Now, what’s tomorrow’s challenge?

APN-009 Rev 1 June 19, 1997

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Functional Overview – WAAS Receiver Subsystem

The WAAS receiver module is based on the NovAtel L1 GPSCard and the L1/L2 GPSCard. To accomplish its high level of

performance, the GPSCards are implemented in a multicard format. The L1/L2 modules are capable of receiving and tracking

the L1 C/A-code, L1 and L2 carrier phase, L2 P-code, and the encrypted L2 P-code of up to 12 satellites. NovAtel has

developed a multipath estimation technology that approaches the theoretical limits of GPS signal reception. This patented

technology, called “Multipath Estimating Delay-Lock-Loop” (MEDLL), utilizes a combination of hardware and software

techniques that are capable of reducing the combined effects of multipath errors by as much as 90% compared to a system

using the Narrow Correlator™.

MEDLL can effectively remove all multipath signals that have a propagation delay of greater than 0.1 chip relative to the

direct path signal. The remaining multipath effect on the C/A code pseudorange measurements is now in the same order of

magnitude as a “P” code GPS receiver.

The performance gains provided by MEDLL versus other types of PRN code signal tracking techniques are illustrated below.

Figure 1 Multipath Error Envelopes for Narrow Correlator vs. MET vs. MEDLL

-20

-40

-60

-80

Multipath Signal:

Amplitude = 0.5

Phase = 0, 180 deg

NovAtel’s Narrow

Correlator

Standard Correlator

MEDLL

MET

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Multipath Delay (C/A Chips)

Tracking Error (m)

Negative values indicate 180 degrees

out-of-phase errors

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Functional Overview – WAAS Receiver

The main building blocks of the WAAS receiver can be seen in the following figure.

Figure 2 WAAS Receiver Subsystem

Functional Block Diagram

All I/O interconnections to the WAAS receiver are

accessed via the unit’s rear panel. These include

system power, optional external reference clock input,

antenna input, 20.473MHz output, timing strobes, and

four RS232 ports: L1-C/A, L1/L2-I, L1/L2-II, and

Time communications. All connectors are available

from the mounted 9-pin D and coaxial connectors.

WAAS Receiver Architecture

The WAAS receiver consists of three separate

subreceivers, MEDLL, L1/L2-I, and L1/L2-II, so the

incoming RF antenna signal must be split for each

receiver unit.

The Slave Cards of the MEDLL receiver receive their GPS/GEO signals in a digitized format generated on the MEDLL

Master Card. The figure below illustrates how the satellite signals are distributed from the MEDLL Master Card to the Slave

Cards. It can also be seen how the signals are split from the antenna input to the MEDLL Master Card and to the L1/L2 cards

simultaneously.

Figure 3 RF Signal Distribution

Clock/

Status

Card

Master

Card

Process

Slave

Cards

L1/L2 I

L1/L2 II

MEDLL Cards

3-Way

Splitter

Back Plane

Digitized

Signal

From Antenna

Digitized Signal

Although the subreceivers (MEDLL, L1/L2-I, and L1/L2-II) can operate independently, their oscillators are locked together

since they receive their clock signals from the same external clock source. The GPSCards of the MEDLL unit (Master Card

and Slave Cards), however, must operate synchronously as a single unit. They receive the GPS/GEO signals in a digitized

CLOCK/

STATUS

MASTER SLAVES L1/L2 POWER

MEDLL L1/L2

External

Power

Supply

Antenna

L1/L2

Cesium

Clock

Host

Computer

3 x RS232

L1 C/A, L1/L2-I, L1/L2-

Strobes:

3 x 1PPS

3 x Measure

1PPS

RS232 (Time)

20.473 MHz

WAAS

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format in order to eliminate interchannel biases. Also, the pseudorange and carrier phase measurements on all MEDLL cards

must happen at the same time.

The pseudorange and carrier phase measurement synchronization and digital signal distribution is performed by an additional

“clock/status” EuroCard that has been added to the multi-card system. The clock/status card has a number of bi-colour LEDs

that are used to indicate go/no-go health status for the individual GPS cards. Red indicates a faulty module, whereas Green

indicates a good module. If any of the LEDs are red, the system should be considered unreliable.

The clock/status card also has an OCXO to provide good quality clock signal for the L1 GPSCards (MEDLL Master and

Slave Cards), and circuitry to accept the high stability external reference clock signal. The green stage of the external

oscillator LED (11

: bottom) indicates the system has locked to the external reference. If no external reference signal is

present, the LED remains dark. Red indicates the system is under the locking process, and may not have a stabilized clock.

Note that the warm-up process may take several minutes, depending on ambient temperature.

The clock/status card also provides a buffered 20.473 MHz clock signal for the external GSV unit. The following diagram

illustrates the clock signal input, frequency conversion, and distribution from the Clock/Status Card to the MEDLL receiver

cards. In this graphic, we can also trace the 1PPS output signal from the three receiver systems, through the Strobes port as

well as the 1PPS output connector on the rear panel.

Figure 4 Clock Tree Distribution

Clock/

Status

Card

Master

Card

Process

Slave

Cards

L1/L2 I L1/L2 II

MEDLL Cards

20.473 MHz

10 MHz In

20.473 MHz Out

1 PPS Port

1 PPS

Strobe Port

Back Plane

20.473 MHz

Splitter

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As mentioned, all the MEDLL cards must take the pseudorange and carrier phase measurements at the same time. The

synchronization is implemented through MKI (Mark Input) and MKO (Mark Output) signals. MKO is generated on the

MEDLL Master Card and is distributed on the backplane as a MKI signal after inversion and buffering. The time

synchronization is implemented using the 1PPS (1 Pulse Per Second) and Time signals. Time is a simplex RS232 channel

between the MEDLL Master Card and the L1/L2 modules. Both the Time and 1PPS output signals are accessible through the

rear panel connectors. The following illustration displays the flow of the measurement strobes and timing signals.

Figure 5 Measurement Strobes and Timing

Clock/

Status

Card

Master

Card

Process

Slave

Cards

L1/L2 I

L1/L2 II

MEDLL Cards

MSR

Strobe Port

Back Plane

MKI

MKO

PPS

Time

1 PPS

MSR

COM2

The MEDLL cards as well as the L1/L2 modules communicate with each other through the Tlink (Transputer link) interface.

Since the MEDLL and the L1/L2 modules operate at a different clock frequency, there is no Tlink connection between the

MEDLL and the L1/L2 modules, although they can communicate using COM2 which is implemented as a simplex Time

connection. The Tlink and Time (COM2) distribution network is illustrated in the figure below.

Figure 6 Communication – TLink and RS-232

Master

Card

Slave

Card 1

Slave

Card 2

Slave

Card 8

TLink

L1/L2

TLink

COM1

COM2

COM1

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As the NovAtel GPSCards are the primary building blocks for the WAAS receiver, the following section is provided to give

the user some insight into the functionality of the GPSCard and is directly applicable to the WAAS receiver.

Functional Overview – L1 GPSCard

The GPSCard modules are composed of two major sections: an RF section (MEDLL Master Card only) and a Digital section

(all L1 GPSCards). In order for the GPSCard module to function as a complete system, you must also connect an external

antenna, external power supply, and external DCE. A brief description of each section follows.

Figure 7 L1 GPSCard System Functional Block Diagram

RF/IF

Section

Signal

Processor

CPU

(32 bit)

System

I/O

External

DCE

GPSCARD OEM ASSEMBLY

TCXO

Master

Oscillator

AGC

External

Power Supply

+12

-12

GND

Controls

COM1

COM2

Input Strobes

Output Strobes

GPSAntenna

Model 501, 511, 521,

or User-Supplied

LNA

NovAtel long

coaxial cable

Short

coaxial

cable

User supplied

0 - 30 V

Optional

LNA

Power

Clock

GPSCard RF/IF Section

The GPSCard receives the filtered and amplified RF signal from the GPS antenna via the external interconnecting coaxial

cable. The RF section of the GPSCard serves the following primary functions:

• Filters the RF signal to reduce noise and interference.

• Down-converts the RF signal to an IF (intermediate frequency) range that is suitable for the A/D converter.

• Amplifies the GPS signal to a level suitable to drive the A/D converter in the digital section.

