Source: http://www.google.com/patents/US20020058478?ie=ISO-8859-1&dq=6,952,563
Timestamp: 2014-03-17 19:44:01
Document Index: 53963475

Matched Legal Cases: ['ARTS 134', 'ARTS 134', 'ARTS 134', 'ARTS 134', 'ARTS 134', 'ARTS 134', 'ARTS 134', 'ARTS 134', 'ARTS 134', 'ARTS 134', 'ARTS 134', 'ARTS 134', 'ARTS 134', 'ARTS 134', 'ARTS 134', 'ARTS 134']

Patent US20020058478 - Return link design for PSD limited mobile satellite communication systems - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsA system and method for managing access to a satellite-based transponder by a plurality of aircraft each having a mobile radio frequency (RF) system. The system employs a ground-based, central control system for managing access to the satellite-based transponder so that the aggregate power spectral density...http://www.google.com/patents/US20020058478?utm_source=gb-gplus-sharePatent US20020058478 - Return link design for PSD limited mobile satellite communication systemsAdvanced Patent SearchPublication numberUS20020058478 A1Publication typeApplicationApplication numberUS 09/884,555Publication dateMay 16, 2002Filing dateJun 19, 2001Priority dateSep 28, 2000Also published asCN1640017A, CN100440755C, DE60126792D1, DE60126792T2, DE60126792T3, EP1320948A1, EP1320948B1, EP1320948B2, EP1772976A2, EP1772976A3, EP1772976B1, US7054593, US7136621, US7630683, US20060040614, US20070026795, WO2002027975A1Publication number09884555, 884555, US 2002/0058478 A1, US 2002/058478 A1, US 20020058478 A1, US 20020058478A1, US 2002058478 A1, US 2002058478A1, US-A1-20020058478, US-A1-2002058478, US2002/0058478A1, US2002/058478A1, US20020058478 A1, US20020058478A1, US2002058478 A1, US2002058478A1InventorsMichael de la Chapelle, Kevin O'BrienOriginal AssigneeDe La Chapelle Michael, O'brien Kevin M.Export CitationBiBTeX, EndNote, RefManReferenced by (27), Classifications (18), Legal Events (3) External Links: USPTO, USPTO Assignment, EspacenetReturn link design for PSD limited mobile satellite communication systemsUS 20020058478 A1Abstract A system and method for managing access to a satellite-based transponder by a plurality of aircraft each having a mobile radio frequency (RF) system. The system employs a ground-based, central control system for managing access to the satellite-based transponder so that the aggregate power spectral density (PSD) of the RF signals of all the mobile systems does not exceed, at any time, limits established by regulatory agencies to prevent interference between satellite systems. This is accomplished by a dual control loop arrangement for monitoring the signal-to-noise ratio (Eb/No) of the RF signal transmitted by the satellite-based transponder. A ground-based control loop is used whereby a ground-based central controller monitors the Eb/No and transmits commands to the aircraft (via the satellite transponder) to maintain the Eb/No of the transmitted signal within a predetermined range. A fast scan angle compensation is used by the mobile system of the aircraft to implement another control loop to further adjust the transmit power. This control loop maintains the Eb/No of the signal transmitted to the satellite-based transponder at the commanded level inbetween updates from the ground-based central controller. Images(12) Claims(18)
[0104] where: [0105] EIRPi(θ)=EIRP of ith mobile system 20 in the direction of θ. [0106] Bs=spreading bandwidth. [0107] N=number of mobile systems 20 simultaneously accessing the system [0108] An example PSD regulatory mask is defined in Table 1 and depicted graphically in FIG. 5. This regulatory mask represents a PSD limit below which the invention must manage the power spectral density. The example regulatory mask is based on FCC requirement 25.209 for very small aperture terminals (VSATs) with −14 dBW/4 KHz power spectral density into the antenna. [0109] The method of the present invention requires that all mobile systems 20 spread their transmit signal over a fixed bandwidth (B) where B is chosen to be large enough so that multiple user terminals can simultaneously access the system without exceeding the regulatory limits on total EIRP spectral density. In one preferred implementation, B is set equal to the bandwidth of the transponder (e.g., satellite transponder 18 a 1). Typical Ku-band transponders have a bandwidth of 27 MHz, 36 MHz or 54 MHz. These bandwidths are typically wide enough to allow multiple mobile systems 20 to simultaneously access a single return link transponder without exceeding regulatory limits. FIG. 6 illustrates how the EIRP from multiple mobile terminals 20 1-20 n is spread over the full transponder bandwidth, and the resultant aggregate PSD is maintained below the regulatory limit. [0110] A second important