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Location technologies represent a major step in the usage of technology these days. This paper discusses the current services that still emerging in the area of global positioning as well as their underlying location technologies. We expose our project's concept in the area of people tracking and control, based on a set of technologies, which are also approached.
Location-based Systems, Location-aware Services, Real-time Information Delivery, Positioning Technologies, People Tracking and Control.
Nowadays, it's amazing the amount of information we can get with just a single piece of data like our current position. This fact ends up justifying the amount of new services than we've seen emerging in the last few years and we'll continue to see in the next ones, since the market potential for such services - based on costumer's location - is huge. Now that location technologies embed in mobile devices had just begun to trivialize into the public market, the interest in location aware services other than traditional navigation systems, has just started to grow up substantially.
Despite the huge popularity in usage of Global Navigation Satellite Systems (GNSS) devices like GPS, there are other solutions available, each one with its advantages and drawbacks, which will be approached more in-depth along this document. The needs of organizations also made the development of LBS (Location-based Systems) to rapidly evolve. Since there are LBS that are unable to operate inside covered areas such as buildings but are much more effective under exterior conditions in contrast to other systems, the mixing of those different technologies expands the current possibilities to a limitless environment where information can be shared and accessed everywhere and by anyone, with virtually zero effort.
This paper is organized as follows: Section 2 exposes several location technologies appliances emerging in the market, from which we have chosen one for our project. Section 3 covers the positioning technologies and techniques available to the mass market development. Section 4 describes the scope of the application to be developed. Sections 5 and 6 discuss possible real-life scenarios and denote the required technologies by the application face to different adversities. Section 7 concludes.
As we all know, the availability of location technologies to the mass market triggered the development of brand new and innovative services. Those ones, often designated as location-aware services, brought new possibilities at different levels :
"You are just about to join a nine kilometer traffic queue. You better turn right on the A2 ahead for a better alternate route."
"Help, my car has broken down. Need assistance, please."
"What is the speed limit on this road where I am?"
Other appliances would be, for instance, connecting citizen's vehicles alarms to police vehicles.
"Where should I shovel snow to find the hydrant?"
"I have free calls on my mobile phone, while I'm in a particular location or the called number is within a certain range."
Package and Asset Tracking
"Where is the package with those new SIM cards?"
"I'm sure I left my PDA on the train, but where is it now?"
"Is there a programmer's job nearby?"
"Where are nearby yard-sales featuring antiques?"
Vehicle Theft Detection and Recovery
"My car has been stolen; where is it?"
"Tell me if my child strays beyond the neighborhood."
"I lost my pet; where is it now?"
The project we're about to develop is mainly focused in the concerning of people tracking. Before taking the reader to a longer explanation of the project itself, we would like to provide some info about a set of technologies we explored at a scientific level in order to choose the ones that would fulfill our project requirements in a feasible way, which follows in the Section 3.
When we talk about positioning technologies, the conventional U.S. space-based global navigation satellite system known as GPS (Global Positioning System) is what usually comes first in our mind. Despite its relatively high accuracy, the truth is that there are other emerging technologies that are more efficient under certain conditions.
GPS is structured down in three parts: about 32 satellites orbiting the Earth, four control and monitoring stations placed on the Earth and the GPS receivers it selves owned by users . Orbiting 12,600 miles above the earth, at least 6 satellites should be visible from any vantage point on it, regarding GPS design. However, only 3 are really needed to determine the device location. A GPS receiver calculates its position by precisely timing the signals sent by those satellites. When a GPS receiver is turned on, it looks for and locks on to visible GPS satellites to begin receiving and decoding the GPS broadcast signal. The GPS ephemeris data provides precise timing and location information for each satellite. Latitude, longitude, time and altitude are the 4 dimensions that must be solved for (usually only the first 3 are used) . Position acquisition time is driven by previously collected GPS information and GPS signal quality. GPS accuracy is about 5-20m depending on environmental conditions . In comparison to other solutions, GPS receivers may take more time to become ready to use by providing a location fix and the power consumption is also a factor to consider especially in mobile devices. Given the fact that a lot of new mobile devices are already featuring GPS receivers, and that as we'll realize along this paper, GPS technology is the most accurate regarding outdoors tracking precision (at least among the other technologies explored in this document), this is definitely a good solution as regards to our project scope.
