Patent Publication Number: US-11650329-B2

Title: Motion sensors in asset travel monitoring

Description:
CROSS-REFERENCE 
     This application claims the benefit under 35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 63/012,995, titled “Motion Sensors in Asset Travel Monitoring, Temperature-Dependent Charging of Asset Tracking Devices, and Asset Travel Monitoring with Linked Asset Tracking Devices”, filed on Apr. 21, 2020, which is herein incorporated by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates to telematics, and in particular to asset tracking devices and asset tracking device management systems for monitoring the movement of assets. 
     BACKGROUND 
     The movement of an asset may be monitored by the placement of an asset tracking device on the asset. An asset tracking device may communicate with a satellite navigation system, such as a Global Positioning System (GPS), Global Navigation Satellite System (GNSS), cellular tower network, Wi-Fi network, or other system which enables the monitoring of the location of the asset tracking device. Such an asset tracking device may periodically obtain its location from such a locating system and transmit its location to an asset tracking device management system that records movements of the asset. Asset tracking devices and asset tracking device management systems may be used to monitor the movement of vehicular assets such as trucks, ships, and cars, and non-vehicular assets such as transport trailers, shipping containers, pallets, shipped goods, or any other asset which may be tracked by an asset tracking device. 
     SUMMARY 
     According to an aspect of the disclosure, a method for asset travel monitoring is provided. The method involves monitoring a motion sensor of an asset tracking device located at an asset to determine whether the asset has entered into a travelling state, upon determination that the asset has entered into the travelling state, monitoring the motion sensor to determine whether the asset has left the travelling state, and, upon determination that the asset has left the travelling state, obtaining a present location of the asset tracking device and transmitting the present location to a remote server. 
     Monitoring the motion sensor to determine whether the asset has entered into the travelling state may involve monitoring the motion sensor to detect initiation of motion of the asset tracking device, and, upon detection of the initiation of motion of the asset tracking device, monitoring the motion sensor for continued motion to determine whether the asset has begun deliberate travel. Monitoring the motion sensor to determine whether the asset has left the travelling state may involve monitoring the motion sensor for suspension of motion of the asset tracking device, and, upon detection of suspension of motion of the asset tracking device, monitoring the motion sensor for continued lack of motion to determine whether the asset has ceased deliberate travel. The method may involve operating a locating device of the asset tracking device in a low-power operating mode and operating a communication interface of the asset tracking device in a low-power operating mode. Transmitting the present location to the remote server may involve a controller of the asset tracking device determining whether the locating device has access to sufficient power to obtain the present location, and, upon determination that the asset tracking device has access to sufficient power to obtain the present location, the controller waking the locating device from its low-power operating mode. The transmitting may involve the locating device obtaining the present location, the controller returning the locating device to its low-power operating mode, the controller determining whether the communication interface has access to sufficient power to transmit the present location to the remote server, upon determination that the communication interface has access to sufficient power to transmit the present location to the remote server, the controller waking the communication interface from its low-power operating mode, the communication interface transmitting the present location as the trip end location to the remote server, and the controller returning the communication interface to its low-power operating mode. The method may involve, upon determination that the asset has entered into the travelling state, transmitting an indication of a travel beginning location of the asset tracking device to the remote server. The method may involve periodically transmitting a heartbeat signal to the remote server to indicate that the asset tracking device is active. The asset may be a non-vehicular asset that is coupleable to a vehicle, the vehicle to control travel of the asset. 
     According to another aspect of the disclosure, an asset tracking device is provided. The asset tracking device includes a motion sensor to detect motion at the asset tracking device, a locating device to locate the asset tracking device, a communication interface to communicate with a remote server, and a controller. The controller is to execute asset travel monitoring instructions to monitor the motion sensor to determine whether an asset at which the asset tracking device is located has entered into a travelling state, upon determination that the asset has entered into the travelling state, monitor the motion sensor to determine whether the asset has left the travelling state, and, upon determination that the asset has left the travelling state, cause the locating device to obtain a present location of the asset tracking device, and cause the communication interface to transmit the present location to the remote server. 
     The asset tracking device may include an energy storage unit to power the asset tracking device, wherein the energy storage unit includes a supercapacitor. The asset tracking device may include an energy harvester to supply energy to the energy storage unit, wherein the energy harvester includes a solar panel. The motion sensor may include an accelerometer. The communication interface may include a cellular modem. The locating device may include a global navigation satellite system (GNSS) device. The asset may be a non-vehicular asset that is coupleable to a vehicle, the vehicle to control travel of the asset. 
     According to yet another aspect of the disclosure, a non-transitory machine-readable storage medium comprising instructions that when executed cause a controller of an asset tracking device to execute a method for asset travel monitoring is provided. The instructions cause the controller to monitor a motion sensor of an asset tracking device located at an asset to determine whether the asset has entered into a travelling state, upon determination that the asset has entered into the travelling state, monitor the motion sensor to determine whether the asset has left the travelling state, and upon determination that the asset has left the travelling state, obtain a present location of the asset tracking device and transmit the present location to a remote server. 
     The instructions may cause the controller to monitor the motion sensor to determine whether the asset has entered into the travelling state by monitoring the motion sensor to detect initiation of motion of the asset tracking device, and, upon detection of the initiation of motion of the asset tracking device, monitoring the motion sensor for continued motion to determine whether the asset has begun deliberate travel. The instructions may cause the controller to monitor the motion sensor to determine whether the asset has left the travelling state by monitoring the motion sensor for suspension of motion of the asset tracking device, and, upon detection of suspension of motion of the asset tracking device, monitoring the motion sensor for continued lack of motion to determine whether the asset has ceased deliberate travel. The instructions may cause the controller to operate a locating device of the asset tracking device in a low-power operating mode and operate a communication interface of the asset tracking device in a low-power operating mode. The instructions may cause the controller to transmit the present location to the remote server by the controller of the asset tracking device determining whether the locating device has access to sufficient power to obtain the present location, upon determination that the asset tracking device has access to sufficient power to obtain the present location, the controller waking the locating device from its low-power operating mode, the locating device obtaining the present location, the controller returning the locating device to its low-power operating mode, the controller determining whether the communication interface has access to sufficient power to transmit the present location to the remote server, upon determination that the communication interface has access to sufficient power to transmit the present location to the remote server, the controller waking the communication interface from its low-power operating mode, the communication interface transmitting the present location as the trip end location to the remote server, and the controller returning the communication interface to its low-power operating mode. The instructions may cause the controller to, upon determination that the asset has entered into the travelling state, transmit an indication of a travel beginning location of the asset tracking device to the remote server. The instructions may cause the controller to periodically transmit a heartbeat signal to the remote server to indicate that the asset tracking device is active. 
     According to yet another aspect of the disclosure, a method for temperature-dependent charging of a supercapacitor energy storage unit is provided. The method involves obtaining a temperature reading measured at an asset tracking device, the asset tracking device located at an asset to monitor travel of the asset, determining a target voltage for a supercapacitor energy storage unit of the asset tracking device based on the temperature reading to balance utilization of a capacity of the supercapacitor energy storage unit against temperature-dependent deterioration of the supercapacitor energy storage unit, and controlling a charging interface of the asset tracking device to charge the supercapacitor energy storage unit to the target voltage. 
     The method may involve obtaining one or more additional previously measured temperature readings measured at the asset tracking device, wherein determining the target voltage is further based on the one or more additional previously measured temperature readings. The method may involve, receiving environmental data from a remote server, the environmental data pertaining to an environmental condition at a present location of the asset tracking device, wherein determining the target voltage is further based on the environmental data. The environmental data may include temperature data that indicates a regional temperature at the present location. The asset tracking device may include a solar panel to provide energy to the supercapacitor energy storage unit through the charging interface, and the environmental data may include sunlight data that indicates an amount of sunlight expected to reach the asset tracking device at the present location. Determining the target voltage may be based on a charge cycle deterioration model of the supercapacitor energy storage unit. 
     According to yet another aspect of the disclosure, an asset tracking device with temperature-dependent charging of a supercapacitor energy storage unit is provided. The asset tracking device includes a temperature sensor to capture temperature readings at the asset tracking device, a supercapacitor energy storage unit to power the asset tracking device, a charging interface to charge the supercapacitor energy storage unit, and a controller to execute temperature-dependent charge control instructions. The instructions are to obtain a temperature reading measured at the asset tracking device, determine a target voltage for the supercapacitor energy storage unit based on the temperature reading to balance utilization of a capacity of the supercapacitor energy storage unit against temperature-dependent deterioration of the supercapacitor energy storage unit, and control the charging interface to charge the supercapacitor energy storage unit to the target voltage. The asset tracking device is located at an asset to monitor travel of the asset. 
     The controller may obtain one or more additional previously measured temperature readings measured at the asset tracking device and determine the target voltage further based on the one or more additional previously measured temperature readings. The asset tracking device may include a communication interface to receive environmental data from a remote server, the environmental data pertaining to an environmental condition at a present location of the asset tracking device, wherein the controller is to determine the target voltage based on the environmental data. The environmental data may include temperature data that indicates a regional temperature at the present location. The asset tracking device may include a solar panel to supply energy to the supercapacitor energy storage unit through the charging interface, wherein the environmental data comprises sunlight data that indicates an amount of sunlight expected to reach the asset tracking device at the present location. The controller may determine the target voltage further based on a charge cycle deterioration model of the supercapacitor energy storage unit. The asset may be a non-vehicular asset that is coupleable to a vehicle, the vehicle to control travel of the asset. 
     According to yet another aspect of the disclosure, a non-transitory machine-readable storage medium comprising instructions that when executed cause a controller of an asset tracking device to execute a method for temperature-dependent charging of a supercapacitor energy storage unit is provided. The instructions cause the controller to obtain a temperature reading measured at the asset tracking device, the asset tracking device located at an asset to monitor travel of the asset, determine a target voltage for a supercapacitor energy storage unit of the asset tracking device based on the temperature reading to balance utilization of a capacity of the supercapacitor energy storage unit against temperature-dependent deterioration of the supercapacitor energy storage unit, and control a charging interface of the asset tracking device to charge the supercapacitor energy storage unit to the target voltage. 
     The instructions may cause the controller to obtain one or more additional previously measured temperature readings measured at the asset tracking device, determine the target voltage further based on the one or more additional previously measured temperature readings. The asset tracking device may include a communication interface to receive environmental data from a remote server, the environmental data pertaining to an environmental condition at a present location of the asset tracking device, and the instructions may cause the controller to controller to determine the target voltage based on the environmental data. The environmental data may include temperature data that indicates a regional temperature at the present location. The environmental data may include sunlight data that indicates an amount of sunlight expected to reach the asset tracking device at the present location, and the asset tracking device may include a solar panel to supply energy to the supercapacitor energy storage unit through the charging interface. The instructions may cause the controller to determine the target voltage further based on a charge cycle deterioration model of the supercapacitor energy storage unit. The asset may be a non-vehicular asset that is coupleable to a vehicle, the vehicle to control travel of the asset. 
