A Survey on Different Methods to Prevent the Manipulation
of Locating-Technologies
Michael Decker
Institute AIFB, University of Karlsruhe (TH), Kaiserstr. 89, 76 128 Karlsruhe, Germany
Location-based Services (LBS), Location-Spoofing, Mobile Computing Security, Positioning.
There are many different locating technologies to determine a mobile device’s current position. Examples
for such technologies are the satellite-based Global Positioning System (GPS), the cell-ID method in mobile
phone networks or WLAN-based approaches. These technologies are the enabler of so called Location-based
Services (LBS): LBS are services to be used with mobile handheld-computers like PDAs or smartphones that
evaluate a mobile user’s position. When locating-technologies are discussed in the LBS community, the focus
is often on the accuracy of the calculated location whereas the resistance to manipulation attempts by the
possessor of the mobile device or by third-parties is almost never considered. But there are examples of LBS
where users or external attackers might have an incentive to manipulate the locating system. This is termed
as location spoofing. This article presents a survey of different technical approaches to prevent or at least to
detect location spoofing.
Location-based Services (LBS) are services for mo-
bile handheld-computers which evaluate the loca-
tion of at least one mobile device during execution.
The standard example for LBS is that of a Point-of-
Interest-Finder (POI-Finder): such a service guides a
user to certain type of facility (e.g. restaurants, petrol
stations, ATM) in his nearer surrounding.
LBS are enabled by the possibility to determine
the location of a mobile device. This is called locat-
ing. Nowadays many descriptions for locating meth-
ods can be found in literature, see Küpper (2007) for
an overview. While in the common considered sce-
narios for LBS like the above mentioned POI-finder
it seems to be quite unlikely that someone has an in-
centive to manipulate the employed locating system
there are several evident examples of LBS for which
this isn’t the case, e.g. location-aware access control,
navigation of military vehicles, location-based billing
or geo-fencing.
The term location spoofing in literature can re-
fer to the manipulation of a locating system by the
possessor of the device
(internal attack, e.g. Mundt
the possessor of the device isn’t necessarily the legal
owner, e.g. if the device was stolen or lost
(2005)) or by an third-party attacker, who is tech-
nically not involved in the provisioning of the LBS
(external attack, e.g. Warner & Johnston (2003)).
In the domain of computer science the term spoof-
ing usually refers to faking ones identity, e.g. DNS-
or IP-spoofing. Spoofing doesn’t include the case of
a denial-of-service-attack where the determination of
the location is just inhibited or jammed; such attacks
are noticed by the mobile user or the LBS provider
as failure of the locating system. During a spoofing
attack the mobile user or service provider obtains a
faked location statement but isn’t aware that the lo-
cation was faked, so spoofing is more dangerous than
just jamming.
The purpose of this article is to give a system-
atic overview of various approaches to prevent lo-
cation spoofing that are described in literature. De-
spite extensive literature research we couldn’t find a
survey article which covers anti-spoofing techniques
from the viewpoint of LBS. To facilitate this presenta-
tion of all the anti-spoofing-approaches we developed
an appropriate classification scheme.
The remainder of the article at hand is organized
as follows: Section 2 is devoted to an explanation
of basic terms and the introduction of a classifica-
tion scheme for anti-spoofing-methods. According to
this scheme we call it the Anti-Spoofing-Tree
Decker M. (2009).
PREVENTION OF LOCATION-SPOOFING - A Survey on Different Methods to Prevent the Manipulation of Locating-Technologies.
In Proceedings of the International Conference on e-Business, pages 109-114
DOI: 10.5220/0002232601090114
the discussion of the identified basic methods to pre-
vent spoofing from section 3 to section 7 is organized.
Since GPS is the most popular used locating system
we devote section 8 to discuss how GPS is secured
against spoofing. The article ends with the obligatory
summary and outlook in section 9.
During our literature research we identified five basic
methods for the preventionof location spoofing which
can be found at the second level of the classification
tree depicted in figure 1: Plausibility checks, tam-
perproof hardware, location keys, request-response-
protocols and radio technology methods. The further
subclassifications shown in the figure are covered in
the corresponding sections in this paper.
