NEO/LEA-M8T - Data sheet

UBX-15025193 - R05

Functional description

Page 13 of 36

Production Information

uncertainty may include unresolved ambiguities; for example, for GPS these might be epoch

(millisecond), bit (20 ms) and sub-frame (6 s). Where the output time-base standard is derived

from a different constellation (for example, GPS-time from GLONASS), the estimate of

uncertainty includes inter-constellation offset uncertainties. The estimate of uncertainty is used

to disable or modify the time-pulse output by comparison with the “tAcc” parameter (after

conversion to distance) configured in message CFG-NAV5.



Multi-GNSS signal reception:

Particularly where sky views are limited, the timing accuracy is improved by combining

measurements from two constellations. Inter-GNSS timing offsets are derived locally by the

receiver whenever a timing fix can be achieved independently from each constellation (locally

derived offsets automatically account for antenna, filter and cable dispersion). These offsets are

then used for subsequent combined fixes. Where inter-GNSS offsets cannot be derived locally,

offsets broadcast by the constellation satellites are used where available.



Fix redundancy (RAIM):

The receiver automatically and continually adjusts the significance of individual signal

measurements in the reported estimate of time according to its quality and consistency. This

ensures that the integrity of the reported time is protected from individual faulty signals or

measurements so long as there are more signals in use than the minimum required. The minimum

number changes depending on the situation but whenever it is exceeded “RAIM active” is set in

message TIM-TP to indicate that this protection is active.



Aiding:

While a GNSS receiver may be able to achieve a vernier (sub-microsecond) time-fix even under poor

signal conditions, it may be slow or unable to resolve higher order ambiguities (especially whole

milliseconds for GPS). Sub-millisecond time aiding may be applied to u-blox NEO-M8T and LEA-

M8T modules by means of a pulse to one of the EXTINT pins in conjunction with a MGA-INI-TIME

message, enabling immediate resolution of ambiguities as well as accelerating time to fix.

1.8.5 Raw data

The NEO/LEA-M8T modules provide raw measurement data for civil L1 band GPS, GLONASS and

BeiDou signals including pseudo-range and carrier phase, Doppler and message payloads. The data

contained in the RXM-RAWX message follows the conventions of a multi-GNSS RINEX 3 observation

file and includes pseudo-range, carrier phase and Doppler measurements along with measurement

quality data. The RXM-SFRBX message provides the demodulated, parity-checked navigation and

signaling message bits for each satellite currently tracked by the receiver including GPS, GLONASS

and BeiDou constellations, SBAS satellites, the QZSS L1S signal and IMES beacons.

Raw measurement data are available once the receiver has established data bit synchronization and

time-of-week. Message data are available for all signals tracked at a sufficient level to achieve data

bit and frame synchronization. For more information, see the u-blox 8 / M8 Receiver Description

including Protocol Specification [3].

1.8.6 Low duty-cycle operation

The NEO-M8T and LEA-M8T low-power timing modules support energy-saving automatic on/off

interval low duty-cycle operation in conjunction with their precision timing features. On/off low duty-

cycle operation is enabled with the power save mode setting in message CFG-RXM and on/off mode

in message CFG-PM2. Through a set of period and time-out parameters defined in the CFG-PM2

message the receiver can be configured to deliver a new time fix at intervals with a limit on total energy

consumed for searches if no fix can be achieved. Note that the time-pulse output is not available while

the receiver is in the “off” section of each cycle.

The duty-cycle of operation may be reduced significantly by:



provision of sub-millisecond time-aiding to accelerate ambiguity resolution (see 1.8.4 above),