     According to yet another aspect of the disclosure, a method for monitoring the travel of assets that travel together is provided. The method involves obtaining a first travel history of a first asset tracking device, obtaining a second travel history of a second asset tracking device, determining, based on the first and second travel histories, whether the first asset tracking device and the second asset tracking device travel together, and, upon determination that the first asset tracking device and the second asset tracking device travel together, linking the first asset tracking device and the second asset tracking device together in an asset tracking database to indicate that the first asset tracking device and the second asset tracking device travel together. 
     Determining that the first asset tracking device and the second asset tracking device travel together may involve determining that a first trip travelled by the first asset tracking device recorded in the first travel history matches a second trip recorded in the second travel history travelled by the second asset tracking device. Determining that the first trip matches the second trip may involve determining that the first asset tracking device and the second asset tracking device were in a vicinity of one another throughout a duration of the first trip and the second trip. Determining that the first trip matches the second trip may involve determining that the first trip and the second trip are coterminous and contemporaneous with one another. The method may involve obtaining a third travel history of the first asset tracking device, obtaining a fourth travel history of the second asset tracking device, determining, based on the third and fourth travel histories, that the first asset tracking device and the second asset tracking device have stopped travelling together, and, upon determination that the first asset tracking device and the second asset tracking device stopped travelling together, unlinking the first asset tracking device from the second asset tracking device in the asset tracking database to indicate that the first asset tracking device and the second asset tracking device have stopped travelling together. The first asset tracking device may be located at a non-vehicular asset, the second asset tracking device may be located at a vehicle, and the vehicle may be to control travel of the non-vehicular asset. The non-vehicular asset may be coupleable to the vehicle. The non-vehicular asset may include a transport trailer. The method may involve compiling trip information from the first travel history together with trip information from the second travel history for display at a display device. The method may involve displaying a visual indication that the first asset tracking device travels with the second asset tracking device. 
     According to yet another aspect of the disclosure, a system for monitoring the travel of assets that travel together is provided. The system includes a first asset tracking device located at a first asset, a second asset tracking device located at a second asset, and a server having access to an asset tracking database. The server is to obtain a first travel history of the first asset tracking device, obtain a second travel history of the second asset tracking device, determine, based on the first and second travel histories, whether the first asset tracking device and the second asset tracking device travel together, and, upon determination that the first asset tracking device and the second asset tracking device travel together, link the first asset tracking device and the second asset tracking device together in an asset tracking database to indicate that the first asset tracking device and the second asset tracking device travel together. 
     The server may determine that the first asset tracking device and the second asset tracking device travel together by determining that a first trip travelled by the first asset tracking device recorded in the first travel history matches a second trip recorded in the second travel history travelled by the second asset tracking device. The server may determine that the first trip matches the second trip by determining that the first asset tracking device and the second asset tracking device were in a vicinity of one another throughout a duration of the first trip and the second trip. The server may determine that the first trip matches the second trip by determining that the first trip and the second trip are coterminous and contemporaneous with one another. The server may obtain a third travel history of the first asset tracking device, obtain a fourth travel history of the second asset tracking device, determine, based on the third and fourth travel histories, that the first asset tracking device and the second asset tracking device have stopped travelling together, and, upon determination that the first asset tracking device and the second asset tracking device stopped travelling together, unlink the first asset tracking device from the second asset tracking device in the asset tracking database to indicate that the first asset tracking device and the second asset tracking device have stopped travelling together. The first asset tracking device may be located at a non-vehicular asset, the second asset tracking device may be located at a vehicle, and the vehicle may be to control travel of the non-vehicular asset. The non-vehicular asset may be coupleable to the vehicle, and the non-vehicular asset may include a transport trailer. The system may include a display device, wherein the server is further to compile trip information from the first travel history together with trip information from the second travel history for display at the display device. The display device may be to display a visual indication that the first asset tracking device travels with the second asset tracking device. 
     According to yet another aspect of the disclosure, a server or monitoring the travel of assets that travel together is provided. The server includes an asset tracking database, a communication interface to communicate with a first asset tracking device and a second asset tracking device, and a controller to obtain a first travel history of the first asset tracking device from the asset tracking database, obtain a second travel history of the second asset tracking device from the asset tracking database, determine, based on the first and second travel histories, whether the first asset tracking device and the second asset tracking device travel together, and, upon determination that the first asset tracking device and the second asset tracking device travel together, link the first asset tracking device and the second asset tracking device together in an asset tracking database to indicate that the first asset tracking device and the second asset tracking device travel together. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of an example system for asset travel monitoring that includes motion sensors to determine whether an asset is in travel. 
         FIG.  2    is a block diagram of an example asset tracking device with motion sensors to determine whether an asset is in travel. 
         FIG.  3    is a block diagram of an example non-transitory machine-readable storage medium that stores instructions that, when executed, cause a controller of an asset tracking device to execute a method for asset travel monitoring in which motion sensors are used to determine whether an asset is in travel. 
         FIG.  4    is a flowchart of an example method for asset travel monitoring in which motion sensors are used to determine whether an asset is in travel. 
         FIG.  5    is a flowchart of an example method for determining whether an asset has entered into a travelling state. 
         FIG.  6    is motion sensor data plot showing example motion sensor data from an asset tracking device that indicates that an asset being monitored by the asset tracking device has entered into a travelling state. 
         FIG.  7    is a flowchart of an example method for determining whether an asset has left a travelling state. 
         FIG.  8    is a motion sensor data plot showing example motion sensor data from an asset tracking device that indicates that an asset being monitored by the asset tracking device has left a travelling state. 
         FIG.  9    is a state diagram of an example process for operating an asset tracking device. 
         FIG.  10    is a flowchart of an example method for obtaining and transmitting a location of an asset tracking device to a remote server. 
         FIG.  11    is a block diagram of an example asset tracking device with temperature-dependent charging. 
         FIG.  12    is a block diagram of an example non-transitory machine-readable storage medium that stores instructions that, when executed, cause a controller of an asset tracking device to execute a method for temperature-dependent charging of an energy storage unit of an asset tracking device. 
         FIG.  13    is a flowchart of an example method for temperature-dependent charging of an energy storage unit of an asset tracking device. 
         FIG.  14    is a schematic diagram of an example supercapacitor degradation model for determining a target voltage to which a supercapacitor energy storage unit of an asset tracking device is to be charged. 
         FIG.  15    is a plot showing an example relationship between temperature and a target voltage to which an energy storage unit of an asset tracking device is to be charged. 
         FIG.  16    is a block diagram of another example asset tracking device with temperature-dependent charging, the asset tracking device including an energy harvester and communication interface. 
         FIG.  17    is a schematic diagram of an example system for monitoring the travel of assets that travel together. 
         FIG.  18    is a flowchart of another example method for monitoring the travel of assets that travel together. 
         FIG.  19    is a schematic diagram showing a data structure of example trip histories of two asset tracking devices that travel together. 
         FIG.  20    is a schematic diagram showing a map of example trip histories of two asset tracking devices that travel together. 
         FIG.  21    is a schematic diagram showing an example user interface depicting a trip history of two asset tracking devices that travel together. 
         FIG.  22    is a block diagram of another example asset tracking device. 
     
    
    
     DETAILED DESCRIPTION 
     An asset tracking device generally operates remotely from any fixed power source. In some cases, an asset tracking device may tap into a mobile power source located directly on an asset that it is tracking. For example, an asset tracking device tracking the movements of a vehicle, such as a car or truck, may draw power from the vehicle battery, which will generally have a capacity that is sufficiently large, and that is renewed sufficiently regularly, to power the asset tracking device indefinitely. 
     However, in many cases, an asset tracking device may be used to monitor an asset that does not have a large mobile power source that the asset tracking device may draw from. For example, an asset tracking device may be placed on a transport trailer, a shipping container, a shipment pallet, or another asset on which there is no usable power supply. In such cases, the asset tracking device may include an onboard power supply such as a battery, and may further include an energy harvesting system such as a solar panel, Peltier device, kinetic energy harvesting device, or other energy harvesting system to power the asset tracking device. In such cases, power management of the asset tracking device is an important factor in preserving the utility of the asset tracking device. 
     Although location tracking may be a primary purpose of an asset tracking device, it may be taxing on its power supply to frequently obtain the location of the asset tracking device, which may be of particular concern in the case where the asset tracking device does not have an outside power source to tap into. Communicating with a locating system such as a GPS or GNSS system to obtain the location of the asset tracking device and transmitting the location to an asset management tracking system may consume a significant amount of power. Therefore, an operating scheme for the asset tracking device that involves regularly and indiscriminately obtaining the location of the asset tracking device may not be conducive to energy conservation and to preserving the utility of the asset tracking device. 
     Thus, the present disclosure provides asset tracking devices and methods to operate asset tracking devices that use motion sensors to determine when to obtain and transmit location information to asset tracking device management systems. The techniques described herein may conserve power as compared to obtaining and transmitting location information on a fixed schedule. These techniques may be particularly useful for the tracking of non-vehicular assets where there is no external power source for an asset tracking device to tap into. 
     The present disclosure further provides asset tracking devices and methods to operate asset tracking devices that use temperature-dependent charging to determine how to charge energy storage units of the asset tracking devices in order to extend the use and/or lifespan of such energy storage units. The techniques described herein may be particularly useful when supercapacitors are used as energy storage units. 
     The present disclosure further provides asset tracking device management systems and methods to operate asset tracking device management systems which provide for the synchronized tracking of groups of assets that travel together. The techniques described herein may involve analyzing travel histories of assets and identifying two or more assets that are likely to be travelling together. Synchronized tracking of assets that travel together may be particularly useful for the tracking of a large group of assets that includes both vehicular and non-vehicular assets where the vehicular assets tend to transport the non-vehicular assets, for example, in the case of transport trucks pulling transport trailers. 
       FIG.  1    is a schematic diagram of an example system  100  for asset travel monitoring. The system  100  includes a locating system  110  for tracking the locations of one or more asset tracking devices, including an asset tracking device  130 . The locating system  110  may include a Global Positioning System (GPS), a Global Navigation Satellite System (GNSS), a cellular tower network, Wi-Fi networks, or another system which enables the monitoring of the location of the asset tracking device  130 . 
