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On LEOs and Starlink
Geoff Huston AM
Chief Scientist, APNIC
November 2023
screenshot from starwatch app
Screenshot: https://asia.nikkei.com/Business/Telecommunication/Elon-Musk-s-Starlink-launches-satellite-internet-service-in-Japan
Screenshot - https://www.theverge.com/2022/8/25/23320722/spacex-starlink-t-mobile-satellite-internet-mobile-messaging
LEOs in the News
Newtonian Physics
• If you fire a projectile with a speed
greater than 11.2Km/sec it will not
fall back to earth, and instead head
away from earth never to return
• On the other hand, if you incline the
aiming trajectory and fire it at a
critical speed it will settle into an
orbit around the earth
• The higher the altitude, the lower
the orbital speed required to
maintain orbit
Solar Radiation Physics
• The rotating iron core of the
Earth produces a strong
magnetic field
• This magnetic deflects solar
radiation – the Van Allen Belt
• Sheltering below the Van
Allen Belt protects the
spacecraft from the worst
effects of solar radiation,
allowing advanced
electronics to be used in the
spacecraft
Low Earth Orbit
https://commons.wikimedia.org/wiki/File:Orbitalaltitudes.jpg GNU Free Documentation
Low Earth Orbit
• LEO satellites are stations between 160km and 2,000km in altitude.
• High enough to stop it slowing down by “grazing” the denser parts of the earth’s ionosphere
• Not so high that it loses the radiation protection afforded by the Inner Van Allen belt.
• At a height of 550km, the minimum signal propagation delay to reach the satellite and back is 3.7ms,
at 25o it’s 7.5ms, and at the horizon it’s 18ms
screenshot from starwatch app
Image - spacex
Starlink Constellation
If you use a minimum angle of elevation of 25o then at an
altitude of 550km each satellite spans a terrestrial
footprint of no more than ~900Km radius, or 2M K2
At a minimum, a LEO satellite constellation needs 500
satellites to provide coverage of all parts of the earth’s
surface
For high quality coverage the constellation will need 6x-
20x that number (or more!)
7
Starlink Constellation
• 4,276 in-service operational spacecraft, operating at an altitude of 550km
https://satellitemap.space/
8
So LEOs are “interesting”!
• They are very close to the Earth – which means:
• They don’t need specialised high-power equipment to send and receive
signals
• Even hand-held mobile devices can send and receive signals with a LEO!
• They can achieve very high signal speeds
• It’s a highly focussed signal beam
• They are harder to disrupt by external interference
• But you need a large number of them to provide a continuous service
• The extremely host cost of launching a large constellation of LEO
spacecraft has been the major problem with LEO service until recently
• Which is why Motorola’s Iridium service went bankrupt soon after launch
What’s changed recently?
• SpaceX’s reusable rocket technology has slashed the cost of lifting
spacecraft into low earth orbit
SpaceX
https://ourworldindata.org/grapher/cost-space-launches-low-earth-orbit
Cost/Kg
(log
scale)
Time
Current and Planned satellite constellations
Tracking a LEO satellite
550km
27,000 km/h
Satellite
horizon to horizon: ~5 minutes
Looking Up
Starlink tracks satellites with a minimum
elevation of 25o.
There are between 30 – 50 visible Starlink
satellites at any point on the surface
between latitudes 56o North and South
Each satellite traverses the visible aperture
for a maximum of ~3 minutes
13
Starlink Scheduling
• A satellite is assigned to a user terminal in 15 second time slots
• Tracking of a satellite (by phased array focussing) works across 11
degrees of arc per satellite in each 15 second slot
client
11o
15 seconds
14
Starlink Scheduling
• Latency changes on each
satellite switch
• If we take the minimum
latency on each 15 second
scheduling interval, we can
expose the effects of the
switching interval on latency
• Across the 15 second interval
there will be a drift in latency
according to the satellite’s
track and the distance relative
to the two earth points
• Other user traffic will also
impact on latency, and also
the effects of a large buffer in
the user modem
Starlink Spot Beams
• Each spacecraft 2,000 MHz of spectrum for user downlink and splits it into 8x channels
of 250 MHz each
• Each satellite has 3 downlink antennas and 1 uplink antennas, and each can do 8
beams x 2 polarizations, for a total of 48 beams down and 16 up.