feature of the invention is the use of a single, central controller 26 a which preferably is part of the NOC 26 (FIG. 1), that manages the use of the communication resources (i.e., the satellite-based transponders 18 a 1-4) and regulates access to the return link from the many mobile systems 20 operating within the coverage region. The invention also involves a control scheme for �Demand Assigned Multiple Access� (DAMA) by which each mobile system 20 requests and releases capacity (data rate) through the central controller 26 a. The central controller 26 a operates to regulate the usage of the satellite-based transponder to achieve maximum efficiency while maintaining regulatory compliance. [0111] Because the PSD contribution from each mobile system 20 is dependent on its location (and scan angle in the case of PAA antennas), and the location of the aircraft 12 will change over time, the PSD contribution from each mobile system 20 will be time varying. Accordingly, the system 10 requires that each mobile system 20 periodically report its position and antenna pointing angle to the central controller 26 a so that the PSD contribution of each mobile system to the aggregate can be updated. However, the PSD of the RF signal from any given mobile system 20 is expected to change slowly with time, even for relatively fast moving mobile platforms such as commercial jet aircraft. Accordingly, the central controller 26 a typically will not need to calculate mobile system PSD patterns more often than once every several minutes. The exception to this statement occurs for mobile antenna that have gain patterns that are very sensitive to scan angle (such as phased array antennas). Mobile systems 20 having these antennas must report their parameters (position and antenna scan angle) more often when the aircraft or mobile system 20 is rapidly changing it's heading or attitude. [0112] Referring to FIG. 9, initially a determination is made, at step 100, whether a request for capacity from a mobile system 20 n has been received by the central controller 26 a or whether the mobile system 20 n is releasing capacity. If a release of capacity has occurred, then the central controller 26 a subtracts the PSD of the mobile system 20 n releasing capacity from the aggregate PSD, as indicated at step 102. [0113] Mobile system 20 n is required to make a request for data rate (power) to the central controller 26 a if it wishes to access the satellite-based transponder 18 a 1 at a higher data rate than previously authorized, or if it wants initial authorization to operate at a specific data rate (power). This request provides the central controller 26 a with the information described above necessary for the central controller to determine the PSD of the RF signal to be transmitted by the mobile system 20 n. At step 104, the central controller 26 a then determines the PSD for both the on-axis (along the geostationary arc) and off-axis PSD of the transmit signal. At step 106, the central controller 26 a adds this PSD to the aggregate PSD of all other mobile systems 20 currently accessing the satellite 18 a. The central controller 26 a then compares the new aggregate PSD against the regulatory PSD limit, as indicated at step 108. If this comparison indicates that the PSD of the mobile terminal 20 n presently requesting access would cause the new aggregate PSD to exceed the predetermined regulatory PSD limit at any on-axis or off-axis offset angle, then access to the system 10 is denied, as indicated at step 110. Optionally, the request for additional capacity could be queued until the central controller 26 a determines that additional capacity is available, as indicated at step 112. Only when sufficient PSD (i.e., capacity) becomes available (for instance by the release of data rate power by another mobile system 20) will the central controller 26 a send an authorization to transmit signal to the mobile system 20 n, as indicated at step 114. [0114] In a similar manner, when a mobile system 20 no longer requires data rate (i.e., power), it is released to the central controller 26 a so that it may be used by other mobile systems 20 sharing the transponder. No authorization by the central controller 26 a is required before any mobile system 20 releases capacity. When the central controller 26 a receives a release of data rate message from any mobile system 20 it subtracts the PSD of the released data rate from the aggregate PSD to form a new aggregate PSD. [0115] In practice, the aggregate PSD monitored by the central controller 26 a will be changing constantly as various mobile systems 20 operating within the coverage region request and release capacity (i.e., data rate) to the system 10, as well as