     The system  100  further includes an asset tracking device management system  120  for storing locations and travel histories of one or more asset tracking devices, including the asset tracking device  130 . The asset tracking device management system  120  may further store information such as associations between asset tracking devices and assets being tracked, user accounts, and other information related to the monitoring of asset tracking devices. For example, the asset tracking device management system  120  may store the types and/or versions of asset tracking devices being monitored, the types of assets being tracked (e.g., vehicles, non-vehicular assets), and other data. The asset tracking device management system  120  may further store travel histories, which may include detailed information collected during the travels of the asset tracking devices, such as motion sensor data, temperature data, speed data, or any other data collected during the trips travelled by the asset tracking devices. The asset tracking device management system  120  includes one or more computing devices, such as a server  122 . The server  122  includes a communication interface to communicate with asset tracking devices via one or more computing networks and/or telecommunication networks, including the asset tracking device  130 , a memory to store data, and a controller to execute the methods performed by the asset tracking device management system  120  as described herein. 
     The system  100  further includes the asset tracking device  130 . The asset tracking device  130  is installed at an asset  102  to monitor movement of the asset  102 . The asset tracking device  130  monitors motion of the asset  102  to determine whether the asset  102  is in travel or at rest. In particular, the asset tracking device  130  monitors motion sensor data  132  from a motion sensor of the asset tracking device  130  for indications that the asset  102  has begun or finished travel. The asset tracking device  130  is also in communication with the locating system  110  to obtain the location  134  of the asset tracking device  130  when appropriate, and is also in communication with the asset tracking device management system  120  to report the location  134  of the asset tracking device  130  when appropriate. 
     Example methods by which the asset tracking device  130  determines whether the asset  102  is in travel, and methods by which the asset tracking device  130  determines when to report the location  134  of the asset tracking device  130  to the asset tracking device management system  120 , are discussed in greater detail below. 
     For exemplary purposes, the asset  102  is shown as a transport trailer connected to a transport truck. The transport truck pulls the transport trailer to initiate and cease travel of the transport trailer. In other examples, the asset  102  may include any non-vehicular asset, such as a transport trailer, shipping container, pallet, shipped good, or any other asset which may be tracked by an asset tracking device. In still further examples, the asset  102  may be a vehicular asset, such as a truck, ship, car, or other vehicular asset that may be tracked by an asset tracking device. The asset  102  may be a non-vehicular asset that is coupleable to, connectable to, or otherwise transported with a vehicle, where the vehicle is to control travel of the asset  102  ((e.g., a transport trailer is connectable to a transport truck). Moreover, the asset  102  may be one of several non-vehicular assets that are coupleable to, connectable to, or otherwise transported with a vehicle, such as one of several rail cars pulled by a train, or one of several tethered transport trailers connected to a transport truck. 
       FIG.  2    is a block diagram of an example asset tracking device  200 . The asset tracking device  200  may be similar to the asset tracking device  130  of the system  100  of  FIG.  1   . The asset tracking device  200  is installed at an asset  202  to monitor travel of the asset  202 , which may be similar to the asset  102  of  FIG.  1   . 
     The asset tracking device  200  includes a motion sensor  210  to detect motion at the asset tracking device  200 . That is, the motion sensor  210  produces motion sensor data, which may be similar to the motion sensor data  132  of  FIG.  1   . This motion sensor data may include indications that the asset  202  has begun travel and indications that the asset  202  has finished travel. The motion sensor  210  may include an accelerometer, such as a three-axis MEMS accelerometer (e.g., a LIS3DHTR). 
     The asset tracking device  200  further includes a locating device  212  to locate the asset tracking device  200 . The locating device  212  may include a GPS module, GNSS module (e.g., an U-BLOX ZOE M8G), or other interface to obtain a location from a locating system, such as the locating system  110  of  FIG.  1   . 
     The asset tracking device  200  further includes a communication interface  214  to communicate with a remote server, such as the server  122  of the asset tracking device management system  120  of  FIG.  1   . The communication interface  214  may include a cellular modem, such as an LTE-M modem (e.g., QUECTEL BG96 or WNC IMA2A), CAT-M modem, or other cellular modem configured for bidirectional communication via the network with which asset tracking devices may communicate with the asset tracking device management system  120 . 
     The asset tracking device  200  further includes a controller  220 . The controller  220  includes one or more of a processor, a microcontroller (MCU), a central processing unit (CPU), microprocessor, processing core, a state machine, a logic gate array, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or similar, capable of executing, whether by software, hardware, firmware, or a combination of such, the actions performed by the controller  220  as described herein. The controller  220  further includes memory that may include any combination of read-only memory (ROM), random-access memory (RAM), flash memory, magnetic storage, optical storage, and similar, for storing instructions and data as discussed herein, including asset travel monitoring instructions  222 . 
     The controller  220  executes asset travel monitoring instructions  222  to monitor travel of the asset  202 . In particular, the instructions  222  are executable to cause the controller  220  to monitor the motion sensor  210  to determine whether the asset  202 , at which the asset tracking device  200  is located, has entered into a travelling state. That is, the controller  220  monitors motion sensor data from the motion sensor  210  to determine whether the asset  202  has begun travel. Monitoring motion sensor data for indications that the asset  202  may have begun travel may be a more energy efficient way to determine whether the asset  202  has begun travel than by determining whether the asset  202  has begun travel based on location information obtained from a locating system such as a GPS or GNSS system. 
     Further, upon determination that the asset  202  has entered into the travelling state, the instructions  222  cause the controller  220  to monitor the motion sensor  210  to determine whether the asset  202  has left the travelling state. That is, the controller  220  monitors motion sensor data from the motion sensor  210  to determine whether the asset  202  has finished travel. As with determining that the asset  202  may have begun travel, monitoring motion sensor data for indications that the asset  202  may have finished travel may be a more energy efficient way to determine whether the asset  202  has finished travel than obtaining location information from a locating system such as a GPS or GNSS system. 
     Further, upon determination that the asset  202  has left the travelling state, the instructions  222  cause the locating device  212  to obtain a location of the asset tracking device  200 , which may be similar to the location  134  of  FIG.  1   , and cause the communication interface  214  to transmit the location to the remote server. That is, the controller  220  obtains the location of the asset tracking device  200  from a locating system, such as the locating system  110  of  FIG.  1   , and transmits the location to an asset tracking device management system, such as the asset tracking device management system  120  of  FIG.  1   . Thus, the energy-costly task of obtaining location information from a locating system and transmitting the location information to an asset tracking device management system may be reserved until a particularly important point in time, namely, when the asset  202  has travelled to a new location. 
     In some examples, obtaining and transmitting location information may be performed both at the beginning and end of travel. That is, upon determination that the asset has entered into the travelling state, the asset tracking device  200  may obtain its location and transmit its location to an asset tracking device management system. Thus, the energy-costly task of obtaining and transmitting location information is reserved until two particularly important pots in time, namely, when the asset  202  starts travel and when the asset  202  has reached its destination. 
     The asset tracking device  200  may further include an energy storage unit (not shown) to power the asset tracking device  200 . The energy storage unit may include a supercapacitor, which may be particularly useful for its properties of non-toxicity, safe failure, long lifecycle, and its ability to operate in high and low temperatures, which may be particularly desirable in asset tracking devices. 
     The asset tracking device  200  may further include an energy harvester (not shown) to supply energy to the energy storage unit. The energy harvester may include a solar panel to harvest solar energy, which may be particularly desirable in an asset tracking device which may be located outdoors for extended periods of time. 
     The asset tracking device  200  may include a housing (not shown) that is designed to resist environmental conditions or other hazardous conditions, including precipitation, wind, dust, debris, water spray, cold and warm weather, or any other adverse condition that may impact the asset tracking device  200  if placed on the exterior of an asset, such as on top of a transport trailer. Further, the housing of the asset tracking device  200  may be designed to fit securely onto the surface of such an asset, such as, for example, between the ribs of a shipping container. 
       FIG.  3    is a block diagram of an example non-transitory machine-readable storage medium  300  which stores example asset travel monitoring instructions  310 . The non-transitory machine-readable storage medium  300  may be understood to be any medium which can store asset travel monitoring instructions  310  to be executable by a processor of a computing device, such as, for example, the controller  220  of  FIG.  2   . The programming instructions  310  may be similar to the instructions  222  of  FIG.  2   , and thus for convenience, the instructions  310  are described with reference to the asset tracking device  200  of  FIG.  2   . However, it is to be understood that the instructions  310  may be executed by another system or device. 
     Thus, the instructions  310  include motion sensor monitoring instructions  312  to monitor the motion sensor  210  to determine whether the asset  202  has entered into a travelling state. The instructions  310  further include motion sensor monitoring instructions  314  to, upon determination that the asset  202  has entered into the travelling state, monitor the motion sensor  210  to determine whether the asset  202  has left the travelling state. The instructions  310  further include location obtention instructions  316  to, upon determination that the asset  202  has left the travelling state, obtain a location of the asset tracking device  200 . The instructions  310  further include location transmission instructions  318  to transmit the location to a remote server. 
     As described above, the instructions  310  may be similar to the asset travel monitoring instructions  222  executable by the controller  220  of  FIG.  2    to monitor travel of the asset  202 . 
       FIG.  4    is a flowchart of an example method  400  for asset travel monitoring. The method  400  may be similar to a method performed by the controller  220  upon execution of the asset travel monitoring instructions  222 . Thus, for convenience, the method  400  is described with reference to the asset tracking device  200 . However, it is to be understood that the method  400  may be performed by other systems or devices. 
     At block  402 , the controller  220  monitors the motion sensor  210  to determine whether the asset  202  has entered into a travelling state. An example method for determining whether an asset has entered into a travelling state is provided in  FIG.  5   , below. At block  404 , where it is determined that the asset  202  has entered into the travelling state, the method  400  proceeds to block  406 . Where it is not determined that the asset  202  has entered into the travelling state, the method  400  returns to block  402  for continued monitoring. 
     In some examples, upon determination that the asset  202  has entered into the travelling state, the locating device  212  may obtain its location, and the communication interface  214  may transmit the location of the asset tracking device  200  at the beginning of travel (i.e., the travel beginning location), to the remote server. 
     At block  406 , upon determination that the asset  202  has entered the travelling state, the controller  220  monitors the motion sensor  210  to determine whether the asset  202  has left the travelling state. An example method for determining whether an asset has left a travelling state is provided in  FIG.  7   , below. At block  408 , where it is determined whether the asset  202  has left the travelling state, the method  400  proceeds to block  410 . Where it is not determined that the asset  202  has left the travelling state, the method  400  returns to block  406  for continued monitoring. 
     At block  410 , upon determination that the asset  202  has left the travelling state, the locating device  212  obtains the location of the asset tracking device  200 . Further, at block  412 , the communication interface  214  transmits the location of the asset tracking device  200  to a remote server. An example method for obtaining and transmitting the location of an asset tracking device is provided in  FIG.  10   , below. 
     As described above, the method  400  may be similar to a method performed by the controller  220  of  FIG.  2    upon execution of the asset travel monitoring instructions  222  to monitor travel of the asset  202 . 
