Gravity File Format (*.grv)
The gravity field of a central body is an extremely important constituent of the total force model on a body. Gravity field information is contained within a formatted ASCII text file; the file has a *.grv extension.
The specification of a gravitational potential has several parts. The header contains information common to all fields including the name and size of the field and some defining parameters. The specification of a tide model is optional, but recommended for high accuracy use. Of primary importance is designation of whether the gravity coefficients to follow are zero tide (including the effects of the permanent tide) or tide free values. The potential model coefficients are given in either normalized or unnormalized form in accordance with the header designation.
Header Information
The header information is required for all gravitational potential functions.
| Header Information | |
|---|---|
| Keyword | Description |
| STK version number | The version of STK software for which the file is formatted to be used. |
| Model keyword phrase | Name of the potential model. |
| Degree keyword phrase | Maximum degree of coefficients in file. |
| Order keyword phrase | Maximum order of coefficients in file. |
| Gm keyword phrase | The gravitational parameter in m3/sec2 |
| RefDistance keyword phrase | The reference distance for the potential model in meters. |
| ReferenceFrame keyword phrase | The central body fixed reference frame in which the gravity field coefficients are defined. Currently supported for lunar gravity fields, where the principal axis reference frames are typically used to specify gravity fields. Typical frames used in the definition of lunar gravity fields include PrincipalAxes_421 and PrincipalAxes_430, where the number at the end of the name corresponds to the JPL/DE ephemeris version used in the realization of the PA frame. |
| Normalized keyword phrase | Yes or No indicating whether the coefficients are normalized. |
| IncludesPermTide keyword phrase | Yes or No indicating whether the gravity coefficients, appearing later in this file, contain the effect of the permanent tide. This keyword can appear in the header block for Earth gravity fields where the IERS tide model will be used. It can also appear within the EarthIERSTideModel keyword block for Earth gravity fields. It appears within the SimpleTideModel keyword block for gravity fields about other central bodies. |
| Begin LoveNumbers Keyword phrase | No longer recommended, included for backwards compatability only. Indicates that the real and imaginary parts of the love numbers and associated degree and order will be specified on the following line (Ex: 2 0 0.3000 0.0). The next line should be End LoveNumbers. Only degree 2 order 0 is valid and only the real component is used. |
Tide Models
Tide model information is optional and may be specified using either the EarthIERSTideModel keyword block for Earth models or using the SimpleTideModel keyword block, applicable to all central bodies. The IERS tide model is the default for Earth gravity fields and will be the model used even if no tide model information is contained within the file. Further specification via the EarthIERSTideModel keyword block is only necessary if a modification to the IERS recommended k20 love number of 0.3 is desired. If the EarthIERSTideModel keyword block is not included in the specification of an Earth gravity field, it is important the IncludesPermTide keyword be appropriately specified in the header.
EarthIERSTideModel Keyword Block
| EarthIERSTideModel Keyword Block | |
|---|---|
| Keyword | Description |
| BEGIN EarthIERSTideModel Keyword phrase | Indicates the start of the IERS tide model specification. This block should only be used for Earth gravity fields. |
| LoveNumbers Keyword phrase | Indicates that the real and imaginary parts of the love numbers and associated degree and order will be specified. Only degree 2 order 0 is valid and only the real component is used. |
| IncludesPermTide Keyword phrase | Yes or No indicating whether the gravity coefficients, appearing later in this file, contain the effect of the permanent tide. For the Earth, only the C2,0 term is affected. |
| END EarthIERSTideModel Keyword phrase | Indicates the end of the IERS tide model specification. |
SimpleTideModel Keyword Block
| SimpleTideModel Keyword Block | |
|---|---|
| Keyword | Description |
| BEGIN SimpleTideModel Keyword phrase | Indicates the start of the simple tide model specification. This block should be used to specify tide model information for central bodies other than the Earth. |
| K2 Keyword phrase | The k2 love number. If not present, the value of 0.0 will be used. |
| K3 Keyword phrase | The k3 love number. If not present, the value of 0.0 will be used. |
| Lamda Keyword phrase | The lag in the tide, expressed as an angle in radians. |
| IncludesPermTide Keyword phrase | Yes or No indicating whether the gravity coefficients, appearing later in this file, contain the effect of the permanent tide. |
| Body Keyword phrase | Specifies the name of a central body to be considered as having a tidal influence. May be repeated as needed. |
| BEGIN TideFreeValues Keyword phrase | Indicates the start of a block of tide free values of coefficients. Tide free values may be specified if the model coefficients are being specified as zero tide values (inclusive of the permanent tide effect) to provide precise control over the application of the tide model. Data lines in this block are formatted identically to the data lines in the model coefficients block. |
| END TideFreeValues Keyword phrase | Indicates the end of the tide free values specification. |
| BEGIN ZeroTideValues Keyword phrase | Indicates the start of a block of zero tide values of coefficients. Zero tide values may be specified if the model coefficients are being specified as tide free values (not inclusive of the permanent tide effect) to provide precise control over the application of the tide model. Data lines in this block are formatted identically to the data lines in the model coefficients block. |
| END ZeroTideValues Keyword phrase | Indicates the end of the zero tide values specification. |
| END SimpleTideModel Keyword phrase | Indicates the end of the simple tide model specification. |
Model Coefficients
The coefficients are given in either normalized or unnormalized form in accordance with the header designation.
