Patent Application
20100124922
The disclosure relates to systems and methods for satellite transmission obfuscation.
It is beneficial for operators of satellite communications to deny adversaries from eavesdropping on their transmissions. One way adversaries eavesdrop is by using a high gain antenna to receive a small amount of stray radio frequency radiation emitted from the side-lobes or back-lobes of satellite communication terminal antenna. The adversaries may then use a spectrum analyzer to monitor the transmit signal power spectral density over time in order to derive useful intelligence. For example, an adversary could associate an increase in power spectral density with greater activity on the terminal.
In one prior art system and method for attempting to deny adversaries from eavesdropping on satellite transmission, as provided in U.S. Pat. No. 7,136,621, code division multiple access (CDMA) signals are transmitted from the satellite communication terminal and intentionally spread in frequency using direct sequence spread spectrum modulation. This reduces the transmit power spectral density thereby lowering the probability of intercept and detection by an eavesdropper. However, if the eavesdropper gets sufficiently close to the transmitting satellite communication terminal, or if the eavesdropper uses a big enough antenna, it is possible to detect and monitor transmission activity on the terminal. The transmitted power and power spectral density may be directly proportional to the data rate of traffic transmitted from the terminal, so the eavesdropper may monitor terminal activity by measuring fluctuations in received power.
In another prior art system and method for attempting to deny adversaries from eavesdropping on satellite transmissions, the satellite communication terminal may be operated at a fixed transmit power and data rate which meets the peak power and data rate demand of the satellite communication terminal. Using this constant activity system and method, an eavesdropper may not detect change in terminal activity. However, this may be inefficient because the average demand for satellite communication terminal data rate is typically a small fraction of the peak demand constant rate.
Another prior art system and method for attempting to deny adversaries from eavesdropping on satellite transmissions in time division multiple access (TDMA) systems is to randomly assign time slots to terminals to randomize the duration and interval between transmission bursts. Without randomizing transmissions, an eavesdropper may determine peak periods of activity, identify unusual or unexpected activity spikes, and identify locations of remote terminals that have remained quiet for a period of time and suddenly experience increased traffic volumes. The adversary may subsequently extrapolate timing, location, and scale of the communications.
Another approach may use frequency division multiple access (FDMA), which may work similarly to the time division multiple access approach, except that the random assignment of unused satellite capacity may occur in the frequency domain rather than in the time domain. Alternatively the allocation may be to a fixed number of frequency slots with null data transmitted in the excess slots. However, this method may be inefficient due to the typically large difference between the actual data rate demand and the peak resource allocation, thereby reducing the useful aggregate data rate of the communication resource.
A system and method is needed that is both effective at obfuscating signal transmissions and efficient in use of satellite resources.
In one aspect of the disclosure, a method is disclosed for obfuscating satellite terminal transmission activity. In one step, a network may be provided comprising a satellite, distributed satellite terminals transmitting signals to and receiving signals from the satellite, and at least one computer in communication with the network. In another step, the computer may compute how much transmission power spectral density each of the distributed satellite terminals is or would be transmitting based on allocated data rates of each of the distributed satellite terminals. In an additional step, the computer may calculate an aggregate transmission power spectral density which is being used or would be used by all of the distributed satellite terminals by adding up the computed transmission power spectral densities which each of the distributed satellite terminals is or would be using based on the allocated data rates. In still another step, the computer may calculate how much unused transmission power spectral density is available to the network for obfuscation by subtracting the aggregate transmission power spectral density from a total network regulatory transmission power spectral density limit. In an additional step, if the calculating steps establish that there is unused transmission power spectral density available to the network, the computer may determine a random distribution of at least a portion of the unused transmission power spectral density to one or more of the distributed satellite terminals in order to provide obfuscation of terminal power transmission activity. In another step, transmission power spectral density levels of one or more of the distributed satellite terminals may be adjusted to distribute the portion of the unused transmission power spectral density according to the random distribution determination of the computer.
In another aspect of the disclosure, a network is provided for obfuscating satellite terminal transmission activity. The network may comprise: a satellite; a plurality of distributed satellite terminals transmitting signals to and receiving signals from the satellite at least one computer in communication with the network; and a computer program. The computer program may compute how much transmission power spectral density each of the distributed satellite terminals is or would be transmitting based on allocated data rates of each of the distributed satellite terminals. The computer program may calculate an aggregate transmission power spectral density that is being used or would be used by adding up the computed transmission power spectral densities that each of the distributed satellite terminals is or would be using. The computer program may calculate the unused transmission power spectral density available for obfuscation by subtracting the aggregate transmission power spectral density from a total network regulatory transmission power spectral density limit. The computer program may determine a random distribution of at least a portion of the unused transmission power spectral density to one or more of the distributed satellite terminals. The computer program may adjust the transmission power spectral density levels of one or more of the distributed satellite terminals to distribute the portion of the unused transmission power spectral density according to the random distribution determination.
