implications of errors in density response time delay on satellite prediction error
DESCRIPTION
Implications of Errors in Density Response Time Delay on Satellite Prediction Error. Rodney L. Anderson and Christian P. Guignet. October 28, 2010, NADIR MURI meeting . Introduction. Orbit prediction relies on the prediction of density from atmospheric models. - PowerPoint PPT PresentationTRANSCRIPT
Implications of Errors in Density Response Time Delay on Satellite Prediction Error
Rodney L. Anderson and Christian P. Guignet
October 28, 2010, NADIR MURI meeting
Introduction Orbit prediction relies on the prediction of
density from atmospheric models. Model predictions can sometimes be
inaccurate, especially during magnetic storms. A previous study by Forbes showed a
premature increase in density predicted by a model caused a as large an error as predicting no variation. J. M. Forbes, “Low-Altitude Satellite Ephemeris
Prediction” Delays of 1-4 hours are not uncommon.
This study seeks to quantify these errors by examining time delays in the density model.
Introduction A simple two body model was used for the
spacecraft integration. Acceleration due to drag:
Velocity relative to the atmosphere:
Introduction Atmospheric model used: NRLMSISE-00
Model densities were computed using values provided from the CHAMP spacecraft.
Densities obtained from observations by the CHAMP satellite were used as truth. Density data used spans 2003 through 2008. CHAMP was in a low Earth orbit with inclination of
approximately 87° and initial altitude of 457 km.
http://science.nasa.gov/media/medialibrary/2010/03/31/champ.jpeg
Outline Introduction
Storms in data
Perfect model comparisons
Delays in model data
Delays in real, smoothed data
Storms in Data Multiple storms are examined to determine possible delays
between model prediction and truth density. The model densities are computed using the CHAMP altitude
and position. A 701 point smoothing is then used to observe the peaks in
the densities.
•October, 2003 – 1.5 hours•November, 2003 – 1.68 hours•July, 2004 – 3.15 hours•November, 2004 – 3.18 hours•Used in delayed orbit study.
Outline Introduction
Storms in data
Perfect model comparisons
Delays in model data
Delays in real, smoothed data
Perfect Model Comparison A spacecraft is integrated twice:
First orbit uses model densities. Second uses CHAMP densities. Initial 400 km, polar orbit.
24 hour integrations are performed over years 2003-2008.
Results are given in the Radial, In-track, and Cross-track directions.
Perfect Model Comparison Largest differences occur in the in-track direction. Errors are in agreement with previous study by Anderson et al.
R. L. Anderson, G. H. Born, and J. M. Forbes, “Sensitivity of Orbit Predictions to Density Variability” Differences are largest during more active times.
Outline Introduction
Storms in data
Perfect model comparisons
Delays in model data
Delays in real, smoothed data
Delays in Model Data How can delays in predicting the density
effect a satellite’s orbit? Delays were introduced into the model by
altering the inputs by a number of hours. 1, 2, and 3 hour delays are examined.
A spacecraft was again integrated as before: Once using perfect model inputs. A second time using the delayed inputs.
Simulation performed for the year 2003.
One hour delay A delay of one hour is added to the model inputs. Largest difference again occurs in the in-track direction Cross-track difference is significantly less than a meter.
Two hour delay The simulation was performed using a 2 hour
delay. Same general behavior was observed with
larger magnitudes.
Three hour delay Behavior similar to 1 and 2 hour delays. The large spikes in the orbit differences occur
during large storms.
Delays in Model Data Very large errors can occur (thousands of
meters).
Time Delays (h) Radial (m) In-track (m) Cross-track (m)
1 0.777 37.94 0.009
2 0.705 32.92 0.007
3 0.998 46.09 0.0104
Mean errors are significant as well (tens of meters).
Mean values of Orbit differences
Outline Introduction
Storms in data
Perfect model comparisons
Delays in model data
Delays in real, smoothed data
Delays in Real, Smoothed Data Quantify the effect of the time delay on orbit
prediction using real-world density fluctuations.
Perform the same simulation before using different densities: First orbit integrated using smoothed CHAMP
density. 701 point smoothing used to remove short term
variations. Second orbit integrated using same density
delayed by a specified amount of time. Simulation performed over 2003-2008
One hour delay
Two hour delay
Three hour delay Orbit differences in this simulation are similar
to those seen in the model simulation. Maximum differences occur during times of
high geomagnetic activity.
Delays in Real, Smoothed Data Maximum errors can reach thousands of
meters. Mean errors are smaller but still significant.
Time Delays (h) Radial (m) In-track (m) Cross-track (m)
1 0.395 20.58 0.0041
2 0.63 40.92 0.0081
3 0.79 61 0.012
Mean values of Orbit differences
Conclusions Model predictions of density can lag behind
actual density values, especially during times of high geomagnetic activity. Delays of several hours are possible.
Density predictions can have a significant effect on satellite orbit predictions.
Orbit errors due to density delays can reach thousands of meters. Mean values of the orbit errors are still significant.