Difference between revisions of "GPS analysis system"

Revision as of 13:55, 20 May 2006

We use a GPS data analysis system based on the GIPSY software developed at JPL. Most of the GIPSY programs are called by shell scripts written by Jeff Freymueller. Using these scripts, we can analyze a large amount of data either as part of network solutions or in Precise Point Positioning (PPP) mode.

Where do you put RINEX files?

RINEX files should be put into the hopper, $RAWDATA/hopper. What, you don't have RINEX files yet? See RINEXing. Once files are in the hopper, you can either let the first processing stages happen automatically overnight (see next section), or run the autofront and autoclean programs manually.

What happens automatically?

Quite a lot.

Autoftp runs every night beginning at 6pm local time and fetches data files. These are placed into the hopper ($RAWDATA/hopper), a directory where all data files are put for entry into the system and processing. Autofront then runs at midnight to process all files in the hopper, including any placed there manually (from campaigns, for example). Finally, autoclean runs at 4am local to carry out automated screening for cycle slips and other bad data.

Autoftp

Autoftp is an efficient data-fetching tool that uses wget to automatically get data from any of several internet GPS data archives. It reads a list of desired sites from a request file, which contains the date in the filename, and attempts to find and download data from as many sites as possible. It is intended to run automatically on a daily basis under cron, and when acccompanied by another simple program to generate a standard request file every day, it can easily fetch a standard set of sites on a daily basis for analysis. Because it keeps track in the request file of sites that it has found already, autoftp can be run multiple times with the same request file and it will not repeatedly fetch data. This is ideal for the real world, in which data from some sites are available rapidly while data from other sites may require many hours or days to become available.

Autofront

Autofront is a script intended to run under cron that carried out the initial "front end" processing on a set of GPS data files. When executed, it will process all files in the hopper directory, and will place each resulting qm file into the appropriate week directory.

Autofront runs the following steps 1. Checks on the validity of RINEX file and repair of some common problems. 2. Depending on receiver type, clockprep -fixtags 3. (optional, presently not default) PhasEdit 4. ninja

Autoclean

Autoclean carries out automated cleaning of cycle slips, based on point positioning solutions. It is quite effective and at present it rarely misses cycle slips unless they are smaller than its minimum tolerance (10 cm). Autoclean operates on an edit-request file, which contains the name of the directory (week directory) and a list of qm files that need to be cleaned. It will clean all files on the list as long as orbits and clocks are available, and it marks off files that have been cleaned so that it can safely be run multiple times.

Autoclean operates in an iterative mode. It's zeroth iteration is to do a pseudorange-only solution and identify and delete extremely bad pseudorange data. In this step it uses a tolerance that catches only grossly biased data. (Explain it). It then carries out 1 or more iterations of screening the phase data. In each iteration, it uses postbreak to identify discontinuities in the residuals of a point positioning solution. Postbreak is run with an adaptive tolerance (minimum 10 cm), and it is critical that my slightly modified version of postbreak be used. If any cycle slips are discovered, they are flagged and another iteration is run. Autoclean runs a maximum of 4 iterations on the phase data.

Where do the data files go?

Data files from each station are stored in the QM format that is native to GIPSY. QM files (and all other) files are stored in directories by GPS week. For each week directory there are several subdirectories; qm files are stored in $ANALYSIS/wwww/qm, where wwww is the 4 character GPS week number (with a leading zero if needed).

Running Solutions

Solve, a very flexible script. (link to detailed help)

Philosophy of solve

Subnet files and campaign files

Standard solutions

(text of standard_Alaska_solution)

Running several days at once: make-make-flt and make-alaska

(text of a standard make-alaska file and variant)

Running several weeks at once

(text of sample rerun-* file)

Cleaning Solutions

Initial Explanation of terms

Expected residuals from a clean solution

Automated screening: postfit, the point file, postbreak

Checking for bad pseudorange data: badp, allbadp

Removing biased pseudorange data: del_pcode_arc

Automatically identified cycle slips: breaks

Quickly scanning through residuals: short_hand

Limitations of short_hand

Manually checking residuals and fixing problems (link)