ALOS -Advanced Land Observing Satellite
Using SLR, The GPS accuracy
verification of ALOS
JAXA
Precise Orbit Determination Team
S Nakamura, N Kudo, R Nakamura, S Katagiri
ALOS -Advanced Land Observing Satellite
Contents
1 Introduction
2 Mission requirement (Necessity of SLR)
3 Restricted laser tracking
4 Analysis
ALOS -Advanced Land Observing Satellite
Introduction
ALOS Launch
Date : 24th January, 2006
Vehicle : H2A
Site : Tanegashima Space Center, Japan
The value of the orbit
Orbit Type
Solarsynchronous,
sub-recurrent, frozen
Height
691.65km (above the
equator)
Period
98.7 min
Eccentricity
1/1000
Inclination
98.16deg
Recurrent days
46 days
Local time of
descending node
AM 10h30m±15m
ALOS -Advanced Land Observing Satellite
Mission Requirement
Mission: High-resolution land observation
To achieve this mission, highly accurate sensor pointing
is required.
Orbit determination accuracy is required
within 1m (peak to peak).
To fulfill this requirement, we performed precise
orbit determination by using onboard GPSR data.
Necessity of SLR
We need to verify that the position which the
onboard GPS receiver shows is right.
Whether or not there is offset
ALOS -Advanced Land Observing Satellite
Restricted Laser Tracking
Before the verification, we analyzed whether laser
transmitted from ground stations damaged ALOS sensors.
We checked it by considering the maximum incident energy
and used the specifications of each station.
The result showed that laser could damage the sensors.
PRISM
Backward
PRISM Nadir
AVNIR-2
PRISM
Forward
Sometimes the ranging pass was divided into 2, 3 or 4.
ALOS -Advanced Land Observing Satellite
Restricted Tracking
We used a restricted tracking technique standardized in ILRS.
• Tracking with Closed Network
We confirmed the interface between JAXA and candidate
SLR stations (TIRV, SLR-SUP file, Go/NoGo file).
And then we asked limited SLR stations to track.
•
Pass start and end time control
The pass start and end time was controlled by SLR-SUP file
interface. Within the visible time of ALOS, the time which
dose not interfere with the sensors was calculated for each
SLR station. We sent the results to SLR stations.
•
Control by the Go/NoGo file
The Go/NoGo file was interfaced with each station as the
method by which all laser ranging should be stopped.
ALOS -Advanced Land Observing Satellite
Participation Stations
CDP pad ID
SLR Stations
ID
7825
Mt. Stromlo
STL3
Australia
1884
RIGA
RIGL
Latvia
7308
Koganei(KOGC)
KOGC
Japan
7838
Simosato
SISL
Japan
7110
NASA/MonumentPeak(Moblas-4)
MONL
USA
7501
NASA/ Hartebeesthoek (Moblas-6)
HARL
South Africa
7090
NASA/ Yarragadee(Moblas-5)
YARL
Australia
7358
JAXA/Tanegashima
GMSL
Japan
7810
Zimmerwald
ZIML
Switzerland
7840
Herstmonceux
HERL
United Kingdom
7105
NASA/GreenBelt (MOBLAS-7)
GODL
USA
7130
NASA/GreenBelt (TLRS-4)
GO4T
USA
Campaign Period (UT): 14th Aug 2006 00:00:00 31st 16:00:00
Thanks to this campaign, we obtained 100 pass, 2979 data.
ALOS -Advanced Land Observing Satellite
Analysis(1)
At first, we confirmed the consistency between SLR data
and GPS orbit.
Difference of SLR data from GPS orbit (one day example)
O-Cグラフ：AL_GAS_20060829_01
0.4
0.3
O-C(m)
0.2
0.1
0.0
-0.1
-0.2
-0.3
-0.4
2006/08/28 2006/08/29 2006/08/29 2006/08/29 2006/08/29 2006/08/30 2006/08/30 2006/08/30
18:00:00
00:00:00
06:00:00
12:00:00
18:00:00
00:00:00
06:00:00
12:00:00
For this analysis, we used all SLR data.
Average difference : -4.78 ±12.03 cm
This result shows GPS and SLR agree within the error (1σ).
ALOS -Advanced Land Observing Satellite
Analysis(2)
That analysis did not have the resolution for each direction
(radial, cross, along direction) in evaluating the GPS orbit.
So we carried out the orbit determination only using SLR data in the
following period, and calculated the difference from the orbit by GPS.
The period is that over 3 stations carried out ranging during a few
revolution.
