Data from the two MODIS instruments have been accurately geolocated (Earth located) to enable retrieval of global geophysical parameters. The authors describe the approach used to geolocate with sub-pixel accuracy over nine years of data from M0DIS on NASA's E0S Terra spacecraft and seven years of data from MODIS on the Aqua spacecraft. The approach uses a geometric model of the MODIS instruments, accurate navigation (orbit and attitude) data and an accurate Earth terrain model to compute the location of each MODIS pixel. The error analysis approach automatically matches MODIS imagery with a global set of over 1,000 ground control points from the finer-resolution Landsat satellite to measure static biases and trends in the MO0lS geometric model parameters. Both within orbit and yearly thermally induced cyclic variations in the pointing have been found as well as a general long-term trend

Nishihama, Masahiro

Wolfe, R. E.

NASA Technical Reports Server

Trends in MO]08S geoNNcatioDDerror analysis
K. E.	 Masahiro Niyhi
^ Earth Sciences Directorate, NASA/G9MC, Greenbelt, MO2O77|
" Sigma Space Corporation, Lanham,MQ,DS/\
ABsTRACT
Data from the two MOD/S instruments have been accurately geolocated (Earth located) menable retrieval of global
geophysical parameters. The authors describe the approach used mgoolocutewith sub-pixel mzumuy over nine
yours of data from M0DIS on lwA3A`o B0S Terra op*ooxrat and seven ycma of data Bnm K40Dl0 on the Aqua
spacecraft. The 4p/nooh uece o geometric model of the \100S imommeotm ' accurate navigation (orbit and
uuhudu)datooudunamcunxcEmth/cnainmodnit000mpu\cUu|ocabnuofumhM0D|Spixci. The error analysis
approach automatically mutobcu M00l3 imagery with u global set of over 1,000 ground control points from the
fioc,000}vdon Loudnm satellite to measure static Nuoeo and trends in the MO0lS geometric model parameters.
Both within orbit and yearly thermally induced cyclic variations in the pointing have been found as well as a general
long-term trend.
Keywords: wiOD|S,EOS.Guohcmion Satellite navigation
INTRODUCTION
Since February 2000, the first Moderate Resolution Imaging Spectroradiometer (PNIODIS) instrument [i] has been
acquiring hoogo of the Earth in 36 spectral bands with nadir spatial resolutions of 0.25 km (2 bands), 0.5 km (5
bands) and \D km (29 bands). The center spectral wm'duugtho uf these bands range from the visible (041 pm) to
the |vog-*yvc infrared (14.2 Two K40Dl8 instruments are currently nn-mhb. The first launched onNASA's
Earth 0bscniu& 8you,m (£0S) Terra epauuo,ob in Cwoumbor 1999 and the second on NASA's EOS Aqua
spacecraft in May 2002. Over nine years of data hmn MOD\S yTcoa and seven years ufk1OD|8/Aguo data have
been accurately geolocated to enable accurate retrieval of global geophysical parameters. In this paper, we discuss
results from the geo|ooutiou error analysis approach that has allowed uutomaintain and improve the geo|ou#iou
accuracy for both instruments over their mission lifetimes.
The MOD[8geolocation and error analysis approach is discussed in detail in [2] with the first on-orbit results
presented in [3]. The approach uses u geometric model o[ the MN)DI8 in000mcmm accurate navigation (orbit and
attitude) data and an accurate Earth terrain model W compute the location of each M0}l8 pixel. The error analysis
up[vo^ob automatically oumchca M0Dl3 boogcg/ with xg|nbu| set vfl.2l4 ground control points from the finer-
resolution Loodouu om6|i/e to measure static biases and omuda in the KJ0D[S geometric model parameters. Later
geo|ocm(ion results are also presented in [4], [ 5 ] and [«]. Both within-orbit and yearly dheonu(|y induced cyclic
variations 1u the pointing have been found ue well oou general long-term trend. Also, details of the control point
mmchio^u|gnd,hmmrdescribed io[6].
MO0S data has been reprocessed several times allowing for improvements in instrument characterization and
algorithms to be incorporated in the standard MK)Di9 products. The current reprocessing started in 2007 and is
called Collection 5 (C5). The next reprocessing, Collection 6 (C6), is expected to begin in early 2010. For
gcoloumioo, each coUmuimo iuu|udux improvements in the geometric characterizations and minor algorithm
improvements ([4]. [5] and [6]). In addition, for C6, an improved Terrain correction algorithm [7] will be added.
Starting with C5, nuvcm| improved ancillary data sets were incorporated: an improved oomm| point iih,my
cnu{uiuin& more than three times the number of points with a better global dimzihminu [6]; an improved
digital terrain model humod on the Shuttle Radar 7cunin Mapping k4ioalou (SBIk4) data [8]; and an improved
MODIS based Land/Water mask. In this paper, we discuss the MODIS/Terra and MODIS/Aqua geolocation results
from the current C5 reprocessing and the results expected ioC6,
https://ntrs.nasa.gov/search.jsp?R=20110007087 2019-08-30T14:46:00+00:00Z
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MODIS/TerruGeolocution Results
The MODIS/TerraC5 geolocation error (Table | and Figure 1)issmall and meets theNODIS geolocation accuracy
&ouof50m(l(7),20%nfnM0D|8250mhmd. 	 Table | gives the C5gcnloouion Roo t Mean Square E,nx
(KK18E) in nadir equivalent units based on an average of269control point mmchuyn per day in the track and noun
(cross-track) direction. 	 Shown in Figure l are
the daily nvougo control point oxiduu|v for Table l. MODIS control point residuals in nadir equivalent
each data-day since launch.	 Also shown are units for C5 and expected C6results.
the g|nbu|, ounhem hemisphere and southern
hemisphere	 ld'dxy	 average	 control	 point Expected C6C5 Results (m)
residuals,	 along
	
