ORBVIEW-2 One Year Orbit Performance Report

Space Exploration Engineering, Inc. planned and executed the orbit-raising task for the SeaStar (ORBVIEW-2) spacecraft launched August 1, 1997. During the previous three years, SEE worked as a subcontractor to Orbital Sciences Corporation (OSC) to design the final target orbit, develop the Rapid Response Mission Analysis Tools (RR-MAT) to allow real-time orbit raising decision to be made, and develop the operational scenario and constraints for the orbit raising task. The following is a re-examination of the final imaging orbit obtained, the performance of the orbit over the first year of operations, an evaluation of the models used for prediction, and a prediction of orbit performance for years 2 through 5. ORBVIEW-2 is owned and operated by ORBIMAGE and this analysis was performed under contract to ORBIMAGE.

 

1.0        Final Imaging Orbit

1.1        Requirements on final orbit

The executed burn plan, using 32 separate motor burns without violation of any of the mission operations constraints, raised the spacecraft to the final target orbit in a period of 31 days while maintaining ample margins in all parameters of interest. Figure 1.3 contains the targeted final orbit, allowable errors, actual final orbit, and actual errors.

 

Mean Orbital Parameters

Predicted Insertion Orbit

Actual Insertion

Orbit

Error

Semi Major Axis

6733.4 km

6679.4 km

-54 km

Eccentricity

0.0061

0.0001

-0.006

True Anomaly

unconstrained

200.0 deg.

N/A

Arg. Of Perigee

unconstrained

241.9 deg.

N/A

Inclination

98.2175 deg.

98.274 deg.

+0.0565 deg.

Longitude of Node

307.675 deg.

307.8 deg

+0.125 deg.

Altitude at Perigee

310.0 km

299.9 km

-10 km

Altitude at Apogee

400.0 km

302.5 km

-97.5 km

Mean Time of Node

11:47:00 AM

11:47:29 AM

+29 sec

Epoch

 

8/2/1997 6:44:00 UTC

 

Table 1.1

 

Thus the orbit achieved was, for all practical purposes, identical to the desired orbit.

 

Orbit Performance at Year 1

Predicted vs. actual

Figures 2.1.1 through 2.1.5 compare the actual orbital performance with predicted performance for the first 345 days of operations at the final imaging orbit. In all charts, the discontinuity near day 110 is due to missing data. Orbit data were provided by an on-board GPS receiver and processed via the Oasys Orbit Determination software provided by Integral Systems Incorporated. Predictive data were produced by a module of SEE's Rapid Response Mission Analysis Tools (RR-MAT) that is a derivative of the NASA Jet Propulsion Laboratory's (JPL) standard "POLOP" code.

 

Figure 2.1.1 OrbView II Actual vs. Predicted Mean Apogee and Perigee Altitude Histories

 

The mean perigee and apogee altitudes (i.e. the eccentricity) are oscillating almost exactly as predicted. The small variations are accounted for by the limitations of modeling and orbit determination accuracy.

Figure 2.1.2 OrbView II Actual vs. Predicted Mean Argument of Perigee History

 

The argument of perigee is, by design, "frozen" near the pole. This assures that, regardless of orbit number, a relatively constant imaging altitude is obtained for any given latitude. Figure 2.1.3 indicates that the Lunar, Solar, and Earth gravitational perturbations on inclination are being modeled properly. Note that the increased short-term amplitude variations between actual and predicted values are on the order of 0.0005 degrees and reflect the limits of orbit determination accuracy.

 

Figure 2.1.3 OrbView II Actual vs. Predicted Inclination History

 

Figure 2.1.4 Actual vs. Predicted Mean Descending Node Local Time of Day History

 

Figure 2.1.5 Orbview II Actual vs. Predicted Mean Semi-Major Axis History

 

The original estimates of semi-major axis predicted the spacecraft would be approximately 200 meters lower at day 345. This is due to a combination of an conservative estimates of both atmospheric density and the average spacecraft projected area subject to atmospheric drag. Since atmospheric density is notoriously difficult to predict, the model allows for it to be adjusted. Thus, the atmospheric density was adjusted to match what was observed, and then used to predict the five year orbit performance.

3. Five Year Performance Predictions

 

3.1 Five year predictions

 

Using the corrected atmospheric density, the five-year predicted performance of the Orbview-2 orbit was calculated using the RR-MAT software and is summarized in Figures 3.1.1 through 3.1.5

 

Figure 3.1.1 Predicted OrbView-2 Five Year Mean Perigee and Apogee Altitude Histories

 

Figure 3.1.2 Predicted OrbView-2 Five Year Mean Argument of Perigee History

 

Figure 3.1.3 Predicted OrbView-2 Five Year Mean Inclination History

 

Figure 3.1.4 Predicted OrbView-2 Five Year Mean Descending Node Local Time of Day History

Figure 3.1.5 Predicted OrbView-2 Five Year Mean Semi-Major Axis History

3.2 Comments and Recommendations

The orbit is performing as designed and should require no trim maneuvers at any point in the nominal five year mission. Ten year predictions (not shown here) using the RR-Mat Software, show that the orbit will stay within mission tolerances for the duration of an extended mission as well.