Geoscience Reference
In-Depth Information
the satellites transmit additional signals at frequencies referred to as L3 and
L4. These signals are classified and for military purposes only (Misra and
Enge 2001: Sect. 2.3).
According to the modernization initiative released in 1999, the Inter-
agency GPS Executive Board concept will be realized with the following
specifications. Future GPS signals will be transmitted by three carriers where
L1 and L2 remain unchanged, and the new carrier L5 is specified as
L5 = 115
f
0
= 1176
.
45 MHz
,
where
f
0
=10
.
23 MHz denotes the basic GPS frequency. The carrier L5,
placed in a protected aeronautical radio navigation service band, was re-
cently allocated by the World Radio Conference organized regularly by the
International Telecommunication Union (Vorhies 2000).
Note that both new civil GPS signals will have two codes. L5 will not
share with military signals and use two equal-length codes in phase quadra-
ture, each clocked at 10.23 MHz. L2 is shared between civil and military
signals. The new L2c signal provides two codes by time multiplexing. The
two codes are of different length (Fontana et al. 2001). The existing military
Y-code will be replaced by new (split) M-codes.
The linear carrier phase combination of L2 with L5 results in a signal
with a wavelength of about 5.9 m. Long wavelengths facilitate ambiguity
resolution. By contrast, the linear combination of L1 with L5 will be used
as ionosphere-free combination because large frequency differences are ad-
vantageous for calculating ionospheric corrections. The common processing
of phase data from all three carriers will be performed in the three-carrier
ambiguity resolution approach (Vollath et al. 1999).
A perspective for the implementation is given in the 2001 Federal Radio-
navigation Plan: IOC (18 satellites in orbit with the new L2c signal and
M-code capability) is planned for 2008 and FOC (24 satellites in orbit) is
planned for 2010. At least one satellite is planned to be operational with the
new L5 capability no later than 2005, with IOC planned for 2012 and FOC
planned for 2014.
5.4
From GPS to coordinates
So far, we have got an introductory GPS overview. Now we are interested in
applying elementary GPS approaches to demonstrate how coordinates are
obtained. Two examples, as simple as possible, are selected: point positioning
and relative positioning.
