Determine the Alternate Navigation File to Download from NASA’s CDDIS

When the auto-download of the ephemeris and alternate navigation is not working you may elect to manually determine the an alternate navigation file to download from the NASA CDDIS site while your IT resolves any connection issues. Click here to determine the ephemeris file to download from NASA’s CDDIS.

  1. Determine the start/end times in UTC or GPS for your mission/Cycle to be processed. The difference between UTC and GPS time is only a few seconds, so it won’t matter for the purpose of selecting an appropriate ephemeris file. For an example to follow, if your data was collected between 13:00 – 17:00 UTC on April 26, 2021.
  2. To determine the alternate navigation file to download, first determine the Julian Day for your observation by using the interactive calendar provided by a third party. This calendar presents dates specific to the GPS community. The data for each day are as follows. The fourth line contains the Julian Day Number that you will need to locate the appropriate file to download from the site.
RowExampleDefinition
First1Calendar day of the month
Fourth183Julian Day Number
Reading the GPS Calendar
Example of GPS calendar showing April 21, 2001 with the corresponding Julian Day highlighted to determine the alternate navigation file download
Example of GPS calendar showing April 21, 2001 with the corresponding Julian Day highlighted
  1. Go to the NASA CDDIS site. You will need to create an EarthData login if this is your first time using the site. navigate to https://cddis.nasa.gov/archive/gnss/data/daily/ if you end up elsewhere during the login process. You will see a list of available years for the daily products.
  2. Select the applicable year for your observation, 2021 in our example.
  3. Scroll down to the bottom and select the BRDC folder.
  4. Locate the applicable BRDC00IGS_R_YYYYJJJ0000_01D_MN.rnx.gz, where YYYY is the year of observation and the JJJ is the Julian Day determined in step 2, 116 for our example. Hence, for the example we would right-click on BRDM00DLR_S_20211160000_01D_MN.rnx.gz and select “save link as” to copy the file local to our machine for processing.
  5. Once the file is downloaded, you will need a program, such as 7-zip, to be able to right-click on the gz file and extract the rnx that has been compressed therein. For our example, it is the BRDM00DLR_S_20211160000_01D_MN.rnx that we would provide to True View EVO or ASPSuite as the alternate navigation file for use during processing of the trajectory solution.
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