Patent Publication Number: US-2023138527-A1

Title: System and method that facilitates precision landing of an aerial vehicle

Description:
RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 63/275,093, filed Nov. 3, 2021, the content of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Field 
     This application generally relates to aircraft navigation systems. In particular, this application describes examples of systems and methods that facilitate the precision landing of an aerial vehicle. 
     Description of Related Art 
     Electric vertical take-off and landing (eVTOL) aircraft are being considered for use as air taxis to facilitate passenger travel between relatively close points of interest, such as between different parts of a large metropolitan area, geographically close cities, etc. Some examples of these aircraft include multiple counter-rotating rotors that are powered by one or more electric motors. Onboard batteries provide power to the electric motors. Some examples of these aircraft are equipped with GNSS receivers, which facilitate navigation. Some of these aircraft are intended to be operated by a pilot, while others are intended to be operated autonomously. 
     SUMMARY 
     In a first aspect, a navigation, take-off, and landing support system (NTLS) that facilitates vertical landing at a landing area by an aerial vehicle comprises a plurality of pseudolites distributed proximate the landing area, at least one monitoring receiver, and at least one control system. Each pseudolite is configured to transmit a radio frequency (RF) signal that facilitates determining, by the aerial vehicle and based on a code phase and a carrier phase of the RF signal, its position and velocity relative to the pseudolite and whether the pseudolite is operating within a nominal operating range. The monitoring receiver is positioned proximate the landing area and is configured to receive RF signals from the plurality of pseudolites, measure the code phase and the carrier phase associated with each of the plurality of pseudolites, and determine corresponding clock bias estimates associated with the RF signals of each pseudolite. The control system is in communication with the plurality of pseudolites and the monitoring receiver and is configured to determine, based on the RF signals received from the monitoring receiver, whether the plurality of pseudolites are operating within a nominal operating range and to indicate to each of the plurality of pseudolites whether the pseudolite is operating within a nominal operating range. 
     In a second aspect, a navigation method that facilitates vertical landing at a landing area by an aerial vehicle comprises communicating, by a plurality of pseudolites distributed proximate the landing area, a radio frequency (RF) signal that facilitates determining, by the aerial vehicle and based on a code phase and carrier phase of the RF signal, its position and velocity relative to the pseudolite. The method further comprises receiving, by at least one monitoring receiver positioned proximate the landing area, RF signals from the plurality of pseudolites, measuring, by the monitoring receiver, the code phase and the carrier phase associated with each of the plurality of pseudolites, and determining, by the monitoring receiver and based on the code phase and the carrier phase of the RF signal, corresponding clock bias estimates associated with the RF signals of each pseudolite. The method further comprises determining, by at least one control system in communication with the plurality of pseudolites and the at least one monitoring receiver and based on the RF signals received from the at least one monitoring receiver, whether the plurality of pseudolites are operating within a nominal operating range. The method further comprises communicating, by a bi-directional RF communication system in communication with the control system, landing area information associated with the landing area to the aerial vehicle. The landing area information indicates particular pseudolites of the plurality of pseudolites that are operating within a nominal operating range and the clock bias estimate, which further facilitates determining, by the aerial vehicle, its position and velocity relative to the pseudolites. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the figures and the following detailed description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  illustrates a navigation, take-off, and landing support system (NTLS) that facilitates vertical landing at a landing area by an aerial vehicle, in accordance with example embodiments. 
         FIG.  1 B  illustrates a group of pseudolites of the NTLS, in accordance with example embodiments. 
         FIG.  2    illustrates an example of a control system. 
         FIG.  3    illustrates operations that facilitate processing a landing request, in accordance with example embodiments. 
         FIG.  4    illustrates operations that facilitate ensuring the integrity of information communicated by pseudolites, in accordance with example embodiments. 
         FIG.  5    illustrates operations performed by one or more devices described herein, in accordance with example embodiments. 
         FIG.  6    illustrates a computer system, in accordance with example embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Various examples of systems, devices, and/or methods are described herein. Any embodiment, implementation, and/or feature described herein as being an example is not necessarily to be construed as preferred or advantageous over any other embodiment, implementation, and/or feature unless stated as such. Thus, other embodiments, implementations, and/or features may be utilized, and other changes may be made without departing from the scope of the subject matter presented herein. 
