FIX

Constrains to fixed height or position

Platform:

OEM719, OEM729, OEM7500, OEM7600, OEM7700, OEM7720, PwrPak7, SPAN CPT7, SMART7, SMART2

Firmware Stream:     

This command is used to fix height or position to the input values. For various applications, fixing these values can assist in improving acquisition times and accuracy of position or corrections. For example, fixing the position is a requirement for differential base stations as it provides the reference position to base the differential corrections from.

  1. It is strongly recommended that the FIX POSITION entered be accurate to within a few meters. This level of accuracy can be obtained from a receiver using single point positioning once 5 or 6 satellites are being tracked.

  2. FIX POSITION should only be used for base station receivers. Applying FIX POSITION to a rover switches it from RTK mode to a fixed position mode. Applying FIX POSITION to the rover does not speed up ambiguity resolution.

  3. Any setting other than FIX POSITION disables output of differential corrections unless the MOVINGBASESTATION command is set to ENABLE.

  4. You can fix the position of the receiver using latitude, longitude and height in Mean Sea Level (MSL) or ellipsoidal parameters depending on the UNDULATION setting. The factory default for the UNDULATION command setting is EGM96, where the height entered in the FIX command is set as MSL height. If you change the UNDULATION setting to USER 0, the height entered in the FIX command is set as ellipsoidal height (refer to Table: FIX Parameters).

Error checking is performed on the entered fixed position by the integrity monitor. Depending on the result of this check, the position can be flagged with the following statuses.

  • SOL_COMPUTED: The entered position has been confirmed by measurement.

  • PENDING: Insufficient measurements are available to confirm the entered position.

  • INTEGRITY_WARNING: First level of error when an incorrect position has been entered. The fixed position is off by approximately 25-50 meters.

  • INVALID_FIX: Second level of error when an inaccurate position has been entered. The fixed position is off by a gross amount.

An incorrectly entered fixed position will be flagged either INTEGRITY_WARNING or INVALID_FIX. This will stop output of differential corrections or RTK measurements and can affect the clock steering and satellite signal search. Checks on the entered fixed position can be disabled using the RAIMMODE command.

Message ID: 44

Abbreviated ASCII Syntax:

FIX type [param1 [param2 [param3]]]

Factory Default:

FIX none

ASCII Example:

FIX none

FIX HEIGHT 4.567

FIX position 51.116 -114.038 1065.0

In order to maximize the absolute accuracy of RTK rover positions, the base station coordinates must be fixed to their known position using the FIX POSITION [lat][lon][hgt] command.

Field

Field Type

ASCII Value

Binary Value

Description

Format

Binary Bytes

Binary Offset

1

FIX header

-

-

Command header. See Messages for more information.

-

H

0

2

type

See Table: Fix Types

Fix type

Enum

4

H

3

param1

See Table: FIX Parameters

Parameter 1

Double

8

H + 4

4

param2

Parameter 2

Double

8

H + 12

5

param3

Parameter 3

Double

8

H + 20

FIX Parameters

ASCII Type Name

Parameter 1

Parameter 2

Parameter 3

AUTO

Not used

Not used

Not used

HEIGHT

Default MSL height 1

(-1000 to 20000000 m)

Not used

Not used

NONE

Not used

Not used

Not used

POSITION

Lat (-90 to 90 degrees) where a ‘-’ sign denotes south and a ‘+’ sign denotes north

Lon (-360 to 360 degrees) where a ‘-’ sign denotes west and a ‘+’ sign denotes east

Default MSL height 1

(‑1000 to 20000000 m)

For a discussion on height, refer to An Introduction to GNSS available on our website.

Fix Types

ASCII Name

Binary Value

Description

NONE

0

Unfix. Clears any previous FIX commands

AUTO

1

Configures the receiver to fix the height at the last calculated value if the number of satellites available is insufficient for a 3-D solution. This provides a 2-D solution. Height calculation resumes when the number of satellites available allows a 3-D solution

HEIGHT

2

Configures the receiver in 2-D mode with its height constrained to a given value. This command is used mainly in marine applications where height in relation to mean sea level may be considered to be approximately constant. The height entered using this command is referenced to the mean sea level, see the BESTPOS log (is in meters). The receiver is capable of receiving and applying differential corrections from a base station while fix height is in effect. The fix height command overrides any previous FIX HEIGHT or FIX POSITION command.

Note: This command only affects pseudorange corrections and solutions.

POSITION

3

Configures the receiver with its position fixed. This command is used when it is necessary to generate differential corrections.

For both pseudorange and differential corrections, this command must be properly initialized before the receiver can operate as a GNSS base station. Once initialized, the receiver computes differential corrections for each satellite being tracked. The computed differential corrections can then be output to rover stations using the RTCMV3 differential corrections data log format. See Transmitting and Receiving Corrections for information about using the receiver for differential applications.

The values entered into the fix position command should reflect the precise position of the base station antenna phase center. Any errors in the fix position coordinates directly bias the corrections calculated by the base receiver.

The receiver performs all internal computations based on WGS84 and the DATUM command is defaulted as such. The datum in which you choose to operate (by changing the DATUM command) is internally converted to and from WGS84. Therefore, all differential corrections are based on WGS84, regardless of your operating datum.

The FIX POSITION command overrides any previous FIX HEIGHT or FIX POSITION command settings.