with	 the	 g|uhu|	 16-duy Sme|-	 Matches Resu lts
oumdmd deviation (gray bars). 	 0vcmU, the |i|r	 per day	 Track	 8uun	 Track	 Scan
loup,-term trend is o|oo= to zero, except for n^^8E	 RN8D	 8^^8E	 kh4S8
periods,	 b	 ^h	 periodsum^ on	 o, xuc	 uo	 opun u	 .
^
Terra	 269	 42	 44	 43	 42
when the overall trend in the scan direction did &uuu	 232	 47	 53	 45	 51
not follow the prediction. This was
io the next update that occurred in early 2006.
An improvement 	 for M00S/Terra C5 was the introduction ofusun-angle correction to 	 for within-
orbit variation seen in the ground control point mwWuoh. Because of this uoneuhom as neoo in Figure ), the
Northern and Southern hemisphere trends are not significantly different from the global trend.
Based on the C5 nsoka uvn improvements will be made in C6. Fiat, u,etrnopeo6ve long-term trend analysis will
bo applied w remove any biases n, extrapolation errors. Second, an improved yuu+mg|eco,reodon will be applied
to cn,mu for residual within-orbit variation. Table i and Figure  show the expected results from these changes.
The overall C6 results are expected to improve in the scan direction, reducing the RMSE by 2 m. This iu primarily n
result o/ the improved ounspouivc fit in 2005. The long-term trend includes uycm|y cyclic term with npenk'u+
peukmnp|itudunf26 mio the track and 10 m in the scan direction. This amplitude will 6c59% and 29% higher bu
Years since Jan. 1, 2000
Fiore 1. 	 C5 control point residuals in the track (top) and scan (bottom)directions.
75
— 50
^
^ 25
^
–` 0
^
_25
^ -50
^
-7 4;
75
-- 50
^ 25
^
-, 8
^
-25
CO
'y '50
-75
O	 )	 2	 3	 4	 5	 6	 7	 V	 9
Years since Jan. |.2000
Figure 2. Expected K40DlS/[err C6 control point residuals lu the track (mn)and scan (bottom)
the track and scan directions, respectively, than [5.
The improved mmmo fit iashown in Figure 3 (black lines). The control point residuals innadir equivalent units
are binned in5"increments (gray filled cim^^^^^^^u^N^m^^^i^^^^b^. T^
po|ynomi^§uuou^dfor ^cum-ung|enm^^^um0"ml2V° and then uUu^ncor^cion^n^m^ndc,ofUn
orbit (1  to 360'). The fit has upcak-,*poak amplitude of40min the track and 4l mio the scan direction. This
amplitude is 43% and 32% amu||,, in the track and scan directions, ,uxpuuivdy, and so oomponoomo for an
ovcrconccdoo that occurred inC5.
0	 }O	 60	 90	 120 150 180	 0	 30	 60	 90	 120 15  180
Sun angle (dug)	 Sun angle (dqg)
Figure 3. MODIS/Terra sun-angle fit in the track (left) and scan (right) directions.
As part of this error analysis, ustudy was done to 	 whether instrument temperature could be used to
correct for with-orbit pointing errors since temperature changes are believed to the primary cause of the
instrument and opmzcxmft pointing changes within an orbit. ^& removing the long-term trend from both the
residuals and the temperature data from six instrument |ocu|ioua, the temperature that had the highest uonr|u|iou
with the control point residuals was the scan mirror temperature (Tp_SA_OCTl_K4iktheonismr). The relationship
between this temperature and the pointing error was -5.3mand -5.6m per l"Ciothe track and scan directions.
respectively. When this temperature fit wus used to remove this within-orbit variation, the overall results were not
quite as good as the muo'ongleUt. Based on this and bconuau o[thcaddidom| work needed *`continuously check
the quality of the temperature values, the sun-angle fit is the approach that will be used for C6.
MODIS/Agua Geolocution Results
The MOD|S/Aqua C5 geolocationorror (Table | and Figure 4)is small and meets the MOD{S geolocation accuracy
g^u| in the oom` direction and almost mee*the goal in the track direc tion. Table } gives the C5 gro|ucmion Kun
Mean Square Error (mN8E)iu nadir equivalent units based onan average of232 control point matchupo per day io
the track and o"uo (cross-track) direc tion. Ovcm|i(hu long-term trend is close mzero, exce pt for some periods,
such as the period in 2007 and 2008, when the overall trend in the scan direction did not follow the prediction. This
was uoncoxod in the next update that occurred in cu,|y 2006. For Aqua, there is u significant with-orbit variation
that causes the northern and southern hemisphere residuals to vary with respect to one another. In addition, there is
o small global yearly cyclic term. Neither of these corrections was used inC5 for Ki0D|8/Aquu.
^ DnUy
	
