Navman 11 Manual Do Utilizador

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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.

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.

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.

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.

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).

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 

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 

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