ModernizedL2CivilSignal.pdf

SYSTEM

Modernized

The Scene: 2008

The meeting started at

9:00 AM in a small con-

ference room at Acme

Industries. Fred, Acme’s

product development manager,

had attended ION GPS-2008 the previous

week, and he wanted an update on the GPS

chipset alternatives for the 2009 product intro-

ductions. He had invited only three other peo-

ple: Charley, who headed Acme’s dual-frequency

and high precision GPS product developments,

Valerie, who headed GPS-based consumer prod-

uct developments, and Albert, from marketing.

Under Fred’s direction, Acme offered a wide

array of GPS and non-GPS products for both

the professional and consumer markets. Years

ago Acme had recognized how important GPS

was for many applications, so it acquired a few

small companies with expertise in designing

and applying positioning technology. By 2008,

Acme had become a major supplier of GPS-

based equipment for high precision, OEM, and

consumer applications, although it had not

entered the aviation or military markets.

At the ION conference, GPS chipset vendors

had impressed Fred with the wide variety of

options available, including single-frequency

and multi-frequency chipsets for all three civil

GPS signals at L1, L2, and L5. He knew Charley

and Valerie were on top of these trends, so

he wanted a better understanding of the new

options and what they might mean for Acme’s

markets.

Fred asked Valerie to explain why she used

only L1 C/A chipsets for consumer applications,

while Charley had been using dual-frequen-

cy chipsets for years.

Valerie said “show slide one,” and her palm-

top transmitted it to the conference room pro-

jector (see Figure 1 ).

L2 Civil Signal

Leaping Forward in the 21st Century

by Richard D. Fontana, Wai Cheung, and Tom Stansell

This article reveals . . .

modernization: a signal suddenly changed.

After years of preparation, modernization

called for:

c implementing military (M) code on the

L1 and L2 frequencies for the Department of

Defense (DoD)

c providing a new L5 frequency in an aero-

nautical radio navigation service (ARNS) band

with a signal structure designed to enhance

aviation applications

c adding the C/A code to L2.

Implementation was underway when the

System Program Director for the GPS Joint

Program Office (JPO) asked whether it was

wise simply to replicate the 20th-century C/A

code in a 21st-century “modernized” GPS.

Responding to this challenge, a truly mod-

ern L2 civil (L2C) signal was designed in a

remarkably short time to meet a much wider

range of applications. The first launch of a Block

IIR-M satellite in 2003 will carry the new sig-

nal, as will all subsequent GPS satellites.

As a result, civil GPS product designers even-

tually will have at least three rather different

types of GPS signals to choose from. It also

would be desirable for GPS III to add a mod-

ern civil signal to L1, further increasing the

number of design choices. Depending on

the application, designers will be able to select

signals based on power, center frequency, code

clock rate, signal bandwidth, code length, cor-

relation properties, threshold performance,

interference protection, and so on.

As well as describing the technical char-

acteristics of the L2C signal, this article inves-

tigates how it will be used, what difference it

will make, and how it will affect both users and

manufacturers. To explore these issues, we

invite you to eavesdrop on a meeting held

on September 16, 2008.

how the new L2 signal, scheduled to

originate from GPS satellites in space

from 2003 onwards, will affect both

marketing and technical decisions

for receiver manufacturers

how L2 ideally suits some consumer

applications

how other applications will continue

using L1, while yet others will wait to

adopt L5, some time after 2008

how the arrival of the L2 civil signal

may prompt a transition to

consumer-level chipsets for high-

precision products.

Consumer Applications

“Thirty satellites now transmit L1 C/A,” Valerie

began, “but as this slide shows, only 20 have

the L2 civil signal, and only nine have the L5

signal. I think Al agrees we can’t sell single-fre-

quency L2 products until there are at least

24 satellites in good orbit positions. Until then,

even with a better signal, we can’t overcome

the geometry advantage a 30-satellite con-

stellation gives L1-only products.” Al made a

note and nodded agreement.

“A year from now, in late 2009,” she con-

tinued, “we expect to see a good 24-satellite

L2 constellation, so I’m starting to design for

L2. But there’s no guarantee. I don’t know

whether we should put all our chips on L2 —

no pun intended — or delay another year until

we’re sure of the constellation, or offer two fla-

28 GPS World September 2001

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The

A funny thing happened on the road to GPS

SYSTEM Challenge

vors of equipment and let our customers decide.

