User Manual

SARA-R4 series-AT commands manual
UBX-17003787 - R16
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23Positioning
Page 269 of 401
23Positioning
23.1NMEA
u-blox cellular modules support reading NMEA strings from the GNSS receiver through AT commands.
Before being able to read a specific NMEA string, it is necessary to activate the storage of the last value of that
particular NMEA string. If storing a particular NMEA string was not activated, the information text response
to the query will be "0,NULL". The last value of a specific NMEA string is saved in RAM and is made available
even after the GNSS receiver switch off.
The NMEA standard differentiates between GPS, GLONASS, GALILEO, BeiDou and multi-GNSS receivers using
a different 'Talker ID'. Depending upon device model and system configuration, the u-blox receiver could output
messages using any one of these Talker IDs.
By default, the receivers configured to support GPS, SBAS and QZSS use the 'GP' Talker ID, receivers configured
to support GLONASS use the 'GL' Talker ID, receivers configured to support BeiDou use the 'GB' Talker ID,
receivers configured to support GALILEO use the 'GA' Talker ID and receivers configured for any combinations
of multiple GNSS use the 'GN' Talker ID.
Even if the NMEA specification indicates that the GGA message is GPS specific, u-blox receivers support the
output of a GGA message for each of the Talker IDs.
As a factory-programmed setting, the cellular modules configure the GNSS receiver through AT+UGPS
AT command to not provide the NMEA sentences.
When reading an NMEA message, if the response value is "1,Not available" then the storing of the NMEA
string is activated but this information has not been still sent to the user, if this persist check that the
relative NMEA message is enabled. To enable it use the +UGUBX command (for further information see
the UBX-CFG-MSG message in the u-blox GNSS protocol specification).
23.2AssistNow services
Users would ideally like GNSS receivers to provide accurate position information the moment they are turned
on. With standard GNSS receivers there can be a significant delay in providing the first position fix, principally
because the receiver needs to obtain data from several satellites and the satellites transmit that data slowly.
Under adverse signal conditions, data downloads from the satellites to the receiver can take minutes, hours
or even fail altogether.
GNSS AT commands provides the means for delivering assistance data to u-blox receivers obtained from the
u-blox AssistNow Online or AssistNow Offline services.
AssistNow Online is u-blox' end-to-end Assisted GNSS (A-GNSS) solution for use cases that have access to
the Internet. Data supplied by the AssistNow Online service can be directly uploaded to a u-blox receiver to
substantially reduce Time To First Fix (TTFF), even under poor signal conditions.
AssistNow Offline service is targeted at use cases that only have occasional Internet access and so cannot
use AssistNow Online. AssistNow Offline speeds up Time To First Fix (TTFF), typically to considerably less
than 10 s. Cellular modules using AssistNow Offline download data from the AssistNow Offline service when an
Internet connection is available. Data are stored locally to the cellular module file system and are subsequently
uploaded to a u-blox receiver, so that it can estimate the positions of the satellites, when no better data is
available. Using these estimates will not provide as accurate a position fix as if current ephemeris data is used,
but it will allow much faster TTFFs in nearly all cases.
Both the AssistNow Online and Offline services use a simple, stateless, HTTP interface. Therefore, they work
on all standard mobile communication networks that support Internet access.
UDP protocol for the AssistNow Online service is deprecated.
Both the AssistNow Online and Offline services are only available for use by u-blox customers. To use the
services, customers will need to obtain an authorization token from u-blox. This token must be issued as a
parameter of +UGSRV AT command.
AssistNow Autonomous feature provides a functionality similar to AssistNow Offline without the need for
a host and a connection. Based on a broadcast ephemeris downloaded from the satellite the receiver can