PROCESSING PSEUDORANGES AND CARRIER PHASES - GPS Satellite Surveying - page 275

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Figure 7.18 Impact of receiver clock errors on GLONASS double-differenced obser-

vations.

[27

Scaling the carrier phases to distances, or to a mean GLONASS frequency, or to
f 1 or f 1 eliminates the receiver clock term but introduces a linear combination of
sin gle-difference ambiguities whose coefficients are nonintegers. For example, we
ca n write

Lin
—
0.3
——
No
PgE

f 1
f 1

f 1
c

f 1
f 1

ϕ rs
km, 1 , GLO

ϕ km, 1 , GLO − ϕ km, 1 , GLO =

rs
km

N km, 1 , GLO − N km, 1 , GLO = ρ +

(7.126)

Th e term N km, 1 , GLO is an integer. In practical applications, one can compute an
ap proximate value N km, 1 , GLO , 0 for the single-difference ambiguity from (7.116) using
sta tion coordinates and receiver clock estimates computed from pseudoranges. Note
th at point positioning with GPS or GLONASS is conceptually the same, i.e., the
GL ONASS point positioning is not burdened with ambiguity issues or extra receiver
clo ck complications. The double-difference GLONASS observation (7.126) can then
be written as

[27

f 1
f 1

f 1
c

ϕ rs
km, 1 , GLO

N km, 1 , GLO , 0 =

rs
km

+ N rs
km, 1 , GLO

rs

+ ρ + η (7.127)

with

f 1

−

f 1

rs

dN km, 1 , GLO ≤ 0 . 01 dN km, 1 , GLO

η =

(7.128)

f 1

term depends on the quality of the initial estimate N km, 1 , GLO , 0 , since The size of the η

dN km, 1 , GLO = N km, 1 , GLO , 0 . If we neglect this term, (7.127) has the same

form as a double-difference equation for GPS. However, neglecting the

N km, 1 , GLO −

term causes
a model error that might make ambiguity fixing difficult, if not impossible, depending
on the accuracy of the approximation N km, 1 , GLO , 0 . The float solution does not require
the

η

η term.