| Model Coefficients | |
|---|---|
| Keyword | Description |
| BEGIN Coefficients Keyword phrase | Indicates the start of the coefficients (CS). |
| Data Point | Each line of the coefficients block contains the <Degree> and <Order> followed by the associated <C> and <S> coefficients. <Degree> always starts at 2 and goes to the highest degree defined in the header, while <Order> starts at 0 and goes to the highest order defined in the header. If a particular pairing (n,m) is not specified in the file, where n <= highest degree and m <= highest order, then the corresponding C and S coefficients are considered to be 0.0. |
| END Coefficients Keyword phrase | Indicates the end of the coefficients (CS). |
Gravity Process Noise
The uncertainty in the gravity model must be provided in order for model specific gravity process noise to be computed. Gravity process noise can be applied in the propagation of covariance matrices. This information is not needed for propagation of the trajectory, propagation of covariance matrices without the addition of gravity process noise or propagation of covariance matrices with the addition of the default gravity process noise model. Process noise information is contained within a keyword block delimited as shown in the following table.
| Gravity Process Noise | |
|---|---|
| Keyword | Description |
| BEGIN ProcessNoise Keyword phrase | Indicates the start of process noise information. |
| END ProcessNoise Keyword phrase | Indicates the end of process noise information. |
Model Uncertainty
The uncertainty in the gravity model coefficients are specified in terms root variances expressed in milligals.
| Model Uncertainty | |
|---|---|
| Keyword | Description |
| BEGIN ErrorRootVariances Keyword phrase | Indicates the start of the coefficient uncertainty information within the process noise keyword block. |
| Data Point | Each line of the ErrorRootVariance block contains the |
| END ErrorRootVariances Keyword phrase | Indicates the end of the coefficient uncertainty information within the process noise keyword block. |
Correlation Time Integrals
The gravity process noise model depends on the use of proper correlation times for errors of omission and errors of comission. These correlation times are computed based on approximating polynomials where a unique set of polynomials are provided for each possible truncation of the model. For a specific degree of truncation, four polynomials are specified: one for radial and cross-track errors of commission, one for in-track errors of commission, one for radial and cross-track errors of omission and one for in-track errors of omission. The independent variable of the polynomials is the distance from the planet center expressed in planetary radii.
