128.0 millimeters.  Therefore, the total offset from the GODE monument to the L1 phase center is
  
    N    E    U      (all in meters)
   0.0  0.0  0.0614
  +0.0 +0.0 +0.110
  ---- ---- -------
   0.0  0.0  0.1714
  The vertical L1-L2 offset is:   0.110 - 0.128 = -0.018 m. The final items to determine are reasonable approximate surface meteorological values.  GODE produces a RINEX met file but, because in the basic processing, a correction to the tropo model will be estimated, only crudely approximate values are needed.  Being February just outside Washington, D.C., a reasonable guess would be 5 degrees C, 1000 millibar and 50% relative humidity. Now, all these items can be combined into the format of a POM file:
  
   echo "    1130773.815   -4831253.590   3994200.397"               > gode.pom
   echo "         0.0           0.0           0.1714      -0.0180"   >> gode.pom
   echo "         5.0        1000.0          50.0"                   >> gode.pom
   echo "AOAD/M_T       JPLA"                                        >> gode.pom
 

Creating a POM File the Easy Way

Several accessory programs can be used to create POM files.  Here, we'll two simpler methods to create POM files provided the information is available in an accepted format.

The first also takes advantage of the information stored in a station's log file and is a three step process using utility programs.  This example will use the most recent log file for SOL1 available at the date this tutorial was written.

  1. First compute the MJD for the data epoch:


    2. mjd 1 33
    51942

  2. Next, use the log2site program to read, interpret and output the pertinent information into a standard format:


    3. log2site sol10008.log

    The log2site program stores the extracted info in a binary file, name log2site.out by default, specifically designed to store station information in a compact format.

  3. Finally, use the sitelist program which can read station information or "site info" files and print information for a station in a human readable format.  Several formats are available include POM file format.


    4. sitelist -p 51942 sol1 log2site.out > sol1.pom

    The "-p" option flag instructs the program to produce a POM file format.  The epoch is entered with this flag so the site coordinates can be expressed at the desired epoch.  The sitelist program searches for and uses the newest ant info to produce the L1 phase center coordinates and L1-L2 offset.  If this file can not be found, the user must enter the ant info file name and path from the command line.  For example:

    sitelist -A /opt/GPS/info/ant_info.003 -p 51942 sol1 log2site.out > sol1.pom

The resulting file looks like:
  
    1173608.7755  -4871160.8478   3933263.0998
          0.0000         0.0000         0.1100        -0.0180
          8.4023      1015.6002        50.0000
AOAD/M_T
sol1  a ARP; L1 OFFSET; MET MODEL @ EPOCH 01/02/02 00:00; CREATED 01/03/12 12:03
  Note that the fifth line is merely a comment automatically generated by sitelist documenting the file's creation.  The site coordinates in a site's log file are approximate only so care must be taken when first using coordinates taken from this source.  sitelist also uses a season meteorological met model to create appropriate surface meteorological values.

The third way is the easiest of all.  Actually it is a variant of the last method but makes use of a standard site info file previously created.  Examples of these files can be downloaded from the software distribution site or from the NGS.  Specifically, the site_info.igs file created and maintained by NGS will be used because this file has up-to-date info on many IGS tracking sites.  A copy of the site_info.igs file is available from GPos. After the site_info.igs file was downloaded, the command to create a pom file for USNA, for example, would be

sitelist -A /opt/GPS/info/ant_info.003 -p 51942 usna site_info.igs > usna.pom

Using the site_info.igs file available at the time of this writing, the resulting POM file for usna would look like

  
    1160668.9418  -4826883.3643   3990863.0254
          0.0000         0.0000         0.1100        -0.0180
          7.9263      1016.6205        50.0000
JPL D/M+CRT
usna  a ARP; L1 OFFSET; MET MODEL @ EPOCH 01/02/02 00:00; CREATED 01/03/12 12:28
  The site_info.igs file uses an older version of the AOAD/M_T antenna ID.  Normally, non-standard antenna type ID's are a cause of concern and source of potential errors but, in this case, the ID was an accepted form and will be recognized in subsequent processing.  If the user is unsure, the file should be edited and the correct ID entered manually.

Notes

The directory ~/tutorial/files should now contain six files:
ls
ant_info.003 fixsite.inp gode.pom hubsite.inp sol1.pom usna.pom
 
© 2009 General Positioning LLC
2009-02-15 18:41 UTC