The number of objects in earth orbit tracked by the US Space Surveillance Network (SSN) has experienced unprecedented growth since March, 2003. Approximately 2000 orbiting objects have been added to the "Analyst list" of tracked objects. This growth is primarily due to the resumption of full power/full time operation of the AN/FPS-108 Cobra Dane radar located on Shemya Island, AK. Cobra Dane is an L-band (23-cm wavelength) phased array radar which first became operational in 1977. Cobra Dane was a "Collateral Sensor" in the SSN until 1994 when its communication link with the Space Control Center (SCC) was closed. NASA and the Air Force conducted tests in 1999 using Cobra Dane to detect and track small debris. These tests confirmed that the radar was capable of detecting and maintaining orbits on objects as small as 5-cm diameter. Subsequently, Cobra Dane was reconnected to the SSN and resumed full power/full time space surveillance operations on March 4, 2003. This paper will examine the new data and its implications to the understanding of the orbital debris environment and orbital safety

Stansbery, Eugene G.

NASA Technical Reports Server

1IAC-04-IAA.5.12.1.03
GROWTH IN THE NUMBER OF SSN TRACKED 
ORBITAL OBJECTS
E. G. Stansbery
NASA Johnson Space Center, Houston, TX 77058, USA
eugene.g.stansbery@nasa.gov
ABSTRACT
The number of objects in earth orbit tracked by the US Space Surveillance Network (SSN) has 
experienced unprecedented growth since March, 2003.  Approximately 2000 orbiting objects have been 
added to the “Analyst list” of tracked objects.  This growth is primarily due to the resumption of full 
power/full time operation of the AN/FPS-108 Cobra Dane radar located on Shemya Island, AK.  Cobra 
Dane is an L-band (23-cm wavelength) phased array radar which first became operational in 1977.  
Cobra Dane was a “Collateral Sensor” in the SSN until 1994 when its communication link with the Space 
Control Center (SCC) was closed.  NASA and the Air Force conducted tests in 1999 using Cobra Dane to 
detect and track small debris.  These tests confirmed that the radar was capable of detecting and 
maintaining orbits on objects as small as 5-cm diameter.  Subsequently, Cobra Dane was reconnected to 
the SSN and resumed full power/full time space surveillance operations on March 4, 2003.   This paper 
will examine the new data and its implications to the understanding of the orbital debris environment and 
orbital safety.
Introduction
The US Space Surveillance Network provides 
collision avoidance screening for manned 
spaceflight, including the International Space 
Station, against all tracked orbiting objects.  For 
many years, the nominal sensitivity of the 
network for low earth orbit objects was 10 cm.  
However, spacecraft are vulnerable to severe or 
catastrophic damage from collisions with debris 
much smaller.  Shielding techniques can protect 
spacecraft from the smallest debris pieces, but 
there is a large gap between what can be 
shielded against and 10 cm.  In recent years, 
the US has begun making efforts to improve the 
sensitivity of the SSN.
Catalog Growth
Tracked objects are maintained in two lists.  
Objects in the official catalog are given, along 
with their international designator, a sequential 
number starting with satellite number 1, the 
Sputnik 1 rocket body.  Currently there are over 
28,000 objects in the regular catalog including 
close to 19,000 objects which have reentered 
the earth’s atmosphere or have left earth orbit.  
Objects must meet certain criteria to be 
included in the catalog.  One criteria is that the 
object must be associated with its original 
launch.  There are many objects which are 
tracked by the SSN which have not yet been 
identified by mission.  This includes many small 
debris fragments.  These objects are maintained 
in an “Analyst” list using numbers from 80,000 
to 89,999.  Objects in the analyst catalog are 
often transient in that they are discovered, 
tracked, identified, and then transferred to the 
regular catalog.  The number in the analyst 
catalog is then reused.  Many objects in the 
analyst list reenter or are administratively 
removed prior to them entering the regular 
catalog.  Objects in both the regular and analyst 
https://ntrs.nasa.gov/search.jsp?R=20060022013 2019-08-29T21:53:28+00:00Z
2catalogs are screened for collision avoidance 
with manned space flights.
Breakups
Figure 1 shows the growth in the regular and 
analyst catalogs of on-orbit objects since 1990.  
Sharp jumps in the regular catalog are usually 
associated with on-orbit fragmentation events.  
The largest single jump shown comes from the 
fragmentation of object 1994-029B, the Pegasus 
HAPS (Hydrazine Auxiliary Propulsion System) 
stage from the STEP II launch which broke up in 
June, 1996.  Over 700 objects were eventually 
cataloged from this event1.  The transient nature 
of the analyst list can be seen in the rapid rise in 
number of analyst satellites tracked immediately 
after the event and the subsequent reduction a 
few months later as the objects were transferred 
to the regular catalog.  Other breakups which 
produced more than 100 (regular) cataloged 
fragments and which show visible sharp jumps 
in Figure 1 are listed in Table 1.  
1990 1992 1994 1996 1998 2000 2002 2004
Year
0
5000
10000
15000
N
um
be
r o
f T
ra
ck
ed
 O
bj
ec
ts
Total Tracked
Regular Catalog
Analyst Satellites Tracked
a
Debris Campaigns
Cobra Dane Full Power Operation
Nimbus 
6 R/B
Cosmos
2227 R/B
Step II R/B
CBERS-1
SACI-1
R/B
TES R/B
Fig. 1: Number of orbiting objects tracked by the SSN.
Common Name International 
Designator
Event Date No. of Catalogued Objects
Nimbus 6 R/B 1975-052B 1-May-91 268
Cosmos 2227 R/B 1992-093B 26-Dec-92 225
Step II R/B 1994-029B 3-Jun-96 709
CBERS-1/SACI-1 R/B 1999-057C 11-Mar-00 316
TES R/B 2001-049B 19-Dec-01 326
Table 1. Fragmentation events since 1990 with >100 cataloged objects.
3Debris Campaigns
There are several sharp increases in the analyst 
satellites list which are not associated with 
fragmentation events.  In 1993 and 1994, the Air 
Force conducted two debris campaigns using 
