HOW TO USE THE PROGRAM MT80TW

MT80TW is a standard routine to compute tidal effects for gravity, tilt or strain. It requires for input a file with the geographical coordinates of the station and, as option, tidal parameters (amplitude factors and phase differences) to be applied on the astronomical tides. If standard options are chosen, the user has only to select the sampling rate and to enter the initial and final epochs.

This program uses as default option the tidal potential development of Tamura (1987) with 1200 waves (285 long periods, 345 diurnals, 377 semi-diurnals, 82 third-diurnal and 10 quarter-diurnals). An evaluation of the precision can be found in Ducarme (2006). The numerical precision is better than 0.02 nm/s² or 10^-5 of the tidal range. The program can also use the lower resolution Cartwright-Tayler-Edden (CTE505) tidal development with 505 waves (128 long periods, 205 diurnals, 155 semi-diurnals and 17 ter-diurnals). The numerical precision is 10 times lower, but is still largely sufficient, as the main limitation of the precision is the imperfect knowledge of the elastic response of the Earth and of the ocean tides loading effect. The discrepancy between different models of the Earth response is at the level of .1%, while the discrepancy between the tidal gravity parameters derived from different ocean tides models can reach half a percent in coastal area.

This program follows the so called "zero tides" tidal correction. It means that the constant tide M₀S₀ is computed with an amplitude factor equal to one i.e. without elastic response of the Earth.

Output is available according to two different formats: ETERNA or MDASGRAPHER. Output has to be subtracted from the observations to suppress the tidal effects.

Note that the program MT80TW.exe has to be executed under the DOS System.

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SUMMARY

Install all files in the same directory.

INPUT

- XXXXmtht.dat or XXXXmthb.dat: control parameters and tidal factors for station (Mtht is to be used with TAMURA tidal development and MTHB with CTE505).

- Grouplim.dat: separation of the tidal groups;

- Etpolut1.dat: pole coordinates and Earth rotation;

- Tdtutc.dat: difference ET - UTC ;

- Tidal potential : CTE505.dat (505 waves) or TAM1200.dat (1200 waves) ;

- Earth model : DPREMZ, HYDRO or NONHYDRO.

Answer to questions during interactive execution. Choose preferably standard options

OUTPUT

- Mt80tw.lst: program execution status

- Mt80te.out: tidal prediction with ETERNA format

- Mt80tm.out: tidal prediction for MDAS_grapher

- MT80tw.out: tidal prediction in old ICET format (12 values per record)

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1. INPUT files

1.1 STATION INPUT FILE (SSSSMTHB.dat or SSSSMTHT.dat)

1.1.1 station record

col 1- 2 tidal component : 0= gravity, 1= tilt NS*, 2= tilt EW, 3= tidal potential,

4= horizontal strain*, 5= vertical strain,

6= vertical displacement, 7= horizontal displacement*

* azimuth can be specified in col 43-52,

col 3-12 longitude, E positive,

col13-22 latitude, N positive,

col23-32 altitude in kilometres,

col33-42 gravity in gal,

col43-52 azimuth in degrees from N to E for tilt or displacement

S to E for strain,

col53-62 scale factor, normally 1,

col63-72 time correction in hours (used to compute tides in local time instead of UT).

1.1.2. header

Any number of records can be inserted.

The last one has to be 99.

1.1.3. station number

A four digit numeric code associated with the station.

1.1.4. tidal parameters (optional)

Records with tidal parameters for the selected tidal groups.

For each group, waves being numbered from 2 to 505 (CTE505) or 2-1200 (TAM1200),

col 1- 4 lower limit of the group,

col 6- 9 upper limit of the group,

col 12-17 name of the group i.e. acronym of the main tidal constituent in the group,

col 32-39 tidal factor i.e. ratio between observed and theoretical amplitude,

col 47-55 phase difference i.e. difference between observed phase and theoretical one.

