GPS Receivers for Navigation and Timing Applications

GPS Receivers
Navigation and Timing Applications
S. Purushotham, J.K. Ray
Accord Software and Systems Private Limited
Bangalore, INDIA
GPS receivers for navigation and timing applications
are being developed and used extensively in India by
various governmental and private organizations/
industries. M/s Accord software has done a pioneering
in this direction. The present paper deals with the
development carried out M/s Accord software.
Accord – an ISO9000-2000 certified company- is
primarily involved in the development of hardware and
software for several of Aerospace applications and real
time embedded systems.
Accord has created an array of GPS-related products in
the areas of Navigation and Timing. Based on the
research carried out over the past decade, Accord has
developed indigenous technologies to address the need
of the industry. Accord’s patented technologies are now
available in the form of different products. The rest of
the paper describes some of Accord’s products in brief.
The SIM card is further upgraded with extensive
data validation logic of SWDT. This is for P5 and
P6 satellites.
Accord is currently developing a ten channel high
dynamics GPS receiver with MIL1553B interface
using UTMC chips. This FPGA-based approach is
likely to render better accuracies, higher number of
channels and improved TTFF performances.
The technical challenges associated with the GPS
receivers for Space applications include,
High Dynamics: The receiver has to function normally
under very high dynamics, such as 8 km/s. At this
dynamics the receiver has to meet the Time To First Fix
performance and sensitivity performance, which is a
Satellite Acquisition: High Dynamics also causes a very
large Doppler shift, thereby the GPS signal is offset
from its nominal frequency by a large amount. GPS
signal acquisition scheme has to search a much larger
frequency range to acquire and track the signal.
Accord’s GPS for Space Application
Accord has a GPS receiver product line for Space
Applications. They have been customized to meet the
requirements of Indian Space Research Organization’s
Low Earth Orbit satellites. The following GPS receiver
based Satellite Positioning Systems are successfully
delivered to ISRO
Accord has designed and developed an eight
channel high dynamics GPS receiver (RCE) and
Spacecraft Interface Module (SIM) for IRS P4
Accord has modified the Spacecraft Interface
Module with a HWDT and a limited data validation
logic to cater for the radiation effects for the TES
satellite, in addition to the GPS receiver.
Accord has further developed a GPS receiver with
MIL 1553B interface with added feature of HWDT.
Stringent Environment: The receiver has to work under
very stringent environmental conditions. This include
temperature cycle, vibration levels, radiation levels,
EMI/EMC requirements, dynamics etc.
Special Interfaces: Appropriate interfaces with the onboard system. The interface include RS-422, ARINC,
MIL 1553B, Biphase telemetry etc.
Satellite Positioning System (SPS), which consists of an
on-board high dynamics GPS receiver gives sustained
operation at high dynamics of satellite. It has inbuilt
monitoring scheme comprising the independent
watchdog timers for each of processors and algorithm
for the navigation processor output parameters
validation. Status information comprising the health of
the system, total number of resets and the reason for
malfunction are continuously posted to the ground
station. Any malfunction of the system is detected with
monitoring system and the recovery sequence will be
initiated by the monitoring system. Also, additional
logics are in-built such that during maneuvering, the
loss of position, if any, is limited for a short duration.
