Navman 11 User Manual

Page 67

If the measurements are lost for a long time, the

EHPE and EVPE will grow until they surpass their

thresholds and the solution fails the validity test for

that reason. Some applications require a solution

to be marked invalid unless it uses three, four, or

more satellites. The OEM can set any of these

thresholds by sending a binary ‘solution validity

criteria’ message (Message 1217) with the number

of satellites required

4.6.4.4. Maximum EHPE validity criterion

The EHPE is the one sigma horizontal position

error estimate for the solution. The validity criterion

default is 10 m. The meaning of the reported

EHPE depends on whether or not DGPS is in

use. If DGPS is in use the EHPE is the estimated

one-sigma error in absolute position accuracy.

When DGPS is not in use, the EHPE and EVPE

are reported with the effects of the satellite User

Equivalent Range Error (UERE) excluded. This

excludes SA induced error from the EHPE and

EVPE.

So in SPS navigation, the EHPE and EVPE serve

as convergence indicators for the Kalman ﬁlter,

not as absolute accuracy limits for the reported

position. The EHPE validity threshold can be set

by the OEM in the binary ‘solution validity criteria’

message (message 1217).

4.6.4.5 Maximum EVPE validity criterion

The EVPE is the one sigma vertical position error

estimate for the solution. The default is 25 m.

The operation and meaning of this criterion is

analogous to the EHPE criterion in section 4.6.4.4.

The threshold can be set in the binary ‘solution

validity criteria’ message (message 1217).

4.6.5 Mean Sea Level (MSL)
MSL is a geoid, or surface of equal gravitational

potential. The height of the MSL geoid above or

below the reference ellipsoid (WGS-84 by default)

is called the geoidal separation. Positive geoidal

separation means that the geoid is above the

ellipsoid.

Values for the geoidal separation are computed at

any receiver position by interpolating on the table

of geoidal separation values provided in the U.S.

Government document, Department of Defense

World Geodetic System 1984.

Altitude or height computation is referenced to the

ellipsoid and is referred to as geodetic altitude.

Altitude referenced to the geoid (usually referred to

as altitude MSL) can be computed as the geodetic

altitude minus the geoid separation.

4.6.6 Magnetic variation
The magnetic variation model used in the receiver

is derived from the full International Geomagnetic

Reference Field (IGRF) 95 magnetic model.

Documentation, tabular data, and test programs

for the IGRF 95 magnetic model can be obtained

from the National Oceanic and Atmospheric

Administration (NOAA) National Geophysical Data

Centre (NGDC) web site (http:/ /julius.ngdc.noaa.

gov / seg/potﬂd/geomag.html).

The magnetic variation is used to convert true

heading to magnetic heading. It is output in

binary Message 1000. To convert the true course

provided in binary Message 1000 to magnetic

heading, the magnetic variation is added to the

true course.

4.7 Support functions

This section describes the support functions of the

GPS receiver.

4.7.1 Serial communication interfaces
Jupiter GPS receivers provide two bi-directional

serial communication interfaces: the host and

auxiliary ports (see Section 3)

4.7.1.1 The host port

The Host port is the primary interface to the

controlling application and provides all the services

for initialising and conﬁguring the system as well

as for the reporting of the navigation solution and

receiver status.

By default, the Host port is conﬁgured for 9600

baud, no parity, 8 data bits, and 1 stop bit. The

Navman binary communications protocol is the

default selection for the Host port.

The default settings (conﬁguration and protocol)

can be overridden with the use of the NMEA

Select control line of the interface. When this line

is asserted, the conﬁguration defaults to 4800

baud, no parity, 8 data bits, and 1 stop bit, and the

communications protocol defaults to NMEA.

Note that the NMEA Select line will override any

previously stored selections for the Host port

conﬁguration and communication settings.

While using the Navman binary communications

protocol on the Host port, a number of application

speciﬁc parameters can be conﬁgured to

customise the receiver for a speciﬁc application.

The ability to freely switch between Navman binary

and NMEA modes provides full capability to all

users.

Host port output messages can be conﬁgured

using the log control messages supported in both

Navman binary and NMEA message protocols.

Changes to the port conﬁguration settings,

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