One kind of attack that is relevant for several lo-
cating systems is the so called rerouting attack (also
called wormhole attack): the principle is that a signal
or message is received at a particular location and then
transmitted (maybe using another medium) to another
location, where it is emitted. A perfect rerouting at-
tack can only be detected by the latency (time lag)
it induces. Another similar attack is the replay at-
tack: for such an attack the signal is recorded and re-
played (maybe several times) with a deliberate time
lag, maybe at a different location.
When performing a plausibility check on the level of
the raw messages (e.g. radio signals) of the locating
system we have the possibility to check the absolute
or the relative signal strength (Warner and Johnston,
2003). The signals emitted by the GPS satellites reach
the earth’s surface with a strength of just 160dBW.
An obvious way to perform an external spoofing at-
tack is to employ earthbound artificial satellites (so
called pseudolites) which broadcast faked GPS sig-
nals with a much higher signal level and thus over-
cast the genuine signals from the satellites. This at-
tacks can be detected by checking the absolute signal
strength of the received signals. More sophisticated
attacks can only be detected by paying attention to rel-
atively weak increase of the received signal strength.
There is another plausibility check method based on
the specific features of GPS: for GPS the approximate
trajectories of the satellites for the next few months
are published in advance over the internet (so called
almanac). So the GPS receiver should check if the
satellites he currently receives at his alleged location
conforms to those listed in the almanac.
A plausibility check can also be performed when
the locating system already calculated the position of
the mobile device based on the received raw mes-
sages: In many scenarios there will be several lo-
cation determinations at different time points so it
can be checked if the mobile device moves with a
reasonable velocity. The route of the mobile device
should also be plausible, e.g. there should be no
alleged movements through obstacles like walls or
buildings. If available the measurements of appro-
priate sensors designed for dead reckoning (e.g. ac-
celerometer, odometer, barometer, (gyro)compass or
speedometer) can be evaluated and checked for con-
cordance with the alleged movement pattern as re-
ported by the primary locating system.
Plausibility checks at the level of the raw mes-
sages are primarily applicable for scenarios where the
plausibility check should be performed on the mobile
device since the raw signals are usually not forwarded
to the backend. Checks of the calculated position can
be done on the device and on the backend.
It is also possible to have dedicated stations in a lo-
cating system for performing plausibility checks, so
called reference stations. Reference stations are an
optional extension of the locating system. For GPS
there are several stations all over the world (e.g. in
Colorado Springs (USA) or on Ascension Island).
These stations know their own position and calculate
their location according to the received locating sig-
nals. If a significant deviation is detected, this implies
a malfunction of the locating system or a spoofing at-
tack. In this case the operator of the locating system
and/or the mobile user have to be warned.
However, an external attacker could emit his
spoofing signals in a way that they only affect the mo-
bile device but not the reference station. Therefore in
literature the suggestion of hidden reference stations
can be found (Capkun et al., 2006), i.e. the locations
of the reference stations are kept secret. Since the se-
cret concerning the location of the reference stations
might be revealed sooner or later there is further the
idea to have mobile reference stations, e.g. motor ve-
hicles equipped with the necessary technical equip-
From the domain of location-aware digital rights
management (DRM) comes the requirement that an
end user device should only be able to playback mul-
ICE-B 2009 - International Conference on E-business
On Device
On Backend
Reference Stations
on Device
on Backend
Based on
Dedicated Non-dedicated
Figure 1: Anti-Spoofing-Tree”: Classification scheme for Anti-Spoofing-Methods.
timedia content (e.g. movies) at particular places
(Mundt, 2005). For set-top-boxes for television sets
there is even the requirementthat the decryption of the
broadcasted content should only be possible within
the subscriber’s private residence but not at public
places like restaurants (Gabber and Wool, 1998). In
this case the owner of the mobile device is also the
potential attacker, so this requires methods for the
prevention of internal attacks. This can only be im-
plemented by using a special hardware module that
is secured against physical manipulation attempts, so
called tamperproof hardware. The tamperproof hard-
ware module has to encapsulate the functionalities for
locating, for making the decision if at the current lo-
cation the content should be playbacked or not (the so
called reference monitor) and the function to decrypt
the content.
