2C or Not 2C: The First Live Broadcast of GPS CNAV Messages

July 8, 2013  - By 0 Comments

By Oliver Montenbruck, Richard B. Langley, and Peter Steigenberger

Over the past several years, some users of the GPS navigation system have already benefitted from the addition of various new signals in addition to the legacy C/A- and P(Y)-code. With the introduction of the Block IIR-M satellites in 2005, a new civil signal (L2C) was transmitted on the L2 frequency, and a new signal on a new frequency (L5) was introduced as a standard signal with the Block IIF satellites beginning in 2010. These new signals provide direct access to dual-frequency observations and thus enable improved ionospheric corrections for civil, including aeronautical, users. In addition, a new Civil Navigation (CNAV) broadcast message has been defined in the GPS Interface Specifications (IS-GPS-200 and IS-GPS-705).

This message will be transmitted jointly on the L2C and L5 signals and provides a variety of useful new parameters. Compared to the legacy L1 C/A-code navigation message, the CNAV message also offers an increased flexibility concerning the type, sequence, and repeat rate of specific messages.

CNAV messages have already been broadcast over the past two years by the Michibiki (QZS-1) satellite of the Japanese Quasi-Zenith Satellite System (QZSS), which shares many aspects of the GPS signal design. In contrast to this, Block IIR-M and IIF GPS satellites have only transmitted dummy messages so far and a fully operational CNAV transmission is only foreseen once the ongoing modernization of the GPS control segment has been completed.

Triggered by various interest groups, the Global Positioning Systems Directorate has just conducted a first test campaign with live CNAV transmissions on L2C and L5 over the two-week period from June 15 to 29 (see Global Positioning System Modernized Civil Navigation (CNAV) Live-Sky Broadcast Test Plan.) It served as a first opportunity for end users and receiver manufacturers to test the decoding and use of the new messages under a wide range of different configurations.

CNAV messages have a common length of 300 data bits and are identified by a message type number that signifies their contents. The messages presently defined for GPS are summarized in Table 1. For QZSS, complementary messages have been established, which enable, among other features, a rebroadcast of GPS-specific data to QZSS users.

Table 1. Summary of CNAV message types transmitted by space vehicles (SVs). Messages marked by an asterisk were transmitted during the recent CNAV test campaign.

Message

Type

CNAV Message Title

Function/Purpose

0*

Default Default message (transmitted when no message data is available)

10*

Ephemeris 1 SV position parameters for the transmitting SV

11*

Ephemeris 2 SV position parameters for the transmitting SV

12*

Reduced Almanac Reduced almanac data packets for seven SVs

13

Clock Differential Correction SV clock differential correction parameters

14

Ephemeris Differential Correction SV ephemeris differential correction parameters

15*

Text Text (29 eight-bit ASCII characters)

30*

Clock, Iono & Group Delay SV clock correction parameters, ionospheric and group delay correction parameters (inter-signal correction parameters)

31

Clock & Reduced Almanac SV clock correction parameters, reduced almanac data packets for four SVs

32*

Clock & EOP SV clock correction parameters, Earth orientation parameters; Earth-centered, Earth-fixed to Earth-centered inertial coordinate transformation

33*

Clock & UTC SV clock correction parameters, Coordinated Universal Time parameters

34

Clock & Differential Correction SV clock correction parameters, SV clock and ephemeris differential correction parameters

35*

Clock & GGTO SV clock correction parameters, GPS to GNSS time-offset parameters

36

Clock & Text SV clock correction parameters, text (18 eight-bit ASCII characters)

37

Clock & Midi Almanac SV clock correction parameters, midi (mid-accuracy) almanac parameters

Other than the legacy L1 navigation message, which adheres to a fixed order of subframes, the sequence of CNAV messages can be varied widely to provide users with an optimized set of low latency information and parameters that change infrequently. As a baseline, the two ephemeris message types 10 and 11 are combined with any of the clock-and-auxiliary data messages (types 30 through 37) to provide users with the orbit and clock data of the received satellites. With a transmission duration of 12 seconds per CNAV message on L2C, a minimum of 36 seconds is required to transfer this information to the user if no other messages are transmitted. On L5, which operates at twice the data rate, a new frame is transmitted once every 6 seconds yielding a minimum of 18 seconds for the broadcast of ephemeris and clock data.

The recent test campaign started at 18:00 GPS Time on Saturday, June 15, 2013, with the transmission of message types 10, 11, 15, and 30 on a first space vehicle (PRN24) and included PRN12 from 18:42 onwards. Other space vehicles were sequentially phased in until all active IIR-M and IIF satellites (except for the recently launched IIF-4 satellite) transmitted CNAV on the supported signals. When the test ended exactly two weeks later (June 29, 18:00 GPST), all participating satellites were transmitting a complex master frame of 15 x 4 = 60 individual messages, which was repeated once every 12 minutes (on L2C). Each minor frame comprised the two ephemeris messages and at least one clock-data message (see Table 2).

Table 2. Sequence of message types in a CNAV master frame.

Message Types

10

11

15

30

10

11

32

33

10

11

12

35

10

11

12

30

10

11

12

33

10

11

12

35

10

11

12

30

10

11

32

33

10

11

15

35

10

11

32

30

10

11

12

33

10

11

12

35

10

11

12

30

10

11

12

33

10

11

12

35

Other messages included a reduced almanac (message type 12) and a text message (message type 15) with dummy content (“THIS IS A GPS TEST MESSAGE.”)

