Background Positioning for Mobile Devices

Background Positioning for Mobile Devices
Background Positioning for Mobile Devices Android vs. iPhone
Denis Huber
School of Electrical and
Computer Engineering
Technical University Berlin
Email: hubeealh@mailbox.tu-berlin.de
Abstract—This seminar paper covers the current techniques
available for mobile phone positioning and their application.
GPS, WiFi and cellular based positioning methods are explained
in detail, and current trends regarding the optimization of
existing algorithms are analyzed. The algorithms talked about
include fingerprinting and different time of arrival methods.
Current solutions in the mobile phone market from Apple
and Google which use several positioning technologies in their
products are investigated. Especially the APIs from Apple and
Google who deliver access to these technologies. Location Based
Services (LBS) which need continuous monitoring of the users
position are leading to a high energy demand. This problem is
not solved yet satisfactory, and different approaches for solving
this problem are available.This paper investigates some of the
proposed algorithmic solutions and gives an outlook to the
future.
Location-based services a lot of different techniques are
available today including GSM, WiFi or GPS positioning.
These different methods as well as new approaches of
improving them are covered in section 2. In section 3 a
look under the hood of the positioning capabilities of the
iPhone and Android OS’s will be provided, as well as the
API calls available for developers. In section 4 the problem
of continuous tracking by LBS’s leading to high power
consumption will be discussed. Different approaches to solve
this problem will be presented. The final section gives an
outlook for the future and concludes this paper.
II. OVERVIEW OF P OSITIONING S YSTEMS
I. I NTRODUCTION
Smartphones with high bandwidth internet access and
GPS receivers have become very widespread these days and
have repressed traditional mobile phones. Two of the leading
companies in the mobile phone operating system market
today are Apple with their iPhone iOS and Google with
Android OS. One of the major features of these operating
systems is the ability of running a wide array of third party
applications available for purchase in special App Stores.
A special kind of applications has emerged in the last few
years, the so called Location-based services (LBS) [1]. These
are applications using the ability of smartphones to localize
the geographical position of their users outdoors as well as
indoors. With applications ranging from showing the users
position on the map or routing to new kinds of services
like Location-based gaming, Location-based advertising or
even mobile dating, many companies are developing LBSs
today. But also security applications like for example the
tracking of 911 calls, which is now mandatory by law in the
United states [2], are using the location information provided
by mobile phones these days. While some applications
may need only very rough position fixes others may need
a very accurate location determination. One of the major
challenges in localization today is to deliver this accuracy
without consuming too much of the limited amount of battery
power available on the mobile devices. In order to realize
Modern smartphones today can use up to three different
methods to locate their position. Satellite Positioning,
Wifi Positioning and Cellular Positioning (Figure 1). In
the following subsections all three will be explained in
detail. Generally these positioning methods have different
characteristics which can be measured by the following
criterias like described by Hightower et. al [3].
•
Accuracy. The difference between the true position and
the calculated position.
•
Precision. The closeness of a number of position values
to their mean value.
•
•
•
Power consumption. The amount of electrical power
needed for determining the position measured in Watts
per second.
Latency. The time needed for obtaining the position
fix. Also generally known as TTFF (Time to first fix)
measured in seconds.
Availability. Not all positioning methods are available
in every situation.
B. Cellular Positioning
Cellular networks have expanded globally in the last decade
and reached almost worldwide coverage. At the moment
there are mainly two types of cellular networks in operation.
The Global System for Mobile Communication (GSM) and
the Universal Telecommunication System (UMTS) often also
referred as 2G and 3G networks. While the next generation
of networks Long Term Evolution (LTE) is already in the
pipeline, it will take again probably two years since this
standard will be integrated in the network infrastructure and
on the mobile devices. While work is already in progress
in specifying positioning for the next generation of cellular
networks, in this paper only techniques used for 2G and 3G
networks are explained.
Fig. 1.
Overview of Position Systems from [4]
A. Satellite Positioning
Satellite Positioning is a technology originally invented
by the US Military. The most prominent satellite positioning
method today is the Global Positioning System (GPS). It
uses up to 32 satellites to determine the position of a Mobile
Station (MS) [5]. There are two types of signals sent by
the satellites. The Precise Positioning Service (PPS) and the
Standard Positioning Service (SPS). While PPS is reserved
for military use and is encoded, SPS is free for public use.
