Planet Spacecraft Operations and Ground Control
Version 1.2 | Last updated September 2015
1.1 Overview of this Document
This document describes Planet’s Spacecraft Operations & Ground Systems. It is intended for users of satellite imagery interested in working with Planet’s initial product offerings.
1.2 Company Overview
Planet was founded in 2011 by three ex-NASA engineers to disrupt the traditional aerospace industry by using modern consumer electronics manufacturing techniques to build a large constellation of nanosatellites. Planet uses an agile aerospace approach for the design of its satellites, mission control and operations systems; and the development of its web-based platform for imagery and information. It is the only fully integrated company that designs, builds, and actively operates satellites while also delivering data to customers via an internally developed web-based platform. Planet employs an “always on” line- scanning image capturing method as opposed to the traditional tasking model used by most satellite companies today.
1.3 Space and Ground Systems Overview
Planet designs, builds, and operates large constellations of nanosatellites. Each individual satellite is 3U form factor (10cm x 10cm x 30cm). The spacecraft payload is an optical system and camera, capable of capturing imagery at 3-5 meter Ground Sample Distance (GSD).
Planet satellites are designed to be frequently updated and replaced; each satellite has an expected operational lifetime of 3 years. Planet uses an “agile aerospace” approach to technology development, using rapid iterative design and frequent testing in space in order to continually deploy improved spacecraft and payloads. Planet has launched three versions of its optical system, Planet Scope 0, Planet Scope 1, and Planet Scope 2.
Satellites are deployed into two types of orbit. International Space Station (ISS) orbits are at a 52 degree inclination at approximately 420km altitude. Sun Synchronous Orbits (SSO) are at a 98 degree inclination or higher at approximately 475km altitude. Large constellations of satellites will be operated in both orbit types.
Planet’s satellite constellations operate in a constant monitoring mode. Individual satellites are not tasked, they remain nadir pointing and they continuously capture imagery of the sunlit portion of the earth’s surface. Planet has developed its own network of ground stations to ensure efficient satellite operations and successful downlink of imagery.
2. Spacecraft and Payload Design
2.1 The “Dove” Spacecraft
The Planet Dove satellite design is based on the “3U” cubesat form factor (10cm x 10cm x 30cm). Planet refers to a group of Doves deployed simultaneously into a single orbit as a flock.
Figure 1: Planet Dove Satellite in Operational Configuration with Solar Panels Deployed and Communications Antenna Flap Opened.
Planet captures imagery using a telescope and camera combination which has been optimized for this form factor. The imaging system aboard each spacecraft captures red, blue and green (RGB) imagery. Planet is currently developing Near Infrared (NIR) imaging capabilities.
2.2 Optical System, Camera and Capture Techniques
2.2.1 Payload Design
Planet satellites each carry a telescope and a frame CCD camera equipped with Bayer-mask filter. The CCD sensor converts filtered photons into electrons, which are then amplified in order to produce a digital number corresponding to each pixel in each color band.
Figure 2: Planet Optical System and Camera.
Planet has flown three generations of optical instruments: Planet Scope 0 (PS0), Planet Scope 1 (PS1), and Planet Scope 2 (PS2). Images have different attributes depending on satellite altitude and instrument type.
Table 1: Spectral Band and Field of View (FOV) Information for PS0, PS1 and PS2 Instruments Flown at Various Altitudes
|Instrument||Spectral Bands||Field of View (FOV) and Ground Sampled Distance (GSD)|
|620km (Altitude of Planet Flock 1c)||475 km (target altitude for future SSO Flocks)||420 km (ISS Flock altitude)|
|PS0 and PS1||Red: 630-714 nm|
Green: 515-610 nm
Blue: 424-478 nm
|HFOV: 16.1 km|
VFOV: 10.7 km
Area: 173 km sq
GSD: 4 m
|NA (Instrument not flown at this altitude)||HFOV: 10.9 km|
VFOV: 7.3 km
Area: 79 km sq
GSD: 2.7 m
|PS2||Red: 630-714 nm|
Green: 515-610 nm
Blue: 424-478 nm
|NA (Instrument not flown at this altitude)||HFOV: 24.6 km|
VFOV: 16.4 km
Area: 405 km sq
GSD: 3.73 m
|HFOV: 21.8 km|
VFOV: 14.5 km
Area: 316 km sq
GSD: 3.3 m
PS0 features a 2 element Maksutov Cassegrain optical system paired with an 11MP CCD detector. Optical elements are mounted relative to the structure of the spacecraft.
PS1 features the same optical system as PS0, aligned and mounted in an isolated carbon fiber/titanium telescope. This telescope is matched with an 11MP CCD detector.
PS2 features a five element optical system that provides a wider field of view and superior image quality. This optical system is paired with a 29MP CCD detector.
220.127.116.11 Spectral Characteristics:
PS0 and PS1 optical systems are designed to collect data in the visible portion (red, blue and green) of the electromagnetic spec- trum. The following figures show the expected RGB spectral characteristics of the PS0 and PS1 systems:
Figure 3: RGB Spectral Bands for PS0/1
Figure 4: RGB Spectral Bands for PS2
Planet is currently testing a near-infrared (770-900nm) sensor.
Figure 5: Expected Spectral Response for PS2 (with NIR capability)
2.2.3 Image Quality Variations between PS0/1 and PS2
Images from PS0 systems exhibit vignetting, or a decline in pixel usability towards the edges of the sensor. Planet provides an alpha mask to discard pixels of unusable quality.
Images from PS1 and PS2 systems exhibit uniformly high pixel quality and usability over the entire sensor.
3. Orbits, Operational Mode and Constellations
3.1 Monitoring Operational Mode
Planet does not employ the traditional “tasking model” for space-based imagery collection. In the traditional model, imagery collections are prioritized and planned based on “targeted” collects with little or no imaging of non-prioritized areas. Planet’s satellites are designed to operate in concert to continuously collect imagery of the sunlit portion of the Earth’s surface. At full constellation, Planet’s monitoring capability is expected to yield approximately one complete global image dataset every day.
Traditional Industry: Tasking
Table 2: Orbit, Constellation and Satellite Specifications
|Description||International Space Station Orbit||Sun Synchronous Orbit|
|Expected Lifetime||1 year per satellite; constellation is replenished over time||2-3 years per satellite; constellation is replenished over time|
|Orbital Insertion Altitude||420 km||475 km (target altitude for future SSO launches|
|Equator Crossing Time||Varies||9:30-11:30am local solar time|
|Sensor Type||Bayer-masked CCD camera||Bayer-masked CCD camera|
|Spectral Bands||Red: 610-700 nm|
Green: 500-590 nm
Blue: 420-530 nm
|Red: 610-700 nm|
Green: 500-590 nm
Blue: 420-530 nm
|Ground Sampling Distance (Nadir)||2.7-3.2 m||3.7-4.9 m|
|Mission Continuity||Maintain up to 55 satellite constellation (continually replenishing/upgrading satellites)||Maintain 100-150 satellite constellation (continually replenishing/upgrading satellites)|
3.2.1 Imagery Archive
Throughout 2014 and the first half of 2016, Planet focused collection capacity on North America, Asia, and South America. Focusing on these areas allowed Planet to establish initial imagery archive to develop and refine imagery processing, imagery quality, imagery mosaic, and API capabilities.
Figure 6: Heat Map as of June 2015, Showing Percent of Quad Tiles Covered by Planet’s Imagery
4. Ground Stations and Network
4.1 Network Architecture
Planet has developed its own global network of ground stations to support both spacecraft mission operations and image data downlink. Each ground station consists of an antenna and a Radio Frequency (RF) system, coupled with a local computer server, connected via secured VPN access to centralized services. Downlinked image files are transferred from local ground station servers to Planet’s cloud infrastructure for ingestion into Planet’s data processing and distribution pipeline.
Figure 7: End-to-End Data Flow
Planet utilizes two low speed UHF systems for satellite operations and high speed X-band system for imagery download. Planet currently operates six X-band ground stations and will increase this number to 28 at 10 separate locations, in order to meet the downlink capacity requirements for a planned constellation of over 200 satellites operating simultaneously.