RMS of fit to the Historical Data

Since we have used nearly all the available MGS data, the only quasi-independent data are the S Band data from the Viking Orbiters and Mariner 9. It is a necessary condition that the new models including MGS data obtain comparable fits to solutions based solely on the historical data, for instance the GMM-1[29], and Mars50c[30] solutions. Both GMM-1 and Mars50c are 50x50 solutions. A series of test arcs of the Mariner 9 and Viking Orbiter data were selected for each distinct orbital inclination and periapse altitude. The orbit geometry and the number of test arcs selected are listed in Table 9. All the orbits were highly eccentric, and the orbit periods ranged from 12 to 24 hours[13]. Two observations are readily apparent upon examination of the RMS of fit for these test arcs. First, the RMS of fit for all the new models with MGS is broadly similar to the GMM-1 and Mars50c models. In fact, for the low altitude periapse data (300 km), the fit to the Viking-1 and Viking-2 data is superior with the MGS derived models. Even the models that exclude all the historical data (MGM0964C18 and MGM0964C20) fit these low altitude data better than GMM-1 and Mars50c. The second observation pertains to the fit to the high altitude periapse data, in particular those from Mariner 9 and Viking Orbiter 1. Here the fits are degraded with the MGS derived models, compared to GMM-1 and Mars50c. So these results suggest an inconsistency in the long-wavelength gravity field to which MGS and the Viking Orbiters and Mariner 9 are sensitive. This inconsistency may reflect a discordance in reference frames, and an inadequacy in the 1991 IAU planet orientation model[17]. New solutions for the Mars orientation are available based on simultaneous analysis of Viking Lander and Pathfinder tracking which could be used instead of the 1991 IAU model.

Table 9: Mariner 9 and Viking Orbiter Orbit Summary
Spacecraft	Periapse	Inclination	No. of Test
	Ht. (km)	(deg)	Arcs
Mariner 9	1600	64	6
Viking-1	1500	39	6
Viking-1	300	39	4
Viking-2	1500	55 & 75	2
Viking-2	800	80	7
Viking-2	300	80	9

Table 10: Mariner 9 and Viking Orbiter: RMS Summary
Orbit	Gravity Models: (RMS of fit in mm/s)
	GMM-1	Mars50c	0890	0890I	0964C5A	0964C18	0964C20
Mariner 9	1.52	1.47	1.96	2.72	1.93	1.73	1.89
VO-1, 1500 km	2.36	2.42	2.35	2.56	4.20	4.20	4.03
VO-1, 300 km	1.77	1.55	1.48	1.63	1.74	1.54	1.62
VO-2, 1500 km	1.93	2.19	1.54	1.56	1.67	1.97	1.82
VO-2 800 km	1.84	1.80	1.79	1.85	2.82	2.41	2.00
VO-2 300 km	1.16	1.17	0.90	0.86	0.84	0.78	0.78