An Internet-Based Global Differential GPS System, Initial Results

Ronald J. Muellerschoen, Willy I. Bertiger, Michael Lough, Dave Stowers
and Danan Dong

Jet Propulsion Laboratory, California Institute of Technology

# Biography

*Ronald Muellerschoen* received a B.S. degree in physics at Rensselaer
Polytechnic Institute and a M.S. degree in applied math at the
University of Southern California. He is currently a Member of the
Technical Staff in the Orbiter and Radio Metric Systems Group at the Jet
Propulsion Laboratory (JPL). His work at JPL has concentrated on the
development of efficient filtering/smoothing software for processing GPS
data and development of wide area differential systems.

*Willy Bertiger* received his Ph.D. in Mathematics from the University
of California, Berkeley, in 1976. In 1985, he began work at JPL as a
Member of the Technical Staff in the Orbiter and Radio Metric Earth
Systems Group. His work at JPL has been focused on the use of GPS,
including high precision orbit determination, positioning, geodesy,
remote sensing, and wide area differential systems.

*Michael Lough* obtained a Ph.D. in Applied Mathematics from the
California Institute of Technology in 1995. Since 1997, he has worked in
JPL\'s Orbiter and Radio Metric Systems Group. His current interests
include the development of techniques for improving GPS-based precise
orbit determination (POD) strategies for low-Earth orbiters as well as
the development of real-time tools for GPS analysis.

David A. Stowers obtained his B.S. in Electrical and Computer
Engineering from California State Polytechnic University, Pomona in
1985. Mr. Stowers is a member of the Satellite Geodesy and Geodynamics
Systems group at the Jet Propulsion Laboratory (JPL). The group\'s
responsibilities include the operation of NASA\'s stations in the Global
GPS Network. Recent activities include development of applications for
handling high-rate GPS ground station data.

*Danan Dong* received Ph.D degree in geophysics at the Massachusetts
Institute of Technology. He is currently a Member of the Technical Staff
in the Satellite Geodesy and Geodynamics Systems Group at the Jet
Propulsion Laboratory (JPL). His work at JPL is focused on the
development of global GPS network operation software, and the
application of GPS on tectonics.

# Abstract

Using a network of 15 global GPS receivers, GPS data is returning to JPL
via the open Internet to determine the orbits and clocks of the GPS
constellation in real-time. Corrections to the broadcast orbits and
clocks are currently communicated in real-time to the user over the open
Internet via a TCP server. Tests of user positioning show real-time RMS
accuracy of \~10 cms RMS in horizontal and \< 20 cms RMS in the
vertical.

# Introduction

The JPL architecture for a GPS global real-time differential system was
first put forward by Yunck et al. \[*1995, 1996*\]. A commercial North
American Wide Area Differential GPS (WADGPS) system based on the JPL
architecture and software was implemented in 1997 \[*Whitehead et al.
1998; Bertiger et al. 1998*\]. In 1996 the Federal Aviation
Administration (FAA) selected the JPL architecture and software for
their prototype Wide Area Augmentation System (WAAS). The core GPS
corrections are computed with the Real-Time Gipsy (RTG) software
licensed from JPL. WAAS as well as all other existing differential
systems are optimized for users with a single-frequency GPS receiver.
These receivers are susceptible to large, unmodeled ionospheric delays.
In order to compensate for this error source, these differential systems
employ a dense network of reference stations over their service area
(for example, WAAS uses 24 reference stations over the continental US),
and produces maps of the total electron content (TEC) which are
transmitted to the users. In contrast, many of NASA's projects are
global in nature and demand the highest possible accuracy. This system
addresses the issues of accuracy and global coverage.

To address the issue of global coverage, we took advantage of the NASA
Global GPS Network (GGN), which is operated and maintained by JPL. The
GGN consists of approximately 50 sites which are already operated in
batch mode over the Internet \[*http://igscb.jpl.nasa.gov*\]. To return
data in real-time a new software set, Real-Time Net Transfer (RTNT), was
developed. Data at a 1 Hz sample rate is currently returned with a
latency of \< 1.5 seconds using the open Internet. Better than 95% of
the data is returned in less than 3 seconds from a network of 15 global
receivers. We expect to expand the network to 30 real-time sites over
the next 6 months. This will increase accuracy and reliability through
redundancy.

The system has been optimized for dual-frequency users. Tests of user
positions currently show 10 cms RMS accuracy in the horizontal and 20
cms RMS accuracy in the vertical.

The global differential corrections are packaged into a 560 bit/sec
message, and can be made available on the open Internet via a TCP server
running at JPL.

