External u-blox GNSS

Doc for Ellipse firmware ≥ 3.0

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This brief document guides you in the process of configuring an external Ublox GNSS receiver for your Ellipse INS products.

Use this document in complement of the Operating Handbooks.

Step 1: GNSS and Ellipse connections

1. Connect GNSS Tx signal(s) pin to the Rx pin on one of the following Ellipse connectors: PORT A, B, C, D or E. Please also connect Ellipse and GNSS ground signals to each other.

2. Connect GNSS PPS signal pin to Sync A, B, C or D input pin.

Step 2: GNSS module configuration

Basic operation - single antenna

Configure the following outputs and output rates on your GNSS receiver:

• UBX-NAV-PVT @ 5 Hz
• UBX-NAV-HPPOSLLH @ 5 Hz
• UBX-NAV-SIG @ 1Hz

Basic operation - dual antenna

RTK is not compatible with external u-blox operating in dual antenna mode.

The two external u-blox receivers should be configured to 921600 bps, with the following outputs:

The table values are configuration settings in the form of divisors, not the actual frequency. For example, setting UBX_NAV2_PVT to 1 means an output frequency of 5 Hz.

PPS signal

For proper operation, a PPS signal must be provided at 1 Hz.

Adding post-processing capability

The following additional message configuration is required for post-processing.

• UBX-RXM-RAWX @ 1Hz

Step 3: Ellipse configuration

In order to configure the Ellipse-E, you need to use the sbgCenter and open the configuration window. Simply follow those instructions:

Set aiding assignment

In this window, you just indicate where you connected your GNSS receiver.

Both communication port and Sync In pin must be set.

Set correct baud rate and mode for serial port

In our example we configured the GNSS to be connected on PORT C in RS‑232 mode using a baudrate of 460800 bauds.

Set logic input configuration for PPS signal

In order to use correctly PPS signal information, you must enable the corresponding logic input. Here we configured PPS on Sync C.

Polarity should be set accordingly with the actual PPS signal (rising edge or falling edge).

Set correct GNSS model and configuration

1. GNSS model should be set to ublox (Passive).

2. GNSS lever arms should be measured accurately FROM the IMU TO the antenna phase center (APC), in the vehicle frame. For RTK applications, the Primary antenna lever arms must be measured with an accuracy of within 1 cm. A larger tolerance may be acceptable when operating in single point positioning mode.

3. In case of Dual antenna system, the secondary antenna lever arm must also be entered FROM the IMU TO the antenna phase center (APC), and the same accuracy requirements as for the primary antenna apply.

4. Finally, each available measurement (position and heading if available i.e. if using a dual antenna receiver) should be configured to be used or not. Automatic is recommended.

Check clock alignment

Finally, you check that the time synchronization reference is set to GPS 1 (default configuration).

Step 4: Checking status

The status and GNSS windows should be checked carefully before going further. These status indicators will give essential hints in case of troubles to get a correct fix. Each step is labeled in red in the following screenshot.

1. Corresponding COM port must be OK.

2. GNSS 1 frame in “Aiding Inputs” tab must show that the data is received. Not seeing this would probably imply that there is a baudrate or wiring issue.

3. After that, you can check if the GNSS solution has been calculated and is consistent.

4. Then you can check the "Clock" section. Input clock must be OK and UTC time should be set to valid after a few minutes in steering mode.