GNSS Source File Types Used in ODTK
ODTK uses the following GNSS navigation inputs:
- SV ephemeris and clock correction estimates
- SV ephemeris and clock correction estimate error covariance
- Model parameters for high-accuracy correlation of GNSS time to UTC time (and TDT time)
The ephemeris and clock correction estimates are required. Other inputs are optional and you can select them if available from the GNSS source. ODTK currently accepts the following GNSS source messages:
| GNSS Source File Types | |
|---|---|
| Type | Description |
| SP3 | This is an ASCII message containing post-pass SV ephemeris but broadcast SV clock parameters. Ephemeris data is in an Earth-fixed (ITRF) frame. The ephemeris data may include both position and velocity, or position only. ODTK will accommodate 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 two or more 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, you can assign the same SP3 file to two or more GNSS constellations that each use a different catalog file to represent a different GNSS constellation. You can obtain SP3 format descriptions and data from different sources. One source is the NIMA Precise Ephemeris (PE) Satellite Phase Center (APC) data. The SP3 data includes ephemeris and clock estimates, but does not include covariance nor the high-accuracy GPS-to-UTC time correlation.
Source information for a particular PRN is marked as invalid in a SP3 file when the positional information is specified as all zeros and when the clock is specifed as all 9's. Once a PRN is marked as invalid, ODTK ignores the rest of the data for that PRN, even if it is marked as valid again later in the dataset. |
| RINEX | RINEX GPS Navigation Message is an 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. You can obtain data from the CDDIS web site. The ephemeris is computed by propagating orbital elements using the ICD-200 20.3.3.4.3 ephemeris determination algorithm.
The data includes ephemeris and clock data; it may also include (depending on the reporting source) the GPS-to-UTC
high-accuracy time correlation parameters. It does not include any covariance data.
ODTK can also read the RINEX 2.10/2.11 GLONASS Nav Message. 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 to minus 15 minutes from the ephemeris epoch. This is the algorithm used by ODTK. 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 the definition and a catalog of files. The data includes only ephemeris and clock data, and does not include covariance nor the high-accuracy GPS-to-UTC time correlation. (See the second 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 the definition and a catalog of files. The data includes only ephemeris and clock data, and does not include covariance nor the high-accuracy GPS-to-UTC time correlation. (See the second note below.) |
| AUX | The Auxiliary User Message, also known as the High Accuracy Navigation User Message (HANU), is defined by ICD-GPS-208. The binary AUX message contains ephemeris and clock data. It also contains covariance data, but ODTK does not use this. |
Almanac refers to a set of reduced precision SV clock, ephemeris and satellite health information as defined in ICD-200 20.3.3.5.2.1.
You should not use GPS almanac data 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. Only use Almanac data for simulation purposes, and only if other data sources are unavailable.
Measurement file types not supported in ODTK
The following measurement types are known, but ODTK does not support them:
- BeiDou pseudo-range and phase (e.g., C1I & L1I)
- QZSS L1-SAIF pseudo-range and phase (C1Z, L1Z)
- GLONASS G2 C/A phase (GLONASS-M L2C)