# Enabling Software

## RTG (Real-Time GIPSY)

RTG provides real-time estimates of the dynamic GPS orbits, and
one-second GPS clocks \[*Bertiger et al. 1998\].* Orbit estimates are
needed less frequently than the clocks due to their slower varying
physical behavior. RTG contains many of the precise models contained in
JPL\'s GIPSY OASIS II (GOA II) software. GOA II has a long history in
precise orbit determination of GPS and other spacecraft equipped with
GPS receivers \[*Muellerschoen, 1994,1995*; *Bertiger,1994*;
*Gold,1994*; *Haines,1994*\], and in precise GPS geodetic applications
\[*Heflin,1994*; *Argus,1995*\]. Post-processing of global GPS data with
GOA II routinely yields better than 10 cms GPS orbits.

Additionally, RTG has been embedded in real-time user equipment for
flight on the X33 sub-orbital vehicle, and has flown on the NASA DC-8
SAR flights \[*Muellerschoen and Bertiger, 1999*\].

## RTNT (Real-Time Net Transfer)

RTNT returns 5 of the 6 GPS data types: CA range, P1 and P2 ranges, and
P2 phase, and either the P1 or CA phase, plus signal to noise ratios.
The data is edited, smoothed, phase breaks detected, and compressed down
to 21 bytes/GPS, with 17 bytes of overhead needed for time-tag, site id,
nav solution, sequence number, and status flags. A remote site tracking
10 GPS satellites would then transmit over the open Internet 227
bytes/sec to a central data daemon. Additionally, the broadcast
ephemeris is included in this transmission when new iode/iodc numbers
are observed.

For improved reliability, the central data daemon keeps track of the
sequence number of these packets from each remote site, and may request
up to 3 retransmissions of missed data epochs. The central data daemon
may have a twin data daemon running on another computer. The central
data daemon relays all of its incoming GPS data to its twin also via
socket communications. Should the twin no longer see any data flow, it
will send out a request to the entire global network to request
re-routing of the real-time data to itself. It would then serve as the
central data daemon until the primary daemon is brought back on-line. It
is also possible to chain these data daemons, in order to export the
real-time GPS data to any other computer on the open Internet, and even
merge streams from various data daemons or additional receivers.

The remote sites minimally have a dual-frequency GPS receiver, a PC
running linux operating system, and connectivity to the open Internet.
RTNT currently supports the data streams from Ashtech Z-12, Turbo-Rogue,
and AOA-ACT Benchmark GPS receivers. A particular data daemon running on
a PC at the remote site establishes communications with the receiver
through its serial port, and places the data in a revolving buffer of
shared memory. A second process that is independent of receiver type,
reads this shared memory and opens a socket connection to the central
data daemon. The data is checked and flagged for phase breaks, and then
sent out the socket. In addition, \"soc\" files are constructed which
contains the socket packets, and can be saved for later batch retrieval.
Conversion routines are available from \"soc\" files to standard rinex
files or turbo-binary format. Turbo-binary can then be converted to
rinex files with for example TEQC \[*Estley, Meertens 1999*\].

# System Overview

## Data Collection

Fig. 1, shows a map of the current real-time receiver network. The
coverage is clearly not very well distributed but is expected to improve
in the near future. Data returned includes dual-frequency phase
measurements with a resolution of 0.02 mm, and dual-frequency range
measurements with a resolution of 1 mm, the receivers solution for its
time, and broadcast ephemeris information. At the central processing
site, these data are collected by another RTNT process which monitors
the state of the whole system. This central data daemon sorts the data
according to timetag, rejects duplicate transmissions, and at a
specified drop dead time, outputs all the data at a common epoch into a
circular buffer of shared memory.

Figure 1.) Network of dual-frequency GPS receivers returning data to JPL
in real-time. Current network consists of 4 AOA-ACT Benchmarks, 2
Turbo-Rogues, and 9 Ashtech Z-12s.

## Data Latencies

Fig. 2, shows a plot of typical latencies from the turbo-rogue site in
the Philippines, and from the Ashtech site in Tidbinbilla, Australia.
The spikes in the Philippines graph are requests for retransmission of
missed data epochs. The 0.1 second scatter in the floor of these plots
is a consequence of an imposed sleep state at the remote sites so as to
minimize CPU usage on the PCs.

![](media/image1.png){width="3.6in" height="1.9319444444444445in"}

![](media/image2.pct){width="3.129861111111111in"
height="3.129861111111111in"}Figure 2.) Data latency is the difference
between the GPS timetag of the data and the central collecting
computer\'s system time.

## Data Reliability

Table 1, shows the percentage of 1-second data obtain from the
Philippines site for a 6 hour period. After 3 seconds, 97% of the data
has arrived at the central data daemon. The Philippines is one of the
more difficult sites since it may require at times 25 hops across
gateways in order for the data to reach JPL.

  -------------------- --------------------------------------------------
  X                    percent of data epochs received from the
                       Philippines in less the X seconds.

  2 second             93.4 %

  3 seconds            97.0 %

  4 seconds            97.7 %

  5 seconds            97.9 %

  6 seconds            98.0 %
  -------------------- --------------------------------------------------

Table 1.) Percentage of GPS data epochs successfully returned from the
Philippines site to JPL.

