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Innovation: A Ground Station in the Lab—Test SATCOM Solutions and Applications Efficiently
by Lars Christensen, Chief Software Architect, GateHouse Telecom

 

Test and validation are crucial to ensure robust satellite communications equipment, including user terminals, applications, and solutions, are viable. 

However, traditional on-air test activities come with a number of challenges. Access to SATCOM equipment may be limited due to equipment and air-time costs or service availability. SATCOM terminals cost thousands of dollars or more, especially within niche markets such as aeronautical SATCOM services.

In addition to the economic challenge, service availability may not be easily attainable. Testing activities are often desired a long time before the satellite communications service is launched, and even then, it may, or may not, be available at the specific physical location of the test engineers.

Testing activities have further requirements, such as having to set up the terminal with good line-of-sight to the communications satellite and the routing of cables with power and connectivity to a convenient testing facility.

Finally, with access to the SATCOM service, and when the test equipment is set up, all are subject to the conditions of the network. Bandwidth may, or may not, be limited by a service plan and the amount of congestion in the network, depending on how many other users are accessing the same channels. These conditions make for an uncontrollable test environment — that can complicate or obstruct a planned test procedure.

To alleviate these challenges, attempts can be made to work around them by looking for options such as moving all, or part, of the testing effort into the lab. Simulation or emulation equipment can be used, similar to the evaluations occurring in the cellular world, where test equipment exists that provides some level of connectivity on the bench. A similar solution has now been developed by a small SATCOM software house for Inmarsat BGAN services.

SATCOM Satellite and Ground Station Emulator
When developing the first generation of user terminals for the Inmarsat BGAN satellite service, GateHouse Telecom A/S and the firm’s partners experienced, first-hand, SATCOM testing challenges. Access to the service was limited. Rigorous testing was time consuming and costly. A better solution was needed.

Leveraging already available RF and modem hardware, GateHouse Telecom was in a unique position to reuse the company’s mobile protocol stack IP to develop a comprehensive network emulator for the Inmarsat BGAN system.

The existing hardware platform was originally developed by Square Peg Communications Inc. to support physical layer and protocol stack conformance testing of BGAN user terminals as a key part of the terminal type approval process. The current generation of the modem hardware is a compact and fully integrated 4U unit supporting four uplink and downlink channel pairs — plenty of capacity to simulate multiple satellites and spot beams.

GateHouse developed the Network Emulator product which augments the Square Peg hardware with emulation of an Inmarsat BGAN ground station. Together, they enable end-to-end testing of solutions on the bench, directly in the lab.

The Network Emulator implements the Inmarsat air-interface protocol that allows a user terminal to log on to the system and activate services. Both IP services (standard data and streaming data) and circuit-switched services (voice and ISDN) are supported.

The current generation platform is the Application Tester for the Inmarsat BGAN service — combining the Square Peg Communications hardware with the GateHouse software into a fully integrated ground station in-a-box.

Toward the user terminal, the emulator is connected via a regular L-band RF interface over a coaxial cable. The appropriate broadcast information is sent to the terminal to simulate a full satellite constellation, allowing the terminal to acquire the signal and log on. A special Test-SIM card is used in the user terminal instead of the normal subscription SIM.

The network emulator is connected via a normal Ethernet cable. When the user terminal activates an IP service, data can be transported from the mobile side to any server accessible via this connection. Any IP-based application or solution can then be tested and validated.

Core Functionality of SATCOM Network Emulation
On the inside, the network emulator simulates a complete I4 satellite constellation. Given the position of the user terminal, it will turn on the signals from the global and regional beams of the satellites visible at that location. When a service is activated by the user via the terminal, the required narrow beam channels are activated.

A Well-Traveled Solution
Satellite service performance also often varies with location, giving different levels of service, depending on the elevation of the communications satellite.

Effects from handovers or satellite switches may affect the application. Such scenarios are quite challenging to reproduce in the real world, but with an emulator, this work becomes trivial.

The user terminal activates GPS to get a position resolution that is used to select the satellite and spot beam to use for the satellite service. Faking this position can be done by using a GPS signal generator, or by overriding the position via a debug interface. Changing the position makes the user terminal operate as if located anywhere on the Earth.

As the terminal reports the position to the emulator, the service level is adjusted accordingly. Depending on the pointing elevation of the antenna, adjustments are made based on the assigned link margins in each elevation band.

When the terminal crosses a spot beam boundary, handovers are performed. When it crosses the edge of coverage, satellites are turned off and on, allowing realistic simulation of switches between each satellite and ocean region.

Rainy Day Testing
The satellite service level may vary depending on network condition, number of users and even be subjected to environmental effects, such as weather fade. Such conditions are not under the user’s control, but in a controlled environment, they can be emulated.

An emulator can simulate degraded performance, error conditions, and user data congestion. This allows testing under a wider range of scenarios, fully under the user’s control.

The robustness of the application when facing a highly congested network can be validated, and the user can verify that the application is giving proper feedback or is performing the appropriate error handling when errors
do occur.

The Future
By bringing the ground station and all the other parts of a SATCOM network into the lab, the user can control the test environment. This enables a far more robust environment that results in better performing products far faster than such would have ever been possible by testing on the live network.

With a world that is constantly in search of ways to secure and guarantee connectivity and up-time, wherever and whenever, testing in the lab seems to have an important role to play in the future of satellite communications.
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