RADARSAT-1 “Super thrust” : Recovery Validation Report

Author:

Wade Albright

Date Published:

2004

Summary

The RADARSAT-1 imaging window has returned to its nominal orbit. There are no significant geolocation differences between pre-anomaly and post-recovery data over the Delta Junction targets.

Introduction

On April 25, 2004 during nominal orbit maneuvers, RADARSAT-1 experienced what was termed a “super-thrust” anomaly. This anomaly pushed RADARSAT-1 out of it’s nominal orbit and by April 27^th, caused noticeable errors in the windowing of the data as well as in the geolocation in the imagery. After considering the options, CSA decided to correct the orbit of the satellite back to it’s original track. On May 17^th-19^th these corrective maneuvers were performed. By June 1^st RADARSAT-1 was back in it’s original orbit. This study was performed in order to assess the geolocation accuracy and windowing accuracy of RADARSAT-1 after June 1^st of 2004.

Dataset

Imagery of Delta Junction, Alaska was used for this study. The Alaska Satellite Facility maintains an array of several corner reflectors in this area that are very useful for point target measurements, such as geolocation. In order to get this study completed in a timely fashion it was determined that only two beam modes need to be supported. We chose Standard 1 and Fine 1. A total of six orbits were acquired over the test site after June 1^st. To compare the results to something meaningful, we used six other orbits over the test site that occurred prior to April 25^th. The table below lists the images that were used for this study.

	Date	Revolution #	ASF Frame #	Platform Direction
Pre-anomaly	02/06/03	37881	160	Ascending
	08/18/03	40647	290	Descending
	08/31/03	40833	290	Descending
	11/22/03	42019	290	Descending
	12/15/03	42340	160	Ascending
	02/14/04	43212	160	Ascending
Post-anomaly	06/01/04	44763	290	Descending
	06/07/04	44849	290	Descending
	06/14/04	44949	290	Descending
	06/24/04	45084	160	Ascending
	06/25/04	45106	290	Descending
	07/07/04	45270	160	Ascending

Tests and Results

Two tests were performed on this data: geolocation and windowing. The geolocation test consisted of comparing the measured locations of corner reflectors within the imagery to known GPS coordinates. A total of 29 targets were measured prior to the anomaly and 34 targets were measure after the anomaly. On the whole the numbers before and after the anomaly were within the definable limits for ASF products (100m). While the results for the post anomaly images were slightly worse than the pre anomaly imagery, the difference between the two was within the standard deviation of the pre anomaly images, if just barely. One may conclude that difference are small enough to be negligible or within the normal variation from image to image. The full results of this test can be seen in the Appendix.

The second test that was completed was the windowing test. This test was performed to determine if the data that was imaged, was the actual dirt that was supposed to be imaged as per the planned acquisition. RADARSAT-1’s orbital geometry allows for an exact repeat of an acquired target area every 24 days. To perform this test an identical repeat orbit was identified with one frame occurring prior to the anomaly and the second frame in the pair, occurring after the anomaly. A common feature was found near the edges of both images and then that feature was measured as to how far it was from the edge of the image. In the range direction this difference was approximately 100m, while in the azimuth direction it was approximately 500m. Nominal space craft movement between maneuvers allows for a ground track offset of +/- 2km.

Conclusion

Based on the tests that were performed here at ASF, it appears that the positioning of the satellite and the imaging area of RADARSAT-1, after the “super thrust” anomaly that occurred on April 25^th, 2004 and it’s subsequent corrections, are very similar to those prior to the anomaly. It can therefore be assumed that there is not a significant degradation of data quality in ASF products due to this anomaly, and products may be distributed as normal operations require.

Appendix

DIFFERENCE (Pre-Post)

		Euclidean (m)	Range (m)	Azimuth (m)
Overall	average	-20.4	-38.4	-13.0
	st.dev	-14.7	-5.9	-5.8
	Student’s T probability*		0.14	0.27

* if probability < 0.05, difference is significant.