• Accepts automatic gain control (AGC) input from the Digital Signal Processor (DSP) to maintain the IF signal at a

constant level.

• Converts the IF signal to a digital format (A/D conversion).

Digital Section

The GPSCard digital section consists of three major subsections: Signal Processor, CPU, and System I/O.

The Signal Processor contains two NovAtel custom ASIC correlator chips, and performs the following primary functions:

• Independent satellite channel tracking

• C/A code and carrier phase tracking

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The CPU is the heart of the GPSCard. All of the system control, processing, and positioning intelligence is in the CPU. It

consists of both hardware and software components. The CPU is summarized below:

• A 32-bit microprocessor

• Custom Real Time Operating System (RTOS)

• Database management

• I/O control

• Position filtering

• Channel/loop control

The I/O section allows two-way communications and timing strobes between outside devices and the GPSCard. All

interfacing and connections to the GPSCard (with the exception of RF input and external oscillator input) are provided

through the on-card 64 (or 96) pin DIN 4162 type B (or C) male connector. A summary of I/O functions is listed below:

• Provides two serial communication ports for interfacing with outside DCE – EIA RS232 configurable.

• Selectable baud rates up to 115.2 KBaud (defaults to 9600 baud)

• Provides input and output timing strobe lines.

• Allows user command input.

• Provides a means of output logging of various data types.

Functional Overview – L1/L2 GPSCard

Each of its 24 tracking channels can independently track a code/carrier combination of a GPS satellite in view and provide a

pseudorange accuracy within 10 cm RMS. It is configurable to track a variety of combinations of GPS L1 and L2 signals; for

example, any combination of L1/L2 pairs up to a maximum of 12, or up to 24 L1 channels. Proprietary correlator circuits

combined with a high-performance 25 MHz 32-bit CPU are capable of generating satellite code and carrier phase data as well

as position solutions at a rate of up to 5 times per second.

NovAtel’s P-Code Delayed Correlation Technology guarantees superior performance even in the presence of P-code

encryption. The L1/L2 modules extend the exceptional performance of NovAtel’s patented narrow correlation process, high

data update rates, multipath-resistant processing, and solid tracking loops to provide reliable centimeter-level positioning.

Exceptional acquisition and re-acquisition times allow this receiver to operate in environments where very high dynamics and

frequent interruption of signals can be expected.

L1/L2 GPSCard RF/IF Section

The GPS signal is fed from the antenna module, through the interconnect cable to an input connector on the L1/L2 receiver

module. The signal is split into two channels and fed to an L1 channel operating at 1575.42 MHz and an L2 channel

operating at 1227.6 MHz. The RF section of the GPSCard serves the following functions:

• Filters and amplifies the RF signal to reduce noise and interference.

• Down-converts the RF signal to an IF (intermediate frequency) range that is suitable for the A/D converter.

• Performs further filtering and amplifies the GPS signal to a level suitable to drive the A/D converter in the digital

section.

• Accepts automatic gain control (AGC) input from the Digital Signal Processor (DSP) to maintain the IF signal at a

constant level.

• Converts the IF signal to a digital format (A/D conversion).

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Figure 8 L1/L2 GPSCard System Functional Block Diagram

Serial

Com.

AGC

CPU

MINOS3

1PPS

A to D

AGC

A to D

RF A

(GPS L1)

SPLITTER

RF B

(GPS L2)

MINOS3

L1/L2

L1/L2 GPSCARD Digital Section

The L1/L2 GPSCard digital section essentially is an upgraded version of the L1 GPSCard.

The Signal Processor contains two NovAtel custom high-density ASIC correlator chips, which are capable of tracking

P-codes as well as C/A-codes.

The CPU operates at a higher clock frequency due to increased performance needs. As was seen for the L1 GPSCard, all of

the system control, processing, and positioning intelligence is in the CPU.

From the interface point of view, the L1/L2 I/O section is 100% compatible with the L1 GPSCard.

For more information:

Write to: NovAtel Inc.

1120 – 68 Ave N.E.

Calgary, Alberta, Canada

T2E 8S5

Phone: 1-800-NOVATEL (in Canada or the U.S.) or +1-403-295-4900

Fax: 403-295-4901

E-mail: support@novatel.ca

Website: www.novatel.ca