The Orbit Propagation Timing and Geometry file (OPTG) file is used by the MSOP navigators to track the orbital parameters that define the current orbit of the spacecraft. Because Mars Global Surveyor is currently in an aerobraking mode, simple closed form equations will not accurately determine the orbit. This is primarily due to the perturbation associated with the drag pass through the Martian atmosphere. For this reason, an expansion series (a Taylor series from elementary calculus is a form of an expansion series ) is used to arrive at a good approximation to the solution of the orbital equations. However, the expansion series solution is only accurate close to the time when the orbital inputs were last known. This means that an OPTG created today will only be accurate for a certain number of orbits into the future, after which time it can no longer be trusted to give correct values. It is for this reason that the Nav Team regularly creates new OPTG files.

At the beginning of the aerobraking process, the period of the orbit
was long (about 20 hours ) compared to the current period of 3.1 hours as
of 10-JAN-99. Therefore, OPTG files must now be created with greater frequency
in order to accurately determine future orbits. **Both
orbital displays** make use of the OPTG file. To generate accurate
values, the applications that drive the orbital displays must also more
frequently pull the latest OPTG file off of the Nav System. The following
file name is associated with the current OPTG file on the Nav System:

optg_i_990109_OD1029-1036_1222_V1 |

The sequence of numbers 990109 indicates that the file was created on
January 9, 1999. OD stands for orbit determination. The sequence 1029-1036
means that the parameters defining the orbit were measured
using Doppler Shift data by the Deep Space Network (DSN)
during orbits 1029 through 1036 (**Orbit Determination Measurement Interval**).
The number 1222 means that it contains solutions up through orbit number
1222. However, the values for the higher orbit numbers are not likely to
be a good approximation and should not be relied upon. *It is this
information that determines whether or not you can trust the values displayed
in the Real
Time Orbit Information page and Detailed
Orbit Information page.*

1) If the **ORBIT NO.** on the Real
Time Orbit page is not close to the **Orbit Determination Measurement
Interval** shown in the most current file listed in the **OPTG
File Download Table**, then you can not trust the values shown on
the Real Time
Orbit Information page.

2) If the **Orbit Number** on the Detailed
Orbit Information page is not close to the **Orbit Determination Measurement
Interval** shown in the gray box at the top of the display,
then you can not trust the values shown on the Detailed
Orbit Information page.

In both cases **"close to the Orbit Determination Measurement Interval"**
is defined to mean that the current **Orbit Number** should not exceed
the **Orbit Determination Measurement Interval** by more than 10 orbits.
So, if the display is based on the OPTG file containing OD1029-1036 and
the current **Orbit Number** is 1047 or greater, then it is likely that
the orbital values displayed are slightly in error.

**In either of these two cases you should send flame mail to: kirk.goodall@jpl.nasa.gov**

Starting today, every morning we will download the latest OPTG file off the Nav System and reinitialize the Real Time Orbit Information display engine. Please be advised that the Detailed Orbit Information display is under the control of the Nav Team. If this display lags too far behind we will notify them to pull down the latest OPTG file.