Are these sites down?
I just posted on the GRX1 thread that I haven't been able to get access to the NGS CORS data or OPUS ... first started trying yesterday, but I've checked back often, and haven't been able to get data.
The FDOT's CORS data site isn't working, either. It will load the pages, but all the data download requests come back as unavailable ...
Read this message from NGS home page.
Notices
NGS web site and related infrastructure will be undergoing maintenance starting at 2:00 pm Friday, December 19th. We expect the maintenance to be complete by 6:00 pm, Sunday, December 21st. Only certain products and services such as the CORS data sets will be available during this time. We appreciate your patience during this maintenance window.
Thanks
I ran in to the same issue this weekend, but really needed the positions, so I tried this site, with excellent results.
http://www.ga.gov.au/bin/gps.pl
OPUS alternatives
There are alternatives to the OPUS tools of the US NGS they include the following:
On-line processing tools
OPUS GPS processing tool - NGS processor using web-based form for input
SCOUT GPS processing tool - Scripps Institution of Oceanography (UCSD) ftp-based automated processing tool
AUSPOS GPS processing tool - Australian processor using web-based form for input
http://www.geod.nrcan.gc.ca/products-produits/ppp_e.php - Online Global GPS Processing Service (CSRS-PPP) of Geodetic Survey Division of Canada
http://apps.gdgps.net/apps_file_upload.php - NASA GDGPS processing tool.
http://gaps.gge.unb.ca/ - University of New Brunswick's GAPS (GPS Analysis and Processing Software)
There are some articles available on line describing and comparing them. FWIW, I took a look myself and the file mentioned is my comparison of results.
file: lab9_2012_summarySolutions.pdf compares results of a data set submitted to all these engines.
I just reprocessed in OPUS, and the results were almost identical: ± 1-2 mm. Nice alternative.