initiate and terminate their communication sessions with the system 10. Optionally, if a request for authorization to transmit from a particular mobile system 20 is denied by the central controller 26 a, the system 10 could assign the requesting mobile system to another transponder having available PSD capacity. No authorization to transmit is provided to any mobile system 20 attempting to access the system 10 unless the central controller 26 a has determined that its RF emissions will not cause the aggregate PSD of all mobile systems 20 currently accessing the system 10 to exceed the regulatory PSD limit. [0116] All mobile systems 20 operating within the coverage region operate to periodically request and release power as their data rates, locations, orientations, etc. change during the course of a communication session. Each mobile system 20 transmits with only as much power as required to close its communication link with the transponder 18 a 1 of the satellite 18 a. This transmit power is a function of the data rate and many other parameters (i.e., slant range, antenna scan angle, etc.). The operation of adjusting the transmit power to maintain communication link closure may be referred to as �power control�. [0117] The system and method of the present invention can be used with any power control method that allows the central controller 26 a to be apprised of power changes (by periodic messaging, for instance). The preferred method of power control is the dual loop power control method described above. [0118] Another method of power control is the open loop approach, where each mobile system uses its known position on the Earth (provided usually via GPS) and its attitude, together with knowledge of the location of the satellite that it wants to communicate with, to determine the appropriate transmit power. Again, the transmit EIRP selected is only that amount that permits the communication link with the satellite to be closed. With the open loop approach, the mobile system 20 must periodically report its transmitted power to the central controller 26 a. With either approach, it is important that the central controller 26 a be apprised of the transmit power of each mobile system 20 accessing the system 10. [0119] Referring now to FIG. 10, an example of the operation of the system and method of the present invention will be described. In this example, three aircraft 12 a, 12 b and 12 c are each in communication with satellite transponder 18 a 1. Aircraft 12 a is over Seattle, Wash., aircraft 12 b is over Houston, Tex. and aircraft 12 c is over Bangor, Me. For this example, further suppose that each aircraft 12 has a different sized phased array antenna (PAA), and that each is accessing the transponder of the satellite 18 a 1 at a different data rate. Aircraft 12 a is using a 256 element (16�16) active phased array antenna and is transmitting at 64 Kbps using an EIRP of 34 dBW. Aircraft 12 b is using a larger 512 element PM and transmits within an EIRP of 39 dBW and a data rate of 256 Kbps. Finally, aircraft 12 c has an even larger aperture 1024 element PM operating at 128 Kbps and 37 dBW. Each of the mobile systems 20 of each aircraft 12 a, 12 b and 12 c are pointing their antennas at the satellite transponder 18 a 1, which is located at 93� East longitude. [0120] The EIRP spectral density of the RF signal from aircraft 12 a is shown in FIG. 11 and indicated by reference numeral 112. The EIRP spectral density of the RF signal from aircraft 12 b is shown in FIG. 12 and indicated by reference numeral 114. The EIRP spectral density of the RF signal from aircraft 12 c is shown in FIG. 13 and indicated by reference numeral 116. FIG. 14 illustrates the aggregate PSD determined by the central controller 26 a. The aggregate PSD from all three aircraft is denoted by waveform 118. From FIG. 14, it can be seen that the aggregate PSD 118 remains below the on-axis regulatory PSD limit (i.e., �mask�) 120 at all points along the geostationary arc. A similar check can be performed for off-axis PSD. [0121] As described previously, the system 10 makes use of a model which enables the central controller 26 a to accurately calculate the radiation pattern of the transmit antenna based on the aircraft-to-satellite beam pointing geometry. In actual operation, this antenna model is used by the central controller 26 a so that antenna gain patterns can be computed for each type of antenna that will be used to access the system 10. Knowing the transmit power, the gain pattern and the spreading bandwidth, a PSD pattern can be calculated for each mobile system 20, as indicated in FIGS. 11-13. It then becomes a routine summing operation to sum the PSD contributions from each mobile system 