     Accordingly, the examples described herein are not meant to be limiting. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations. 
     Further, unless the context suggests otherwise, the features illustrated in each of the figures may be used in combination with one another. Thus, the figures should be generally viewed as component aspects of one or more overall embodiments, with the understanding that not all illustrated features are necessary for each embodiment. 
     Additionally, any enumeration of elements, blocks, or steps in this specification or the claims is for purposes of clarity. Thus, such enumeration should not be interpreted to require or imply that these elements, blocks, or steps adhere to a particular arrangement or are carried out in a particular order. 
     Moreover, terms such as “substantially” or “about” that may be used herein are meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including, for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those skilled in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide. 
     As noted above, some electric vertical take-off and landing (eVTOL) aircraft are equipped with a GNSS receiver that facilitates navigation. However, such a navigation system does not provide the level of integrity, precision, and availability that is anticipated to be required by various regulatory agencies that regulate these activities. In particular, such navigation systems will not support, for example, vertical take-off/landing in dense urban areas where room for a landing area is limited. Landing in such areas is expected to require a horizontal and vertical position accuracy of 0.5 meters and 0.1 meters, respectively, with an expected failure probability of less than 10 -9 . 
     These and other issues are ameliorated by various navigation, take-off, and landing support system (NTLS) examples described herein. Some examples of the NTLS comprise several pseudolites (short for pseudo satellite) distributed proximate a landing area such as a landing area in a densely populated area. In an example, at least four pseudolites are provided to facilitate the triangulation of the aerial vehicle. Each pseudolite is configured to transmit a radio frequency (RF) signal that facilitates determining, by the aerial vehicle, a distance between the pseudolite and the aerial vehicle. Some examples of the NTLS include a monitoring receiver positioned proximate the landing area, and that is configured to receive the RF signals from the pseudolites. Some examples of the NTLS include a control system that is in communication with the pseudolites and the monitoring receiver. The control system is configured to determine, based on the RF signals received from the monitoring receiver, whether the pseudolites are operating within a nominal operating range. 
     Some examples of the NTLS include a number of pseudolites that is greater than the minimum number of pseudolites (e.g., four) needed to perform trilateration. For instance, an example of the NTLS includes ten pseudolites. The extra pseudolites facilitate improved location accuracy. Further, in the event that a particular pseudolite fails or otherwise begins to operate outside of a nominal operating range, it can be taken offline, and the number of remaining pseudolites is still sufficient to facilitate the performance of trilateration. In some examples, pseudolites that are operating outside of a nominal operating range are taken offline or instructed to broadcast an indication of the same (e.g., via the RF signals). 
     Some examples of the NTLS include additional and/or redundant monitoring receivers. In some examples, when the RF signals received by a majority of the monitoring receivers indicate that a particular pseudolite is in a particular state of operation (e.g., inoperative), the control system is configured to determine the particular pseudolite is in that particular state of operation. In some examples, when a particular monitoring receiver indicates a state of pseudolite operation that is different from the state of pseudolite operation indicated by the majority of monitoring receivers, the system is configured to determine that the monitoring receiver is operating outside of a nominal operating range. 
     Some examples of the NTLS include a primary control system, a redundant control system, a primary network, a redundant network, a primary power system, and a redundant power system. The primary and redundant networks facilitate communications between the control systems, pseudolites, and monitoring receivers. The primary and redundant power systems are configured to supply power to the networks, control systems, pseudolites, and monitoring receivers. In some examples, the primary control system and the redundant control system are configured to determine whether the primary network and/or the primary power system are inoperable and to utilize the redundant network for communications when the primary network is inoperable and to use the redundant power system to supply power when the primary power system is inoperable. In some examples, the redundant control system is configured to determine whether the primary control system is inoperable and to control operations of the NTLS when the primary control system is inoperable. 
