The Flight Dynamics Facility (FDF) at the NASA Goddard Space Flight Center (GSFC) has provided operational spacecraft orbit support for many years, currently generating orbit products for about 20 satellites. To date, operational orbit determination in the FDF has been performed on the ground using data from ground-based or space-based tracking systems. Current development of spaceborne Global Positioning System (GPS) receivers is projected to have a significant effect on the support needed for operational satellite navigation. This paper identifies the functions performed in spacecraft navigation and examines and quantifies how the functions and support levels will be affected as onboard GPS receivers are implemented on spacecraft. Cases are considered spacecraft using or not using NASA ground and space networks resources

Hart, Roger C.

Hartman, Kathy R.

English

NASA Technical Reports Server

N95- 27794
Operational Considerations of Using GPS for Spacecraft Navlgauon
Roger C. Hart and Kathy R. Hartman
Goddard Space Flight Center (GSFC)
Greenbelt, MD, USA
Abstract
The Flight Dynamics Facility (FDF) at the NASA Goddard Space Flight Center (GSFC)
has provided operational spacecraft orbit support for many years, currently generating
orbit products for about 20 satellites. To date, operational orbit determination in the FDF
has been performed on the ground using data from ground-based or space-based
tracking systems. Current development of spacebome Global Positioning System (GPS)
receivers is projected to have a significant effect on the support needed for operational
satellite navigation. This paper identifies the functions performed in spacecraft
navigation and examines and quantifies how the functions and support levels will be
affected as onboard GPS receivers are implemented on spacecraft. Cases are
considered spacecraft using or not using NASA ground and space networks resources.
1.0 Introduction
Interest in use of the Global Positioning System (GPS) for spacecraft navigation has grown considerably in recent
years with the flight of several experiments and new spacecraft committing to the operational use of GPS. Often
cited as drivers for this movement are reductions in ground operations, including elimination of traditional
tracking, orbit determination, and state vector uploads to the spacecraft. Evidence is often anecdotal, focusing on
only one or two issues. This paper examines the functions performed in support of spacecraft navigation and
assesses the effect on ground systems.
1.t Flight Dynamics Functions
The Flight Dynamics Facility (FDF) at the NASA Goddard Space Flight Center (GSFC) provides orbit, attitude,
and TDRSS or station acquisition support for about 20 NASA and non-NASA spacecraft. FDF functions include
analysis for mission planning, launch support, and routine operational support. FDF receives tracking data and
telemetry and generates orbit, attitude, and acquisition products that are distributed to spacecraft control centers,
scientists, and tracking networks. Figure 1 illustrates key interfaces and functions of the FDF.
FDF involvement with a spacecraft continues from the conceptual phases through the end of mission life. Support
includes both analysis and operations. The following are considered as high-level navigation functions performed
in the FDF:
Mission design and orbit analysis
Orbit determination
Trajectory Control
Scheduling and planning aid generation
Acquisition data operations
Calibration and verification of onboard system
Onboard compute table generation
Metric tracking data evaluation
Anomaly resolution.
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https://ntrs.nasa.gov/search.jsp?R=19950021373 2020-06-16T07:52:34+00:00Z
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1.1.1 Mission design and orbit analysis
Given mission objectives, analysis is performed to determine an orbit that meets all requirements and constraints.
Selection of the mission trajectory influences sensor placement on the spacecraft as well as power, attitude, and
propulsion system design and requires close coordination between instrument developers, spacecraft designers and
flight dynamics engineers in the spacecraft design phase.
Maneuver strategies must be developed for missions requiring propulsion for altitude or ground track maintenance,
station keeping, rendezvous, reentry, etc. Elements of these studies may include trajectory optimization,
minimization of fuel usage, orbital decay/lifetime projection, and reentry targeting. These activities continue
through the mission in response to altered mission requirements or changes in spacecraft performance.