The system described by Mundt (2005) is based
on GPS and also includes a clock that is implemented
as tamperproof hardware. He assumes that the signals
emitted by the satellites have a time stamp and are
digitally signed so the locating module would be able
to detect rerouting attacks since rerouting of radio
waves leads to an unavoidable time lag. But to detect
the time lag caused by rerouting a highly precise clock
is necessary, because radio waves travel with light
speed so even rerouting over large distances causes
a time lag in the order of several milliseconds (e.g.
100 km in 0,3 milliseconds). Such highly precise
time measurements can only be provided by atomic
clocks, but they are too heavy and too expensive so
they cannot be integrated into mobile consumer de-
vices. Mundt’s system therefore uses a quartz clock
that is synchronized with an external time source ev-
ery couple of hours so it isn’t necessary to have a per-
manent internet connection. For this time synchro-
nization a special cryptographic protocol is used.
For the case that the location information is cal-
culated on the mobile device and then forwarded to
the stationary backend of a service provider it can be
reasonable to employ tamperproof hardware on the
mobile client, too: the tamperproof module in this
case not only performs the calculation of the location
but also signs the determined location with a private
key. With the corresponding public key the service
provider then is able to verify the authenticity of the
received location information. The private key has to
be stored inside the tamperproof module because if an
attacker would be able to obtain it he could generate
faked location messages with a valid digital signature.
As location key we regard information that is only
available at particular locations. The mobile user has
to forward this information to some backend system
as proof that he is actually at the location where he
claims to be. This principle is especially suited for
cases where the mobile device calculates its location
and provides this to the LBS provider but the provider
wants to be sure that this calculation wasn’t faked.
We distinguish at the first level whether a location
key is of either natural or artificial origin. “Natural”
means that the key isn’t emitted as result of human
activity. For artificial keys we can further distinguish
if it is a dedicated or a non-dedicate key: dedicated
keys are generated only for the purpose of the preven-
tion of spoofing while non-dedicated keys are emitted
for other purposes and their employment as mean to
prevent spoofing is a spin-off effect.
One anti-spoofing system that works with non-
dedicated location keys is the so called CyberLoca-
tor (Denning and MacDoran, 1996). The system was
designed to supplement GPS. Using this system the
mobile device is equipped with a GPS receiver and
thus can determine the position by itself, but it has
also to forward the raw signals (radio fingerprint) re-
ceived from the GPS satellites at the backend. It is
not possible to predict the pattern of the raw signals
PREVENTION OF LOCATION-SPOOFING - A Survey on Different Methods to Prevent the Manipulation of
receivedat a particular spot on earth’s surface at a par-
ticular time instant because the signals are affected by
many different influences, e.g. by the ionosphere or
the weather conditions. The trajectory of the satel-
lites is defined in advance and this information is pub-
lished in the GPS signals in form of the so called
ephemeris and almanac data; however, the actual tra-
jectory of the satellites isn’t exactly the one defined
in advance and subject to random influences. At the
backend the raw signal reported from the mobile de-
vice will be compared to those reported by trusted
reference stations in the proximity of the alleged lo-
cation of the mobile device. The authors of the Cy-
berLocator paper state that the distance between ref-
erence station and mobile device shouldn’t be larger
than 3.000 kilometres; unfortunately the authors of
CyberLocator don’t explain how they calculated this
maximum distance between mobile device and refer-
ence station. Also rerouting attacks (see section 2) are
considered, i.e. a colluding user that actually stays
at the alleged location forwards the received raw sig-
nals to the attacker. However, for the CyberLoca-
tor system it is demanded that the radio fingerprint
is forwarded within 5 milliseconds because it is as-
sumed that rerouting would cause additional latency
beyond that threshold. It seems quite demanding to
meet the maximum latency time of 5 ms even when
not performinga rerouting attack, since UMTS-HSPA
causes a latency of 150 ms, and even wire-bound in-
ternet connections over DSL have a latency of at least
20 ms.
