Washington, October 11- 12, 2005
Positioning and Navigation On the Moon
Walking On the Moon Items to be addressed
Premises
• Positioning and Navigation services on the Moon: key issues for MOON BASE Program • Integration of localisation with other services (e.g.: TLC, MO): a “must” for Program sustainability Understanding of the problem
• “Indoor” and “outdoor” positioning: local / wide range, global. Viable solutions and respective domain and advantages Next steps
• Exploiting the partnership with Europe / Italy • Activity Plan to satisfy MOON BASE Program
2 Positioning and Navigation needs on the Moon
3 Positioning and Navigation needs
• Moon Base program perspective of permanent settlements on the Moon, both for scientific as well as industrial resources exploitation, implies:
– Large infrastructures set up, which requires precise positioning for surveying, – Navigation capability for automatic movement of robots inside infrastructures, which requires local, precise navigation, – Navigation capability to move among different settlements, which requires medium precision, wide range or global navigation, – Geographic information availability for Lunar resources exploitation which require medium or low precision, and global navigation. – Search and rescue services, which requires precise positioning. – Precise positioning of scientific instrumentation, which in some cases is required both in a global, selenocentric as well as a geocentric system
4 Drivers for Local and Global Positioning
• Surveying and Positioning for complex infrastructure set up • Indoor/outdoor navigation of people and automatic drive of unmanned rovers and crews • Lunar far side and polar regions operations • Need to improve personal safety in an adverse and risky environment, with limited or no support means and infrastructures • Lunar resources localisation and exploitation • Safe spacecraft flight and exploration • Definition/adoption of a Global Lunar Reference System, linked to the terrestrial one with known transforms • Science ⇒Summary of Needs
5 Integrating techniques to exploit “services”
6 Integration to exploit “combined services”
• A Lunar GNSS system is the final step of a process, which implies the use of different methods and instruments for positioning and navigation, locally and globally • As far as the Lunar GNSS constellation and facilities is concerned, the possibility of multi-service / multi-function constellations has to be explored, addressing the integration and cooperative work, on the same platform, of different payloads, covering functions that span from navigation, to communication and Lunar observation • Necessity to implement personal safety concepts (e.g.: global search and rescue), with a return channel available, is also felt as a crucial aspect and “service” • Instrument remote control and data return, up to Earth laboratories, is also to be addressed
7 Viable solutions for “Local” and “Global” Positioning
8 Where am I?
Localisation
Indoor Outdoor
Define user position in 3D, local tangent coordinates, with Local a restricted coverage Define user position in 3D, selenocentric coordinates, Global over the whole satellite
9 Local Positioning: the initial step
Local
Indoor Outdoor
ons Pseudolites i ut ol s
e t Optical Signal Generators da ndi Ca ⇒Pseudolites
10 Global Positioning (1)
• Global Positioning requires to define and adopt a 3D Global Reference Frame. • Various approach and Space Geodesy techniques can be used to realise the Global Reference System:
– Lunar Laser Ranging (LLR) technology, based upon use of retroreflectors placed on Moon surface establishing reference points – Very Long Baseline Interferometry to measure 3D baseline vectors among points on the moon and on the Earth; – Lunar positioning systems based on a GNSS constellation
11 Global Positioning (2)
• Using LLR and VLBI techniques, it will be possible to measure reference points on the cislunar face of the Moon; a Lunar Global Reference Frame can be defined, with respect to the Earth Reference System • To extend such a network of lunar reference points to the translunar face of the Moon implies to measure reference points on the Lunar surface directly, from the Moon itself • The Moon centre of mass is the focus of any Lunar satellite orbit. It is the origin of force model to be used for computing orbits. GNSS constellation can “locate” the Moon barycentre, allowing to place properly the origin of the Global reference Frame • Such a capability of “measuring the Moon from the Moon itself” is crucial: merging LLR and/or VLBI information together with raw navigation ranging data, obtained from the Lunar GNSS constellation already orbited, will be the key to achieve a precise definition of Lunar Global Frame and obtain a precise user positioning accuracy
12 System Concepts and Figures of Merit
⇒Principles of Positioning
⇒Dilution of Precision
⇒Constellation Value
13 Sample Lunar Constellation (1)
Constellation with 8 satellites on 2 orbital planes (4 satellites per plane), at a distance of about 3 Lunar radii
3 or 4 satellites are simultaneously visible only at Lunar polar regions.
At lower latitudes, only 1 or 2 satellite signals can be accessed at the same time Possible augmentations • More polar satellites ? • 2 Additional satellites at L4, L5 Lagrange points ? • Displacement of complementary ⇒Lagrange Points Lunar Pseudolites ?
14 Sample Lunar Constellation (2)
Initial constellation (*) with 12 satellites on 2 orbital planes (6 satellites per plane)
It guarantees four-signal coverage over most areas of interest
However, there are two “blind” spots, where visibility reduces down to 1 satellite.
