**HISTORICAL MOBLAS**

**SYSTEM CHARACTERIZATION**

**Van S. Husson**

> Abstract \-- This paper is written as a direct response to the
> published NASA LAser GEOdynamic Satellite (LAGEOS) orbital solution
> SL7.1 \[Smith et al., 1991\], in order to close the data information
> loop with an emphasis on the NASA MOBile LAser Ranging System\'s
> (MOBLAS 4,5,6,7,8) LAGEOS fullrate data since November 1, 1983. A
> preliminary analysis of the supporting information (i.e. satellite
> laser ranging system eccentricities and system dependent range and
> time bias corrections) contained in SL7.1 indicated CentiMeter (cm)
> level discrepancies. In addition, a preliminary analysis of the
> computed monthly MOBLAS range biases from SL7.1 appear to show cm
> level systematic trends, some of which appear to be \"real\",
> particularly in the 1984 to 1987 time period. This paper is intended
> to be a reference document for known MOBLAS systematic errors
> (magnitude and direction) and for supporting MOBLAS information
> (eccentricities, hardware configurations, and potential data problem
> periods). Therefore, this report is different than your typical system
> characterization report \[Pearlman, 1984\], but will be more valuable
> to the user. The MOBLAS error models and supporting information
> contained in this paper will be easily accessible from the Crustal
> Dynamics Data Information System (CDDIS).

**1. INTRODUCTION**

In the late 1970\'s, NASA developed five, second generation mobile laser
ranging systems in support of the SEa Altimeter SATellite (SEASAT)
mission. The goal of the SEASAT mission was to map the surface of the
oceans using the onboard satellite altimeter. Satellite Laser Ranging
(SLR) data was used to calibrate the SEASAT altimeter. During the
1980\'s, the MOBLAS systems underwent an extensive upgrade program that
improved the data accuracy and precision an order of magnitude, from the
100 MilliMeter (mm) to the 10 mm level. Since the mid-1980\'s, LAGEOS
data has been compressed into 2 minute bins (normal points), which
statistically reduces the single shot precision level \[Smith et al.,
1985\]. MOBLAS LAGEOS normal point precision levels have been at the 1-2
mm level since the late- 1980\'s.

During the 1980\'s, cm level systematic biases were discovered through
special and collocation analysis techniques developed by Bendix in
support of the Goddard Laser Tracking Network (GLTN) and the Crustal
Dynamics Satellite Laser Ranging (CDSLR) mission contracts. When
centimeter level systematic errors were discovered in NASA SLR data, the
NASA management philosophy was to identify the source of the bias; to
eliminate or model the bias; and to document the nature of the bias. Due
to cost considerations, historical NASA SLR data were not repaired (in
most cases) to remove known cm level systematics. In addition, the
documentation of these bias problems was not maintained in a central
database and had a limited distribution. The intent of this paper is to
provide one reference source for known uncorrected MOBLAS (LAGEOS)
systematic errors that will be available from the CDDIS.

The original MOBLAS major hardware components were the General Photonics
(GP) q-switched laser, the Amperex dynode-chain PhotoMultiplier Tube
(PMT), the Ortec 934 discriminator, and the Hewlett Packard (HP) 5360
time interval counter. In the 1983-1984 time frame, the largest
systematic unrecoverable error source, the GP laser, was replaced with a
Quantel mode-locked laser. The HP5370 counter was usually incorporated
as part of the Quantel laser upgrade. In the 1986-1987 timeframe, the
largest systematic error source, the Amperex 2233B and Ortec
discriminator, was replaced with the ITT F4129 gated three-stage
Micro-Channel Plate (MCP)PMT and Tennelec TC454 discriminators. The
systematic error sources associated with the hardware components
mentioned above are discussed in Degnan \[1985\] and Varghese \[1985\].

It is virtually impossible to recover any hardware related systematic
errors in the GP laser era due to the domination of
\"wavefront-distortion\" errors \[Degnan, 1985\]. In the Quantel laser
pre-MCPPMT era, the largest recoverable systematic hardware related
error source was signal strength (0-30mm) of the returning pulse
\[Heinick, 1984\]. Discriminator and PMT \"timewalk\" characteristics,
coupled with calibration signal strengths not distributed the same as
LAGEOS signal strengths induced a systematic bias. The magnitude and
direction of the bias was system dependent, and decreased in magnitude
as calibration methods improved. A model has been developed to recover
this error based on the review of historical calibration and LAGEOS
\"timewalk\" curves. Unfortunately, only a small sample of these curves
ever existed. Signal strength effects did vary from pass-to-pass and
within a pass. The MCPPMT and dual Tennelec discriminators package
developed by Varghese, coupled with proper calibration techniques
essentially eliminated signal strength as a bias source at the mm level.

In 1988, another recoverable hardware systematic error source was
discovered by See and Sneed. They discovered the measurement of the
laser transmit delay (TDEL) was in error, because the TDEL electronics
did not measure the cable delay (\<0.5 microseconds) between the time
code generator and the system computer. In addition, they and McCollums
discovered that the TDEL electronics had other TDEL sub-microsecond
accuracy ambiguities, which would have masked this bias. In the
1989-1990 timeframe, the TDEL electronics were modified to remove these
sub-microsecond ambiguities. The cable delay has been measured in
MOBLAS-4,7,8 and is being modelled in the pre-processing. As of this
writing, the MOBLAS-5 and 6 cable delays have not been measured.

Other mm level hardware errors are known to exist in the current MOBLAS
hardware (i.e. HP5370 \[Selden, 1992\] and optical path changes as a
function of pointing angle \[McCollums, 1986\]), but are presently
unrecoverable. Not all systematic error sources in SLR systems are
hardware related. Survey of ground targets, survey of monument offsets,
atmospheric modelling, data pre-processing, and orbital modelling are
other SLR error sources. Fortunately, many of these errors sources can
be recovered to the 10-20mm level, using historical processing results.

In the GP laser era, calibration targets were large boards at distances
of 2-3 KiloMeters (Km). These boards were mounted on metal poles which
were guyed. The height of these poles was system dependent. Cube corners
also existed and were mounted on various structures and in most cases
were in close proximity (a few meters) to the calibration boards. The
reason calibration boards were used versus cube corners was to average
the \"wavefront-distortion\" effects of the GP laser. Historical ground
tests results \[Brogdon et. al.\] indicated up to 20cm and 60cm
differences in system delay between the board and cube corner, and as a
function of pointing angle, respectively. When the mode-locked (Quantel)
lasers were installed, ground tests results were dramatically improved,
and for the first time, 10-20mm survey errors to the ground targets and
10-20mm ground target movement could be detected \[Wroe et. al.\].
Within a few months of the MCPPMT\'s (1986-1987) being installed, the
primary calibration targets became the original cube corners. In the
1988-1989 timeframe, long (2-3Km) Nelson piers became the primary
calibration targets. In the 1991-1992 timeframe, the primary targets
became short (100-200 meters) Nelson piers with the advent of the
translator and the short target ranging electronics \[Eichinger et. al.,
1990\].

In the early to mid 1980\'s, cm level movement of the original targets,
coupled with survey uncertainties and infrequent surveys, severely
limited the absolute accuracy of MOBLAS data. \"Average\" target
movement and survey uncertainties, in most cases, is recoverable to the
10- 20mm level using historical ground test results. In the mid-1980\'s,
surveying equipment and frequency of surveying was improved, but the
problem still existed with unstable targets. In the late-1980\'s, the
target stability problem was minimized to the millimeter level, when the
current day target calibration structure, the Nelson pier, were used as
the primary calibration target.

The MOBLAS eccentricity measurement techniques were improved in the
mid-1980\'s \[Nelson, 1986\]. The infrequency of eccentricity surveys
was a mm level systematic error source, because long term temporal
changes in the eccentricity data were at this level. Some of these
errors are traceable based on MOBLAS log records or special analysis
techniques.

Neutral Density (ND) filters have always been used during calibration to
attenuate the signal. During satellite ranging the filters were removed,
thus inducing a difference in the \"effective\" calibration and
satellite optical paths \[Crawford, 1985\]. This difference was not
modelled, but is recoverable to the 1mm level. In 1991-1992 the Optical
Attenuation Mechanisms (OAM) \[Silva et. al., 1991\] were installed in
the MOBLAS, which minimizes the use of ND filters. The ND filter bias is
currently being modelled in the pre-processing.

