Abstract:
Falling drone warning apparatuses and methods are disclosed. The apparatus may be attached to a drone and may measure acceleration during the drone&#39;s operation in order to ascertain whether the drone is free falling. If the apparatus detects that the drone is free falling, the apparatus may activate an audible alarm to warn people on the ground of the potential danger and to afford them the opportunity to take action to avoid the drone&#39;s impact or minimize its effect.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of provisional application Ser. No. 62/349,079, filed Jun. 12, 2016, the disclosure of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure generally relates to unmanned aerial vehicles, hereinafter referred to as “drones,” and apparatuses and methods for warning people that a drone may be falling to the ground. 
       BACKGROUND 
       [0003]    As background, drones are remotely operated aircraft which use wireless communications to an operator for command and control. Drones are also called “unmanned aircraft systems” or “unmanned aerial vehicles.” Most drones feature geo-positioning using a global navigation satellite system (GNSS) such as the United States&#39; global positioning satellite (GPS) system. Drones have been in use for over a century for military purposes, such as missile targets; yet their existence and usage were relatively unknown until America&#39;s war on terror following the Sep. 11, 2001, terrorist attacks. Shortly thereafter, the Predator® aircraft became a household name for its well-publicized missions in intelligence, surveillance, and reconnaissance (ISR) and for its use in remote missile attacks. However, only in the past five years or so have drones become ubiquitous, primarily with vertical-take-off-and-landing (VTOL) aircraft featuring multiple propellers (e.g., quadcopters, octocopters), such as the Phantom® from DJI®, but also with more traditional aircraft featuring a single propeller and forward flight. Aided greatly by several technological advances and economies of scale, consumer- and commercial-grade drones have become available at an affordable cost, enabling the adoption by law enforcement, various industries, educational institutions, hobbyists, and even household consumers. 
         [0004]    Drones are now in use in industries as varied as law enforcement, emergency management, photography, videography, motion picture and television, real estate, farming and agriculture, construction, mining, sports, railroad, utilities, oil and gas, and so forth. The applications for drones continue to grow at a remarkable rate; and, thus, it is only time before their use and presence in our skies become widespread. With this advancement comes a growing safety threat, as these machines can suffer from unexpected catastrophic failures as well as “fly-aways” when the user loses complete control of the drone, and it travels out of visual range without warning. Several injuries have been reported as well as widely publicized near-misses. In one such example, a professional skier was nearly struck by a falling videography drone during a skiing event in December, 2015, in Italy. Other people have not been so lucky. In another example, an athlete was reportedly struck on the head by a drone in April, 2014, during a triathlon in Western Australia. It is very likely that accident rates will rise as more and more people begin flying drones. 
         [0005]    While drone manufacturers are beginning to build in safety features, such as proximity sensors to help prevent crashing into objects and geo-fencing to prevent drone usage in certain geographical areas or airspace (e.g., the White House in Washington, D.C.), they may fall short of mitigating the threat of catastrophic failure in which either the electronics and/or the battery stops working and the drone falls victim to gravity. 
         [0006]    Existing add-on drone safety devices on the market are currently limited to automatic parachutes, which while novel and promising for reducing the risk of serious injury, are much heavier solutions and do not warn people on the ground of the impending impact. Thus, there exists a void in the market for a compact electronic device that intelligently senses a drone&#39;s flight status and provides advance warning to those on the ground if there is a catastrophic failure. Such a device must be small and light enough to attach to a drone without significantly impacting the drone&#39;s battery life or aerodynamics. The present disclosure is directed toward providing such apparatuses and methods. 
         [0007]    The apparatuses and methods disclosed herein may significantly reduce the risk of innocent bystanders (i.e., people and animals) being unaware of an impending crash from a drone falling from the sky. The apparatuses may feature three-axis sensing of acceleration as well as other sensor inputs to determine if a drone is in free-fall. The apparatuses may further include a loud audible alarm which is intended to immediately direct one&#39;s attention to the sky, similar to the effect of an ambulance or police siren since the natural reaction is to look in the direction of audible alarm like a siren. The audible alarm may be augmented by an optional visual alarm to visually cue bystanders to look up in the sky and take immediate action, such as to run or dive out of the way of the falling object. The visual alarm would add the ability for individuals to visually identify the current position and trajectory of the falling drone, especially in low-light conditions. Further, this apparatus may operate completely independently from the drone, except for the physical attachment. Thus, the apparatus may not derive power or sensor inputs directly from the drone, which may provide an inherent redundancy to any native safety features provided by the drone itself. 