1.2 Separation of tidal groups (GROUPLIM.dat)

1.3 Difference ET-UTC (TDUTC.dat*)

1.4 Pole coordinates and Earth rotation (ETPOLUT1.dat*)

1.5 Tidal potential (CTE505.dat and TAM1200.dat)

1.6 Earth tidal response (DPREMZ.dat, HYDRO.dat, NONHYDRO.dat)

* files to be regularly updated

2. DIALOG

During the execution of the program it is necessary to answer the following questions :

“ FILENAME FOR STATION SSSSMTHT.dat or SSSSMTHB.dat ”

answer with the input file name with the complete access path from the current directory (maximum 80 alphanumeric characters).

MTHT corresponds to TAM1200 and MTHB to CTE505

“ STANDARD OPTIONS : Yes or No ”

answer Y for yes or N for no.

Y : recommended option for user comfort

N : you will have to choice additional parameters i.e. :

MAT10 “base option for potential:MAT10 / 1(CTE) or 9(TAM)”

STANDARD is 9, 1 will provide quicker but lower accuracy computations

“ basic options MAT1 MAT3 ”*

MAT1 0 screen display of computations, 1 (STANDARD) output in file MT80TW.out

MAT3 0 do not consider tidal parameters in input file

>0 tidal parameters required

GRAVITY

1 inertial correction applied on tidal parameters

7 (STANDARD) no inertial correction

OTHER COMPONENT

7 (STANDARD)

“ introduce computing step : ”, “ 0= 1 hour, 1= 30minutes, 2= one minute, 99=END ”

“ initial epoch : year, month, day ”*

“ final epoch : year, month, day ”*

The program is computing full days with 24 hourly values or 48 half-hourly or 1440 minutes values. When the final epoch is entered, it stops at the end of the previous day.

e.g. : initial epoch 2001 5 6

final epoch 2001 5 7

will provide tidal data for only one day i.e. May sixth, 2001.

After computing the program branches again at “ introduce computing step : ”, “ 0= hour, 1= 30minutes, 2= one minute, 99=END ” :

to stop the program enter 99,

to compute tidal prediction for another time interval enter 0, 1 or 2,

if several time intervals are computed sequentially the results will be separated by interruption records in the output files(3.3).

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* Parameters in an input list are separated by blanks or comma's.

3. OUTPUT

Program execution status can be found in mt80tw.lst.

A 12 data per record listing is given in mt80tw.out

Data are stored in 2 different output files with slightly different format:

- ETERNA format compatible with Tsoft or ANALYZE (file mt80e.out),

- MDASGRAPHER format (file mt80m.out)

3.1. header

The header of the input file is copied.

Additional information related to the options is given as comments in the header.

3.2. data blocks

Each computed value is given in a single record as follows :

year, month, day, hour, minute, second, value in :

nm/s² (0.1µgal) ð Gravity

milliarcsecond (mas) ð Tilt

nanostrain (10^-9) ð Strain

ATTENTION : output is always multiplied by the scale factor introduced in station record of input file, see 1.1. If no tidal parameters are available it can be used to compute more realistic values taking into account the elastic response of the Earth e.g. 1.16 for gravity and 0.7 for tilt. For horizontal strain, a model is already introduced with values h=0.62 and l=0.09.

3.3. end of file and interruption

Specific interruption records are inserted between blocks as well as end of file record according to the ETERNA or MDASGRAPHER format.

REFERENCES

Cartwright, D.E. and R. J. Tayler, 1971: New computations of tide generating potential. Geophysical Journal, 23, 1.

Cartwright, D.E., Edden, A.C., 1973: Corrected tables of tidal harmonics. Geophysical Journal, 33, 3.

Ducarme B., 2006: Comparison of some tidal prediction programs and accuracy assessment of tidal gravity predictions. Bull. Inf. Marées Terrestres, 141, 11175-11184

Tamura Y., 1987: A harmonic development of the tide-generating potential. Bull. Inf; Marées Terrestres, 99, 6813-6855.