National Aeronautics and Space Administration

An Introduction to Global Navigation
Satellite Systems
Ben Ashman
Navigation and Mission Design Branch
Goddard Space Flight Center, Greenbelt, MD

T151-40 & T131-24 | Pathum Thani, Thailand | 6-11 January 2020

Introduction to Satellite Based
Navigation

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T151-40 & T131-24 | January 2020

Navigation•

Navigation is the process of determining position and direction•

Generalization of the problem: estimate unknown parameters based on related observations= parameter vector (e.g., Cartesian position and velocity, our “state”)= observation vector (i.e., set of measurements)= observation noise vector (i.e., measurement error)= relation between parameter set and observation set (i.e., measurement model)•

Given a parameter set, we seek an observation set, a relation between our parameters andobservations, and an estimator•, in order to form an estimate:

Elegant and effective solutions have been devised by humans and other species for millennia

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From left: day and night bird migration [20], astrolabe 1619 [21]T151-40 & T131-24 | January 2020

Navigation (continued)•

Relative navigation
Dead reckoning: monitor rate of travel and heading using a compass; prone to error, especially at sea
Landmark bearings: angles to two known landmarks will constrain position in two dimensions•

Absolute navigation: latitude and longitude (clocks vs. celestial)
Latitude: Measure the elevation of pole star above the horizon with a sextant or astrolabe
Longitude: Very good clock or celestial (sextant for the elevation of celestial bodies above the horizon,
accurate clock to determine the time of observations, almanac to find the predicted position of the body,magnetic compass to determine azimuth and maintain course continuity between celestial observations)

Latitude (left) and longitude (right) [1]
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Radionavigation•

Measurements: distances from known
transmitter locations via the measurementof radio frequency signal transit time
Solution to the estimation problem:
trilateration, the determination of absolute

400

or relative locations of points by300

measurement of distances using the200

geometry of circles, spheres, or triangles•

Ground based:
LORAN (1940s), Omega (1960s)•

Satellite-based:
Sputnik I (1957), Parus and Tskikada,Transit, MOSAIC, and SECOR (1960s)

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Y•

receiverbeacon 1
beacon 2
beacon 3

500

100
0
-100
-200
-200

0

200

400

X
Trilateration in 2 dimensions

T151-40 & T131-24 | January 2020

600

GNSS
•

Global Navigation Satellite Systems (GNSS): radionavigation perfected•FeaturesAccuracy: 3D accuracies of a few meters and down to millimeters for users withspecialized equipment and processing
Availability: signal availability anywhere on Earth with a clear view of the sky
Integrity: the assurance that expected performance will be realized
SPACE SEGMENTCONTROL SEGMENT

Monitor Station
Ground Antenna
MasterControlStationNational Aeronautics and Space Administration

USER SEGMENT

Components of Global Positioning System (GPS) [3]T151-40 & T131-24 | January 2020

GNSS (continued)•

Space segment
Constellation of satellites in near-circular, Medium Earth Orbits (~20,000 km) or
Geosynchronous Earth Orbits (~36,000 km), each satellite equipped with atomic clocks•

Control segment
Network of ground stations and antennas that perform monitoring of the constellation, checkfor anomalies, generate new orbit and clock predictions, build and send upload to spacecraft•

User segment
GNSS receivers—specialized radios that track GNSS signals and produce position andvelocity solutions, typically with low-cost clocks

GPS Control Segment
Greenland
AlaskaSchriever AFB
Colorado
Vandenberg AFB
California

New Hampshire
USNO Washington

United Kingdom

Cape Canaveral
FloridaSouth Korea

Bahrain

Hawaii

Guam

Ecuador

Ascension

Uruguay

Master Control Station
Ground Antenna
Air Force Monitor Station

GPS Space Segment [4]National Aeronautics and Space Administration

Kwajalein

Diego Garcia

South Africa

Australia

New
Zealand

Alternate Master Control Station
AFSCN Remote Tracking Station
NGA Monitor Station

GPS Receivers [6]

GPS Control Segment [5]Updated May 2017

T151-40 & T131-24 | January 2020

GNSS constellations•

GNSS is an umbrella term for satellite constellations that broadcast signals from space forradionavigation
Systems with global coverage: GPS (United States), Galileo (European Union), GLONASS(Russia), BeiDou (China)
Systems with regional coverage: NAVIC (India), QZSS (Japan)

NAVIC
GNSS constellations, augmentations, and regional constellations [7]National Aeronautics and Space Administration

