Global Positioning System Reference
In-Depth Information
sor for making pseudorange measurements), but generally can get by with less
embedded RAM and read-only memory (ROM) as the firmware required to com-
pute the position solution exists elsewhere in the network. The network transmits a
very short assistance message to the MS, consisting of time, visible satellite list, pre-
dicted satellite Doppler, and code phases, among other things. This visible satellite
list helps the embedded GPS sensor reduce its acquisition time considerably, since
the receiver does not need to use what is, in essence, a trial and error approach to
determining the visible satellites. In addition, the other parameters will enable signif-
icant reduction of the search regions used for the visible satellites. This point will be
explained in detail in Section 9.4.3. The MS-assisted handset simply acquires the sig-
nals and returns the measured pseudorange data for all detected satellites to the net-
work. There, a position determining entity (PDE) such as a server does the work of
computing the position solution. MS-assisted solutions are inherently differential in
nature, since the PDE can have access to DGPS corrections, either from a local
receiver or via the Internet.
In the MS-based method, the position solution is computed in the handset. The
MS-based handset solution maintains a fully functional GPS receiver in the handset.
This requires the same functionality as described in MS-assisted handset with the
additional means for computing the position of the mobile station. Computing posi-
tion locally to the handset generally adds to the handset's total memory (RAM, ROM)
requirements in addition to increasing the loading on the host processor [e.g., as might
be measured in millions of instructions per second (MIPS)]. The MS-based handset
may work in an autonomous mode as well, providing position solutions to the user or
embedded applications without the cellular network provided aiding data. MS-based
methods are better for applications requiring the position solution in the handset, an
example of which is personal navigation that can provide the user with turn-by-turn
real-time directions. Turn-by-turn navigation is difficult in MS-assisted technology
due to required network interaction between each position update. In the MS-based
case, significantly more data needs to be delivered to the handset in the form of the
precise satellite orbital elements (ephemeris, or its compressed equivalent), but once it
is transferred to the handset, little or no additional data is needed to perform periodic
fixes as long as the ephemeris remains vaild (several hours). MS-based solutions can
be differentially corrected if DGPS corrections are sent to the handset, but DGPS is not
implemented in most current MS-based networks.
These two methods, and the supporting handset input and output data messages
corresponding to each, are depicted in Figure 9.35.
In both cases, some portion of the solution is accomplished by the in-network
element. The network server needs, at a minimum, access to GPS satellite informa-
tion in the form of the real-time ephemeris data so that it can deliver it to the
MS-based handset when needed; or it needs to be able to compute predicted Doppler
and code phase estimates for the MS-assisted handset. The simplest implementation
installs a fixed-site GPS receiver with clear-view of the sky that can operate continu-
ously somewhere in the network. This reference network is also connected with the
cellular infrastructure and continuously monitors the real-time constellation status
and provides precise data such as satellite visibility, Doppler, and even the
pseudorandom noise code phase for each satellite at a particular epoch time. At the
request of the mobile phone or location-based application, the assist data derived
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