       FIG.  5    is a flowchart of an example method  500  for determining whether an asset has entered into a travelling state. The method  500  may be understood to be one example of a way in which the block  402  of the method  400  of  FIG.  4    may be performed. Thus, for convenience, the method  500  is described with reference to the asset tracking device  200  of  FIG.  2   , but this is not limiting, and the method  500  may be performed by other devices or systems. 
     At block  502 , the controller  220  monitors the motion sensor  210  for initiation of motion of the asset tracking device  200 . That is, the controller  220  monitors motion sensor data from the motion sensor  210  for an initial or preliminary indication that the asset  202  may have begun travel. An example of motion sensor data that includes a preliminary indication that an asset may have begun travel is provided in  FIG.  6   , below. At block  504 , where an indication of initiation of motion is detected, the method  500  proceeds to block  506 . Where no indication of initiation of motion is detected, the method  500  returns to block  502  for continued monitoring. 
     At block  506 , the controller  220  monitors the motion sensor  210  for continued motion to determine whether the asset  202  has begun deliberate travel (i.e., substantial, purposeful, intentional, or directed travel). That is, the controller  220  continues to monitor motion sensor data from the motion sensor  210  to determine whether the initial or preliminary indication of motion is followed by further indication that the asset  202  has actually begun travel, and that the initial indication of motion is not a false positive. An example of motion sensor data that includes further indication that an asset is in deliberate travel is provided in  FIG.  6   , below. At block  508 , where it is determined that the asset  202  has begun deliberate travel, the method  500  is ended. Where it is not determined that the asset  202  has begun travel, the method  500  returns to block  502  for continued monitoring. 
       FIG.  6    is a plot that shows example motion sensor data  600 . The motion sensor data  600  may be similar to the motion sensor data monitored by the controller  220  of  FIG.  2   , and thus, for convenience, description of the motion sensor data  600  is made with reference to the asset tracking device  200  of  FIG.  2   . 
     The motion sensor data  600  includes an indication of initiation of motion of the asset  202 , and further includes an indication that the asset  202  has begun deliberate travel. For example purposes, the motion sensor data  600  is shown as a measurement of a magnitude of motion sensor data from the motion sensor  210 , measured in arbitrary units between −1 and +1, over arbitrary units of time. 
     The motion sensor data  600  includes a period  602  during which the motion sensor  210  indicates substantially no motion at the asset tracking device  200 . That is, the magnitude of the motion sensor data  600  during the period  602  is substantially zero. The controller  220  may periodically read the motion sensor data  600  to determine whether the magnitude of the motion sensor data  600  remains substantially near zero. 
     The motion sensor data  600  may appear to be substantially zero where, for example, the asset tracking device  200  is located on a vehicular asset that is at rest (e.g., the asset  202  is a land vehicle that is parked or stopped), or where the asset tracking device  200  is located on a non-vehicular asset that is at rest (e.g., the asset  202  is a transport trailer or shipping container that is in storage, or that is connected to a vehicle that is at rest). 
     The motion sensor data  600  further includes an instant or period  604  during which initiation of motion at the asset tracking device  200  is detected. That is, the magnitude of the motion sensor data  600  during the instant or period  604  is greater than a first threshold  610 . In some examples, the motion sensor  210  may be configured to alert the controller  220  when a magnitude of the motion sensor data  600  is detected above the first threshold  610 , and in other examples, the motion sensor  210  may periodically read the magnitude of the motion sensor data  600  to determine whether the magnitude of the motion sensor data  600  exceeds the first threshold  610 . When the magnitude of the motion sensor data  600  exceeds the first threshold  610 , block  504  of the method  500  of  FIG.  5    may be satisfied, as the initiation of motion of the asset tracking device  200  is detected. 
     The motion sensor data  600  may surpass the first threshold  610  where, for example, the asset tracking device  200  is located on a vehicular asset that begins to travel (e.g., the asset  202  is a land vehicle that begins to move from a parked or stopped position into a travelling state), or where the asset tracking device  200  is located on a non-vehicular asset that beings to travel (e.g., the asset  202  is a transport trailer or shipping container that is moved from storage to being on or connected to a vehicle, or the asset  202  is on or connected to a vehicle that begins motion). 
     However, the preliminary indication that the asset  202  may have begun travel may be a false positive. Thus, the controller  220  continues to monitor the motion sensor data  600  for a period  606  during which continued motion at the asset tracking device  200  may be detected, which may be taken to indicate that the asset  202  has begun deliberate motion. In other words, the motion sensor data  600  is monitored to determine that the initial indication of motion is not a false positive (e.g., motion caused by a vehicle door closing, or by environmental factors such as wind). 
     Continued motion during the period  606  may be determined if the magnitude of the motion sensor data  600  exceeds a second threshold  612  a predetermined number of occurrences (indicated as counts  614 ,  616 ) within a predetermined duration. A series of subsequent occurrences in which the second threshold  612  is exceeded may indicate that the asset  202 , whether a vehicular asset or a non-vehicular asset on or connected to a vehicular asset, is undergoing starts, stops, turns, bumps in the road, and other forms of motion that are indicative of deliberate travel. 
     Once triggered by detection of the initiation of motion, in some examples, the motion sensor  210  may be configured to alert the controller  220  when the magnitude of the motion sensor data  600  exceeds the second threshold  612 , and in other examples, the controller  220  may periodically read the magnitude of the motion sensor data  600  to determine whether the magnitude of the motion sensor data  600  exceeds the second threshold. 
     When there are a sufficient number of occurrences during which the magnitude of the motion sensor data  600  exceeds the second threshold  612 , block  508  of the method  500  of  FIG.  5    may be satisfied, as continued motion indicative of deliberate travel of the asset  202  is detected. Continued indications of motion at the asset tracking device  200  may indicate that the asset  202  is in deliberate motion and that the initial indication of motion was not a false positive. 
     The thresholds  610 ,  612 , may be predetermined, and may be set based on the type of asset tracking device  200 , type of asset  202 , the particular asset tracking device  200 , the particular asset  202 , and/or other factors that may influence how the motion sensor data  600  is indicative of the initiation of motion or of continued motion of the asset  202 . The second threshold  612  may be equal to, greater than, or less than, the first threshold  610 . Similarly, the duration during which indications of continued motion are monitored may be predetermined, and may be based on the type of asset tracking device  200 , type of asset  202 , the particular asset tracking device  200 , the particular asset  202 , and/or other factors that may influence how the motion sensor data  600  is indicative of the initiation of motion or of continued motion of the asset  202 . 
       FIG.  7    is a flowchart of an example method  700  for determining whether an asset has left a travelling state. The method  700  may be understood to be one example of a way in which the block  406  of the method  400  of  FIG.  4    may be performed. Thus, for convenience, the method  700  is described with reference to the asset tracking device  200  of  FIG.  2   , but this is not limiting, and the method  700  may be performed by other devices or systems. 
     At block  702 , the controller  220  monitors the motion sensor  210  for suspension of motion of the asset tracking device  200 . That is, the controller  220  monitors motion sensor data from the motion sensor  210  for an initial or preliminary indication that the asset  202  may have finished travel. An example of motion sensor data that includes a preliminary indication that an asset may have finished travel is provided in  FIG.  8   , below. At block  704 , where an indication of suspension of motion is detected, the method  700  proceeds to block  706 . Where no indication of suspension of motion is detected, the method  700  returns to block  702  for continued monitoring. 
     At block  706 , the controller  220  monitors the motion sensor  210  for continued lack of motion to determine whether the asset  202  has ceased deliberate travel. That is, the controller  220  continues to monitor motion sensor data from the motion sensor  210  to determine whether the initial or preliminary indication of suspension of motion is followed by further indication that the asset  202  has finished deliberate travel. In other words, the controller  220  determines that the initial indication of suspension of motion is not a false positive. An example of motion sensor data that includes further indication that an asset has ceased deliberate travel is provided in  FIG.  8   , below. At block  708 , where it is determined that the asset  202  has ceased deliberate travel, the method  700  is ended. Where it is not determined that the asset  202  has ceased travel, the method  700  returns to block  702  for continued monitoring. 
       FIG.  8    illustrates example motion sensor data  800 . The motion sensor data  800  may be similar to the motion sensor data monitored by the controller  220  of  FIG.  2   , and thus, for convenience, description of the motion sensor data  800  is made with reference to the asset tracking device  200  of  FIG.  2   . 
     The motion sensor data  800  includes an indication of suspension of motion of the asset  202 , and further includes an indication that the asset  202  has ceased deliberate travel. For example purposes, the motion sensor data  800  is shown as a measurement of a magnitude of motion sensor data from the motion sensor  210 , measured in arbitrary units between −1 and +1, over arbitrary units of time. 
     The motion sensor data  800  includes a period  802  during which the motion sensor  210  indicates continued motion at the asset tracking device  200 . That is, the magnitude of the motion sensor data  800  during the period  802  is substantially greater than zero. 
     The motion sensor data  800  may appear to be substantially greater than zero where, for example, the asset tracking device  200  is located on a vehicular asset that is in motion (e.g., the asset  202  is a land vehicle that is driving), or where the asset tracking device  200  is located on a non-vehicular asset that is in motion (e.g., the asset  202  is a transport trailer or shipping container that is on or connected to a vehicle that is driving). 
     The motion sensor data  800  further includes an instant or period  804  during which suspension of motion at the asset tracking device  200  takes place. That is, the magnitude of the motion sensor data  800  during the instant or period  804  is below a third threshold  810 . In some examples, the motion sensor  210  may be configured to alert the controller  220  when the magnitude of the motion sensor data  800  falls beneath the third threshold  810 , and in other examples, the controller  220  may periodically read the motion sensor data  800  to determine whether the magnitude of the motion sensor data  800  is beneath the third threshold  810 . When the magnitude of the motion sensor data  800  falls below the third threshold  810 , block  704  of the method  700  of  FIG.  5    may be satisfied, as the suspension of motion of the asset tracking device  200  is detected. 
     The motion sensor data  800  may fall beneath the third threshold  810  where, for example, the asset tracking device  200  is located on a vehicular asset that stops moving (e.g., the asset  202  is a land vehicle that stops travelling for either a brief period or an extended duration), or where the asset tracking device  200  is located on a non-vehicular asset that stops moving (e.g., the asset  202  is a transport trailer or shipping container that is connected to a vehicle that stops travelling). 
     However, the preliminary indication that the asset  202  may have ceased travel may be a false positive. Thus, the controller  220  may continue to monitor the motion sensor data  800  for a period  806  during which continued lack of motion at the asset tracking device  200  may be detected, which may be taken to indicate that the asset  202  has ceased deliberate motion. The motion sensor data  800  being beneath the third threshold  810  for an extended duration may indicate that the asset  202 , whether a vehicular asset or a non-vehicular asset on or connected to a vehicular asset, has stopped for an extended period of time (e.g., parked or entered into storage), as opposed to having merely suspended motion temporarily. 