PRE ANOMALY ACQUISITIONS


Date	Orbit	Direction	Reflector #	Euclidean (m)	Range (m)	Azimuth (m)
38031	43212	Asc	1	60.5	-13.7	59.0
38031	43212	Asc	15	53.2	-4.5	53.0
38031	43212	Asc	19	58.6	-7.1	58.2
38031	43212	Asc	20	61.4	-7.8	60.9
37970	42340	Asc	1	74.2	-27.3	69.0
37970	42340	Asc	4	62.9	-23.1	58.5
37970	42340	Asc	9	76.1	-44.3	61.9
37970	42340	Asc	15	80.9	-44.3	67.6
37970	42340	Asc	19	75.7	-43.2	62.2
37970	42340	Asc	20	78.5	-30.6	72.3
37970	42340	Asc	24	96.1	-67.2	68.6
37947	42019	Desc	1	50.2	14.8	48.0
37947	42019	Desc	15	53.9	18.4	50.6
37947	42019	Desc	19	53.5	13.2	51.8
37947	42019	Desc	20	53.4	9.4	52.5
37864	40833	Desc	1	77.6	60.5	48.7
37864	40833	Desc	4	79.2	63.9	46.9
37864	40833	Desc	15	64.7	57.2	30.3
37864	40833	Desc	19	62.0	47.1	40.2
37864	40833	Desc	20	66.5	52.1	41.3
37864	40833	Desc	24	36.5	28.8	22.5
37851	40647	Desc	1	45.3	27.6	36.0
37851	40647	Desc	15	40.5	30.5	26.7
37851	40647	Desc	19	38.2	25.4	28.5
37851	40647	Desc	20	39.4	24.2	31.1
37658	37881	Asc	15	46.7	-39.3	25.2
37658	37881	Asc	19	43.8	-39.2	19.4
37658	37881	Asc	20	40.4	-30.5	26.6
37658	37881	Asc	24	66.8	-61.9	25.2
		Overall	average	59.9	-0.4	46.3
			st.dev	15.5	38.4	16.2
		Asc	average	65.1	-32.3	52.5
			st.dev	15.4	19.0	18.5
		Desc	average	54.4	33.8	39.7
			st.dev	14.1	18.7	10.4

POST ANOMALY ACQUISITIONS


Date	Orbit	Direction	Reflector #	Euclidean (m)	Range (m)	Azimuth (m)
38175	45270	Asc	1	51.8	-15.9	49.3
38175	45270	Asc	9	49.8	-9.0	49.0
38175	45270	Asc	10	63.2	-40.4	48.6
38175	45270	Asc	15	44.9	-4.5	44.7
38175	45270	Asc	19	60.0	-8.1	59.5
38175	45270	Asc	20	49.6	-9.7	48.6
38163	45106	Desc	1	77.2	50.1	58.7
38163	45106	Desc	11	86.3	-85.3	13.1
38163	45106	Desc	13	73.3	46.6	56.6
38163	45106	Desc	14	67.3	35.1	57.4
38163	45106	Desc	15	75.1	50.3	55.8
38163	45106	Desc	19	74.6	47.0	58.0
38163	45106	Desc	20	69.7	45.9	52.5
38162	45084	Asc	1	69.1	21.5	65.7
38162	45084	Asc	4	65.2	35.7	54.6
38162	45084	Asc	9	67.0	16.6	64.9
38162	45084	Asc	15	67.4	25.6	62.3
38162	45084	Asc	19	58.0	18.0	55.1
38162	45084	Asc	20	68.0	15.1	66.3
38162	45084	Asc	24	62.3	-2.2	62.3
38152	44949	Desc	1	147.1	100.2	107.7
38152	44949	Desc	4	145.9	100.6	105.7
38152	44949	Desc	15	136.3	97.0	95.9
38152	44949	Desc	19	139.1	90.2	105.8
38152	44949	Desc	20	137.0	92.9	100.7
38152	44949	Desc	24	110.4	70.6	84.9
38145	44849	Desc	4	121.7	111.3	49.2
38145	44849	Desc	15	94.0	82.8	44.6
38145	44849	Desc	20	86.7	77.4	38.9
38145	44849	Desc	24	77.3	65.7	40.7
38139	44763	Desc	1	61.1	43.8	42.7
38139	44763	Desc	15	58.9	43.6	39.6
38139	44763	Desc	19	58.0	40.4	41.7
38139	44763	Desc	20	56.1	42.2	36.9
		Overall	average	80.3	38.0	59.3
			st.dev	30.2	44.2	22.0
		Asc	average	59.7	3.3	56.2
			st.dev	8.2	20.9	7.7
		Desc	average	93.0	59.4	61.3
			st.dev	31.9	41.2	27.4