Prior to January 1, 1988, the tropospheric range correction applied to
the fullrate data was computed based on the MOBLAS elevation encoder
measurements. These measurements could be in error by a few millidegrees
and would thus induce a systematic error in the tropospheric refraction
correction \[Husson et. al., 1987\]. This correction could be in error
up to 20mm. The error was system and satellite dependent, but is fully
recoverable to the sub-mm level. To remove this error in pre-1988 MOBLAS
data, the applied tropospheric refraction must be removed and reapplied
using the computed elevation angle. All MOBLAS data taken after January
1, 1988, were pre-processed using the computed elevation angle from a
short arc fit to the ranges.

LAGEOS orbital modelling errors have improved significantly over the
last 12 years along with the quality of SLR data \[Smith et. al.,
1991\]. Orbital modelling is limited by the quality and distribution of
the global SLR dataset, but in some instances, is the only hope for
recovering the \"net\" historical SLR biases.

In the remainder of this article, each MOBLAS system is addressed
individually. Each system\'s eccentricities and recoverable bias sources
are discussed and summarized in tables, which are contained at the end
of this article. The bias model tables include the major system
configuration changes (laser, PMT, discriminators, counter) and
important pre- processing information (i.e. calibration target and
calibration range). The tables and this article will be available from
the CDDIS.

**2. MOBLAS-4**

**2.1 MOBLAS-4 Eccentricities**

Since August 15, 1983, MOBLAS-4 has only occupied one monument/marker
(7110). Prior to August 1983, 7110 was occupied by MOBLAS-3. There
exists 5 sets of MOBLAS-4 eccentricities and 10 MOBLAS-4 Site Occupation
Designators (SOD\'s) at 7110. SOD\'s 71100402, 71100403, and 71100410,
each have two sets of eccentricities.

The 3mm change in the Up ecc., between the 1985 and 1988, surveys could
be caused by settlement of the pad and monument \[Nelson, 1988\]. There
were no measurements of the eccentricities between 1985 and 1988. The
MOBLAS-4 mount was releveled twice in 1988 (June 16 and December 13),
but apparently there was no change in eccentricities, based on the April
1988 and November 1989 surveys. In February 1992, the eccentricities
were remeasured prior to the removal of the mount. There was a 2mm
unexplained change in the North ecc. In March 1992, the MOBLAS-4 mount
was removed from system to be refurbished, and in April 1992, the
refurbished mount was re-installed and the ecc. were remeasured. Table 1
contains the recommended set of MOBLAS-4 eccentricities with their
corresponding SOD(s) and effective starting dates.

**2.2 MOBLAS-4 LAGEOS Biases**

The net MOBLAS-4 recoverable biases (range and time) are presented in
Table 2. To correct MOBLAS-4 LAGEOS data, the known range and time
biases (in Table 2) should be subtracted from the LAGEOS range and
timetags, respectively. Any time there is the potential for a change in
the bias(es), even though the bias(es) did not change, a new entry has
been added. A component breakdown of the biases is presented, followed
by a section describing known problem data, and followed by a section
describing potential problem data.

**2.2.1 MOBLAS-4 ND filters**

MOBLAS-4 LAGEOS data was biased short by 3mm + 1 and 1mm + 0mm from
August 15, 1983 through January 17, 1992, and from January 18 through
May 31, 1992, respectively, due to ND filters. On January 18, 1992, the
OAM was installed in MOBLAS-4 reducing the ND filter bias. After June 1,
1992, this bias has been removed through modelling.

**2.2.2 MOBLAS-4 signal strength**

MOBLAS-4 LAGEOS data was biased long by 20mm + 10, 10mm + 5, 6mm + 3,
and 4mm + 2 from August 15, 1983 through June 30, 1984; July 1, 1984
through August 31, 1984; September 1, 1984 through July 31, 1985; and
August 1, 1985 through November 19, 1986; respectively, due to signal
strength. This bias would vary from pass-to-pass and would have a slight
elevation dependence. Low elevation data would be biased longer by
several mm than high elevation data. Signal strength effects were
reduced as the MOBLAS-4 calibration techniques improved, and were
eliminated when the MCPPMT package was installed on November 20, 1986.

Prior to January 1, 1988, the tropospheric range correction applied to
the fullrate data was computed based on the MOBLAS-4 encoder
measurements. To remove this error in pre-1988 MOBLAS-4 data, the
applied tropospheric refraction must be removed and reapplied using the
computed elevation angle.

**2.2.4 MOBLAS-4 calibration targets/distances**

MOBLAS-4 has used 4 different calibration targets (1 board and 3 cube
corners) at marker 7110. From August 15, 1983 through December 31, 1987;
from January 1, 1987 through January 28, 1988; from January 29 through
April 29, 1988; from April 29, 1988 through December 5, 1991; and from
December 6, 1991 to the present; the MOBLAS-4 primary calibration
targets were the 2Km board, the 2Km cube corner, the 2Km board, the 2Km
Nelson pier, and the 200m Nelson pier, respectively.

MOBLAS-4 has been surveyed 5 times (1983, 1985, 1988, and twice in 1992)
at marker 7110. Sometime between the 1983 and the 1985 surveys, vandals
removed the bracing support of the MOBLAS-4 calibration board tower
\[Nelson, 1985\], which could explain the 13mm difference between the
1983 and 1985 calibration board distances. When the new calibration
board values were used in data processing, there would be an
automatically induced 13mm discontinuity in the MOBLAS-4 range bias. The
new board values were used with all data taken after June 20, 1985. Data
just prior to June 20, 1985 would have been biased longer by 13mm than
data after June 20, 1985 (This 13mm change does not appear in the range
bias model, because the exact date of the change is unknown). The cube
corner became the operational target in January 1987 (The exact day is
unknown at this time).

The distance (2Km) and the design/stability of the original MOBLAS-4
targets (a calibration board mounted on a tower and a cube corner
mounted on wooden posts) were limiting factors to surveying accuracy and
MOBLAS-4 data accuracy. The survey accuracy of these original targets
was at the 10-15mm level \[Nelson\]. Historical ground test results
indicate up to 20mm differences in \"apparent\" system delays between
the MOBLAS-4 calibration board and the cube corner, which indicates both
targets could move at least 10mm in either direction from month-
to-month and from day-to-day. LAGEOS orbital analysis appears to be the
only viable means for recovering the \"true\" average MOBLAS-4 monthly
target distance to better than 20mm in the October 1983 to April 1988
timeframe.

Between November 1, 1987 and January 28, 1988, the MOBLAS-4 operational
calibration target, cube corner, was very unstable, which caused unusual
changes (up to 90mm) in the \"apparent\" system delay. Individual
MOBLAS-4 LAGEOS passes could be biased long (up to 90mm) during this
period. Eanes \[1987\] originally discovered this data problem. The bias
could change (up to 30mm) from pass-to-pass and from day-to-day. The
problem was eliminated when the previous calibration target (board) was
used starting January 29, 1988. On April 30, 1988, the MOBLAS- 4
calibration target became the 2Km Nelson pier.

**2.2.5 MOBLAS-4 TDEL**

All MOBLAS-4 data between January 5, 1990 and May 31, 1992 contains a
time bias of -0.4 microseconds. To correct this time bias, add +0.4
microseconds to each observation\'s timetag. This correction is only
valid after January 5, 1990, because on this date, the MOBLAS-4 TDEL
electronics were modified to eliminate other TDEL sub-microsecond
accuracy ambiguities, which would have masked this bias. This bias has
been eliminated through modelling, effective June 1, 1992.

**2.2.6 MOBLAS-4 known problem data**

Between April 12, 1984 and May 24, 1984, individual MOBLAS-4 passes
contained discontinuities (100mm up to 600mm) due to the range gate
window being changed during the pass. The magnitude of the
discontinuities depends upon where the return occurred in the range gate
window \[Varghese, 1985\]; therefore, data during this period should not
be used. This problem was originally detected by Eanes \[1985\] and was
resolved when the system adhered to standard tracking procedures.

