Global Positioning System Reference
In-Depth Information
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TABLE 3.5
Elements of Subframes 2 and 3
M
0
Mean anomaly at reference time
∆
n
Mean motion difference from computed value
e
Eccentricity
√
a
Square root of the semimajor axis
Ω
0
Longitude of ascending node of orbit plane at beginning of week
i
0
Inclination angle at reference time
ω
Argument of perigee
Ω
Rate of right ascension
IDOT
Rate of inclination angle
C
u
c
,C
us
,C
rc
,C
rs
,C
ic
,C
is
Amplitude of second-order harmonic perturbations
t
oe
Ephemeris reference time
IODE
Issue of data (ephemeris)
[83
id
entical to those listed in Table 3.5. The algorithm of Table 3.6 applies, using zero
fo
r all elements that are not included in the almanac and replacing the reference time
t
oe
by
t
oa
.
The mean anomaly, the longitude of the ascending node, the inclination, and UTC
(if
desired) are formulated as polynomials in time; the time argument is GPS time.
Th
e polynomial coefficients are, of course, a function of the epoch of expansion. The
re
spective epochs are
t
oc
,
t
oe
,
t
oa
, and
t
ot
.
The navigation message contains other relevant information, such as the user range
er
ror (URE). This measure equals the projection of the ephemeris curve fit errors onto
th
e user range and includes effects of satellite timing errors (and possibly SA).
Lin
—
1
——
No
PgE
[83
3.
2.3 GPS Modernization
GPS modernization becomes possible because of advances in technology as used
in the satellite and the receiver. The additional signals transmitted by modernized
satellites will improve the antijamming capability, increase protection against anti-
spoofing, shorten the time to first fix, and provide a civilian “safety of life” signal
(L5) within the protected Aeronautical Radio Navigation Service (ARNS) frequency
band. The new L2C signal will increase robustness of the signal, improve resistance
to interference, allow for longer integration times in the receiver, thereby reducing
tracking noise and increasing accuracy, as well as providing better positioning inside
buildings and in wooded areas. The second civil frequency will eliminate the need
to use inefficient squaring, cross-correlation, or other patented techniques currently
used by civilians in connection with L2. Once the GPS modernization is completed,
the dual-frequency or triple-frequency receivers are expected to be in common use
and affordable to the mass market.
At the same time, new military codes called the M-codes will be added to L1 and
L2, but will be spectrally separated from the civilian codes. There is no military code
planned on L5.
