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Ephemeris

The satellite constellations and the transmitted navigation data of each satellite are dynamically built, once you start the scenario or the signal generation. The constellation and the navigation data is based on RINEX data stored in the unit, or uploaded to the unit. The constellation orbits can be refined by providing precise orbit information in SP3 format (for details, see below).

GPS and QZSS almanac data may optionally be provided in the form of YUMA files (for details, see below).

In addition, SBAS message files are also supported (see SBAS Satellites and User-Uploaded Ephemeris below for more details).

Under the menu item Select > Select Scenario > Configure > Ephemeris, there are two or three options to choose from (as described below), in order to select a source for your scenario navigation data:

  • Default
  • Download
  • User-uploaded files.

Ephemeris selection

Default Ephemeris

The default RINEX data for GPS and GLONASS is based on the CDDIS GNSS archive, using the brdc files. The non-redundant brdc file merges the individual site navigation files into one, and thus can be used instead of the many individual navigation files.

This data is complemented by GLONASS almanac data downloaded from ftp://www.glonass-iac.ru/MCC/ALMANAC/, covering the same period (file names are prefixed by receiver types, e.g. MCCT_ , MCCJ_, GG-24, or TOPCOM_).

The default navigation data begins Jan 8, 2012 and runs for 33 consecutive days.

For Galileo, BeiDou, and IRNSS, the GSG unit comes shipped with its own ephemeris data set.

When the ephemeris setting is set to Default, the GSG unit builds all scenarios, any start date, using the default data. If there is an exact match for the scenario Start time and preloaded navigation files, that navigation data will be used. If an exact date match is not found, then the GSG unit will use the first preloaded navigation data with the same day of the week as the scenario’s start time. Further simulation days will use consecutive in date navigation data.

In general, the start time of the scenario always supersedes the time stamps in the navigation data files. If file date and scenario start time do not match, then the loaded data is transformed accordingly to match the scenario’s start time. If the scenario defines a GPS almanac files only, the YUMA files will define the almanac and the ephemeris will be derived from the default RINEX data.

Download Ephemeris

The user can let the unit automatically download navigation data from official websites. The navigation data, brdc files and GLONASS almanac files are retrieved from the same sites as mentioned under Default Ephemeris.

For this feature to work, the following requirements must be met:

  1. The GSG unit must have access to the Internet.
  2. The correct DNS address must specified, either by setting Options > Interfaces and Reference > Network > Obtain IP autom. = Yes, or—when using a static IP configuration—by manually entering the correct DNS address.
  3. The scenario start time must be in the past.

The downloaded navigation data will be locally stored on unit. On subsequent simulations the GSG unit will first look for previously downloaded files before attempting to retrieve them again. Hence once scenarios have run once they can also be replayed at later occasions even if the Internet connection is no longer available.

Note, however, that the unit performs automatic clean-up of downloaded files and that this clean-up will occur when free disk space is less than 20% of the total disk space.

Download cannot be used in conjunction with Galileo, INRSS and/or BeiDou simulation. The download functionality does not support the downloading of GPS almanac files.

User-Uploaded Ephemeris

User-specified RINEX and SP3 files can be uploaded to the unit. Multiple files may be selected. The uploaded RINEX files will be used to build both constellation, and navigation data for the satellites. If SP3 data is provided, it will override RINEX data for the definition of satellite orbits in the constellation. If no SP3 data is available, the constellation orbits will be built, using provided or built-in RINEX data.

The number of RINEX files necessary depends on the scenario’s start time and duration, and must be equal to the total number of simulation days (including start/end days utilizing less than 24 hours).

In the event that dates for the user-specified data do not match the scenario’s start time, then GSG will transform the start time in order to resolve the conflict.

If a satellite system (e.g., GPS, or Galileo) is selected (i.e., number of satellites selected is not 0) and no navigation files are selected for that particular satellite system, then GSG will use default data for that satellite system.

The RINEX format support includes version 2.x and 3.0.

The file extension for SP3 files must be *.sp3 (not case sensitive).

YUMA

Optionally, GPS and QZSS almanac data may also be provided in the form of YUMA files, which are identified by their .alm file extension. GPS and QZSS almanac files are identified by a first-letter file naming convention:

  • g*.alm: If the first letter of the file name is a ‘g’, GSG assumes the file contains Galileo satellite almanac data.
  • b*.alm: If the first letter of the file name is a ‘b’, GSG assumes the file contains Beidou satellite almanac data.
  • q*.alm: If the first letter is ‘q’, then GSG assumes the file contains QZSS satellite almanac data.
  • qg*.alm: If the first 2 letters are ‘qg’, then GSG assumes the file contains both GPS, and QZSS satellite almanac data.

  • *.alm: If the first letter is anything other than ‘g, b, or q’, GSG assumes the file contains only GPS almanac data.

YUMA almanac data can be used with custom RINEX files, or default ephemeris data. If no custom RINEX files are provided, the default data will be used.

This allows testing using GPS and QZSS satellites with the same, or different GPS almanac data. The GSG supports multiple GPS and QZSS almanac files. The YUMA almanac is considered valid for ±3.5 days from the TOA value (Time-of-almanac) listed in the YUMA almanac.

The scenario is restricted to start times within this range. If a scenario runs beyond this range of time, no new satellites will be added. If the user specifies a start time outside this range, a dialog will advise the user that the ephemeris and almanac are dates are mismatched. The SCPI error “Data out of range" will be logged to indicate this issue for remote control users.

 

Note: For GLONASS, matching ephemeris and almanac files must be specified (only the 2-line AGL format is supported, see ftp://ftp.glonass-iac.ru/MCC/FORMAT/Format.agl). In addition, GLONASS almanac files must be named *YYMMDD.agl (i.e., a date must be provided at the end of the file name).

Note: The GLONASS data at this publicly available FTP site is known to contain errors. These can cause the GSG to generate signals that are deemed ‘bad’ by a receiver and may not be used in a fix or for navigation. This data is not maintained by Spectracom and is not guaranteed.

Note: The GPS and QZSS almanac files specified must comply with the YUMA file format and match the first 5 characters exactly for field identification. The spacing to the rightmost column of data must be preserved. If the file fails to be processed, verify that the Af0 and the Af1 lines do not contain a space between these prefixes and the (s/s). For example, the line must be Af0(s/s), not Af0 (s/s).

Note: RINEX data files in most cases must be full day files. However, when GPS almanac files are provided, the RINEX records can be of shorter duration. RINEX files of less than a day duration without supporting GPS and QZSS YUMA almanac files are limited to start times times only after 1400 hours, and may operate for limited times.