These and other features, aspects and advantages of the disclosure will become better understood with reference to the following drawings, description and claims.
The following detailed description is of the best currently contemplated modes of carrying out the disclosure. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the disclosure, since the scope of the disclosure is best defined by the appended claims.
The computer program 18 may compute how much transmission power spectral density 24 each of the distributed satellite terminals 12 is or would be transmitting based on allocated data rates 26 of each of the distributed satellite terminals 12. The distributed satellite terminals 12 may all use spread spectrum modulation, and may all spread their transmission power spectral density 24 approximately evenly across a shared channel bandwidth. The shared channel bandwidth may remain constant as satellite terminal power levels 24 and data rates 26 change, and may result in satellite terminal transmission power spectral density that is approximately equal to the transmit power level divided by the channel bandwidth. The distributed satellite terminals 12 may share the channel bandwidth using the method of code division multiple access (CDMA).
The computer program 18 may adjust the transmission power spectral density levels 24 of one or more of the distributed satellite terminals 12 to distribute the portion of the unused transmission power spectral density 30 randomly to one or more of the distributed satellite terminals 12 according to the random distribution determination 34. In one embodiment, the computer program 18 may adjust the transmission power spectral density levels 24 of one or more of the distributed satellite terminals 12 to distribute all of the unused transmission power spectral density 30 according to the random distribution determination 34 in order to obfuscate the network 10. If the computer program 18 determines that there is no unused transmission power spectral density 30 available to the network 10, the computer program 18 may not determine a random distribution 34 of unused transmission power spectral density 30, and may instead reduce the transmission power spectral density levels 24 of the distributed satellite terminals 12 in order to avoid exceeding the total network regulatory transmission power spectral density limit 32.
The computer program 18 may continuously adjust in real-time for changes in the transmission power spectral density 24 and the data rates 26 of each of the distributed satellite terminals 12. In one embodiment, the computer program 18 may not adjust the allocated data rates 26 of the distributed satellite terminals 12 as the computer program 18 adjusts the transmission power spectral density levels 24 of the one or more distributed satellite terminals 12. In another embodiment, the computer program 18 may adjust the allocated data rates 26 of the distributed satellite terminals 12 by inserting non-information carrying bits into the data stream. These data rate 26 adjustments may be made to be proportional to the adjustments the computer program 18 makes to the transmission power spectral density levels 24 of the distributed satellite terminals 12.
In still another embodiment, if one or more of the distributed satellite terminals 12 requests from the network 10 a data rate 26 adjustment, the computer program 18 may determine if there would be unused transmission power spectral density 30 available to the network 10 if the network 10 accommodated the data rate 26 adjustment. If the computer program 18 determines that there would not be unused transmission power spectral density 30 available to the network 10 if the network 10 accommodated the data rate 26 adjustment, the network 10 may not allow the requested data rate 26 adjustment. If the computer program 18 determines that there would be unused transmission power spectral density 30 available to the network 10 if the network 10 accommodated the data rate 26 adjustment, the computer program 18 may allow the requested data rate 26 adjustment, and the computer program 18 may determine a random distribution 34 of at least a portion of the unused transmission power spectral density 30 at the allowed, requested data rate 26 adjustment. The network 10 may adjust the transmission power spectral density levels 24 of one or more of the distributed satellite terminals 12 to distribute the portion of the unused transmission power spectral density 30 according to the random distribution determination 34 at the allowed, requested data rate 26 adjustment.
In an additional embodiment, if the computer program 18 determines that there would be unused transmission power spectral density 30 available to the network 10 if the network 10 accommodated the data rate 26 adjustment, the computer program 18 may allow the requested data rate 26 adjustment for the one or more distributed satellite terminals 12 which requested the adjustment, but may not adjust the allocated data rates 26 of the remaining distributed satellite terminals 12 which did not request the adjustment. In another embodiment, if the computer program 18 determines that there would be unused transmission power spectral density 30 available to the network 10 if the network 10 accommodated the data rate 26 adjustment, the computer program 18 may allow the requested data rate 26 adjustment for the one or more distributed satellite terminals 12 which requested the adjustment, and may also adjust the allocated data rates 26 of the remaining distributed satellite terminals 12 which did not request the adjustment by inserting non-information carrying bits into the stream to increase satellite transmission data rates. These adjustments may be made to be proportional to the adjustments which were made to the transmission power spectral density levels 24 of the distributed satellite terminals 12.
