Problems encountered in developing a high speed trajectory data processor for the shuttle ascent and entry phases are described. The development of a 19 state acceleration filter for the processor is reported

Lear, W. M.

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Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19770011254 2020-03-22T10:29:04+00:00Z
JULY 1976 YEARLY FINAL. PROGRESS REPORT FOR
"SPACE SHUTTLE NAVIGATION FILTER DEVELOPMENT"
by:
Wm. M. Lear
The purpose of this contract is to assist NASA/JSC in developing naviga-
tion software for the Space Shuttle. The purpose of this report is to sum-
marize the work done from 1 July 1975 to 31 July 1976.
Early in the reporting period a major effort was expended in finding
simple but accurate refraction corrections for radar tracking data for
vehicles in and out of the atmosphere and for low to negative elevation
angles. During the course of this study it was found that NASA was computing
atmospheric scale height incorrectly.
The noise statistics of the NASA/FRC FPS-16 radar were analyzed from
skin tracking data of the Skylab. This radar is a prime radar for the
Orbital Flight Tests. The noise statistics are needed for simulations and
or the real-time filter inputs.
In September 1975 we looked at the effect of very large, unmodelled IMU
errors on navigation accuracy during reentry and landing. The unmodelled errors
were:
0.7 deg/hr gyro drift
12000 ppm accelerometer scale factor errors
1500 jig accelerometer bias errors 	 -
Briefly, the gyro drift and accelerometer bias errors caused the navigation
filter very little trouble. The scale factor errors required a manual over-
	