Assisted GPS (A-GPS)
Assisted GPS is a technology that allows mobile devices which are connected to a wireless network to improve the speed of GPS signal acquisition. This is accomplished through A-GPS by simply taking the known location of the base station at which the mobile device is connected to and then providing synthetic or seed data to the mobile device's GPS chip so that if it knows it should locate only a specific set of satellites at its current position, it will be able to reduce the search of signals to acquire. Note that in order for A-GPS to work properly, an initial position estimate is required (traditionally acquired via the operator's Cell-ID database) and that device chipset support is required as well . Most of today's devices featuring GPS receivers are A-GPS enabled devices, so it is substantially faster to get a position fix when comparing to standalone GPS receivers without A-GPS support.
Triangulation is a calculation that is done by measuring the distance between the device and other 3 sensors (at least) close to it. In theory, the position determination is then given by the intersection of the distances computed for each of the 3 sensors.
Signal strength-based NT
The usage of RF (Radio-frequency) signal strength is another approach in order to provide improved positioning determination. The concept behind this is quite simple: to evaluate RF signal strength of signals received by the device and then to incorporate network information. This way, RF fingerprinting takes the locations of known base stations, then calculates the propagation of RF signals, and uses the resulting RF base map and signals received at the device to estimate the devices location . Note also that, along with RF signal-strength readings, RF fingerprinting also considers other RF characteristics such as reflection, attenuation, and multi-path .
Like GPS which is able to use timing information provided by satellites, there are some position technologies that make use of signal timing information from the network in order to provide position determination. This can be accomplished by making use of some techniques like Time Difference of Arrival (TDOA) and Advanced Forward Link Triangulation (AFLT). However, due to the amount of signaling traffic required to make a position estimate, the first technology is not a feasible alternative in the commercial LBS services area. Regarding AFTL, it is a CDMA specific technology that uses the signaling characteristics of a CDMA to make a positioning determination, being also a good alternative to A-GPS lack or failure when the network is requested to make a location determination. To determine location, the device takes measurements of signals from nearby cellular base stations (at least 3 are usually needed), reporting the time/distance readings back to the network, which are then used to triangulate an approximate location of the handset .
Hybrid signal-strength and time based NT
Another technology that makes use of both signal strength and timing received from the GSM signal is Enhanced Cell-ID (E-CID). By doing that, it is possible to make an in-network calculation of location which is more accurate than Cell-ID alone. This accuracy improvement is achieved by using additional Timing Advance (TA) and Network Management Records (NMR), or information derived from the wireless network. The precision of the location using this technique could be as large as several kilometers, depending on the distribution of base stations.
In the Table 1, there's a comparison between the different technologies mentioned above, regarding some of their key factors like location precision rate, time to first fix and underlying requirements.
As organizations needs grow in terms of mobility of their collaborators, a new obvious need come up: the ability to control. Control is the key aspect of our project. It's also wise to think that this kind of solution may please a great number of parents with some piece of mind, as it should provide parental control functionalities over their children. The goal we are trying to approach, is to end up with a remote solution able to track device's locations at any time, using a set of technologies which were prudently selected not only for economic reasons but also to ensure the proper and more accurate positioning status to the end user.
The application running on two or more mobile devices would be able to track the position of each device. Privacy issues have also to be considered given who is supposed to monitor and who is expected to be monitored.
Let's consider that a certain company's scope involves constant travelling to outside countries by its collaborators. Depending on the importance level of the job performed by each one of them, there might be a necessity to control their position at any time. The application should be able to perform that task as main functionality. At any time, a supervisor will be able to track the current location of all the collaborators' devices which were properly configured to provide its position to the supervisor's device.