In another step 144, the computer 16 may compute how much transmission power spectral density 24 each of the distributed satellite terminals 12 is or would be transmitting based on allocated data rates 26 of each of the distributed satellite terminals 12. In an additional step 146, the computer 16 may calculate an aggregate transmission power spectral density 28 which is being used or would be used by all of the distributed satellite terminals 12 by adding up the computed transmission power spectral densities 24 which each of the distributed satellite terminals 12 is or would be using based on the allocated data rates 26. In still another step 148, the computer 16 may calculate how much unused transmission power spectral density 30 is available to the network 10 for obfuscation by subtracting from a total network regulatory transmission power spectral density limit 32 the aggregate transmission power spectral density 28.
In another step 150, if the calculating steps 146-148 establish that there is unused transmission power spectral density 30 available to the network 10, the computer 16 may determine a random distribution 34, using an algorithm 27, of at least a portion of the unused transmission power spectral density 30 to one or more of the distributed satellite terminals 12 in order to provide obfuscation of terminal power transmission activity. During step ISO, the computer 16 may additionally adjust the transmission power spectral density levels 24 of one or more of the distributed satellite terminals 12 to distribute the portion of the unused transmission power spectral density 30 according to the random distribution determination 34. In one embodiment, step 150 may comprise determining a random distribution 34 of all of the unused transmission power spectral density 30, and setting the transmission power spectral density levels 24 of one or more of the distributed satellite terminals 12 to distribute all of the unused transmission power spectral density 30 according to the random distribution determination 34.
If the computer program 18 determines that there is no unused transmission power spectral density 30 available to the network 10, in step 152, the computer program 18 may not determine a random distribution 34 of unused transmission power spectral density 30 and may not adjust the transmission power spectral density levels 24 of the distributed satellite terminals 12 in order to avoid exceeding the total network regulatory transmission power spectral density limit 32.
In one embodiment, if the calculating steps 146-148 establish that there is unused transmission power spectral density 30 available to the network 10, in optional step 154, the allocated data rates 26 of the distributed satellite terminals 12 may not be adjusted when the transmission power spectral density levels 24 of the one or more distributed satellite terminals 12 are adjusted to obfuscate satellite terminal transmission activity on the network 10. In another embodiment, if the calculating steps 146-148 establish that there is unused transmission power spectral density 30 available to the network 10, in optional step 156, the allocated data rates 26 of the distributed satellite terminals 12 may be adjusted, to be proportional to the adjustments made to the transmission power spectral density levels 24 of the distributed satellite terminals 12, by inserting non-information carrying bits into their data streams.
In step 268, if the computer 16 determined that there would be unused transmission power spectral density 30 available to the network 10 if the network 10 accommodated the data rate 26 adjustment, the network 10 may do the following: allow the requested data rate 26 adjustment; follow step 150 of method 140 of
In optional step 270, if the computer 16 determined that there would be unused transmission power spectral density 30 available to the network 10 if the network 10 accommodated the data rate 26 adjustment, and the network 10 subsequently allowed the requested data rate 26 adjustment to the requesting satellite terminal 12, the network 10 may not further adjust the allocated data 26 rates of the remaining non-requesting distributed satellite terminals 12 when the transmission power spectral density levels 24 of the distributed satellite terminals 12 are/were adjusted to obfuscate satellite terminal transmission activity on the network 10.
In optional step 272, if the computer 16 determined that there would be unused transmission power spectral density 30 available to the network 10 if the network 10 accommodated the data rate 26 adjustment, and the network 10 subsequently allowed the requested data rate 26 adjustment to the requesting satellite terminal 12, the network 10 may also adjust the allocated data rates 26 of the non-requesting distributed satellite terminals 12 by inserting non-information carrying bits into the bit stream to increase satellite terminal transmission data rates. These adjustments may be proportional to the adjustments made to the transmission power spectral density levels 24 of the distributed satellite terminals 12, when the transmission power spectral density levels 24 of the one or more distributed satellite terminals 12 are/were adjusted to obfuscate the satellite terminal transmission activity on the network 10.
One or more embodiments of the disclosure may reduce one or more problems of one or more of the prior art systems and methods by randomizing the transmission power/activity spectral density of code division multiple access satellite communication terminals (which are discussed in U.S. Pat. No. 7,136,621) by varying the transmit power to the terminals, using unused transmission power spectral density, while maintaining the aggregate transmission power spectral density of the network to levels that are at or below established regulatory limits. This may allow for communications to be obfuscated on code division multiple access satellites without degrading quality of service, while efficiently managing satellite communication resources by distributing the resources to the satellite terminals that need it, taking it away from those that don't, and randomly allocating the excess resources to the terminals in order to obfuscate the satellite communications.
It should be understood, of course, that the foregoing relates to exemplary embodiments of the disclosure and that modifications may be made without departing from the spirit and scope of the disclosure as set forth in the following claims.
