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Using SBAS in a Simulation

GSG will select SBAS SV based on their elevation with respect to the user position. When the scenario is running the SBAS satellite positions and speed will be updated with the information found in the SBAS messages. In particular, for each MT9 message, the satellite’s position and speed are updated.

Although PRN120 - PRN158 are all reserved for SBAS systems, only a few of them are actually used by satellites. When determining the elevation angle of SBAS satellites, GSG unit looks for the SBAS satellites listed below. This is in contrast to the signal generator mode where you can specify any SBAS PRNs to be simulated.

The currently supported SBAS satellites are:

  • EGNOS: 120, 124, and 126
  • WAAS: 133, 135, and 138
  • MSAS: 129, 137
  • GAGAN: 127, 128

The simulator uses two approaches for SBAS messages:

  1. Default SBAS messages (MT63)
  2. EGNOS/WAAS message files

The default SBAS messages are always available. These messages should be recognized by SBAS-compatible receivers. However, they carry no information and will therefore not enable the receiver to correct GPS signals.

SBAS message files for both EGNOS, and WAAS are supported. EGNOS files (.ems) are ASCII and hourly, while WAAS files are typically in binary format and cover a whole day. Both systems share the same format of the messages and details can be found in http://www.navipedia.net/index.php/The_EGNOS_SBAS_Message_Format_Explained.

When the scenario has Ephemeris set to “Download”, the GSG unit will download the SBAS messages from official sites and match these messages to the time of the scenario. The SBAS messages broadcast by these satellites are downloaded automatically from these public FTP sites:

GSG uses an anonymous login. However, note that both FTP sites are likely to track and record all FTP access, including access by the GSG-55.

The SBAS download starts when the constellation simulation of the scenario has started; not during initialization of the scenario.

If a scenario needs SBAS messages that cannot be downloaded from these FTP sites, the scenario continues, but the GSG unit transmits null-messages (SBAS message type: MT63). An SBAS-compatible receiver should still be able to see the SBAS signals, but it will not find any useful information (range corrections, time offsets, etc.) in the SBAS messages.

It follows that SBAS scenarios run best with a live Internet connection. Furthermore, since the aforementioned FTP sites store only a limited amount of SBAS records, the start time of SBAS scenarios has to be chosen carefully. Usually, SBAS records that are less than a year (EGNOS)/6 months (WAAS) old can be found on the aforementioned FTP sites. Select a start time that is not older than one year for EGNOS scenarios, and not older than 6 months for WAAS scenarios. Moreover, the start time shall not be too close to the current time. For EGNOS, there can be a one day delay before the SBAS messages are published on the FTP site. For WAAS the delay can possibly be longer (up to 3 or 4 days).

The Internet connection is not always needed. All downloaded ephemeris data and SBAS data will be locally stored on the unit once they are downloaded. So, the next time the same scenario runs, the ephemeris data and SBAS messages are read from the local storage and no Internet connection is needed. The unit performs automatic clean-up of downloaded files. Such clean-up will occur when free disc space is less than 20% of the total disc space.

Note: Currently SBAS corrections are not ‘applied backwards’ to the outputted GPS signals, even though the corrections will be transmitted in the SBAS signal.