We also don’t know what our competitors will

do, so our options seem to be either picking

one signal and taking the market risk or spend-

ing the extra money to cover both options.”

“Why are there so many more L2 than L5 sig-

nals?” Fred asked.

Valerie showed slide two (see Figure 2 ) and

explained, “Before the first IIR-M ‘modernized’

satellite was launched in 2003, GPS provid-

ed only three navigation signals, right from the

first Block I in 1978 through the last unmodi-

fied IIR. Of the three signals, only the L1 C/A

was designated for civil use.

“Twelve IIR satellites were modernized into

IIR-Ms to speed-up the availability of the mil-

itary M code on L1 and L2 and the civil code

on L2. However, it wasn’t feasible to put L5 on

the modified satellites. That had to wait for the

IIF series now being launched. So, until the

twelve IIR-M satellites reach end of life and are

replaced, L5 won't be on every satellite. Any

delay is a shame, however, because L5 is a great

signal.”

Why an L2 Civil Signal?

A White House press release on March 30, 1998, announced that a civil signal would be added

to the GPS L2 frequency. Instead of replicating the C/A code, as many expected, a modern signal

structure, better matched to 21st-century capabilities and requirements, will be used. Although

the new signal will be available for all GPS applications, two primary requirements drove the

design.

First, the signal must serve the current large and growing population of dual-frequency

civil users, estimated to employ about 50,000 receivers for high-value professional or

commercial applications. Although this number seems small compared with handheld or

automobile use, the purchase value of these receivers is about a billion dollars, not count-

ing spares, application software, communication systems, and so on.

More importantly, these products are at work adding value to society. Applications

include:

c scientific projects to monitor earthquakes, volcanoes, continental drift, and weather

c cadastral and construction land survey

c guidance and control of mining, construction, and agricultural machines

c land and offshore oil and mineral exploration

c marine survey and construction, etc.

The most important objective was to eliminate the need for the marginal and fragile

semi-codeless tracking technique by placing a civil code on the L2 frequency. A C/A code

replica would meet this requirement, but L2C enhances performance by having no data on

one of its two codes, which improves threshold tracking performance by 3 dB and pro-

vides ‘full-wavelength’ carrier phase measurements without the 180 degree phase ambigu-

ity inherent in GPS signals which carry data.

The second key objective was to make L2 valuable for a host of single-frequency GPS

applications which so far have been served only by the L1 C/A signal. The primary need

was to eliminate the unacceptable 21 dB crosscorrelation performance of the C/A code,

which allows a strong GPS signal to interfere with weak GPS signals. The L2C signal

achieves this by having a worst-case crosscorrelation performance of 45 dB (over 251

times better). Furthermore, L2C lowers the data demodulation threshold, making it possi-

ble to read the message when barely tracking the signal. As a result, L2C is likely to

become the signal of choice for applications like E911 positioning inside buildings, person-

al navigation in wooded areas, or vehicle navigation along tree-lined roads. If this predic-

tion comes true, embedded GPS in wireless phones will make L2C the most widely used of

all GPS signals.

High-Precision Applications

Charley then explained why his dual-frequen-

cy products had used the new L2 civil signal

since the first IIR-M launch in 2003. “When the

government defined the new L2 signal in 2001,

we had to respond pretty fast. We didn’t want

our competitors saying they were compatible

with the new signal when we weren’t. That

would have made even our newest products

seem obsolete.

“The avionics manufacturers had a similar

but even more difficult problem. Common prac-

tice was for avionics to be supported for 20

years after installation on a commercial air-

plane. Can you imagine the dilemma of know-

ing that signals which had never been launched

would fill the sky years before the avionics was

replaced? There was disagreement in the FAA

about whether to use the L2 civil signal or not,

because it isn’t in an ARNS (Aeronautical Radio

Navigation Service) band, even though it would

be available years earlier than L5 and, by increas-

ing signal redundancy, would give substan-

tial protection against GPS interference. Also,

it wasn’t clear whether or when WAAS or LAAS

would support either or both of the new sig-

nals. One solution was a modular design, sup-

porting future upgrades, including software

upgrades, by adding or exchanging plug-in

components.”