## Orbit Determination and Clock Estimation

RTG reads the shared memory output of the central data daemon process.
Orbit and troposphere estimates at the reference stations are computed
once per minute by RTG\'s \"slow\" process. These corrections are then
placed into another revolving buffer of shared memory so that they may
be read by the clock correction process known as \"fast\". Clock
corrections are computed every second by fast.

Figure 3.) RSS 3-D orbit errors for GPS22 for a 24 hour period with RTG
and a 15 station global network.

Fig. 3, shows GPS 22 orbit errors for a 24 hour period. Even when the
GPS s/c is poorly tracked as in the middle of the day, orbit errors are
still held in check due to the dynamic/predictable nature of the orbits.
The real-time orbit represented in Fig. 3 does not yet include dynamic
partials for solar scale factors and y-bias parameters. These partials
are in the RTG integrator and we are in the process of tuning the
stochastic attributes of these parameters. Also, the orbit integrator at
this time did not include the effects of solid earth tides, pole tides,
or ocean tides. These models have decimeter to centimeter level effects
on the orbits. These tide models have been coded and are currently
undergoing testing.

## Global Differential Correctors

![](media/image3.pct){width="3.05in" height="2.8875in"}Orbit and clock
estimates are differenced with the broadcast ephemeris and broadcast
clock. Corrections to these broadcast elements are packaged into a 560
bit message. Each message contains 4 orbit solutions, 4 meter level
clocks, and 32 cm level clock corrections. It takes 8 seconds then to
initialize all 32 prn orbit and clock estimates. The total clock
correction is the sum of meter level clock and the cm level clock
corrections. The resolution of the clock corrections is 1.5625 cms. This
is 4 times better than the resolution of the Satloc corrections and 8
times better than the resolution of the WAAS corrections. The orbit
correction resolution is 6.25 cms which is the same for Satloc and WAAS.
iode/iodc ephemeris changes are held until 2 minutes has past when new
iode/iodc\'s are observed. This gives the user sufficient time to
accumulate the newest ephemeris message.

# User Point Positioning Tests

To test the accuracy of these global differential GPS correctors, we
point position stationary GPS receivers at known locations. The same RTG
s/w used on the DC-8 flights \[*Muellerschoen, Bertiger 1999*\] is used
with the global differential GPS correctors obtained through a socket
connection from the TCP server.

Fig. 4, shows a plot of kinematic positioning of a static user at JPL.
The large variations at cold start indicates that the phase biases have
not been well determined due to multipath. After about 10 minutes, the
graphs settle down as mulitpath is averaged over time. In this
particular plot, after 30 minutes, there remains a -28 cm bias in the
East component. This is due to the GPS orbits at this point in time not
yet being well determined by RTG. As more stations are added to the
system and with the assistance of dynamic partials for solar scale and
y-bias parameters, these biases are expected to be reduced to below the
10 cm level.

Table 2, shows a later result of positioning a dual-frequecy receiver in
the state of Florida with the global real-time corrections. The 40 cm
RMS vertical error can be attributed to troposphere error correlating
with the vertical component. For slowly moving real-time users such as
trucks, ships, and farm equipment a troposphere parameter can be
estimated. Estimating a zenith troposphere delay as a slowly varying
stochastic parameter reduces the vertical to under 20 cms RMS. This is
indicated in Fig. 5.

![](media/image4.pct){width="3.3868055555555556in" height="3.5in"}

Figure 4.) 1-Hertz positioning of a static receiver at JPL with global
differential corrections.

+--------------------+----------------+----------------+---------------+
| #                  | # Mean         | # Sigma        | # RMS         |
+--------------------+----------------+----------------+---------------+
| # East             | # -0.04        | # 0.12         | # 0.13        |
+--------------------+----------------+----------------+---------------+
| # North            | # 0.00         | # 0.06         | # 0.06        |
+--------------------+----------------+----------------+---------------+
| # Vertical         | # 0.21         | # 0.31         | # 0.37        |
+--------------------+----------------+----------------+---------------+

Table 2.) Results of 10 hours of 1-Hertz positioning of static receiver
at Fort Pierce, Florida with global differential corrections. No zenith
troposphere parameter is estimated. Units are in meters.

# 

![](media/image5.pct){width="3.640277777777778in"
height="3.591666666666667in"}Figure 5.) Results of 15 hours of 1-Hertz
positioning of a static receiver at Fort Pierce, Florida with global
differential corrections. Estimating a zenith troposphere parameter
reduces the vertical error to under 20 cms RMS.

# Summary

The open Internet is a reliable choice to return GPS data for a
state-space dual-frequency global differential system. Initial results
suggest 10 cms horizontal accuracy and 20 cms vertical accuracy are
achievable. As more stations come on line and with better orbit
modeling, horizontal errors are expected to be further reduced.

# Acknowledgments 

The work described in this paper was carried out in part by the Jet
Propulsion Laboratory, California Institute of Technology, under
contract with the National Aeronautics and Space Administration.

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