16
“Unveiling Beamforming Strategies of Starlink LEO Satellites”
https://people.engineering.osu.edu/sites/default/files/2022-10/Kassas_Unveiling_Beamforming_Strategies_of_Starlink_LEO_Satellites.pdf
Starlink’s reports
$ starlink-grpc-tools/dish_grpc_text.py -v status
id: ut01000000-00000000-005dd555
hardware_version: rev3_proto2
software_version: 5a923943-5acb-4d05-ac58-dd93e72b7862.uterm.release
state: CONNECTED
uptime: 481674
snr:
seconds_to_first_nonempty_slot: 0.0
pop_ping_drop_rate: 0.0
downlink_throughput_bps: 16693.330078125
uplink_throughput_bps: 109127.3984375
pop_ping_latency_ms: 49.5
Alerts bit field: 0
fraction_obstructed: 0.04149007424712181
currently_obstructed: False
seconds_obstructed:
obstruction_duration: 1.9579976797103882
obstruction_interval: 540.0
direction_azimuth: -42.67951583862305
direction_elevation: 64.61225128173828
is_snr_above_noise_floor: True
17
Reported Capacity & Latency
18
Reported Capacity
& Latency
• This is going to present some interesting issues for conventional TCP
• TCP uses ACK pacing which means it attempts to optimize its sending
rate over multiple RTT intervals
• The variation in latency and capacity occurs at high frequency, which
means that TCP control is going to struggle to optimize itself against a
shifting target
19
Link Characteristics
1-second ping
Micro-drops
Highly unstable jitter
24 Hours
20
How well does all this work?
Speedtest measurements:
We should be able to get
~120Mbps out of a starlink
connection. Right?
21
TCP Flow Control Algorithms
“Ideal” Flow behaviour
for each protocol
22
iperf3 – cubic, 60 seconds
CUBIC
Loss
23
Qperf – quic (with cubic)
QUIC/CUBIC
Loss
24
iperf3 - bbr
BBR
Loss
25
Cubic, Quic/Cubic, BBR
26
Protocol Considerations
• Starlink services have two issues:
• Very high jitter rates
• High levels of micro-loss
• Loss-based flow control algorithms will over-react and pull back the
sending rate over time
• Short transactions work very well
• Paced connections (voice, zoom, video streaming) tend to work well most of
the time
• Bulk data transfer, not so much
• You need to move to less loss-sensitive flow control algorithms, such
as BBR to get good performance out of these services
What about Starlink Gen2?
• These satellites are larger, heavier and operate at a higher power level
• More bandwidth available, better SNR, and high achievable data
speeds
• Multiple orbital plans at a collection of discrete altitudes
• Incorporates 5G cellular services
• Will use inter-satellite laser connectors to support packet routing
across satellites – few details as yet
I don’t want to leave you with
the wrong impression of Starlink
Starlink is perfectly acceptable for:
• short transactions
• video streaming
• conferencing
• The service can sustain 10 – 20Mbps delivery for long-held sessions
• The isolated drop events generally do not intrude into the session state
• And it can be used in all kinds of places where existing wire and mobile
radio systems either under-perform or aren’t there at all!
• Its probably not the best trunk infrastructure service medium, but it’s a
really good high speed last mile direct retail access service, particularly for
remote locations!
• Its also very challenging to jam!
Questions?

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40th TWNIC OPM: On LEOs (Low Earth Orbits) and Starlink Download

  • 1. On LEOs and Starlink Geoff Huston AM Chief Scientist, APNIC November 2023
  • 2. screenshot from starwatch app Screenshot: https://asia.nikkei.com/Business/Telecommunication/Elon-Musk-s-Starlink-launches-satellite-internet-service-in-Japan Screenshot - https://www.theverge.com/2022/8/25/23320722/spacex-starlink-t-mobile-satellite-internet-mobile-messaging LEOs in the News
  • 3. Newtonian Physics • If you fire a projectile with a speed greater than 11.2Km/sec it will not fall back to earth, and instead head away from earth never to return • On the other hand, if you incline the aiming trajectory and fire it at a critical speed it will settle into an orbit around the earth • The higher the altitude, the lower the orbital speed required to maintain orbit
  • 4. Solar Radiation Physics • The rotating iron core of the Earth produces a strong magnetic field • This magnetic deflects solar radiation – the Van Allen Belt • Sheltering below the Van Allen Belt protects the spacecraft from the worst effects of solar radiation, allowing advanced electronics to be used in the spacecraft
  • 6. Low Earth Orbit • LEO satellites are stations between 160km and 2,000km in altitude. • High enough to stop it slowing down by “grazing” the denser parts of the earth’s ionosphere • Not so high that it loses the radiation protection afforded by the Inner Van Allen belt. • At a height of 550km, the minimum signal propagation delay to reach the satellite and back is 3.7ms, at 25o it’s 7.5ms, and at the horizon it’s 18ms screenshot from starwatch app Image - spacex
  • 7. Starlink Constellation If you use a minimum angle of elevation of 25o then at an altitude of 550km each satellite spans a terrestrial footprint of no more than ~900Km radius, or 2M K2 At a minimum, a LEO satellite constellation needs 500 satellites to provide coverage of all parts of the earth’s surface For high quality coverage the constellation will need 6x- 20x that number (or more!) 7
  • 8. Starlink Constellation • 4,276 in-service operational spacecraft, operating at an altitude of 550km https://satellitemap.space/ 8