Another system to prevent spoofing is called Lo-
cation Aware Access Control (LAAC) and was de-
vised by Cho and colleagues (2006) . Unlike the Cy-
berLocator this system is based on dedicated location
keys that are emitted by the base stations of a wire-
less local area network (WLAN). The location keys
are randomlychosen bit sequences which are renewed
periodically (e.g. every ve seconds). These location
keys are reported to the backend system. The mobile
device has to combine all the location keys it receives
from different base stations within a given timeframe
and combine them by using the XOR-function. Af-
terwards the result of this calculation is then the input
for a hash function whose output is the actual location
key that has to be transmitted to the backend. Since
the backend knows all the current location keys used
by the base stations it is able to calculate the hash val-
ues like the mobile device to verify the correctness of
the received location keys. A further feature of the
system is that the radiation angle of the base stations
can be controlled by using special antennas. If we
have two base stations with a radiation angle of 90° it
is possible to arrange these base stations in a way so
that the area where the waves of both stations can be
received has a rectangular shape. This is an interest-
ing feature to obtain regions that cover the premises
of a business like a restaurant, a hotel or a theme park
where the currently present customers should be able
to access a particular wireless service (e.g. free in-
ternet access, special information services). The au-
thors of LAACs don’t describe arrangements to pre-
vent rerouting attacks but this has to be interpreted by
considering the application scenario that is primarily
addressed, namely to restrict free wireless internet ac-
cess to users staying in a particular area. For rerouting
the colluding attackers usually need a fast data trans-
mission connection; however, if this connection is al-
ready available there is no need to perform a spoofing
attack to gain internet access.
Malaney (2007) proposes a system based for in-
door WLANs. The aim of this system is that mobile
devices should be able to prove that they are within
a building. It is assumed that only authorized people
can enter the building (e.g. because there is a gate-
keeper) and should have access to the WLAN. The
mobile devices have to calculate their position (e.g.
based on GPS or a special indoor locating-system)
and measure the signal strength of all the WLAN ac-
cess points they can receive at the current location.
These values have to be reported to a central server
that makes the decision if the mobile device should
get access or not: it is checked if the reported position
lies within the building and then if the reported signal
strength pattern matches the signal strength pattern
for that location. In this scenario the signal strength
pattern of the WLAN access points at a particular lo-
cation can be considered as non-dedicated location
key because unauthorized people cannot get into the
building to measure the signal strength. There are
simulation models to calculate an estimation of a sig-
nal strength pattern at a given location; however, to
work with these models it is necessary to know the
building plan, the locations of the access points and
the specific attenuation characteristics of the walls
and furniture in the building.
In literature so far no anti-spoofing-approach can
be found that is based on natural location keys. How-
ever, the cosmic background radiation could act as
one because it is receivable at each point of the earth’s
surface for a given time instant with a specific pat-
tern. A further dimension for the discrimination of
location-key methods would be to differentiate be-
tween singular and multiple keys. For singular keys
(e.g. CyberLocator) each location key stands for one
area while for multiple keys (e.g. Malaney’s system)
the location keys may overlap for some regions.
ICE-B 2009 - International Conference on E-business
The basic principle of request-response-protocols is
as follows: the mobile device (prover) receives a re-
quest message from a trusted base station (verifier);
the message contains a non-predictablerandom bit se-
quence (called nonce) and the prover has to send a
response message that is based on this bit sequence.
We distinguish two cases: In runtime-based proto-
cols the prover’s response message won’t arrive in
time at the verifier’s position if the prover is further
away from the prover than alleged because longer
distances lead to longer runtimes. If request and re-
sponse are sent over radio waves it isn’t possible to
sent a message over a larger distance in the same pe-
riod of time because radio waves travel at the speed of
light ( 3× 10
m/s), which constitutes the maximum
speed that is possible according to today’s knowledge.