Operation mode of the SPS can be controlled with the
ground station tele-commands or through MIL1553B
bus from the onboard computer. The system has the
onboard data storage facility, which can be downloaded
when the satellite is visible. Interface and the storage
related issues are implemented in Spacecraft interface
Module (SIM)
SPS system allows multiplexing so that any one of the
dual onboard systems can be selected using the ground
commands. Mechanical, environmental and electrical
issues relating to high dynamics are considered in the
system design to give superior performance in on-orbit
Figure 1 shows Accord’s satellite positioning system for
Figure 1: Satellite positioning system for TES
The following gives a brief specification of Accord’s
GPS receiver on-board satellite
No of channels
Receiver Mask angle
Carrier Doppler frequency
Doppler Rate
System Storage Memory Capacity
TTFF: Cold Start
Position Accuracy (Without SA)
Velocity accuracy – (Without SA)
Position solution mode
Position update rate
Watch-dog timer for SIM and
+/- 85 deg
from Zenith
+ / - 64KHz
70 Hz / sec
2 Orbit Data
100 s (typical)
15 m (1 sigma)
0.15 m/s (1
1 Hz
RCE processor with disable
provision through Tele-command
Processor Reset Provision through
Pulse Command
Provision for Pitch (+/- 45 deg)
and Roll (+/- 26 deg) maneuver
inputs to SPS through pulse
commands on SIMCARD
Provision for RAIM
Provision for Monitoring the SPS
command status in RT/ PB / HK
Provision for RCE reset counter
(This is used to count the number
of RCE resets)
Provision for SIM hour counter
(This counter is used to tell when
SIM was last reset)
Provision For SIM reset counter
in RCE (updated upon SIM reset)
Hardware Watchdog timer
availability for SIMCARD
RCE should be capable of
accepting and Transmitting the
Mil-1553B messages.
Storage of ephemeris in
SIMACARD as a part of PBbuffer.
Provision of Hardware watch dog
timer in RCE
Provision of Data validation logic
in SIM for RCE
Operating temperature
Storage temperature
-10 to +55
degrees C
-40 to +85
degrees C
Accord’s GPS for Aerospace Applications
Accord has developed a GPS-WAAS Receiver for
aerospace application. It is compatible with other
Satellite Based Augmentation Systems (SBAS)
including EGNOS, MSAS and GAGAN. It is developed
around a Digital Signal Processor with optimum usage
of hardware and software in the system. The receiver
architecture is designed in such a way as to engineer it
for avionics application after FAA certification for TSO
in the En-route, Terminal and Non-precision Approach
mode of operations.
The important aspects of the receiver is reliable
hardware and added safety features. The failure analysis
of the receiver hardware for various navigation
functionalities as per ARP4761 shows acceptable
performance. The power-on self test and continuous
online tests to detect hardware failure enhances the
safety and reliability of usage of the equipment.
Additional hardware to detect failure improves the test
coverage of the equipment. The receiver accuracy,
acquisition and tracking sensitivity, dynamics, time-tofirst-fix and other characteristics meet the required
performance specifications outlined in DO-229B as well
as DO-208 for en-route, terminal, non-precision
approach.. The receiver software is engineered as per
DO-178B and eventually certified to make the system
usable in en-route, terminal and non-precision approach
applications in an aircraft.
Figure 2 shows Accord’s GPS receiver for aerospace
applications for en-route, terminal and non-precision
mode of operations.
Message format
Magnetic Variation
50000 Hours
Power-on Self Test and Online
Messages as per ARINC 743a
Through serial port without
removing the GPS card
As per DO-160D at the Card
Accord’s GPS for Automotive Applications
Accord has partnered with Analog Devices Inc., USA,
to develop GPS receiver chipset for the automotive and
hand-held market segment. Accord’s research in the
GPS area were primarily driven by this goal over the
past decade. Accord has created several reference
designs suitable for automotive applications and have
done technology transfer of those design to various
OEMs in India and far east.
Some of the reference designs created by Accord for
Automotive segment are
Figure 2: GPS Receiver for Aerospace applications
The following table shows some of the performance
characteristics of this receiver.
No of channels
Horizontal (SA off)
either GPS or UTC
(SA off)
Update rate
Receiver Specification
C/A code Sensor with WAAS
RTCA-DO-178B, Level B
Beta Class 1: LNAV
NAV2400 and NAV2500 are the latest offerings from
Accord. NAV2400 has a low-gate-count accelerator to
expedite the signal acquisition and tracking and at a
very low signal strength. This reference design is also
used to develop algorithms to acquire and track GPS
signals indoor or under foliage environment at less than
–150 dBm.
Total: 12 (GPS: 10 WAAS: 2)
15 m (95%) without SA
2 m (95%)
150 ns
-135 dBm (GPS)
-138 dBm (GPS)
1 Hz
FD and FDE in absence of
WAAS integrity
WAAS integrity, if available
All as per DO-229C
Figure 3: NAV2500 : GPS for Automotive
Figure 3 shows NAV2500 - Accord’s GPS receiver for
Automotive applications
NAV2500 is a Blackfin based reference design, which
is now integrated with an on-board solid stage
Gyroscope to derive a GPS-DR integrated solution. It is
further integrated with GSM and car telematics to arrive
at an Integrated Car Telematics System.
The following are some of the important specifications
of the NAV2500 receiver.
Reacquisition time
DR performance
Technical Specifications
12 channel L1 C/A code
Hot start: 12 sec
Warm start: 40 sec
Cold start: 65 sec
Position: 10 m (90%)
Velocity: 0.1 m (90%)
< 1 sec for less than 3 min
3% to 5% error of distance
traveled up to 1 km
Accord has indigenously developed a Fleet
Management System called eZfleet. The components of
eZfleet are,
Mobile Units (MU) or in-vehicle equipment
Fleet management server at the control station (CS)
Web based User terminal
CS and MUs communicate through the GSM network.
The overall architecture of the eZfleet is as shown in
Figure 4
GPS Antenna
GSM Antenna
In-Vehicle Equipment
User Terminal
GSM Driver and
User Terminal
Figure 4: eZfleet components
The advantages of this software-based correlator are
to continuously improve the receiver’s accuracy
and availability characteristics without always
having to invest in new hardware development
and/or customization of silicon
to make the complete GPS receiver function
available as a library on a family of instruction-setcompatible programmable DSPs from Analog
Devices so that reference designs can be created
for a wide variety of GPS applications by
integrating the GPS receiver library with other
communication and multimedia programs
to achieve scalable performance characteristics by
exploiting the availability of faster DSPs with lower
power consumption
to make the base band processing architecture
independent of the RF down converter architecture
to make the software available on both floating
point as well as fixed point families of DSPs from
Analog Devices, so that system developers /
integrators have a wider range of DSPs to choose
The MU is built with Accord's NAV2300R GPS
receiver. The highlights of the GPS receiver are its low
cost, small form factor, and higher sensitivity. These
advantages along with its lower power consumption
make it ideal for vehicle tracking applications.
The fleet management firmware is embedded on the
GPS core using the Unique Programmatic interface
provided by the GPS receiver chipset. The hardware
and software resources of the GPS chipset are shared
along with the application. This avoids usage of
additional hardware in the MU required for the Fleet
Management Application thus reducing the associated
Fleet Server runs at the control station and
communicates with the vehicles in the fleet through
GSM Driver.
The CS comprises of two software components, namely
GSM driver and User terminal.
In the User terminal, the web-based interface is
implemented on Java technology. The GSM driver and
the Fleet Server are implemented in Microsoft VC++.
Accord’s GPS for Fleet Management Applications
Figure 5 shows the plot of a trial conducted from
Bangalore (southern region) to Delhi (northern region).
The trip covered a total of 6000 kilometers. The MU
was configured for an update every one hour. The
Coverage of GSM is poor in the northern regions.
The following gives the technical specifications of the
high accuracy GPS receiver.
Technical Specification
GPS L1 C/A code receiver
No of channels
12 channels
Digital Signal Processor with
FPGA and RF Front-end
Horizontal (SA
10 m (95%) without SA
1 m (95%)
receiver noise
20 cm (RMS) at nominal signal
strength (-130 dBm)
Carrier phase
1 cm (RMS) at nominal signal
strength (-130 dBm)
-135 dBm
-138 dBm
Update rate
10 Hz
Figure 6 shows a plot of position error using this
receiver due to receiver noise only and in the nonsmooth mode of operation.
