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Downloading GPS Data
The Reciever
INdependent EXchange formats are well documented and have
become the de facto standards for distributing high accuracy GPS data.
As a result, many GPS processing software suites can read
RINEX
data files. All data distribution centers mentioned here use this format
although the files themselves may be compressed to speed transmission.
Most GPS receiver manufacturers and some third party development groups
provide convertors to
RINEX
so the use of this format should not be an obstacle.
Data from a receiver is commonly converted into four
RINEX
files, each with a specialized task. The formats for these files are
described in the
RINEX
standards. The naming convention for these files is:
where
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SSSS |
is the four character site ID |
| |
DDD |
is the three digit day-of-year |
| |
# |
is a session code |
| |
|
0 |
= |
zero = all data for that site for that day |
| |
|
a-x |
= |
hourly subset: a = 00:00 - 01:00 UT,
b = 01:00 - 02:00 UT, ...
There is no redundant, overlap data between these consecutive
hourly data sets. |
| |
|
1-9 |
= |
session (although, historically, other characters
have been used) |
| |
YY |
is the two digit year |
| |
C |
is the file type designation |
| |
|
d |
= |
Hatanaka compressed observations |
| |
|
m |
= |
surface meteorological data |
| |
|
n |
= |
broadcast navigation message |
| |
|
o |
= |
lower case letter O = observations |
| |
|
s |
= |
data status and quality control information |
Only the observation files are required for these tutorials with one exception;
one broadcast navigation file is also necessary. The navigation file, also known
as the nav, broadcast or RINEX
"n" file, from any one of the sites to be processed could be used.
Many data centers also generate a global navigation created from many
nav files from a globally distributed set of sites and suitable for use
with any data set.
Tutorial
Retrieving data taken locally, by one's own group or institution, should
be straight forward and may be as simple as copying the file into the
appropriate location from some other directory or uploading the data from a
PC. Such cases will not be discussed here. Downloading data from a remote
repository via the internet is only slightly more difficult. In this
tutorial, we'll download data regional or global data
centers, not because it is necessary but, rather, to sample some of the
institutions redistributing data as a service to the community. Every data
center has a policy about how long recent data are kept on-line, on
reloading historical data and/or transfer via other media. Contact the data
center directly to inquire about those policies.
This begs the question of why the GPS data available through these centers are
of interest beyond the project for which they were collected? Often it is
desirable to include data from one or more sites with accurately known
coordinates in the processing. Holding these sites' coordinates to their known
values, along with the GPS satellites' coordinates, provides a connection to an
accepted, common reference frame, such as the NAD83, WGS84 or ITRF2000, and
forces the site coordinates computed as part of the processing
to be consistent with that frame.
Obviously, sites with well determined coordinates and long histories are most
desirable for the purpose of being a reference site. In turn, collecting,
distributing and archiving these data becomes a valuable service provided to the
GPS community.
Three examples of downloading GPS data from regional or global data
centers are shown. The date arbitrarily selected for these tutorials
is February 2, 2001.
- The first stop will be the CORS
data center, operated by the NGS, which redistributes GPS data from
government and private sites predominately in North America. CORS, by the
way, stands for Continuously Operating Reference Station. The site to be
downloaded, SOL1, is located in Solomons Island, MD.
CORS data from this center can be selected and
downloaded via ones web browser from the
CORS data center
web page along with maps, site logs and a wealth of other
useful information. Unfortunately, only one file at a time can be retrieved
using this method from this or any of the data centers. Using FTP will
almost certainly be quicker for downloading several files. Note that scripts
facilitating downloading data files from the CORS site are available.
The CORS FTP
site also contains site logs, ASCII files which give information about a
site and its hardware. These are stored in the station_log subdirectory.
Retrieve SOL1 data from the CORS
anonymous FTP site:
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