Survey market. “For our survey market, we

still use semi-codeless tracking of the military

P/Y signals to get L2 measurements from old

satellites which don’t have the L2 civil sig-

nal, but until now that’s always been a neces-

FIGURE 1 Introduction of new signals (authors’ estimate, as funding not finalized)

FIGURE 2 New signal availability

www.gpsworld.com

GPS World September 2001 29

SYSTEM

sary evil. You remember that semi-codeless

requires the receiver to track the P code on L1

and on L2 and crosscorrelate the L1 and L2

measurements, in a 500 kHz bandwidth, which

hammers the signal-to-noise ratio. Semi-code-

less works OK, but the signal margins are slim,

they drop 2 dB for every 1 dB of real signal loss,

high ionospheric dynamics stresses the track-

ing loops, L2 acquisition is much slower, and

it’s a lot more expensive to build, mostly because

there’s no high-volume consumer market for

these chips. Other than that, semi-codeless is

great!

“There’s nothing like having a civil code on

L2 to solve all these problems. Unfortunately,

we’ll have to keep a semi-codeless capability

in our products for a few more years, at least

until nearly every satellite has the L2 civil

signal. Even after that there’ll be compatibil-

ity problems with really old receivers which

don’t have the L2 codes, but there’ll be so few

of them, and they’ll be so obsolete by then, I

don’t think we should worry about that much

backward compatibility.” Al made a note and

nodded agreement.

able to detect that mode and react properly.

“Switch B allows the C/A code to be trans-

mitted either with or without a navigation mes-

sage. Having no message is much better for

dual-frequency applications, because we get

the message on L1 anyway, and, by using a

phase-locked loop rather than a Costas loop

on L2, we get a 6 dB tracking threshold advan-

tage. Maybe you don’t remember, Fred, but bi-

phase data modulation forces the receiver

to use a squaring, or Costas, carrier loop, which

has a 6 dB-worse tracking threshold. Without

IIF Options

“There were two main concerns in the begin-

ning,” Charley continued. “I’ll illustrate with

block diagrams for the IIF and the IIR-M satel-

lites. The first (see Figure 3 ) shows the L2 sig-

nal options built into IIF satel-

lites. Although we haven’t

seen the C/A code on L2, at

least for a very long time now,

switch A allows the satellite

to transmit the old C/A code,

and our receivers must be

Why Two Codes?

Notice that both new civil GPS signals have two codes. The con-

cept was first adopted for L5, although its origin is an old idea

revisited. The world’s first navigation satellite system was the

Navy Navigation Satellite System, usually called Transit. Its devel-

opment began in 1958 (triggered by launch of the first Sputnik

the previous year), it became operational in 1964, and it was

switched off at the end of 1996 after nearly 32 years of depend-

able service.

Transit did not use bi-phase data modulation. Instead, the

carrier phase had three states, 0 8 , +60 8 , and -60 8 . The modu-

lation pattern put 44 percent of the signal power into data

bits and a bit clock, but 56 percent of the power remained in

a coherent, unmodulated carrier component which Transit

receivers tracked with a simple phase locked loop.

Bi-phase data modulation, which has been the GPS prac-

tice, removes the carrier component, forcing the receiver to

use a squaring (Costas) loop to create a second harmonic of

the carrier, which can be tracked. Although this may be ideal

for a data communication channel, it worsens the phase-

tracking threshold by 6 dB, that is, four times more signal

power is required to maintain phase lock than if there were

no modulation.

Following the Transit precedent, L5 was designed with two

equal power signal components, one with data and one with-

out. Although each component has only half the total power

( 2 3 dB), the 6 dB threshold advantage of tracking a data-less

signal gives an overall 1 3 dB tracking improvement. With the

resultant better phase reference and by using forward error

correction (FEC), the data error rate is the same as if all the

power were in just one data-modulated code. Since L5 is not

shared with military signals, it achieves the power split by

using two equal-length codes in phase quadrature, each

clocked at 10.23 MHz.

L2 is shared between civil and military signals. Therefore,

L2C is limited to a single bi-phase component in phase quad-

rature with the P/Y code. Also, L2C is limited to a 1.023 MHz

clock rate in order to maintain spectral separation from the

new military M code. Even so, as stated above, having two

codes provides an important advantage. L2C achieves this by

time multiplexing two codes, which in this case are of differ-

ent length. The moderate length code (CM) has 10,230 chips,

repeats every 20 msec, and is bi-phase modulated with data.

The long code (CL) has 767,250 chips, repeats every 1.5 sec.,

and has no data modulation. The composite signal is clocked

at 1.023 MHz and alternates between chips of each code

(chip-by-chip time multiplexed).