—|6-duyGlobal
16-day South Hem i --------- 16-day North Hemi
— 75
^
^ 50
25
^ 0
Z^ -25
^
-50
-75
75
50
^
^ 25
^
–'	 U
-25
^lu ~50
-75
Years since Jan. |.Z000
Figure 4. MNJD[S/AquuC5 control point residuals iothe track (mp) and scan (bottum)
Based oo the C5 results, three improvements will hc made ioC6 First, u retrospective |on '-'term trend analys i s will
hc applied m remove any biases o« extrapolation errors. Second, u yearly cyclic fit will ho used m remove the globa l
ywsh/ variation. The third change will be to implement u ouo'mng|o correction to correct for the within-orbit
variation. Tuh|c | and Figure 5 show the expected results from these changes. The overall C6 results are enneo/cd
to improve in both the track and scan directions, reducing the KK18E by 2 m for each. The new long-term trend
yearly cyclic terms will have omu|i peak-to-peak amplitudes of | m in dbctrack and j m in the scan direction. As
000n in Figure 5 ` the retrospective fit ofthe long-term trend that includes the cyclic term bring the global msidnu|y
close to zero in 6mb the track and uouo directions. In the track direction, u|momoU of the within-orbit variation is
removed by the 26 in peak--to-peak amplitude sun-angle unnccbon (Figure 6). There appea rs to be uxmm|i two-
cycle per year residual error remaining in the track direction. |u the scan direction, the within-orbit variation iu
reduced 6y the 62mpouk-m'puukauo-omg|ecorrection. However, there remains u small north/south difference that
appears early in each year starting with 2004. Some additional adjustments will also be made in the early post-
launch period to correct for the initial on-orbit bias.
Da iLy	 16-day Global
16-day South Hem i --------- 16-day North Hem i
E! 75
50
25
0
-25
-50
-75
75
50
25
0
-2 5
-50
-75
2	 3	 4	 5	 6	 7	 8	 9
Years since Jan. 1, 2000
Figure 5. Expected MODIS/Aqua C6 control point residuals in the track (top) and scan
(bottom) directions.
The first MODIS/Aqua sun-angle fit is shown in Figure 6. The polynomial fits are used for the sun-angle range
from 0' to 140' and then a linear correction for remainder of the orbit (140' to 360'). The MODISIAqua scan
direction sun-angle fit is similar in sign and has a 51% larger amplitude than the MODIS/Terra scan fit. The
MODIS/Aqua track direction sun-angle fit, however, has the opposite sign and an amplitude that is 35% smaller
than the MODIS/Terra track fit.
100 100
75 75
50 50
25 25
0 71 0
-25 25
-50 -50
-75 y = 0.00271 x 2 -0.382.X 4- 39.4 -75
_100 -100
0	 30	 60	 90	 120	 150	 180	 0	 30	 60	 90	 120	 150	 180
Sun angle (deg)	 Sun angle (deg)
Figure 6. MODIS/Aqua sun-angle fit in the track (left) and scan (right) directions.
As part of the MODIS/Aqua C5 error analysis, a study like the study for MODIS/Terra was done to determine
whether instrument temperature could be used to correct for within-orbit pointing errors. After removing the long-
term trend from both the residuals and the temperature data from six 	 locations, there was no temperature
