Navman 11 - Manual (Page 68)

68 MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.
communications protocol, and message controls
are stored in non-volatile EEPROM.
4.7.1.2 The auxiliary port
The auxiliary port is used exclusively for the receipt
of differential corrections in RTCM SC-I04 serial
message format.
By default, the auxiliary port is configured for 9600
baud, no parity, 8 data bits, and 1 stop bit. There is
no data output from this port.
4.7.2 EEPROM services
The EEPROM services provide for the non-volatile
storage and retrieval of system configuration
parameters and data that vary, but are generally
fairly constant for short periods of time (a few
weeks).
The configuration and operational data stored in
EEPROM is only read during system initialisation
if the complimentary SRAM data is invalid. When
data is stored in EEPROM, a checksum is stored
with it to validate the data when it is read. If the
data read from EEPROM during initialisation is
invalid, default values from ROM will be used to
initialise the system.
EEPROM data blocks are updated/refreshed
when the corresponding system data changes
significantly. The qualification of a significant
change varies for each data block. In the
case of user configurable data items (datum
selection, user-defined datums, platform class,
communication parameters, etc.), simply receiving
new inputs is all that is required for the data to be
refreshed in the EEPROM.
In the case of slowly changing data (position,
almanac, frequency standard data, etc.), additional
constraints of distance moved, change in value,
and/or elapsed time are imposed on the EEPROM
update. The various parameters and data
maintained in the Jupiter receiver’s EEPROM are
listed in Table 4-2.
4.7.3 RTC services
The RTC services provide for the storage of time/
date data, maintained while the system is in an
“idle” state. As long as external power is provided
to the RTC device, it will keep the time/ date data
current, providing the system with accurate time
initialisation as needed.
The time/ date data is only read from the RTC
during system initialisation. When the time/ date
data is stored in the RTC, a snapshot of the data
is stored with a checksum in the RAM space of
the RTC device (RTC- RAM). The snapshot data
in the RTC-RAM is used to determine if the RTC
was kept alive, and therefore if the time/ date data
is valid. If the clock data is not valid at system
initialisation, the ‘‘last known time” stored in SRAM
will be used if it is available, otherwise time will be
invalid.
The time/ date data is updated in the RTC
periodically while the system is in its Kalman filter
navigation mode.
4.7.4 Differential GPS (DGPS)
DGPS techniques can be used to eliminate errors
introduced by Selective Availability (SA) and other
error sources. DGPS requires one GPS receiver
to be located at a precisely surveyed location.
This receiver, often referred to as a ‘base station’
or ‘reference station’, calculates corrections to
the measured pseudo-range and delta-range
measurements from each of the satellites it is
tracking.
These corrections are then broadcast over a
communications link to remote GPS receivers in
the field which apply these corrections to their
Configuration data Satellite management parameters Navigation data
Serial port configuration (both ports) UTC and ionosphere model parameters last known position
solution validity criteria frequency standard data user-entered altitude
selected datum almanac data
platform class
cold start control
satellite elevation Mask Angle
satellite candidate List
differential GPS control
default serial output messages
user defined datums
navigation control
Table 4-2 Parameters and data maintained in EEPROM