L5-Like CNAV

message

25 bits/sec

Rate 1/2 FEC

Legacy NAV

message

50 bits/sec

10,230 chip

code generator

code

Chip by chip

multiplexer

511.5kHz clock

767,250 chip

code generator

code

Transmitted

signal

1/2

C/A code

generator

1.023MHz

clock

FIGURE 3 L2 signal options in IIF satellites

L5-Like CNAV

message

25 bits/sec

Rate 1/2 FEC

Legacy NAV

mesage

25 bits/sec

Legacy NAV

message

50 bits/sec

10,230 chip

code generator

code

Chip by chip

multiplexer

511.5kHz clock

767,250 chip

code generator

code

Transmitted

signal

1/2

C/A code

generator

1.023MHz

clock

FIGURE 4 L2 signal options in IIR-M satellites

30 GPS World September 2001

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SYSTEM

the data, we can use a non-squaring phase-

locked loop, which doesn’t suffer the 6 dB loss.

However, the receiver must be able to sense

message data on the C/A code and switch to

the Costas loop if it’s there. As long as the switch-

es are in the satellites, we have to be ready for

any combination. Fortunately, all satellites

have the A switch in the ‘1’ position, so we’re

able to use the standard two-code L2 civil

signal.

“As you probably know, we get two advan-

tages with this signal. We can track the long

data-less code (CL) with a phase-locked loop

for better threshold performance, and even

though the data rate on the moderate length

code (CM) is half that of the L1 NAV message,

the performance actually is better because the

demodulation threshold is lower and the mes-

sage structure is more compact.”

L2C Abbreviations

L2C the L2 Civil Signal

CM the L2C moderate length code

is 10,230 chips long, repeats

every 20 milliseconds, and

is bi-phase modulated with

message data

CL the L2C long code is 767,250

chips long, repeats every 1.5

second, and has no data

modulation

NAV the legacy navigation message

provided by the L1 C/A signal

CNAV the L2C navigation message

structure, like that adopted for

atively low volume. That’s because there has-

n’t been a mass market for L2, because semi-

codeless is so complex, and because every

semi-codeless technique is slightly different and

protected by patents. We’re approaching a time

when every GPS satellite will have the L2

civil signal, and there will be a mass market for

L2 chipsets. Val will help make that happen.

“We’d like to buy consumer L1 and L2 chipsets

to use in our most sophisticated profession-

al products. Our chip designers won’t like that,

but the change is inevitable. However, the

chipsets must have excellent performance char-

acteristics, including common clocks, low

phase noise, wide bandwidth, and multipath

mitigation correlators. If this means they’re

slightly different than run-of-the-mill consumer

parts, we should be able to use Val’s higher vol-

umes to persuade chipset vendors to meet our

needs. I think the timing will work out OK.” Al

made a note and nodded agreement.

“Unfortunately, this transition brings a big

risk,” Charley stated. “Using consumer chipsets

in professional products will reduce the cost

a lot, mostly because we won’t have to con-

tinue designing and improving our own every

couple of years. However, the patent and semi-

codeless ‘complexity’ barriers to market entry

will be gone. Any company, worldwide, will be

able to buy the same chipsets and jump into

the high-precision dual-frequency market. With

so many ambiguity resolution survey software

packages available to license, that won’t be

much of a barrier, either, like it was many years

ago.”

IIR-M Options

“The next chart ( Figure 4 ) shows that IIR-M satel-

lites have even more options. It must have been

a real squeeze to add new signals to the IIR

birds, because there were several backup mes-

sage options. As you can see, all the IIF options

are included, but in addition there are two other

message options. One, with switch position

C2, puts the L1 NAV message on the moderate

length code at 50 bits per second. The other,

with switch positions C1 and D2, uses the L1

NAV message but at 25 bits per second and

with forward error correction. Fortunately,

these options aren’t needed, because all the

IIR-M satellites operate with the C and D switch-

es both in the ‘1’ position. However, until all

the IIR-M satellites are phased out, I think

we should keep these options in our chipsets,

just in case.”

Compatibility During Transition. “For us,

that wasn’t the end of the story. In the begin-

ning, we had to assure compatibility between

new receivers able to use any of these signal

options and legacy receivers with only semi-

codeless tracking. We couldn’t mix L2 phase

measurements from receivers with different

tracking techniques. On small networks the

coordination could be done by manual com-

mands, even though that introduced human

error. On larger reference networks, espe-

cially government-provided networks, the coor-

dination had to be automatic. Information

about the reference stations was added to phase

differential messages, but automatic detection

became necessary. Many of our receivers pro-

vide both kinds of measurements in order to

adapt to any situation, whether as a reference

station or as a rover. Fortunately for everyone,

both the National Geodetic Survey (NGS) and

the Radio Technical Commission for Maritime

Services (RTCM) grabbed the ball and helped

everyone get through the transition, but it was

difficult. Fortunately, Val won’t have to face that

problem.

