TITLE: Wireless Assisted GPS (WAG) for Wireless Internet Location Services
SOURCE: SnapTrack, Inc.
4040
Moorpark Ave., Suite 250
San
Jose, CA 95117
(408)
556-0400
(408)
556-0404 (fax)
AUTHORS:
|
Kirk Burroughs |
Justin McGloin |
|
Len Sheynblat |
Kamil A. Grajski, Ph.D. |
DATE: January 3, 2000
ABSTRACT: SnapTrack
Wireless Assisted GPS technology provides a seamlessly integrated, extensible,
scalable core technology for fast response, high accuracy position determination
for Wireless Internet Location Services.
It is recommended that the WAP Forum and the W3C leverage recently
completed (IS-801) and in-progress (GSM LCS) wireless position determination
standards, and support a wide variety of position determination technologies.
DISTRIBUTION: W3C Members
COPYRIGHT
STATEMENT:
Ó Copyright
1997-2000 SnapTrackÔ
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1 Background
SnapTrack, Inc. develops a wireless assisted GPS technology for location determination. The technology is based on a client-server architecture and is air interface independent. The technology offers highly accurate location determination of wireless calls, even inside buildings where conventional GPS cannot operate. This paper details the relative advantages of wireless assisted GPS over conventional GPS technology. In addition, this paper talks briefly about the types of location services being considered and suggests that the support for a wide range of location determination technologies is necessary. The intent is to ensure that the standardization process that is beginning in the W3C and the WAP Forum allow for this wide range of location determination technologies and that they leverage the existing work ongoing in the various telecommunications standards bodies.
2 Wireless-Assisted GPS Technology
When wireless assisted GPS (WAG) technology is activated, the wireless network or user application sends an estimate of the location of the mobile device, i.e., PDA or cellular phone, to a wireless infrastructure or Internet-based Location Server (LS). The LS responds with a GPS Acquisition Assistance message to the handset listing which GPS satellites are in its area, and provides other acquisition assistance data. The handset takes a "snapshot" of the GPS signal, calculates distance to all satellites in view and sends this information back to the LS. LS software performs complex error correction and calculates the caller's precise latitude, longitude and altitude. For Internet-based Location applications, the server sends the coordinates to a third-party service provider, individual website, or back to the handset. The process takes just a few seconds, requires minimal point-to-point network bandwidth, and is available to support commercial applications today.
The following items detail the performance improvements that wireless assisted GPS technology provides over conventional, or standalone, GPS receiver technology.
1.
Conventional GPS Limitations Drive Need
for Wireless Assisted GPS
A standalone GPS receiver requires strong signals (>= -135 dB) over time periods of 18-30 seconds, or more, to perform its functions without any external assistance. The satellite navigation message is extracted from the GPS signal -- after it has been acquired and tracked. Strong, continuous signal reception is required for the duration of the message being received from all satellites in the navigation solution set. Conditions like these are rarely encountered by a mobile consumer device in an indoor or urban canyon setting. SnapTrack GPS technology is proven to meet or significantly exceed FCC Phase II E-911 requirements in these conditions.
2.
WAG Delivers High Sensitivity Performance
Field studies indicate that receiver sensitivity on the order of –150dBm is required to achieve acceptable position accuracy. This sensitivity is achieved using calibrated integration periods. In any GPS solution, a 1-second snapshot integration period is required to achieve –150dBm signal sensitivity. Conventional receivers use a limited number of hardware correlators. These correlators provide samples of the received C/A code at the times around the correlator peak time. With the SnapTrack snapshot approach, the entire C/A-code spectrum is simultaneously sampled and analyzed for the presence of direct and in-direct signals. This is the equivalent processing power of 8000 hardware correlators. With the additional 15dB sensitivity over the conventional GPS, wireless assisted techniques can detect GPS satellite signals where conventional receivers see only noise.
3.
WAG Delivers Rapid Acquisition
While reacquisition is important for vehicle tracking applications, acquisition time is important for consumer applications. “Cold start” acquisition implies no prior information is available in the handset. This is the likely starting point for a portable consumer device that must meet strict power consumption requirements. The distributed architecture of SnapTrack technology delivers time to first locate on the order of several seconds.
4.