However, measuring the runtime of radio waves re-
quires extreme precise clocks because the travel time
of radio waves for typical application scenarios of
locating technologies are very short. In proximity-
based protocols a prover too far away won’t be able
to receive the verifier’s message because of signal at-
A nice example for a anti-spoofing-system em-
ploying a runtime-based request-response-protocol is
the one described by Sastry & colleagues (2003): The
prover can start the protocol by sending a radio mes-
sage with his alleged location to the verifier. If the
verifier receives this messages and deems itself as re-
sponsible for the alleged location he generates a nonce
and sends a request message containing this nonce
over radio back to the prover; the verifier also starts
a precise clock at the time instants he begins to send
the request message. When the prover receives the
request message he sends back the nonce as response
as soon as possible; however, for this way back ul-
trasonic waves are used instead of radio waves. The
verifier will stop the clock when he received the com-
plete response. He verifies that the response indeed
contains the nonce and tests if the calculated runtime
is small enough for the distance between his location
and the alleged location of the prover. The special fea-
ture of this protocol is that it uses two different kinds
of waves for the transmission of the messages, namely
radio waves and ultrasonic waves. This stems from
the requirement that the protocol should be robust
with regard to particular measurement errors of the
messages’ roundtrip time, especially the processing
time required by the prover to produce the response
message. If all the messages would be transmitted
over radio these measurement errors would lead to
distance inaccuracies that would render the protocol
useless for the purpose of prevention location spoof-
ing. That’s why ultrasonic waves are used for the
transmission of the response message. Ultrasonic
waves travel at a velocity that is six orders of magni-
tude smaller than lightspeed (331m/s), so a measure-
ment error of 0.1 seconds results only in a distance er-
ror of 33 meters, while for radio waves the distance
error would be 30.000 km for the same time span.
A distance error of 33 meters should be acceptable
even for most indoor locating scenarios. Since the
speed of ultrasonic waves is relatively low the return
way of the nonce is prone to a rerouting attack with
an out-of-band-transmission over radio waves; this is
why the authors of this protocol opted to use the ultra-
sonic waves for the response message and not for the
request message because in their opinion it requires
much more effort to perform a rerouting attack for the
response message than for the request message.
Waters & Felten (2003) describe another request-
response-protocol for the prevention of location
spoofing that is based on runtime measurements.
However, in their protocol radio waves are used for
all message exchanges. To discern protocols that use
one kind of wireless waves from protocols that use
different kind of waves we further introduce two fur-
ther sub-classes of request-response protocols that are
mutual exclusive, namely symmetric respective asym-
metric protocols.
An example of a request-response-protocol based
on proximity (i.e. without measuring the runtime of
signals) can be found in Vora et al. (2006). In their
system several verifiers are installed within a partic-
ular target region. The system’s purpose is to detect
mobile clients who falsely claim they are within that
target region. To make the system more secure there
are so called rejection verifiers outside the target re-
gion; if one of them receives a signal from a prover
the verifier is considered as spoofer.
There are special approaches concerning the radio
technology employed for locating systems that can
help to prevent spoofing attacks:
Spreading of Frequence Spectrum. The basic prin-
ciple of spread spectrum techniques is to transform
a narrow-banded signal into a signal with a broader
bandwidth. It requires more effort to jam or forward a
broadbanded radio signal than a narrow-banded one.
Examples for spread spectrum methods are Frequence
Hopping Spread Spectrum (FHSS) and Direct Se-
PREVENTION OF LOCATION-SPOOFING - A Survey on Different Methods to Prevent the Manipulation of
quence Spread Spectrum (DSSS). A simple form to
spread signals is to assign a different frequency to
each sender like for GLONASS: in this system each
satellite broadcasts on his own frequency.