“But we also have a big transition problem

coming up,” Charley went on. “Until now we’ve

had to design our own chipsets and essen-

tially beg foundries to produce them in our rel-

Competition Up, Prices Down

“Expect competition to get worse and prices

to fall. Our main hope for holding market share

is to provide the best service and the best func-

tionality for our customers. We know this mar-

ket a lot better than any of our would-be com-

petitors, so we should concentrate on and

improve our application leadership, while there's

time.” Al looked concerned, but he jotted a

note and nodded agreement.

“What about L5, then?” Fred asked.

Charley answered, “We’re excited about L5

and, if possible, we want to include it as part

of the overall transition to consumer chipsets.

Unfortunately, we may have to do something

sooner than consumer chips are available,

because we wouldn’t want our competitors

to have it first. Like Val said, so far there aren’t

very many signals, but in the future our high-

end products will use all three frequencies.

This will speed up ambiguity resolution and

extend baselines by permitting better ionos-

pheric corrections over long distances. This is

an important improvement, but it certainly has

challenged the antenna designers to maintain

low-multipath patterns with good sensitivity

Carrier

Code

Forward

Civil

Frequency Length

Clock

Bit Rate

Error

Signal

(MHz)

(chips)

(MHz)

Phases

(BPS) Correction

1,575.42

1,023

1.023

Bi-Phase

1,227.60 10,230 (CM)

1.023

Bi-Phase

Yes

767,250 (CL)

1,176.45

10,230

10.23

Quad-Phase

Yes

10,230

Relative

Civil

Fully

Ionospheric

Correlation Data Recovery

Carrier Tracking

Signal

Available Error Ratio Protection (dB) Threshold (dB)

Threshold (dB)

Now

1.00

> 21

0.0

~ 2011

1.65

> 45

+2.7

+0.7

~ 2015

1.79

> 30

+5.7

+6.7

FIGURE 5 Comparisons of the three civil signals

32 GPS World September 2001

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SYSTEM Challenge

and tightly controlled phase center charac-

teristics for all three frequencies!”

A Better Signal. Fred then asked Valerie

why she seemed so motivated to use the L2

civil signal for consumer products rather than

stick with the tried-and-true L1.

“It’s a better signal,” she replied. “That does-

n’t mean it’s better for everything, so we won’t

abandon L1, and for many future applications

we’ll use L5. That’s the beauty of having three

rather different civil signals to choose from,

because we can choose the best signal for each

application.”

An interesting market. “This also makes the

market more ‘interesting,’ because our cus-

tomers and our competitors might think a dif-

ferent signal is better for a particular applica-

tion. And the choices won’t be static, because

we expect things to change, starting with the

GPS III signals. For example, in the future we

expect more power for the L2 civil signal and

perhaps an L2-like signal on L1 in addition to

just C/A. This business won’t get boring!”

“Show the comparison charts,” Val com-

manded, and her palmtop complied (see Figure

5 ). “Because it doesn’t hurt to think about future

choices as well as what we can use today, the

next chart compares all three civil signals. The

top table shows the basics, like the center

frequency, the length of the code or codes, the

overall code clock rate, and whether the sig-

nal consists of one bi-phase component or two

components in phase quadrature. It also shows

the data bit rate for each signal and whether

the data has forward error correction (FEC) or

not. As you can see, the signals have rather dif-

ferent characteristics.” Al made a note and nod-

ded agreement.

“The second table concentrates on func-

tional differences between the signals. The sec-

ond column estimates when there will be about

as many satellites with the new signal as there

are with L1 C/A today. That, of course, is dif-

ficult to predict.

“The third column shows one effect of the

different frequencies. Because ionospheric

refraction error is inversely proportional to fre-

quency squared, ionospheric error at L2 is 65%

larger than at L1, and at L5 it’s 79% larger. If

a local differential GPS correction signal (DGPS)

is available, that’s not too much of a prob-

lem. However, partly because satellite orbit

and clock accuracy have improved so much,

the ionosphere is the largest source of sin-

gle-frequency navigation error, and we’ll approach

another solar maximum in about three years.

Therefore, we should continue to use L1 for all

applications where single-frequency, non-DGPS

accuracy is the primary concern. If GPS III car-

ries a better L1 signal, then a dozen years or so

from now we won’t have that problem either.”

Correlation Protection. “The fourth column

shows the correlation protection character-

istics for each signal. Because L2 has the longest

code, it gives the best correlation protec-

tion. L1, with the shortest code, has the worst.

This is really important for situations where

some satellite signals are very strong and oth-

ers are very weak, like wireless E911 inside

buildings or for navigation in or along heavily

wooded areas.

“The problem with L1 C/A is that a strong

signal from one satellite can crosscorrelate

with the codes a receiver uses to track other

satellites. A strong signal thus can block recep-

tion of weak signals. Also, the receiver must

test every signal so it doesn’t falsely track a

strong signal instead of a weak signal. With

more than 45 dB of crosscorrelation protec-

tion, there’s no such problem with L2, and it

has headroom for increases in L2 power from

GPS III satellites. Also, better correlation prop-

erties help L2 receivers reject narrowband inter-

ference signals.

“Relative to L1, the raw signal power on

L2 is 2.3 dB weaker, but on L5 it’s 3.7 dB stronger,

for a 6 dB advantage of L5 over L2. We hope

these differences will slowly disappear as more

GPS III satellites with increased L1 and L2 power

are launched. Both L2 and L5 use FEC, and the

data rate on L2 is 25 bits per second versus 50

on L1 and L5. Signal tracking threshold on both

L2 and L5 is improved because one of the codes

has no data. The fifth and sixth columns give

the bottom line for data recovery and for thresh-

old signal tracking: L2 is better than L1 C/A but

not as good as L5, simply because L5 has

four times more power than L2.”

L2 Advantages

“One obvious conclusion is that L5 will be a

very attractive signal in a few years when the

number of signals in space catches up. However,

right now it appears that L2 is superior to L1

for many applications, it’s available years

earlier than L5, and it may be better than L5 for

a lot of future applications, even after there are

enough L5 signals.

“Getting back to your original question, Fred,

I’m motivated to use L2 because it has the best

crosscorrelation protection of all, and relative

to L1 it has a lower tracking threshold, it has a

lower data demodulation threshold, and it pro-

vides a better message structure.

“Also, like L1 C/A, the L2 codes have an over-

all 1.023 MHz chip rate, ten times slower

than L5. On the surface this might seem like a

disadvantage, but for many low-power appli-

cations it’s a real advantage. As you know,

the code clock rate strongly influences GPS

chipset power consumption. That may

not be a problem for vehicle-mounted equip-

ment with plenty of power, but for wristwatch

and cell phone navigation, battery drain is

a major issue. Also, chip size often is driven

more by thermal dissipation than by the

number of gates, so a slower clock helps with

miniaturization.

Signal Tracking Must Be Coordinated

Today, every civil dual-frequency receiver uses codeless or semi-codeless techniques to make L2

measurements. In the 2008 meeting depicted in this article, Charley explains the problems of

semi-codeless tracking, and codeless tracking is 13 dB worse. These problems will be eliminated

with a civil code on L2. However, the signal transition will be difficult, because high-precision

differential processing requires the same tracking technique to be used on each satellite signal

by all receivers in a common survey.

Assume a survey is underway with two modernized receivers; one is the reference sta-

tion at a known location and the other is the rover. Both can use the new L2C signal. After

the first IIR-M satellite is in service, these receivers will continue to use semi-codeless to

track every other satellite, but they will use L2C to track the IIR-M. Tracking with L2C is far

more robust than with semi-codeless, so this advantage will be achieved one satellite at a

time as they are launched.

However, suppose a third receiver without L2C capability is added as another rover. It

must track the IIR-M with semi-codeless, so these data probably can’t be combined with

the reference station data from the IIR-M.

Unless ways can be found to calibrate the precise phase difference between semi-code-

less and L2C tracking of the same satellite, much care must be taken when mixing different

generations of equipment. In particular, recorded data must be marked automatically to

show if L2C is being used, and the same flag should be provided in differential messages

from every reference station. This becomes particularly difficult for networks of mixed ref-

erence stations. One interim approach might be to track new satellites in both ways and

provide the phase difference as an additional message.

The high-precision GPS industry should address these issues immediately, perhaps

through RTCM SC-104 activities. Signal simulators and prototype receivers will be needed

as soon as possible to quantify the extent of this transition problem.

www.gpsworld.com

GPS World September 2001 33

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