ODTK uses the following GNSS navigation inputs:
The ephemeris and clock correction estimates are required. Other inputs are optional and can be selected if available from the GNSS source. ODTK currently accepts the following GNSS source messages:
|GNSS Source File Types|
|SP3||ASCII message containing post-pass SV ephemeris, but broadcast SV clock parameters. Ephemeris data is given in an Earth-fixed (ITRF) frame. The ephemeris data may include both position and velocity, or position only. ODTK will accomodate either set. It is important to be aware of whether the ephemeris represents the location of the spacecraft center of mass or antenna phase center and that the SVEphemerisReference.ReferencePoint setting reflects the appropriate value. If emphemeris representing the center of the mass is used, the antenna location will be determined using the antenna offset in conjunction with the attitude profile of the spacecraft where both are specified in the system specific catalog file. If the assigned SP3 file contains data from multiple GNSS constellations, only the data associated with the GNSS constellation identified by the assigned catalog file will be exposed to ODTK. In such a case, the same SP3 file can be assigned to multiple GNSS constellations which each use a different catalog file to represent a differnt GNSS constellation. SP3 format descriptions and data can be obtained from multiple sources. One source is the NIMA Precise Ephemeris (PE) Satellite Phase Center (APC) data. Note that the SP3 data includes ephemeris and clock estimates, but does not include covariance, nor the high accuracy GPS to UTC time correlation.|
|RINEX||RINEX GPS Navigation Message: ASCII message containing the
navigation data defined in ICD-GPS-200. RINEX (Receiver INdependent
EXchange) format is an international format standard. ODTK follows
version 2.10 and later. The format description is given by Werner
Gurtner, "RINEX: The Receiver Independent Exchange Format Version
2.10" 8 June 2001, found at this NOAA
web site. Data can be obtained, for example, from the CDDIS web
site. The ephemeris is computed by propagating orbital elements
using the ICD-200 188.8.131.52.3 ephemeris determination algorithm.
Note that this data includes ephemeris and clock data; it may also
include (depending on reporting source) the GPS to UTC high
accuracy time correlation parameters. It does not include any
The ability to read the RINEX 2.10/2.11 GLONASS Nav Message has also been added. The NAV file generally contains ephemeris data, position and velocity, and clock data at 15 or 30 minute intervals. Not all SVs are reported at all times. The GLONASS ICD identifies an ephemeris extension algorithm, valid for plus/minus 15 minutes from the ephemeris epoch. This is the algorithm used by ODTK. So valid ephemeris data is available for SVs for minus/plus 15 minutes of an ephemeris record. However, to support simulations, data will be returned if an ephemeris record is within a day of the requested time with an error message written to the message viewer identifying the data is unreadable.
|SEM||The SEM Almanac is an ASCII message containing the almanac information in SEM format. Refer to the U.S. Coast Guard Navigation Center web site for definition and catalog of files. Note that this data includes only ephemeris and clock data, and does not include covariance, nor the high accuracy GPS to UTC time correlation. (See 2nd note below.)|
|YUMA||The YUMA Almanac is an ASCII message containing the almanac information in YUMA format. Refer to the U.S. Coast Guard Navigation Center web site for definition and catalog of files. Note that this data includes only ephemeris and clock data, and does not include covariance, nor the high accuracy GPS to UTC time correlation. (See 2nd note below.)|
|AUX||Auxiliary User Message, also known as High Accuracy Navigation User Message (HANU), defined by ICD-GPS-208. The binary AUX message contains ephemeris and clock data. It also contains covariance data, but ODTK does not currently use this.|
NOTE: Almanac refers to a set of reduced precision SV clock, ephemeris and satellite health information as defined in ICD-200 184.108.40.206.2.1.
NOTE: GPS almanac data should not be used as the basis for precision orbit determination. The almanac data only provides positional accuracy good to a kilometer or so, whereas ephemeris from the navigation data is accurate to within a few meters. Almanac data should only be used for simulation purposes, and only if other data sources are unavailable.
The following measurement types are known, but are not supported in ODTK: