this hosting will confirm the on-orbit performance to enable DSAC to be used ... sensing business side of Surrey, DMCii (Disaster Monitoring Constellation) was.
AAS 14-075
Hosting the Deep Space Atomic Clock (DSAC) on the Orbital Test Bed (OTB-1) satellite F. Brent Abbott, PE; William Thompson Surrey Satellite Technology US Todd A. Ely Jet Propulsion Laboratory, California Institute of Technology
29th ANNUAL AAS GUIDANCE AND CONTROL CONFERENCE February 1-5, 2014 Breckenridge, Colorado
Sponsored by Rocky Mountain Section
AAS Publications Office, P.O. Box 28130 - San Diego, California 92198
AAS 14-075
Hosting the Deep Space Atomic Clock (DSAC) on the Orbital Test Bed (OTB-1) satellite F. Brent Abbott, PE, William Thompson, Surrey Satellite Technology US Todd A. Ely, Jet Propulsion Laboratory, California Institute of Technology This paper will share the experiences, ongoing work and lessons learned in hosting the DSAC instrument on a relatively standard satellite bus, OTB-1. As DSAC is a great advancement in navigation, this hosting will confirm the on-orbit performance to enable DSAC to be used for future operational systems. Payload performance and operational requirements will be discussed. The process in which JPL and Surrey US work together with requirements and bus design to optimize maximum return on on-orbit testing will be presented with focus on GN&C systems.
STARTING A HOSTED PAYLOADS PROGRAM A new internal need for a supplemental instrumentation to support the remote sensing business side of Surrey, DMCii (Disaster Monitoring Constellation) was identified. This business is built on products developed from the multispectral cameras known as the SLIM-6. These have provided 32m multispectral data over a wide swath and more recently a 22m resolution. With SSTL (Surrey UK) moving forward with Tech Demo Sat (TDS-1, a wholly hosted payload mission) and the US market moving to embrace Hosted Payloads in the forms of HoPS at SMC and programs at NASA STMD (Space Technology Mission Directorate), Surrey US (SST-US) saw an opportunity to reduce the cost of the new DMC satellite, fly company development hardware and start their own hosted payload leasing of the additional space and capacity on the new bus.
The Slim-6 wide swath multispectral instrument and the initial OTB-1 bus
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TechDemoSat-1, SSTL’s first Hosted Payloads Mission utilizing a 150 bus. Launch 2014
In the following chart we can see the typical equipment content on the SSTL-150 bus to be used for the initial OTB-1 bus design: TTC Comms
MTQ-X
OBDH
MTQ-Y
MTQ-Z
ASS-0
ASS-1
STAR TRACKER
ASS-2
CAN
5V
SGR-10 28V
28V
CAN
5V 28V
PDM
BCR 2
PCM 1
28V
-Z
CAN
Payload Data Storage
5V
BCR 1
-X
BCR 4
-X
BCR 5
BATT- 0
Power System 28V
5V CAN
X-Tx 0 28V
CAN
X-Tx X-Tx 1 1 28V
CAN
Payload Comms
80 Mbps data
CAN
28V
28V
Temp sensor
28V
HSDR1
PROP CTRL
PROP SYS
HSDR0
BCR 3
CAN
PROP CTRL 28V
CAN
-Z
Separation switch. Two switches in parallel.
CAN
CAN
Propulsion
CAN
PCM 0
+X
28V
5V
CAN
W HL-2
W HL-3
CAN
BCR 0
CAN
CAN
CAN
5V
CAN
W HL-0 28V
W HL-1
28V
+X
CAN
MAG-1
5V
S-Rx 1
CAN
AOCS
PPS to: Payload OBCs SSDRs Star Tracker
CAN
CAN
28V
28V
5V
28V
CAN
28V
5V
OBC386 1
ADCS1
ADCS0 28V
S-Rx 0
ST ELEC
MAG-0
5V
28V
CAN
LVDS
Payload 28V
28V
Payload
Connections to redundant PCM/PDM power supply
Serial connections to S-Band Rx’s and Tx’s
Connections to redundant CAN bus
Connections within power subsystem Analogue connection to ADCS module
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5V
OBC386 0
5V
CAN
S-Tx 1
5V
S-Tx 0 28V
The typical specifications for the 150 bus are below. Surrey’s bus architectures have been developed primarily for remote sensing applications that usually require good attitude control, a large amount of on board storage and strong downlinking capability. SSTL150 Payload Mass
50kg
Payload Power (OAP/Peak)
50/100W
Pointing Control
36 arcsec
Stability
1.5 arcsec/sec
Data Downlink
80Mbps (X)
Data Storage
16GB
Delta-V
36 m/s
Design Life
7 years
# on orbit
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SST-US (hereafter Surrey) started an outreach campaign in 2012 to solicit other payloads across their customer base and made a press release in April 2012 publicizing the offering and directing candidates to our website to fill out a payload requirements sheet. This sheet was a first order questionnaire to cover payloads needs such as mass, power, attitude, ops, field of view, volume and other requirements so that Surrey could quantify the ability to host and insure compatibility with existing payloads. In addition to smaller candidates looking for a position on OTB-1, JPL inquired about a hosting position for their Deep Space Atomic Clock (DSAC). DSAC had recently lost its hosted position on IRIDIUM and was increasingly looking for hosting opportunities as the payload was maturing and ready for a flight demonstration needed to increase its Technology Readiness Level from 5 to 7. The Deep Space Atomic Clock mission is developing a small, highly stable mercury ion atomic clock with an Allan deviation of at most 1e-14 at one day, with current estimates near 3e-15. This stability enables one-way radiometric tracking data with accuracy equivalent to or better (under certain conditions) than current two-way deep space tracking data; allowing a shift to a more efficient and flexible one-way deep space navigation architecture. The project is building a demonstration unit of the mercury ion atomic clock and the associated payload that will used to validate the clock’s performance.
