The size and shape of the satellites retroreflector arrays have a major impact on the distribution and scatter of the return signal; this can be seen clearly when reaching sub-cm ranging accuracies and when using Single-Photon Detectors with single- or multi-photon returns; for other receiver systems (using MCP's), it should be checked also. As a consequence, the necessary center-of-mass correction for some satellites will differ, depending on the receiver systems. While this effect is not yet visible on small satellites or small retro-reflector arrays (like STARLETTE, ERS1), it can be in the order of centimeters on AJISAI or ETALON

Kirchner, Georg

Koidl, Franz

English

NASA Technical Reports Server

N94-15558
Work at Graz on
Satellite Signatures
Georg KIRCHNER, Franz KOIDL
SLR Graz / Institute for Space Research
Austrian Academy of Sciences
Observatory Lustb_hel, Lustb_helstr. 46
A-8042 GRAZ, AUSTRIA
Tel.: +43-316-472231; Fax: +43-316-462678
E-Mail: kirchner@flubiw01.tu-graz.ac-at
Abstract
The size and shape of the satellites retroreflector arrays
have a major impact on the distribution and scatter of the return
signal; this can be seen clearly when reaching sub-cm ranging
accuracies and when using Single-Photon Detectors with single- or
multi-photon returns; for other receiver systems (using MCP's) it
should be checked also. As a consequence, the necessary center-
of-mass correction for some satellites will different, depending
on the receiver systems.
While this effect is not yet visible on small satellites or
small retro-reflector arrays (like STARLETTE, ERSI), it can be in
the order of centimeters on AJISAI or ETALON.
1.0 Introduction
The SLR station Graz uses, since some years, Single Photon
Avalanche Diodes (SPAD) to detect return signals from satellites;
these diodes can detect single photons - which is used for
ranging to ETALON, or, in worse atmospheric conditions, also for
lower satellites -, but usually we use multiphoton returns to get
maximum return 2ates; this results in better accuracies when
using Normal-Point-Methods, and helps to avoid part of the
2-15
https://ntrs.nasa.gov/search.jsp?R=19940011085 2020-06-16T19:18:26+00:00Z
satellite signatures
described later.
in the return signal distribution, as
2.0 Ranging tests to the calibration target
Standard calibration to our target (distance about 400 m)
gives an RMS of 6 to 7 mm for the routine SLR setup (with HP5370A
counter, SPAD at i0 V above break, cooled to -27°C; 2.5a limits);
an example is shown in fig. i, upper histogram; the distribution
shows nice symmetry, with an RMS of 5.8 mm at 2.5a ; there is no
significant change of the mean value when using different sigma
criteria, also indicating a proper distribution.
To check influences of the ranging system itself on the
distribution of the return signals, different tests with various
misadjustments of the involved devices were made; a worst-case
example is shown in fig. i, lower histogram: A similar calibra-
tion as before, but with misaligned SPAD, lower voltage above
break, no cooling, and non-optimum start puls detection; this
results in a non-symmetric distribution, higher RMS, and measu-
rable shift of the mean value for varying sigma criteria.
After verifying the maximum contribution of the system itself,
the distribution of returns from different satellites were
analyzed.
3. ERSI and STARLETTE
Due to their small •size of the retroreflector arrays, there is
no significant satellite signature visible in ERSI and STARLETTE
data; the average RMS is between 8 and 9 mm, close to the values
obtained from the calibration target.
The distribution of the data (fig. 2, upper histogram) is more
or less symmetric, with small irregularities due to lower number
of returns; the 3 mm bin width for all these histograms was
chosen in coincidence with the 20 ps resolution of the HP 5370A
Counter.
2-16
4.0 AJISAI
As a contrast to the previous satellites, AJISAI shows quite
significant signatures; these are very dependant of the return
signal strength (fig. 3).
With strong (multiphoton) return signals, AJISAI shows low RMS
(11.8 mm, upper histogram), and only slightly non-symmetric
distribution; when reducing the return signal level to mostly
single photon-electrons (by switching off the last laser
amplifier, and opening divergence; the return signal level is
checked in real time by watching the return signal rate and/or
the number of semi-train returns on a graphic screen), the RMS
increases to 22 mm (lower histogram), while the distribution now
follows the shape of the satellite.
Both histograms in fig. 3 are shown with editing criteria of