Figure 5: eZfleet field trials from Bangalore to Delhi
Accord’s GPS for Survey, Attitude Determination
Accord has developed the technology (patent pending)
for a high accuracy carrier phase GPS receiver for
survey and attitude determination applications.
The basic characteristics for the RF front-end for High
Accuracy GPS are,
High IF bandwidth for sharper correlation function.
The current version uses 8 MHz RF bandwidth
Figure 6: Position error (north-south)
High sampling frequency to utilize the IF
bandwidth and also to prevent aliasing. The current
version uses 22.5 MHz sampling frequency
More than single bit digitzed signal. The current
version uses 2-bits.
The high accuracy GPS uses a novel technique, in
which it exploits the asymmetry of the correlation
function and relates it to the amount of multipath error.
Accord’s Timing Receivers
Accord has developed GPS time source built around
Accord’s GPS receiver. In absence of GPS signal, the
unit continues to provide 1 PPS signal and maintains the
GPS time within 20 microsecond accuracy for at least 4
Accord has created several GPS based products, which
are now well accepted by the industry. Accord plans to
continue building products for the industry and upgrade
the products in tune with the modernization plans of
GPS and Galileo.
The unit generates a 1 PPS signal from an internal high
stability clock and synchronizes with the 1 PPS signal
output from the GPS receiver.
Accord’s Publications in International Journals and
The Unit is specifically designed to mee the
The following shows the specifications of the timing
Technical Specification
12 channel L1 C/A code GPS
100 ns (typical)
Pulse width: 5 msec
Level: TTL
Rise time: 10 nsec
Power supply
28 Vdc with option for AC
supply source
The unit can be customized as
per customer’s needs
Figure 7 shows Accord’s GPS clock.
Ray, J.K., M.E. Cannon and P. Fenton (2000),
Code and Carrier Multipath Mitigation Using a
Multi-Antenna System, IEEE Transactions on
Aerospace and Electronic Systems, 37, 1, pp. 183195.
Ray, J.K., and M.E. Cannon (2000), Synergy
Between GPS Code, Carrier and SNR Multipath
Errors, AIAA Journal of Guidance, Control and
Dynamics, 24, 1, pp. 54-63
Murali Krishna S, Madhukar B R, Ashok Kamath
(2000), DSPs in GPS receivers, GPS Solutions,
Vol 4, Summer 2000, pp. 67-71.
Ray, J., M.E. Cannon and P. Fenton (2000),
Mitigation of Carrier Phase Multipath Effects
Using Multiple Closely-Spaced Antennas,
NAVIGATION: Journal of the Institute of
Navigation, Vol. 46, No. 4, pp. 193-201.
Ray, J.K., O. Salychev and M.E. Cannon (1999),
The Modified Wave Estimator as an Alternative to
a Kalman Filter for Real-time GPS/GLONASSINS Integration, Journal of Geodesy, Vol. 73, No.
10, pp. 568-576.
Ray, J.K., K.V. Kalligudd (2001), Development
and Test Results of a Cost Effective Inverse DGPS
System, ION GPS-2001, Salt Lake City,
September 11-14.
Sudhir, N.S., Vimala C., J.K. Ray (2001),
Receiver Sensitivity Analysis and Results, ION
GPS-2001, Salt Lake City, September 11-14.
Figure 7: Accord’s GPS Clock
Kiran, S., Raghu Char, Rengasamy T.D., J.K. Ray
(2001), GPS-GSM based Fleet Management
System for Sparse GSM Networks, ION GPS2001, Salt Lake City, September 11-14
Vimala, C., C.S. Nagraj, M.S. Hemanth, M.S.