A mobile device equipped with a GPS Receiver catches up
at least four of these satellite signals which are broadcasting
time and orbital information and calculates the position from
these. GPS is very accurate and precise technology, providing
accuracy up to 20 meters, but it consumes also a lot of power
on the end device. Another downside of GPS is that satellite
signals can not be received indoors, and therefore it is not
suited for indoor positioning. Also the TTFF can be up to 15
minutes long because when the device was off for a longer
period of time the complete Almanac database (location of
all satellites in the orbit) must be reloaded into the device.
Most smartphones today however use Assisted GPS (A-GPS)
like described in [4], a technique where the traditional GPS
Receiver is assisted by cellular positioning methods and
the assisting data is downloaded in the background via IP
connection. This results in better accuracy, faster TTFF times
and less power consumption. However IP connectivity may
not be available in all situations and power consumption is
still higher than in other positioning methods.
As GPS Receivers have become less and less cheap they
are integrated in nearly every smartphone today becoming a
standard. Beside that a lot of improvement in the Hardware
of GPS Receivers has been done lately, and recent studies
have shown that they can be functional in certain indoor
environments which can bee seen in [6].
Cellular networks are basically divided into cells which
ideally have a round shape. Each of this cells is associated
with a cellular tower (base station). As a user moves through
the cells his mobile device will connect to the base station
with the best signal strength available, which should be in
most cases the nearest one according to his position. The
basic form of cellular positioning therefore is to use the
location of this base station, whose position is already known
and stored in a database. This method is referred as cell
identification (cell ID). The accuracy delivered from this
method relies on the cell size which can be up to 35 km
in radius for 2G networks and a little bit less in 3G due to
technical restraints. For this reason a method called timing
advance (figure 2) is used. This technique measures the time
signals take to arrive from the mobile device to the base
station. The mobile station can then be located in a circle
around the base station. This can be improved even further
with the use of multiple antennas which divide the circle
into sections. These method is referred as enhanced cell ID
(E-CID) [7].
When mobile devices are in reach of multiple base stations
more complex algorithms can be used, which utilize the Time
Difference of Arrival (TDOA) or Angle of Arrival (AOA).
See Kuepper [8] for a complete overview of these techniques.
Generally all of these methods need a high density of cell
towers to function properly and TDOA, due the need of
timing measurements reliability, is not well suited for 3G
networks.
The accuracy of cellular positioning lies far behind GPS,
but regarding it’s power consumption, nonavailability indoors
and additional hardware costs, research in cellular positioning
techniques regarding the improvement of their accuracy is
considered well. For example Ibrahim and Youssef propose
a new algorithm based on a hidden Markov model which is
especially suited for low end GSM phones and which delivers
quite good results [9] and Nurmi et. al provide a grid based
solution [10]. Also a probabilistic method based on RSSI
fingerprinting is provided by Ibrahim and Youssef [11]
C. WiFi Positioning
In general WiFi positioning takes a similar approach as
the cell ID algorithm in cellular based positioning. Instead
of cell towers the geographical information of WiFi Access
Points (AP) is kept in a database which can be looked up for
positioning. This information is usually gathered through war
driving. This means that people equipped with GPS Receivers
and WiFi clients are driving around collecting the data. As a
key the unique MAC Address of the access points is being
used. As WiFi networks have only an average range of 60
meters a good accuracy is provided naturally. Also the TTFF
times are low, while no connection must be established to the
Wifi Network and the database lookup may only take some
seconds. Traditionally WiFi positioning was regarded as a
indoor only solution, but as wireless Internet access gained
more and more popularity, now there’s also a very good
outside coverage in urban areas. Jones et.al provided a good
research work in analyzing the density of access points in the
United states [12].
WiFi positioning can either be performed passive or
active from the mobile terminal viewpoint. The active
method uses the uplink of a 802.11 WiFi enabled device.