**2.2.7 MOBLAS-4 potential problem data**

Between December 29, 1988 and January 27, 1989, MOBLAS-4 LAGEOS data is
questionable due to a bad time interval unit (HP5370). The RMS scatter
of this data was cyclic and dependent upon the LAGEOS range \[Heinick
et. al., 1989\]. The data is believed to be unbiased and just
systematically noisy. This problem was resolved by replacing the HP5370.

**3. MOBLAS-5**

**3.1 Eccentricities**

Since July 1, 1979, MOBLAS-5 has only occupied one monument/marker
(7090). There exists only 3 unique sets of MOBLAS-5 eccentricities and
12 MOBLAS-5 SOD\'s at 7090. SOD 70900508 has 2 sets of eccentricities.

The MOBLAS-5 mount was releveled on September 5, 1985. No survey was
performed after the releveling til 1987. The 8mm change in the Up ecc.,
between the 1979 and 1987 surveys, remains unexplained. It is not known
how far back the 1987 Up ecc. can be backdated. The MOBLAS-5 mount was
also releveled on August 5, 1989, but the eccentricities were not
remeasured. In January 1992, the mount was re-installed and the
eccentricities were remeasured. Table 3 contains the recommended set of
MOBLAS-5 eccentricities with their corresponding SOD(s) and effective
starting dates.

**3.2 MOBLAS 5 LAGEOS Biases**

The net MOBLAS-5 recoverable biases (range and time) are presented in
Table 4. To correct MOBLAS-5 LAGEOS data, the known range and time
biases (in Table 4) should be subtracted from the LAGEOS range and
timetags, respectively. Any time there is the potential for a change in
the bias(es), even though the bias(es) did not change, a new entry has
been added. A component breakdown of the biases is presented, followed
by a section describing known problem data, and followed by a section
describing potential problem data.

**3.2.1 MOBLAS-5 ND filters**

Prior to June 1, 1992, MOBLAS-5 LAGEOS data was biased short by 3mm + 1
due to ND filters. After June 1, 1992, this bias has been removed
through modelling.

**3.2.2 MOBLAS-5 signal strength**

MOBLAS-5 LAGEOS data was biased short by 3mm + 2 from July 27, 1983
through April 22, 1987, due to signal strength. This bias would be
fairly constant from pass-to-pass. This bias was eliminated when the
MCPPMT package was installed in MOBLAS-5 on April 23, 1987.

**3.2.3 MOBLAS-5 elevation angles**

Prior to January 1, 1988, the tropospheric range correction applied to
the fullrate data was computed based on the MOBLAS-5 encoder
measurements. To remove this error in pre-1988 MOBLAS-5 data, the
applied tropospheric refraction must be removed and reapplied using the
computed elevation angle.

**3.2.4 MOBLAS-5 calibration targets/distances**

MOBLAS-5 has used 4 different calibration targets (2 boards and 2 cube
corners) at marker 7090. From August 1, 1979 through June 7, 1980; from
June 8, 1980 through May 31, 1987; from June 1 through July 31, 1987;
from August 1 through August 25, 1987; and from August 26, 1987 through
June 30, 1992; the MOBLAS-5 primary calibration targets were the 2Km
calibration board, the 3Km calibration board, the 1Km cube corner, the
3Km calibration board, and the 3Km Nelson pier, respectively.

MOBLAS-5 has been surveyed only 3 times in 13 years (1979/80, 1987, &
1992) at marker 7090. Between the 1979/80 and 1987 surveys, the 1Km cube
corner and the 3Km board ranges changed by +30mm and -9mm, respectively.
The 2Km board was never resurveyed and the 3Km board blew away in August
1987. In early January 1992, the mount was replaced and a new survey was
performed.

The system delay differences obtained from the 1Km cube corner and the
3Km board in historical minico tests since June 1985 were 39mm + 5 and
0mm + 5 \[Husson and Wroe, 1987\] using the 1980 and 1987 surveys,
respectively. Therefore, the 1987 survey values can be backdated to at
least June 1985, which means MOBLAS-5 data between June 1985 through May
31, 1987 (all board calibrated data) was biased long by 9mm +5mm. All
data since June 1, 1987, until the mount replacement in 1992, was
processed with the 1987 surveyed ranges and should be bias free.
Research into minico results prior to June 1985 still needs to be
performed.

**3.2.5 MOBLAS-5 TDEL**

The MOBLAS-5 cable delay causing this bias has not been measured as of
this writing. On January 30, 1990, the MOBLAS-5 TDEL electronics were
modified to eliminate other TDEL sub- microsecond ambiguities.

**3.2.6 MOBLAS-5 PMT Gating**

Prior to June 8, 1980, MOBLAS-5 LAGEOS data was biased short by 20mm+5
due to calibration data falling in a non-linear region of the Amperex
2233B PMT gate. This bias was characterized by Varghese \[1985\]. This
bias was eliminated, effective June 8, 1980, when a longer calibration
board (\>3Km) was used.

**3.2.7 MOBLAS-5 known problem data**

Between December 19, 1991 and January 3, 1992, MOBLAS-5 data quality was
degraded due to a stuck bit. The bit was stuck low (100 picosec.) for
LAGEOS. It does not appear that this data is recoverable, because of
it\'s unknown effect on calibration data.

**3.2.8 MOBLAS-5 potential problem data**

Between August 12, 1988 and January 31, 1989, MOBLAS-5 system delays and
pre-to-post calibration shifts were erratic \[Heinick et al., 1989\].
Therefore, the uncertainty in the system delay is increased during this
period; however, this should be a random error than should average over
several passes. This problem disappeared during February 1989.

**4. MOBLAS-6**

**4.1 MOBLAS-6 Eccentricities**

Since March 3, 1983, MOBLAS-6 has only occupied one monument\\marker
(7122). There exists 4 sets of MOBLAS-6 eccentricities and 9 MOBLAS-6
SOD\'s at 7122. SOD 71220604 has two sets of eccentricities.

The May 1988 survey sheet had the wrong sign for the East ecc. It should
be negative, not positive. Based on the 1991 survey, there was no
apparent change in the eccentricities when the MOBLAS-6 mount was
releveled on August 5, 1989. Table 5 contains the recommended set of
MOBLAS-6 eccentricities with their corresponding SOD(s) and effective
starting dates.

**4.2 MOBLAS-6 LAGEOS Biases**

The net MOBLAS-6 recoverable biases (range and time) are presented in
Table 6. To correct MOBLAS-6 LAGEOS data, the known range and time
biases (in Table 6) should be subtracted from the LAGEOS range and
timetags, respectively. Any time there is the potential for a change in
the bias(es), even though the bias(es) did not change, a new entry has
been added. A component breakdown of the biases is presented, followed
by a section describing known problem data, and followed by a section
describing potential problem data.

**4.2.1 MOBLAS-6 ND filters**

All MOBLAS-6 LAGEOS data was biased short by 3mm + 1 due to ND filters.

**4.2.2 MOBLAS-6 signal strength**

MOBLAS-6 LAGEOS data was biased long by 12mm + 4, 10mm + 3, and 4mm + 2
from May 1, 1984 through March 31, 1986; from April 1 through August 31,
1986; and September 1, 1986 through January 27, 1987; respectively, due
to signal strength. This bias would vary slightly (several mm) from
pass-to-pass and would have a slight elevation dependence. Low elevation
data would be biased longer than high elevation data. Signal strength
effects were reduced as system calibration techniques improved, and were
eliminated when the MCPPMT package was installed in MOBLAS-6 on January
28, 1987.

**4.2.3 MOBLAS-6 elevation angles**

Prior to January 1, 1988, the tropospheric range correction applied to
the fullrate data was computed based on the MOBLAS-6 encoder
measurements. To remove this error in pre-1988 MOBLAS-6 data, the
applied tropospheric refraction must be removed and reapplied using the
computed elevation angle.

**4.2.4 MOBLAS-6 calibration targets/distances**

MOBLAS-6 has used 4 different calibration targets (1 board and 3 cube
corners) at marker 7122. From May 1, 1983 through March 26, 1987; from
March 27, 1987 through August 16, 1988; from August 17, 1988 through
October 3, 1989; and from October 4, 1989 through April 30, 1991; the
MOBLAS-6 primary calibration targets were the 2Km calibration board, the
2Km cube corner, the first 2Km Nelson pier, and the second 2Km Nelson
pier, respectively.