     Some examples of the NTLS include a bi-directional RF communication system that is in communication with the control system. In some examples, the control system is configured to receive, via the bi-directional RF communication system, a landing request from the aerial vehicle. The control system then determines whether the aerial vehicle is authorized to land at the landing area and, if so, communicates an encryption key to the aerial vehicle that facilitates decoding, by the aerial vehicle, information in the RF signals. 
     Some examples of the control system are configured to communicate landing area information associated with the landing area to the aerial vehicle via the bi-directional RF communication system. Some examples of the control system communicate one or more waypoints to the aerial vehicle that facilitates navigating the aerial vehicle from a current location to the landing area. Some examples of the waypoints specify a location and a time at which the aerial vehicle  150  should arrive at the location specified by the waypoint. 
     Some examples of the landing area information indicate particular pseudolites of the NTLS that are operating within a nominal operating range. This, in turn, allows the aerial vehicle to base its location on RF signals received from the nominally operating pseudolites and to reject RF signals from those pseudolites that are operating outside of a nominal operating range. As noted above, some examples of pseudolites communicate their respective operational status in the RF signal and in some examples, the aerial vehicle determines the operational state of the pseudolites based on this information in the RF signal. 
       FIG.  1 A  illustrates an example of a navigation, take-off, and landing support system (NTLS)  100  that facilitates vertical landing at a landing area  105  by an aerial vehicle  150 . The NTLS  100  includes a plurality of pseudolites  110 , one or more monitoring receivers  115 , and one or more control systems  102 . Some examples of the NTLS  100  include a bi-directional RF communication system  117 , one or more networks  108 , and one or more power systems  104 . 
     Some examples of the power systems  104  are configured to supply power to the various subsystems of the NTLS  100 . Some examples of the power systems  104  receive power from a power utility and are configured to generate power in the case of utility power loss. In this regard, some examples of the power systems  104  comprise batteries or gas generators from which power is derived in the case of utility power loss. Some examples of the power systems  104  operate as redundant power systems  104  and are configured to supply power to the various subsystems of the NTLS  100  when one or more primary power systems  104  fail. 
     Some examples of the networks  108  facilitate communications between the various subsystems of the NTLS  100 . Some examples of the networks  108  correspond to wireless, wired and/or fiber networks. Some examples of the networks  108  operate as redundant networks  108  and are configured to facilitate communications between the various subsystems of the NTLS  100  when one or more primary networks  108  fail. 
     Some examples of the pseudolites  110  are distributed proximate the landing area  105 . For instance, some examples of the pseudolites  110  are distributed along the perimeter of the landing area  105 . In this regard, some examples of the landing area  105  comprise one or more landing area regions that accommodate the simultaneous landing of a corresponding number of aerial vehicles  150 . 
     Some examples of the pseudolites  110  are configured to transmit a radio frequency (RF) signal  125  that facilitates determining, by the aerial vehicle  150 , a distance between the pseudolite  110  and the aerial vehicle  150 . Some examples of the RF signal  125  comprise information that facilitates determining by the aerial vehicle  150  whether the pseudolite  110  is operating within a nominal operating range. Some examples of the aerial vehicles  150  utilize information from several pseudolites  110  (e.g., four or more) to precisely locate the landing area  105  or a particular landing area region within the landing area  105 . In this regard, some examples of the aerial vehicle  150  perform trilateration based on the RF signals  125  to locate the landing area  105 . 
     Some examples of the RF signal  125  comprise location data that specifies a location of a corresponding pseudolite  110 . Some examples of the location data correspond to geospatial coordinates of the pseudolite  110 . In this regard, in some examples, the location of each pseudolite  110  is precisely determined by surveying equipment and, in some cases, is accurate to within several centimeters. 
     Some examples of the RF signal  125  comprise integrity data that specifies whether the pseudolite  110  is operating within a nominal operating range. For instance, some examples of the RF signal  125  indicate whether the pseudolite  110  should be used for location determination, whether the pseudolite  110  is scheduled for maintenance, etc. 
