Global Positioning System Reference
In-Depth Information
Table 9.16
Position Response Data Element
Information Element/
Group Name
Type and Reference
Semantics Description
Latitude sign
Enumerated (north, south)
The IE value (
N
) is derived by this formula:
N
Degrees of latitude
Integer (0...2
23
−
1)
1
X
is the latitude in degrees (0º…90º)
≤
2
23
X
/90
<
N
+
The IE value (
N
) is derived by this formula:
N
Degrees of longitude
Integer (-2
23
…2
23
−
1)
1
X
is the longitude in degrees (
≤
2
24
X
/360
<
N
+
−
180º…
+
180º)
Altitude direction
Enumerated (height, depth)
The IE value (
N
) is derived by this formula:
N
Altitude
Integer (0…2
15
−
1)
1
a
is the altitude in meters
≤
a
<
N
+
Uncertainty semimajor
Integer (0…127)
The uncertainty
r
is derived from the uncertainty
code
k
by
r
=
10
×
(1.1
k
−
1)
Uncertainty semiminor
Integer (0…127)
The uncertainty
r
is derived from the uncertainty
code
k
by
r
=
10
×
(1.1
k
−
1)
The IE value (
N
) is derived by this formula:
2
N
Orientation of major axis Integer (0…89)
1)
a
being the orientation in degree (0º…179º)
≤
a
<
2(
N
+
The uncertainty in altitude,
h
, expressed in
meters, is mapped from the IE value (
K
), with
the following formula:
h
=
C
((1
Uncertainty altitude
Integer (0…127)
x
)
K
+
−
1) with
C
=
45 and
x
=
0.025.
Confidence
Integer (0…100)
In percentage
Source:
[80].
One example of how the cellular over-the-air protocol can be used to solve a
particular handset application problem is demonstrated by the “Real-Time-Integ-
rity” acquisition assist data element. In Section 7.5, the importance of ensuring
integrity for GPS is discussed, since GPS satellite clocks can fail, resulting in signifi-
cant error in unprotected receiver solutions. A GPS receiver embedded in a cellular
handset cannot generally be expected to perform its own RAIM (e.g., see Section
7.5.3) function, since signal reception conditions may be poor and the luxury of
redundant measurements may not exist. For this reason, the cellular standards have
allowed for integrity information to be communicated to the handset, since a net-
work-based GPS receiver will certainly be able to perform the RAIM function and
identify which satellites are failed or failing. It should be noted that the historical
failure rate of GPS satellites or ephemeris uploads to those satellites has been very
low—approximately one event every 18 to 24 months. However, when GPS is used
for high-frequency E-911 emergency location function, it is certain that someone
will need the system at precisely the time a satellite fails. Consequently, a real-time
integrity function was added to the Radio Resources Location Services Protocol
(RRLP) to prevent such failures.
An MS-based handset can be particularly vulnerable to GPS satellite failures.
The handset can request real-time ephemeris data from the cellular network and
then subsequently use the data for several hours. One mode that may be used by the
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