T151-40 & T131-24 | January 2020

GPS modernization: space segment status

As of April 24, 2019, there were a total of 31 operational satellites in the GPS
constellation
https://www.gps.gov/systems/gps/space/#generations
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T151-40 & T131-24 | January 2020

GNSS Fundamentals

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GPS signal structure•

What is required of a radionavigation signal?
1. Propagation delay between transmitter and receiver can be measured
2. Transmitters can be distinguished, enabling geometric diversity
3. Modulation allowing the signal to propagate through space•

For any signal p(t) combined with Additive White Gaussian Noise (AWGN) n(t),correlation with a copy of p(t) maximizes the output signal to noise ratio (SNR) (i.e., optimal
estimator in the Maximum Likelihood sense), so p(t) is designed to have a correlation shape thatsatisfies signal requirements 1 and 2•

Delay estimation
Consider a known, continuous-time signal p(t) generated at the transmitter that arrives at thereceiver with some delay tau:
In order to estimate tau, a local replica of p(t) is formed at the receiver with test delay tau tilde.
The delay estimate, tau hat, is the test delay that maximizes the average (over TI) of theinner product:

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GPS signal structure: code•

Autocorrelation in terms of alignment error,:

The ideal autocorrelation function would be•

Multiple signals are required in order to form a position estimate, however. The trilateration
problem relies on geometric diversity. One means of distinguishing transmitters is to minimize
the cross correlation of signals from different transmitters:•

These auto- and cross-correlation properties could be achieved with infinitely long random
sequences of +1 and -1, known at the transmitter and receiver, and unique to each transmitter

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GPS signal structure: code•

This is accomplished using Pseudorandom Noise (PRN) codes
Must be deterministic and finite for practical implementation, but sufficiently long and noiselike to approximate the desired autocorrelation and cross-correlation properties•

GPS Coarse Acquisition Code (C/A code) solution: Gold codes (modulo-2 sum of two linear
feedback shift registers)
Periodic sequence of {+1,-1} pulses called chips, unique to each GPS satellite, length 1023
with period of 1 ms (i.e., fchip = 1.023 MHz)

Selection of GPS C/A code [8]National Aeronautics and Space Administration

T151-40 & T131-24 | January 2020

GPS signal structure: carrier•

Third navigation signal requirement: modulation allowing the signal to propagate through space•

Radio frequencies used for satellite navigation—must penetrate atmosphere•

Apparent frequency at the receiver is Doppler shifted due to the relative motion of the transmitterand receiver

where

and

EM spectrum [9]National Aeronautics and Space Administration

T151-40 & T131-24 | January 2020

GPS signal structure (continued)•

Finally, signal is also modulated with 12.5 minute navigation message, a 50 bps binary
sequence containing time tags, GPS satellite ephemerides (i.e., transmitter locations), etc.

•

Time domain signal:+1 GPS Data Message

20ms

d (t )
50bps

-1
+1

Repeating 1023 Chip “PRN Spreading Code” (20 per data message bit)p(t)1.023Mbps-1

CarrierwL1 =1.57542 GHz

+1

-1

+1

Composition of GPS signal in time domain [3]•Received L1 frequency signal from the i-th satellite

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GNSS receivers•

Receiver has three main tasks:
1. Acquisition: Determine which satellites are visible and estimate the propagation delay
and Doppler associated with each
2. Tracking: Refine the delay and Doppler estimates and track these features as theychange over time
3. Navigation: Use measurements from all visible signals to estimate the receiver’s positionand velocity•

GNSS observables (i.e., receiver outputs)
1. Pseudorange: propagation delay plus receiver clock bias (measured from the PRN code
to a fraction of a chip: ~meter level accuracy)
2. Doppler: measured frequency shift of the received carrier
3. Carrier phase: measured fractional and accumulated whole cycle phase of the carrier(measured to small fraction of 19 cm cycle: ~mm precision)
4. C/N0: carrier to noise spectral density estimate in dB-Hz

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T151-40 & T131-24 | January 2020

GNSS receivers: acquisition•

Acquisition seeks to determine whether a particular satellite is visible (via its unique PRN) andestimate its delay (modulo one code period, 1 ms) and Doppler•

Correlation of an incoming signal with a local replica, mismatched in frequency and delay, forms
what is known as an asymmetric ambiguity function:

Ambiguity function magnitude complete [3] (left) and zoomed in [11] (right)•

Delay and Doppler values are tried over a search space. Correlation magnitudes are comparedto the noise floor—if the carrier to noise spectral density exceeds a threshold, the signal isdetermined visible and the delay and Doppler at the correlation peak are used to seed tracking.