20 to calculate the aggregate PSD as shown in FIG. 14. In this example, the aggregate PSD is less than the regulatory PSD limit so additional mobile systems 20 can be admitted access to the system 10 or existing users may increase their transmit power (i.e., data rate). Since data rate is proportional to transmit power, which is proportional to PSD, it can be said that the present invention manages power, PSD, data rate or capacity. [0122] Referring now to FIGS. 15-18, a more detailed description of the system 10 for monitoring and controlling the aggregate PSD of all aircraft 12 will be provided. The present invention 10 incorporates a return link power controller (RLPC) 130. The RLPC 130 includes a scan angle compensator 132 and an airborne receive/transmit subsystem (ARTS) 134. The scan angle compensator 132 comprises a software program which is an important component of the RLPC 130. This component will be discussed in greater detail in the subsequent drawing figures, but it is essentially implemented in software that resides onboard the aircraft 12 and interfaces to other hardware on the aircraft. It compensates for the relatively fast rolling and pitching motion of the aircraft 12. More specifically, it compensates for changes in transmit antenna 74 scan angle which are the direct result of aircraft motion. It is referred to as a �fast� scan angle compensator because it generates correction commands at a rate of approximately 10 commands per second which, when compared to other portions of the RLPC 130, is about 10 times faster than such other portions. The input to the scan angle compensator 132 is the actual transmit antenna scan angle. The output from scan angle compensator 132 represents a time series of correction commands in the form of ARTS 134 antenna power levels. [0123] The ARTS 134 is a hardware component which is in communication with the communications subsystem 52 (FIG. 2). The ARTS 134 accepts commands either from the ground station 22 or from the onboard scan angle compensator 132 for setting antenna 74 power levels and generating an output power level as close as possible to the commanded power level. The inputs to the ARTS 134 are the actual antenna scan angle, the power commands from the scan angle compensator 132, and the power commands from the ground-based central controller 26 a. The output of the ARTS 134 is simply a simulated value of Eb/No. The ARTS 134 may output more than just the value of Eb/No, but for the present discussion, the Eb/No is all that is needed. [0124] Block 136 represents an input level of Eb/No that the system 10 is intended to control to. In actual practice of the RLPC 130, this value will typically be set by some external entity and accepted by a ground component of the RLPC 130. The output of block 136 represents a time series of commanded Eb/No values. [0125] The RLPC 130 further includes a summing component 138 and a reporting algorithm 140. The summing component 138 takes the difference between the commanded (desired) Eb/No, represented by block 136 and the value that was measured and reported from reporting algorithm 140 (to be discussed momentarily), thereby generating an error used to drive the RLPC system 130. Summing component 138 resides in software running on one or more computers of a data center 155 shown in FIG. 1, which forms a portion of the ground station 22. The output of summing component 138 represents a time series of error values that reside completely in software. [0126] The reporting algorithm 140 comprises a major portion of the RLPC 130. It represents a software program residing on computer equipment associated with the data center 155. It is used to sample the Eb/No measurements that are generated by a ground receive/transmit system (GRTS) 143. The GRTS 143 is not a part of the RLPC 130. The reporting algorithm 140 limits the size of the Eb/No measurements to ensure that occasional spurious measurement data is used by the RLPC system 130. The output from the reporting algorithm 140 is simply a repeat of the input Eb/No measurement except that the output is taken only at specific and regular time intervals. [0127] The output of the summing component 138 is input to a slow loop ground controller 142 which also forms an important component of the RLPC system 130. The slow loop ground controller 142 contains many subcomponents which will be discussed momentarily. It is implemented in software that resides on computers of the data center 155 (FIG. 1). [0128] The slow loop ground controller 142 compensates for any form of disturbance in Eb/No that can be measured by the computers of the data center 155. It is referred to as �slow� because it essentially can only generate power corrections