     Some examples of the RF signal  125  comprise a pseudorandom code that facilitates deriving, by the aerial vehicle  150 , a code-phase and a carrier-phase that facilitate measuring a distance between the aerial vehicle  150  and the particular pseudolite  110 . In an example, the code-phase facilitates determining the location of the landing area  105  (or landing area region within the landing area  105 ) to within several meters of accuracy, and the carrier-phase facilitates refining this measurement to within centimeters of accuracy. 
     In some examples, the pseudorandom code associated with a group of pseudolites  110  distributed proximate a particular landing area  105  (e.g., a particular city or location in the city) is the same. In some examples, the pseudorandom code associated with a different group of pseudolites  110  distributed proximate a different landing area  105  (e.g., in a different city or different location of the city) is different from the pseudorandom code associated with the first group of pseudolites  110 . In some examples, the RF signal  125  comprises a carrier wave that is in the gigahertz range (e.g., 1 GHz, 2 GHz, etc.) 
     Referring to  FIG.  1 B , some examples of pseudolites  110  include a pseudorandom code generator  155  configured to generate the pseudorandom code specified in the RF signal. Some examples of the pseudorandom code generator  155  derive timing information from a common clock source  151  so that pseudorandom codes generated by each pseudolite  110  have the same phase and frequency. In this regard, in some examples, the common clock is communicated (e.g., via the network  108 ) to each pseudolite  110  from a particular monitoring receiver  115 , a particular pseudolite  110  (e.g., master pseudolites), the control system  102 , etc. 
     Some examples of the pseudorandom code generator  155  derive timing information from clock sources that are asynchronous. In this regard, some examples of the pseudolites  110  comprise a precision clock source (e.g., an atomic clock) that generates a clock signal. In some examples, the frequency and phase of the clock signal are calibrated from time to time (e.g., by the control system  102  and based on information received from the RF monitoring station  115 ) to minimize differences between respective frequencies and phases of the clock signals. Additionally, or alternatively, in some examples, timing information (e.g., relative differences in frequencies and phases) associated with the pseudorandom code of each pseudolite  110  is communicated to the aerial vehicle  150  to facilitate performing trilateration based on RF signals received from the pseudolites  110  that take into account this timing information. 
     Some examples of the monitoring receivers  115  are positioned proximate the landing area  105  and are configured to receive the RF signals  125  from the pseudolites  110 . The monitoring receivers  115  are configured to receive the RF signals  125  from the pseudolites  110  to facilitate determining whether the pseudolites  110  are operating within a nominal operating range. For instance, in some examples, aspects of the RF signal  125  (e.g., carrier frequency, phase, pseudorandom code, etc.) are compared with expected values. In some examples, when values associated with these aspects for a particular pseudolite  110  deviate by a predetermined amount, the pseudolite  110  is determined to be operating outside of a nominal operating range. In this regard, some examples of the monitoring receivers  115  are configured to measure the code phase and the carrier phase associated with each of the plurality of pseudolites and determine, based on the code phase and the carrier phase of the RF signal, corresponding clock bias estimates associated with the RF signals of each pseudolite  110 . 
     Some examples of the bi-directional RF communication system  117  implement a bi-directional communication channel  130  that facilitates transmitting information to the aerial vehicle  150 , such as landing area  105  information associated with the state of equipment at the landing area  105  (e.g., the operational status of pseudolites  110  at the landing area  105 ). Some examples of the bi-directional RF communication system  117  are configured to facilitate processing a landing request communicated from an aerial vehicle  150  via the bi-directional communication channel  130 . 
       FIG.  2    illustrates an example of a control system  102 . The control system  102  includes a processor  225 , a memory  227  in communication with the processor  225 , and an input/output (I/O) subsystem  210 . 
     The processor  225  is in communication with the memory  227 . The processor  225  is configured to execute instruction code stored in the memory  227 . The instruction code facilitates performing, by the control system  102 , various operations that are described herein. In this regard, the instruction code may cause the processor  225  to control and coordinate various activities performed by the different subsystems of the control system  102  and/or the NTLS  100 . Some examples of the processor  225  can correspond to a stand-alone computer system such as an ARM®, Intel®, AMD®, or PowerPC® based computer system or a different computer system and can include application-specific computer systems. The computer system can include an operating system, such as Windows ®, Linux ®, Unix ®, or a different operating system. 