Considering another scenario and exploring yet another expected feature, there might be parents which may not only want to know where their children are at any time, but also to be notified when they are not where they are supposed to be at some predefined time of the day (i.e.: skipping the school, etc). Let's suppose that a child should be at school between 8:00 am and 12:00 pm. This location should be remotely acquired and predefined when, for example, parents know the real position of their child. Only the monitoring device (the parents' one) is needed to perform such task and it has to be done only once. When the child gets away from the school by a predefined radius in meters and considering that it is respecting the time span specified by the monitoring device, this last device will be notified.
The application is also expected to contain security measures in order to avoid abusive behaviors by its users, especially the ones which are supposed to be monitored (i.e.: application shutdown, changing settings, etc).
Privacy is a major concerning, especially when we're talking at business levels. Therefore, a device's location should only be provided to other trusted devices. In order to accomplish that, the application will have a "white list" where it should be specified the device or set of devices with which the current device should share its position.
REQUIRED SUPPORTING COMMUNICATIONS TECHNOLOGIES
After exploring a number of possibilities, we decided to use GPS (though considering the usage of A-GPS in order to improve the tracking precision). Given the battery drain problem caused by the relatively high power consumption of GPS receivers, there is a necessity to keep it idle (or switched off) until there is a mandatory task that requires its usage, that is, a necessity to get device's current position coordinates. This situation leads to another problem. The GPS receiver working alone will take some time (from seconds to several minutes) to fix its position every time it is switched on (and the problem worsens when we move from the position of the last fix to another distant one, between the time span where the GPS receiver was switched off). This will result in having to wait an undesirable amount of time to get the current position of a monitored device. In this case we rely on Assisted GPS to decrease the time to first fix, but also to improve location positioning when satellite signals are blocked or weakened by buildings or walls, situation often experienced in the cities.
Another issue that we can predict in advance, is the problem of not getting any location coordinates by intercepting no GPS satellite signals at all (i.e.: inside buildings, under rooftops, etc). An attempt to avoid that is to rely on CELL-IDs to provide positioning. It would also function as a workaround for mobile devices not featuring a GPS receiver, allowing them to take advantage of this application. Actually, Google Maps (which we are going to use to show devices' positions) has a huge database containing the entire world's CELL -IDs of which our application can take advantage . The real problem here is the precision of such technique. As we can see at Table 1, the location precision's margin of error is very high comparatively to other technologies like GPS. In the second scenario denoted on Section 5, where we expect to be notified when a monitored device gets away from a specific location by a certain radius in meters, the usage of CELL-IDs to acquire the device's position could generate several false warnings/notifications because of the weak location precision. That said, we will have to reflect on all those factors to consider if it is feasible or not to implement such functionality on the application.
All the communications between devices will pass through a central web server which will be responsible for establishing connection between the devices, controlling its status and hold information about the logged devices such as their IP addresses providing transparency to the end-user. To accomplish that, and given the small amount of data to be carried between the end-points, GPRS communications between devices and the web server will be used, trying to assure the economic benefits of this communication technology over other ones like GSM. Note that GPRS was already required in order to download data from Google Maps.
In conclusion, we must say that we truly believe that tracking applications have a well defined spot in the market, and even more when they are able to take advantage of set of technologies working in a hybrid way, which makes it possible to provide functionality under several restrictive conditions at which human beings are exposed every day. With the development of our application, we expect to provide a multipurpose application capable of spreading into different market niches keeping always in mind the different needs of each of them.
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- Jim McGeough, President & CEO, Digital Earth Systems, Inc. (Dec. 27, 2001) - Originally published and presented at GITA conference, July 26, 2001.
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- Table of values that gives the positions of astronomical objects in the sky at a given time or times .
- Mobile phone standard which use "Code Division Multiple Access" as an underlying channel access method .
- Length of time a signal from the mobile device takes to reach the base station .
- Time to First Fix (TTFF) describes the time and process required for a GPS device to acquire enough usable satellite signals and data to provide accurate navigation .
- TTFF - Time to First Fix (refer to footer note 4).