Manchester-Coding. For an attacker it is usually
harder to eliminate a set bit in a radio message than
to set an unset bit. Therefore the idea of the so called
Manchester-Coding is to replace each bit of the origi-
nal message by two bits. This coding is done in a way
that also unset bits are mapped to codewords with set
bits, so an attacker who cannot “delete” bits isn’t able
to alter the navigation message in a consistent way.
Another approach in this class is to use multiple
antennas at a station or device to detect if signals are
arriving from the wrong direction.
All of the nominal 24 satellites of the GPS-system
emit signals carrying navigation messages on the
same two radio frequencies called L1 and L2. The
navigation messages contain amongst other things the
identification number of the satellite that produced the
message and parameters to describe the trajectories of
the individual satellites. The code division multiple
access (CDMA) method is used to encode the mes-
sages so a receiver is able to obtain the message of a
single receiver. For CDMA the messages are encoded
using a spread code sequence. The GPS has two types
of such code sequences (Küpper, 2007): The Coarse
Acquisition Code (C/A-code) is for the Standard Po-
sitioning Service (SPS), which can be used by civilian
users; it is only broadcasted on the L1 frequency. The
Precise Code (P-code) is for the Precise Positioning
Service (PPS) that should only be accessible by mili-
tary users. It is possible to use a secret Y-key to obtain
the P(Y)-code. The P-code is broadcasted on both fre-
quencies and can be considered as symmetric key be-
cause for encryption and decryption the same key is
used. If someone wants to perform a GPS-spoofing-
attack including the PPS he has to know the secret Y-
key. This means that the secret key has to be stored in
all military GPS receivers that are intended to use the
PPS. If only one of this receivers is compromised (e.g.
gets lost or is stolen) the whole systems gets prone to
spoofing attacks. To prevent this the Y-key is replaced
every 24 hours (rekeying). Nowadays techniques are
available to benefit from the higher locating accuracy
provided by the PPS messages even if the secret Y is
not known, e.g. so called kinematic or codeless re-
In the article we gave a survey on different meth-
ods to prevent the manipulation of location-methods
which was based on a classification schema. Since
there is a large variety of different attack methods
as well as anti-spoofing-methods it is not possible to
recommend a single approach for the prevention of
location-spoofing; rather the decision for one or more
spoofing-prevention-techniques has to be made after
an analysis of the respective application scenario and
its specific security threats and requirements.
Capkun, S., Cagalj, M., and Srivastava, M. (2006). Secure
localization with hidden and mobile base stations. In
Proceedings of IEEE INFOCOM 2006, pages 1–10.
Cho, Y., Bao, L., and Goodrich, M. T. (2006). LAAC: A
Location-Aware Access Control Protocol. In Mobile
and Ubiquitous Systems, pages 1–7.
Denning, D. E. and MacDoran, P. F. (1996). Location-
Based Authentication. Computer Fraud & Security,
Gabber, E. and Wool, A. (1998). How to prove where you
are. In ACM Conference on Computer and Communi-
cations Security, pages 142–149.
Küpper, A. (2007). Location-based Services. John Wiley &
Sons, Chichester, U.K.
Malaney, R. A. (2007). Securing Wi-Fi Networks with Po-
sition Verification (Extended Version). International
Journal of Security Networks, 2(1-2):27–36.
Mundt, T. (2005). Location Dependent Digital Rights Man-
agement. In ISCC 2005, pages 617–622.
Sastry, N., Shankar, U., and Wagner, D. (2003). Secure Ver-
ification of Location Claims. In 2nd ACM Workshop
on Wireless Security, pages 1–10, San Diego, USA.
Vora, A. and Nesterenko, M. (2006). Secure Location Veri-
fication using Radio Broadcast. IEEE Transactions on
Dependable and Secure Computing, 3(4):377–385.
Warner, J. S. and Johnston, R. G. (2003). GPS Spoofing
Countermeasures. Technical Report LAUR-03-6163,
Los Alamos National Laboratory (USA).
Waters, B. R. and Felten, E. W. (2003). Secure, pri-
vate proofs of location. Technical Report TR-667-03,
Princeton University.
ICE-B 2009 - International Conference on E-business