# Presentation: 266184

## Interference ProblemsCurrent and Future

**Overview**
- Charles Wende
- Office of Earth Science
- NASA HQ

## 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.

## 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).

## 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., 6:00 AM, 10:00 AM, and 1:30 PM local satellite time).

## 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.

## Physics

## High Latitude Advantage

## NASA Polar Ground Network

## Physics

## 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.

## Interference

**In-band Contention**

## Congestion

- The 8025-8400 MHz band is getting crowded:
- 1990’s:		2000’s ===================
- Landsats 		Landsat-5/7 		AQUA
- SPOTs			SPOTs			AURA
- IRSs			IRSs			CALIPSO
- ERS-1,-2		ERS-2			ENVISAT
- ADEOS-1		QUIKSCAT		EO-1
- JERS-1		TERRA		ICESAT
- RADARSAT		RADARSAT-1/2	KOMPSAT
- 				ADEOS-2		SAC-C

## Congestion-II

- The 8025-8400 MHz band is getting more crowded!
- ====================== 2000’s
- ALOS				IKONOS
- ESSP (?)			ORBVIEW
- EUMETSAT(?) 	 	QUICKBIRD
- NPP/NPOESS(?) 		EROS-A1,A2,B1,...B6
- 					AVSAT-1
- 					M5 Constellation (?)
- 					Resource 21 (?)

## 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.

## New Paradigm

- The NASA morning train (10:00 AM southbound):
- Landsat-7 (10:00 AM), X-band wideband, EROS Data Center and international cooperators, some Polar Ground Network (Fairbanks, AK and Svalbard).
- EO-1 (10:01 AM), X-band, Polar Ground Network
- SAC-C (10:31 AM), X-band, Argentine ground station
- TERRA (10:34 AM), TDRSS wideband, X-band Direct Broadcast, X-band wideband for backup/proficiency.
- This formation is up and operating now.

## New Paradigm

- The NASA afternoon train (1:30 PM northbound):
- AQUA (1:30 PM), X-band, wideband via polar ground network, direct broadcast otherwise.
- Cloudsat (  1:32:32 PM), AFSCN S-band
- CALIPSO ( 1:32:45 PM), X-band, USN in AK, HA
- PARASOL ( 1:34 PM), S-band (?)
- AURA (1:45 PM), X-band, wideband via polar ground network.
- Only AQUA has been launched.

## 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

## 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)

## Interference

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

## Deep Space Network Situation

- 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.

**8025                                                                                          8400     8450        8500**

- EESS                              SRS   SRS
-                                        (DSN)

## 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).

## Lines-of-Sight of DSN

## 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).

## 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).

## 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.

## 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_.

## 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)

## 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