         [0008]    As such, the present disclosure describes a warning apparatus which may operate independently from the drone, including sensing and characterizing the drone&#39;s flight status and providing its own power. Such a warning apparatus may provide advance warning to people on the ground of a potential impact from a falling drone and may afford them the ability to vacate the area of imminent impact or at least to mitigate the damage of the impact by protecting the head and/or neck area. Furthermore, the warning apparatus may be expected to minimize injuries and possibly reduce deaths causes by unexpected drone impacts. 
       SUMMARY 
       [0009]    In one embodiment, a falling drone warning apparatus comprises a structure, a processor, an accelerometer, an audible alarm, and a power source, wherein: the processor, the accelerometer, the audible alarm, and the power source are mechanically coupled to the structure, and the structure is configured to be attached to a drone; the power source supplies electrical power to the processor, the accelerometer, and the audible alarm; the accelerometer is capable of measuring acceleration in three orthogonal axes and is electrically coupled to the processor such that the processor is capable of reading a measured acceleration for each of the three orthogonal axes; the audible alarm is electrically coupled to the processor such that the processor is capable of activating the audible alarm; and the processor activates the audible alarm in a first audible manner based on whether the measured acceleration of one or more of the three orthogonal axes or a combination of the measured acceleration of any two or all of the three orthogonal axes is greater than a first predetermined threshold. 
         [0010]    In another embodiment, a falling drone warning apparatus comprises a structure, a processor, an accelerometer, an acoustic sensor, an audible alarm, and a power source, wherein: the processor, the accelerometer, the acoustic sensor, the audible alarm, and the power source are mechanically coupled to the structure, and the structure is configured to be attached to a drone; the power source supplies electrical power to the processor, the accelerometer, the acoustic sensor, and the audible alarm; the accelerometer is capable of measuring acceleration in three orthogonal axes and is electrically coupled to the processor such that the processor is capable of reading a measured acceleration for each of the three orthogonal axes; the acoustic sensor measures one or more sound characteristics of one or more propellers of the drone, and the acoustic sensor is electrically coupled to the processor such that the processor is capable of reading the one or more measured sound characteristics from the acoustic sensor; the audible alarm is electrically coupled to the processor such that the processor is capable of activating the audible alarm; and the processor activates the audible alarm in a first audible manner based on whether the measured acceleration of one or more of the three orthogonal axes or a combination of the measured acceleration of any two or all of the three orthogonal axes is greater than a first predetermined threshold and based on the one or more measured sound characteristics. 
         [0011]    In yet another embodiment, a falling drone warning method comprises: attaching a falling drone warning apparatus to a drone, wherein the falling drone apparatus comprises an accelerometer capable of measuring acceleration in three orthogonal axes, an audible alarm, a visual alarm, and a power source, and the power source supplies electrical power to the accelerometer, the audible alarm, and the visual alarm; measuring the acceleration in each of the three orthogonal axes by the accelerometer; and activating the audible alarm in a first audible manner and activating the visual alarm in a first visual manner based on whether the measured acceleration of one or more of the three orthogonal axes or a combination of the measured acceleration of any two or all of the three orthogonal axes is greater than a first predetermined threshold. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The embodiments set forth in the drawings are illustrative and exemplary in nature and not intended to limit the inventions defined by the claims. The following detailed description of the illustrative embodiments can be understood when read in conjunction with the following drawings, where like structure is indicated with like reference characters and in which: 
           [0013]      FIG. 1  depicts a drone warning apparatus according to one or more embodiments shown and described herein; 
           [0014]      FIG. 2  illustrates a drone warning apparatus affixed to a drone according to one or more embodiments shown and described herein; and 
           [0015]      FIG. 3  shows a method for warning of a falling drone according to one or more embodiments shown and described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The embodiments described herein generally relate to falling drone warning apparatuses and methods, which may warn people on the ground of a potential impact from a falling or out-of-control drone. Such apparatuses and methods may also allow searchers to locate the fallen drone after it has landed on or crashed to the ground. 