Other important considerations include error analysis to ensure that orbit accuracy requirements are met with the
given tracking inputs, and analysis to determine launch windows.
1.1.2 Orbit Determination
Spacecraft trajectories may be estimated from a variety of observational types--ranges, Doppler, angles, and
vectors are routinely used. As shown in Figure 1, FDF receives tracking data from NASA ground and space
networks as well as Department of Defense (DOD) sites and others. Tracking data is automatically captured, pre-
processed, and stored in the FDF tracking database for orbit determination by the Goddard Trajectory
Determination System (GTDS).
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Formost FDF-supported missions routine orbit determination is performed three day per week in a largely
automated process, including automated quality assurance. Orbit operations personnel are on call or provide
support during launches or critical orbit maneuvers.
1.1.3 Trajectory Control
Maneuver sequences are planned to adjust an orbit to meet mission requirements. The ideal maneuver in terms of
the mission orbit may conflict with communications, power, or other constraints, so the spacecraft operators are
closely involved with maneuver planning and give final approval of maneuver plans. Following a maneuver, the
achieved orbit is analyzed to determine actual performance of the spacecraft thrusters. Thruster calibration and
bookkeeping of fuel used are important in identifying thruster malfunction and planning subsequent maneuvers,
particularly for the first maneuvers following launch when the performance is not well known and may be
changing. Special post-maneuver orbit determination for rapid evaluation or thruster performance is performed
following critical maneuvers.
1.1.4 Scheduling and Planning Aids
Ephemerides from the definitive orbit determination process serve as the basis for generating predictive orbits and
scheduling and science planning aids. Users of NASA's space network, ground network, or Deep Space Network
are required to deliver projected orbits as far as two weeks in advance for network scheduling. Orbit-based products
are generated to meet the requirements for spacecraft operation and science instrument management. Events such
as eclipses or view periods and geometrical relations between the spacecraft and sun, for example, are computed
from the predicted orbit. Attitude and orbit information are often combined to generate products for antenna
pointing or scheduling viewing for instruments.
1.1.5 Acquisition Data Operation
Acquisition data are generated for the ground and space networks during launch and routine operations phases.
Acquisition data are used for antenna pointing to a spacecraft.
1.1.60nboard system calibration
This analysis is performed during launch and routine operations to ensure the integrity of the onboard navigation
algorithms. Onboard vectors returned in telemetry are compared to the ground-based orbit to validate
performance.
1.1.70nboard Computer Support
Spacecraft typically obtain orbit information onboard from polynomial fits to a predicted orbit or an orbit
propagator. Both cases require uploading information to the spacecraft onboard computer (OBC). In the first, a
table of coefficients is uploaded, and, in the second, a single state vector is used.
1.1.8 Metric Tracking Data Evaluation
This work is performed during launch and routine operations phases to evaluate the tracking network integrity in
support of the navigation, and to help resolve ground tracker equipment problems.
1.1.9 Anomaly Resolution
While no specific activities will be identified here, anomaly resolution is mentioned to acknowledge the need to
deal with unexpected events during all phases of support.
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1.2 Baseline support levels
Two missions were examined to provide representative levels of Flight Dynamics support. The Upper Atmosphere
Research Satellite (LIARS) and the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) were
selected because they bracket the range of products produced by FDF in number and type. UARS performs
periodic restoration of its frozen orbit, is supported by Tracking and Data Relay Satellite System (TDRSS), and
requires many planning products for its multiple instrmnents. SAMPEX is tracked from the ground, has no
propulsion, and has few product deliveries. Table 1 shows Flight Dynamics support staffing for the two spacecraft
in the defined classifications.