•

Pre-detection integration time, TI, is an important parameter in detecting weak signals.

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GNSS receivers: tracking•

Tracking seeks to refine the delay and Doppler estimates produced by acquisition
1. Input signal is correlated with a local replica
2. Correlation result is filtered to produce error terms that quantify the difference between the
input and local signal
3. A feedback process makes adjustments to the local signal replica according to the error
terms

•

In addition to converging on the input signal delay and Doppler parameters, the tracking of a
dynamic signal allows for measurements of changing signal features and more accurate
estimates of the signal to noise ratio•

Most receivers compute three correlations
per signal: Early, Prompt, and Late
Phase of prompt corr. gives error signal for
carrier tracking
Comparing size of Early and Late corr.
gives error signal for PRN code tracking•

Coupled feedback loops DLL and PLL
maintain lock on code and carrier signal
parameters

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Code and carrier tracking [3] from [11]T151-40 & T131-24 | January 2020

GNSS measurements: pseudorange•

Pseudorange measured from the i-th satellite (“pseudo” because of receiver clock bias):

time of reception
according toreceiver clock

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time oftransmission
according tosatellite clock

T151-40 & T131-24 | January 2020

GNSS measurements: pseudorange•

Pseudorange measured from the i-th satellite (“pseudo” because of receiver clock bias):

time of reception
according toreceiver clock•

time of transmission
according tosatellite clock

Transmission and receive times each expressed as a sum of the “true” time (i.e., the time
according to a common time standard, such as GPST) plus an unknown bias

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GNSS measurements: pseudorange•

Pseudorange measured from the i-th satellite (“pseudo” because of receiver clock bias):

true receiver
position•broadcast satellite
position

Propagation delay:ri is the geometric range between the i-th transmitting satellite and the receiver, |x – xit|
Qi is the satellite orbit error
IiL1 is the delay due to the ionosphere, a region of ionized gas in the upper atmosphere where
the time varying density of free electrons and ions introduces a dispersive (frequency
dependent) delay
Ti is the delay due to the troposphere, the lowest region of the atmosphere, a non-dispersive
medium consisting of dry gases and water vapor

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GNSS measurements: pseudorange (cont.)•

Propagation delay:

Acquisition and tracking measure code phase, i.e., ambiguous time of transmission modulo one code period(1 ms for GPS C/A code, or approximately 300 km):
The navigation message must be decoded to form a pseudorange•

Navigation message is organized into six
second subframes, each beginning with an
8-bit Telemetry word (TLM) and Hand-overword (HOW), the latter of which contains
the satellite time the subframe wastransmitted
svtime_of_transmission =svtime_of_subframe+bits_since_subframe_start*0.02+whole_cacodes_since_bit_start*0.001+whole_chips_into_current_cacode/1.023e6+frac_code_phase_chips/1.023e6;

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Calculation of pseudorange using four satellites [12]T151-40 & T131-24 | January 2020

GNSS measurements (cont.)•

Measured Doppler shift is a combination of the changing geometric line of sight and theunknown receiver clock drift. Time derivative of the pseudorange:

recall:•unknown receiverclock drift

transmitter clock
drift approximately
known

Carrier phaseCan be measured with much higher precision than code phase (i.e., pseudorange), ~cm for
GPS, but ambiguous on the order of carrier cycles, 19 cm for GPS. Combine with code
measurements or use for precise measurement of change (Accumulated Delta Range)
Ionosphere also induces a delay, but opposite in sign relative to pseudorange, leading to acode/carrier divergence

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Navigation solution: typical GPS error budget•

Error Source

Basic single frequency

Precise dual-freq, assisted

Ionosphere (< 1000 km)~3 m (single frequency,
using broadcast model)

Dual frequency<1 cm

Troposphere (< 20 km)

0.1-1 m

1 cm level using estimators,
advanced models

GPS orbits<2.0 m (broadcast ephem)

1 cm, Int. GNSS service (IGS)

GPS clocks<2.0 m (broadcast clock)1 cm (IGS)

Multipath (“clean”
environment)

0.5-1 m code

0.5-1 cm carrier

Receiver Noise0.25-0.5 m code

1-2 mm carrier

RSS range error

4m

2 cm

Typical GDOP2

2

RSS solution error

8m

4 cm

Disclaimer: for illustration purposes only

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T151-40 & T131-24 | January 2020

