Global Positioning System Reference
In-Depth Information
receiver clock time,
t
rcv
, to obtain GPS system time,
t
E
. (Note that in the development
of the user position solution in Section 2.4.1, GPS system time was denoted as
T
u
,
which represented the instant in system time when the satellite signal reached the
user receiver. However, we need to represent GPS system time at
any particular time
and will use the parameter
t
E
to do so.)
Expressing receiver clock time at any particular time:
t
=+
t
t
rcv
E
u
So that:
t
=−
t
t
E
rcv
u
From IS-GPS-200 [4], UTC (USNO),
t
UTC
, is computed as follows:
t
=−∆
t
t
UTC
E
UTC
where
t
LS
and a fractional esti-
mate of the difference between GPS system time and UTC (USNO) modulo 1 second
denoted herein as
∆
t
UTC
represents the number of integer leap seconds
∆
t
A
. [The control segment provides polynomial coefficients (
A
0
,
A
1
, and
A
2
) in the navigation data message that are used to compute the fractional
difference between GPS system time and UTC (USNO) [4].]
Therefore, UTC (USNO),
t
UTC
, can be computed by the receiver as follows:
δ
t
=−
=−−
=−−
t
∆
t
UTC
E
UTC
t
t
∆
∆
t
rcv
u
UTC
t
t
t
−
δ
t
rcv
u
LS
A
2.6.3.2 Mobile Users
Mobile users compute UTC (USNO) using the exact methodology described earlier
except that they need to solve the system of (2.20)-(2.23) to determine the receiver
clock offset,
t
u
.
References
[1]
NAVSTAR
GPS
Joint
Program
Office
(JPO),
GPS
NAVSTAR
User's
Overview
,
YEE-82-009D, GPS JPO, El Segundo, CA, March 1991.
[2]
Langley, R., “The Mathematics of GPS,”
GPS World Magazine
, Advanstar Communica-
tions, July-August 1991, pp. 45-50.
[3]
Long, A. C., et al., (eds.),
Goddard Trajectory Determination System (GTDS) Mathemati-
cal Theory
, Revision 1, FDD/552-89/001, Greenbelt, MD: Goddard Space Flight Center,
July 1989.
[4]
GPS Navstar Joint Program Office,
Navstar GPS Space Segment/Navigation User Inter-
faces, IS-GPS-200
, Revision D, GPS Navstar Joint Program Office, El Segundo, CA,
December 7, 2004.
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