     Continued lack of motion during the period  806  may be determined if the magnitude of the motion sensor data  600  remains beneath the third threshold  810  for a predetermined duration. In some examples, the motion sensor  210  may be configured to alert the controller  220  when the magnitude of the motion sensor data  800  falls beneath the third threshold  810 , and in other examples, the controller  220  may periodically read the motion sensor data  800  to determine whether the magnitude of the motion sensor data  800  is beneath the third threshold  810 . 
     When the magnitude of the motion sensor data  800  remains beneath the third threshold  810  for a predetermined duration, block  708  of the method  700  of  FIG.  7    may be satisfied, as continued lack of motion indicative of cessation of deliberate travel of the asset  202  is detected. Continued indications of lack of motion at the asset tracking device  200  may indicate that the asset  202  has ceased deliberate motion and that the initial indication of suspension of motion was not a false positive. 
     The thresholds  610 ,  612 , and  810  may be predetermined, and may be set based on the type of asset tracking device  200 , type of asset  202 , the particular asset tracking device  200 , the particular asset  202 , and/or other factors that may influence how the motion sensor data  600  and/or  800  are indicative of the initiation of motion, continued motion, suspension of motion, and/or continued lack of motion of the asset  202 . The third threshold  810  may be equal to, greater than, or less than, the first threshold  610  and/or the second threshold  612 . Similarly, the duration during which indications of continued lack of motion are monitored may be predetermined, and may be based on the type of asset tracking device  200 , type of asset  202 , the particular asset tracking device  200 , the particular asset  202 , and/or other factors that may influence how the motion sensor data  600  is indicative of continued lack of motion of the asset  202 . 
       FIG.  9    is a state diagram of an example process  900  for operating an asset tracking device. The process  900  may be employed by the asset tracking device  200  to monitor travel of the asset  202 . Thus, for convenience, the process  900  will be described with reference to the asset tracking device  200  of  FIG.  2   . However, this is not limiting, and the process  900  may be employed by other systems or devices. 
     At the outset of the process  900 , the controller  220 , the locating device  212 , and the communication interface  214  may each be operating in a low-power (i.e., “sleep”) mode that conserves energy. At block  902 , the controller  220  wakes from its low-power operating mode by some wakeup source or triggering event. 
     At block  904 , the controller  220  determines whether the asset tracking device  200  has access to sufficient power to carry out further steps of the process  900 , which involve determining the wakeup source and taking follow-on actions. Where it is determined that the asset tracking device  200  does not have access to sufficient power, the controller  220  is returned to its low-power operating mode at block  908 . The asset tracking device  200  may gain access to sufficient power to proceed with the process  900  at a later time, such as, for example, by an energy source of the asset tracking device  200  being charged. 
     Where it is determined that the asset tracking device  200  has sufficient power to carry out further steps of the process  900 , the controller  220  determines the reason for the controller  220  waking (i.e., the “wakeup source”) at block  906 . The wakeup source may be either a timer which periodically wakes the controller  220  or an indication of movement at the asset tracking device  200 . 
     Where the wakeup source is movement of the asset tracking device  200 , the controller  220  attempts to confirm that the detected movement is indicative of deliberate travel of the asset  202  at block  910 . For example, the controller  220  may execute the method  500  of  FIG.  5    to determine whether the asset tracking device  200  has begun deliberate travel. 
     Where it is determined that the asset  202  has begun deliberate travel, the controller  220  continuously tracks the travel of the asset  202  at block  912 . For example, the controller  220  may execute the method  700  of  FIG.  7    to determine when the asset tracking device  200  finishes travel. When tracking the travel of the asset  202  is complete, the controller  220  returns to its low-power operating mode at block  908 . Where it is not determined that the asset  202  has begun deliberate travel, the controller  220  is returned to its low-power operating mode at block  908 . 
     Where the wakeup source is a timer, the controller  220  causes a heartbeat signal to be transmitted to a remote server to indicate that the asset tracking device  200  is active, at block  914 . After the heartbeat signal is sent, the controller  220  returns to its low-power operating mode at block  908 . The timer may be set to wake the controller  220  to transmit a heartbeat signal on a periodic basis, such as, for example, once or twice per day. 
     Thus, the asset tracking device  200  may be operated in a low-power operating mode for energy conservation, waking only to track movement of the asset  202  or to transmit a heartbeat signal to a remote server. 
       FIG.  10    is a flowchart of an example method  1000  for obtaining and transmitting a location of an asset tracking device to an asset tracking device management system. The method  1000  may be performed by the asset tracking device  200  while monitoring travel of the asset  202 , and thus for convenience, the method  1000  will be described with reference to the asset tracking device  200  of  FIG.  2   . However, this is not limiting, and the method  1000  may be followed by other systems or devices. 
     As discussed above with reference to  FIG.  9   , the asset tracking device  200  may be operated with the controller  220 , the locating device  212 , and the communication interface  214  operating in low-power modes that conserve energy. By the method  1000 , these components may be selectively awakened from their low-power operating modes when instructed to perform a given action, and returned to their low-power operating modes to continue to conserve energy. 
     At block  1002 , the controller  220  determines whether the locating device  212  has access to sufficient power to obtain the present location of the asset tracking device  200 . Where it is not determined that the locating device  212  has access to sufficient power, the method  1000  is ended. Where it is determined that the locating device  212  has access to sufficient power, the method  1000  proceeds to block  1004 . 
     At block  1004 , the controller  220  wakes the locating device  212  from its low-power operating mode. At block  1006 , the locating device  212  obtains the present location of the asset tracking device  200 . At block  1008 , the controller  220  returns the locating device  212  to its low-power operating mode. 
     At block  1010  the controller  220  determines whether the communication interface  214  has access to sufficient power to transmit the present location of the asset tracking device  200  to a remote server. Where it is not determined that the communication interface  214  has access to sufficient power, the method  1000  proceeds to block  1012 , where the controller  220  stores the present location for later transmission to the remote server. Where it is determined that the communication interface  214  has sufficient power, the controller  220  wakes the communication interface  214  from its low-power operating mode, and the method  1000  proceeds to block  1016 . 
     At block  1016 , the communication interface  214  transmits the present location of the asset tracking device  200  to the remote server. At block  1018 , the controller  220  returns the communication interface  214  to its low-power operating mode. 
     Thus, as described above, it can be seen that an asset tracking device may operate its components in low-power operating modes, and may use energy-efficient methods to determine whether the asset that it is tracking is beginning or finishing travel. An asset tracking device may sparingly report the location of the asset to an asset tracking device management system only at the appropriate times and under the appropriate circumstances in order to conserve energy. 
     As described below, the energy capacity and lifecycle of an asset tracking device with an on-board energy storage unit may be improved by employing temperature-dependent charging of the energy storage unit. Temperature-dependent charging of an energy storage unit may be particularly applicable where the energy storage unit includes a supercapacitor, which the life cycles of which may be impacted when charged to different voltages at different temperatures. In many cases, a supercapacitor, when used as an energy storage unit, may be charged below capacity (e.g., at 80% of capacity) as a heuristic to reduce the deterioration of the lifecycle of the supercapacitor under adverse temperature conditions. However, such techniques are often based on predetermined rules which are not temperature-dependent, and which often do not allow the supercapacitor to be utilized to its full capacity under a given temperature condition. As described herein, a supercapacitor may be charged to a target voltage that is determined to balance utilization of the capacity of the supercapacitor against temperature-dependent deterioration of the supercapacitor. 
       FIG.  11    is a block diagram of an example asset tracking device  1100  with temperature-dependent charging of a supercapacitor energy storage unit. The asset tracking device  1100  may be similar to the asset tracking device  200  of  FIG.  2   , and thus, may be located at an asset to monitor travel of the asset, similar to the asset  202  of  FIG.  2   . 
     The asset tracking device  1100  includes a location monitoring system  1110  to track the location of the asset tracking device  1100 . The location monitoring system  1110  may include a motion sensor, locating device, and communication interface, similar to the motion sensor  210 , locating device  212 , and communication interface  214  of the asset tracking device  200  of  FIG.  2   . However, this is not limiting, and the location monitoring system  1110  may include other components and employ other techniques for monitoring location. 
     The asset tracking device  1100  further includes a supercapacitor energy storage unit  1114  to power the asset tracking device  1100 . The supercapacitor energy storage unit  1114  includes one or more supercapacitors, such as an electric double layer capacitor (EDLC) supercapacitor. 
     The asset tracking device  1100  further includes a temperature sensor  1112  to capture temperature readings at the asset tracking device  1100 . The temperature sensor  1112  may be located near the supercapacitor energy storage unit  1114  to measure an ambient temperature near the supercapacitor energy storage unit  1114 . The asset tracking device  1100  further includes a charging interface  1116  to charge the supercapacitor energy storage unit  1114 . 
     The asset tracking device  1100  further includes a controller  1120  to execute temperature-dependent charge control instructions  1122  to control the charging interface  1116  to charge the supercapacitor energy storage unit  1114  in a temperature-dependent manner. The controller  1120  is similar to the controller  220  of  FIG.  2   , and thus may include one or more of a processor or similar, and memory, as described above, to execute the instructions  1122 , and to perform other actions, such as control of the location monitoring system  1110 . 
     The instructions  1122  are executable to cause the controller  1120  to obtain a temperature reading measured at the asset tracking device  1100 . The temperature reading may have been measured by the temperature sensor  1112 . The temperature reading may be the most recent temperature reading measured by the temperature sensor  1112 . In some examples, the instructions  1122  may cause the controller  1120  to obtain one or more previously measured temperature readings measured at the asset tracking device  1100 . 
     The instructions  1122  further cause the controller  1120  to determine a target voltage for the supercapacitor energy storage unit  1114  based on the temperature reading (and/or any previously measured temperature readings). A supercapacitor exhibits a predictable relationship (a substantially quadratic relationship) between voltage and energy storage, and thus the voltage held by a supercapacitor is an indication of the amount of energy stored in the supercapacitor. 
     The target voltage is determined to balance utilization of the capacity of the supercapacitor energy storage unit  1114  against temperature-dependent deterioration of the supercapacitor energy storage unit  1114 . That is, the target voltage is selected so that the supercapacitor energy storage unit  1114  is charged to the highest safe voltage without significant deterioration of the lifecycle of the supercapacitors thereof. The target voltage may be determined based on a supercapacitor degradation model of the supercapacitor energy storage unit  1114 , for example, as described in  FIG.  14   , below. 
     The instructions  1122  further cause the controller  1120  to control the charging interface  1116  to charge the supercapacitor energy storage unit  1114  to the target voltage. That is, the supercapacitor energy storage unit  1114  is charged up to the target voltage, and no further. 