• Augmentations with pseudolites are obviously possible
(*) College of Engineering, Utah State University, “AEGIS” System
15 Sample Lunar Constellation (3)
Final constellation (*) with 18 satellites on 3 orbital planes (6 satellites per plane). Two orbits are polar, one equatorial
It guarantees five-signal coverage over all surface, with zones were up to 11 satellites contemporary visible
• RAIM (Receiver Autonomous Integrity) techniques start to be applicable
(*) College of Engineering, Utah State University, “AEGIS” System
16 Bringing GNSS satellites to the Moon
• To reach polar Lunar orbit from sun-synchronous earth-orbit requires a plane change of 6.5°. • Equatorial Lunar orbit requires a plane change of 7.5° from GTO. • It is necessary to determine where the plane change for the Lunar mission should take place. • The plane change should take place while in Moon orbit rather than Earth orbit, due to a smaller spacecraft velocity • The transfer spacecraft use a Hohmann transfer to reach lunar orbit. Shortly before reaching the point of Lunar orbit insertion, the plane change will take place
17 Exploiting the partnership with Europe / Italy
18 Exploiting partnership (1) • Cooperation between the two sides of the Atlantic Ocean in the field of Space Geodesy is lasting more than twenty years.
• As a result of this Heritage, many geodetic fundamental stations accredited in the international community are based in Europe and Italy. In particular very few of these stations can support VLBI, SLR and GNSS Techniques; Matera Space Geodesy Centre of the Italian Space Agency can furthermore support Lunar Laser Ranging. • We believe that such a model of cooperation should be extended to the establishment of a GNSS system on the moon
19 Heritage and Background (3)
• Italy is active as well in implementation of satellite navigation systems since 1997, with the successful test of a minimal infrastructure –the Mediterranean test Bed– , built up and operated at Telespazio, Fucino Space Centre, to generate and uplink a SBAS (MOPS DO-229A compliant) complementary signal
• Today, Telespazio cooperates with ESA, providing RIMS data to ESTB CPF and having settled two EGNOS signal uplink stations at Fucino and Scanzano Space Centres
20 Road Map to satisfy the Moon Base Program
21 Development Perspectives
• The deployment of Lunar GNSS system is a cost effective approach to satisfy Moon Base Program Global Positioning needs. From a logical stand point it could be the last step of a road map where other local positioning techniques are applied. In facts the Global Positioning requirements would follow the progress of colonization and deployment of infrastructures • However, the possible synergies with communication and Lunar Surface Observation needs, could justify an earlier deployment of navigation payloads on board of a multi mission constellation. • The large experience gained by the international community needs to be adapted and transferred from the Earth to the Moon surface. All the steps to support such a transfer must be timely undertaken. • System studies first must be pursued to identify the potential configurations and then the needed technologies selected for pre- development
22 A Road Map for Future Studies • Feasibility studies has to be undertaken quite soon to address at least the following issues: – Analysis of the advantages and drawbacks of different technical solutions of positioning needs depending upon definition and refinement of Lunar Mission profile – Analysis of the feasibility of the deployment of multi-service/multi- function satellites constellation including Moon Observation, TLC and Navigation/Positioning capability. – Go more in depth with benefits, needs, requirements related to positioning and navigation and arising from identified sub-mission chimneys, e.g.: Search & Rescue, model of territory, autonomous guidance on Moon, radioastronomy from Moon, … – Analysis of the applicability of existing radio-navigation constellations on Earth for navigating as well on the Moon as some author is claiming. – Characterization and modelling of the propagation effects of navigation signal in the mission environment (e.g. indoor)
23 References
Cosimo LA ROCCA [email protected] +39 06 41799289
Galileo Industries S.p.A. Via G.V. Bona 85 Franco DI STADIO 00156 Roma (Italy) [email protected] +39 06 4079 3236
Marco FERMI Stefano LAGRASTA [email protected] [email protected] +39 06 4069 6500 +39 06 4079 6315
Galileian Plus Telespazio S.p.A. Via Tiburtina 755 Via Tiburtina 965 00159 Roma (Italy) 00156 Roma
24 Add-On Material
25 Navigation needs depending on application Application / Operation Area Navigation / Positioning needs
In Space Transportation Continuous Navigation availability
Advanced Telescopes and Observatories Navigation can be provided by combination of autonomous and linked methods Robotic Access to Planetary Surfaces Navigation can be provided by combination of autonomous and linked methods Human Planetary Landing Systems Navigation can be provided by combination of autonomous and linked methods Human Exploration Systems and Mobility Critical dependence on highly reliable, highly available navigation Autonomous Systems and Robotics Navigation provided by combination of autonomous and linked methods Transformational Spaceport/Range Navigation provided by combination of autonomous and linked methods Scientific Instruments and Sensors Precise Localisation might be needed or not, depending case by case
