NavCom Sapphire Rev.J User Manual
Page 43
Sapphire Technical Reference Manual Rev. J
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slot numbers above 24 are allocated to on-orbit spares.
The NMEA sentences describe the satellite population using the following naming convention:
$GAxxx, describes data generated from Galileo satellites only
$GPxxx, describes data generated from GPS satellites only
$GLxxx, describes data generated from GLONASS satellites only
$GNxxx, describes data generated from mixed GPS, GLONASS, and Galileo satellites
The following are some common definitions that appear in NMEA sentences in particular, and in
GPS frequently. Each represents a value that is accurate, but does not necessarily conform to
any given mathematical range limits.
Dilution of precision is a figure of merit describing the navigation efficiency provided by the
satellite geometry. This value manifests in one, two or three dimensions, and is always "the
lower the better", with 1 being the ideal (best) value, and usually anything over about 20 is
bad.
Geoidal height and mean sea level form virtual boundaries that define the surface of the
Earth. These values grow in tables accrued by continuous surveying.
DGPS correction age is the number of seconds since the last differential correction packet
arrived from a reference station. A few seconds is okay, but many seconds indicate the fix is
degrading over time, and becoming less and less accurate.
A standard deviation is used to measure the error in any calculation, for example latitude or
longitude. If the measurement is good, the standard deviation will be small. If not, it will be
large.
The signal to noise ratio is a number that represents how "loud" the information is when
compared to the ambient noise. This number is specific to the measurement.
Speed over ground is the actual speed the GNSS unit is moving over the ground. This may
differ from airspeed or nautical speed due to such things as head winds or sea conditions.
Delta values for Solid Earth tides are governed by the Earth, the Moon, and other factors
that also affect ocean tides. There is no specific range.
These messages are output messages only. The receiver does not process
NMEA-0183 input data.
Refer to the fore-matter for the address of the headquarters of the National Marine
Electronics Association (NMEA). The NMEA messages listed in this manual begin
with Section 8,