System Start-Up and Alignment Techniques

The SPAN system requires an initial position, velocity and attitude estimate to start the navigation filter. This is called system alignment. On start-up, the system has no position, velocity or attitude information. When the system is first powered up, the following sequence of events happens:

  1. The first satellites are tracked and coarse time is solved.

  2. Enough satellites are tracked to compute a position.

  3. Receiver “fine time” is solved, meaning the time on board the receiver is accurate enough to begin timing IMU measurements.

  4. The INS Status field changes from INS_INACTIVE through DETERMINING_ORIENTATION and WAITING_INITIALPOS.

  5. Once a GNSS position is available, an initial IMU bias estimate is conducted. During this period, the INS Status field reports INITIALIZING_BIASES.

  6. After the bias estimate is complete, the coarse alignment routine starts. Bias estimates will be refined and a static coarse alignment completed, if IMU quality allows. During this period the INS Status field reports INS_ALIGNING.

  7. If static coarse alignment is not possible or is locked out by command, the system will require an external azimuth input, either from vehicle motion, dual antenna input or command input. While in this state the INS Status field reports WAITING_AZIMUTH.

    If vehicle movement is detected, any ongoing static coarse alignment will be reset, and the INS Status field will report MOTION_DETECT.

  8. After an alignment is completed, the INS Status field changes to INS_ALIGNMENT_COMPLETE. The system transitions to Navigation Mode.

  9. The solution is refined using updates from GNSS. Once the system starts estimating the IMU errors after some vehicle movement, the INS Status field will change to INS_SOLUTION_GOOD indicating INS solution has achieved a basic level of convergence. Note that it is advised to continue converging dynamics until the convergence flag is set to 1 in the Extended Solution Status word (see Table: Extended Solution Status). This indicates that the INS filter has successfully converged to within performance specifications.

SPAN Alignment Methods

The following sections describe the different alignment routines available. By default, the first available method to align is used. Options for configuring a specific alignment methods are outlined below.

Static Coarse Alignment

The static coarse alignment is the default alignment routine for SPAN. The alignment starts automatically when a GNSS solution is available, the receiver has computed fine time, the IMU is connected and configured, and initial bias estimates have been completed. During the static coarse alignment, accelerometer and gyro measurements are averaged over a period of time to measure Earth rotation and gravity. From these averaged measurements, initial estimates of roll, pitch and heading are computed. Because the static coarse alignment uses averaged sensor output, the vehicle must remain stationary for the duration of the alignment, which is approximately 45 seconds. The attitude estimates solved by the alignment are larger than the system specified attitude accuracy and vary upon the characteristics of the sensor and the geographic latitude of the system. Attitude accuracy converges with motion after the coarse alignment is complete (see Navigation Mode).

All Grade 1 IMUs, as well as the HG4930 400Hz variant, and products that use these IMUs, cannot perform static coarse alignments, as these IMUs cannot accurately measure Earth rotation (see IMU Grade. For these IMUs, the default alignment routine is kinematic alignment. Refer to Kinematic Alignment. If a stationary alignment is required, refer to options for Dual Antenna Alignment, NVM Seeded Alignment, or Manual Alignment.

Static Coarse alignments may be degraded due to environment, vibration or IMU age effects. Any error in initial alignment can cause performance issues.

Kinematic Alignment

An alternate form of aligning the SPAN system is a kinematic alignment. A kinematic alignment can be used for any SPAN system and is the best alignment alternative for lower performance sensors (Grade 1 IMUs, as well as the HG4930 400Hz variant, and products that use these IMUs). The kinematic or moving alignment is performed by estimating the heading from the GNSS velocity vector and injecting it into the SPAN filter as the initial system heading.

For the kinematic alignment routine to work optimally, the course-over-ground azimuth must not include any bias from the defined Vehicle Frame. (For example, a plane being blown in the wind has a large ‘crab angle’ and the course-over ground trajectory will not match the direction the IMU is pointing.)

The rotation from the IMU Body frame to the Vehicle frame (RBV) must be set to enable kinematic alignment. In order to simplify this configuration it is strongly suggested that you mount the IMU in parallel to the vehicle frame. The Y axis marked on the IMU enclosure, should point in the direction of travel.

When using the default RBV configuration, the X-axis of the CPT7 should point in the direction of travel.