that had u good correlation with the control point residuals. So, 	 correction based on
temperature was possible.
Other Collection 6 Changes
Several additional changes will made for geolocatiuin0. First, minor improvements in the overall gmlocation
accuracy will hc obtained hy making *wu| wUuouncnm in acvom| instrument parmnuters(iu. yaw bias, pitch hiaa
auun rate), Second, as mentioned nhnve. on improved terrain correction algorithm [7] will be implemented that
improves I km geolocation accuracy in areas with rough terrain. This change will be coupled with a higher quality
terrain model and on improved |nnd/wm|c, mask, both with 0.5 km ,coo|mion. PivaUy, in addition to the l km
geolocation, geolocation will be computed at 0.5 km and stored as offsets from the | kmgco|ocmioovalues.
Other Geeloc*tioo
Both the Terra and Aqua spacecraft perform monthly attitude roll for calibration purposes. Additionally,
mhh maneuvers, drag makeup and inclination adjustment, are performed movcod times uycu. For u short time
before a maneuver and for up to 1.5 hours afte r a maneuver, the satellite orbit and attitude data are not as accurate,
so the goohco|iom has larger errors mentioned above. 8ocuuuc nf this, these maneuver poiode are cxc|udcd
when the standard MN)DiS products are produced inC5. A different approach ia being considered for C6 that would
allow more data near maneuvers tnheused.
For near-real-time users ofMOD|S/Aqua data, the predicted orbit data provided in the direct broadcamdata stream
is not amaccurate as the definitive orbit 	 that is ava i lable 12 to 1-4 	 kuc,. Th is is not 
an issue for
MODIS/Terra because the direct broadcast data stream contains definitive orbit data.
Summary and Conc lusion
The error analysis approach described above is being used maccurately geolocate data from both N00S
instruments. The current gcolocmioo results were discussed along with the &*ohcatino improvements that are
expected in the next MODUS reprocessing (C6) oc1 to begin in early 2010. Overall, the accuracy of the MOD|8
gcn|ucmiou data meum the tough accuracy Avu| mc\ by the M0 p|S Science Team. The consistent sub-pixel
geolocation accuracy enables the accurate retrieval of geophysical pararneters globally. Lcuxuou learned from this
error analysis approach is being used for the Visible Infrared Imager Radiometer Suite (VUR8) instruments aboard
the National pn|apu,hihng0peratiovu| Covimomou`u\ Satellite Syamm(N p0ESS) Preparatory Project (NPP) and
will hc used for future operational NP0B8Smissions.
Acknowledgement
The authors are grateful for the support of the MODIS science team, the MODI8calibration support team, the
MODAPS production team and the MODS science data support team, in particular Janes K"ypc. This work was
performed under the direction of the K4ODlS Science Team in the Terrestrial Informat ion Systems Branch (Code
6|4.5)u/ NASA GS[C.
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Trends in MODIS Geolocation Error Analysis

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