MOBLAS-6 was surveyed 4 times in 8 years (1983, 1988, 1989, & 1991) at
marker 7122. In October 1983, several months after the first survey, a
hurricane passed through the site. Between the 1983 and 1988 surveys,
the ranges to the 2Km cube corner and the 2Km board changed by +6mm and
-17mm, respectively. The system delay differences obtained from the
these two targets in historical minico tests, since the Quantel laser
installation (July 1984), were 33mm + 15 and 10mm + 15 \[Husson and
Wetzel, 1987\] using the 1983 and 1988 surveys, respectively. The 1988
surveyed target ranges for the cube corner and the board are only
accurate to the 10mm level. The 10mm difference which remains in the
historical minico results can be attributed to survey. Making this
assumption and distributing the 10mm difference equally to both targets,
the 1988 survey (with adjustment) can be backdated to the hurricane
(October 1983). This means MOBLAS-6 data from October 1, 1983 through
March 26, 1987; from March 27, 1987 through May 2, 1988; and from May 3
through August 16, 1988; is biased long by 22mm + 10, short by 11mm +
10, and short by 5mm + 10, respectively.

In 1988, the first Nelson pier was built; however, the pier foundation
was not built to specifications. The pier moved 14mm between 1988 and
1989 due to a poor foundation and being on a steep slope \[Nelson,
1989\]. Nelson believes this movement would have been linear over the 13
months between the 1988 and 1989 surveys. MOBLAS-6 data between August
17, 1989 and June 23, 1989, was biased long by 4mm + 1mm/month. On
October 4, 1989, the primary MOBLAS-6 calibration target became the
second Nelson pier, which eliminated target movement as a MOBLAS-6
systematic bias source.

**4.2.5 MOBLAS-6 TDEL**

The MOBLAS-6 cable delay causing this bias has not been measured as of
this writing. On January 11, 1990, the MOBLAS-6 TDEL electronics were
modified to eliminate other TDEL sub- microsecond ambiguities.

**4.2.6 MOBLAS-6 known problem data**

All MOBLAS-6 satellite RMS\'s between February 1 and March 1, 1984, were
high caused by a bad distribution amplifier \[Oldham\]. Data during this
period should not be used. On March 2, 1984, the distribution amplifier
was replaced which eliminated the problem.

**4.2.7 MOBLAS-6 potential problem data**

All MOBLAS-6 data between February 8 and May 12, 1989, was affected by a
bad Setra barometer. Setra barometric data was in error by up to 30
millibars. All MOBLAS-6 data during this interval was corrected and
believed to be accurate to at least 3 millibars.

All MOBLAS-6 calibration data between April 1 and September 26, 1990,
was unusually noisy (twice the normal RMS). The LAGEOS data during this
period is suspected to be bias free. The calibration data returned to
normal when some adjustments were performed on the HP5370 in late
September \[MOBLAS-6, 1990\].

**5. MOBLAS-7**

**5.1 MOBLAS-7 Eccentricities**

Since January 1, 1981, MOBLAS-7 has only occupied one monument/marker
(7105); however, two other laser systems (TLRS-2, 71051206, and
MOBLAS-2, 71050207) have been referenced to marker 7105. There exists 14
sets of MOBLAS-7 eccentricities and 21 MOBLAS-7 SOD\'s at 7105.

The reason for the 5mm change in the east ecc. between the 1981 and 1984
survey is assumed to be caused by movement of the survey monument to the
East or by movement of the MOBLAS support pad to the West \[Nelson,
1984\].

During the 1985 pre-collocation survey of MTLRS-1 and MOBLAS-7, a 2 cm
discrepancy was found in the 7105 Up eccentricity \[Nelson, 1985\]. The
original determination of the Up eccentricity is suspected to be in
error \[Nelson, private communication\]; and therefore, the 1985 Up
eccentricity can be backdated to Jan 1, 1981.

Prior to the TLRS-1 and MOBLAS-7 collocation in 1986 \[Nelson, 1986\],
MOBLAS-7 eccentricities were measured on three different occasions, once
in December 1985 and twice in January 1986 (before and after the
MOBLAS-7 mount was releveled on January 18, 1986). There were millimeter
level changes between these surveys. This would give SOD 71050709 five
unique sets of eccentricities.

During October 1989, a 1-2 cm bias existed between MOBLAS-7 and TLRS-3
\[Varghese and Husson, 1989\] and a follow-up survey indicated cm level
movement of the MOBLAS-7 reference point. Polyquick collocation analysis
revealed that the movement appeared to have occurred in August 1989;
therefore, the October 12, 1989 survey can be backdated to August 25,
1989.

MOBLAS-7 was down for approximately 6 months in early-1990 due to a
major slip ring failure. During this period that the slip rings were
being repaired, MOBLAS-7 was removed from it\'s pad in order that the
pad could be reinforced. Prior to ranging and after MOBLAS-7 re-occupied
marker 7105, the eccentricities were remeasured. Table 7 contains the
recommended set of MOBLAS-7 eccentricities with their corresponding
SOD(s) and effective starting dates.

**5.2 MOBLAS-7 LAGEOS Biases**

The net MOBLAS-7 recoverable biases (range and time) are presented in
Table 8. To correct MOBLAS-7 LAGEOS data, the known range and time
biases (in Table 8) should be subtracted from the LAGEOS range and
timetags, respectively. Any time there is the potential for a change in
the bias(es), even though the bias(es) did not change, a new entry has
been added. A component breakdown of the biases is presented, followed
by a section describing known problem data, and followed by a section
describing potential problem data.

**5.2.1 MOBLAS-7 ND filters**

All MOBLAS-7 LAGEOS data was biased short by 3mm + 1,1mm + 1,3mm + 1,
and 1mm + 1 from January 1, 1981 through July 9, 1991; from July 10
through 23, 1991; from July 24 through October 17, 1991; and from
October 18, 1991 through May 31, 1992; respectively, due to ND filters.
The OAM was installed on July 10, 1991, removed on July 24, 1991, and
reinstalled on October 18, 1991. After June 1, 1992, this bias has been
removed through modelling.

**5.2.2 MOBLAS-7 signal strength**

MOBLAS-7 LAGEOS data was biased long by 30mm + 15, 4mm + 2, 7mm + 3,
30mm + 10, 5mm + 3, 3mm + 2, and 4mm + 2 from August 22, 1983 through
February 29, 1984; from March 1 through July 31, 1984; from August 1
through September 30, 1984; from October 1, 1984 through January 25,
1985; from January 26, 1985 through March 31, 1985; from April 1, 1985
through March 30, 1986; and from February 1, 1988 through April 30,
1988; respectively, due to signal strength. This bias would vary
slightly (several mm) from pass-to-pass and would have a slight
elevation dependence. Low elevation data would be biased longer than
high elevation data. Signal strength effects were reduced as system
calibration techniques improved, and were eliminated when the MCPPMT
package was installed in MOBLAS-7 on March 31, 1986.

**5.2.3 MOBLAS-7 elevation angles**

Prior to January 1, 1988, the tropospheric range correction applied to
the fullrate data was computed based on the MOBLAS-7 encoder
measurements. To remove this error in pre-1988 MOBLAS-7 data, the
applied tropospheric refraction must be removed and reapplied using the
computed elevation angle.

**5.2.4 MOBLAS-7 calibration targets/distances**

MOBLAS-7 has used 3 different calibration targets (1 board and 2 cube
corners) at marker 7105. From January 1, 1981 through September 19,
1986; from September 20 through December 5, 1986; and from December 6,
1986 to the present; the MOBLAS-7 primary calibration targets were the
3Km board, the second 3Km cube corner, and the 200m Nelson pier,
respectively.

MOBLAS-7 has been surveyed at least once per year since January 1984.
The February 1981 and the June 1981 surveyed board ranges were
incorrect, causing all MOBLAS-7 data from March 1 through June 30, 1981
and from July 1 through December 31, 1981 to be biased long by 49mm + 20
and 22mm + 20, respectively.