     Some examples of the I/O subsystem  210  include one or more input/output interfaces configured to facilitate communications with subsystems of the NTLS  100 . An example of the I/O subsystem  210  includes wired or wireless communication circuitry configured to facilitate communicating information. An example of the wireless communication circuitry includes cellular telephone communication circuitry configured to communicate information over a cellular telephone network such as a 3G, 4G, and/or 5G network. Other examples of the wireless communication circuitry facilitate communication of information via an 802.11 based network, Bluetooth®, Zigbee®, near field communication technology or a different wireless network. 
       FIG.  3    illustrates examples of operations  300  that facilitate processing the landing request received via the bi-directional communication channel  130 . In some examples, these operations are performed by the control system  102 , the bi-directional communication system  117 , and/or both systems operating in cooperation with one another. The operations at block  305  involve receiving the landing request from the aerial vehicle  150 . In this regard, in an example, the bi-directional communication system  117  broadcasts information associated with the landing area  105 . Examples of this information facilitate the identification of the bi-directional communication channel  130  for receiving landing requests, information that specifies geospatial coordinates of the landing area  105  or different landing regions within the landing area  105 , the operational status of the pseudolites  110  at the landing area  105 , the weather conditions at the landing area  105 , etc. In some examples, the aerial vehicle  150  communicates the landing request via the bi-directional communication channel  130  specified in the broadcast information. 
     The operations at block  310  involve determining whether the aerial vehicle  150  is authorized to land at the landing area  105 . In some examples, the landing area  105  is restricted to particular aerial vehicles, pilots (if any), and/or passengers. In some examples, the landing request includes identifying information that identifies the aerial vehicle  150 , pilot, and/or passengers and the control system  102  determines whether the aerial vehicle  150 , pilot, and/or passengers are authorized to land at the landing area  105  based on the identifying information. In some examples, communication of the landing request triggers a series of communications (e.g., to conduct a financial transaction) that facilitate obtaining authorization to land at the landing area  105 . 
     If, at block  310 , the aerial vehicle  150  is not authorized to land, then the operations at block  315  are performed. These operations involve communicating information (e.g., via the bi-directional communication channel  130 ) to indicate to the aerial vehicle  150  that it is not authorized to land. In this case, additional information that would otherwise facilitate landing at the landing area  105  is not communicated to the aerial vehicle  150 . 
     If, at block  315 , the aerial vehicle  150  is authorized to land, then the operations at block  320  are performed. These operations involve communicating pseudolite  110  encryption key information (e.g., via the bi-directional communication channel  130 ) to the aerial vehicle  150 . In some examples, the encryption key information specifies the pseudorandom code associated with the pseudolites  110  at the landing area  105 . Communication of the pseudorandom code facilitates determining the code-phase and carrier-phase associated with the RF signals  125  communicated from the pseudolites  110 . This, in turn, facilitates determining, by the aerial vehicle  150 , its location relative to the locations of the pseudolites  110 . 
     In some examples, the pseudorandom code used by the pseudolite  110  is changed periodically (e.g., once a day) to require periodic re-authorization to land at the landing area  105 . Some examples of the pseudolites  110  output multiple RF signals  125 , and each RF signal  125  is associated with a different pseudocode. This, in turn, facilitates the simultaneous authorization of several aerial vehicles  150  to land at the landing area  105  without sharing the pseudocode between the aerial vehicles  150 . 
     The operations at block  325  involve communicating navigation information to the aerial vehicle  150  (e.g., via the bi-directional communication channel  130 ). In this regard, in some examples, the aerial vehicle  150  communicates its current location to the control system  102  (e.g., via the bi-directional communication channel  130 ), and the control system  102  determines a path (e.g., waypoints) the aerial vehicle  150  should follow that will guide the aerial vehicle  150  to the landing area  105 . Some examples of the navigation information specify spatial coordinates (e.g., x, y, and z) for each waypoint and a time at which the aerial vehicle  150  should reach the waypoint or a speed at which the aerial vehicle  150  should travel to the waypoint. Some examples of the control system  102  take the flight paths of other aerial vehicles  150  into consideration in determining the waypoints. 
