PDS_VERSION_ID      = PDS3                                                    
RECORD_TYPE         = FIXED_LENGTH                                            
RECORD_BYTES        = 80                                                      
OBJECT              = TEXT                                                    
  PUBLICATION_DATE    = 1998-04-11                                            
  NOTE                = "Description of GEODYN SPK Files"                     
END_OBJECT          = TEXT                                                    
END                                                                           
                                                                              
GEODYN SPK files (data type GDN) are QUICK LOOK orbits produced by the        
MOLA POD (Precise Orbit Determination) team.  They are used for preliminary   
analysis of the MOLA data, and have been made available to other MGS          
project participants.  They are produced on a best efforts basis, as time     
and resources permit.                                                         
                                                                              
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.                                               
                                                                              
File names have the format  gdn_qlnn.ymmddhhmm.ymmddhhmm.tsp  where the       
components refer to:                                                          
   gdn          derived from analysis using the GEODYN orbit processor;       
   ql           quick-look;                                                   
   nn           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/quick-look-orbits/  and retrieve the data of          
interest.                                                                     
                                                                              
Each orbit was produced from a preliminary analysis of the Doppler            
and range data supplied by the DSN.  They are preliminary because             
certain ancillary information may have been missing at the time the           
orbits were reconstructed -- for example, the dates and times of              
momentum dumps, weather data (used for Earth troposphere refraction           
corrections), and finalized Earth orientation (polar motion) values.          
The  a priori  modeling included the GMM-1 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 JPL DE403           
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.                                                                  
                                                                              
The orbits will be refined on a schedule yet to be determined.                
                                                                              
Questions about the GDN files may be directed to:                             
       F.G. Lemoine    flemoine@olympus.gsfc.nasa.gov   or                    
       D.E. Smith      dsmith@tharsis.gsfc.nasa.gov