Between the 1984 and 1985 surveys, the board range changed by 17mm,
however; historical minico results support the 1984 survey value
\[Heinick, 1985\] for the board range (not the 1985 value). On February
7, 1986, the guy wires were tensioned on the calibration board before
the board was resurveyed (12mm change); however, the new surveyed
calibration board range was not used until March 2, 1986. Therefore, all
MOBLAS-7 LAGEOS passes from August 1, 1985 through February 6, 1986, and
from February 7 through March 1, 1986, is biased long by 17mm + 20 and
short by 12mm + 20, respectively, except for the LAGEOS passes in Table
9. Occasionally due to ground fog conditions, the board could not be
ranged, so the secondary calibration target, the 3km cube corner, was
used. All passes in Table 9 were calibrated using the original 3Km cube
corner and are believed to have the correct calibration distance.
Between the January and October 1989 surveys, the Nelson pier target
range changed by 15mm due to movement of the MOBLAS-7 reference point.
Apparently, the MOBLAS-7 reference point moved around August 25, 1989
based upon intercomparison with TLRS-3 \[Varghese and Husson, 1989\].
All MOBLAS-7 data from August 25 through October 11, 1989 is biased
short by 15mm.

**5.2.5 MOBLAS-7 TDEL**

All MOBLAS-7 data from July 13, 1989 through September 27, 1990, and
from September 28, 1990 through May 31, 1992 contains a time bias -0.5
and -0.3 microseconds, respectively. This correction is only valid after
July 13, 1989, because on this date, the MOBLAS-7 TDEL electronics were
modified to eliminate other TDEL sub-microsecond accuracy ambiguities,
which would have masked this bias. On September 28, 1990, the cable
length causing this bias was shortened, which caused a decrease in the
magnitude of the bias. This bias has been eliminated through modelling,
effective June 1, 1992.

**5.2.6 MOBLAS-7 known problem data**

All MOBLAS-7 data between January 13 through January 17, 1986, has a
reference frequency problem and should not be used. The HP5370 frequency
was on internal frequency during this period of time.

**5.2.7 MOBLAS-7 potential problem data**

All MOBLAS-7 LAGEOS data between June 7 and June 13, 1984 was noisy
(100-130mm) and should be used with caution. The cause of the high noise
is unknown.

All MOBLAS-7 LAGEOS data between September 25 and October 5, 1984 was
noisy (80-120mm) and should be used with caution. The cause of the high
noise is unknown.

**6. MOBLAS-8**

**6.1 MOBLAS-8 Eccentricities**

Since October 1, 1981, MOBLAS-8 has only occupied one monument/marker
(7109). In July 1982, the MOBLAS-4 and MOBLAS-8 moms vans were switched.
There exists 9 sets of MOBLAS-8 eccentricities, and 15 MOBLAS-8 SOD\'s
at 7109.

On August 4, 1982 the system was releveled, but there was not a
follow-up survey until 1984, which indicated a 17mm change in the Up
ecc. Nelson \[1984\] recommends that the 1984 survey be backdated to
1982. There was a 4mm change in the Up ecc between 1985 and 1988,
possibly due to settlement of the laser support slab \[Nelson ,1985\].
Table 10 contains the recommended set of MOBLAS-8 eccentricities with
their corresponding SOD(s) and effective starting dates.

**6.2 MOBLAS-8 LAGEOS Biases**

The net MOBLAS-8 recoverable biases (range and time) are presented in
Table 11. To correct MOBLAS-8 LAGEOS data, the known range and time
biases (in Table 11) should be subtracted from the LAGEOS range and
timetags, respectively. Any time there is the potential for a change in
the bias(es), even though the bias(es) did not change, a new entry has
been added. A component breakdown of the biases is presented and
followed by a section describing known problem data.

**6.2.1 MOBLAS-8 ND filters**

All MOBLAS-8 LAGEOS data was biased short by 3mm + 1 and 1mm + 1 from
September 1, 1981 through April 7, 1992, and from April 8, 1992 through
May 31, 1992, respectively, due to ND filters. On April 7, 1992, the OAM
was installed in MOBLAS-8 reducing the ND filter bias. After June 1,
1992, this bias has been removed through modelling.

6.2.2 MOBLAS-8 signal strength

MOBLAS-8 LAGEOS data was biased long by 5mm + 3, 10mm + 5, 6mm + 3, and
3mm + 2 from August 4, 1982 through December 31, 1983; from January 1
through March 31, 1984; April 1 through July 31, 1984; and from August
1, 1984 through September 25, 1986; respectively, due to signal
strength. This bias would vary slightly (several mm) from pass-to-pass
and would have a slight elevation dependence. Low elevation data would
be biased longer than high elevation data. Signal strength effects were
reduced as system calibration techniques improved, and were eliminated
when the MCPPMT package was installed in MOBLAS-8 on September 26, 1986.

**6.2.3 MOBLAS-8 elevation angles**

Prior to January 1, 1988, the tropospheric range correction applied to
the fullrate data was computed based on the MOBLAS-8 encoder
measurements. To remove this error in pre-1988 MOBLAS-8 data, the
applied tropospheric refraction must be removed and reapplied using the
computed elevation angle.

**6.2.4 MOBLAS-8 calibration targets/distances**

MOBLAS-8 has used 5 different calibration targets (2 boards and 3 cube
corners) at marker 7109. From September 1, 1981 through August 19, 1985;
from August 20, 1985 through December 31, 1986; from January 1, 1987
through March 22, 1989; from March 23, 1989 through April 7, 1992; and
from April 8, 1992 to the present; MOBLAS-8 primary calibration targets
were the original 2Km board, the second 2Km board, the second 2Km cube
corner, the 2Km Nelson pier, and the 200m Nelson pier, respectively.

MOBLAS-8 was surveyed 7 times (1981, 1982, 1984, 1985, 1988, and twice
in 1991) at marker 7109. During August 1985, new calibration targets (a
2Km board and 2Km cube corner) were built to replace the vandalized 2Km
board and 2Km cube corner \[Nelson, 1985\]. The guy wires had been cut
on both targets. The newly constructed board became the operational
target, effective August 20, 1985. The preliminary board range and the
final surveyed board range published in the November 1985 survey sheet
were different by 10mm. It is not known at this writing when the
\"official\" number was used for fullrate data processing. Sometime
between October 1986 and January 1987, the cube corner became the
operational target. In December 1988, these targets were resurveyed
along with a newly constructed 2Km Nelson pier. Between the 1985 and
1988 surveys, the board and cube corner ranges changed by 14mm and 11mm,
respectively. Historical minico results, since January 1987, suggest
that 1985 and 1988 cube corner surveyed ranges were in error by -20mm
and -9mm, respectively. Therefore, MOBLAS-8 data was biased long by 20 +
15 and 9 + 10mm from January 1, 1987 through December 11, 1988, and from
December 12, 1988 through March 22, 1989, respectively. More research is
needed into data processing results prior to January 1987.

**6.2.5 MOBLAS-8 TDEL**

All MOBLAS-8 data from November 22, 1989 through May 31, 1992, contains
a time bias of +0.3 microseconds. This correction is only valid after
November 22, 1989, because on this date, the MOBLAS-8 TDEL electronics
were modified to eliminate other TDEL sub-microsecond accuracy
ambiguities, which would have masked this bias. This bias has been
eliminated through modelling, effective June 1, 1992.

All MOBLAS-8 data from August 4, 1982 (the date of the moms van swap
with MOBLAS-4) through October 26, 1986 was biased late depending on the
interval of a second the laser was fired \[MOBLAS-8 crew and Heinick,
1986\]. The transmit delays (TDEL) on the .0, .2, .4, .6, .8 second
intervals were reading high by 0.0, 3.2, 6.4, 9.6, and 11 microseconds,
respectively, due to a wiring problem. Since MOBLAS-8 data was equally
distributed in these different time intervals, a mean offset of 6.0
microseconds can be post-applied to the data. The TDEL wiring problem
was fixed on October 27, 1986.

**6.2.6 MOBLAS-8 known problem data**

All MOBLAS-8 data between January 1 through March 4, 1982, exhibited
very high RMS\'s (300- 600mm) due to poor hardware performance. This
data probably contains significant biases and should not be used. The
hardware was repaired on March 5, 1982, and the RMS\'s returned to
nominal levels.