existing sensors in the SSN.  The 1994 
campaign was conducted from October 11 – 
November 8, 19942.  The campaign was 
conducted in two phases.  The first phase 
concentrated on detecting new objects while the 
second phase emphasized follow up tracking 
(although many additional objects were detected 
during Phase 2).  Over 1100 element sets were 
added to the analyst list during the campaign 
although 350 of these were later identified as 
duplicates or as previously known objects.  This 
left about 800 unique new objects tracked during 
the campaign.  The campaign required special, 
manpower intensive, operating procedures to 
maintain the orbits of these objects.  For 
instance, more than 40% of the new objects 
detected in the campaign were in high 
eccentricity orbits (> 0.1).  Orbits for these 
objects are more difficult to maintain than 
circular orbits because there are fewer sensor 
viewing opportunities.  Additionally, the 
campaign specifically looked for low inclination 
orbits where normal SSN coverage is sparse.  
When the campaign ended and the SSN 
returned to its normal operational procedures, 
the large majority of the 800 new objects were 
lost.  Hence, about six months after the end of 
the campaign, most of the campaign objects 
were administratively removed from the analyst 
list of tracked objects3.
Cobra Dane
The largest rapid increase in the number of 
tracked objects occurs starting in March, 2003.  
This timing correlates to the resumption of full 
power/full time operation of the AN/FPS-108 
Cobra Dane radar located on Shemya Island, 
AK (52.7° N latitude, 174.1° E longitude).  In 
contrast to the 1994 debris campaign where the 
gains were the result of temporary operating 
procedures, the addition of Cobra Dane to the 
SSN will provide long-term improvement to the 
total tracked population.  
Cobra Dane (Figure 2) is an L-band (23-cm 
wavelength) phased array radar which first 
became operational in 1977.  The radar 
generates approximately 15.4 MW of peak RF 
power (0.92 MW average) from 96 Traveling 
Wave Tube (TWT) amplifiers arranged in 12 
groups of 8. This power is radiated through 
15,360 active array elements.  Cobra Dane 
actually was part of the SSN, as a “Collateral 
Sensor,” until 1994, but operational procedures 
limited the size object it was able to detect and 
report to the Space Surveillance Center (SSC) 
which processes observations from the entire 
SSN.  In 1994, the communication link from the 
radar to the SSC was closed and Cobra Dane 
was limited to its primary intelligence role.  
Fig. 2: AN/FPS-108 Cobra Dane L-band phased 
array radar.
In an effort to explore methods of improving the 
sensitivity of the SSN, NASA and the Air Force 
conducted tests in August and September, 1999 
using Cobra Dane to detect and track small 
debris4.  These tests paid special attention to the 
region below 600 km altitude utilizing a broad 
“debris fence” and operating the radar at full 
power.  Again, special operating procedures 
were employed to create element sets from the 
Cobra Dane radar since the normal 
communications link between the radar and the 
SSC no longer existed.  Despite this fact, 
element sets for over 560 objects were created, 
of which more than 500 could be tracked on a 
regular basis.  This test confirmed that the radar 
was capable of detecting and maintaining orbits 
on objects as small as 5-cm diameter.  Nearly 
one-fourth of the routinely tracked objects 
crossed human space flight altitudes.  
4Subsequently, Cobra Dane was used in 
international debris measurement campaigns 
coordinated by the Inter–Agency Space Debris 
Coordination Committee (IADC).  These 
campaigns confirmed the capacity and 
sensitivity demonstrated during the August-
September, 1999 tests.
The communications link between Cobra Dane 
and the SSC was restored in October 1999.  
However, the radar was only operating at ¼ 
power and was not specifically tasked by the 
SSC to search for debris.  Full power, full time 
operation began at Cobra Dane on March 4, 
2003.  Full power operation allowed the radar to 
allocate radar resources not only to its primary 
mission, but also to a debris search fence.  On 
January 1, 2003, the SSN was routinely tracking 
~11,000 objects including over 2000 objects in 
the analyst list.  The number peaked in late 
June, 2003 with over 13,500 total objects 
including close to 5000 analyst satellites.  Many 
of these objects are below the sensitivity of 
other sensors and can only be tracked by Cobra 
Dane.  The additional radar resources needed to 
maintain the orbits of these objects has slowed 
the addition of new objects to the analyst list.
With over 5000 objects in the analyst list, new 
procedures were implemented to purge objects 
which had not been tracked for a specified time 
period.  This procedure was applied to all 
objects in the analyst catalog.  When 
implemented in late 2003, the number of analyst 
satellite orbits maintained dropped by over 
1000.  By the end of May, 2004 the total number 
of tracked objects was again approaching 
13,000.
Satellite Distributions
In order to assess the effects of the increased 
SSN sensitivity, catalogs (including analyst 
satellites) from three dates were chosen.  The 
first data set was from April, 2002, prior to full 
power operation of Cobra Dane.  The second 
dataset was from September, 2003, near the 
peak in the satellite count.  And finally, the most 
recent catalog available at the time of this 
writing, August, 2004.
Figure 3, shows the distribution in inclination for 
the three catalogs over the inclination range 
visible to Cobra Dane.  Because of the high 
latitude location of the radar and its orientation, 
Cobra Dane can only detect objects with 
inclinations from 55° - 125°.  Also, only objects 
with perigee altitudes below 6000 km are 
plotted.  Figures 4 and 5 show the eccentricity 
and perigee altitude distributions using the same 
limitations and datasets shown in Figure 3. 
55 65 75 85 95 105 115 125
Inclination (deg)
0
1000
2000
3000
4000
N
um
be
r o
f S
at
el
lit
es
Apr '02
Sep '03
Aug '04
 
