Globalstar Product Management – Technical Support

Are Simplex Messages Secure?

Question:  What technical features make the interception of a Globalstar simplex message difficult? What about the level of security during a handover?

Answer:   Globalstar’s simplex system (Remote Telemetry Service) is based on the robustness of Code Division Multiple Access (CDMA), an implementation of direct sequence spread spectrum techniques. Simplex data communications use special codes to spread the low rate communications over a 2.5 MHz frequency band. In order to receive this information, the receiver must know which codes to use to “de-spread” the communications. Intercepting this data is considered to be extremely difficult, not only because the data is spread using an unknown code, but also because the power level of the message is very low, well below the level of ambient electro-magnetic noise and is embedded in multiple messages sharing the same frequency channel. This provides low probability of detection (LPD), since the received power level is well below noise. The low power data being spread with unknown code provides additional low probability of intercept (LPI). The burst duration for the message is only 1.44 seconds long which makes it difficult for the receiver to synchronize with the message which has been spread with unknown code. Also, in this asynchronous system, it is difficult to determine when the user actually transmitted the data.The messages appear as random, short duration bursts with the signal levels well below the noise. These Low-Probability-of-Intercept (LPI) and Low-Probability-of-Detection(LPD) features which are inherent with the CDMA waveform provide over-the-air security between the Simplex Terminal and the Gateway.

There are no hard or soft hand-offs in the Globalstar simplex system. The user transmits asynchronously without having any information about the Globalstar constellation. There is no additional signaling information exchange between the terminal and the Gateway to assist handoffs.

It is feasible for subscribers requiring additional guaranteed privacy to make use of external encryption devices to encrypt the messages to send over the air.

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Written by Joseph Crowley

February 9, 2009 at 5:37 pm

Posted in Uncategorized

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