**7. CONCLUDING REMARKS**

The present article has been the first attempt to characterize known
biases contained in historical MOBLAS LAGEOS data; however, more
research into historical ground test results and calibration processing
information is needed to resolve potential biases. These bias models and
eccentricity information should improve the quality of the historical
MOBLAS LAGEOS dataset. We encourage other global data producing centers
to characterize their historical biases.

In the 1980-1983 timeframe, the laser was the dominant MOBLAS systematic
error source. In the 1984-1986 timeframe, target stability, survey
related issues, and signal strength effects were the dominant error
sources. In the 1987-1988 timeframe, target stability and survey related
issues were the dominant error sources. In the 1989-1992 timeframe, the
HP5370 counter and ND filters appear to have been the dominant error
sources. Some of the above mentioned systematic errors are recoverable
(to some level) using historical processing results; however, orbital
analysis may be the only way to unravel the \"net\" biases in the global
SLR dataset to the 10mm level. These bias models will evolve as
historical SLR data is better understood by close cooperation of the
data producer and the data user.

**8. ACKNOWLEDGEMENTS**

The material in this article has been amassed through years of close
interaction with a highly talented group of colleagues at the Bendix
Field Engineering Corporation in Greenbelt and Columbia, Md; at the NASA
Goddard Space Flight Center in Greenbelt, Md; and at the Center for
Space Research in Austin, Texas, and much of their work is referenced
here.

**REFERENCES**

Brogdon O. L., Heinick J. M., MOBLAS Ground Test Results, Bendix Field
Engineering Corporation internal reports, 1980-1983.

Crawford W. F., LTN Data, Bendix Field Engineering Corporation internal
report, Nov. 1985.

Degnan J. J., Satellite Laser Ranging: Current Status and Future
Prospects, IEEE Trans. Geosci. Remote Sensing, Vol GE-23 Number 4, pp
398-413, Jul. 1985.

Eichinger R. A., Oldham T. J., MOBLAS 4, 6, 7, 8 Antiparallax Optical
Path Differences, Bendix Field Engineering Corporation internal reports,
1990-1992.

Heinick J. M., Receive Energy Dependence and Correction at MOBLAS 4, 5,
7, and 8, Bendix Field Engineering Corporation, presented at the spring
1984 AGU, Feb. 1984.

Heinick J. M, Husson V. S., Wroe S. A., Brogdon O. L., Wetzel S. L.,
Clarke C. B., Six Month Data Processing Report, Bendix Field Engineering
Corporation internal report, Nov. 1989.

Heinick J. M., MOBLAS-7\'s calibrations using the corner cube and the
most recently published calibration board range, Bendix Field
Engineering Corporation internal report, Oct. 1985.

Husson V. S., Wroe S. A., MOBLAS-5 Processing Dilemma Update, Bendix
Field Engineering Corporation, presented at the weekly CDPSLR meeting,
Oct. 1987.

Husson V. S., Wetzel S. L., MOBLAS-6 Processing Dilemma Update, Bendix
Field Engineering Corporation, presented at the weekly CDPSLR meeting,
Oct. 1987.

Husson V. S., Wroe S. A., Wetzel S. L., TLRS-1/MOBLAS-7 Collocation
Analysis Report, Bendix Field Engineering Corporation internal report,
1987.

McCollums D. L., private communication, Bendix Field Engineering
Corporation.

McCollums D. L., Varghese T. K., MOBLAS-7 System Characterization
Reports, Bendix Field Engineering Corporation, 1986-1988.

MOBLAS-8 Crew, Heinick J. M., MOBLAS-8 Transmit Delay Investigation,
Bendix Field Engineering Corporation internal report, Oct. 1986.

Murphy B., Nelson V. E., MOBLAS 5 Survey Reports, Australian Division of
National Mapping, Bendix Field Engineering Corporation, 1979/80, 1987,
1992.

Nelson V. E., MOBLAS 4 Survey Reports, Bendix Field Engineering
Corporation, 1983, 1985, 1988, 1992.

Nelson V. E., MOBLAS 6 Survey Reports, Bendix Field Engineering
Corporation, 1983, 1988, 1989, 1991.

Nelson V. E., MOBLAS 7 Survey Reports, Bendix Field Engineering
Corporation, 1981, 1982, 1984, 1985, 1986, 1987, 1988, 1889, 1990, 1991,
1992.

Nelson V. E., MOBLAS 8 Survey Reports, Bendix Field Engineering
Corporation, 1981, 1982, 1984, 1985, 1988, 1989, 1991.

Nelson V. E., private communication, Bendix Field Engineering
Corporation.

Nelson V. E., System Eccentricity Measurement Procedures for MOBLAS
Systems, Bendix Field Engineering Corporation internal report, 1986.

Oldham T. J., private communication, Bendix Field Engineering
Corporation.

Pearlman M. R., Laser System Characterization, Smithsonian Institution
Astrophysical Observatory, Proc. 5th Int. Workshop on Laser Ranging
Instrumentation, Sep. 1984.

Selden M. D., Varghese T. K., Heinick J. M., Oldham T. J., Preliminary
Results from the Portable Standard Satellite Ranging Intercomparison
with MOBLAS-7, Proc. 8th Int. Workshop on Laser Ranging Instrumentation,
May 1992.

See D., Sneed R. S., Suggestions for new or improved LRC logic, Bendix
Field Engineering Corporation internal report, Oct. 1988.

Silva T. D, Malitson P. H. Eichinger R. E., Optical Attenuation
Mechanism, Bendix Field Engineering Corporation internal report, Feb.
1991.

Smith D. E., Kolenkiewicz R., Dunn, P. D., Klosko S. M., Robbins J. W.,
Torrence M. H., Williamson R. G., Pavlis E. C, Douglas N. B, Fricke S.
K., LAGEOS Geodetic Analysis\--SL7.1, NASA Technical Memorandum 104549,
Sep. 1991.

Varghese T. K., Heinick J. M., Sub-cm Multiphotoelectron Satellite Laser
Ranging, Bendix Field Engineering Corporation, Proc. of 6th Int.
Workshop on Laser Ranging Instrumentation, 1986.

Varghese T. K., Husson V. S., TLRS-3: Engineering Upgrades and
Performance Evaluation through Intercomparison with MOBLAS-7 from Oct.
88 - Nov. 89., Bendix Field Engineering Corporation internal report,
Nov. 1989.

Wroe S. A., Heinick J. M., Brogdon O. L., Wetzel S. L., Clarke C. B,
Cheng G. S., Horvath J. E., MOBLAS Ground Test Results, Bendix Field
Engineering Corporation internal reports, 1984- 1992.

Table 1. MOBLAS-4 Eccentricities for SOD 711004xx

Occ. Start North East Up Revis.

No. Date (M) (M) (M) Date Comments

2 08/15/83 -0.033 -0.015 3.209 01Aug92 First survey

2-3 06/20/85 -0.033 -0.015 3.210 01Aug92 1mm change in Up

3-10 04/27/88 -0.033 -0.016 3.213 01Aug92 1 & 3mm change in East and Up

10 02/01/92 -0.031 -0.016 3.213 01Aug92 2mm change in North, no survey

sheet published

11 04/21/92 -0.026 -0.019 3.189 01Aug92 Mount replacement

Table 2. MOBLAS-4 LAGEOS Biases at 7110

Range Time Revis.

