Using Assisted-GNSS to locate
handsets in wireless networks DR NEIL HARPER
This paper discusses the current state of Assisted-GNSS for locating mobile
handsets and some results of a Hybrid A-GNSS investigation.
There are many different types
of technologies employed in
calculating the location of
handsets in wireless networks
with various levels of success and
accuracy. Assisted-GPS (A-GPS)
is a positioning technology that is
presently used for locating handsets
in wireless networks and is gaining
traction in the market. An A-GPS
server provides assistance data to the
handset in order for it to have a low
Time to First Fix (TTFF), weak signal
acquisition and optimize handset
battery use. A-GPS is used as a location
technology in isolation or hybridized
with other positioning technologies
that provide range-like measurements.
In this paper positioning and
standards for A-GPS in wireless
networks is discussed along with
some results of simulations of
Hybrid A-GPS positioning.
An A-GPS server provides data to a
wireless handset that is specific to the
approximate location of a handset.
The assistance data helps the handset
lock onto satellites quickly and also
potentially allows the handset to lock
onto weak signals. The handset then
performs the position calculation or optionally returns the measured
code phases to the server to do the
calculation. The A-GPS server can
make use of additional information
such as round-trip timing measurements
from the cellular base station to
the handset in order to calculate a
location where it may otherwise not
be possible, for example when there
are not enough GPS satellites visible.
Assisted Global Navigation Satellite
System (A-GNSS) extends the concept
to other satellite navigation systems
besides GPS. There could be 80
GNSS satellites orbiting the planet
within 10 years, all transmitting a
variety of signals. This includes GPS,
GLONASS, Galileo and other satellites.
This will give a receiver access to
many more satellites which will
improve both accuracy and yield. More
satellites means that position accuracy
is less susceptible to satellite geometry
and also provides greater redundancy
when doing the position calculation.
A simplified A-GNSS architecture
is shown in Figure 1. A Wide Area
Reference Network (WARN) is a
network of GNSS receivers that
are placed geographically over the
coverage area of the wireless network.
The WARN collects the broadcast
navigation message from the GNSS
satellites and provides it to the AGNSS
Server for caching. An AGNSS
User makes an emergency call
or a service is invoked that requires
location and a message is sent to the
A-GNSS Server. The A-GNSS server
calculates the GNSS assistance data
required using the location of the
radio access tower as the approximate
location and provides it to the handset.
Applications
A-GNSS is useful for determining
the location of cellular phones in
an emergency situation and also for
location based services. Deployment
of A-GPS in the United States is the
result of the Federal Communications
Commissions' (FCC) Enhanced 9-
1-1 mandate. That mandate requires
that for network-based solutions:
100 meters accuracy for 67 percent
of calls, 300 meters accuracy for 95
percent of calls; for handset-based
solutions: 50 meters for 67 percent
of calls, 150 meters for 95 percent
of calls [1]. When an emergency call
is initiated, an emergency services
coordination centre - Public Safety
Answering Point (PSAP) will make
use of the location that is calculated
in the MLC. In Europe and Asia
deployment is being driven by
Location Based Services (LBS), though
requirements for emergency service
cellular location have been or are
being established in these regions.
Standards
The different components of an AGPS
server are defined in J-STD-
036 [2]. As a real-world example,
the Andrew Corporation Geometrix
Mobile Location Center (MLC)
product supports A-GPS functionality.
The MLC is deployed as part of a
wireless network and its purpose
is to determine the location of
handsets within the network.
The MLC runs in GSM/GPRS and
UMTS networks and can act as a
Serving Mobile Location Center
(SMLC), Gateway Mobile Location Center (GMLC), Standalone AGPS
SMLC (SAS) or SUPL
Location Platform (SLP) and
various combinations of these.
The MLC supports all handsetbased
and network-based wireless
location methods, including AGPS
in both handset-based and
handset-assisted versions.
There are several different standards
for the A-GPS messaging with the
handsets. For GSM networks RRLP
[3], UMTS networks RRC [4] and
CDMA networks PDDM [5] and [6].
These protocols are different ways of
encoding the same basic information
but are specific to the radio technology.
