The following paper was presented at the ION GPS-96
conference, session F1.
For more information see Geomatics Canada.
Mark Caissy, Pierre Heroux, Francois Lahaye and Ken MacLeod are Research
Officers in the Active Control System Section, Geodetic Survey Division of
Dr. Josef Popelar is the Chief of the Active Control System Section,
Geodetic Survey Division of Geomatics Canada.
John Blore is the Technical Program Manager for Systems Integration with
Hewlett-Packard (Canada) Ltd. in Ottawa, Ontario.
Dean Decker is a consultant with Hewlett-Packard (Canada) Ltd. in Ottawa,
Ray Fong is the president and owner of tesserNet Systems Inc. of Calgary,
The real-time GPS correction (GPS.C) service is based on the Canadian
Active Control System (CACS) network of Real-Time Active Control Points
(RTACP) and a Real-Time Master Active Control Station (RTMACS). UNIX
computer servers connected via land and satellite communication facilities
are used for real-time GPS data acquisition and wide area GPS correction
computation. The GPS.C corrections are verified at Integrity Monitoring
Stations (IMS) and distributed in real-time via Virtual Active Control
Points (VACP). The Real Time Application Platform (RTAP) software
facilitates distributed process control, data communication, processing
The Canadian Active Control System (CACS) was established to improve the
accuracy and efficiency of GPS positioning and to provide a direct access
to the Canadian Spatial Reference System (CSRS). Precise GPS satellite
ephemerides, clock corrections, and earth orientation parameters have been
produced on a daily basis since 1992 with current precision of about 10
cm, 1 ns and 0.2 mas respectively. These CACS products facilitate geodetic
positioning at the 1 cm level and single point positioning at the 1 metre
level in post-processing with several days delay.
The Geodetic Survey Division (GSD) of Geomatics Canada, has developed a
prototype real-time GPS Correction Service (GPS.C) based on the CACS
stations shown in Figure 1. The system uses Hewlett-Packard UNIX servers,
frame relay data communications and the Real-Time Application Platform
(RTAP) enabling technology; add-on products and customized automation
solutions have been provided by tesserNet Systems Inc. Operational tests
have confirmed the viability of the system architecture, the selected
platforms, data communication infrastructure and the application software
The real-time CACS is designed to facilitate continuous real-time
positioning and navigation over Canada and adjacent areas. To meet these
stringent requirements, the system incorporates state of the art hardware
and communications technology. The system is also scaleable as far as data
processing and area coverage are concerned. The technology is described in
the sections which follow.
2. PHYSICAL SYSTEM OVERVIEW
The physical system serves the following primary functions:
Detail descriptions of the components are provided below.
An RTACP consists of an HP E35 UNIX server and console, an RM-12
TurboRogue GPS receiver, an external frequency reference (rubidium, cesium
or hydrogen maser), a meteorological station, a Cisco 2400 communications
router, an uninterruptable power supply (UPS) and a power manager as
illustrated in Figure 2. The RTACP is used to acquire, validate, store and
forward data. In addition to the primary tasks, the RTACP also smoothes
and decimates the 1 second GPS data to 30 second data points for post
processing applications and applies corrective action in the event of
Exception events are logged and reported to the RTMACS.
The RTMACS consists of two HP E55 UNIX servers each with its own console,
X-terminals, Cisco 2500 series routers, Local Area Network (LAN)
interfaces and bridges, a redundant array of inexpensive disks (RAID) and
a UPS in a high availability configuration as illustrated in Figure 3. The
RTMACS is used to acquire GPS and meteorological data from all RTACPs.
This data is verified and processed using predicted GPS orbits, resulting
in GPS.C corrections including satellite clock corrections and an
ionospheric vertical delay grid. These corrections, together with updates
to broadcast and predicted ephemeris, are then available to all VACP and
IMS stations within the wide area network via the multicasting TCP/IP
service. The RTMACS also facilitates computer network management including
the reception and transmission of diagnostic messages using Simple Network
Management Protocol (SNMP).
The high availability configuration eliminates single points of failure.
The CPUs are configured with HP M/C Service Guard software to switch over
in the event of a failure of the primary CPU MACSA. A hot swap RAID disk
is used for data storage to prevent loss of data and facilitate faulty
disk replacement without operation interruption. Two routers and two LAN
interfaces are available to provide redundant access to both the LAN and
the WAN frame relay communications. In the event of a power failure a UPS
maintains all critical components before diesel generator power is
A VACP consists of an HP E35 UNIX server and console, a meteorological
station, a Cisco 2400 communications router, an uninterruptable power
supply (UPS) and a power manager. The VACP receives the GPS.C correction
and ephemeris multicast from the RTMACS. Figure 4 illustrates the VACP
configuration which is used as the primary distribution point of GPS.C
corrections for service providers and users. The GPS.C corrections can be
localized and update rates customized for multiple user communication
interfaces (internet, datapac, modems, etc.). The VACP logs activity
exceptions and reports these to the RTMACS.
An RTACP and IMS are similar in their physical design (Figure 2) and in
addition to the functionality of an RTACP and a VACP, has the capability
to monitor on a continuous basis the differences between the observed
local GPS corrections and the GPS.C derived corrections. GPS observations
from IMS stations are not included in the GPS.C processing to provide
independent real-time quality control of the integrity of the GPS.C
service which is reported to the RTMACS. IMS stations also provide standby
RTACP capability since converting an IMS into an RTACP is a matter of
changing configuration flags within the RTAP database.
In the event of an RTACP failure, the nearest IMS can be turned into an
RTACP thereby minimizing the impact of such failures on the GPS.C service.
3. CACS COMPUTER NETWORK
Frame relay is used as the communications media for both data collection
and GPS.C correction distribution. Frame relay has become the standard
telecommunications service for Wide-Area Network (WAN) connectivity by
maximizing bandwidth and minimizing costs. Optional data multicasting is
used at the RTMACS for data distribution to keep bandwidth constant
regardless of the number of receiving stations. The committed information
rate (CIR) and data prioritizing features provide for optimization of WAN
links in the CACS network. Figure 5 illustrates the real-time network
4. REAL-TIME CACS SYSTEM INTEGRATION
The end to end data processing within the CACS network is based on the
Real-Time Application Platform (RTAP). Data collection, processing and
distribution is accomplished using this industrial process control tool
which incorporates the following features:
5. REAL-TIME CACS PERFORMANCE
Tests have been conducted on the Real-Time CACS prototype to gather critical performance information. The results confirm the viability of the system architecture, the selected platforms, data communication infrastructure and the application software technologies.
Each of the physical system components RTACP/IMS, RTMACS and VACP have
been tested to ensure that all primary and secondary tasks function as
designed and that the selected server platforms have sufficient resources
to accommodate future growth.
Tests carried out using 1 second GPS data from five RTACPs have shown
total processing and data communication delays between two to three
seconds for the Canada wide network.