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ride of the residual edit in order to prevent data lock-out, which caused
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the filter to diverge.
In October a major technical report on methods of generating the state
transition matri, was completed. In the middle of October a week long meeting
	 J
on the onboard shuttle software was attended at Rockwell International at
Downey, California. Also a two day trip to Houston was made to discuss the
development of new ground navigation software for the shuttle ascent and entry.
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(NASA-CR-151178) SPACE SHUTTLE NAVIGATION N77 -18196
FILTER DEVELOPMENT Final Report (TRW
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In November another major technical report was generated concerning the
mathematical modelling of data from an S-band station. Work was also stlrted
on a prototype, high-speed-trajectory-data processor for the shuttle ascent
and entry phases. A major decision had to be made on how to model the inte-
grated S-band Doppler data in this processor. There are two ways to model
the integrated Doppler data: range with a large bias and small measurement
noise, and the range difference model in which the large bias subtracts out
but the measurement noise is now timewise correlated. The first method has a
large nonlinear effect associated with it which tends to make the filter
diverge. However, it is the sirr^lest and most accurate model since in the
range difference model, the timewise correlation of the measurement noise is
neglected. Also the range difference model is more complicated to implement.
After much study we were able to utilize the simple and more accurate model
by using a large amount of "measurement underweighting" for nonlinear com-
pensation. The more sophisticated method of using second partials for non-
linear compensation did not work well.
During January 1976 a first penciled draft of a prototype ground track-
ing data processor for the Space Shuttle ascent and entry was completed. This
was a real-time program requiring real time inputs of tracking data and
various flags to run. In order to supply tracking data inputs to this program
a 0.5 second trajectory tape supplied by NASA/JSC was used for entry to
generate a 0.1 second data tape which could be fed into the bench program of
the High Speed Trajectory Data processor. The bench program was completed
in February.
Upon completion of the bench program, the high speed data tape was fed
into the program to determine the best set of statistical parameters to model
the entry trajectory Also an ascent data tape was made to determine the best
set of statistical parameters for this trajectory. Originally the program
modelled jerk of the vehicle in earth-fired coordinates as an exponentially
correlated random variable. By minimizing the velocity error we determined
the best time constant and standard deviation to use for the jerk. Sur-
prisingly we got the same, set of best constants for both ascent and descent.
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r
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ff
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They were
rJ = 4. seconds
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	c3= .55 meters/se
We then tried deleting jerk as a state variable and modelling the acceleration
of the vehicle in earth-fixed coordinates as an exponentially correlated
random variable. We got only a 1% increase in velocity error using this
lower order (and faster) filter. The optimum acceleration constants again
turned out to be the same for both ascent and entry. They were
__Ta = 40. seconds
as = 6. meters/sect
Thus the decision was made to use a 19 state, acceleration filter for
the High Speed Trajectory Determination Processor. The states are:
R, position of the shuttle in earth-fixed coordinates
R, velocity of the shuttle in earth-fixed coordinates
R, exponentially correlated random variables
bpl , range bias from first C-band station
bAl , azimuth bias from first C-band station
bEl , elevation bias from first C-band station
bp2, range bias from the second C-band station
bA2 , azimuth bias from second C-band station
bE21 elevation bias from second C-band station
bp3, range bias for the S-band station
bax , X angle bias for the S-band station
bay, Y angle bias for the S-band station
ID , Doppler integration constant for the S-band station.
.	 a
Since we currently have an old 23 state prototype real-time program,
and since many of the ground rules have changed, we are`just now starting
to write a final version of the above 19 state filter.
The third week of January 1976 was spent at Patrick Air Force Base
in Florida. The meeting was concerned with the types of Department of
Defense tracking data that would be available to support shuttle missions.
The first week in February was spent at NASA/JSC discussing and modifying
the level B documentation for the High-Speed Trajectory Determination Pro-
cessor. A similar meeting was attended in the middle of March where IBM
made an issue that the current filter cycle times were too long, and sug-
gested a two station filter with no measurement biases. TRW presented a
report showing that in some instances, biases can be very helpful. Later,
In April we discovered a 213 m/sec glitch in the velocity estimate when
Bermuda came on during ascent and when no biases were modelled.
A considerable effort was made during April 1976 to generate a 0.2
second trajectory tape from the standard NASA/JSC orbital flight test 1
(OFT-1) entry tape. The NASA tape contained various AT's and had weird time
tags. This was done, and the program (deck of cards) and the documentation
of the program was sent to JSC. Note that 0.2 second was now the baseline
AT for the filter cycle, even though the data comes in at a AT = 0.1 second.
In May a tracking data tape of OFT-1 entry was made to NASA/JSC speci-
fications and sent to them. Using the OFT-1 entry data, a new study was
made to see the effect of a reduced sample rate (from AT = 0.2 to AT = 0.4
second). The reduced sample rate did not affect the accuracy of the position
estimates but velocity accuracy was reduced by 15 0.. Near the middle of May I
attended a 3 day meeting at JSC which was concerned with shuttle onboard
navigation software.
Early in June 1976 a study was completed which showed the navigational
accuracy when processing ground tracking data during the OFT-1 entry. A
short study was made to show the effect of deleting the Doppler measurement
from a S-band site for OFT-1„entry. Also a short study was made to determine
the filter cycle times for a three station filter when 1, 2, or 3 stations
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were tracking. A copy of the current, 23 state, prototype, real-time computer
program for ground navigation of the Space Shuttle, along with pencilled
documentation, was sent to NASA/JSC.
In July a report showing the filter convergence times was sent to JSC.
A meeting was held at JSC to review the latest ground rules for the High
Speed Trajectory Data Processor. Work was started on a new 19 state proces-
sor.
Documentation issued during the reporting period is shown below.
1. Wm. M. Lear, letter to Robert T. Savely of NASA/JSC concerning the
calculation of atmospheric scale height, 31 July 1975.
2. Wm. M. Lear, "Refraction Corrections for An Exponential Atmosphere",
TRW Defense and Space Systems Group Technical Report 27054-6009-TU-00,
8 August 1975.
3. Wm. M. Lear, letter to Robert T. Savely of NASA/JSC concerning,
"Effect of Degraded Double Precision Accuracy in Multiplication and
Division on One-Way Doppler Calculations", 27 August 1975.
4. Wm. M. Lear, "Noise Statistics of Skin Tracking with the FPS-16 Radar
at NASA/FRC", TRW Defense and Space Systems Group Technical Report
27054-6010-TU-00. 29 August 1975.
5. Wm. M. Lear, letter to Robert T. Savely of NASA/JSC concerning,
"Effect of Large IMU Errors During Reentry", 30 September 1975.
6. Wm. M. Lear, "Methods of Generating the State Transition Matrix",
TRW Defense and Space Systems Group Technical Report 27054-6011-TU-00,
24 October 1975.
7. Wm. M. Lear, "Mathematical Models for Short-Range Data from a Single
5-Band Station", TRW Defense and Space Systems Group Technical Report
27054-6012-TU-00, 14 November 1975.
8. Wm. M. Lear, presentation to NASA/JSC on "The Effect of AT and Biases",
17 March 1976.
9. Wm. M. Lear, letter to Paul T. Pixley of NASA/JSC concerning
"Optimum Filter Constants", 26 March 1976.
10. Wm. M. Lear, letter to Paul T. Pixley of NASA/JSC concerning ascent
accuracies with and without radar biases, 1 April 1976.
6i
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11. Wm. M. Lear, letter to B.F. Cockrell of NASA/JSC concerning calculation
of dynamic pressure from drag acceleration measurements, 6 April	 1976.
12. Wm. M.	 Lear, "Mathematical Models of Short Range Data from a Single
S-Band Station", TRW Defense and Space Systems Group Technical Report 3
27054-6012-TU-00,	 14 November 1976	 (revised 16 April	 1976). >,
13. Wm. M. Lear,	 "Shuttle Trajectory Tape Interpolation Program",
TRW Defense and Space Systems Group Technical Report 27054-6013-TU-00,
26 April	 1976.	 Also a computer deck was delivered with this report. )
14. Wm. M. Lear, notes on "OFT-1 Reentry Data Tape User Information", I
7 May 1976.	 Also a data tape was delivered. y
15. Wm. M. Lear,	 letter to Paul T.	 Pixley concerning "Effect of Data
Sample Rate on Navigation Accuracy During OFT-1 Reentry", 7 May 1976.
16. Wm. M. Lear,	 letter to Bruce ^,illiamson of NASA/JSC concerning
"Obtainin g Atmospheric Scale Height, HS ", 2 June 1975.
17. Wm. M. Lear,	 letter to Paul T. Pixley of NASA/JSC concerning effect
on navigation accuracy of deleting the Doppler Measurement, and
filter cycle times for 1,	 2, or 3 station tracking using a 3 station
filter, 8 June	 1976.
18. Wm. M. ,Lear,	 "Ground Tracking Accuracy for OFT-1 Reentry", TRW Defense
and Space Systems Group Technical Report 27054-6014-TU-00, 	 11 June
1976.
19. Wm. M. Lear, pencilled draft of a prototype, real-time, computer
program for ground navigation of the Space Shuttle, 22 June 1976.
Also a computer listing was delivered at the same time.
20. Wm. M. Lear, letter to Paul T. 	 Pixley of NASA/JSC concerning
"Convergence Times for the High Speed Tracking Data Processor"',
1	 July 1976.
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Space shuttle navigation filter development

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