26 Pseudolites (Pseudo- Satellites) (1)
• Pseudolites (PLs) are ground-based transmitters broadcasting GPS- like / Galileo-like signals, to be demodulated by a GPS / Galileo receiver • PL transmitters can, in line of principle, complement or “fully replace” a constellation of satellites for radio-assisted navigation • The geometry of PL placement can be adjusted for an optimisation of user positioning accuracy • All PL instruments can be easily monitored, managed and also maintained, whilst this is not the case of the space vehicles (on-board redundancies adopted to handle hardware failures) PL equipment: Integrinautics IN200C-XL PL equipment: Antenna (from NASA; NSTL)
27 Pseudolites (Pseudo- Satellites) (2)
Practical use of Pseudolites has some drawbacks • ‘Near-far’ problem: when signal is originated by a constellation of navigation satellites, the average power received by the user has a narrow “dynamics”. When using PLs, a receiver can experiment a broad range of received power levels: when user comes to the “near limit”, the closest PL exerts a jamming on signals from the other generators; when at the “far limit”, the user receiver sees a PL signal power very close to background noise level • There is in principle the need to “transfer the clock offset” of each PL (w.r.t. a reference time scale), to the user receiver • The location coordinates of the pseudolites need to be precisely determined. This is a difficult task when placing the pseudolites on another planet, where no geodetic networks already exist
28 Pseudolites (Pseudo- Satellites) (3)
Viable solutions for use of pseudolites out of Earth • The PL equipment is not a “uniquely defined” hardware configuration: one speaks of Free Running PL, Synchrolites, PL Transceivers, etc. • Properly configuring output signal ( e.g.: using a pulsed emission, according to a “duty cycle” ), the near-far range has been significatively steered. One goes from hundred meters to tens of miles • Systems and related algorithms have been analysed, able to solve accurately for their coordinates and determine individual “clock offsets”, without the need of any pre-existing infrastructure
• For instance: Self-Calibrating Pseudolite Array ( SCPA ) considered for the case of Mars exploration, potentially fits Moon as well
29 Principles of Positioning
• A receiver should resolve 4 (four) unknowns in order to determine its position. Unknowns are: the X,Y,Z cartesian coordinates (or, equivalently: latitude, longitude, elevation w.r.t. a reference ellipsoid), and its “time error” ∆t • Four separate equations dependant on the unknowns { X, Y, Z, ∆t } or { lat, lon, alt, ∆t } can be derived from the signals from four satellites • At least one of the satellites “in view” must be in a different orbital plane, otherwise four equations will not be sufficient (inversion appears to be singular) • A receiver can partially resolve for its position using only three signals, e.g.: assuming alt ≅ 0 and solving for { lat, lon, ∆t } • Otherwise, it may access a fourth signal from other source (e.g.: pseudolite)
30 Dilution Of Precision (DOP) (1)
• At each calculation cycle of the receiver, measurement errors ε ε 1, .., m affecting read-outs from m navigation satellites are ε ε ε ε ∆ converted into errors x, y, z, ∆t respectively on X, Y, Z, t The relationship is linear, through a transform matrix M . ε ε • Assume 1, .., m to be gaussian noises and have the same variance σ2 , and denote variances of resulting inaccuracies σ 2 σ 2 σ 2 σ 2 as x , y , z , ∆t . One has: ε ε σ 2 x 1 x ε ε σ 2 y 2 y T 2 = M ⋅ = M ⋅M ⋅σ ε ⋅⋅⋅ σ 2 z z ε ε σ 2 ∆t m ∆t
31 Dilution Of Precision (DOP) (2)
• GDOP ( Geometric Dilution Of Precision ) is defined as: GDOP = trace of (M ⋅MT )
• TDOP ( Time Dilution Of Precision ) is defined as: TDOP = element (4,4) of (M ⋅MT ) = ×σ 2 = σ 2 Variance of timing error TDOP ∆t
• PDOP ( Position Dilution Of Precision ) is defined as: PDOP = GDOP − TDOP = ×σ 2 = (σ 2 +σ 2 +σ 2 ) Variance of positioning error PDOP x y z
32 Dilution Of Precision (DOP) (3)
Good configuration: Bad configuration: low “DOP” high “DOP” values values
33 Costellation Value (CV)
• It is clear that “DOP coefficients” ( Dilution Of Precision, that are: GDOP, PDOP, TDOP ) are amplification factors for measurement inaccuracies. The constellation design must be such, that they appear the lowest achievable
• Given a constellation, they exist minimum thresholds for DOP coefficients, occurring (at a given time instant) over the most favourable location(s) of planet
• One defines CV ( Constellation Value ) the percentage of planet surface, where a given DOP coefficient never
threspasses an assigned upper limit, DOPmax
34 Moon parameters of interest
Mean Distance from Earth 384,403 km (238,856 mi)
Diameter 3480 km (2160 mi)
Period of revolution 27.322 earth days
Eccentricity of orbit 0.055
Inclination of orbit 5°9´
Rotation period (sidereal day) 27.322 earth days
Period of phases 29.53 earth days
Mass (earth = 1) 0.012
Mean density (water = 1) 0.605
Gravitational parameter µ 4.90x103 km3/sec2
⇒Lagrange Points
35 Lagrange Points for Earth-Moon Couple
448 921 km from Earth
326 385 km from Earth
Mean orbit radius: 384 405 km from Earth
36