         [0017]      FIG. 1  depicts a falling drone warning apparatus  10  according to one or more embodiments shown and described herein. The apparatus  10  may include a structure  20 , power source  30 , a processor  40 , an accelerometer  50 , an acoustic sensor  60 , an audible alarm  70 , and a visual alarm  80 . The power source  30 , processor  40 , accelerometer  50 , acoustic sensor  60 , audible alarm  70 , and visual alarm  80  may be mechanically coupled to the structure  20 . The structure  20  may comprise an enclosure which may be constructed of plastic, carbon fiber, or any other suitable lightweight material which is capable of being physically attached to a drone. The structure  20  may be attached to the outside or inside of a professional, semi-professional (i.e., “prosumer”), or consumer-grade drone in such a fashion to reduce the likelihood that the apparatus  10  impacts the drone&#39;s flight characteristics or aerodynamics. The structure  20  may include one or more mounting features  20   a  which may facilitate the attachment of the apparatus  10  to the drone. The mounting features  20   a  may include eyelets, holes, hooks, and other similar devices, as is known in the art. In general, the apparatus  10  may be attached to the drone via any suitable means, including cable ties, hooks, screws, and the like. 
         [0018]    The power source  30  may supply electrical power to the processor  40 , accelerometer  50 , acoustic sensor  60 , audible alarm  70 , and visual alarm  80 . For example, the power source  30  may comprise a battery which is capable of suppling sufficient voltage  30   a  and current to the processor  40 , accelerometer  50 , acoustic sensor  60 , audible alarm  70 , and visual alarm  80 . In one embodiment, the power source  30  may comprise a coin cell lithium battery, such as a CR1632 battery which has a nominal voltage of 3.0 Volts. Other types of batteries may be used as well. Furthermore, two or more batteries may be used. The battery may be removeably inserted in a battery holder, such as model number 3012TR from Keystone® Electronics, which is designed to accommodate a CR1632 battery which is approximately 16 mm in diameter and 3.2 mm thick. The power source  30  may include voltage regulators, DC/DC converters, and other voltage converters. As such, the power source  30  may be capable of supplying one voltage level to the processor  40 , the accelerometer  50 , and the acoustic sensor  60 , and a different voltage level to the audible alarm  70  and visual alarm  80 . 
         [0019]    The processor  40  may comprise a microcontroller, including 8-bit, 16-bit, and 32-bit microcontrollers. In one embodiment, the processor  40  comprises an ARM® Cortex® MO+microcontroller from NXP Semiconductor, model number LPC812M101JTB16X, which has 16 kilobytes of on-chip flash programming memory. The processor  40  may also include RAM, timers, serial interfaces, oscillators, and other peripherals. The on-chip flash programming memory may be designed to execute a program comprising software or firmware instructions. The processor  40  may include a programming interface, such as a Joint Test Action Group (JTAG) interface, which permits a user to load the program into the processor  40 . 
         [0020]    Continuing to refer to  FIG. 1 , the accelerometer  50  may be a 3-axis type, which is capable of measuring acceleration in three orthogonal axes, typically called the x-axis, y-axis, and z-axis. These three orthogonal axes may be disposed at right angles to each other so that the accelerometer  50  is capable of measure acceleration in any direction. Because the accelerometer  50  may be mechanically coupled to the structure  20 , and the structure  20  may be attached to the drone, the accelerometer  50  may be capable of measuring the acceleration of the drone itself. The accelerometer  50  may be electrically coupled to the processor  40  via electrical signals  50   a . The electrical signals  50   a  may include a serial data bus, such as SPI or I 2 C. In one embodiment, the electrical signals  50   a  may include an I 2 C bus which allows the processor  40  to bi-directionally communicate to the accelerometer  50 . The electrical signals  50   a  may also include a signal (e.g., an interrupt signal) which may inform the processor  40  when the measured acceleration greater than (or less than) a predetermined threshold. As used herein, the term “measured acceleration” includes the measured acceleration of any one or more of the three orthogonal axes or of a combination of the measured acceleration of any two or all three axes; also “a combination” of the measured acceleration of any two or all three orthogonal axes may include the sum of any two axes (e.g., x plus y, y plus z, and/or x plus z) or may include the sum of all three axes. In one embodiment, the accelerometer  50  may comprise model number MMA8652FC from Freescale® Semiconductor; this model is capable of measuring acceleration in the x-, y-, and z-axis and converting the measured acceleration for each axis into a digital number by employing a 12-bit analog-to-digital converter. As such, the processor  40  may be able to read the measured acceleration for each of the three orthogonal axes or a combination thereof. 