       FIG.  4    illustrates examples of operations  400  that facilitate ensuring the integrity of information communicated by the pseudolites  110 . As noted above, some examples of the NTLS  100  comprise redundant pseudolites  110  and redundant monitoring receivers  115 . The operations at block  405  involve receiving pseudolite  110  information via one or more monitoring receivers  115 . As noted above, each pseudolite  110  communicates an RF signal  125 . Some examples of the RF signal  125  comprise a pseudorandom code that facilitates deriving, by the aerial vehicle  150 , a code-phase and a carrier-phase that facilitates determining a distance between the aerial vehicle  150  and the pseudolite  110 . In some examples, each monitoring receiver  115  determines the information within the RF signal  125  (e.g., the code-phase, carrier-phase, pseudorandom code, etc.) and communicates the information to the control system  102 . 
     The operations at block  410  involve determining, based on the information communicated by the monitoring receivers  115 , whether particular pseudolites  110  are operating within a nominal operating range. In some examples, when only one monitoring receiver  115  is used, the control system  102  compares the values in the information with predetermined values associated with a nominally operating pseudolite  110 . When the values in the information are within a threshold range of the predetermined values, the pseudolite  110  is determined to be operating within a nominal operating range. 
     In some examples, when several monitoring receivers  115  are used, the values in the information from each monitoring receiver  115  are compared with the predetermined values, as described above. When the information provided by a majority of the receivers indicates that a particular pseudolite  110  is operating within a nominal operating range, the pseudolite  110  is determined to be operating within a nominal operating range. For example, the pseudolite  110  is determined to be operating within a nominal operating range when the information from two out of three monitoring receivers  115  indicates the pseudolite  110  is operating within a nominal operating range. The integrity of this determination is increased by increasing the number of monitoring receivers  115 . For example, the integrity is increased when nine of ten monitoring receivers  115  indicate the pseudolite  110  is operating within a nominal operating range. 
     If at block  410 , the pseudolite  110  is determined to be operating within a nominal operating range, then the operations at block  420  are performed. These operations involve indicating by the control system  102  that the pseudolite is operating within a nominal operating range. In some examples, this involves specifying within the RF signal  125  associated with the pseudolite that the pseudolite  110  is operating within a nominal operating range. Some examples of an aerial vehicle  150  monitor this aspect prior to using the pseudolite  110  for distance determinations. In some examples, the indication is specified via the bi-directional communication system  117 . For example, the indication is specified in a broadcast communication or in a reply to the landing request referred to above. 
     If at block  410 , the pseudolite  110  is determined to be operating outside of a nominal operating range, then the operations at block  425  are performed. These operations involve indicating by the control system  102  that the pseudolite  110  is operating outside of a nominal operating range. Following the examples above, the indication is specified within the RF signal  125  associated with the pseudolite  110 , a broadcast communication by the bi-directional communication system  117 , and/or in a reply to the landing request. In some examples, when the aerial vehicle  150  determines, based on the indication, that the pseudolite  110  is operating outside of a nominal operating range, the aerial vehicle  150  will not use the RF signal  125  from that pseudolite  110  for distance determinations. 
     The operations at block  430  involve indicating whether any monitoring receivers  115  require service. For instance, following the examples above, when the information provided by a majority of the receivers indicates that particular operating state of a pseudolite  110  (e.g., operating within a nominal operating range), but the information provided by a particular monitoring receiver  115  indicates a different state of operation (operating outside of a nominal operating range), that monitoring receiver  115  may be determined, by the control system  102 , to be operating outside of a nominal operating range. In some examples, the monitoring receiver  115  may be indicated as such by the control system  102 . In some examples, the control system  102  takes the monitoring receiver  115  offline and/or communicates a service request to a technician to service the monitoring receiver  115 . 