| Correlation Time Integrals | |
|---|---|
| Keyword | Description |
| BEGIN I1Polynomials Keyword phrase | Indicates the start of the correlation time information within the process noise keyword block. |
| Data Point | Each line of the I1Polynomials block contains the |
| END I1Polynomials Keyword phrase | Indicates the end of the correlation time information within the process noise keyword block. |
The following is a sample Earth gravity file:
stk.v.6.0 Model EGM96 Degree 70 Order 70 Gm 3.986004415E+14 RefDistance 6378136.3 Normalized Yes IncludesPermTide No # # NOTE: The 22 March 2006 update to the IERS Conventions (2003), # Technical Note 32 no longer differentiates the official # k20 love number used by EGM96 (whose value is 0.30000) from the IERS # value used in deriving the its solid tide model # (which uses k20 = 0.30190) when applying its solid tide model. # By default, the IERS value is used. # If the user wishes to use the official EGM96 k20 value, then # uncomment out the following lines by removing the leading # # #Begin EarthIERSTideModel # IncludesPermTide No # LoveNumbers 2 0 0.30000 0.0 #End EarthIERSTideModel # # Note: The gravitational parameter and reference distance defined # within this file are only used in gravity field calculations, # they are not applied to the central body # Source: http://cddisa.gsfc.nasa.gov/926/egm96/readme.egm96 # Source: http://cddisa.gsfc.nasa.gov/926/egm96/egm96.html # BEGIN Coefficients 2 0 -4.841653717360e-04 0.000000000000e+00 3 0 9.572541737920e-07 0.000000000000e+00 4 0 5.398738637890e-07 0.000000000000e+00 5 0 6.853234756300e-08 0.000000000000e+00 6 0 -1.499579947140e-07 0.000000000000e+00 7 0 9.097893714500e-08 0.000000000000e+00 8 0 4.967116673240e-08 0.000000000000e+00 9 0 2.767143008530e-08 0.000000000000e+00 10 0 5.262224885690e-08 0.000000000000e+00 11 0 -5.096137075220e-08 0.000000000000e+00 . . . 70 68 -3.183171226730e-09 1.844470166990e-09 70 69 -2.203530405520e-09 9.940130691520e-10 70 70 -4.703751388260e-10 -6.483061378330e-10 END Coefficients
The following is a sample Lunar gravity file:
stk.v.6.0
Model LP100K
Degree 100
Order 100
Gm 4.90280023800e+012
RefDistance 1.73800000000e+006
Normalized Yes
IncludesPermTide No
Begin SimpleTideModel
IncludesPermTide Yes
k2 0.03
k3 0.0000
Lamda 0.0000
Body Earth
Body Sun
Begin TideFreeValues
2 0 -9.08737911083000e-005 0.00000000000000e+000
2 2 3.46255299432000e-005 1.14220072938000e-008
End TideFreeValues
End SimpleTideModel
BEGIN Coefficients
2 0 -9.09242692346000e-005 0.00000000000000e+000
2 1 8.38314678481000e-009 7.70654861339000e-009
2 2 3.47130031541000e-005 1.14220072938000e-008
3 0 -3.17936010672000e-006 0.00000000000000e+000
3 1 2.63488799252000e-005 5.46699265983000e-006
.
.
.
100 98 1.78631398823000e-007 1.21939810983000e-007
100 99 3.76004697898000e-008 -3.88808101803000e-007
100 100 -9.44185641598000e-008 2.41042325838000e-008
End Coefficients
BEGIN ProcessNoise
BEGIN ErrorRootVariances
#
# degree, omission(mgal), commission(mgal)
#
2 15.7841 0.00147306
3 10.8334 0.00481507
.
.
.
99 20.914 5.54801
100 19.0221 5.31415
END ErrorRootVariances
BEGIN I1Polynomials
1 0 0.0
0 0.0
5 -108173.5991 354340.0647 -459350.1035 296204.8228 -94654.1545 11998.9284
5 -203634.2681 658555.5212 -844673.1151 540060.6513 -171322.6634 21581.2126
2 5 -37.5126 392.7222 542.2504 -123.4510 25.0298 -2.3919
5 -79.3377 830.5961 1146.8490 -261.0977 52.9381 -5.0589
5 -58272.3331 183310.7889 -229554.7611 143997.8041 -44929.1475 5578.6753
5 -84416.6858 264016.2817 -329627.4490 206904.6392 -64635.8213 8039.0587
3 5 -35.4860 343.7082 360.9908 1.1422 -8.2141 0.9844
5 -76.7510 708.8905 669.1933 48.7384 -25.7840 2.5934
5 -25164.5844 79102.7046 -99356.1541 62786.3692 -19731.5328 2467.1668
5 -58428.8202 184385.0764 -232286.4240 147066.4888 -46287.6182 5794.6683
.
.
.
99 5 5661.4470 -28340.2517 51015.7584 -42736.1499 17008.4438 -2558.5367
5 -11480.0746 28917.8522 -24673.2619 6779.2666 1074.2592 -518.0863
5 -1.0038 10.5089 14.5102 -3.3035 0.6698 -0.0640
5 -1.9547 20.4640 28.2554 -6.4327 1.3042 -0.1246
100 5 5623.6185 -28213.4224 50847.5610 -42625.8245 16972.6373 -2553.9346
5 -11555.5669 29170.9800 -25008.9786 6999.4885 1002.7809 -508.8989
0 0.0
0 0.0
END I1Polynomials
END ProcessNoise