  • 9. So LEOs are “interesting”! • They are very close to the Earth – which means: • They don’t need specialised high-power equipment to send and receive signals • Even hand-held mobile devices can send and receive signals with a LEO! • They can achieve very high signal speeds • It’s a highly focussed signal beam • They are harder to disrupt by external interference • But you need a large number of them to provide a continuous service • The extremely host cost of launching a large constellation of LEO spacecraft has been the major problem with LEO service until recently • Which is why Motorola’s Iridium service went bankrupt soon after launch
  • 10. What’s changed recently? • SpaceX’s reusable rocket technology has slashed the cost of lifting spacecraft into low earth orbit SpaceX https://ourworldindata.org/grapher/cost-space-launches-low-earth-orbit Cost/Kg (log scale) Time
  • 11. Current and Planned satellite constellations
  • 12. Tracking a LEO satellite 550km 27,000 km/h Satellite horizon to horizon: ~5 minutes
  • 13. Looking Up Starlink tracks satellites with a minimum elevation of 25o. There are between 30 – 50 visible Starlink satellites at any point on the surface between latitudes 56o North and South Each satellite traverses the visible aperture for a maximum of ~3 minutes 13
  • 14. Starlink Scheduling • A satellite is assigned to a user terminal in 15 second time slots • Tracking of a satellite (by phased array focussing) works across 11 degrees of arc per satellite in each 15 second slot client 11o 15 seconds 14
  • 15. Starlink Scheduling • Latency changes on each satellite switch • If we take the minimum latency on each 15 second scheduling interval, we can expose the effects of the switching interval on latency • Across the 15 second interval there will be a drift in latency according to the satellite’s track and the distance relative to the two earth points • Other user traffic will also impact on latency, and also the effects of a large buffer in the user modem
  • 16. Starlink Spot Beams • Each spacecraft 2,000 MHz of spectrum for user downlink and splits it into 8x channels of 250 MHz each • Each satellite has 3 downlink antennas and 1 uplink antennas, and each can do 8 beams x 2 polarizations, for a total of 48 beams down and 16 up. 16 “Unveiling Beamforming Strategies of Starlink LEO Satellites” https://people.engineering.osu.edu/sites/default/files/2022-10/Kassas_Unveiling_Beamforming_Strategies_of_Starlink_LEO_Satellites.pdf
  • 17. Starlink’s reports $ starlink-grpc-tools/dish_grpc_text.py -v status id: ut01000000-00000000-005dd555 hardware_version: rev3_proto2 software_version: 5a923943-5acb-4d05-ac58-dd93e72b7862.uterm.release state: CONNECTED uptime: 481674 snr: seconds_to_first_nonempty_slot: 0.0 pop_ping_drop_rate: 0.0 downlink_throughput_bps: 16693.330078125 uplink_throughput_bps: 109127.3984375 pop_ping_latency_ms: 49.5 Alerts bit field: 0 fraction_obstructed: 0.04149007424712181 currently_obstructed: False seconds_obstructed: obstruction_duration: 1.9579976797103882 obstruction_interval: 540.0 direction_azimuth: -42.67951583862305 direction_elevation: 64.61225128173828 is_snr_above_noise_floor: True 17
  • 18. Reported Capacity & Latency 18
  • 19. Reported Capacity & Latency • This is going to present some interesting issues for conventional TCP • TCP uses ACK pacing which means it attempts to optimize its sending rate over multiple RTT intervals • The variation in latency and capacity occurs at high frequency, which means that TCP control is going to struggle to optimize itself against a shifting target 19
  • 21. How well does all this work? Speedtest measurements: We should be able to get ~120Mbps out of a starlink connection. Right? 21
  • 22. TCP Flow Control Algorithms “Ideal” Flow behaviour for each protocol 22
  • 23. iperf3 – cubic, 60 seconds CUBIC Loss 23
  • 24. Qperf – quic (with cubic) QUIC/CUBIC Loss 24
  • 27. Protocol Considerations • Starlink services have two issues: • Very high jitter rates • High levels of micro-loss • Loss-based flow control algorithms will over-react and pull back the sending rate over time • Short transactions work very well • Paced connections (voice, zoom, video streaming) tend to work well most of the time • Bulk data transfer, not so much • You need to move to less loss-sensitive flow control algorithms, such as BBR to get good performance out of these services
  • 28. What about Starlink Gen2? • These satellites are larger, heavier and operate at a higher power level • More bandwidth available, better SNR, and high achievable data speeds • Multiple orbital plans at a collection of discrete altitudes • Incorporates 5G cellular services • Will use inter-satellite laser connectors to support packet routing across satellites – few details as yet
  • 29. I don’t want to leave you with the wrong impression of Starlink Starlink is perfectly acceptable for: • short transactions • video streaming • conferencing • The service can sustain 10 – 20Mbps delivery for long-held sessions • The isolated drop events generally do not intrude into the session state • And it can be used in all kinds of places where existing wire and mobile radio systems either under-perform or aren’t there at all! • Its probably not the best trunk infrastructure service medium, but it’s a really good high speed last mile direct retail access service, particularly for remote locations! • Its also very challenging to jam!