Accessing the Data

ETL 1110-1-184

01 Oct 1998

c. Multiple Points per site. Some of the sites (most are

also USCG Radiobeacon Sites) within the CORS network contain two

points. For example, the Brunswick, Maine site has points bru1

and bru2. Each provides RINEX 2 data.

B-5.

Accessing the Data.

a. Before Downloading Data. There are several steps to

consider before downloading CORS data. It is important to check

the CORS homepage to make sure the desired reference station is

up and running. The user's post-processing software must be

capable of accepting raw GPS data in RINEX 2 format. If using

CORS stations as part of a high precision network and the user's

GPS receiver antenna is different from the CORS station's

antenna, the post-processing software must be able to accept or

account for the phase center offsets in the different antenna

types. Mixing of various types or different manufacturer's

antennas can cause an error of up to 10 cm in the height

component of the GPS baseline solution if not modeled properly.

Information about the type of GPS antenna and the phase center

offsets for each station's antenna can be found on the NGS CORS

homepage.

b. Downloading Data within last 31 days. The data can be

downloaded directly through the Data menu on CORS Homepage. The

user selects the site, option (type of data: RINEX 2,

Coordinates...) and the date for the desired data. Selecting

Find_Files button will bring up the ftp directory for the data

file(s) for the selection. Sites that log data on a daily basis

will only list one data file per site per day. Using the file

naming convention from section B-3, select the file or files that

span the time desired. Remember that these files are based on

UTC time. For each time block there are three files listed, a

summary file (contains a "s" in the filename), an observation

file (contains a "o" in the filename), and a navigation file

(contains a "n" in the filename). Downloading the files to a DOS

or Windows computer will drop the ".gz" from the filename.

c.

Associated Software. There are three utility software

programs

are needed to make use of CORS data. They are GZIP386,

JOIN24PC

and INTERPO. These software programs can be downloaded

from the

Software Menu on the CORS Homepage.

(1) GZIP386. Since the downloaded files are compressed,

they require the program GZIP386 to uncompress these files.

Before uncompressing the downloaded files, they have to be

renamed so that a "z" is the last letter of the extension. For

example, rename filename.95o to filename.oz. To uncompress the

files, use the command gzip386 -d filename (where the filename is

B-4