MATLAB Code - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 144

calculate
pseudoranges.m

no

first position
solution?

decode the
ephemeris using
the 1500 bits
following the
preamble

yes

set smallest
travel time to 68
milliseconds

update TOW
by time
increment

define preambles
and preallocate
arrays

truncate
remaining travel
times relative to
smallest travel
time

compute travel
time:
(abs sample num) /
(sampling freq)

update first
subframe by time
increment

correlate
navigation bits
with preamble
to identify first
subframe

find next
preamble
location

compute
pseudoranges:
(travel time *
speed of light)

record the
millisecond of the
first received
subframe

compute travel
time:
(abs sample num) /
(sampling freq) preamble

found 300
bits later?

no

yes return

all channels
processed?

all channels
processed?

no yes yes no

TOW &
HOW pass
parity?

no

yes

FIGURE A.5. Flow diagram for computing pseudoranges.

The output of the function is a set of pseudoranges and the signal transmission
time (SOW) for all measured pseudoranges.

A.5.2 Position Computation
The function leastSquaresPos computes the position of the receiver from the mea-
sured pseudoranges. If for some reason the entire ephemeris for a satellite is not
available, the pseudorange is excluded from the computation (see Figure A.6).
At the first computation, the elevation angles of all satellites are set to the max-
imum. This is necessary to estimate the receiver position and to compute the true
elevation angles of all satellites. All subsequent computations of position will
exclude pseudoranges from satellites with elevation angles lower than elevation-
Mask .
The function postNavigation uses several other functions that are slightly mod-
ified versions of functions from the Easy Suite; see Borre (2003).
At the end of the postNavigation function, the ECEF coordinates are trans-
formed to UTM and geodetic coordinate systems. The results are stored in struc-
ture navSolutions .