     Thus, the supercapacitor energy storage unit  1114  may be charged to a voltage that utilizes a significant portion of the capacity of the supercapacitor energy storage unit  1114  without overcharging to a point that would be unduly detrimental to the lifecycle of a supercapacitors at the given temperature reading. 
       FIG.  12    is a block diagram of an example non-transitory machine-readable storage medium  1200  which stores example temperature-dependent charge control instructions  1210 . The non-transitory machine-readable storage medium  1200  may be understood to be any medium which can store temperature-dependent charge control instructions  1210  to be executable by a processor of a computing device, such as, for example, the controller  1120  of  FIG.  11   . The programming instructions  1210  may be similar to the instructions  1122  of  FIG.  11   , and thus for convenience, the instructions  1210  are described with reference to the asset tracking device  1100  of  FIG.  11   . However, it is to be understood that the instructions  1210  may be executed by another system or device. 
     Thus, the instructions  1210  include temperature reading obtention instructions  1212  to obtain a temperature reading measured at the asset tracking device  1100 . 
     The instructions  1210  further include temperature-dependent target voltage determination instructions  1214  to determine a target voltage for the supercapacitor energy storage unit  1114  of the asset tracking device  1100  based on the temperature reading to balance utilization of a capacity of the supercapacitor energy storage unit  1114  against temperature-dependent deterioration of the supercapacitor energy storage unit  1114 . 
     The instructions  1210  further include energy storage unit charge control instructions  1216  to control the charging interface  1116  of the asset tracking device  1100  to charge the supercapacitor energy storage unit  1114  to the target voltage. 
     As described above, the instructions  1210  may be similar to the asset travel monitoring instructions  1122  executable by the controller  1120  of  FIG.  11    to monitor travel of the asset  1102 . 
       FIG.  13    is a flowchart of an example method  1300  for temperature-dependent charging of a supercapacitor energy storage unit. The method  1300  may be similar to a method performed by the controller  1120  upon execution of the temperature-dependent charge control instructions  1122 . Thus, for convenience, the method  1300  is described with reference to the asset tracking device  1100 . However, it is to be understood that the method  1300  may be performed by other systems or devices. 
     At block  1302 , the controller  1120  obtains a temperature reading measured at the asset tracking device  1100 . The temperature reading may be obtained from the temperature sensor  1112 , or may be obtained from memory. In some examples, the controller  1120  may obtain one or more additional previously measured temperature readings measured at the asset tracking device  1100 . 
     At block  1304 , the controller  1120  determines a target voltage for the supercapacitor energy storage unit  1114  of the asset tracking device  1100  based on the temperature reading. The target voltage is to balance utilization of a capacity of the supercapacitor energy storage unit  1114  against temperature-dependent deterioration of the supercapacitor energy storage unit  1114 . The determination may be made based on a supercapacitor degradation model of the supercapacitor energy storage unit  1114 , for example, as described in  FIG.  14   , below. 
     In some examples, where the controller  1120  obtains additional previously measured temperature readings, the controller  1120  may determine the target voltage further based on the one or more additional previously measured temperature readings. 
     At block  1306 , the controller  1120  controls the charging interface  1116  of the asset tracking device  1100  to charge the supercapacitor energy storage unit  1114  to the target voltage. 
     As described above, the method  1300  may be similar to a method performed by the controller  1120  of  FIG.  11    upon execution of the temperature-dependent charge control instructions  1122  to charge the supercapacitor energy storage unit  1114 . 
       FIG.  14    is a schematic diagram of an example supercapacitor degradation model  1400 . The supercapacitor degradation model  1400  may be stored in memory of an asset tracking device and referenced when charging a supercapacitor energy storage unit of the asset tracking device. For example, the supercapacitor degradation model  1400  may be stored in memory accessible by the controller  1120  of the asset tracking device  1100  of  FIG.  11    to determine the voltage to which the supercapacitor energy storage unit  1114  is to be charged. For convenience, the supercapacitor degradation model  1400  will be described with reference to the asset tracking device  1100  of  FIG.  11   , but this is not limiting, and the supercapacitor degradation model  1400  may be used by other systems or devices. 
     The supercapacitor degradation model  1400  takes as input the supercapacitor temperature of the supercapacitor(s) of the supercapacitor energy storage unit  1114 . The supercapacitor degradation model  1400  may also take as input the supercapacitor voltage  1402  of the supercapacitor(s) of the supercapacitor energy storage unit  1114 . The supercapacitor degradation model  1400  may also take as input past operating conditions  1406 , which may include past voltage readings and/or past temperature readings of the supercapacitor(s) of the supercapacitor energy storage unit  1114 . The past operating conditions  1406  may be stored in memory at the asset tracking device. The supercapacitor degradation model  1400  may also take as inputs additional factors, such as the number and types of supercapacitor(s) in the supercapacitor energy storage unit  1114 , and any properties thereof, such as the energy capacities, maximum voltages, and number of previous charge cycles, of such supercapacitor(s). 
     The supercapacitor degradation model  1400  computes a target voltage  1408  to which the supercapacitor energy storage unit  1114  is to be charged, based on the inputs, in order to balance utilization of a capacity of the supercapacitor storage unit against temperature-dependent deterioration of the supercapacitor storage unit. 
     The computation of the target voltage  1408  may involve any combination of a number of techniques, some examples of which are discussed here. The computation of the target voltage  1408  may involve referencing a table that lists temperature ranges and voltages to be targeted when the supercapacitors are within the listed temperature ranges. The computation of the target voltage  1408  may involve reading the target voltage  1408  from a temperature-voltage curve, such as in the plot shown in  FIG.  15   , below. The computation of the target voltage  1408  may involve the evaluation of a function that takes as arguments any combination of the supercapacitor voltage  1402 , the supercapacitor temperature  1404 , and past operating conditions  1406 , or any other factor described above, to mathematically compute the target voltage  1408 . The computation of the target voltage  1408  may involve application of a machine learning model that is trained to output the target voltage  1408 , the machine learning model having been trained to determine the target voltage  1408  that achieves a target balance between utilization of the capacity of the supercapacitor energy storage unit  1114  and longevity of the supercapacitor energy storage unit  1114  throughout a range of temperature conditions. 
     Determination of the target voltage  1408  may also involve referencing a charge cycle deterioration model of the supercapacitor energy storage unit  1114  that provides a model for how the supercapacitor(s) of the supercapacitor energy storage unit  1114  deteriorate after repeated charge cycles. Such a charge cycle deterioration model may be expanded or enhanced by the inclusion of temperature information. Thus, the use and lifecycle of asset tracking devices may be extended by the use of supercapacitor energy storage units that are charged according to temperature-dependent charging techniques. 
       FIG.  15    is an example temperature-voltage plot  1500 . The plot  1500  includes an example temperature-voltage curve  1502  which represents a function that relates temperatures of supercapacitors to target voltages of supercapacitors. The temperature-voltage curve  1502  may be referenced to determine a target voltage to which a supercapacitor is to be charged when the supercapacitor is at a given temperature in order to achieve a high degree of utilization of the capacity of the supercapacitor without significant degradation of the supercapacitor. The plot  1500  also includes a heuristic line  1504  which defines a heuristic amount to which a supercapacitor may be charged at any temperature (i.e., when temperature is unknown). 
     There are points along the temperature-voltage curve  1502  which are lower than the heuristic line  1504 , and there are points along the temperature-voltage curve  1502  which are higher than the heuristic line  1504 . Where the temperature-voltage curve  1502  is lower than the heuristic line  1504 , reference to the temperature-voltage curve  1502  indicates that a supercapacitor is to be charged to a lower voltage than the heuristic amount in order to conserve longevity of the supercapacitor. Where the temperature-voltage curve  1502  is higher than the heuristic line  1504 , reference to the temperature-voltage curve  1502  indicates that a supercapacitor is to be charged to a higher voltage than the heuristic amount in order to take advantage of a greater proportion of the capacity of the supercapacitor. Thus, reference to the temperature-voltage curve  1502  may be had to charge a supercapacitor in a manner that balances utilization of the capacity of a supercapacitor without unduly deteriorating the supercapacitor. 
     In the example temperature-voltage plot  1500  shown, the target voltage increases with temperature from about 0° C. until about 20° C., at which point one or more supercapacitors are to be charged to its/their highest recommended amount (e.g., about 5V), and decreases at higher temperatures. Thus, a supercapacitor is to be charged to a lower voltage in lower temperatures (e.g., temperatures below about 20° C.), to a higher voltage in moderate temperatures (e.g., temperatures near 20° C.), and to a lower voltage in higher temperatures (e.g., temperatures higher than about 20° C.). It is to be emphasized that the particular temperature-voltage plot  1500  shown is for illustrative purposes only, and other relationships between supercapacitor temperature and target voltage may be used. 
       FIG.  16    is a block diagram of another example asset tracking device  1600  with temperature-dependent charging. The asset tracking device  1600  is similar to the asset tracking device  1100  of  FIG.  1   , with like components numbered in the “1600” series rather than the “1100” series, and therefore includes a location monitoring system  1610 , a controller  1620  to execute temperature-dependent charge control instructions  1622 , a temperature sensor  1612 , a supercapacitor energy storage unit  1614 , and a charging interface  1616 , and is located at an asset  1602  to monitor travel of the asset  1602 . 
     The asset tracking device  1600  further includes a communication interface  1617 . The communication interface  1617  is to receive environmental data from a remote server that may pertain to an environmental condition at the asset tracking device  1600  that may be relevant to the charging of the supercapacitor energy storage unit  1614 . For example, the environmental data may include temperature data that indicates a regional temperature (i.e., a forecasted temperature) at the location of the asset tracking device  1600 . The controller  1620  may incorporate this temperature data into its determination of the target voltage to which the supercapacitor energy storage unit  1614  is to be charged. For example, the controller  1620  may use a weighted average of one or more temperature readings taken by the temperature sensor  1612  and the temperature data. The temperature data may include forecasts for temperature in the area of the asset tracking device  1600  over the upcoming hours, or days, in the region, which may be relevant to the determination of the target voltage of the supercapacitor energy storage unit  1614 . The environmental data may be obtained from an asset tracking device management system or from other systems. 
     The asset tracking device  1600  further includes an energy harvester  1618  to supply energy to the supercapacitor energy storage unit  1614 . The energy harvester  1618  supplies energy to the supercapacitor energy storage unit  1614  through the charging interface  1616 . The energy harvester  1618  may include a solar panel to harvest solar energy. Where the energy harvester  1618  includes a solar panel, the environmental data obtained by the communication interface  1617  may be particularly relevant to the energy that could be expected to be harvested from the energy harvester  1618 . For example, the environmental data may include sunlight data that indicates an amount of sunlight expected to reach the asset tracking device  1600  at the present location. The controller  1620  may incorporate this sunlight data into its determination of the target voltage to which the supercapacitor energy storage unit  1614  is to be charged. For example, if the sunlight data indicates that the asset tracking device  1600  is expected to receive a great amount of sunlight in the upcoming days, and temperature data indicates that the asset tracking device  1600  is expected to be at adversely high temperatures in the upcoming days that would risk deteriorating supercapacitors if charged to a high voltage, the controller  1620  may determine that the supercapacitors can be maintained at low voltage (to protect longevity in high temperatures) with little risk of the energy of the supercapacitors being depleted (due to the ongoing charging to be provided by the solar panel over the upcoming days). Thus, the temperature-dependent charge control instructions  1622  may include such logic that determines the target voltage for the supercapacitor energy storage unit  1614  based, at least in part, on the environmental data obtained by the communication interface  1617 . 
