Some physics

Orbital propagation model includes:1. Models for the Earth atmosphere (estimates of the Earth atmosphere i.e. composition, density etc. at a given LAT/LONG/HEIGHT position)2. Models for the Earth gravity (estimates of the Earth gravity at a given LAT/LONG position)3. Models for the Earth magnetic field (estimates of the Earth magnetic field at a given LAT/LONG position)

Problems: All the three above are constantly changing and the models have to be periodically updated with the new coefficients Solar pressure is hard to be modelled and can change very fast i.e. a solar storm occurs

There are two types of propagator models:1. Using numerical solutions (the real physical models, very accurate but heavy computation)2. Using analytical solutions (simplified models, light for computation, good solution for short term estimations but not so good for long term i.e. the deviation increases exponentially with time)We use the version 2 based on the SGP/SDP 4 model and a solution with the re-initialization of the condition with the orbital position loaded as a TLE (two-line-elements) file.The TLEs are broadcasted daily by NORAD and can be retrieved automatically from the website: https://www.space-track.org/auth/login

Each number is in a specified fixed column. Spaces are significant. The last digit on each line is a mod-10 check digit, which is checked by the program. The program also checks the sequence numbers (first column), and checks each orbital element for reasonable range. This is a very good set of checks, so this format is very safe, and robust.There seems to be some disagreement about how the "+" character is figured into the check digit. If we have trouble with checksum failures on element sets with "+" signs in them, its better to try replacing all the "+" signs with spaces.Data for each satellite consists of three lines in the following format: AAAAAAAAAAA1 NNNNNU NNNNNAAA NNNNN.NNNNNNNN +.NNNNNNNN +NNNNN-N +NNNNN-N N NNNNN2 NNNNN NNN.NNNN NNN.NNNN NNNNNNN NNN.NNNN NNN.NNNN NN.NNNNNNNNNNNNNN

Line 1Line 1 is a eleven-character name.Actually, there is some disagreement about how wide the name may be. Some programs allow 12 characters. Others allow 24 characters, which is consistent with some NORAD documents.Some sources encode additional information on this line, but this is not part of the standard format. There is no checksum on this line.

Line 2Line 2

ColumnDescription

01Line number of element data

03-07Satellite number

08Classification (U=Unclassified)

10-11International designator (Last two digits of launch year)

12-14International designator (Launch number of the year)

15-17International designator (Piece of the launch)

19-20Epoch year (last two digits of year)

21-32Epoch (Day of the year and fractional portion of the day)

34-43First time derivative of the mean motion divided by 2 or ballistic coefficient (depending on the ephemeris type)

45-52Second time derivative of the mean motion divided by 6 (decimal point assumed)

54-61BSTAR (B*) drag term (decimal point assumed) if SGP4 general perturbation theory was used and radiation pressure coefficient otherwise

63Ephemeris type

65-68Element type

69Checksum (Modulo 10) (Letters, blanks, periods, plus signs=0; minus signs=1)

9.2 TLE file line 2

The checksum is computed as follows: Start with zero. For each digit in the line, add the value of the digit. For each minus sign, add 1. For each plus sign, add 2 (or maybe 0, depending on who created the element set and when) For each letter, blank, or period, don't add anything. Take the last decimal digit of the result (that is, take the result modulo 10) as the check digit. All other columns are blank or fixed.Note that the International Designator fields are usually blank, as issued in the NASA Prediction Bulletins.

Line 3

Line 3

ColumnDescription

01Line number of element data

03-07Satellite number

09-16Inclination (degrees)

18-25Right ascension of the ascending node (degrees)

27-33Eccentricity (decimal point assumed)

35-42Argument of perigee (degrees)

44-51Mean anomaly (degrees)

53-63Mean motion (revs per day)

64-68Revolution number at epoch (revs)

69Checksum (Modulo 10)

9.3 TLE file line 3

The same checksum algorithm is used.All other columns are blank or fixed.

Example Following are the Two-Line Elements and Translated Orbital Data for the Bird satellite as of 2004/05/11 (21:00:29 GMT):

BIRD1 26959U 01049C 04310.87534173 .00002752 00000-0 016752-3 0 042 26959 97.7004 43.4227 0017968 93.1216 267.2070 15.11048608167322

Name.........................................................BIRDNORAD ID#.............................................26959Epoch Year...............................................2004Epoch Day................................................310.87534173 Altitude (km)............................................536.7Period (min)..............................................95.298Velocity (km/s).........................................7.59Inclination (degrees).......................97.7004Right Ascension of AscendingNode (RAAN, degrees)..............................43.4227Eccentricity.................................................0.0017968Argument of Perigee (degrees)...................93.121Mean Anomaly (degrees)............................267.207Mean Motion (revs. per 24-hr. day)............15.110486Ballistic coefficient.0.00002752Epoch Revolution (since Zarya launch)..... 81673

Space Alliance propagator page current state

Space Alliance propagator page - future architecture

Data retrievalSpace Track website

Satellite data table

TLE dataSat nameSat ID

Google EarthGoogle EarthGoogle EarthJava AppletQuery the dataDisplay interfaceSearch interfaceMySQL databaseSat IDSat nameTLE data

Some satellite tracking pages as exampleshttp://www.heavens-above.com/Satellites.aspx?lat=0&lng=0&loc=Unspecified&alt=0&tz=UCT

http://www.n2yo.com/database/