Time Span Bias(mm) Bias(us) Date Comments

08/15/83 - 04/11/84 17 0 01Aug92 Quantel, 2233B, 5370, Ortec, & board

(1954.6037m)

04/12/84 - 05/24/84 17 0 01Aug92 Gating problem causing discontinuous
data,

see 2.2.6

05/25/84 - 06/30/84 17 0 01Aug92 Data quality returned to normal

07/01/84 - 08/31/84 7 0 01Aug92 Improvement in amplitude modelling

09/01/84 - 06/19/85 3 0 01Aug92 Improvement in amplitude modelling

06/20/85 - 07/31/85 3 0 01Aug92 New target range (1954.5910m), see 2.2.4

para. 2

08/01/85 - 11/19/86 1 0 01Aug92 Improvement in amplitude modelling

11/20/86 - 12/31/86 -3 0 01Aug92 MCP & Tennelec disc. installed

01/01/87 - 10/31/87 -3 0 01Aug92 Cube corner (1955.469m)

11/01/87 - 12/31/87 -3 0 01Aug92 Unstable cube corner, see 2.2.4 para. 4

01/01/88 - 01/28/88 -3 0 01Aug92 Computed angles used, see 2.2.3,
unstable

cube corner, see 2.2.4 para. 4

01/29/88 - 04/29/88 -3 0 01Aug92 Cal. board (1954.5910m)

04/30/88 - 12/28/88 -3 0 01Aug92 Long Nelson pier (1955.2677m)

12/29/88 - 01/27/89 -3 0 01Aug92 Questionable data, bad counter, see
2.2.7

01/28/89 - 11/06/89 -3 0 01Aug92 New counter installed

11/07/89 - 11/11/90 -3 0 01Aug92 New range (1955.2682m), cube replaced

11/12/90 - 01/04/90 -3 0 01Aug92 Cube vandalized and replaced
(1955.2600m)

01/05/90 - 12/05/91 -3 -0.4 01Aug92 Transmit delay modification see
2.2.5

12/06/91 - 01/17/92 -3 -0.4 01Aug92 Translator installed, short Nelson
pier

(186.9920m)

01/18/92 - 04/20/92 -1 -0.4 01Aug92 OAM installed

04/21/92 - 05/31/92 -1 -0.4 01Aug92 Mount replaced, new target range

(186.9986m)

Table 3. MOBLAS-5 Eccentricities for SOD 709005xx

Occ. Start North East Up Revis.

No. Date (M) (M) (M) Date Comments

1-8 08/01/79 0.003 0.011 3.185 01Aug92 First survey

8-10 10/15/87 0.003 0.010 3.177 01Aug92 1mm & 8mm change in East and Up

11-12 01/12/92 -0.011 0.020 3.181 01Aug92 Mount replaced

Table 4. MOBLAS-5 LAGEOS Biases at 7090

Range Time Revis.

Time Span Bias(mm) Bias(us) Date Comments

08/01/79 - 06/07/80 -23 0 01Aug92 GP laser, 2233B, 5360, Ortec, & board

(2065.491m), gating problem see 3.2.6

06/08/80 - 07/26/83 -3 0 01Aug92 New board (3100.2532m) used

07/27/83 - 05/31/85 -6 0 01Aug92 Quantel & 5370 installed

06/01/85 - 04/22/87 3 0 01Aug92 Suspect 9mm error in target range, see

3.2.4

04/23/87 - 05/31/87 6 0 01Aug92 MCP & Tennelec disc. installed

06/01/87 - 07/31/87 -3 0 01Aug92 Cube corner (1257.9630m) used

08/01/87 - 08/25/87 -3 0 01Aug92 Board (3100.2442m) used

08/26/87 - 12/31/87 -3 0 01Aug92 Long Nelson pier (3116.8969m) used

01/01/88 - 08/11/88 -3 0 01Aug92 Computed angles used, see 3.2.3

08/12/88 - 01/31/89 -3 0 01Aug92 High cal. shifts, see 3.2.8

02/01/89 - 01/29/90 -3 0 01Aug92 Cal. shifts return to normal

01/30/90 - 07/22/90 -3 0 01Aug92 Transmit delay modification, see 3.2.5

07/23/90 - 12/18/91 -3 0 01Aug92 New range (3116.8974m), cube replaced

12/19/91 - 01/03/92 -3 0 01Aug92 Bad data, stuck bit see 3.2.7

01/04/92 - 01/11/92 -3 0 01Aug92 Stuck bit repaired

01/12/92 - 05/31/92 -3 0 01Aug92 Mount replaced, new survey (3116.9047m)

Table 5. MOBLAS-6 Eccentricities for SOD 712206xx

Occ. Start North East Up Rev.

No. Date (M) (M) (M) Date Comments

1-4 05/01/83 0.001 -0.007 3.182 01Aug92 First survey

4-7 05/03/88 0.002 -0.006 3.181 01Aug92 1mm change in all eccs., see 4.1

8-9 06/23/89 0.002 -0.006 3.181 01Aug92 No change

9 04/30/91 0.002 -0.006 3.181 01Aug92 No change, final survey

Table 6. MOBLAS-6 LAGEOS Biases at 7122

Range Time Revis.

Time Span Bias(mm) Bias(us) Date Comments

05/01/83 - 09/30/83 -3 0 01Aug92 GP laser, 2233B, 5360, Ortec, & Board

(2229.1998m)

10/01/83 - 01/31/84 19 0 01Aug92 Hurricane, suspect target movement, see

4.2.4, para 2

02/01/84 - 03/01/84 19 0 01Aug92 Bad distribution amplifier, see 4.2.6

03/02/84 - 04/30/84 19 0 01Aug92 Distribution amplifier replaced

05/01/84 - 03/31/86 31 0 01Aug92 Quantel laser and HP5370 installed

04/01/86 - 08/31/86 29 0 01Aug92 Improvement in amplitude modelling

09/01/86 - 01/27/87 23 0 01Aug92 Improvement in amplitude modelling

01/28/87 - 03/26/87 19 0 01Aug92 MCP and Tennelec installed

03/27/87 - 12/31/88 -14 0 01Aug92 Cube corner used (2229.4086m)

01/01/88 - 05/02/88 -14 0 01Aug92 Computed angles used, see 4.2.3

05/03/88 - 08/16/88 -8 0 01Aug92 New cube corner range used (2229.4150m)

08/17/88 - 08/31/88 1 0 01Aug92 First Nelson pier used (2229.2215m)

09/01/88 - 09/30/88 2 0 01Aug92 Nelson pier movement, see 4.2.4 para 3

10/01/88 - 10/31/88 3 0 01Aug88 Nelson pier movement

11/01/88 - 11/30/88 4 0 01Aug92 Nelson pier movement

12/01/88 - 12/31/88 5 0 01Aug92 Nelson pier movement

01/01/89 - 01/31/89 6 0 01Aug92 Nelson pier movement

02/01/89 - 02/28/89 7 0 01Aug92 Nelson pier movement, bad barometric
data

03/01/89 - 03/31/89 8 0 01Aug92 Nelson pier movement, bad barometric
data

04/01/89 - 04/30/89 9 0 01Aug92 Nelson pier movement, bad barometric
data

05/01/89 - 05/12/89 10 0 01Aug92 Nelson pier movement, bad barometric
data

05/13/89 - 05/31/89 10 0 01Aug92 Barometer repaired

06/01/89 - 06/22/89 11 0 01Aug92 Nelson pier movement

06/23/89 - 11/16/89 -3 0 01Aug92 Second Nelson pier used (2232.7419m)

11/17/89 - 01/10/90 -3 0 01Aug92 New range (2232.7447m), cube replaced

01/11/90 - 03/31/90 -3 0 01Aug92 Transmit delay modification

04/01/90 - 09/26/90 -3 0 01Aug92 Calibration RMS\'s are high

09/27/90 - 04/30/91 -3 0 01Aug92 Calibration RMS\'s return to normal
levels

Table 7. MOBLAS-7 Eccentricities for SOD 710507xx

Occ. Start North East Up Rev.

No. Date (M) (M) (M) Date Comments

1-4 01/01/81 0.016 -0.026 3.169 01Aug92 First survey, Up was in error

5 03/22/84 0.017 -0.031 3.169 01Aug92 1 & 5mm change in North & East

8 05/07/85 0.017 -0.031 3.169 01Aug92 No change

9 07/29/85 0.017 -0.032 3.169 01Aug92 1mm change in East

9 12/13/85 0.017 -0.032 3.170 01Aug92 1mm change in Up

9 01/17/86 0.016 -0.031 3.168 01Aug92 1, 1 and 2mm changes in North,

East, and Up

9 01/22/86 0.017 -0.031 3.168 01Aug92 1mm change in North

9-10 03/17/86 0.017 -0.031 3.168 01Aug92 No change

10-11 10/06/86 0.017 -0.031 3.168 01Aug92 No change

11-12 06/18/87 0.017 -0.031 3.168 01Aug92 No change

13 01/09/89 0.017 -0.031 3.168 01Aug92 No change

15-16 08/25/89 0.035 -0.040 3.162 01Aug92 System moved, 18mm, 9mm, 6mm

changes in North, East, and Up

17-18 07/25/90 -0.014 -0.033 3.153 01Aug92 Relocation

19-23 12/10/90 -0.014 -0.033 3.153 01Aug92 No change

Table 8. MOBLAS-7 LAGEOS Biases at 7105

Range Time Revis.