     Thus, as described above, it can be seen that an asset tracking device may include an onboard supercapacitor energy storage unit that may be intelligently charged based on temperature-dependent charge control instructions. The supercapacitor energy storage unit may be charged to a target voltage that utilizes a high proportion of the capacity of the supercapacitors thereof without undue degradation of the supercapacitors caused by factors relating to temperature. The supercapacitor energy storage unit may be charged according to rules that consider temperature, sunlight, and other environmental conditions as factors, thereby enabling the asset tracking device to maintain a usable store of energy when deployed in the field for an extended period of time. Asset tracking devices that are able to operate in the field for an extended period of time may be particularly useful when used as part of a large group of asset tracking devices, some of which may track non-vehicular assets that may be deployed in the field for particularly extended periods of time. 
     As described below, large groups of assets may contain smaller groups of assets that travel together in observable ways. For example, a transport truck with an asset tracking device connected to the truck may pull a transport trailer which is tracked by a separate asset tracking device. By observing the travel histories of these two asset tracking devices, the two asset tracking devices can be linked, paired, grouped, or associated together in an asset tracking device management system, thereby allowing the movement of each of these assets to be more effectively tracked. Information related to assets that travel together may be presented to a viewer in a more concise and organized fashion if the asset tracking devices are linked together and the information is presented in a combined way. Further, greater insights may be obtained from the data collected from each of the asset tracking devices that travel in a group if such information is combined or compiled than if the information were analyzed separately. 
       FIG.  17    is a schematic diagram of an example system  1700  for asset travel monitoring that involves the monitoring of asset tracking devices that travel together. The system  1700  may be similar to the system  100  of  FIG.  1   , with components numbered in the “1700” series rather than the “100” series, and therefore includes an asset tracking device management system  1720  with a server  1722 , a first asset tracking device  1730 - 1  to monitor travel of a first asset  1702 - 1 , and further includes a second asset tracking device  1730 - 2  to monitor travel of a second asset  1702 - 2 . The asset tracking devices  1730 - 1 ,  1730 - 2 , transmit location information  1734 , which includes location information about each of the asset tracking devices  1730 - 1 ,  1730 - 2  (which each may be similar to the location  134  from  FIG.  1   ), to the asset tracking device management system  1720 . 
     The asset tracking device management system  1720  compiles the location information  1734  into travel histories  1724  (containing travel histories  1724 - 1  and  1724 - 2  of asset tracking devices  1730 - 1  and  1730 - 2  respectively) in an asset tracking database  1726 . The travel histories  1724  contain historical records of the travels of one or more asset tracking devices, including each of the asset tracking devices  1730 - 1  and  1730 - 2 , including trip start locations, trip end locations, and trip durations. The travel histories  1724  may also include more detailed travel information, such as motion sensor data, temperature data, speed data, collected during the trips travelled by the asset tracking devices  1730 , and any other information collected from asset tracking devices. 
     In the example shown, the first asset tracking device  1730 - 1  is located at a first asset  1702 - 1 , shown for example to be a transport trailer. Further, the second asset tracking device  1730 - 2  is located at a second asset  1702 - 2 , shown for example to be a transport truck connected to the transport trailer. Thus, the first asset  1702 - 1  and second asset  1702 - 2  travel together, as the second asset  1702 - 2  moves the first asset  1702 - 1 . 
     The asset tracking device management system  1720  is configured to identify asset tracking devices, such as the asset tracking devices  1730 - 1  and  1730 - 2 , that travel together. Once identified, the asset tracking device management system  1720  links (or “tethers”) together the asset tracking devices that travel together (e.g., asset tracking devices  1730 - 1  and  1730 - 2 ) in the asset tracking database  1726 . In other words, a flag or association between the linked asset tracking devices is stored. Once linked (or “tethered”), information related to the travel of the two assets may be more effectively presented to a viewer in a more concise and organized fashion, and greater insights may be obtained by compiling data collected by the two asset tracking devices. An example of a method by which the asset tracking device management system  1720  may identify is provided in  FIG.  18   , below. 
     In the present example, the assets  1702 - 1  and  1702 - 2  are shown to be a transport truck and a transport trailer pulled by the transport truck, respectively. In general, the first asset tracking device  1730 - 1  may be located at a non-vehicular asset (i.e., the transport trailer, asset  1702 - 1 ), the second asset tracking device may be located at a vehicle (i.e., the transport truck, asset  1702 - 2 ), where the vehicle is to control travel of the non-vehicular asset (i.e., the transport truck pulls the transport trailer). Further, the non-vehicular asset may be coupleable to the vehicle (e.g., the transport trailer is coupleable to the transport truck), or may be storable on, or otherwise transportable by, the vehicle (e.g., a shipping pallet may be stored in and transported by a transport truck). 
     However, it is to be understood that either of the assets  1702 - 1 ,  1702 - 2  may be a vehicular or a non-vehicular asset. For example, both assets  1702 - 1  and  1702 - 2  may be vehicles that have been identified to travel together, such as, for example, in the case where the first vehicle is a tow truck that is identified to have towed the second vehicle, or where the first vehicle is a delivery truck that delivers vehicles. As another example, both assets  1702 - 1  and  1702 - 2  may be non-vehicular assets that have been identified to travel together, such as, in the case where both assets are shipping containers travelling on the same ship, or in the case where both assets are train cars pulled by the same locomotive, or in the case where both assets are shipping pallets being transported by the same truck, or in any combination of these and similar cases. In each case, it may be advantageous to link together each of the non-vehicular assets for logistical purposes (e.g., to track the movement of shipments or other logistical assets) or for gathering insights from the data collected from the asset tracking devices tracking each of the assets. 
       FIG.  18    is a flowchart of an example method  1800  for monitoring the travel of assets that travel together. The method  1800  may be understood to be one example of a method performed by the server  1722  of the asset tracking device management system  1720  of the system  1700  of  FIG.  17    to monitor the travel of assets that travel together. Thus, for convenience, the method  1800  is described with reference to the system  1700  of  FIG.  17   . However, it is to be understood that the method  1800  may be performed by other systems or devices. 
     At block  1802 , the server  1722  obtains a first travel history  1724 - 1  of the first asset tracking device  1730 - 1 . For example, the server  1722  obtains the first travel history  1724 - 1  from the database  1726 . 
     At block  1804 , the server  1722  obtains a second travel history  1724 - 2  of the second asset tracking device  1730 - 2 . For example, the server  1722  obtains the second travel history  1724 - 2  from the database  1726 . 
     At block  1806 , the server  1722  determines, based on the first and second travel histories  1724 , whether the first asset tracking device  1730 - 1  and the second asset tracking device  1730 - 2  travel together. 
     The determination of whether the first and second asset tracking devices  1730 - 1 ,  1730 - 2  travel together may be made in any of a number of ways. For example, it may be determined that the asset tracking devices  1730 - 1 ,  1730 - 2  travel together by determining that a first trip travelled by the first asset tracking device  1730 - 1  recorded in the first travel history  1724 - 1  matches a second trip recorded in the second travel history  1724 - 2  travelled by the second asset tracking device  1730 - 2 . Identifying such a match may involve determining that the first asset tracking device  1730 - 1  and the second asset tracking device  1730 - 2  were in a vicinity of one another throughout a duration of the first trip and the second trip. This may be determined with reference to location information (e.g., latitude/longitude information) recorded in the travel histories  1724  (see, for example,  FIG.  19   ). Such a match may also be identified by determining that the first trip and the second trip start and finish at the same time and at the same place, or, in other words, are coterminous and contemporaneous with one another. Again, this may be determined with reference to location information (e.g., latitude/longitude information) recorded in the travel histories  1724  (see, for example,  FIG.  19   ). 
     At block  1808 , where it is determined that the asset tracking devices  1730 - 1 ,  1730 - 2  travel together, block  1810  is executed, else the method  1800  is ended. 
     At block  1810 , upon determination that the first asset tracking device  1730 - 1  and the second asset tracking device  1730 - 2  travel together, the server  1722  links the first asset tracking device  1730 - 1  and the second asset tracking device  1730 - 2  together in the asset tracking database  1726  to indicate that the first asset tracking device  1730 - 1  and the second asset tracking device  1730 - 2  travel together. For example, a flag or association between the linked asset tracking devices  1730 - 1 ,  1730 - 2  is stored in the database  1726 . 
     The method  1800  may be repeated periodically to update linkages between asset tracking devices  1730 , including to link together additional asset tracking devices  1730  into larger groups when additional asset tracking devices  1730  are determined to travel together, or to remove linkages between asset tracking devices  1730  which are determined to no longer travel together. 
     In the case where a linkage between asset tracking devices  1730  is removed, the method  1800  may involve the server  1722  obtaining a third travel history of the first asset tracking device  1730 - 1  (e.g., an update to the travel history  1724 - 1 ), obtaining a fourth travel history of the second asset tracking device  1730 - 2  (e.g., an update to the travel history  1724 - 2 ), and determining, based on the third and fourth travel histories, that the first asset tracking device  1730 - 1  and the second asset tracking device  1730 - 2  have stopped travelling together. For example, the third and fourth travel histories may include more recent trips during which the asset tracking devices  1730 - 1 ,  1730 - 2 , were not in the vicinity of one another, or did not end at the same location at the same time. 
     Upon determination that the first asset tracking device  1730 - 1  and the second asset tracking device  1730 - 2  have stopped travelling together, the method  1800  may further involve the server  1722  unlinking the first asset tracking device  1730 - 1  from the second asset tracking device  1730 - 2  in the asset tracking database  1726  to indicate that the first asset tracking device  1730 - 1  and the second asset tracking device  1730 - 2  have stopped travelling together. 
       FIG.  19    is a schematic diagram showing a data structure  1900  of example trip histories of two asset tracking devices that travel together. The data structure  1900  includes latitude and longitude information for a first asset tracking device and a second asset tracking device, which may be similar to the first and second asset tracking devices  1730 - 1 ,  1730 - 2 , of  FIG.  17   . Thus, the data structure  1900  includes location information that describes the travel of two asset tracking devices. The data structure  1900  may include trip histories similar to the travel histories  1724  of  FIG.  17   . The location information and timestamps presented are for exemplary purposes only. 
