Quantifying the Number for GNSS Acquisition Engine Architectures

Innovation: Counting Equivalent Correlators


Quantifying the Number for GNSS Acquisition Engine Architectures
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Jan 1, 2009
By: Angelo Genghi, Fabio Pisoni, Filippo Spalla, Joel Brenner
GPS World

INNOVATION INSIGHTS with Richard Langley



WITH THIS ISSUE, GPS World and the Innovation column begin their 20th yea=
r of publication.=20
The magazine's first issue in January/February 1990 carried advertisement=
s from a number=20
of GPS receiver manufacturers touting their latest products. Most of thes=
e receivers had=20
only a few correlator channels. Some had only one or two, having to seque=
nce through the=20
visible satellites in order to make their measurements. One manufacturer =
introduced a=20
receiver with an amazing 12-channel sophistication =E2=80=94 acquiring an=
d tracking all GPS=20
satellites in view.

These days, most conventional GPS receivers have at least a dozen channel=
s; some have even=20
50 or more. But these receivers must still dedicate at least one channel =
to each satellite=20
being searched =E2=80=94 sequentially offsetting the Doppler frequency an=
d code phase of the=20
replica signal until the satellite is found and then coherently integrati=
ng the detected=20
signal. With this approach, there are limitations to the sensitivity of t=
he receiver (and,=20
to some degree, the time to first fix). And, as a result, GPS signals can=
not be readily=20
tracked indoors or in other locations with challenging signal conditions.=


For GPS to work indoors and downtown, receivers need to acquire weak sign=
als quickly and=20
to collect enough signal power to produce reliable measurements for a pos=
ition fix. This=20
can be achieved by designing a GPS receiver's baseband processor with mor=
e full correlator=20
channels, each of which could search simultaneously for a satellite using=
different=20
Doppler-frequency and code-phase offsets and, when a signal is detected, =
could coherently=20
integrate it with the coherent integrations then being combined through a=
noncoherent=20
integration. However, a much more efficient and less costly approach is t=
o use the concept=20
of correlator "fingers" or "taps" where certain features of a correlator =
channel are=20
shared amongst the taps, resulting in a large number of effective correla=
tors. Such=20
designs have produced processing engine architectures with tens to hundre=
ds of thousands=20
of effective correlators.

But different chip and receiver manufacturers have approached the task of=
designing such=20
engine architectures differently and use...

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574529