         [0021]    In one embodiment, the processor  40  may receive a signal from the accelerometer  50  which indicates that the measured acceleration for all axes combined is greater than a first predetermined threshold and/or that the measured acceleration is less than a second predetermined threshold. In this case, the accelerometer  50  itself may perform the task of determining whether the measured acceleration is greater than a first predetermined threshold and, if so, may inform the processor  40  via the signal (e.g., an interrupt signal). In another embodiment, the processor  40  may periodically read the measured acceleration from the accelerometer  50  (e.g., via the SPI or I 2 C bus), and the processor  40  may determine whether the measured acceleration is greater than a first predetermined threshold. The processor  40  may do this by executing software instructions which compare the measured acceleration with the first predetermined threshold. In any case, the processor  40  may be able to ascertain whether the measured acceleration is greater than a first predetermined threshold or less than a second predetermined threshold. 
         [0022]    Because the processor  40  may periodically read the measured acceleration for each of the three orthogonal axes, the processor  40  may also be capable of determining a rate of change of the measured acceleration for each axis. That is, the processor  40  may be able to determine how quickly the acceleration is changing. The processor  40  may determine whether the drone is in a free-fall condition based in part on whether the rate of change of acceleration of one or more of the three orthogonal axes or a combination thereof is greater than a predetermined rate-of-change threshold. 
         [0023]    For the purposes of this disclosure, “acceleration” and “measured acceleration” are measured with respect to the earth&#39;s acceleration due to gravity, which is approximately 9.8 m/s 2  (meters per second squared). That is, an object resting on the ground has zero acceleration with respect to gravity. The aforementioned MMA8652FC accelerometer measures absolute acceleration, which includes gravity; thus, it measures 9.8 m/s 2  when resting on the ground, and it reads close to zero m/s 2  when in free-fall. Thus, it is to be understood that this disclosure anticipates measuring acceleration in any suitable manner, although the use of “acceleration” and “measured acceleration” herein are defined to be with respect to gravity, unless otherwise stated. 
         [0024]    Furthermore, the process of measuring acceleration may include filtering, either by the processor  40  and/or by the accelerometer  50 . The filtering may remove or reduce unwanted noise introduced by the measuring process and/or introduced by sudden maneuvers by the drone which could be misinterpreted as the drone going into a free-fall condition. The filtering may be adjusted for a particular type and size of drone. For the purposes of this disclosure, the measured acceleration may or may not include filtering. As an example, the accelerometer  50  may measure acceleration by averaging the most recent eight samples from its analog-to-digital converter. Other types of filtering may be employed, as is known in the art. 
         [0025]    The accelerometer  50  may send an interrupt to the processor  40  when a potential free-fall condition is detected. The processor may pull the x-, y-, and z-axis data into its register; and the processor may perform multiple calculations on the combined x-, y-, and z-axis acceleration values over time to determine whether the combined measured acceleration is greater than a predetermined single- or multi-axis threshold, which would be more indicative of a free-fall as opposed to a momentary acrobatic maneuver, such as an intentional acrobatic flip. 
         [0026]    The acoustic sensor  60  may be capable of measuring one or more sound characteristics (i.e., frequency and amplitude) of the one or more propellers of the drone. When the drone is flying in a normal, controlled fashion, the rotating propellers (and the air moved by the propellers) may emit a sound having particular frequency and amplitude characteristics. These characteristics may depend on a number of factors, including the shape, size, and number of propellers as well as their operating rotational speed. As such, the frequency and amplitude characteristics may vary between different types of drones. In one embodiment, the expected sound characteristics may be determined by simply permitting the acoustic sensor  60  to record the sound of the one or more propellers when the drone is operating normally. 
         [0027]    By measuring the one or more sound characteristics emitted by the one or more propellers of the drone, the acoustic sensor  60  may determine whether the measured frequency and amplitude characteristics are those expected from normally operating propellers. If the measured frequency and amplitude characteristics are close to an expected sound characteristic pattern, then the acoustic sensor  60  may assume a normal operational status of the one or more propellers. Otherwise, the acoustic sensor  60  may assume a failed operational status of the one or more propellers, which may indicate that one of the one or more propellers are either no longer rotating or are rotating at speed insufficient to prevent the drone from free falling. When this occurs, the drone may be entering into a free-fall condition. The acoustic sensor  60  may be electrically coupled to the processor  40  via electrical signals  60   a,  which may include a serial data bus, such as SPI or I 2 C. 
