PDS_VERSION_ID      = PDS3                                                    
RECORD_TYPE         = FIXED_LENGTH                                            
RECORD_BYTES        = 80                                                      
OBJECT              = TEXT                                                    
  PUBLICATION_DATE    = 1998-06-26                                            
  NOTE                = "Description of Final GEODYN SPK Files"               
END_OBJECT          = TEXT                                                    
END                                                                           
                                                                              
Final GEODYN SPK files (data type GDF) are FINAL orbits produced by the       
MOLA POD (Precise Orbit Determination) team.  They are used for final         
analysis of the MOLA data, and have been made available to other MGS          
project participants.  They are usually improvements over corresponding       
GDN (quick-look) GEODYN SPK files.                                            
                                                                              
Each file is in SPK format, compatible with readers in the SPICE toolkit      
(see companion files TK_FTP.TXT, TK_DESC.TXT, and TK_INSTL.TXT in this        
DOCUMENT directory for more information on the SPICE toolkit and files).      
The data were orginally written out by GEODYN (the orbit processor) at        
five or ten second intervals.  They were converted from the original          
true of date (IAU of date) coordinates to J2000, and then were written        
in an SPK-format (type 8, discrete states, evenly spaced, Lagrange            
Interpolation).  A TOPEX-like/POE (ASCII) format is available upon            
request from the MOLA POD Team.                                               
                                                                              
Original file names have formats like  gdf_vnnn.ymmddhhmm.ymmddhhmm.tsp       
where the components refer to:                                                
   gdf          derived from analysis using the GEODYN orbit processor;       
   vnnn         version number;                                               
   yymmddhhmm   start and stop times                                          
   tsp          suffix designating an SPK transfer file                       
                                                                              
Files can also be obtained via anonymous ftp from  geodesy.gsfc.nasa.gov      
(node number 128.183.201.91).  Change directories (Unix command "cd")         
to directory  /dist/MGS/final-orbits/  and retrieve the data of               
interest.                                                                     
                                                                              
Each orbit was produced from an analysis of the Doppler and range data        
supplied by the DSN.  It is the opinion of the MOLA POD team that no          
significant improvements can be made in these orbits; but that does           
not preclude release of new versions of these files at a later date.          
Modeling includes the current best Mars gravity field, the 1991 IAU           
model, the Stewart (LASP-JPL Internal Rep., PO\#NQ-802429, Jet Propulsion     
Laboratory, Pasadena, CA, March 1987) drag model, and the current best        
JPL planetary ephemeris.  For each arc, the state vector, a drag              
coefficient, and pass by pass range biases were estimated.  Empirical         
parameters were adjusted at regular intervals to absorb orbit error and       
other mismodeling.                                                            
                                                                              
Files released in late June 1998 span the MGS Assessment Phase in             
October-November 1997, when aerobraking had been temporarily suspended.       
The gravity model used was MGM0827E, a solution combining MGS tracking        
data from the Assessment Phase and first Science Phasing Orbit (SPO-1)        
with earlier Viking Orbiter and Mariner 9 data.  Comparing orbits at          
points where they overlap yields the following errors (radial, cross-,        
and along-track rms in units of meters), which are indicative of the          
accuracy in the solutions:                                                    
                                                                              
delout.mgs971013drL.6b.mgs971016drL.6b:  16.212189  52.025091  256.61398      
delout.mgs971016drL.6b.mgs971019drL.6b:  34.114237  22.612096  150.13345      
delout.mgs971019drL.6b.mgs971022drL.6b:  63.274255 103.24701   784.84708      
delout.mgs971022drL.6b.mgs971025drL.6b:  56.340671 130.60698  1309.5290       
delout.mgs971025drL.6b.mgs971028drL.6b: 189.50996    4.079704  118.75692      
delout.mgs971028drL.6b.mgs971031drL.6b:  28.034890  23.903310  506.09557      
                                                                              
Questions about the GDF files may be directed to:                             
       F.G. Lemoine    flemoine@olympus.gsfc.nasa.gov   or                    
       D.E. Smith      dsmith@tharsis.gsfc.nasa.gov