     As shown, during timestamps 0:00-0:02, the first and second asset tracking devices are not in the vicinity of one another. However, during timestamps 0:03-0:010, the first and second asset tracking devices are in substantially the same location at substantially the same time, and start and stop a trip together from timestamp 0:03 to 0:10. In other words, from 0:03 to 0:10, the trip history of the first and second asset tracking devices match, and therefore, the first and second asset tracking devices may be determined to be travelling together during this period. 
     As further trip information is collected, the data structure  1900  may expand with additional location information that indicates that the first and second asset tracking devices are not in the vicinity of one another and/or do not start and end trips at the same time and place, and therefore may be determined to no longer travel together. 
     It is to be understood that in order to be determined to travel together, the first and second asset tracking devices need not have recorded precisely the same location information, and that the first and second asset tracking devices may be determined to be travelling together if the location information of the two asset tracking devices is only sufficiently similar within an acceptable margin of error. 
       FIG.  20    is a schematic diagram showing a map  2000  of example trip histories of two asset tracking devices that travel together. The map  2000  may be understood to be a visual representation of a trip similar to the trip travelled by the first and second asset tracking devices of  FIG.  19   . Thus, it can be seen that the first and second asset tracking devices follow trips  2002 ,  2004  respectively, that start and stop a trip in the same place at the same time, and are in the vicinity of one another throughout the duration of the trip. 
       FIG.  21    is a schematic diagram showing an example user interface  2100  that depicts a trip history of two asset tracking devices that travel together. The user interface  2100  provides trip information about a first asset tracking device and a second asset tracking device that track assets that have been determined to travel together, such as, for example, by the techniques described above. The first and second asset tracking devices that travel together may be similar to the first and second asset tracking devices  1730 - 1 ,  1730 - 2  of  FIG.  17   , and thus, for convenience, description of the user interface  2100  will be made with reference to the system  1700 , and the asset tracking devices  1730 - 1  and  1730 - 2 , of  FIG.  17   . 
     The user interface  2100  may be displayed at a display device, such as a display device of a computing device with access to the asset tracking device management system  1720  of  FIG.  17   . 
     The user interface  2100  includes a map  2102  onto which a trip path  2104  which visually represents a trip travelled by first and second asset tracking devices  1730 - 1 ,  1730 - 2  is overlain. Since the first and second asset tracking devices  1730 - 1 ,  1730 - 2  travel together, and thus the travel of the first and second asset tracking devices  1730 - 1 ,  1730 - 2  overlap, the trip path  2104  is shown as a single trip. The trip path  2104  may be generated by trip information included in the travel histories  1724 . Thus, the server  1722  compiles trip information from the first travel history  1724 - 1  together with trip information from the second travel history  1724 - 2  for display at a display device. 
     The user interface  2100  further includes a first visual indication  2110  of the first asset  1702 - 1  (tracked by the first asset tracking device  1730 - 1 ), and a second visual indication  2112  of a second asset  1702 - 2  (tracked by the second asset tracking device  1730 - 2 ). Each visual indication  2110 ,  2112  may include a depiction of the respective asset  1702 - 1 ,  1702 - 2  that is being tracked. In the present example, the first asset  1702 - 1  may be a transport trailer, and thus, the first visual indication  2110  includes a depiction of a transport trailer. The second asset  1702 - 2  may be a transport truck that pulls the transport trailer, and thus the second visual indication  2112  includes a depiction of a transport truck. 
     In the present example, the first and second visual indications  2110 ,  2112  are shown in visual association with one another, such as, for example, by the visual indications  2110 ,  2112  being placed adjacent or near one another in the user interface  2100  to visually indicate that the assets  1702 - 1 ,  1702 - 2  are linked together and travel together. Thus, the user interface  2100  includes a visual indication that the first asset  1702 - 1  (and the first asset tracking device  1730 - 1 ) travels with the second asset  1702 - 2  (and the second asset tracking device  1730 - 2 ). In some examples, the first and second visual indications  2110 ,  2112  may be combined into a single visual indication of a transport truck travelling with a transport trailer. 
     The user interface  2100  further includes a trip history component  1220  in which trip information for the trip path  2104  from the travel histories  1724  is compiled and presented. The trip history component  1220  may display the travel history  1724  of one of the asset tracking devices  1730 - 1 ,  1730 - 2 , or an average, combination, or compilation of the location information in the two travel histories  1724 - 1 ,  1724 - 2 . 
     Thus, information related to the travel of the group of assets that travel together more effectively presented to a viewer in a more concise and organized fashion than if the information were presented about each of the assets individually. Travel histories may be compiled, and trip paths may be combined, so that visual space in the user interface  2100  may be conserved, and so that redundant computations and the storage of redundant data may be avoided. 
     Further, where information from one of the linked asset tracking devices is lost, similar information from one of the other linked asset tracking devices may provide useful data redundancy for the missed information. For example, where one asset tracking device loses power or network connectivity and therefore stops transmitting location data or other useful data (e.g., temperature data, motion sensor data), the lost information may be inferred from the information gathered from a linked asset tracking device. Thus, the location of a disconnected asset tracking device, temperature at a disconnected asset tracking device, or motion taking place at a disconnected asset tracking device may be estimated based on similar information received from a linked asset tracking device. 
     Further, greater insights may be obtained by compiling data collected by each of the asset tracking devices that travel in the group. The information obtained from one of the asset tracking devices can be checked, compared against, or combined with the information obtained from the second asset tracking device. For example, where each asset tracking device collects information related to environmental conditions, such as temperature or weather data, a more reliable understanding of the environmental conditions at the asset tracking devices may be discerned upon analysis of the data collected by both asset tracking devices. For example, an average of the temperature data collected by two linked asset tracking devices may be used as an estimate for the actual temperature in the vicinity of both asset tracking devices. 
     Where environmental data is gathered at one asset tracking device that may be relevant to the second asset tracking device (which may not collect the same environmental data either by fault or by lack of capability), that environmental data may be made available to the control of the second asset tracking device. For example, a determination of whether an asset being tracked by one asset tracking device has started or finished travel may be made based on the motion sensor data collected at a linked asset tracking device. As another example, where one asset tracking device collects temperature data that may be relevant to the charging of a supercapacitor energy storage unit of a linked asset tracking device, that temperature information may be used in the determination of the target voltage to which the supercapacitor energy storage unit is to be charged. Such sharing of information may be mediated by an asset tracking device management system or by direct communication between the linked asset tracking devices. 
     Further, where a vehicle and another linked asset are involved in an accident or collision (e.g., in the case of a transport truck pulling a transport trailer where each are equipped with asset tracking devices), information that may be relevant to accident recreation (e.g., motion sensor data) from each of the asset tracking devices may be compiled and analyzed for improved accident recreation, or information from one of the asset tracking devices may be used for redundancy if information from the other is faulty. In other words, more accurate and more reliable information may be gathered for accident recreation from the asset tracking device on the transport truck as well as the asset tracking device on the transport trailer. Thus, accident recreation techniques may be improved by the combination of data gathered by two linked asset tracking devices. 
       FIG.  22    is a block diagram of another example asset tracking device  2200 . The asset tracking device  2200  may be understood to be one example implementation of an asset tracking device that may perform any functionality of an asset tracking device described herein, and thus may be similar to the asset tracking device  130  of  FIG.  1   , the asset tracking device  200  of  FIG.  2   , the asset tracking device  1100  of  FIG.  11   , the asset tracking device  1600  of  FIG.  16   , or the asset tracking devices  1730  of  FIG.  17   . 
     The asset tracking device  2200  includes an accelerometer  2202  to detect motion at the asset tracking device  2200 , a GNSS module  2204  to locate the asset tracking device  2200 , and a cellular modem  2206  to communicate with a remote server. The cellular modem  2206  may include an LTE-M cellular modem. The GNSS module  2204  has access to memory  2205  to store configuration settings, assistance data, and other data for the operation of the GNSS module  2204 , and access to the memory  2221  to store location data obtained from a locating system. The memory  2205  and/or  2221  may include a flash memory. 
     The asset tracking device  2200  further includes a temperature sensor  2208  to capture temperature readings at the asset tracking device  2200 , a supercapacitor energy storage unit  2210  to power the asset tracking device  2200 , a charging interface  2212  to charge the supercapacitor energy storage unit  2210 , and a solar panel  2214  to supply energy to the supercapacitor energy storage unit  2210  through the charging interface  2212 . The supercapacitor energy storage unit  2210  may include two 75 F supercapacitors and an active balancing module to balance energy stored at the two supercapacitors. 
     The asset tracking device  2200  further includes a controller  2220  to perform functionality described herein. The controller  2220  has access to memory  2221  to store programming instructions, temperature data from the temperature sensor  2208 , and motion sensor data from the accelerometer  2202 . The controller  2220  may be configured to monitor the voltage outputted from the solar panel  2214  and the voltage at the supercapacitor energy storage unit  2210 , and may further be configured to modulate the charging of the supercapacitor energy storage unit  2210 , and to control low-power operating modes of the controller  2220 , GNSS module  2204 , and cellular modem  2206 , appropriately, to conserve energy. For example, the controller  2220  may operate the cellular modem  2206  and GNSS module  2204  for data transmission only when the transmission of location information is appropriate. 
     The asset tracking device  2200  further includes a first power converter  2216  to provide adequate voltage from the supercapacitor energy storage unit  2210  to the controller  2220 , temperature sensor  2208 , accelerometer  2202 , memory  2221 , GNSS module  2204 , and memory  2205 . The first power converter  2216  may include a DCDC buck converter. The asset tracking device  2200  further includes a second power converter  2218  to provide adequate voltage from the supercapacitor energy storage unit  2210  to the cellular modem  2206 . The second power converter  2218  may include a low-dropout (LDO) regulator. 
     The controller  2220  may execute programming instructions to perform any of the functionality of an asset tracking device described herein. For example, the controller  2220  may execute programming instructions to perform the method  400  for asset travel monitoring of  FIG.  4   , the process  900  of  FIG.  9    for operating the asset tracking device  2200 , the method  1300  of  FIG.  13    for temperature-dependent charging of the supercapacitor energy storage unit  2210 , and related methods and actions. Thus, the asset tracking device  2200  may determine whether the asset it is tracking has started or finished travel based on motion sensor data from the accelerometer  2202 , may charge the supercapacitor energy storage unit  2210  to a target voltage based on temperature data from the temperature sensor  2208  and/or environmental data received via the cellular modem  2206 , and may feed location information and other data to an asset tracking device management system which may link the asset tracking device  2200  together with other asset tracking devices that it travels with. 
     It should be recognized that features and aspects of the various examples provided above can be combined into further examples that also fall within the scope of the present disclosure. The scope of the claims should not be limited by the above examples but should be given the broadest interpretation consistent with the description as a whole.