GNSS multipath•

Propagation delay:•

Multipath
Reflected signals are received as delayed, attenuated replicas of the direct signal
Correlation shape of the combined signal causes an error in the code tracking loop that
depends on geometry, number and strength of reflections, and tracking loop design
E.g., one signal, noncoherent DLL:

Direct, building-reflected, and ground-reflected signal paths in multipath interference scenario [19]National Aeronautics and Space Administration

Direct, reflected, and composite correlations [10]T151-40 & T131-24 | January 2020

GNSS multipath (cont.)•

Carrier phase (cont.)
Wall reflection

Ground reflection

Direct, building-reflected, and ground-reflected signal paths in multipath interference scenario [19]National Aeronautics and Space Administration

T151-40 & T131-24 | January 2020

GNSS time reference•

GNSS requires a common time scale forcomputing ranges•

GPS Time (GPST) is the operational timescale of GPS•

To keep satellites on GPST adequately,
atomic clocks are required
Corrections in the navigation message areused to synchronize satellites to GPSTFor example, to limit clock error to 1 m over
12 hrs requires drift < 8 x 10-14 s/s••

Tidal friction and other processes that cause a

GPST coarsely steered to align withsignificant redistribution of mass are slowing

Universal Consolidated Time (UTC) as

the Earth’s rotation, lengthening the solar day

maintained by the US Naval Observatory via

by ~2 ms / century

corrections in the navigation message•

Time systems [14]•UTC incorporates leap seconds to maintain

Traceability to UTC USNO enables precisealignment with sidereal time (UT1), but GPST

time and frequency transfer on a global

does not. This difference is a persistentscale

challenge for receiver designers and users.

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T151-40 & T131-24 | January 2020

Navigation solution•

Position estimation with pseudorange
Want to estimate receiver position and clock bias at some instant in time:

Given N > 4 pseudorange measurements (corrected for transmitter clock bias):
Standard approach is to solve as a non-linear least squares (NLLS) problem by GaussNewton method:

1. Linearize about initial guess
2. Solve linear least squares problem for3. Set4. Iterate

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Navigation solution: DOP•

In general, when solving the linear least squares problem•

The covariance of the least squares solution•

W (the inverse Gramian matrix) transforms measurement noise into solution noise•

In GPS, the i-th row of H is•Thus, W is determined by the geometry of the visible transmitters. Dilution of Precision (DOP):•

Examplesis

If transmitters are in a plane, H is rank deficient andIf transmitters are located at corners of a tetrahedron
National Aeronautics and Space Administration(minimum for N = 4)
T151-40 & T131-24 | January 2020

Improving performance•

Multi-frequency receivers
Eliminate ionosphere as an error source through “ionosphere-free combination”•

Carrier phase observables
Millimeter rather than meter level measurement noise and negligible multipath error•

Differential measurements: Receivers in close proximity can be used to cancel common error
sources (e.g., Differential GPS, DGPS)
Ionosphere, troposphere, satellite orbit/clock can be cancelled by differencing measurements or solutions
Solution is relative•

Precision GNSS orbits and clocks
Available from global networks of reference receivers (e.g., International GNSS Service, ICG) for postprocessing and in near real-time•Augmentations
Additional transmitters and measurements enhance geometry•

Filtering
Incorporate dynamic constraints or additional measurement sources (e.g., inertial sensors)•

A combination of these techniques enables cm to mm level solutions

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T151-40 & T131-24 | January 2020

GPS modernization: new civilian signals
L2C
L5
L1C

New civilian signal specs [24]National Aeronautics and Space Administration

T151-40 & T131-24 | January 2020

GPS modernization: new civilian signals
L2C
L5
L1C

New civilian signal specs [24]National Aeronautics and Space Administration

T151-40 & T131-24 | January 2020

GPS documentation• System technical docs available on www.gps.gov• GPS IS-200:

Spec. of legacy C/A & P codes and NAV message
Rev E and beyond adds L2C and CNAV
• GPS IS-800:

Specification of L5, and L5 CNAV• SPS & PPS Performance standards

Defines the guaranteed level of performance in terms of Signal in Space(SIS) accuracy and Constellation design
Current system performance surpasses minimum spec and is improving.

