increased sensitivity with help of A-GPS

Hello,

Is it true that A-GPS increases the sensitivity of a GPS-Receiver
because there is no need to encode data from the satellites? If yes,
which improvement can you expect?

Thanks for the information!

Reinhold Buchinger wrote:


Background
http://en.wikipedia.org/wiki/Assisted_GPS

Reinhold Buchinger wrote:


Hi Reinhold,

yes, A-GPS can increase sensitivity. Without assistance, the
navigation data modulated onto the signal is disturbing the
acquisition of the signal and thus lowers the sensitivity.

To achieve higher sensitivity, a receiver must collect and
process (correlate) longer chunks of signal. Navigation data
bits are 20ms long. If the receiver processes data across a
bit edge, it loses sensitivity if the next bit is different
from the previous one.

A non-aided receiver may correlate over up to 10ms at a time,
thus guaranteeing that every second measurement does not cross
a bit edge.

With assistance information, the receiver can predict the
navigation data bits and can compensate. This allows for longer
correlation time and thus higher sensitivity.

For the simplest level of aiding data - accurate time - you can
expect the acquisition sensitivity to be similar to the tracking
sensitivity.

More complex A-GPS implementations also include the navigation
data itself into the aiding signal, which allows a receiver to
correlate over several bit periods. It also allows 'snapshot'
operation, as all the required information is available within
seconds.

However, there is a price to be paid for all this. Higher
sensitivity means longer correlation times. The receiver can
compensate for the satellite movement, but it can't do much
about the receiver movement.

That is, high sensitivity modes assume that the receiver is
standing still during the signal correlation. If you move,
sensitivity drops.

Kind regards,

Iwo

Thanks for the answers! That was exactly what I was looking for.
Do you know some links where I can read more information about
improvements (beside TTFF) if I supply a receiver with additional data.
Most of the sources I've found (like wikipedia) speak mainly about
improved TTFF.

Thx,
Reinhold


Iwo Mergler schrieb:

Reinhold wrote:


Don't know about links, but I can give it a go...

If you regard a classic GPS receiver, the difference
between "factory start", "cold start", "warm start",
"hot reacquisition", etc. is the amount and quality
of aiding information the receiver has. The information
affects the TTFF ("xxx-start"-time) and the sensitivity
(tracking vs. acquisition).

Some of this information decays with time. If you
are familiar with modern GPS chipsets, e.g.
SirfStarIII, you may have noticed that they will
work indoors in some cases, but left there for long
enough, stop eventually.

A standalone GPS receiver can collect the required
data given enough time and signal strength. Some
of this can't be 'refreshed' during high sensitivity
operation.

A-GPS provides the same information via a different
communication channel, like a mobile phone network.

You can think of A-GPS as instantly getting the receiver
into hot-start mode, or even into a hot-reacquisition mode.

Of course the true application for A-GPS is to use
the increased sensitivity to save $2 and replace
the GPS antenna with a piece of wire. ;^)

Kind regards,

Iwo