Navman 11 User Manual

66

MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.

These errors induce velocities in the receiver’s 

state estimate even if the receiver is motionless. 

The magnitude of these velocities depends on the 

geometry and number of satellites in track. Typical 

values are between one-half and two metres per 

second. The position estimate of the receiver will 

vary within a circle of approximately 100 m (the 

2Drms position error specified in the GPS SPS 

Signal Specification). 

This varying of the position estimate while the 

receiver is static is undesirable for users who 

display their position on a map or for those with 

similar applications. The receiver’s proprietary 

navigation software is capable of “pinning” the 

position output when the estimated velocity is low 

and the navigation solution is of good quality. This 

prevents the varying behaviour of the solution 

in static situations, but introduces some risk 

that actual platform motion will be mistaken for 

deliberate SA induced error. 

By default, the position pinning feature of the 

Jupiter GPS receiver is enabled. The OEM can 

turn it off using the ‘nav configuration’ message 

(binary Message 1221).

Without the use of DGPS, satellite measurements 

are corrupted by SA and do not generate a 

consistent estimate for receiver position. Typical 

range errors are on the order of 30 m. The receiver 

processes all satellites above the mask angle to 

minimise the effects of the range error. 

Changes occur in the set of satellites being 

processed because of blockage, and the rising 

and setting of satellites. When a change occurs, 

the position estimate formed from averaging these 

errors is shifted. These shifts can be substantial, 

typically 10 m to 20 m, but up to 100 m or more in 

bad geometry.

The receiver has a proprietary compensation 

technique for these constellation switch 

effects which maintains a smooth estimate of 

the platform’s ground-track and altitude. This 

smoothing is accomplished without any loss of 

dynamic response.

However, this method is not required and is 

automatically disabled when DGPS is available. 

Without DGPS, the method is enabled by default. 

Most users should leave it enabled. One reason for 

disabling it would be to compare the solution with 

a point solution from another receiver or a solution 

calculated off-line.

Ground-track smoothing can be disabled 

using the ‘nav configuration’ message (binary 

Message 1221). The configuration word is stored 

in EEPROM, so the setting for ground-track 

smoothing will be retained through power-off and 

reset cycles.

4.6.4 Solution validity
The validity of navigation solution outputs is 

determined by user-defined criteria. The OEM can 

specify which criteria to apply for the particular 

application. There are five possible criteria used to 

validate a solution: 

1 Was an altitude measurement used? 
2 Was DGPS used? 
3 How many satellite measurements were 

used? 
4 What is the Expected Horizontal Position 

Error (EHPE)? 
5 What is the Expected Vertical Position Error 

(EVPE)? 

The OEM cannot change the validity criteria in 

NMEA mode. These criteria can be set using 

only the binary ‘solution validity criteria’ message 

(Message 1217) and they are retained through 

power-off and reset cycles in EEPROM.

The altitude measurement validity criterion allows 

the OEM to specify if solutions that make use 

of an altitude measurement should be marked 

valid. Altitude is not used in the solution unless it 

is necessary because of deteriorating EVPE or 

untracked satellites. When it is required, the OEM 

may wish this to be an indication that the solution 

is invalid for purposes of the specific application. 

The default is that solutions using altitude 

measurements may be marked valid

The DGPS used validity criterion indicates an 

invalid navigation solution if DGPS is unavailable 

after it has been required. The system default is 

that DGPS is not required for a valid solution.

The number of satellites used validity criterion 

indicates an invalid navigation solution if the 

minimum number of satellites required to be in the 

solution is not met. The default for this test is zero. 

A solution may be reported as valid with no 

measurements used so long as the EHPE and 

EVPE criteria pass and a Kalman filter solution 

has been previously computed. The reason the 

default is set to zero is to allow the receiver to 

coast through brief outages without declaring the 

solution invalid (e.g. for example, under a freeway 

overpass).