Interference Problems Current and Future

1
Interference ProblemsCurrent and Future

• Overview
• Charles Wende
• Office of Earth Science
• NASA HQ

2
Physics

• Frequency is logarithmic (DC to light and
  beyond), but usable frequencies are finite and
  are allocated internationally to specific
  services.
• If too many people try to use the
• Same frequency/frequencies at the
• Same place at the
• Same time
• Interference will result.

3
Physics

• It has happened before with
• 1 ground station, which had
• 2 antennas tracking
• 2 satellites, and
• bad timing
• (both spacecraft simultaneously lost lock for 1
  minute).
• Now they schedule more carefully, but
• They scheduled both spacecraft (good news), and
• They are in a very popular location (very bad
  news).

4
Physics

• For good reasons, most Earth Science Spacecraft,
  both public and private, are
• In sun-synchronous orbits,
• Many at a preferred altitude of 705 km (98
  minute orbital period),
• With preferred equatorial crossing times (e.g.,
  600 AM, 1000 AM, and 130 PM local satellite
  time).

5
Physics

• All the good reasons conspire to increase the
  likelihood of conflicts.
• Sun-synchronous spacecraft with the same
  equatorial crossing time but different altitudes
  will walk across one another.
• Sun-synchronous satellites at the same altitude
  but different equatorial crossing times will
  cross their orbits at high latitudes where the
  prime ground stations are located.

6
Physics
7
High Latitude Advantage
8
NASA Polar Ground Network
9
Physics
10
Interference

• There are two different type of interference
• Contention with other in-band spacecraft
  affecting the ability of both to downlink data.
• Frequency diversity may help (unless everyone is
  using all the band), and
• Geographic diversity of ground stations can help.
• Out-of-band emissions interfering with other
  services.
• Carefully designed equipment can help.

11
Interference

• In-band Contention

12
Congestion
The 8025-8400 MHz band is getting
crowded 1990s 2000s Lands
ats Landsat-5/7 AQUA SPOTs SPOTs AURA IR
Ss IRSs CALIPSO ERS-1,-2 ERS-2 ENVISAT ADEO
S-1 QUIKSCAT EO-1 JERS-1 TERRA ICESAT RADARSAT
RADARSAT-1/2 KOMPSAT ADEOS-2 SAC-C
13
Congestion-II
The 8025-8400 MHz band is getting more
crowded!
2000s ALOS IKONOS ESSP (?) ORBVIEW EUMETSAT(
?) QUICKBIRD NPP/NPOESS(?) EROS-A1,A2,B1,B6
AVSAT-1 M5 Constellation
(?) Resource 21 (?)
14
New Paradigm

• Faster, Better, Cheaper has led to the concept
  of formation flying.
• Multiple smaller spacecraft replacing one very
  expensive spacecraft, reducing risk and
  (hopefully) costing less,
• All crossing over the same ground scene, and
• Requiring careful orbit maintenance and increased
  ground support.
• NASA has two such formations.

15
New Paradigm

• The NASA morning train (1000 AM southbound)
• Landsat-7 (1000 AM), X-band wideband, EROS Data
  Center and international cooperators, some Polar
  Ground Network (Fairbanks, AK and Svalbard).
• EO-1 (1001 AM), X-band, Polar Ground Network
• SAC-C (1031 AM), X-band, Argentine ground
  station
• TERRA (1034 AM), TDRSS wideband, X-band Direct
  Broadcast, X-band wideband for backup/proficiency.
  
• This formation is up and operating now.

16
New Paradigm

• The NASA afternoon train (130 PM northbound)
• AQUA (130 PM), X-band, wideband via polar ground
  network, direct broadcast otherwise.
• Cloudsat ( 13232 PM), AFSCN S-band
• CALIPSO ( 13245 PM), X-band, USN in AK, HA
• PARASOL ( 134 PM), S-band (?)
• AURA (145 PM), X-band, wideband via polar ground
  network.
• Only AQUA has been launched.

17
New Paradigm

• NASA realized that they had to synchronize the
  two trains
• Both are at the same altitude (705 km) and
  period.
• Orbits will cross near the poles and in
  line-of-sight of polar ground stations.
• They could overload the available ground stations
  (and maybe collide).
• NASA set a minimum of 15 minutes between trains.
• TERRA leads AQUA by 20 minutes
• AURA will lead Landsat-7 by 40 minutes

18
New Paradigm

• However, the two trains are being operated by/for
  NASA independent of any other band users.
• NASA was forced by the physics to address the
  contention problem.
• Other users of 705 km orbits are not coordinated
  with these trains.
• There is no central frequency-orbit-ground
  contact coordination mechanism in place today.
• Something similar is in place for geosynchronous
  spacecraft (position and frequency are jointly
  assigned)

19
Interference

• Out-of-Band Emissions
• into a Nearby Service

20
Deep Space Network Situation
EESS SRS SRS
(DSN)
8025
8400
8450 8500

• All 8025-8500 MHz bands are space-to-Earth
  downlinks.
• Deep Space Network (DSN) has very low power
  density limit for interference (-220 dBW/Hz).
• No guard band between EESS and DSN.

21
Deep Space Network Situation

• EESS X-band band is already heavily used.
• Only wide EESS downlink band with infrastructure,
• More missions upcoming, and
• Most missions want most of the bandwidth.
• DSN X-band band usage is increasing.
• Rarely is any replay available, and
• Relocking on lost signal is difficult (very very
  low S/N).

22
Lines-of-Sight of DSN
23
Deep Space Network Situation

• Simple solution is turning off within
  line-of-sight of DSN stations
• Goldstone, California, USA,
• Madrid, Spain, and
• Canberra, Australia.
• Line-of-sight turnoff of real time data means
• No Western USA (fire service will object),
• No Western Europe, and
• No Eastern Australia (Australian Weather Service
  will object).

24
DSN and TERRA and AQUA

• TERRA was launched on December 18, 1999.
• Wideband playback data sent via TDRSS (X-band
  backup system), and
• Real time data sent via X-band (Direct
  Broadcast).
• AQUA was launched on May 4, 2002.
• Wideband playback data sent via X-band,
• Real time data sent via X-band (Direct Broadcast).

25
DSN and TERRA and AQUA

• Both TERRA and AQUA have a Direct Broadcast
  mode which transmits real time data directly to
  ground stations
• Relatively narrow bandwidth (15-20 Mb/s) relative
  to Playback (150 Mb/s),
• On all the time (almost),
• To a worldwide community, with
• Some quasi-operational use.

26
DSN and TERRA and AQUA

• TERRA did not meet DSN power density limits.
• Operational coordination is required between
  TERRA schedule and DSN schedule.
• AQUA design met DSN power density limits.
• Operational coordination is believed to be not
  required.

27
Future

• A large number of new bandwidth-hungry missions
  are considering using 8025-8400 MHz.
• Some scientific
• Earth System Science Pathfinder mission(s).
• Some operational
• NPOESS, the next generation weather satellites
• Some commercial Earth remote sensing
• Radarsat-2 (-3?)
• M5 constellation of 4 S/C (DigitalGlobe)
• Resource 21 (one or more satellites)

28
Next Steps

• Further discussion of TERRA and AQUA
• Discussion of alternative bands
• Available infrastructure
• Available technology
• Technological help
• Data compression technology
• Modulation technology
• Regulatory situation