         [0028]    The measured acceleration and the measured sound characteristics of the one or more propellers may be combined in order to determine whether the drone is in a free-fall condition. Regarding the acceleration, the processor  40  may ascertain whether the measured acceleration is greater than the first predetermined threshold, and the processor  40  may combine this with how closely the sound characteristics match to an expected sound characteristic pattern. In one embodiment, the processor  40  may weight the measured acceleration by 70% and the sound characteristics by 30% in order to determine whether the drone is in free-fall condition. Other percentages may be used as well. In one embodiment, the processor  40  may weight the measured acceleration by determining how close it is to the threshold (above or below). 
         [0029]    The audible alarm  70  may be capable of emitting a sound  70   b  when activated such that the sound  70   b  may be capable of attracting the attention of people on the ground. The audible alarm  70  may be electrically coupled to the processor  40  via electrical signals  70   a  such that the processor  40  is capable of activating the audible alarm  70 . The audible alarm  70  may include an amplifier (e.g., an operational amplifier) in order to allow the processor  40  to efficiently drive the audible alarm  70 . In one embodiment, the audible alarm  70  may be model number PKLCS1212E24AO-R1 from Murata Electronics; models from other suppliers may be used as well. 
         [0030]    The audible alarm  70  may be activated in a number of audible manners in which the frequency, amplitude, and other characteristics of the sound  70   b  may be defined. In one embodiment, the audible alarm  70  may be activated in a first audible manner which the sound  70   b  may be similar to a siren emitted by a typical police car or ambulance. For example, activating the audible alarm  70  in the first audible manner may cause it to generate a sound  70   b  which rapidly alternates between two frequencies. Such a type of sound  70   b  may attract the attention of people on the ground and may audibly warn them of an impending impact from a falling drone. In another embodiment, the audible alarm  70  may be activated in a second audible manner in which the sound  70   b  may comprise a periodic “chirp” at a fixed rate (e.g., one second). Such a type of sound  70   b  may assist searchers looking for the drone after it has crashed to the ground. Other audible manners of activating the audible alarm  70  are anticipated. 
         [0031]    The visual alarm  80  may be capable of emitting visible light  80   b  when activated such that the visible light  80   b  may attract the attention of people on the ground, which may permit them to ascertain the location and trajectory of the falling drone and to take appropriate action. The visual alarm  80  may be electrically coupled to the processor  40  via electrical signals  80   a  such that the processor  40  is capable of activating the visual alarm  80 . The visual alarm  80  may include an amplifier or a driver circuit in order to allow the processor  40  to efficiently drive the visual alarm  80 . In one embodiment, the visual alarm  80  may comprise one or more light-emitting diodes (LEDs); other types of light-emitting devices may be used as well. In one embodiment, the visual alarm  80  may include one or more LEDs, model number APT1608SURCK or APT1608SGC from Kingbright. 
         [0032]    Similar to the audible alarm  70 , the visual alarm  80  may be activated in a number of visual manners in which the frequency and other characteristics of the visible light  80   b  may be defined. In one embodiment, the visual alarm  80  may be activated in a first visual manner in which a bright, white light is rapidly turned on and off. For example, activating the visual alarm  80  in the first visual manner may cause it to generate a bright, white light which turns off and on at a rate of  500  milliseconds. Such a visible light  80   b  may attract the attention of people on the ground and may also permit them to ascertain the location and trajectory of the falling drone and to take appropriate action. In another embodiment, the visual alarm  80  may be activated in a second visual manner in which the visible light  80   b  comprises a periodic “flicker” at a fixed rate (e.g., one second). Such a visible light  80   b  may assist searchers looking for the drone after it has crashed to the ground. Other visual manners of activating the visual alarm  80  are anticipated. 
         [0033]    The power source  30 , processor  40 , accelerometer  50 , acoustic sensor  60 , audible alarm  70 , and visual alarm  80  do not necessarily have to be directly affixed to the structure  20  in order to be mechanically coupled thereto. For example, the processor  40 , accelerometer  50 , acoustic sensor  60 , and audible alarm  70  may be mounted on a printed circuit board (PCB), which may be affixed to the structure  20 . Other means of mechanically coupling these components to the structure  20  are contemplated as well. 
         [0034]    Regarding the predetermined thresholds, the first predetermined threshold and the second predetermined threshold may be based on the size and type of drone. For example, the first predetermined threshold may be set to 9.0 m/s 2 . That is, the processor  40  may ascertain that the drone is entering a free-fall condition if the measured acceleration is greater than 9.0 m/s 2 , and it may activate the audible alarm  70  in a first audible manner and the visual alarm  80  in a first visual manner. As another example, the second predetermined threshold may be set to 0.1 m/s 2 . That is, the processor  40  may ascertain that the drone has landed on or crashed to the ground if, after the processor  40  has ascertained that the drone is in a free-fall condition, the measured acceleration is less than 0.1 m/s 2 . In this case, the processor  40  may activate the audible alarm  70  in a second audible manner and activate the visual alarm  80  in a second visual manner. 
