GNSS Satellites

A GNSS satellite is represented in the Browser and in the ODTK toolbar by the icon. There are also menu options that let you add and remove GNSS satellites.

Note: See ODTK 5 Applied to GNSS Satellite Orbits and Clocks for an overview of and guide to the application of ODTK 5 to GNSS satellite and clock estimation.

Properties Available for GNSS Satellites Only

The following properties can be configured for a GNSS satellite:

GNSSSatellite Properties
Property Description
PRN A GNSS satellite can be identified by its SV identifier or its pseudorandom noise (PRN) code identifier. The PRN number is in the range from to 1-31 and can be reused after a satellite becomes inactive or decommissioned.
SVN The Space Vehicle Number associated with the PRN. The SVN increases for each GNSS SV launched, whereas the PRN stays within the fixed range 1-31.
BlockType SV Block Type, e.g. II, IIA, IIR, etc. Read-only.
BeamFOV Defines a conical field of view with the vertex at the GNSS satellite and axis passing through the center of the Earth. During simulation of GNSS measurements, a user satellite is considered to be visible to the GNSS satellite only if it is not obstructed by the Earth and is within the field of view.
GroupDelay The SV group delay differential between L1 and L2. This correction term only applies to single frequency measurements (reference ICD- GNSS-200, 20.3.3.3.3.2). The group delay is unique to each SV. It is initially calculated via a ground calibration and then can be updated to reflect on-orbit performance.
ClockControls See table below.
ClockUncertainty See table below.
OrbitClockCorrelations A 9x9 ephemeris/clock error covariance.
PartitionControls See description below.

Clock Controls

In the following clock controls, phase is the current time, frequency is the first derivative of phase, and aging is the second derivative. Thus, aging represents a long term constant rate of change in frequency and a corresponding quadratic change in phase. The Aging_Sigma attribute specifies the uncertainty in the known long term trend in frequency (a clock that runs faster and faster or slower and slower), and the AgingWN attribute is the process noise. The clock models are extremely sensitive to these quantities.

There is no Epoch attribute for a GNSSSatellite (in contrast to a GNSSReceiver). Thus, the initial phase estimate defined for the GNSSSatellite will be considered to be the value at the state creation time, whatever that time may be, even if the state creation time is different from the satellite epoch.

ClockControls Properties
Property Description
ClockType Displays the type of clock (cesium or rubidium) being used by the PRN. This display is currently for information purposes only. The displayed data derives from the GNSSCatalog.txt file. Typically the clock type is indicative of clock stability statistics, which in turn impact the noise statistics for measurement processing.
Estimate Read-only. Defaults to true.
PhaseBias Clock phase (time) offset estimate at state creation time (see note below).
FreqBias Clock frequency offset estimate at state creation time (see note below).
AgingBias Clock aging offset estimate at state creation time (see note below).
A0
Frequency-modulated (FM) clock white noise statistic.
Aminus1 FM clock flicker noise statistic (not currently modeled).
Aminus2 FM clock random walk statistic.
AgingWN FM clock drift statistic.
ClockResets A list of clock phase, frequency and/or aging reset events.

Note: If state creation time (e.g. simulator or filter start time) is not the same as the satellite orbit epoch, then, while the satellite orbit is propagated forward or backward from the orbit epoch to the state epoch, the clock parameters are not. They are used at the state epoch as-is.

Clock Uncertainty

The following are the clock uncertainty properties:

ClockUncertainty Properties
Property Description
Phase_sigma Uncertainty associated with the phase bias estimate at the initial filter time. Used to initialize the covariance when the GNSS Constellation attribute to estimate clocks is set to true. If the GNSS Constellation attribute to estimate clocks is set to false, then this uncertainty is applied in the deweighting of measurements dependent upon the GNSS satellite clock phase (such as pseudorange).
Freq_sigma Uncertainty associated with the frequency bias estimate at the initial filter time. Used to initialize the covariance when the GNSS Constellation attribute to estimate clocks is set to true. If the GNSS Constellation attribute to estimate clocks is set to false, then this uncertainty is applied in the deweighting of measurements dependent upon the GNSS satellite clock frequency (such as carrier phase).
Aging_sigma Uncertainty associated with the aging bias estimate at the initial filter time. Used to initialize the covariance when the GNSS Constellation attribute to estimate clocks is set to true.
PhaseFreq_correlation
PhaseAging_correlation
FreqAging_correlation
Cross-correlations among the above attributes.

In the case of GNSS Satellites, the orbit uncertainty is used to initialize the covariance when the GNSS Constellation attribute to estimate orbits is set to true. If the GNSS Constellation attribute to estimate orbits is set to false, then this uncertainty is applied in the deweighting of measurements dependent upon the GNSS satellite position (such as pseudorange and carrier phase).

Partition Controls

When the PartitionControls.SeparatePartition attribute is set to true, the current SV is partitioned from the remaining SVs. Measurements involving this SV are used in estimating the orbit and clock of this SV only and will not affect the estimation of any other state parameter. However the state and covariance estimates for the monitor stations (clock, troposphere, location, etc.) are used in the modeling of the SV measurements. This partitioning applies only to processing of non-differenced range and phase measurements. It is ignored in the processing of single-differenced or double-differenced measurements.

Note: This capability supports taking one or more satellites "offline" in the GNSS MCS application. The use case is as follows: The GNSS constellation has a "anomalous" satellite (e.g. the satellite is in a clock swap recovery period). The satellite is to be kept "separate" from all the others, but it is desirable to have the filter take advantage of the estimates for monitor station clocks and tropospheric refraction as determined by the remaining "healthy" constellation.

Properties Shared with Other Satellites

The following categories of properties are available for satellites generally, including GNSS satellites:

* Some properties in this category are not available for GNSS satellites. See the linked page.

Adding & Removing GNSS Satellites

When a GNSSConstellation object is selected in the Object Browser, a GNSS Constellation menu appears in the main menu bar. That menu provides options that let you add and remove GNSS satellites:

Adding GNSS Satellites

Select Add GNSS Satellites from the GNSS Constellation menu to add GNSSSatellite objects as children of the selected GNSSConstellation object. Note the following:

  1. This item requires that the GNSSConstellation object be selected, that there are currently no GNSSSatellites objects defined under the GNSSConstellation object, and that a valid SVCatalog.Filename and Source.Filename have been entered.
  2. GNSS Satellite Objects will be created for all PRNs defined in the GNSS Source file.
  3. Note: To add just a single satellite, simply select one from the Object Catalog.

  4. The SVEstimatedStates.InitialStateEpoch time is used to define the state epoch, and the PRNs in the Source file are matched against the GNSS Catalog SV Launch/Decommission times to determine the SV number applicable to this PRN. SV Catalog parameters for this SV are then used as defaults for the GNSSSatellite object. If no PRN/SV match can be made in the Catalog, then no GNSSSatellite object for that PRN is generated.
  5. Orbit and Clock Initial State Estimates at the InitialStateEpochTime are set from the Source file.
  6. Orbit and Clock Initial State Covariance are set either from the GNSSConstellation object or from the SV Catalog, depending on the SVDefaultCovSource setting.
  7. Orbit, SRP, and Clock estimate flags are set from the SVEstimatedStates settings.

Removing GNSS Satellites

Select Remove GNSS Satellites from the GNSS Constellation menu to delete all GNSSSatellite objects from the selected, parent GNSSConstellation object.

Note: If you remove GNSS satellites in this way, none of the changes you have made in them will be saved.

ODTK 6.5