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T151-40 & T131-24 | January 2020

GNSS Applications

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T151-40 & T131-24 | January 2020

GNSS Science: Reflectometry•

Reflectometry material from Rashmi Shah(rashmi.shah@jpl.nasa.gov)•

Concept: Multistatic radar system• Transmitters are GNSS satellites (Lband suitable for all weather)• Receivers are dedicated platforms
equipped with specialized GNSS
receiver/processor and antenna system• Reflected signal is influenced by thesurface type, underlying medium, andatmosphere traversed (e.g., ionosphere)•

GPS TRANS

GNSS-R [27]

GPS REC
VEL

Scattered field is cross correlated with a code
replica• Coherency: The closer the shape to atriangle, the more coherent the scattering• Scatterometry: Sensitivity of shape toroughness etc.
• Altimetry: Delay value corresponding to
specular point enables measurement ofrange associated with reflection
geometryYX

ISODOPPLER CURVESPowerSpecular Point

Delay, t

Reflected signal integrated along the Doppler coordinate: not a symmetric triangle [27]National Aeronautics and Space Administration

T151-40 & T131-24 | January 2020

GNSS-R: Delay Doppler Maps
Tx : GPS satellite

Rxv

Coherent reflection:~0.5 km

Incoherent reflection:~25 kmNote: Colorbars differ by an order of magnitude

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GNSS-R: CYGNSS•Constellation of 8 nanosatellites to study
the inner core of tropical cyclones• Selected by NASA in EVM-1
competition• Launched in 2016
• 8 satellites at 500 km altitude, 35
deg inclination•

Objectives• Measure ocean surface wind speed
in all precipitating conditions,
including those experienced in thetropical cyclone eye wall
• Measure ocean surface wind speed
in the tropical cyclone inner corewith sufficient frequency to resolve
genesis and rapid intensification•CYGNSS reflectivity broadly agrees withSMAP soil moisture measurements
CYGNSS [27]National Aeronautics and Space Administration

T151-40 & T131-24 | January 2020

GNSS Signals in the Space Service Volume (SSV)•The Terrestrial Service Volume (TSV) is
defined as the volume of space including thesurface of the Earth and LEO, i.e., up to
3,000 km•

The Space Service Volume (SSV) is definedas the volume of space surrounding the Earthfrom the edge of LEO to GEO, i.e., 3,000 km
to 36,000 km altitude•

The SSV overlaps and extends beyond the
GNSS constellations, so use of signals in thisregion often requires signal reception fromsatellites on the opposite side of the Earth –main lobes and sidelobes•

Use of GNSS in the SSV is increasing
despite geometry, Earth occultation, andweak signal strength challenges

National Aeronautics and Space Administration

The Space Service Volume [25]T151-40 & T131-24 | January 2020

Benefits of Real-Time GNSS Navigation in the SSV
Benefits of GNSS use in the TSV and SSV:
• Supports fast trajectory maneuver recovery (from: 5-10 hours to: minutes)• Significantly improves real-time navigation performance (from: km-class to: meter-class)• GNSS timing reduces need for expensive on-board clocks (from: $100sK-$1M to: $15K–$50K)• Supports increased satellite autonomy, lowering mission operations costs (savings up to $500750K/year)• Enables new/enhanced capabilities and better performance for High Earth Orbit (HEO) andGeosynchronous Earth Orbit (GEO) missions, such as:

Earth Weather Prediction using
Advanced Weather Satellites

Space Weather Observations

Precise Relative Positioning

Launch Vehicle Upper Stages
and Beyond-GEO applications

Formation Flying, Space Situational
Awareness, Proximity Operations

Precise Position Knowledge
and Control at GEO

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High Altitude GNSS
Transition from experimentation to operational use:•

1990s: Early flight experiments demonstrated basic feasibility – Equator-S, Falcon Gold•180
2000: Reliable GPS orbit determination
demonstrated at GEO
a bent pipe
MMSemploying(Phase 2B)(NASA Navigator)
160
architecture and ground-based
receiver (Kronman 2000)•

Falcon Gold
2001: AMSAT OSCAR-40 mapped
GPS main and sidelobe signals (Davis et al. 2001)(NAVSYS TIDGIT)

altitude (1000km)

140
120

GOES-16(GD Viceroy 4)

EQUATOR-S(GD Viceroy)•

2015: MMS employed GPS100operationally at 76,000 km and recently 150,000 km•

180(NASA Navigator)(Trimble TANSat
Vector)2016: GOES-16
employed 80
GPS operationally
GEO
MMS (Phase
2B)
160
140

160

120

altitude (1000km)

180

altitude (1000km)

140

100
80

40

60

20

40

0

GIOVE-A(Surrey SGR-GEO)- Operational use of GPS

MMS (Phase 2B)(NASA Navigator)

Falcon Gold20(NAVSYS TIDGIT)- Experimental use of GPS

GOES-160
EQUATOR-S
1997
1999
2001
2003
2005
2007
2009 4)2011 2013 2015 2017 2019
(GD
Viceroy(GD Viceroy)
year of activity