Navigation Function
1. Mission design an_d_o_rb_it_mm.l__sis
2. Orbit Determination
_3_.T_ra__ecLt_o_r_ Control
_4_.sc_h_ed_  _and_ Odu
__ Acquisition data operations
6. Calibration and verification of onboard
_s2/stem
7. Onboard computer table generation
8. Metric trackin 8 data evaluation
Total
UARS SAMPEX
0.2 0.2
0.2
0.3
0.8
0.1
,-O
0.1
0.2
1.9
0.2
0
0.3
0.1
--0
0.1
0.2
1.1
Table 1. Recurring operations support levels per year of mission support (man-years)
2.0 Effects of GPS on FDF support
At this time, the first spacecraft are implementing GPS receivers for operational use. Without long-term
operational history, a level of performance was assumed for this study. GPS receivers have been proposed, ranging
from simple data collection devices to units capable of full orbit determination and trajectory control. The receiver
assumed here produces output suitable for use by onboard control systems and of sufficient accuracy to replace the
definitive ephemeris.
2.1 Flight Dynamics Functions
The following sections assess the impact of a spacecraft using GPS on functions performed within the FDF.
2.1.1 Mission design and orbit analysis
Little changes in this area for spacecraft navigating using GPS. Mission design presents unique requirements for
each spacecraft. For many missions in typical low, circular orbits for which post-processing is not required, error
analysis can be eliminated assuming that the receiver has been proven to meet its specifications, and the
specifications satisfy mission requirements.
2.1.2 Orbit determination
GPS has the potential to eliminate the need for routine definitive orbit determination for most missions. The
onboard real-time and definitive requirements are met by the GPS standard positioning service levels of
performance of 100m horizontal and 156m vertical at 95% probability. For missions with tighter requirements,
post-processing will still be required. While GPS positioning is adequate for most missions, the velocity accuracy
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stated for many receivers is insuffident for ephemeris prediction and generation of long-term planning products. It
is recommended that a navigation filter be included in the GPS receiver to improve the velocity solution over point
solutions.
While orbit determination may be eliminated, operational savings are expected to be small given the high degree of
automation in the ground support process. Orbit determination is one step in the product generation process, and
its elimination amounts largely to a small reduction in CPU utilization. For facilities without orbit determination
capability, this represents a greater savings in providing independence from network requirements, although orbit
determination software is becoming more commonly available with commercial mission analysis software.
2.1.3 Trajectory Control
With the assumed autonomous navigation receiver, maneuver planning is still done on the ground. Again, little
changes in this function except for the source of the input ephemeris. Once a labor-intensive activity, maneuver
planning is recently becoming an automated process within the FDF.
The need for thruster calibration and fuel bookkeeping are not affected by GPS. Thruster calibration will require
new techniques because post-maneuver orbit determination is necessary to evaluate thruster performance during a
maneuver. The strength of GPS in trajectory control is that post-maneuver recovery of position knowledge is
nearly instantaneous, so table uploads for pest-maneuver conditions are not needed. If confidence in the
propulsion system and GPS receiver are high real-time support for maneuvers may be reduced or eliminated.
The real-time nature of GPS orbit determination holds promise for completely autonomous navigation, including
orbit maintenance and stationkeeping. Early development autonomous orbit control is under way.
2.1.4 Scheduling and planning Aids
Requirements for predictive products are not changed by the incorporation of GPS. Input to the process is switched
from the definitive ephemeris to a GPS-derived state. It should be noted that GPS standard positioning service
(SPS) performance for unfiltered, point solutions of velocity is less accurate than the traditionally determined
ephemeris, and the effect on predictions should be considered against mission requirements. As a result of more
capable spacecralt computers and instruments, more orbital event and pointing functions are being performed
onboard, reducing ground support requirements.
2.1.5 Acquisition Data Operations
With the current NASA networks, GPS has minor impact on acquisition data operations. Again, the source of the
input state vectors changes, but the generation and delivery do not. Changes to network operations designed to
simplify scheduling and take advantage of GPS-derived states are under consideration.
2.1.60nboard System Calibration
The nature of the onboard system calibration function will change for GPS users, but the need to perform periodic
check on onboard navigation performance remains. This function would migrate from the FDF to the mission
operations centers.