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DSAC mission architecture
JPL found that a one year mission using OTB-1 could work for verifying the clock stability in LEO. Surrey discovered that DSAC and the primary SLIM-6 payload were incompatible in resources needed although there was a way to make their operations compatible. The SLIM-6 business case was revisited and found that in order to accommodate DSAC, the SLIM-6 would be pushed to the next OTB mission but other smaller company payloads could still fit. Launch changes also came up at this time. With all the changes driven by the new launch ride share and new primary payload, the bus design made a large change to the existing SSTL-150 ESPA design (A bus that had been launched with Orbital Express, used an ESPA ring adapter, and is now still flying for Los Alamos Labs). CDR PAYLOADS By CDR, January 2014, the following are payloads of record for the OTB-1: • • • • • • • •
DSAC – Clock, USO, TRIG GPS, Antenna/choke and Payload interface unit FlexRX - New Surrey flexible com system Radiation Monitor – New Surrey design Terminator Tape deorbit device Electronics Test Bed – Surrey Space Center CUSP – CU and Surrey Three unidentified payloads High Rate S-band transmitter
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Below is the OTB-1 based on the 150 ESPA. The most noticeable change is the addition of deployable solar arrays made necessary by the power demands and change in orbit inclination. Also shown are OTB-1 in its launch position on the ESPA ring and OTB-1’s system block diagram.
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GNC CHANGES AND MORE The rideshare launch opportunity carried with it a specified and perhaps disadvantageous orbit of 720 km and 24 degrees. This put constraints on the TT&C and power as most missions this bus was originally designed for are sun synchronous and most Surrey ground stations are at high latitudes. A yaw maneuver will also be required every 6 months in this orbit. Propulsion has been removed as orbit maintenance will not be required.
24 degree inclination orbit and associated beta angles chart
The lower inclination, the limitation of a 5 minute TT&C pass per day, and the accommodations needed for the payloads has driven additional changes to the GN&C. There has been power negative spins identified that have to be avoided upon launch tip off and possible loss of control and communications through life. The following are some of the avionics changed or added: • • •
STIM-210 Gyros for pointing during eclipse Two Active Safety Monitors Moog Bradford sun sensors
Active Safety Monitors are used in the case of an anomaly and will control OTB. The watch dog timer going off will turn these units on with redundant unit cold. Only the ASMs, AIM, X and Y wheels and heaters will be powered to regain safe mode. Given the removal of the standard star trackers and the above modifications OTB-1 has the following control specifications.
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Sunlit Case
Roll (°, 1 σ)
Pitch (°, 1 σ)
Yaw (°, 1 σ)
Knowledge Error Requirement
0.600
0.800
0.600
Knowledge Error
0.493
0.653
0.535
Control Error - Requirement
2.000
2.000
2.000
Control Error
0.493
0.653
0.535
Eclipse Case
Roll (°, mean + 1 σ)
Pitch (°, mean + 1 σ)
Yaw (°, mean + 1 σ)
Knowledge Error Requirement
1.500
1.500
1.500
Knowledge Error
1.158
1.397
1.120
Control Error - Requirement
2.000
2.000
2.000
Control Error
1.158
1.394
1.120
GPS The needs of DSAC required upgrading the typical SGR-10 GPS receiver with the TRIG-POD, which became part of the DSAC payload. Also, the GPS antenna with choke rings (to minimize multipath) has been mounted to the anti-nadir deck to give a maximum number of GPS satellites in view.