A beacon, which is basically a broadcast signal, is sent by
the terminal.The access points receive this information and
evaluate it in the network.This method is generally referred
as network-based positioning.The other method available
uses the downlink.The terminal passively scans beacons from
the access points, known as Basic Service Set Identifier’s
(BSSI), which include several information including their
signal strength. If no beacon information is available for a
longer period of time, the terminal can send a probe request
and all access points available in range will send their BSSI
back. This is referred as active scanning. Generally WiFi
positioning algorithms which use upstream or downstream
can be divided in three categories as described in [8].
•
•
•
Proximity sensing.The position of the access point with
the best signal quality received is adopted.
Lateration.The distance between the access point and
the terminal is determined by the path loss experienced
by the beacon during transmission.
Fingerprinting.A database with RSS patterns observed
earlier is compared with the actual RSS pattern received
by the access points. The best match is then adopted as
the actual position.
From these methods proximity sensing is the easiest to
implement. Though it suffers from accuracy as the range of
a signal broadcasted by access points can vary from some to
a hundred meters. Accuracy and precision can therefore not
be guaranteed and the algorithm is only suited for granular
positioning. Lateration on the other side is only possible
when the access points are aligned in a certain way. As this
is far from reality in most cases this method is not used in
daily practice. The last method remaining is fingerprinting.
In this method a radio map,in which the signal strength
for a certain position is stored, is generated offline with
the already introduced uplink method. This information is
stored in a database. The database can be either stored on
the device or in the network, the latter being the better
option because of limited processing power and memory
available on the handsets. In the online phase, the terminal
requests it’s position from the network in sending the actual
measured RSSI information. This information is matched
against the existing data from the radio map in the database.
The point with the minimum distance to the received one
from the terminal is then calculated and will be taken. This is
referred as the Nearest Neighbor in Signal Strength method
[13]. Beside this simple deterministic approach also different
probabilistic methods are possible which can deliver better
results but are more complex to implement. Honkarvirta et.
al provide a very good overview of these algorithms [14].
RSSI fingerprinting is very promising in terms of accuracy
when enough access points are used, as already existing
implementations like RADAR [13] show very good results
with an accuracy of up to only a few meters. This can surely
be improved in the future as a lot of researchers are currently
active in this area. A major advantage of WiFi positioning
however is that no additional hardware is required, as most
devices already implement the 802.11 standard and the
access points are already present and don’t need any further
modification.
D. Summary
Fig. 2.
Cell ID with timing advance and sectored antennas
As seen in this section so far, all of the positioning methods
have different characteristics. With GPS in advance in terms
of accuracy and position but weak in energy consumption,
and WiFi best suited for indoor application but not available
everywhere. While cellular positioning is almost available
everywhere today, and very energy efficient it still lacks
accuracy and precision. An in depth comparison of all
positioning techniques on the iPhone 3G is provided by
Zandbergen [7].
III. S MARTPHONE S PECIFIC P OSITIONING
In this section an overview of the current smartphone
positioning techniques is provided, by looking at the two
most popular mobile Operating Systems (OS) today. These
are Google’s Android OS and Apple’s iOS running on the
iPhone. Also third party solutions available for different
platforms will be covered.
A. Apple iPhone
Apple recently introduced the fourth version of the iPhone.
Simultaneously they also released their renewed operating
system iOS, currently available in Version 4.2, which added
new features including multitasking. The first generation of
iPhones did not have GPS Receivers. Location determination
was realized with GSM and WiFi positioning using Google’s
location databases. With the second and third models Apple
built in GPS Receivers and used Skyhook as their main
location data service. Skyhook is a company specialized in
providing location data services, and will be discussed in the
Third Party section. On the newest iPhone 4 model however
Apple now use their own database of location information,
and like tests have shown the location determination accuracy
and performance has been improved with it [17].
Developers can access the positioning features of iOS
4.2 via the Core Location API provided by Apple [18].
There are two major options available. The first one is
the standard location service (Figure 3). For the standard
location service a new object of the CLLocationManager
class has to be created.Then a delegate object of type
CLLcationManagerDelegate and a desired accuracy value
must be assigned to it. Also a distanceFilter is set to the
value in meters when the location should be updated. Finally
the startUpdatingLocation method is called, and the device
begins to track it’s position, updating it when the user
moved out of the distance set in the distanceFilter. The
assigned delegate will be notified when location information
becomes available or changes. This method as proposed by
Apple works best for positioning applications running in the
foreground e.g. Turn by Turn Navigation. The higher the
value in the CCLocationManagerAccuracy property is set
the more precise the position data. This also leads to higher
energy consumption as GPS will be used when available.