2.5 a, which is used for our routine ranging procedures; using
other a values (4 a, 3 a, 2 a), the mean value of the histogram
will move - in the worst cases - between 1 and almost 2 cm.
5.0 LAGEOS
As expected from size and shape of LAGEOS, the satellite
signature is much less visible than with AJISAI, but is still
present (fig. 2); it is the main contribution to the increase of
the RMS, from 6 to 7 mm from the target, to ii to 14 mm from the
satellite (again, this is valid for our single-photon detection
system, using single photon and/or multi-photon return
signals!!) , with lower signal levels resulting in higher RMS.
When using different editing criteria again, the non-symmetric
distribution causes a shift of the mean value of about 1 to 2 mm,
only in extreme cases up to 4 mm.
6.0 ETALON
To complete the satellite's list, we show also the signature
coming from ETALON ranging data, this time using a different way
2-17
of demonstrating the non-symmetric distribution. While fig. 4
shows the residuals of an ETALON-2 pass (demonstrating also the
advantage of using the semitrain!), in fig. 5 the residuals are
plotted after "folding", polynomial fitting and 2.Sa-editing. Due
to the low return signal level from these satellites (most of the
returns are single photons: We are ranging with 5 to i0 mJ
Semitrain - this is about 2 or 3 mJ for the first pulse! - and 50
cm receiver to the ETALONS, still getting return rates of up to
i000 returns per hour), we see the full satellite's size and
shape in the data, with an RMS of 3.5 to 4 cm
7.0 Conclusion
To keep systematic errors due to the influence of the
satellite's shape as low as possible, we
- keep calibration and satellite return signals in the same level
- use the same editing criteria (2.5 u) for calibration and for
satellite data;
- try to minimize any contributions of the system itself to non-
symmetric distributions.
As far as we know, in all analyzing calculations the
satellites are treated as a "flat" reflecting surface, with a
fixed center-of-mass correction, which was determined before
launch using the detectors, techniques and accuracies available
at that time. However, with the improvements in accuracy, we can
see now the shape of the satellites in our data, which in turn
can influence the values of the center-of-mass corrections; so it
seems necessary to determine the center-of-mass correction with
respect to different receiver systems, and using different values
for the analysis.
2-18
TARGET 1 CALIB: 1500 Rets
Good SPADAUGNmENT/ 10 Vab /-2Z°C
Re_du,18 ptr 6in
4.0 SIGMA: -6.8 mm 9.1 mm
_l.O SIGMA: -10.0 mm e.l mm .................................
................................................................................g2.8 $1GMA_ -10.0 mm l.S mm
2-0 SIGMA: -0.9 mm 4.7 mm
- • . T -- --. , lit h',l'l 'lll'tt _l _ _ , i ,
I I I I I I I I | I I I I I I I i I I I I 1 I I I I
Bin Width: 3 [mm]
1992-05-08
TARGET 1 CALIB: 1500 Rets
Weak SPA_ ALIGNMENT / 5 V_b / _15°C
Residu&is I:er Bin
RM8:8.6 mm
Bin Width: 3 [ram]
1gg_-05-08
Figure i: Histograms of calibration ranging to the target
2-19
ERS1 RESIDUALS DISTRIBUTION
IR12820 / 4,34 Returns / 10 Vab / -27°C
.........................._! _ _' '-_.." --".--.'-....... ....._ ........[_, ,°o-,.,0,. I.i
No Llmlh 4_4 0.0 mm 1_.0 mm I1_ II
I I U
S.O Si_lmS. 416 -1.6S mm 9.2 mm I I nt"_
...._2","_ii_;;;;';_;;'"-_;'_;;'"_2:F;_;;.........""'_11IIIB.............................................................._:
ItH!
I t t I I i I t t I I I I I t I i I i I llitlltlll;llllln ,.,
Bin Width= 3 [mm]
1992-05-08
LAGEOS RESIDS DISTRIBUTION
L_622 / 2552 F_e_u_'n8 / "_OVab / -27°?
-; , . ,{I I¢il|_l
1992-05-08
I i i I
_Risiduils per Bin !
2 mm
I lll II IIllll _ Vr_-___.__- :_.__ ____-
IllJtJ}rltllllllltlllJl'i'llllltilti'i-Jlll,.
Bin Width: 3 [mini
Figure 2: Histograms of ERSI and LAGEOS returns
AJISAI RESIDS DISTRIBUTION
JGTOGO / 13_0 Returns / 10 Vab / -27 °
Strong Return Signal ,,,, Small Influence of Satellite Shape
Ti I I i i I 1 I i I I
lgg2-OS-lS
)-
RMS: 11.8 mm
tl itilIIlill illtltlilil il Ill I1tl I I I I Ii II
Bin Width: 3 [mini
AJISAi RESIDS DISTRIBUTION
f Weak Return Signal --, Large Influence of Satellite Shape
!IH!
RMS: 22.0 mm
J ...................
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i i i I I I I 1 I I I I I I I I I I i i , t I I I I I I i I I I I I i I I i tl
Bin Width: 3 [mm]
1992-05-13
Figure 3: Histograms of AJISAI returns
2-21
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2-22


Work at Graz on satellite signatures

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