       FIG.  5    illustrates an example of operations  500  performed by some examples of the devices described herein. The operations at block  505  involve communicating, by a plurality of pseudolites  110  distributed proximate a landing area  105 , a radio frequency (RF) signal that facilitates determining, by the aerial vehicle  150  and based on a code phase and a carrier phase of the RF signal, its position and velocity relative to the pseudolite  110 . 
     The operations at block  510  involve receiving, by at least one monitoring receiver positioned proximate the landing area  105 , RF signals from the plurality of pseudolites  110 . 
     The operations at block  512  involve measuring, by the at least one monitoring receiver  115 , the code phase and the carrier phase associated with each of the plurality of pseudolites  110 . 
     The operations at block  514  involve determining, by the at least one monitoring receiver  115  and based on the code phase and the carrier phase of the RF signal, corresponding clock bias estimates associated with the RF signals of each pseudolite  110 . 
     The operations at block  515  involve determining, by at least one control system  102  in communication with the plurality of pseudolites  110  and the at least one monitoring receiver and based on the RF signals received from the at least one monitoring receiver, whether the plurality of pseudolites  110  are operating within a nominal operating range. 
     The operations at block  520  involve communicating, by a bi-directional RF communication system in communication with the at least one control system  102 , landing area  105  information associated with the landing area  105  to the aerial vehicle  150 , wherein the landing area  105  information indicates particular pseudolites  110  of the plurality of pseudolites  110  that are operating within a nominal operating range and the clock bias estimate, which further facilitates determining, by the aerial vehicle  150  it’s position and velocity relative to the pseudolites  110 . 
     In some examples, the at least one monitoring receiver is one of a plurality of monitoring receivers  115  in communication with the at least one control system  102 . These examples involve when the RF signals received by a majority of the plurality of the monitoring receivers  115  indicate that the particular pseudolite  110  is in a particular state of operation, determining, by the at least one control system  102 , the particular pseudolite  110  is in the particular state of operation. 
     Some examples of the operations further involve when the RF signals received by the majority of the plurality of monitoring receivers  115  indicate that the particular pseudolite  110  is in a particular state of operation, and a particular monitoring receiver indicates that the particular pseudolite  110  is in a different state of operation, determining, by the at least one control system  102 , the particular monitoring receiver to be operating outside of a nominal operating range. 
     Some examples of the operations further involve receiving, by the at least one control system  102  and via the bi-directional RF communication system, a landing request from the aerial vehicle  150 , determining, by the at least one control system  102 , whether the aerial vehicle  150  is authorized to land at the landing area  105 , and responsive to determining that the aerial vehicle  150  is authorized to land at the landing area  105 , communicating, by the at least one control system  102 , an encryption key to the aerial vehicle  150  that facilitates decoding, by the aerial vehicle  150 , information in the RF signals that facilitates determining, by the aerial vehicle  150 , distances to the pseudolites  110 . 
     In some examples, the landing request specifies a current location of the aerial vehicle  150 . These examples further involve communicating, by the at least one control system  102  and to the aerial vehicle  150 , one or more waypoints that facilitates navigating, by the aerial vehicle  150 , the aerial vehicle  150  from the current location to the landing area  105 . 
     In some examples, communicating, by the plurality of pseudolites  110 , the RF signal involves communicating, by the plurality of pseudolites  110 , an RF signal that comprises location data that specifies a location of the particular pseudolite, integrity data that specifies whether the pseudolite  110  is operating within a nominal operating range, and a pseudorandom code that facilitates deriving, by the aerial vehicle  150 , a code-phase and a carrier-phase that facilitate measuring a distance between the aerial vehicle  150  and the particular pseudolite. 
     In some examples, each of the plurality of pseudolites comprises a pseudorandom code generator  155  configured to generate the pseudorandom code. These examples further involve deriving, by the pseudorandom code generator  155 , timing information from a common clock source  151  of the NTLS. 
     In some examples, each of the plurality of pseudolites comprises a pseudorandom code generator  155  configured to generate the pseudorandom code. These examples further involve operating corresponding pseudorandom code generators  155  of the plurality of pseudolites asynchronously with respect to one another; and communicating, by the at least one control system, timing information associated with corresponding pseudorandom code generators  155  of the plurality of pseudolites to the aerial vehicle to facilitate performance of trilateration by the aerial vehicle. 
