HMR3500 User's Guide
Every one of the corresponding working variables can be changed by commands issued from the host computer, and this
is how they should be set as much as possible. The flash memory in which the default values are written is not intended
for frequent writing, and is not intended to support a large number of writes. Use SDFLT sparingly!
The SDFLT command can also be used merely to read out start-up default values without actually writing to the flash
memory. A quantity whose bit in the REQ field is zero will not be changed. The current value, however, will be returned in
Initialize Magnetic Declination
Request Data: 3 bytes:
This command returns or sets the difference between true north and magnetic north for the user's current location.
"Variation" is normal nautical terminology; "declination" is often used on land to mean the same thing. The value varies
with latitude, longitude, altitude, and slowly with time (years). Magnetic declination (variation) may be obtained from a
mathematical magnetic model of the earth, or from other sources such as margin information on maps.
Magnetic declination, like mounting offset, is added to the magnetic compass heading to report true azimuth.
Consequently, if magnetic north is east of true north, declination is a positive quantity. This is consistent with traditional
Correct magnetic declination is important to accurate magnetic compass headings, and the local declination should
always be loaded at start-up. As an alternative, the start-up default for declination can be set. (See Set Power-Up
Defaults, on Page 11.)
World Magnetic Model – WMM
The compass unit is supplied with a built-in World Magnetic Model (WMM) and the ability to calculate magnetic declination
from it. The WMM consists of 168 spherical-harmonic Gauss coefficients relating the earth's magnetic field to latitude and
longitude, and an additional 168 coefficients to estimate time variations. The model is approved by both the United States
and United Kingdom Departments of Defense and is intended for use only during a five-year "epoch." In the absence of
The local WMM magnetic declination is a function of latitude, longitude, altitude, and date. These input values are
specified in the WMM message. When the compass receives a WMM message, it will compute the local WMM magnetic
0 = Return current variation
(ignore next field)
non-zero = Set current variation
Magnetic variation (declination).
Copy of field from IMVAR.
Current variation (declination).