Fig. 3: Inclination distribution.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Eccentricity
1
10
100
1000
10000
N
um
be
r o
f S
at
el
lit
es
Apr '02
Sep '03
Aug '04
Fig. 4: Eccentricity distribution.
0 1000 2000 3000 4000 5000 6000
Perigee Altitude (km)
1
10
100
1000
10000
N
um
be
r o
f S
at
el
lit
es
Apr '02
Sep '03
Aug '04
Fig. 5: Perigee altitude distribution.
One area noted in examination of the early 
catalog and the peak catalog is an increase in 
the population near 65° at mean altitudes near 
9000 km.  A Gabbard diagram (Figure 6) of 
5these objects shows perigees near 300-500 km 
altitude with apogees in the 15,000-20,000 km 
range.  A number of known breakups of Proton-
Block DM Ullage motors have occurred with 
these orbital parameters.  However, very few 
objects were cataloged from these breakups.  
Cobra Dane is currently maintaining orbits on 
about 100 of these objects.
0
5000
10000
15000
20000
25000
250 300 350 400
Period (min)
Al
tit
ud
e (
km
)
Fig. 6: Gabbard diagram of “analyst” satellites 
with orbital inclinations between 60° - 70° 
and with mean altitudes between 8,000 – 
11,000 km.
Conclusions
The addition of the Cobra Dane radar to the 
SSN has increased the number of tracked 
orbiting objects by over 2000.  Many of these 
objects are smaller than the 10-cm diameter 
nominal pre-Cobra Dane sensitivity of the SSN.  
Although this does marginally improve the 
safety of manned spaceflight by increasing the 
number of objects for which collision avoidance 
screening is performed, it does not fully close 
the gap between what can be shielded against 
and what can be tracked and avoided.  Further, 
because of Cobra Dane’s high latitude location, 
not all inclinations are covered to Cobra Dane’s 
nominal sensitivity of 5-cm.  Additional assets 
are needed to further improve the sensitivity of 
the SSN and to increase the number of orbiting 
objects tracked.
References
Johnson, N. L., et. al.  “History of On-orbit 1.
Satellite Fragmentations.”  13th Ed.  Orbital 
Debris Program Office, NASA/Johnson 
Space Center, Houston, TX.  May, 2004.
“1994 Space Debris Campaign Final 2.
Report.”  SenCom Corp., Colorado Springs, 
CO.  14 July 1995.
Devere, G. T.  Air Force Satellite 3.
Assessment Center.  Personal 
Communication.
Robertson, S. A., Ed. “Small Debris 4.
Observations by the Cobra Dane Radar.”  
Orbital Debris Quarterly News, Vol. 4 No. 4, 
October, 1999.


Growth in the Number of SSN Tracked Orbital Objects

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