about once every second. The input to the slow loop ground controller 142 is an error signal and its output is the computed power level commands which are transmitted to the aircraft 12. [0129] Referring now to FIG. 16, the scan angle compensator 132 is shown in greater detail. The scan angle compensator 132 includes a �scan angle measurement interval� subsystem 144 which is contained in software on board the aircraft in the ARTS 134. This subsystem essentially samples the scan angle measurement at regular intervals. The presently preferred sampling interval is 100 milliseconds. Thus, every 100 milliseconds, a new sample of the scan angle is taken. During the period when a sample is not being taken, the last sampled value is held on the output of subsystem 144 until the next sample is taken. [0130] Block 146 represents a �backlash�. This block is contained in software associated with the ARTS 134 onboard the aircraft 12. It is used to provide backlash to its input. That is, the output from block 146 will not change unless the input changes beyond a certain value. When this happens, the output changes as much as the input changes. If the input changes direction, the output will not change until the input changes by a predetermined magnitude. This function is helpful for making sure the RLPC system 130 does not react to very small noise spikes. Currently the preferred backlash �deadzone� is zero; therefore, block 146 has no affect on its input. It is illustrated, however, as an optional element that is available for fine tuning the performance of the system RLPC 130. [0131] The �cosine� block 148, also is contained in the software of the ARTS 134 onboard the aircraft 12 and is used to output just the cosine of its input. The �cosine power� block 150 is also contained in the software onboard the aircraft 12. Block 150 outputs a constant value (preferably a value of 1.2) which is used to take the output of block 148 to a particular power. Its function is to try to approximate the actual behavior of the transmit antenna 74 because its own gain is affected by the scan angle in the form of cos(θ)1.2, where �θ� is the scan angle. Therefore, the scan angle compensator 132 can predict what the antenna 74 is doing to try to counter the effects of this behavior. [0132] The outputs from blocks 148 and 150 are input to a �raise-to-power� block 152, which is also an important part of the scan angle compensator 132. Block 152 is contained in software in the ARTS 134 onboard the aircraft and is used to raise the value of the output from cosine block 148 to that of the output of cosine power block 150. Block 152 is also used to help the scan angle compensator 132 to predict what the antenna 74 is doing and to try to counter the effects of this behavior. [0133] The output from the raise to power block 152 is input to a �reciprocal� block 154, an important part of the invention. Block 154 is contained in the software in the ARTS 134 onboard the aircraft 12 and it outputs the reciprocal of its input. This is done because the output of the fast scan angle compensator 132 will eventually multiply the actual desired power level from the ARTS 134 (FIG. 15). Thus, when this value (1/x) is multiplied by the actual value (which should be close to x, which is what blocks 148-152 are trying to predict), the results should be close to 1. This means that no matter what the scan angle does, the final output will nearly always be 1. This value will be used to multiply other values within the system 130, so if it is kept near 1, then the final value of the overall system will not change much. [0134] Block 156 is a decibel conversion block that is contained in the software of the ARTS 134 onboard the aircraft 12. Block 156 converts the signal on its input to decibels (dB), which is the common unit of measurement in most communication systems. Depending upon the precise architecture of the RLPC 130, block 156 may not be needed. [0135] Block 158 performs an �aggregation� function on the output from block 156. Block 156 actually is a combination of a �quantizer� 158 a and a �diff1� block 158 b. At every sample time, the output of diff1 block 158 b is the difference between the input from the previous sample and the input from the current sample. Aggregation block 158 functions to output the change in its input at each time step. In this case, because of the 100-millisecond sampling of block 144, a time step is every 100 milliseconds. Every 100 milliseconds blocks 158 a and 158 b compute that change in input from the previous 100 millisecond period and output this change. The quantizer 158 a ensures that the changes are at least of a specific level (currently 0.1 dB) before a change is reported. The