Time Span Bias(mm) Bias(us) Date Comments

01/01/81 - 06/30/81 46 0 01Aug92 GP laser, 2233B, 5360, Ortec, & board

(3225.460m)

07/01/81 - 12/31/81 19 0 01Aug92 Slyvania laser installed, board

(3225.433m)

01/01/82 - 01/31/83 -3 0 01Aug92 Board (3225.411m)

02/01/83 - 08/21/83 -3 0 01Aug92 HP5370 counter installed

08/22/83 - 02/29/84 27 0 01Aug92 Quantel laser installed

03/01/84 - 03/21/84 1 0 01Aug92 Improvement in amplitude modelling

03/22/84 - 06/06/84 1 0 01Aug92 New board (3225.421m)

06/07/84 - 06/13/84 1 0 01Aug92 Data suspect, high RMS\'s, see 5.2.7

06/14/84 - 07/31/84 1 0 01Aug92 RMS\'s return to normal

08/01/84 - 09/24/84 4 0 01Aug92 Increase in amplitude dependence

09/25/84 - 09/30/84 4 0 01Aug92 Data suspect, high RMS\'s, see 5.2.7

10/01/84 - 10/05/84 27 0 01Aug92 Increase in ampl. dependence, high
RMS\'s

10/05/84 - 01/25/85 27 0 01Aug92 RMS\'s return to normal

01/26/85 - 03/31/85 2 0 01Aug92 Improvement in amplitude modelling

04/01/85 - 07/31/85 0 0 01Aug92 Improvement in amplitude modelling

08/01/85 - 01/12/86 17 0 01Aug92 New range (3225.438m), see 5.2.4 &
Table 9

01/13/86 - 01/17/86 17 0 01Aug92 5370 on internal frequency, see 5.2.6

01/18/86 - 02/06/86 17 0 01Aug92 5370 put back on external frequency

02/07/86 - 03/01/86 -12 0 01Aug92 Guy wires on board tensioned,
Secondary

cal. target new cube corner (3482.552m)

03/02/86 - 03/30/86 0 0 01Aug92 New board range (3225.450m)

03/31/86 - 09/30/86 -3 0 01Aug92 MCP and Tennelec disc. installed

10/01/86 - 12/05/86 -3 0 01Aug92 Cube corner used (3482.547m)

12/06/86 - 12/31/87 -3 0 01Aug92 Nelson pier used (223.385m),
antiparallax

hardware installed

01/01/88 - 01/31/88 -3 0 01Aug92 Computed angles used 5.2.3

02/01/88 - 04/30/88 1 0 01Aug92 Amplitude dependence (+4mm)

05/01/88 - 01/08/89 -3 0 01Aug92 Elimination of amplitude dependence

01/09/89 - 06/14/89 -3 0 01Aug92 New range (223.3868m)

06/15/89 - 07/12/89 -3 0 01Aug92 New antiparallax hardware installed

07/13/89 - 08/02/89 -3 -0.5 01Aug92 Transmit delay modification, see
5.2.5

08/03/89 - 08/24/89 -3 -0.5 01Aug92 Original antiparallax hardware
installed

08/25/89 - 10/11/89 -18 -0.5 01Aug92 Apparent movement of system, see
5.2.4

10/12/89 - 07/23/90 -3 -0.5 01Aug92 New range (223.4018m)

07/24/90 - 09/27/90 -3 -0.5 01Aug92 New range (223.3922m), cube replaced

09/28/90 - 12/09/90 -3 -0.3 01Aug92 TDEL cable shortened, see 5.2.5

12/10/90 - 07/09/91 -3 -0.3 01Aug92 New antiparallax hardware installed

07/10/91 - 07/23/91 -1 -0.3 01Aug92 OAM installed

07/24/91 - 10/17/91 -3 -0.3 01Aug92 OAM removed

10/18/91 - 05/31/92 -1 -0.3 01Aug92 OAM reinstalled

Table 9. MOBLAS-7 Cube Corner Calibrated LAGEOS Passes (The range biases
for these passes would be 0)

Date Time Date Time Date Time

08/09/85 5:20 09/07/85 8:03 10/16/85 11:34

08/09/85 8:51 09/10/85 3:59 10/17/85 13:22

08/14/85 5:51 09/10/85 7:31 12/03/85 21:32

08/28/85 4:11 09/11/85 2:52 12/04/85 0:59

08/29/85 6:16 09/11/85 6:15 02/13/86 22:07

08/29/85 9:42 09/11/85 9:37

08/30/85 8:30 09/17/85 5:04

08/31/85 7:08 09/18/85 10:38

09/04/85 5:10 09/20/85 8:01

09/04/85 8:36 09/21/85 9:59

09/06/85 5:55 10/01/85 10:22

09/06/85 9:22 10/07/85 12:49

09/07/85 4:35 10/08/85 11:24

Table 10. MOBLAS-8 Eccentricities for SOD 710908xx

Occ. Start North East Up Rev.

No. Date (M) (M) (M) Date Comments

1 09/01/81 0.012 0.011 3.225 01Aug92 First Survey

1 07/22/82 -0.029 0.011 3.124 01Aug92 M8 & M4 mom\'s van swap

1-2 08/04/82 -0.029 0.011 3.141 01Aug92 System releveled, 17mm change in

Up, based on 84 survey

2-3 09/24/84 -0.029 0.011 3.141 01Aug92 No survey sheet

3-5 08/20/85 -0.029 0.011 3.142 01Aug92 1mm change in Up

6-11 12/12/88 -0.027 0.012 3.138 01Aug92 2, 1, & 4mm change in North,

East, and Up

12 12/01/91 -0.019 0.005 3.184 01Aug92 Mount replacement

13-15 12/12/91 -0.035 -0.003 3.184 01Aug92 System releveled, 16 & 8mm

change in North and East

Table 11. MOBLAS-8 LAGEOS Biases at 7109

Range Time Revis.

Time Span Bias(mm) Bias(us) Date Comments

09/01/81 - 12/31/81 -3 0 01Aug92 GP laser, 2233B, 5360, Ortec, & board

(2394.087m)

01/01/82 - 03/04/82 -3 0 01Aug92 Very high RMS\'s (300-600mm), see 6.2.6

03/05/82 - 08/03/82 -3 0 01Aug92 Hardware repaired, data nominal

08/04/82 - 12/31/83 2 +6.0 01Aug92 M4/M8 van switch, Quantel, HP5370,

transmit delay problem see 6.2.5, board

(2394.120m)

01/01/84 - 03/31/84 7 +6.0 01Aug92 Increase in amplitude dependence

04/01/84 - 07/31/84 3 +6.0 01Aug92 Improvement in amplitude modelling

08/01/84 - 08/19/85 0 +6.0 01Aug92 Improvement in amplitude modelling

08/20/85 - 09/25/86 0 +6.0 01Aug92 New board used (2389.506 or
2389.496m),

see 6.2.4

09/26/86 - 10/26/86 -3 +6.0 01Aug92 MCP & Tennelec disc. installed

10/27/86 - 12/31/86 -3 0 01Aug92 Transmit delay wire connected, see
6.2.5

01/01/87 - 12/31/87 17 0 01Aug92 Cube corner used (2389.519m), see 6.2.4

01/01/88 - 12/11/88 17 0 01Aug92 Computed angles, see 6.2.3

12/12/88 - 03/22/89 6 0 01Aug92 New cube corner range (2389.508m)

03/23/89 - 11/06/89 -3 0 01Aug92 Nelson pier used (2401.1450m)

11/07/89 - 11/21/89 -3 0 01Aug92 New range (2401.1458m), cube replaced

11/22/89 - 11/30/91 -3 -0.3 01Aug92 Transmit delay modification, see
6.2.5

12/01/91 - 12/11/91 -3 -0.3 01Aug92 Mount re-installed, Nelson pier

(2401.1392m)

12/12/91 - 04/06/92 -3 -0.3 01Aug92 Mount releveled, Nelson pier
(2401.1576m)

04/07/92 - 05/31/92 -1 -0.3 01Aug92 Short range Nelson pier used
(207.4742m),

OAM installed