Navman 11 User Manual

Page 62

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).

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