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A new anti-jam device is described based on substitution of a more efficient automatic gain control (AGC) function into the front end of a transponder. Smart AGC discriminates between jammed and unjammed signals based solely on amplitude of the incoming signals. It is a broadband device which does not frequency-distort the transponder's passband, nor require modification of existing ground station modulation formats. In the absence of large interference signals, Smart AGC will revert back to a linear input-output relationship and the transponder will appear to be an ordinary non-regenerative repeater.The key to operation of Smart AGC is a broadband power measurement of the composite input signal which is then used to control a nonlinear amplifier/limiter characteristic, preventing the satellite HPA from saturating. Calculations and simulation results show potentially large improvements in output Jammer-to-Signal (J/S) ratio. Where a conventional MILSAT transponder under jamming suffers 6dB small signal suppression and downlink power robbing, a protected transponder can have small sianal enhancement relative to the jammer by tens of dB. These improvements could be traded off for reduced dependence on spread spectrum bandwidth, reduced need for high up-link e.i.r.p., increased data rates, and reduced dependence on techniques such as antenna beam steering and nulling.1) Creation of intermodulation distortion falling inband with wanted signals.2) Power suppression of wanted signals by unwanted signals.3) Transponder capture which shifts most of the transponder's power over to the largest signals present. 4) Large signal-to-noise ratio (SNR) reduction seen at the demodulator.The first occurs in any non-regenerative transponder operating with multiple inputs such as FDMA. Every satellite, commercial or military, will experience a signal to noise ratio reduction when jammed, and unless the satellite has sophisticated on-board demodulation equipment, which very few have, they also experience the second and third effects listed above. Smart AGC has the potential to mitigate all of these harmful effects, making it particularly beneficial in the following applications:--As an applique upgrade to an existing MILSATCOM family whose networks currently employ anti-jam techniques.--Protection of ?T&C systems.--Creation of dual-purpose military/commercial transponder packages transparent to existing user group signal designs.--Protection of transponders used in high natural or man-made unintentional interference.--Automatic, on-board optimization of transponder power sharing in Frequency Division Multiple Access (FDMA) transponders.The paper describes the basics of Smart AGC operation, and presents results obtained with a simulator developed by Comsat Systems Division. Implications for improved performance of future UHF and X-Band satellites are discussed.The trade offs which benefit future MILSATCOM systems will be reduced dependence on spread spectrum bandwidth, antenna nulling, high Power transmitters, and lower data rates, as...

This study verified the capability of a new anti-jam approach (proprietary trademark name: Smart AGCY that could be used for. MILSATCOM transponders; it assessed performance in controlled jamming scenarios, and surveyed the capability of current technology to support near-term implementation. Smart AGCTM employs time-dependent, RF "center stripping", the size of which is determined by a fast measurement of the jammer's amplitude. Results with constant envelope (CE) and slowly varying jamming confirm that the output jammer-plus-intermod-to-noise ratio, (J+IM)/S, can be reduced to several dB above the combination of background noise level and wanted signal level. With Smart AGCW, a pure unmodulated carrier (CW) jammer at the center of the band would no longer be the optimum jammer for direct sequence spread spectrum modulation (DSSS), nor could a CE signal produce 6dB small signal suppression, nor could it capture the transponder's EIRP. Smart AGCm does not employ radically new technology. RF delay, on the order of a few hundred nanoseconds, needed to compensate for delay in the tracker filters, is not new technology for spacecraft. The center stripping function, while new, employs variable back-biased diodes or the equivalent (such as GaAs MESFETs), but these have been deployed in transponders before. The use of a wide dynamic range linear driver amplifier is the same technology used in current hard limiting transponders. 1 INTRODUCIION OBJECrrVE OF STUDY Beginning in 1989, Comsat implemented a new technique for nonlinear analysis called the Z M Microscope [l] which simplifies solutions for intermodulation distortion and power division in piece-wise linear, time-varying nonlinearities. It was essential to analyzing the capabilities of Smart AGCW. Preliminary analytical results were published in MILCOM '91 [2] and results of the R&D investigation were presented at the AIAA conference in Washington, D.C, March 1992 131.The basic Smart AGCm concept, described is shown in Figure 1. In response to a measurement on the envelope, the null zone is activated. Th of this RF nonlinearity is to strip the center fr input signal, as the inset shows for a simplified case: one large and one small sine wave.The objective of this study was to verify AJ capability through simulation tests of the output (J + IM)/S performance vs. input J/S of Smart AGCTM under controlled jamming scenarios including the following: -CONSTANT ENVELOPE: CW AND PHASE MODULATED -TWO SINE WAVES ("BEAT FREQUEN -ADDITIVE WHITE GAUSSIAN NOISE (A In the course of the study, sev alvorithms were investigated including: -ORIGINAL TRACKER (Figure 1 and 4) -RMS TRACKER (Figure 6) -LOW PASS AND BANDPASS -ABOVE, WITH "BYPASS Not all combinatio suppression provided the in about 6 dB [41. For am a hard limiting transponder. Thus for example two CWs having an average J/S of 20 dB, if converted to a CW with the same average power, would have a benchmark performance of (J + IM)/S of 26dB. CHARACTERISTICS OF THE SIMULATION, THE TRACKERS, AND TEST SIGNALS SIMULATION A N D TE...