The SMART7-S simplifies the alignment configuration by setting the default translation and rotation values. To make use of these defaults ensure the SMART7-S is mounted in the right orientation. The SMART7-S should be oriented as shown in Figure: SMART7 Orientation.

The kinematic alignment begins when the receiver has a good GNSS position, fine time is solved, the Body to Vehicle frame rotation is known and a GNSS velocity of at least 5 m/s (~ 18 km/h) is observed (0.5 m/s for SMART7-S). For optimal performance, it is recommended to keep the vehicle fairly level during kinematic alignment.

5 m/s (~ 18 km/h) (0.5 m/s for SMART7-S) is the default alignment velocity. If a different alignment velocity is required, it can be changed using the SETALIGNMENTVEL command.

While it is possible to lower the minimum speed required for kinematic alignment to take place, the higher the value, the more closely the course-over-ground estimate will match the vehicle heading.

The accuracy of the initial attitude of the system following the kinematic alignment varies and depends on the dynamics of the vehicle. The attitude accuracy will converge to within specifications once some motion is observed by the system. This transition can be observed by monitoring the INS Status field in the INS logs.

Dual Antenna Alignment

SPAN can also use heading information available from a NovAtel Dual Antenna ALIGN solution to rapidly perform an alignment. Refer to SPAN with Dual Antenna for details.

NVM Seeded Alignment

SPAN can use the alignment information from a previous power up to quickly perform an alignment. For information about enabling and using an NVM Seeded Alignment, refer to INS Seed / Fast INS Initialization.

Manual Alignment

If the initial azimuth of the system is known, it can be entered manually using the SETINITAZIMUTH command.

A manual alignment should be used only if the azimuth is accurately known.

Configuring SPAN Alignment Method

The SPAN-enabled receiver can be configured for different alignment routines depending on the motion conditions experienced during the alignment period. For example, in marine applications, the dynamics required for either a static coarse or kinematic alignment cannot be guaranteed, so a different alignment routine will be required. This is achieved with the ALIGNMENTMODE command.

NVM Seeded alignment and Manual alignments are valid for all alignment modes and will supersede all other options if valid and available.

Automatic Alignment Mode (default)

The automatic alignment mode is the default setting for a SPAN-enabled receiver. This mode is designed to allow alignment of the system as quickly as possible, using any of the alignment methods (Static Coarse, Kinematic, NVM Seeded, Dual Antenna or Manual), regardless of its relative quality. No additional configuration is required to use this alignment routine. If a specific alignment technique is desired, it must be selected manually to guarantee that alignment method.

Unaided Alignment Mode

The unaided alignment mode sets the SPAN system to use only single antenna and superseding alignment options (Static Coarse, Kinematic, NVM Seeded or Manual alignment).

To use this alignment mode, the ALIGNMENTMODE command must be sent to the receiver.

ALIGNMENTMODE UNAIDED

Aided Transfer Alignment Mode

The aided transfer alignment mode sets the SPAN system to use only dual antenna and superseding alignment options (Dual Antenna, NVM Seeded or Manual alignment).

This alignment routine is the preferred dual antenna alignment method.

If the vehicle is not stationary during the alignment, such as may be the case on a ship, use the Aided Transfer Alignment routine. This alignment method uses the ALIGN baseline solution to perform an instantaneous alignment of the vehicle azimuth.

The alignment happens instantaneously after the receiver establishes communication with the IMU and computes a verified, fixed integer, ALIGN solution.

To guarantee the use of this alignment mode, the ALIGNMENTMODE command must be sent to the receiver:

ALIGNMENTMODE AIDED_TRANSFER

Static Coarse Alignment Mode

The static coarse alignment mode sets the SPAN system to use only static coarse and superseding alignment options (Static Coarse, NVM Seeded or Manual alignment).

To guarantee the use of this alignment mode, the ALIGNMENTMODE command be sent to the receiver:

ALIGNMENTMODE STATIC

Kinematic Alignment Mode

The kinematic alignment mode sets the SPAN system to use only kinematic and superseding alignment options (Kinematic, NVM Seeded or Manual alignment).

To guarantee the use of this alignment mode, the ALIGNMENTMODE command must be sent to the receiver:

ALIGNMENTMODE KINEMATIC