Venkatesh, M.R. Shenoy (2000), Accord’s Next
Generation High Performance GPS/WAAS
Receiver Based on the Soft-Correlator , ION
GPS-2000, Salt Lake City, September 19-22) (in
Cannon, M.E., J.K. Ray, and J. Deschamps (2000),
Attitude Determination Using Multipath Mitigation
on Multiple Closely-Spaced Antennas, ION GPS2000, Salt Lake City, September 19-22) (in press).
Kulkarni, P.A., K.N. Sudharshan (2000), An
Efficient Algorithm to Compute the Inverse of a
Fourth Order Positive Definite Syemmetric Matrix,
ION GPS-2000, Salt Lake City, September 19-22)
(in press).
Nayak, R. (2000), Urban Vehicular Multipath
Detection Using Multiple Antennas and Reliability
Analysis, Proceedings of ION GPS 2000, Salt Lake
City, Utah. September 19-22, (in press), (winner of
an ION Best Paper Award).
Nayak, R., M. E. Cannon, and C.Wilson, (2000),
Use of Multiple GPS Antennas and A Low Cost
IMU for Reliable And Continuous Urban
Navigation, Proceedings of ION, IAIN World
congress 2000, San Diego CA. June 26-28, pp. 44 –
R.A.Nayak and G.Lachapelle (2000), Attitude
determination with GPS-Aided Inertial Navigation
System, Proceedings of IAIN World Congress in
association with ION Annual Meeting, San Diego
CA, June 26-28, pp. 705 – 711.
Nayak, R., M. E. Cannon, C.Wilson, and G. Zhang
(2000), Analysis of Multiple GPS Antennas for
Multipath Mitigation in Vehicular Navigation,
Proceedings of ION NTM 2000, Anaheim CA,
January 26-28, pp. 284 - 293.
Ray, J. K. (1999), Use of Multiple Antennas to
Mitigate Carrier Phase Multipath in Reference
Stations, Proceedings of ION GPS-99, Nashville,
September 14-17, pp. 269-280, (winner of an ION
Best Paper Award).
Madhukar, B. R., R. A. Nayak, J.K. Ray and M.R.
Shenoy (1999), GPS-DR Integration Using Low
Cost Sensors for Land Vehicle Applications, ION
GPS-99, Nashville, September 14-17, pp. 537-544.
Characterization of GPS Carrier Phase Multipath,
Proceedings of ION National Technical Meeting,
San Diego, January 25-27, pp. 343-252.
Ray, J.K., M.E. Cannon and P. Fenton (1998),
Mitigation of Static Carrier Phase Multipath
Effects Using Multiple Closely-Spaced Antennas,
Proceedings of ION GPS-98, Nashville, September
15-18, pp. 1025-1034, (winner of an ION Best
Paper Award).
International Patent Pending for “High Performance
Low Cost Global Positioning System Receiver’.
Application filed on January 15, 2002
Indian Patent Pending for “High Performance Low
Cost Global Positioning System Receiver”.
Application Number: 907/MAS/99. Application
filed on September 22, 1999.
Ray, J.K., M.E. Cannon and P. Fenton (2001)
System and Method for Mitigating Static Carrier
Phase Multipath Effects, United States Patent No.
R.A.Nayak and G.Lachapelle (2000), Low Cost
INS/GPS Integration: Concepts and Testing,
Proceedings of ION NTM 2000, Anaheim CA,
January 26-28, pp. 98 - 105.
Ray, J.K., M.E. Cannon and P. Fenton (1999),
Code Range and Carrier Phase Multipath
Mitigation Using SNR, Range and Phase
Measurements in a Multi-Antenna System. ION
GPS-99, Nashville, September 14-17, pp. 713-725.
Sudhir N.S. M.R. Shenoy, S. Murali Krishna, R.
Anjan, S. Kiran (1999), NAV2300- Accord's high –
performance low cost receiver technology with
soft-correlator and programmatic Interface, ION
GPS – 99 , Nashville, September 14-17.