2.1.70nboard computer Support
GPS can potentially eliminate some of the current table or vector uploads. The savings here will be seen in
reduced system complexity, not in operations costs; with recent spacecraft, vector uploads amount to a mouse click
to select from a list of available vectors. The GPS receiver represents a new spacecraft system, that requires
management by the flight operators, and depending on the receiver design, the number of uploadable parameters
may actually increase to accommodate initialization, modes of operation, and tuning parameters, although the
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uploads are exp_ed to be less frequent than current practice. Orbit propagation may still be necessary if the
spacecraft cannot power the receiver continuously or as a backup capability.
2.1.8 Metric tracking data evaluation
The current function is eliminated for a GPS user spacecraft. The equivalent function would be autonomous
integrity monitoring within the receiver. Depending on the level of acceptable risk, the operations center may
require external information on GPS integrity.
2.2 Support Levels
Table 2 gives an update of the mission support levels from Table 1 to reflect the effects of using onboard GPS
navigation with no definitive post-processing. In addition support is estimated for a mission using its own ground
station and having no propulsion.
Navigation Function
1. Mission design and orbit analysis
2. Orbit Determination
3_ Traj_ec_to_ry..Control
4. Scheduling_and_p.l_ _a_nn_i_"n_ roducts
_5_ A_cq__sition data operations
6. Calibration and verification of onboard system
7. Onboard computer table generation
8. Metric tracking data evaluation
Total
0.2
0
0.3
0.3
0.1
.1
-4)
--0
1.5
UARS SAMPEX
0.2
0
0
0.3
0.1
0.1
independent
0.2
0
0
0.3
0
0.1
-4)
--0 --O
0.7 0.6
Table 2. Recurring operations support levels per year of mission support (man-years)
3.0 Other considerations
This survey has focused on functions performed within the FDF, but related functions are performed within the
mission operations centers. Figure 1 shows a clear separation between FDF and the mission operations centers.
However, as the FDF transitions from mainframe to distributed systems, the separation is becoming less distinct,
and generation of scheduling and planning aids is becoming more common within the mission operations centers,
saving some overhead and special support.
Spacecralt clock maintenance has been performed by flight operations teams using data provided by the FDF.
Here, too, the process has become automated, so minimal savings will be realized in terms of operator workload by
a GPS-user spacecrafl. The compelling argument is more in the simplification of design for spacecraft and ground
support systems realized by using GPS for timing.
Also neglected have been the FDF attitude determination functions for spacecraft using GPS as an attitude sensor.
GPS attitude determination for spacecraft is in the experimental stages, so impacts on ground attitude support are
not yet clear. Some missions may realize substantial reductions in hardware costs with GPS attitude
determination.
4.0 Conclusions
In reviewing the functions performed in the FDF it is seen that support levels for a spacecraft using NASA network
resources for communication are moderately reduced, primarily in the areas of routine orbit determination and
tracking data evaluation. The trend in automation of ground-based orbit determination will reduce the degree of
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savings. Generating products and planning maneuvers are larger efforts than orbit determination, and greater
operations cost savings can be achieved by including the functions onboard the spacecraft or by automating the
processes on the ground. Automation minimizes workload, but it does not reduce support system complexity. GPS
offers the possibility of eliminating some functions and simplifying the support system.
Less capable ground stations, at university, for example, will realize the full benefit of GPS navigation, being
independent of the constraints and costs of network utilization for tracking and communications scheduling.
Autonomous orbit control opens the potential for automatic orbit maintenance and relative navigation, including
autonomous rendezvous and formation flying.
Current methods meet the requirements of presently supported spacecraft and can meet the requirements of all but
a few future missions. Substantial reductions in operations support can be realized with continued automation on
the ground and improved spacecraft flight software; the more compelling arguments for GPS spacecraft navigation
are, then, not in elimination of functions but in an expansion of the realm of mission possibilities.
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Operational considerations of using GPS for spacecraft navigation

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