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MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.
GDOP. The DOP fields in message 1003 are
set to maximum values when GDOP cannot be
computed.
4.5.2 Acquisition modes
Two methods of satellite acquisition are used by
the Jupiter GPS receiver: sequential acquisition
and parallel acquisition.
4.5.2.1 Sequential acquisition
Sequential acquisition describes the acquisition
of a satellite with all non-tracking channels. An
example of this acquisition mode is Cold Start, in
which individual satellite acquisitions are attempted
one at a time using all available channels to
cover the wide Doppler uncertainty. As satellites
are acquired, they stay in track on one channel
with the remaining channels available for the
next acquisition. Sequential acquisition is always
used to acquire the first satellite. The receiver will
automatically transition to parallel acquisition after
the first satellite is acquired during a Warm Start or
an Initialised Start.
4.5.2.2 Parallel acquisition
Parallel acquisition describes the acquisition of
a satellite with a single non tracking channel. An
example of this acquisition mode occurs after the
first satellite is acquired in Warm Start, in which all
of the visible satellites are assigned a channel and
acquisitions are attempted simultaneously. Note
that even though a single channel is being used, a
large Doppler uncertainty can still be covered with
extended search time.
4.5.2.3 Adaptive threshold-based signal detection
To extend the weak signal reception capability of
the receiver, an adaptive noise threshold-based
detection scheme has been implemented in the
receiver software. With this approach, a variable
detection threshold is computed from the average
cross-correlation value of the received signal with
a Pseudo-Random Noise (PRN) code. This PRN
code is similar in structure to the GPS satellite
PRN codes but uses a PRN ID that is not assigned
to any of the GPS satellites. The computation of
the received C/No power is also based on the
cross-correlation value as determined above.
This scheme lowers the average detection
threshold for weak signals, thus improving the
receiver’s ability to acquire and track satellites
under these conditions. Conversely, this scheme
sets a higher threshold when strong signals are
received. This method results in more reliable
acquisition of satellites and a corresponding
reduction in TTFF over a wider variation of GPS
signal strength conditions.
4.5.2.4 Overall search process
Figure 4-1 depicts the overall search process as
it interacts with the visible satellite list generation
described in section 4.5.I. Sequential or parallel
acquisition is selected based on channel
availability and the required frequency search
range (the number of Doppler bins) for each
satellite.
4.5.3 Data collection
Sub frame data collection is a continuous
process once a satellite is in track. This technique
guarantees that current ephemeris and almanac
information are always available to an operating
GPS receiver (making identification of unhealthy
satellites easy).
4.5.3.1 Ephemeris
Ephemeris data is gathered and maintained on
a per satellite basis. For continuously tracked
satellites (no blockage), it will take between 18 and
36 seconds to gather the data set. Once gathered,
it is used to compute high accuracy satellite
position, velocity, and acceleration (PVA) states for
navigation and re-acquisition processes.
Note that this data is only maintained in SRAM due
to its limited time validity.
4.5.3.2 Almanac
Almanac data is gathered and maintained on
a per satellite basis. For continuously tracked
satellites (no blockage), it will take a minimum of
12.5 minutes to gather the complete data set for all
satellites. The primary function of almanac data is
to provide approximate satellite PVA states for the
acquisition process.
Note that this data is maintained in EEPROM due
to its validity over an extended time range (weeks)
4.5.3.3 UTC and ionospheric corrections.
This data is gathered and maintained
independently of the satellite from which it was
obtained (one set is used for all). For continuously
tracked satellites (no blockage), it will take a
minimum of 12.5 minutes to gather an updated
data set.
UTC corrections are used to compute the
exact time offset between GPS and UTC
time. Ionospheric corrections are used by the
navigation process to compensate for the effects
of the satellite signal passing through the Earth’s
ionosphere.
Note that this data is maintained in EEPROM due
to its validity over an extended time range (a few
weeks).