MMS (Phase 1)
AMSAT-OSCAR 40(NASA Navigator)(Trimble TANS Vector) GOES-16(GD Viceroy 4)
GIOVE-A
MMS (Phase 1)(Surrey SGR-GEO)(NASA Navigator)

AMSAT-OSCAR 40(Trimble TANS Vector)

GIOVE-A(Surrey SGR-GEO)
1997

20

(NASA Navigator)

60
40

100 Gold
Falcon(NAVSYS TIDGIT)
80
EQUATOR-S(GD Viceroy)60

120

MMS (Phase 1)

AMSAT-OSCAR 40

1999

2001

2003

2005

2007 2009 2011
year of activity

2013

2015

0
1997

1999

2001

2003

National Aeronautics and Space Administration

2005

2007 2009 2011
year of activity

2013

2015

2017

Altitude[km]

Altitude[RE]GPS

20,2003

GEO

36,0005.6

MMS 1

76,00012

MMS 2

153,000

24

Moon

378,000

60

2017

2019

2019

T151-40 & T131-24 | January 2020

Recent Activities: GPS ACE
GPS Antenna Characterization Experiment (GPS ACE)
• GPS L1 C/A signals from GEO are available at a groundstation through a “bent-pipe” architecture• Goal: Map side lobes by inserting advanced, weak-signal
tracking GPS receivers at ground station to record
observations from GEO• Method: Trace path of GEO vehicle in antenna frame of each
GPS vehicle and reconstruct full gain pattern after monthsof tracking• In-flight averaged over all SVNs in block in 1 deg x 1 deg bins
• Remarkable similarity between average flight and groundmeasurements

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GPS Antenna Patterns from ACE [26]T151-40 & T131-24 | January 2020

Recent Activities: GPS ACE
GPS Antenna Characterization Experiment (GPS ACE)
• GPS ACE architecture permits tracking of extremely weak signals over long duration• Characterized full gain patterns from Blocks IIA, IIF for the first time
• Additional analysis of pseudorange deviations indicate usable measurements far into side lobes• Dataset available at: https://esc.gsfc.nasa.gov/navigation

IIR

IIF

IIA

GPS Antenna Patterns from ACE [26]National Aeronautics and Space Administration

T151-40 & T131-24 | January 2020

Recent Activities: MMS
Magnetospheric Multi-Scale (MMS)•

Launched March 12, 2015•

Four spacecraft form a tetrahedron near
apogee for performing magnetosphericscience measurements (space weather)•

Four spacecraft in highly eccentric orbits•

Phase 1: 1.2 x 12 Earth Radii (Re) Orbit(7,600 km x 76,000 km)•

Phase 2B: Extends apogee to 25 Re(~150,000 km) (40% of way to Moon)•

2019: Apogee raise to 1.2x29 RE

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MMS Navigator System
GPS enables onboard (autonomous)navigation and near autonomous stationkeeping• MMS Navigator system exceeds all
expectations• Two Guinness world records:
highest reception of signals and onboard
navigation solutions by an operational
GPS receiver in space•fastest operational GPS receiver in space,at velocities over 35,000 km/h

T151-40 & T131-24 | January 2020

Recent Activities: MMS•

Results from 8-day period early in Phase
2B shown here•

Sidelobes dominate signals tracked
above the GPS constellation•

Long term trend shows average of ~3
signals tracked near apogee, with up to
8 observed.

•

Visibility exceeds preflight expectations
significantly

C/N0 vs. time, near apogee

Signals tracked

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Recent Activities: GOES-R Series Weather Satellites•

GOES-R, -S, -T, -U: 4th generation
NOAA operational weather satellites•

GOES-R/GOES-16 Launch: 19 Nov 2016•

GOES-S/GOES-17 Launch: 1 March 2018•15 year life, series operational through mid-2030s•

Features new CONOPS over previous
generation:
Daily low-thrust station-keeping maneuvers,
rather than annual high-thrust events
Continuous data collection through maneuvers,<120 min of outage per year
Tighter navigation accuracy requirements andfaster cadence needed to support highly
increased operational tempo•

Employs on-board GPS at GEO to meet stringent
navigation requirements•

Utilizes GPS sidelobe signals to increase SSV
performance and ensure continuous availability

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GOES-16 Image of Hurricane Maria
Making Landfall over Puerto Rico