output from the aggregation block 158 is input to then transmitted to the ARTS 134. [0136] Referring now to FIG. 17, the slow loop ground controller 142 of FIG. 15 will be described in greater detail. Referring initially to block 160, this block is contained in software in the data center 155. It receives the input error signal from the summing component 138 (FIG. 15) and generates an output signal in accordance therewith. [0137] The output from block 160 is input to an error noise filter 162 and also to a control filter system 164. Block 162 is contained in the software in the data center 155. Block 162 filters its input to reduce the effects of noise. It comprises a discrete first order low-pass filter with a sampling rate of preferably 10 Hz. The output of block 162 represents a filtered version of its input. [0138] The output from the error noise filter 162 is input into a symmetric relay with hysteresis 166. Block 166 is also contained in the software associated with the computers used in the data center 155. Block 166 outputs either a �1�, �0�, or �−1�, depending on the history of the input. If the input is greater than some given value (or less than the negative of this value), then the output is 1 (or −1). If the input is less than another given value (or greater than the negative of this value), then the output is 0. If the input is between these two values, the output is whatever the previous output was. The values used in block 162 are capable of being modified if needed to effect fine tuning of the RLPC system 130. Block 166 is used to test if the output of the filtered error from block 162 is too large (in either the positive or negative direction). If so, a non-zero value is output, which will indicate to the rest of the RLPC system 130 that power corrections are required. [0139] Block 168 is contained in software on the ground. The output of block 168 is the absolute value of its input, which is either �1�, �0� or �−1�. This is done so that the final output of the three blocks 162, 166 and 168 is either �1� or �0�. A �1� indicates too large of an error. A �0� indicates the error is currently acceptable. [0140] The control filters block 164 is also contained in software on the ground and also represents an important subsystem of the invention. The control filters block 164 is shown in detail in FIG. 18, and will be discussed momentarily. Essentially, however, the function of this block 164 is to compute the required power correction once the error has been determined to be too large. The output is a power correction command to be sent to the aircraft 12. [0141] Block 170, which is optional, functions to create command increments from absolute commands, and is also contained in software of the data center 155 computers. Block 172 performs the identical function of block 158 of FIG. 16. This block 172 is also optional for the slow loop ground controller 142. [0142] Block 172 receives the output from block 172 (or from block 164 if block 170 is omitted). Block 172 is also contained in software associated with computers of the data center 155. It outputs its input into the ARTS 134 in FIG. 15. In the actual implementation, the transmission of the correction command will likely proceed through several intervening elements prior to going out to the satellite transponder and back to the aircraft 12, which is the primary source of any time delay experienced in transmitting the correction command. These intervening elements are not part of the invention. They will be elements typically associated with the ground computer inter-network (such as Ethernet cards, routers, switches, firewalls, etc.), as well as elements associated with the communications system 52 (such as modulators, up-converters, encoders, antennas, etc.). They all function cooperatively to route and transmit the power commands from block 172 to the ARTS 134. Therefore, block 172 is simply an interface to all the rest of these intervening elements, and its details are hidden within the final implementation of the system 10. [0143] Referring now to FIG. 18, the control filters block 164 is shown in greater detail. Essentially, this block represents a typical discrete second order filter with anti-windup and a sample period (T) equal to one second. An enable switch 174 is contained in the software of the computers of the data center 155 and allows the control filters block 164 to be executed only when the output from the ABS block 168 (FIG. 17) is greater than or equal to one. By tracing the signal flow on this chart, it can be seen that the enable switch 174 allows execution of the control filter block 164 only when the filtered input error is too large. This is an important part of the RLPC 130 