Observation classifications are used in SNAP to define parameters that are used to determine the expected (calculated) value of the observation. These parameters are the reference frame for GPS observations, the refraction coefficient for zenith distance observations, and optionally bearing orientation errors for bearing and azimuth data, and scale factor errors for distance data.
The following commands define how these parameters are applied in the adjustment:
Selection of classification to use: refraction_coefficient use classification_name distance_scale_error use classification_name bearing_orientation_error use classification_name Definition of value: refraction_coefficient [calculate] code [value [?]] distance_scale_error [calculate] code [value [?]] bearing_orientation_error [calculate] code [value [?]] systematic_error [calculate] code [value [?]] reference_frame_scale_error [calculate] [value [?]] Forcing coefficients to have the same value: refraction_coefficient code1 = code2 distance_scale_error code1 = code2 bearing_orientation_error code1 = code2 systematic_error code1 = code2
The first three commands define what classification will be used for the observation. The default classifications are ""refraction_coef_code" for refraction coefficients, "distance_scale_code" for the distance scale error, and "bearing_error_code" for the bearing orientation error.
The second set commands specify how the refraction coefficients, distance scale errors, bearing orientation errors, user defined systematic errors, and reference frame scale error are calculated. The parameters of the commands are:
calculate
If present, specifies that the value of the named refraction coefficient is to be calculated in the adjustment
code
Is the code of the additional parameter. The code can include * and ? wildcard characters. ? matches any single character, and * matches any number of characters (including none). For example, if a refraction coefficient code is specified as DAY* then the command will apply for any refraction coefficient starting DAY. Codes such as DAY, DAY_ONE, and DAYTIME would be affected.
value
Is the initial value used for the coefficient. If it is followed with "?" then the value will be calculated (equivalent to the "calculate" keyword). If the parameter is being calculated then the value specified is used as an initial trial value.
Examples would be
refraction_coefficient * 0.08 refraction_coefficient calculate DAY distance_scale_error DI20 0.5 ? reference_frame_scale_error calculate
The third set of commands allows different parameter codes to be treated as equivalent. These specify that two or more codes will be treated as identical in the adjustment. The effect of this command is that the additional parameter (or parameters) corresponding to code1 are made identical to code2. Furthermore, if code2 is being adjusted, then all the codes will be adjusted to the same value.
Note that the = must have a space on either side.
code1
The code of parameters that are to be modified. This can include wildcard characters * and ?. ? matches any single character. * matches any number of characters (include no characters).
code2
The parameter that the code1 parameter is being matched with.
For example to do an adjustment in which all the refraction coefficients are treated equally, use the commands
refraction_coefficient calculate DEFAULT 0.075 refraction_coefficient * = DEFAULT