Example
Aug 29
1.5
1
0.5
0
0:00:00
-0.5
-1
-1.5
2:24:00
4:48:00
7:12:00
9:36:00
12:00:00
14:24:00
16:48:00
19:12:00
21:36:00
0:00:00
ALOS -Advanced Land Observing Satellite
Analysis (3)
We calculated the difference between SLR orbit and
GPS orbit. (unit: cm)
Radial
direction
8/21
8/22
8/23
8/24
8/25
8/28
8/29
A
B
A
B
A
B
8/30
Cross Track
direction
2.92±7.31
-24.07±28.68
4.08±1.69
-2.77±17.62
4.20±4.51
6.84±13.84
6.50±31.60
-0.50±11.47
-11.36±12.12
2.57±13.18
2.13±8.37
6.97±8.70
2.38±7.77
-7.35±30.39
12.94±14.31
21.60±4.24
4.05±9.81
-64.46±49.76
-14.03±27.01
5.23±4.77
9.02±7.79
14.27±20.23
Along Track
direction
difference(3D)
RMS
MAX
11.01±18.25
-4.81±44.63
-11.89±5.06
-1.98±4.27
6.40±4.86
-5.60±20.06
-51.38±44.95
-12.15±3.89
7.24±22.36
-5.14±13.09
-3.40±13.21
25.00
57.79
33.11
26.20
24.14
27.52
109.96
34.36
29.12
22.33
28.90
52.43
119.65
45.23
42.65
29.27
55.48
168.62
51.97
37.00
29.29
56.57
OD Arc
（min）
257
204
88
120
186
400
199
108
348
202
254
The weighted mean (cm)
Radial
3.58±1.47
Cross
10.80±2.45
Along
-5.16±3.02
Distance
12.50
??
We compared this result with the accuracy of GPS orbit determination.
ALOS -Advanced Land Observing Satellite
Analysis (4) GPS OD Accuracy
20
12
0
20
12
0
20
12
0
OD Arc
OD Arc
OD Arc
OD Arc
Over Lap Period
UT 1:00 --7:00
Over Lap Period
UT 1:00 --7:00
Over Lap Period
UT 1:00 --7:00
We used Overlap Method to calculate the accuracy of GPS OD
We defined the RMS value as an orbit determination (OD) accuracy.
OD Accuracy (GPS)
7
6
4
3
2
1
0
20
06
/8
/2
1
20
06
/8
/2
2
20
06
/8
/2
3
20
06
/8
/2
4
20
06
/8
/2
5
20
06
/8
/2
6
20
06
/8
/2
7
20
06
/8
/2
8
20
06
/8
/2
9
20
06
/8
/3
0
(cm)
5
OD accuracy (RMS value)
by GPS is 2 cm --- 7cm.
ALOS -Advanced Land Observing Satellite
Analysis (5)
We compared the Accuracy of GPS OD (analysis(4)) with the
Difference between SLR orbit and GPS orbit (analysis(3)).
(cm)
Radial Direction
40
30
20
10
0
-10
-20
-30
-40
-50
-60
06/8/19 06/8/21 06/8/23 06/8/25 06/8/27 06/8/29 06/8/31
Cross Direction
Along Direction
60
40
40
20
20
0
-20
-40
-60
-80
-100
06/8/19 06/8/21 06/8/23 06/8/25 06/8/27 06/8/29 06/8/31
0
-20
-40
-60
-80
-100
-120
06/8/19 06/8/21 06/8/23 06/8/25 06/8/27 06/8/29 06/8/31
Radial direction
Along direction
Cross direction
Blue point with long error bar : Difference SLR – GPS orbit
Pink point with short error bar : Accuracy of GPS OD
These graphs show that Accuracy of GPS OD (including error)
is within the error of difference between SLR orbit and GPS orbit.
ALOS -Advanced Land Observing Satellite
Analysis (6)
The weighted mean of each direction
40
(cm)
30
20
10
0
-10 0
-20
0.5
1
1.5
2
2.5
3
3.5
1. Radial
2. Cross
3. Along
-30
-40
-50
The accuracy of GPS OD (including error) is within the error of
difference between SLR orbit and GPS orbit.
This results show that the accuracy of GPS OD is about a few (27) cm (within mission requirement : 1m), and there is no offset in
any directions within the resolution of SLR in this campaign.
ALOS -Advanced Land Observing Satellite
Conclusion
We needed to verify the accuracy of ALOS orbit determination by
the onboard GPS receiver by comparing with SLR.
Because laser might damage ALOS sensors, we performed the
restricted laser tracking campaign (about 2 weeks).
As the result of this verification, the accuracy of ALOS orbit
determination by GPS is about a few cm and there is no offset in any
directions.
This result also fulfills the requirement from ALOS mission.
Acknowledgement :
ALOS tracking campaign was performed successfully
with the cooperation of ILRS and participating SLR
stations, to all of which we would like to express our deep
appreciation.