T151-40 & T131-24 | January 2020

Recent Activities: GOES-R Series Weather Satellites
GPS Visibility• Minimum SVs visible: 7• DOP: 5–15
• Major improvement overguaranteed performance
spec(4+ SVs visible 100% of time)
Navigation Performance• 3σ position difference
from smoothed groundsolution:• Radial: 14.1 m• In-track: 7.4 m• Cross-track: 5.1 m• Compare to requirement:(100, 75, 75) m

Source: Winkler, S., Ramsey, G., Frey, C.,
Chapel, J., Chu, D., Freesland, D.,
Krimchansky, A., and Concha, M., “GPSReceiver On-Orbit Performance for the
GOES-R Spacecraft,” ESA GNC 2017, 29
May-2 Jun 2017, Salzburg, Austria.
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Future Applications: Lunar Missions
2018 Global Exploration Roadmap reaffirms intention of 14 space agencies to go to theMoon in the next decade
Increased understanding of signal performance at high altitudes has informed GNSS
studies that suggest sufficient signals are available for navigation at the Moon. Recent
NASA publications:AAS GN&C 2019: “GPS Based Autonomous Navigation Study for the Lunar Gateway,”
Winternitz et al.
• AAS GN&C 2018: “Exploring the Limits of High Altitude GPS for Future Lunar Mission,”
Ashman et al.
• ION GNSS+ 2017: “New High-Altitude GPS Navigation Results from the MagnetosphericMultiscale Spacecraft and Simulations at Lunar Distances,” Winternitz et al.
•

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Future Applications: Lunar Missions
Ashman et al. 2018 lunar GPS study• Near Rectilinear Halo Orbit (NRHO) is one proposed
orbit for the Gateway; this is used here for the lunar
simulation with only the outbound cruise• Outbound lunar NRHO visibility with 22 dB-Hz acq/trk
threshold:Peak Antenna Gain

1+

4+

Maximum Outage

7 dB

63%

8%

140 min

10 dB

82%

17%

84 min

14 dB

99 %

65%

11 minA modest amount of additional antenna gain orenhanced GNSS receiver sensitivity increases coveragesignificantly
Winternitz et al. 2019 lunar GPS study•

Number of satellites visible over altitude fordifferent antenna gains

GPS signals tracked for one orbit period for one
case from the simulation

MMS-like GPS navigation system with an Earth pointed
high-gain antenna (~14dBi) would provide strong
onboard navigation for Gateway• Compared to using DSN: far fewer ground station
tracking measurements available and much larger
tracking gaps than with GPS tracking•

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References•

Luke Winternitz, “Introduction to GPS and other Global Navigation Satellite Systems,” in Proceedings of the
43rd Annual Time and Frequency Metrology Seminar, Boulder, CO, 14 June 2018.

•

James Garrison, AAE575: Introduction to Satellite Navigation and Positioning, Purdue University, Fall 2011.

•

Pratap Misra and Per Enge, Global Positioning System, 2nd ed. Lincoln, MA: Ganga-Jamuna Press, 2005.

•

Space Service Volume booklet published by the United Nations’ International Committee on GNSS:
http://www.unoosa.org/res/oosadoc/data/documents/2018/stspace/stspace75_0_html/st_space_75E.pdf

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Benjamin Ashman, Ph.D.
Navigation and Mission Design Branch
Code 595, NASA Goddard Space Flight Center
Greenbelt, MD 20771
benjamin.w.ashman@nasa.gov

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Image references
1.

M. Pratap and P. Enge, Global Positioning System, 2nd ed. Lincoln, MA: Ganga-Jamuna Press, 2005.

2.

https://mk0spaceflightnoa02a.kinstacdn.com/wp-content/uploads/2014/12/GPS_IIF.jpg

3.

Luke Winternitz, “Introduction to GPS and other Global Navigation Satellite Systems,” in Proceedings of the 43rd Annual Time and Frequency Metrology
Seminar, Boulder, CO, 14 June 2018.

4.
5.

https://www.gps.gov/multimedia/images/constellation.jpg
https://www.gps.gov/multimedia/images/GPS-control-segment-map.pdf

6.

http://www8.garmin.com/aboutGPS/

7.

http://www.elenageosystems.com/GNSS.aspx

8.

https://natronics.github.io/blag/2014/gps-viz-1/

9.

https://smd-prod.s3.amazonaws.com/science-blue/s3fs-public/thumbnails/image/EMS-Introduction.jpeg

10.

B. Ashman, “Incorporation of GNSS Multipath to Improve Autonomous Rendezvous, Docking, and Proximity Operations,” Ph.D. dissertation, Purdue
University, 2016.