The CNAV data were recorded by selected multi-GNSS monitoring stations of the German Aerospace Establishment (Deutsches Zentrum für Luft- und Raumfahrt or DLR) and the University of New Brunswick (UNB), which were specifically configured to record raw GPS navigation frames in addition to the normal observation data. The stations are located at Singapore (SIN0); Sydney, Australia (UNX2); Maui, U.S.A. (MAO0); and Hartebeesthoek, South Africa (HRAG); as well as Fredericton, Canada (UNB) and are equipped with either Javad Delta-G2/G3TH or NovAtel OEM6 receivers. Following initial validation, the raw and decoded data from the CNAV test will be made available to interested users through the Multi-GNSS Experiment (MGEX) of the International GNSS Service (see http:/igs.org/mgex) to facilitate the development of user software and suitable data formats (such as an extended RINEX navigation message format).

The CNAV orbit and clock data were updated once every two hours and offer a slightly higher bit resolution than their legacy counterparts. However, the accuracy of the ephemeris data has not yet been evaluated nor compared to that of the L1 C/A-code navigation data.

As indicated above, the CNAV data can also provide a particularly compact form of almanac data known as the reduced almanac. It does not offer clock information (that is not normally required for a signal search) and assumes a circular orbit, which reduces the overall accuracy. Still, it can be transmitted (and repeated) in a much shorter time interval than the legacy almanac, which requires a total of 12.5 minutes. Each reduced almanac message (message type 12) provides orbit information for a total of seven satellites and it takes a set of five such messages to convey information for a complete constellation. For the master frame layout described above, the full constellation reduced almanac is repeated twice within 12 minutes on L2C (and half this time on L5).

Novel types of CNAV data not covered by the legacy navigation message include the differential code biases (also known as inter-system corrections or ISCs), which are required for pseudorange-based positioning with signals other than the legacy P(Y)-code (in addition to the established Timing Group Delay parameter or TGD). An overview of TGD and ISC values broadcast by the various satellites participating in the CNAV test is given in Table 3.

Table 3. Differential code biases (in nanoseconds) of GPS Block IIR-M and IIF satellites broadcast during the test campaign as part of the message type 30 CNAV messages.

SV Type

SVN

PRN

TGO

ISC L1CA

ISC L2C

ISC L5I5

ISC L5Q5

IIR-M

48

07

-10.71

-0.84

6.52

IIR-M

50

05

-10.24

-0.32

5.41

IIR-M

52

31

-13.04

-0.55

7.36

IIR-M

53

17

-10.24

-0.84

6.17

IIR-M

55

15

-10.24

-0.47

5.62

IIR-M

57

29

-9.31

-0.76

5.06

IIR-M

58

12

-12.11

-0.76

6.64

IIF

62

25

5.59

-2.07

-5.24

-0.38

-0.87

IIF

63

01

8.38

-2.30

-7.57

0.38

2.15

IIF

65

24

2.79

-0.26

-2.27

2.27

3.70

Another important achievement is the provision of Earth orientation parameters (EOP) in message 32, which provides GPS users with access to the celestial reference frame.  EOPs were transmitted during the second test week and updated on a daily basis (see Table 4). Knowledge of these parameters is of particular interest for GPS-based orbit determination and navigation of spacecraft (in low Earth orbit), which is preferably referred to an inertial rather than an Earth-fixed coordinate system.

Table 4. Daily Earth orientation parameters from the CNAV test campaign (pole coordinates and dUT1 (UT1-UTC) time differences and derivatives).

Epoch (GPST)

x_p

(arcseconds)

x_p_dot

(arcseconds per day)

y_p

(arcseconds)

y_p_dot

(arcseconds per day)

dUT1

(seconds)

dUT1_dot

(seconds per day)

June 22, 0:00

0.13517

0.00104

0.39657

-0.00054

0.06341

-0.00046

June 23, 0:00

0.13621

0.00102

0.39604

-0.00056

0.06295

-0.00049

June 24, 0:00

0.13740

0.00101

0.39535

-0.00058

0.06231

-0.00053

June 25, 0:00

0.13815

0.00099

0.39487

-0.00060

0.06164

-0.00063

June 26, 0:00

0.13846

0.00096

0.39443

-0.00062

0.06078

-0.00067

June 27, 0:00

0.13885

0.00094

0.39381

-0.00064

0.06004

-0.00067

June 28, 0:00

0.13947

0.00093

0.39310

-0.00066

0.05909

-0.00063

June 29, 0:00

0.13987

0.00090

0.39246

-0.00068

0.05842

-0.00053

Overall, CNAV offers exciting prospects for improved GPS utilization and users may look forward to the next test campaigns, which will tentatively be conducted once every six months.

As a side note, it should be mentioned that individual satellites could be observed to transmit various types of non-standard CNAV messages as well as CNAV messages with improper data (such as an invalid week count) after the end of the main test campaign. Various receivers in the MGEX network, which were processing the received CNAV messages internally and which put full confidence in their proper contents, were mislead by such information. During the actual test campaign, all data appeared fully valid and no problems were reported by the stations.


OLIVER MONTENBRUCK is the head of the GNSS Technology and Navigation Group at DLR’s German Space Operations Center in Oberpfaffenhofen, Germany.

RICHARD B. LANGLEY is a professor in the Department of Geodesy and Geomatics Engineering at the University of New Brunswick, Fredericton, New Brunswick, Canada.

PETER STEIGENBERGER is  a staff member in the Institut für Astronomische und Physikalische Geodäsie of the Technische Universität München (TUM) in Munich, Germany.

 

GPS World staff

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