OTB-1 undeployed showing GPS antenna and middle payload bay on right for DSAC modules and TRIG-POD
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MAGNETORQUERS DSAC also needed a stable magnetic field environment and is shielded for this purpose, but the magnetorquer induced fields still required studied to make sure they were below the sensitivity levels of the payload. With the flexibility of the layout the magnetorquers were moved away from the atomic clock and an analysis was done to show that the simulated fields were tolerable.
OTHER CHANGES
Magnetic field simulation
The reaction wheels were taken from 4 to 3. Maneuvers and safe modes can be accomplished with 2 wheels and torque rods. The wheels also have a great on orbit heritage and this saves space. Propulsion was eliminated because of the relatively high orbit and no need to maintain it. On the other hand a de-orbit device will be employed to make sure the bus will come down in the required time frame. The relatively small demand for downlink allowed Surrey to drop the X band system and use a S-Band system including the new High Rate system being tested on-board. LESSONS LEARNED PAYLOADS CHANGE Payloads will be late and miss the bus; some won’t get funding or able to sign commitments, some will fit better together for a more optimal load, including the prime payload – and there is usually a prime payload. Expect parameters like Mass and Power to change for the worse. Suggestions: Be flexible with launch, GN&C system, and have more payloads ready to make sure you hit critical mass and close the business case. Contractually
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control and envelope payload specs, especially power and mass. Require mass models to take payloads place if they don’t deliver in time. SOMEONE HAS TO STEP UP – Chicken or the Egg We found that Surrey had to step up, own and prime the program to go forward instead of hoping to herd enough cats and their commitments to a program. PAYLOADS CAN ADD VALUE TO THE PROGRAM Payloads, especially ones that are typically experimental satellite subsystems can be advantageous and capability or potential back up to standard bus operations, but one has to be careful not to count on something that may not work or be delivered HOSTING CHARGES ARE NOT JUST A % CAPABILITY COST When pricing space on OTB-1 we found it to be a combination of % of bus capacity used, NRE for integration and testing; and operations. At its base, a payload is first evaluated on how much of the chosen bus it “uses”. This includes con-ops duty cycle, downlink, power, volume and the like. The highest % use of the satellite capacity in any of the categories drives the base cost of the host up to 100% that is similar to, if not the same as, a dedicated free flyer. In the case of OTB-1 the power capability was driven by DSAC. Surrey was able to add 4 panels to accommodate this. Suggestions: Be sure to capture all cost estimates from a turnkey mission including any extra bus NRE or modifications and mission ops including the capacity of the bus used. PAYLOAD INTERATIONS While payloads may seem to have synergistic, or at least compatible relationships, be prepared to look at all possible interactions. These issues, if not found before contracting, can be at the prime’s expense. Suggestions: Formally review requirements and possible interactions between payloads like magnetics, EMI, thermal, vibration and so forth to keep prime cost risks low. VERTICAL INTEGRATION HELPS FLEXIBILITY We believe that Surrey’s vertical integration has helped mitigate cost and schedule risks as we trade subsystems vs. a constantly changing manifest and launch uncertainties. Suggestions: Use a core bus at max capacity options to solicit as many payloads as possible and have options for key subsystems including lesser capability to save money if not used
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MISSION OPS Have a well-developed system for mission operations, data delivery and principal investgator payload control. Surrey had a system developed with TDS-1 that the US organization will duplicate. LEOP IS SERIAL When planning and executing LEOP keep in mind that payloads will most likely need to be commissioned serially. Suggestions: Collect and study commissioning needs early, synergize and combine where possible and plan on the extended overall commissioning. See below.
CONCLUSION SST-US’s first payload hosting mission of its OTB line has been and will continue to be a great learning experience for the company. Payload Hosting looks to be a good business that will address the market need for more cost effective ways to get more payloads in space. Priming a dedicated hosted payload mission requires bus and GN&C flexibility to a much higher degree than a typical dedicated satellite mission or free flyer. Contracting and Mission Ops will also demand more work and discipline to “Herd Cats” to get a closed business case and successful mission. ACKNOWLEDGEMENTS Portions of this research were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
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REFERENCES 1. 2. 3.
The Deep Space Atomic Clock Mission, 23rd International Symposium on Space Flight Dynamics, Pasaden, CA; Todd A. Ely, Timothy Koch, Da Kuang, Karen Lee, David Murphy, John Prestage, Robert Tjoelker, Jill Seubert Expected Performance of the Deep Space Atomic Clock Mission, AAS 14-254, AAS Space Flight Mechanics Meeting;Todd A. Ely, David Murphy, Jill Seubert, Julia Bell, Da Kuang Exploiting Hosted Payload Opportunities: Surrey’s Lessons Learned from OTB and other missions, IEEE Aerospace Conference 2014; Anita Bernie, John Paffett, Marissa Brummitt
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