SATCOM ON THE MOVE (SOTM)
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INTRODUCTION
Today there is greater and greater demand for communication anywhere at anytime, in commercial multi-media requirements and military intelligence, surveillance and reconnaissance communication requirements. The need to maintain persistent mobile communications regardless of location in the world has driven significant growth in Satellite Communications (SATCOM) and particularly for SATCOM on-the-move (SOTM) products and service offerings.
SATCOM is the use of a network of geostationary satellites for the relay of radio communications between a transmitter and receiver located at two different points on the earth. The key challenge for any SATCOM system is maintaining the line-of-sight (LOS) connection with the target satellite. Fortunately, there is no need to track the target satellite, as its orbit is in sync with the earth's rotation and appears fixed in the sky. However, to maximize the number of satellites in geostationary orbit, these satellites are placed very close together with adjacent satellites typically located within a few degrees of each other. Due to this, high-accuracy pointing is required from a SATCOM terminal to send and receive large amounts of data and prevent interference on neighboring satellites. In fact, in order to maximize the transmission power of the antenna and remain within regulatory requirements as stated in the FCC Vehicle-Mounted Earth Station (VMES) rules, the antenna pointing error must be less than 0.2° (3𝞂), as illustrated in Figure 1.
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Figure 1: Antenna Pointing Error
SOTM terminals for ground vehicles typically feature a satellite antenna mounted to an elevation over azimuth (el/az) gimbal to rotate and pitch the antenna to the correct orientation. In order to determine the pointing vector, the terminal requires data from a GNSS receiver and inertial sensors to measure the location and speed of the vehicle as well as the orientation of the gimbal. With data from all of these sources, the control system in the terminal is able to determine the required pointing vector and correctly orient the SATCOM antenna.
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Figure 2: Satcome Terminal Wireframe
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Figure 3: Satcom Terminal- Simplified Illustration
Sensor Category Selection
We have already established that we require position, velocity and attitude data in order to point our antenna, the question then becomes which GNSS/INS solution should be used, a single-antenna system or a dual-antenna system. SATCOM systems primarily get used on ground vehicles or large vessels, such as marine vessels or large aircraft, which typically do not have sufficient motion that is required to sustain dynamic alignment in a single-antenna GNSS/INS. Due to this, a dual-antenna GNSS/INS system is the best solution for this application as it utilizes a GNSS Compass to obtain an accurate heading estimate during static or low dynamic situations.
Utilization of Inertial Sensor
There are three approaches that can used to obtain a precise navigation and pointing solution utilizing a GNSS/INS, as shown in Figure 1 and described in-depth here.
- Inner-Axis Only - placing the INS on the inner axis
- Gimbal Base with Encoders
- Two GNSS/INS
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| Figure 4: Approach 1 | Figure 5: Approach 2 | Figure 6: Approach 3 |
Approach 1 is the simplest, providing a direct measurement of the pointing in question while also providing feedback for gyro stabilization, however can be only applied to a limited number of applications. The most common approach in SOTM applications is to use approach 2, the GNSS/INS in the base of the gimbal providing absolute position and orientation of the base, with encoders providing the relative orientation between the base and the inner-axis. This approach works well but has two main limitations due to the encoders. Approach 3 provides the benefits of both Approach 1 and 2 and will result in the best possible performance for this application.
GIMBAL STABILIZATION AND POINTING APPLICATION NOTE
To learn more about the three approaches please download our Gimbal Stabilization and Pointing Application Note.
Error Budget
Besides the navigation system itself, there are several other components of the error budget in a gimballed application that must be accounted for:
- Misalignments between the camera and the GNSS/INS. This is typically calibrated on a part-by-part basis during manufacturing.
- Encoder errors & gimbal misalignments. As discussed earlier, this only impacts Approach B, but in that case they can be significant.
- Timing errors. Especially in high-dynamic applications (eg. airborne), time synchronization between the GNSS/INS and the pointing payload (eg. SATCOM antenna) is critical for accurate absolute pointing. This is typically done through a sync signal either generated by the GNSS/INS and received by the gimbal controller (eg. GNSS PPS), or vice versa (eg. sync trigger).
- Control errors. For target tracking applications with a small FOV sensor, keeping eyes on target requires both accurate navigation and accurate control
Let's take a quick look at Timing errors
The pointing accuracy of the controller will depend upon many different factors, including what type of motors are used to control the gimbal, how the controller is designed, the rigidity of the SATCOM system, the amount of backlash present in the system, and the time latency of the dual-antenna GNSS/INS system. While many of these elements are chosen at the discretion of the system designer, the timing latency is dictated by the navigation system and will play a major role in the pointing error contribution due to controls.
Timing errors in a SATCOM system are caused by an offset in time between when a measurement from the navigation system occurs and when the controller in the SATCOM terminal actually receives this Measurement.
A sample gimbal pointing trajectory for a SOTM terminal has been reconstructed in Figure 3 to illustrate the implications that timing errors can have in a SATCOM system. In this figure, the true pointing angle is plotted with the pointing angle derived from the delayed measurements alone as well as with the pointing angle estimated using the angular rate propagation. Note the time offset in the delayed measurement due to latency in the dual-antenna GNSS/INS system and in the SATCOM controller. This figure also reveals the dramatic impact of including the propagation of the angular rate into the SATCOM controller, as the estimated gimbal pointing trajectory aligns almost perfectly with the true pointing angle of the gimbal.
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| Figure 7: Pointing error impact of delayed measurements |
SATCOM GIMBAL APPLICATION NOTE
To learn more about sensor selection and implementation for SATCOM applications take a look at our in-depth application note.
Conclusion
Since SATCOM applications require high-accuracy attitude data, a standalone IMU or AHRS will not suffice and thus a GNSS-aided INS solution is required. Furthermore, because SATCOM applications are primarily used on ground vehicles or large, slow moving vessels, a dual-antenna GNSS/INS system will provide the best solution. When integrating this navigation system into the SATCOM terminal, the sensor should be mounted on the inner axis of the el/az gimbal to obtain a direct measurement of the gimbal's pointing
In calculating the error budget for the gimbal pointing solution, there a few different error sources that need to be taken into account to capture the maximum pointing error possible in the system, including errors from the dual-antenna GNSS/INS, misalignment errors, and timing and control errors. If you would like to learn more about the recommended solution for your particular application as well as the impacts that various design choices may have, please contact VectorNav for a more in-depth discussion.
HIGH PRECISION ANTENNA POINTING
VectorNav’s VN-200 GPS-Aided Inertial Navigation System (GPS/INS) provides critical, high accuracy, low latency position and attitude data to enable SkyTech Research to maintain pointing accuracy for SATCOM On-The-Move (SOTM) application.
VECTORNAV SUPPORT LIBRARY
Everything you need to know about inertial navigation