WAG Delivers High Accuracy
The following table lists representative results of numerous, audited field trials of SnapTrack GPS technology.
|
Environment |
Conditions |
Yield |
68.3% Hzntl. Error |
|
Outdoors |
Open site |
100% |
4
m |
|
Urban Street,
Shinbashi Tokyo |
2-10 story
buildings, narrow streets and alleys |
100% |
15
m |
|
Inside Sport
Utility Vehicle |
Parking lot surrounded by red wood trees and two-story buildings. Antenna on inside shoulder |
100% |
17
m |
|
Two Story
House |
Center of
basement |
100% |
20
m |
|
Two Story
Office Building |
1st
floor; interior room |
94% |
22
m |
|
Urban Canyon,
Denver CO |
20-30 story buildings, wide streets |
98% |
29
m |
|
50-story
building |
Glass/Steel building, 21st floor, 14 ft from outside wall |
89% |
84
m |
5.
WAG Enables Multipath Mitigation and
Processing of Reflected Signal
Increased sensitivity is a necessity in delivering high accuracy position information for mobile consumer devices. It is the ability to acquire and process attenuated direct and indirect multipath components that support high fix yields in severe environments. SnapTrack multipath mitigation algorithms dramatically reduce errors associated with multipath in urban canyons (often by a factor of 3 or more) as well as greatly increasing fix yield (often from 50% to nearly 100%).
6.
WAG Impact on Network Bandwidth is Very
Low
High sensitivity, fast time-to-first-fix results are obtained using shared resources on the mobile handset and an infrequent downlink Satellite Assistance data message of approximately 60-80 bytes. The data message is infrequent in that it is required only once per cold-start. That data message has time value up to several tens of minutes. As a result, wireless assisted technology has minimal impact on wireless network data bandwidth. The downlink and uplink messages are consistent in size and frequency with other existing wireless services.
7.
SnapTrack WAG Minimizes Incremental
Mobile Device Costs
Several important components
of the SnapTrack architecture, such as a stable frequency reference source and
DSP processor, may be shared with similar components found in a wireless
communication system, greatly lowering the price of combining SnapTrack technology with a mobile device. The ability
of SnapTrack technology to share circuitry also minimizes the size increase
over the communication device alone.
8.
SnapTrack WAG is Air Interface
Independent
Unlike other proposed
technologies for wireless location services, SnapTrack WAG is air interface
independent. For example, whether the
underlying network is analog or digital (synchronized and unsynchronized),
SnapTrack WAG works. SnapTrack WAG has
been tested in all of the major global air interfaces, including AMPS, CDMA,
GSM, PDC, PHS, etc. In certain air
interface standards, data helpful to WAG is available and can be used to
further enhance performance, such as the availability of accurate GPS time in
CDMA networks.
3 Location Services
Although the initial service considered for location determination technologies was E911, location detection services other than E911 are emerging as potential value-added services and network management aids. Some examples of value added services are directions, concierge, traffic, weather, personnel tracking, location specific advertising, etc. Many of these services, minus location sensitivity, are available today via Wireless Internet Services based on the WAP architecture. One of the most commonly discussed “Killer Applications” for Wireless Internet Services is Location Services. Specifically, the ability to provide better and more personalized content based on the content provider knowing the exact location of the end user.
Organizations like the WAP Forum and the W3C have just begun the standardization process for supporting location determination in any such Wireless Internet Services. However, much work has been performed in the telecommunications standards bodies with respect to E911, or more generically, the basic ability to perform location determination independent of the actual service.
Some key aspects of the work in the telecommunications standards bodies are as follows:
· Functional elements distinguishing application end points and position determining entities
· Request / Response message paradigm
· Mandatory and optional messages and fields that represent, at least for GPS, the expert technical opinion from a global community
· Support for a wide range of location determination technologies
4 Recommendations
It is recommended that the W3C and the WAP Forum allow for a wide range of location determination technologies and that they leverage the existing work ongoing in the various telecommunications standards bodies. Specifically, recently completed (IS-801) and in-progress (GSM LCS) wireless position determination standards. Additional work of interest is TR45.2 AHES PN-3890.
5 References
[1] TIA/EIA/IS-801. Position Determination Service Standard for Dual‑Mode Spread Spectrum Systems.
Publication Version. October 15, 1999.
[2] T1P1.5/99-556r5. Digital Cellular Telecommunications System
(Phase 2+): Location Services (LCS): Mobile Station (MS) – Serving Mobile
Location Center (SMLC): Radio Resource LCS Protocol (RRLP) (GSM 04.31 Version 1.0.0 Release 1998)
[3] TR45.2 AHES PN-3890, Enhanced Wireless 9-1-1 Phase 2, Rev 10