However the real value of precision relies on the availability
of the methods on the infrastructure side and can change
drastically during runtime. The second method for tracking is
called significant update change. Like in the standard method
a CLLocationManager object must be created and associated
with a delegate. But instead of calling startUpdatingLocation
the method startMonitoringSignificantLocationChanges is
called. With this method the application can be suspended
and is woken up automatically if a significant location
change took place. This is the method proposed by Apple if
location services should run in the background. It consumes
much less energy as it is relying on cellular positioning
only, but is much more inaccurate and therefore not suited
if slight position changes must be tracked [19]. With the
new multitask feature it is also possible to run the standard
location service in the background. But as this consumes
lots of power Apple describes it as the least desirable option
[20]. Another new feature interesting for LBS developers
is the new monitoring region feature introduced with iOS
4. With this new feature it is possible to monitor a shaped
region which can be highlighted on a map. This is realized
via the startMonitoringForRegion method provided by the
API. When the mobile phone reaches the boundary of the
monitored region either the didEnterRegion or didExitRegion
will be called depending if the user wants to be notified
entering or exiting the region. However this method tracks
the user continuously, hence consumes a lot of battery
power. As we can see developers do not have much control
over the positioning hardware. Instead the API manages
and encapsulates the decision which technology is used.
Developers have either the option to use the more precise
standard location service resulting in high energy consumption
or use the significant update feature with less precision. Apple
however provides no data about accuracy and precision of
the positioning features of the iPhone. Results of a field test
done by Zandbergen with the older model iPhone 3 can be
looked up here [7]
Fig. 3.
Starting a standard location service in Core Location [18]
B. Google Android
Google recently unveiled the newest Version of Android
which is version 2.3 now. In contrary to Apple, Google
is focusing their attention on the mobile operating system
Android only, and does not produce any hardware. This means
that their operating system is used on a lot different hardware
devices from companies like HTC, Motorola, Samsung and
others and not all positioning methods may be available on
a specific phone e.g. GPS. Google use their own location
databases for localization. Though it is possible for vendors
of Android powered phones to use third-party localization like
Skyhook, Google makes effort in preventing this [21].
Developers can obtain location data using the Android API
[22] in the following way (Figure 4). First a reference to
LocationManager must be acquired from the system. Then
a new LocationListener object is assigned to it, which has to
implement several callback methods. With the call of requestLocationUpdates the device begins to locate it’s position. The
parameters passed to this method are very essential here. The
first parameter expects a LocationProvider. This can be either
Network Provider or GPS Provider. The first option means
that only cell and WiFi based location methods will be used,
while with the second option only GPS will be utilized. It is
also possible activating both with calling requestLocationUpdates twice with both different parameters. With the second
parameter a minimum time interval between location updates
can be set. The third parameter allows to set the minimum
change in distance before getting new location updates. With
the fourth parameter the LocationListener instantiated before
is handed over. As we can see Android developers have a little
more control over the Hardware in comparison to iPhone developers. They can decide which positioning method they want
to use, in choosing the desired LocationProvider. Also the
fact that LocationManager operates systemwide allowing the
developers to run only the LocationListeners in the background
is a good solution. But there are still shortcomings in terms
of controlling the energy consumption and accuracy. Choosing
only between two location providers is still too granular and
in the effort of building energy saving applications which on
the other hand can provide good accuracy. However Google
doesn’t provide any data about the position accuracy on
Android devices.
C. Third Party APIs
There are a couple of third-party solutions for localization
on the market like Skyhook Wireless’s XPS System, SoftGPS
from Devicescape, PlaceEngine, Navizon or Polaris Wireless
[16]. The most prominent and most successful till now being
Skyhook. Skyhook has a large amount of location data from
Wireless LAN access points which they gathered through
wardriving or information by private person’s who can
provide Skyhook their location data in form of MAC-Address
and relating GPS coordinates through their website [24].
