Lunette: Satellite to Satellite Gravity Mapping of the Moon

Transcription

1 Lunette: Satellite to Satellite Gravity Mapping of the Moon Maria Short 9th ILEWG International Conference on Exploration and Utilisation n of the Moon Authors: M. Short, C. Short, A. Philip, J. Gryzmisch, R. Zee, H. Spencer, and J. Arkani-Hamed

2 Space Flight Laboratory Who are We? Unique university lab in Canada focusing on microspace systems research and development Microspace = disciplined small team approach to using the latest commercial technologies in space Developed key subsystems for MOST and supported integration, test and operations Canadian Advanced Nanospace experiment (CanX( CanX) nanosatellite program to train students and provide for low cost space access Technology research in communications, propulsion, radiation testing Full-time professional staff with microspace systems expertise Facilities to support the development and qualification of space systems sfl.net Lunette: Satellite to Satellite Gravity Mapping of the Moon 2

3 Introduction Farside gravity map of the Moon is not as good as the nearside map; Gravity map is important for navigation and exploration Lunette is a mission concept involving a nanosatellite in formation with a parent satellite around the Moon. Whole sphere maps are possible e with radio range-rate rate measurements between the satellites UTIAS/SFL has developed the Generic Nanosatellite Bus (GNB) in support of BRITE Constellation (CanX-3) and the CanX-4/CanX 4/CanX-55 formation flying mission UTIAS/SFL has customizable separation systems, XPODs, that can be used to eject GNB satellites from launch vehicles or parent satellites GNB and XPOD technology can be used to support the Lunette mission. Lunette is a portable nanosatellite mission Lunette: Satellite to Satellite Gravity Mapping of the Moon 3

4 Mission Concept Science mission To map Lunar farside gravity field, to mgal gravity surface anomaly Free-flying nanosatellite, ejected from and flying in formation with a parent satellite, both in low Lunar orbit, measuring relative range rate using radio tracking Science instrument Ranging radio transponder Bus needs 3-axis 3 attitude control and propulsion Phase A Study for European Student Moon Orbiter (ESMO) under ESA s SSETI Program. Lunette: Satellite to Satellite Gravity Mapping of the Moon 4

5 Analysis Method The range rate signal is approximately proportional to selenopotential difference along the track of the two satellites The proportionality constant is the orbital velocity. U 12 Δv v orbit Where U 12 is the potential difference between the locations of the two satellites, Δv is the measured range-rate rate and v is the orbital velocity Lunette: Satellite to Satellite Gravity Mapping of the Moon 5

6 Local Sensitivity Model Range Rate Accuracy at τ = 10s (mm/s) Distance from Leading Satellite to Mascon (km) Lunette: Satellite to Satellite Gravity Mapping of the Moon 6

7 Tracking Approach Range Rate Measurements The range-rate rate tracking algorithm performs accurate measurements of the relative speed of Lunette with respect to ESMO Range-rate is derived by measuring the Doppler shift caused by relative motion on a carrier transmitted by ESMO and in turn sent back by Lunette. Lunette: Satellite to Satellite Gravity Mapping of the Moon 7

8 Tracking Approach Range Rate Measurements f1δv Δv f D = 2 + Mc c Lunette: Satellite to Satellite Gravity Mapping of the Moon 8

9 Goal of communications design Perform high precision range-rate rate measurements Perform low precision range measurements Low-speed two-way way data link between Lunette and ESMO Communications Lunette: Satellite to Satellite Gravity Mapping of the Moon 9

10 Propulsion Perilune Altitude Evolution Propulsion required to maintain circular orbit and perform 1 1 plane change maneuver Estimate about one thrust per week is required to maintain orbit Plane change maneuver is about half the available V Using four thrusters propulsion system is beneficial for momentum dumping of the three axis attitude control system Perilune Attitude (km) Time (days) Lunette: Satellite to Satellite Gravity Mapping of the Moon 10

11 Attitude Determination and Control System Attitude goals during science measurements: Null rotation rates to prevent data corruption Coarse Pointing of antennas Achieved with: Rate Sensors Sun Sensors Occasional Star tracker usage Reaction Wheels Expected effect of ADCS errors on range rate measurement: 0.038mm/s Lunette: Satellite to Satellite Gravity Mapping of the Moon 11

12 On-Board Computers The OBC baseline will be identical for Analyzer and Lunette Simpler integration and workload Centralized Computer Architecture Uses three computers Two on Lunette sub-satellite satellite One on Analyzer Must be able to store data for up to 15 hours Only transfer data to ESMO over poles twice a day Lunette: Satellite to Satellite Gravity Mapping of the Moon 12

13 Power System Power Generation with solar cells 27.5% efficiency Power Storage with Li-ion ion batteries 20 Wh energy capacity Direct Energy transfer Always within 5% of peak power at operating point for mission life To increase power generation, satellite will be edge pointing to the sun Battery Depth of Discharge always kept above 50% Lunette: Satellite to Satellite Gravity Mapping of the Moon 13

14 Structural and Thermal Structural 25x25x25 cm 3 Layout dictated by orthogonal antenna/star-tracker tracker placement and large fuel tank Thermal Cannot survive prolonged lunar eclipse (limits life to ~= 6months) Must survive both a dawn-dusk orbit and a noon-midnight orbit due to lack of precession and 4 week lunar day Lunette: Satellite to Satellite Gravity Mapping of the Moon 14

15 Separation System Need to protect Lunette from radiation dictated design Radiation shielding on board the sub-satellite satellite would increase weight precluding orbital maintenance maneuvers, therefore satellite is to be encapsulated within ESMO Separation system design based on SFL s XPOD deployment system Lunette: Satellite to Satellite Gravity Mapping of the Moon 15

16 Conclusion 20mGal accuracy is feasible with microsatellite/nanosatellite technology ESMO and Lunette is an ideal pairing of student projects Completed a Phase A study for ESMO. Subsequent phases subject to funding and approval by ESA and CSA The challenges in measuring the lunar gravity field are not primarily overcoming technical hurdles, but instead simply the opportunity to achieve this goal Lunette: Satellite to Satellite Gravity Mapping of the Moon 16