Energy Efficient GPS Sensing with Cloud Offloading
Jie Liu, Bodhi Priyantha, Ted Hart
Microsoft Research
Redmond, WA 98039, USA
{liuj,bodhip,tedhar}@microsoft.com
Heitor S. Ramos, Antonio A.F. Loureiro
Federal University of Minas Gerais
Belo Horizonte, MG,
{hramos,loureiro}@dcc.ufmg.br
Qiang Wang
Harbin Institute of Technology
Harbin, China
Abstract
Location is a fundamental service for mobile computing.
Typical GPS receivers, although widely available, consume
too much energy to be useful for many applications. Ob-
serving that in many sensing scenarios, the location infor-
mation can be post-processed when the data is uploaded to
a server, we design a Cloud-Offloaded GPS (CO-GPS) solu-
tion that allows a sensing device to aggressively duty-cycle
its GPS receiver and log just enough raw GPS signal for post-
processing. Leveraging publicly available information such
as GNSS satellite ephemeris and an Earth elevation database,
a cloud service can derive good quality GPS locations from a
few milliseconds of raw data. Using our design of a portable
sensing device platform called CLEO, we evaluate the accu-
racy and efficiency of the solution. Compared to more than
30 seconds of heavy signal processing on standalone GPS
receivers, we can achieve three orders of magnitude lower
energy consumption per location tagging.
Categories and Subject Descriptors
C.3 [SPECIAL-PURPOSE AND APPLICATION-
BASED SYSTEMS]: [Real-time and embedded systems]
General Terms
Design
Keywords
location, assisted GPS, cloud-offloading, coarse-time
navigation
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1 Introduction
Location is a fundamental service in mobile sensing. In
outdoor applications such as wildlife tracking [28, 26], par-
ticipatory environmental sensing [20], and personal health
and wellness applications, GPS is the most common modal-
ity for tagging data samples with their locations. GPS receiv-
ing, although becoming increasingly ubiquitous and lower in
cost, is processing-intensive and energy-consuming.
Take ZebraNet sensor nodes [28] as an example. On av-
erage, one GPS location fix requires turning on the GPS chip
for 25 seconds at 462mW power consumption, which domi-
nates its energy budget. As a result, the unit is equipped with
a 540-gram (1.2 pound) solar cell array and a 287-gram (0.6
pound) 2A-h lithium-ion battery in order to support one GPS
position reading every 3 minutes. Power generation and stor-
age accounts for over 70% of the sensor unit’s total weight
of 1151 grams (2.5 pounds).
Recent mobile sensing applications, especially those
leveraging participatory sensing paradigms, typically use
smart phones as sensors. While smart phones have built-
in GPS and cellular-based communication capabilities, their
battery life is rarely longer than a few days. A typical smart
phone will completely drain its battery in about 6 hours if the
GPS is running continuously [14, 21]. This prevents them
from being used in unattended deployments for long periods
of time.
As we will elaborate in section 2, there are two reasons
behind the high energy consumption of GPS receivers: 1) the
time and satellite trajectory information (called Ephemeris)
are sent from the satellites at a data rate as low as 50bps.
A standalone GPS receiver has to be turned on for up to 30
seconds to receive the full data packets from the satellites for
computing its location. 2) The amount of signal processing
required to acquire and track satellites is substantial due to
weak signal strengths and Doppler frequency shifts. As a
result, a GPS chip cannot easily be duty-cycled for energy
saving. In addition, it requires a powerful CPU for post-
processing and least-square calculation.
In this paper, we address the problem of energy consump-