         [0035]    In  FIG. 2 , a portion of a drone is shown to which the falling drone warning apparatus  120  may be attached. In this figure, the portion of the drone shown includes a drone landing support  100  and a drone landing leg  110 . It is anticipated that the apparatus  120  may be attached to other parts of the drone. In one embodiment, a falling drone warning apparatus  120  may comprise a structure  130  which may comprise an enclosure as shown. The structure  130  may have one or more mounting features  140  to facilitate the attachment of the apparatus  120  to the drone. The mounting features  140  may accommodate mounting screws, washers and nuts, cable ties, or other attachment means. In this embodiment, the apparatus  120  may be attached to the drone landing leg  110  via cable ties  140   a.  The apparatus  120  may also include a power switch  150  which may allow the user to turn the apparatus  120  on and off as needed. The apparatus  120  may include a status indicator  160  which may indicate an operating status of the apparatus  120 . For example, the status indicator  160  may indicate that the apparatus  120  is turned on and is operating normally. The apparatus  120  may further include one or more openings  170  to permit the noise generated by the audible alarm to transfer to the outside of the enclosure more efficiently, thus reducing the potential sound attenuation from the enclosure itself. The openings  170  may include holes, slots, and so forth. Finally, the apparatus  120  may also comprise a battery compartment  180  which may facilitate the installation and removed of a battery or other power source. 
         [0036]      FIG. 3  illustrates a flow chart  200  of the operation of the falling drone warning apparatus. The process starts at step  210  when the apparatus is turned on. The next step  220  is a built-in test (BIT), wherein the apparatus performs a self-test to determine whether the circuitry is fully operable. At step  230 , the status of the BIT is determined. If the BIT passes, the process moves to step  250 ; if the BIT fails, the process moves to step  240  where a BIT fault is indicated (e.g., by a status indicator). At step  250 , the process enters “sensing mode.” At step  260 , the process determines whether the measured acceleration exceeds a first predetermined threshold and/or reads the one or more sound characteristics from the acoustic sensor. At step  270 , the process determines whether the drone is in free-fall in the x-, y-, and/or z-axis based on whether the measured acceleration of any of these axes or combination thereof exceeds a first predetermined threshold and/or based on the one or more sound characteristics from the acoustic sensor. At step  270 , when the process determines that the drone is in a free-fall condition, the process moves to step  280 , where the process enters “catastrophic alert mode.” If, at step  270 , the process determines that the drone not in a free-fall condition, the process moved back to step  260 . 
         [0037]    At step  280 , the method activates the audible alarm and the visual alarm. The audible alarm may be activated in a first audible manner in which the audible alarm may produce a noise that is similar to a siren. The visual alarm may be activated in a first visual manner in which the visual alarm flashed visible light at a fixed frequency. The audible alarm and visual alarm continue to be activated in this manner when the process moves to step  290 , in which the process ascertains whether the measured acceleration of one or more of the three orthogonal axes or combination thereof is less than a second predetermined threshold. At step  300 , if the measured acceleration is less than a second predetermined threshold, the processor determines that the drone has crashed, at which time, the process moves on to step  310 . At step  300 , if the process ascertains that the measured acceleration is greater than or equal to a second predetermined threshold, the process moves back to step  290  and the process repeats. At step  310 , the process enters “recovery mode,” in which the process may provide a unique indication through the audible alarm and/or the visual alarm. In this mode, the operation of the audible alarm and/or the visual alarm may be different than in “catastrophic alert mode.” For example, in this mode, the audible alarm may be activated in a second audible manner, and the visual alarm may be activated in a second visual manner. For example, the audible alarm activated in the second audible manner may emit a noise at an intermittent fashion; and the visual alarm activated in the second visual manner may flash at a slower frequency. 
         [0038]    While particular embodiments and aspects of the present invention have been illustrated and described herein, various other changes and modifications may be made without departing from the spirit and scope of the invention. Moreover, although various inventive aspects have been described herein, such aspects need not be utilized in combination. It is therefore intended that the appended claims cover all such changes and modifications that are within the scope of this invention.