According to Skyhook their location data covers up to 70 %
of the US rural areas and many places around the world with
over 250 million access points in their database. Developers
can download their Software Development Kit (SDK) for
a number of platforms including Android, but excluding
the iPhone. For platforms where the SDK is not available,
Skyhook provides a portable Core Engine for developers and
Fig. 4.
Define a Location Listener in Android [23]
manufactures which can be set up on top of the underlying
OS [25]. Once the SDK or the Core Engine is running on the
device, developers are enabled to use a the Skyhook API for
location tracking. Skyhook’s XPS provides hybrid location
determination with GPS, WiFi and cellular techniques and the
company holds several patents most of them regarding WiFi
positioning. They indicate that their hybrid system offers an
accuracy of up to 10 meters with an availability of 99.8 %
and a TTFF time of only 1 second [26] like shown in table
I, though accurate information under what circumstances
this data was collected is not provided. Also no information
about energy consumption is provided. Their website gives
an overview with sample codes how one time location fixes
on certain platforms can be achieved, but no information
regarding constant location tracking and energy saving can be
found there [27]. Skyhook as already mentioned in a section
before lost a big customer with Apple and it’s relationship
with Google is not the best, so their future remains unclear
at the moment.Apple and Google seem to take the location
market seriously and do not want to share it with other
company’s, though they might face some patent trial issues
with Skyhook [28].
TABLE I
S KYHOOK ’ C ORE E NGINE PERFORMANCE ACCORDING TO THEMSELVE
[26]
Core Engine
GPS
A-GPS
Accuracy
10 meters
10 meters
30 meters
Availability
99,8%
80%
95%
TTFF
1 sec
65 sec
30 sec
IV. LBS AND E NERGY C ONSUMPTION
As already mentioned in the introduction Location-based
services are gaining more and more popularity these days.
Many of these applications do need a location fix at a time
or need only rough position estimation like for example the
city the user is currently staying in. In this case energy
consumption is not a big problem, while GPS is only turned
on for a short period of time or not needed at all because
the accuracy of other methods is already sufficient. However
another category of applications has evolved in the last years
which are known as proactive Location-based services. The
most prominent examples here are navigation applications.
These applications show the users current position on a map
and therefore need to track him constantly while he is moving
around. As high accuracy is needed in this case GPS is
constantly turned on which leads to a very high energy
consumption and the battery of the mobile phone will be
drained quickly (Figure 5).
fixes energy consumption can be reduced significantly (Figure
6), though accuracy suffers. But in comparison to Enloc the
accuracy achieved is better especially in urban areas with a lot
of turns (Figure 7). Furthermore accuracy can be improved wit
map matching where the position data calculated is mapped
against a street map. This technique is already widely used
in car navigation systems. Youssef et al. recently gained an
technology award from Google for this project. Google also
will implement this system in future versions of Android.
Fig. 6.
Fig. 5.
Energy Consumption with GAC from [31]
Power consumption of typical Location Based Services from [29]
This problem has currently gained a lot of attention in
the LBS research community [29]. One solution provided by
Constandache et al. [30] is to turn on GPS only in certain
time intervals and calculate the track between them with probabilistic methods. This leads to less energy consumption, but
the position errors are quite high in tracks with a lot of turns.
The idea has been improved by Youssef et al. with their GAC
Project [31]. The main idea here is that most of the modern
smartphones are equipped with compasses and accelerometers
which can be used to calculate the track between two GPS
position fixes. With longer time intervals between the GPS
Fig. 7.
Accuracy with GAC from [31]
V. C ONCLUSION AND O UTLOOK
Positioning methods have improved a lot in the recent
years, but there are still many challenges to master. GPS
can be improved for indoor positioning and cellular based
algorithms have to become more accurate. Of course also
the infrastructure plays an important role and the density of
WiFi and cellular stations will certainly increase in next years
leading to better accuracy especially with WiFi. Also new
technologies like LTE or the announced european satellite
positioning system Galileo will play an important role in
the future of positioning. On the manufacturer side Apple
and Google should make positioning more transparent for
developers and provide a better energy management. This
could be possible with the solutions presented in this paper.
A good approach would also be to implement a power
simulation already integrated in the SDK, so that developers
can test their applications for energy consumption already in
the early stage of development.
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