which helps to reduce the number of times a command is sent from the central controller 26 a, thereby reducing the use of otherwise saleable bandwidth. [0144] Block 176, which is optional, is also contained in the software in the central controller 26 a. Block 176 functions to transmit the measurement error signal into the control filters block 164 (FIG. 17). It represents a point of reference showing where from the containing block (block 142) the signal enters the block. [0145] The output of block 176 is input to a proportional gain amplifier 178. Amplifier 178 is also contained in software in the central controller 26. The proportional gain amplifier 178 outputs the input it receives multiplied by a given value. This value is important to the design of the RLPC system 130, although it can be changed in response to tuning needs. [0146] A second proportional gain amplifier 180 receives the output from amplifier 178. Proportional gain amplifier 180 is also contained in software in the central controller 26 a. This amplifier 180 performs the same function as amplifier 178 but multiplies its input by a different value. [0147] Block 182 represents a �limited discrete time integrator� which is contained in the software on the ground. Block 182 produces the time integral of its input on its output. The integration is done in discrete time fashion using the so called �Forward Euler� method. The sample period of this integrator is one second. The integrator is limited (so-called �anti-windup�) in that it stops integrating when the output goes above a given value (or below the negative of that value). It will start integrating again when the input reverses its sign, thereby reducing the output from its limited value. [0148] Block 184 is a multiplier which is contained in software of the GRTS 143. This block performs the same function as block 178, but multiplies its input by a different value. [0149] The outputs from multipliers 180 and 182 are fed into a summing junction 186 which sums these values and outputs the summed value to proportional gain amplifier 188. Proportional gain amplifier 188 is contained in the software of the data center 155 and performs the same function as amplifier 178, but rather multiplies its input by a different value. [0150] Referring further to FIG. 18, a discrete time integrator 190 receives the output from proportional gain amplifier 188. Discrete time integrator 190 is contained in the software of the data center 155 computers. This integrator 190 performs the same function as integrator 182 (with the same sample time and integration method) but is not limited as block 182 is. Interface block 192 receives the output from the discrete time integrator 190. The output from block 192 is input to block 170 in FIG. 17. [0151] The slow loop ground controller 142 implements filters instead of a well-known �dead-bang� control method, which would require very low noise and/or an extensive knowledge of various system parameters. The full loop ground controller 142 also provides strong stability and analytical tractability. It also reacts better to model uncertainties and variations that can be easily tuned on line for optimum performance. Advantageously, the slow ground loop controller 142 creates command �increments� which end up requiring less bandwidth to be utilized when transmitting these increments to the aircraft 12. The enable switch 174 further limits the generation of commands by only executing filters when the error is above a settable limit. The enable switch 174 further acts to enable or disable each and every block within block 164. The slow loop ground controller 142 further makes use of hysteresis, contained within block 166, to prevent jitter and �hunting�. [0152] The method and apparatus of the present invention thus provides a means for managing and monitoring communications from a variety of mobile RF transmitting platforms to ensure that the aggregate PSD of all of the mobile platforms does not exceed predetermined regulatory limits. It is also an important advantage of the present invention that a central controller is used to receive and monitor requests for access to the system 10 from each of the mobile systems 20 so that close control can be maintained over the on-axis and off-axis aggregate PSD. By causing each mobile system 20 to transmit with only that amount of power needed to maintain communication link closure, the efficiency of the system 10 is maximized, thus allowing a large number of mobile systems to access the system 10 without causing the aggregate PSD to exceed regulatory limits. [0153] Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and following claims. 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