MATLAB implementation

In this implementation we use object-oriented approach. It means that we 
create new data types - objects that have similar behaviour as Matlab 
built-in data types. For example, for all date and time operations we 
created class DateTime, that enables to store and convert different date 
and time formats. Command epo = DateTime(2005, 1 , 7, 14, 0, 0); 
creates object epo with properties (variables within the object) that store 
the input arguments: year, month, day, hour, minute, second. The 
command also calls methods (functions defined for class) that use the 
input arguments to compute GPS week, seconds within GPS week, 
modified Julian date and day of the week. If you then type command epo, 
Matlab automatically calls method display, which displays properties of 
epo:

>> epo
Year                : 2005
Month               : 1
Day                 : 7
Hour                : 14
Minute              : 0
Seconds             : 0.00000
GPS week            : 1304
Seconds of GPS week : 482400.00000
Day of week         : 5
MJD                 : 53377.58333

The class DateTime has defined operators plus and minus; so for example 
finding number of seconds between epochs epo1 and epo2 is as easy as 
typing command epo1 - epo2. Complete functionality of all classes can 
be found in the published code.
The first step in computation of satellites' coordinates is reading in 
ephemeris. For that purpose we designed class CoordEph. To read in 
precise ephemeris, first we have to create an object of class CoordEph:

>> PEph = CoordEph;

Then we call method that reads in data from specified file(s) and store 
them in PEph:

>> PEph = ReadEphPrecise(PEph, 'filename');

It is possible to read in more files using this method by adding more 
arguments, for example:

>> PEph = ReadEphPrecise(PEph, 'filename1', ' filename2');

where filename is name of the file containing precise ephemeris in sp3-
format.
There is a similar method for reading in broadcast ephemeris:

>> BEph = CoordEph;
>> [BEph, OrbPar] = ReadEphBroadcast(BEph, 'filename', Interval);

This method reads in broadcast ephemeris from RINEX-format file 
filename, computes tabular orbit in Interval seconds interval and store 
them in BEph. It also stores all available orbit parameters in object 
OrbPar. 
In the next step we compute standard orbits using method CompStdOrb:

>> StdPe = StdOrb;
>> StdPe = CompStdOrb(StdPe, PEph, stTgd);

This method fits polynomial function to all possible fit intervals in PEph 
and stores the coefficients in object StdPe. The order of polynomial, the 
length of fit and validity interval is specified in the definition 
(constructor) of class StdOrb. The standard orbit for broadcast ephemeris 
is computed in the same way.
 
Finally, the coordinates of a satellite for a given epoch are computed as:

>> coords = GetSatCoord(StdPe, Prn, epo);

Where Prn is satellite number, epo is DateTime object that specifies for 
which epoch should the coordinates be computed. The method returns 
structure containing coordinates and velocity in Cartesian coordinate 
system and satellite clock correction.