11.

M. Moreau, “GPS Receiver Architecture for Autonomous Navigation in High Earth Orbits,” Ph.D. dissertation, University of Colorado, 2001.

12.

http://insidegnss.com/wp-content/uploads/2018/01/IGM_janfeb12-Solutions.pdf

13.

https://link.springer.com/chapter/10.1007/978-3-319-42928-1_6

14.

http://www.navipedia.net/index.php/Transformations_between_Time_Systems

15.

https://youtu.be/DbYapFLJsPA

16.

https://en.wikipedia.org/wiki/Orbital_elements#/media/File:Orbit1.svg

17.

B. Ashman, J. Parker, F. Bauer, M. Esswein, “Exploring the Limits of High Altitude GPS for Lunar Missions,” AAS GN&C Conference, Breckenridge, CO,
American Astronautical Society, February 2018.

18.

J. Miller and J. Parker, “NASA GNSS Activities,” International Committee on GNSS 12, Kyoto, Japan, December 2017.

19.

G. McGraw, P. Groves, and B. Ashman, "Robust Positioning in the Presence of Multipath and NLOS GNSS Signals," Chapter 21 in 21st Century PNT,Jade Morton editor, 2018.

20.

National Geographic March 2018

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Image references (cont.)
21.

https://timeandnavigation.si.edu/navigating-at-sea/navigating-without-a-clock/celestial-navigation

22.

Winkler, S., Ramsey, G., Frey, C., Chapel, J., Chu, D., Freesland, D., Krimchansky, A., and Concha, M., “GPS Receiver On-Orbit Performance for the
GOES-R Spacecraft,” ESA GNC 2017, 29 May-2 Jun 2017, Salzburg, Austria.

23.

By Persimplex - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=18957671

24.

https://www.gps.gov/systems/gps/modernization/civilsignals/

25.

Ashman B., Bauer F. H., Parker J., Donaldson J, “GPS Operations in High Earth Orbit: Recent Experiences and Future Opportunities,” 2018 SpaceOps
Conference, p. 2568, 2018.

26.

Donaldson J., Parker J., Moreau M., Highsmith D., Martzen P., “Characterization of On-Orbit GPS Transmit Antenna Patterns for Space Users,” 2018.

27.

Shah, Rashmi, “GNSS-R,” IEEE GNSS+R 201928.

Yang et al., ”Recent Advances in Remote Sensing of Natural Hazards-Induced Atmospheric and Ionospheric Perturbations”, 2015 AGU.

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GNSS science: radio occultation•

Radio occultation material from Oscar
Yang (oscar.yang@jpl.nasa.gov)•

Concept: Ground-based Total ElectronContent (TEC) measurements
represent all perturbations along theline of sight (LOS) through theionosphere

Ground-based measurement of ionospheric disturbance [28]National Aeronautics and Space Administration

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!
TEC(t) & ripp (t) @ each sampling time.

GPS Remote Sensing of TIDs
satellite

TEC =∫ N (l)dle

receiverIonosphere Pierce
Points (IPPs)

Ionospheric Disturbances

Electron
AltitudeLine of Sight (LOS)

F - region

Airglow
Atmospheric Waves

Tsunami Waves

GNSS Network
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Electron Density

Recent Activities: GRACE•

Gravity Recovery and Climate Experiment (GRACE) measured gravity anomalies to show how
Earth’s mass is distributed over time, enabling study of the planet’s ocean, geology, and climate•

NASA/JPL BlackJack GPS receiver modified to track gravity-sensing crosslinks, and to form
star-camera solutions, while producing cm-level POD and 0.1 nanosecond relative time transfer•

Data significantly improved understanding of: the global water cycle, mass and energy
exchange within and between the Earth System components, the changes in ocean mass, thechanging dynamics of polar ice caps and large continental aquifers and improved the prospects
for assimilation of mass change data into climate models

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Planned Activities: Restore-L and PACE
Restore-L• Launch date 2022, polar low Earth orbit• Autonomous, real-time navigation system used for
rendezvous• Autonomous grasping with telerobotic refueling andrelocation• Propellant transfer system will deliver fuel to the client
spacecraft at the right temperature, pressure, and rate

Plankton, Aerosol, Cloud, Ocean Ecosystem (PACE)• Assesses ocean health through the distribution ofphytoplankton and continues key measurements
related to air quality and climate• Launch readiness date: Fall 2022
• Sun-synchronous, polar orbit, 675 km orbital altitude• Global coverage every two days

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