Navman 11 - Manual (Page 58)

58 MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.
4.0 Jupiter GPS receiver
operation
This section presents a detailed operational
description of the Jupiter series of GPS receivers.
An overview is provided for the navigation and
receiver support functions. Each of the receiver’s
internal storage devices are described as well as
how each one is initialised and used to control
operational configurations. This section also
provides a description of start-up modes and
satellite management.
4.1 Internal (on board) data sources
Internal data sources are the “built-in” information
storage capabilities of the GPS receiver. The
on-board receiver firmware maintains these data
sources for use on a continuing basis.
4.1.1 Static Random Access Memory (SRAM)
SRAM is used to store all firmware variables
manipulated by the GPS receiver. If external
power has been supplied to SRAM when the main
power is disconnected, this data will be available
to the initialisation functions at start-up. Satellite
ephemeris, last position, and frequency standard
parameters are important data that helps to
minimise TTFF if the data is available in SRAM.
4.1.2 Real-time clock (RTC)
Along with SRAM, an on board RTC is a valuable
source of data at system start-up. If external
power has been applied to the RTC when the main
power is disconnected, time/date information will
be available to the initialisation functions at start-
up. Valid time/date is a key component used to
compute satellite visibility and to minimise TTFF.
Note: A value of ‘last known time’ is available in
SRAM. On the Jupiter board, the RTC is powered
whenever SRAM is powered (see section 4.7 for
more information about the RTC).
4.1.3 Electrically Erasable Programmable
Read- Only Memory (EEPROM)
On board EEPROM is useful for long-term storage
of data that varies somewhat over time but is, in
general, fairly constant over short periods of time
(weeks). Unlike SRAM and the RTC, power is
not required to maintain data during ‘idle’ states.
Important data in EEPROM that helps to minimise
TIFF includes satellite almanac, last known
position, and frequency standard parameters.
Note: EEPROM is used only if the required data is
not available from SRAM (see section 4.7 for more
information about the EEPROM).
4.1.4 Read-Only Memory (ROM)
On board ROM is only used as a data source if
SRAM and EEPROM are unavailable. Satellite
almanac and frequency standard parameters can
be obtained from ROM with limited usefulness.
4.2 Initialisation
4.2.1 Definition
Initialisation is defined as the set of data or actions
that provide time varying information for use by
the GPS receiver at start-up. The most common
example is Position, Velocity, and Time (PVT)
initialisation. For a GPS receiver installed in an
automobile, this information is constantly changing
as time progresses and the vehicle moves from
location to location.
Initialisation data is required when the on
board data sources are old or invalid. Serial
input messages are prepared by the user and
transmitted to the GPS receiver. In general, the
GPS receiver is capable of ‘bootstrapping’ itself
without any valid data sources, but TTFF times are
extended.
4.2.2 Position, Velocity, Time (PVT) data
The most common form of user supplied
initialisation data is position and time (velocity
is normally included in this group, but it is only
required for higher dynamic operations). Accurate
PVT and valid almanac/ephemeris data are
required to generate the current satellite visibility
list and appropriate acquisition uncertainties,
resulting in optimal TTFF performance.
4.2.3 Satellite ephemeris
Unlike user PVT information, satellite ephemeris
data is available from every satellite that is
continuously tracked (18 seconds minimum
collection time). The ephemeris is maintained
in SRAM. This ‘over-the-air’ availability means
that the user does not normally have to supply
ephemeris data.
4.2.4 Satellite almanac
Almanac information for all satellites is available
from each tracked satellite (12.5 minute collection
time for the complete set) and is maintained in the
on board EEPROM and SRAM. Like ephemeris
data, ‘over-the-air’ availability means that the user
does not normally have to supply almanac data.
4.2.5 Universal Time Coordinated (UTC) and
ionospheric parameters
UTC and ionospheric correction parameters are
available from every tracked satellite (broadcast
once every 12.5 minutes) and are maintained in
the on board EEPROM and SRAM. Like almanac
and ephemeris data, ‘over-the-air’ availability
means that the user does not normally have to
supply UTC and ionospheric data.