The Secure User Plane Location
(SUPL) architecture [7] is defined by
the Open Mobile Alliance (OMA)
and allows bypass of many of the
traditional telecommunication
elements to be bypassed by relying
on SUPL functionality within the
target device. This makes network
interoperability simpler and is a
platform that is likely to get wide
acceptance as the requirement to also
support legacy, non-location capable,
devices diminishes. The SUPL Enabled
Terminal (SET) makes a secure TCP/
IP connection directly with the SUPL
Location Platform (SLP). The SET
and the SLP exchange messages using
the UserPlane Location Protocol
(ULP) [8] which is the transport for
the underlying RRLP messaging.
ULP optionally supports RRC [4]
and the CDMA PDDM standards
[5] and [6]. Operators are starting
to deploy SLPs and quite a few
handset manufacturers offer SETs.
Present standards cater for GPS L1 C/A
code assistance data. There are two
Global Navigation Satellite Systems
(GNSS) presently in service; GPS and
GLONASS. The European Galileo
project is also underway and will meet
Full Operational Capacity (FOC) within
several years. The SUPL specification
facilitates Assisted-GNSS (A-GNSS)
operation but the underlying RRLP,
RRC and PDDM protocols do not
currently support it. This is presently
being worked by the standards bodies. Once the A-GNSS mechanism has been
defined for these protocols, an A-GNSS
server will be able to provide assistance
data to different types of handsets (for
example a Galileo handset Assisted-
Galileo) and handsets that have
receivers capable of detecting satellites
from multiple GNSSs (for example a
combined GPS and Galileo receiver).
Standards are also being developed
for providing location in Voice over
Internet Protocol (VoIP) networks. VoIP
allows users to make telephone calls
over the IP network instead of switched
circuit network. Some VoIP providers
presently do not even provide access
to emergency services such as 9-1-1
in the USA. The National Emergency
Number Association (NENA) and
Internet Engineering Task Force (IETF)
are developing the architecture and
standards for delivery of the location
within VoIP. It is likely that the handset
will talk to the Location Information
Server (LIS) via a protocol that will
encapsulate RRLP or some derivative
messaging at its deepest layer.
Handset-based A-GPS and
Handset-assisted A-GPS
There are two primary modes of
operation for A-GPS. In handsetbased
A-GPS mode, the handset
requests assistance data from the AGPS
server which is used to lock onto
the satellites in view and calculate the
position of the handset. In handsetassisted
A-GPS, the handset requests
assistance data from the A-GPS
server which it uses to lock onto the
GPS satellites. It then sends the GPS
measurements to the A-GPS server
for it to do the position calculation.
An example of messaging between
an A-GNSS server and handset is
shown in Figure 2. The messaging
shown is one way that RRLP is used
to achieve GNSS positioning of the
handset. A location request could
be network-initiated in the case of a
Location-Based Application (LBA)
providing a value-added service.
It could also be handset-initiated
in the case of the user making an emergency call or an application
running from the handset itself.
When the location request is received at
the A-GNSS Server, it sends a Measure
Position Request message to the
handset. If the handset has sufficient
cached GNSS assistance data then, in
the case of handset-based GPS, the
handset will lock onto the satellites,
perform the position calculation and
return the location in the Measure
Position Response message. In the
case of handset-assisted GNSS the
handset will lock onto the satellites and
return the satellite measurements to the
server to do the position calculation.
Generally however, the A-GNSS
Handset will not have sufficient
assistance data and will send a Measure
Position Response to the A-GNSS
Server with the assistance data types
that it requires. The A-GNSS Server
will then send another Measure
Position Request to the handset with
the requested assistance data types.
In the case of handset-based A-GNSS,
the primary assistance data type is
the navigation model (which contains
the ephemeris). The handset uses
the information in the ephemeris to
calculate where the satellites are in
order to refine its search and also as
input to its position calculation.
In the case of handset-assisted AGNSS,
the primary data type is the
much more compact Acquisition
Assistance data type. The Acquisition
Assistance data tells the handset
which satellites to search for and
provides a search window in the
time and frequency domain for each
satellite. The handset treats the search windows as relative search windows
once it locks onto the first satellite.
In this mode, the handset has no
need to calculate where the satellites
are since it is not doing a position
calculation, instead it returns the GPS
measurements to the A-GNSS server
in the Measure Position Response.
The measurement consists of the
approximate time, the code-phase chip
measurements, Doppler, RMS error
of the measurements, and a multipath
indicator. The server can then do a
self-contained position calculation or
make use of additional measurements
and perform a hybrid location.
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