     Some examples of the operations involve adjusting, by the at least one control system, timing information associated with the pseudorandom code generators  155  to minimize differences between respective frequencies and phases of pseudorandom codes generated by the respective pseudorandom code generators  155 . 
     In some examples, communicating, by the plurality of pseudolites  110  distributed proximate the landing area  105 , the RF signal involves communicating an RF signal that indicates whether a corresponding pseudolite  110  is operating within a nominal operating range. 
       FIG.  6    illustrates an example of a computer system  600  that can form part of or implement any of the systems and/or devices described above. Some examples of the computer system  600  include a set of instructions  645  that the processor  605  can execute to cause the computer system  600  to perform any of the operations described above. Some examples of the computer system  600  operate as a stand-alone device or can be connected, e.g., using a network, to other computer systems or peripheral devices. 
     In a networked example, some examples of the computer system  600  operate in the capacity of a server or as a client computer in a server-client network environment, or as a peer computer system in a peer-to-peer (or distributed) environment. Some examples of the computer system  600  are implemented as or incorporated into various devices, such as a personal computer or a mobile device, capable of executing instructions  645  (sequential or otherwise), causing a device to perform one or more actions. Further, some examples of the systems described include a collection of subsystems that individually or jointly execute a set, or multiple sets, of instructions to perform one or more computer operations. 
     Some examples of the computer system  600  include one or more memory devices  610  communicatively coupled to a bus  620  for communicating information. In addition, in some examples, code operable to cause the computer system to perform operations described above is stored in the memory  610 . Some examples of the memory  610  are random-access memory, read-only memory, programmable memory, hard disk drive, or any other type of memory or storage device. 
     Some examples of the computer system  600  include a display  630 , such as a liquid crystal display (LCD), organic light-emitting diode (OLED) display, or any other display suitable for conveying information. Some examples of the display  630  act as an interface for the user to see processing results produced by processor  605 . 
     Additionally, some examples of the computer system  600  include an input device  625 , such as a keyboard or mouse or touchscreen, configured to allow a user to interact with components of system  600 . 
     Some examples of the computer system  600  include a drive unit  615  (e.g., flash storage). Some examples of the drive unit  615  include a computer-readable medium  640  in which the instructions  645  can be stored. Some examples of the instructions  645  reside completely, or at least partially, within the memory  610  and/or within the processor  605  during execution by the computer system  600 . Some examples of the memory  610  and the processor  605  include computer-readable media, as discussed above. 
     Some examples of the computer system  600  include a communication interface  635  to support communications via a network  650 . Some examples of the network  650  include wired networks, wireless networks, or combinations thereof. Some examples of the communication interface  635  facilitate communications via any number of wireless broadband communication standards, such as the Institute of Electrical and Electronics Engineering (IEEE) standards 802.11, 802.12, 802.16 (WiMAX), 802.20, cellular telephone standards, or other communication standards. 
     Accordingly, some examples of the methods and systems described herein are realized in hardware, software, or a combination of hardware and software. Some examples of the methods and systems are realized in a centralized fashion in at least one computer system or in a distributed fashion where different elements are spread across interconnected computer systems. Any kind of computer system or other apparatus adapted for carrying out the methods described herein can be employed. 
     Some examples of the methods and systems described herein are embedded in a computer program product, which includes all the features that facilitate the implementation of the operations described herein and which, when loaded in a computer system, cause the computer system to perform these operations. A computer program as used herein refers to an expression, in a machine-executable language, code or notation, of a set of machine-executable instructions intended to cause a device to perform a particular function, either directly or after one or more of a) conversion of a first language, code, or notation to another language, code, or notation; and b) reproduction of a first language, code, or notation. 
     While the systems and methods of operation have been described with reference to certain examples, it will be understood by those skilled in the art that various changes can be made, and equivalents can be substituted without departing from the scope of the claims. Therefore, it is intended that the present methods and systems are not limited to the particular examples disclosed but that the disclosed methods and systems include all embodiments falling within the scope of the appended claims.