GNSS - GPS Modernization

[Summary]

- With old navigational satellites phased out, new advanced communicaiton technologies are applied to new navigation satellites.

- GPS's positioning accuracy has been steadily improved.

- GPS modernizaiton: Beginning from 2000, the present constellation of Block II satellites will be all replaced with new Block III models by 2021.

- Modernized GPS signals at a glance:

1) By receiving multiple GPS signals in the receiver, postitioning accuracy and stability of signal reception are greatly improved.

2) Application of advanced navigational data format(CNAV-2)

3) Application of the FEC error correction algorithm

4) Increased radiated power from the satellite

4) Interoperability between different GNSS(beginning with the L1C signal)

6) For military GPS signals, wide/spot beams are introduced to improve anti-jamming and security(-158dBW/-138dBW received power at earth)

7) Greatly improved accuracy in satellite clocks

8) Addition of carrier pilot signal: reduced signal acquisition time, improved S/N at the correlator input

9) Continued emission of the old GPS signals

[Status of GPS Modernization Program(as of Mar. 2012)]

Mode

SVN

Launch Year

Number of SVs

as of Mar 2012

Transmitted Signals

Remarks

Block I

01-12

78-85

0

-

1st generation

Block II

13-21

89-90

0

-

2nd generation

Block IIA

22-40

90-97

10

L1 C/A, L1Y, L2Y

Block II improvement

Block IIR

41-61

97-04

12

L1 C/A, L1Y, L2Y

Block II/IIA replacement

Block IIR-M

05-09

7

L1 C/A, L1Y, L2Y, L2C, L1M, L2M

Block IIR modernization

Block IIF

62, 63

10-

2

L1 C/A, L1Y, L2Y, L2C, L1M, L2M, L5

Replace Block IIA before Block IIIA

becomes operational. Satellite clock

accuracy improved.

Block IIIA

N/A

14-

0

L1 C/A, L1Y, L2Y, L2C, L1M, L2M, L5, L1C

Final modernized GPS

 

[New Civilian GPS Signals]

- L2C Signal: For improved accuracy in civilian GPS. 1227.60MHz. Used together with L1 C/A, ionospheric errors are reduced. BPSK modulation. Introduction of a new dedicated channel for codeless tracking. Will be available from 2016 on with a total of 24 satellites transmitting the L2C signal.

L2C waveform

- L5 signal: For precision navigational data for airplanes, trains, ships and automobiles. 25MHz bandwidth at 1176.45MHz. Radiated power doubled over L1 C/A (-154dBW received with 0dBic antenna). Positioning possible with weak signal environment. Increased anti-jamming performance with increased 25MHz bandwidth. Modernized CNAV message (multiple messages introduced). FEC error correciton algorithm. When used with L1 signal, ionospheric error is reduced. Use of three signals L1 C/A (or L1C), L2C, and L5 will give sub-meter accuracy (0.1m is the goal) with trilaning technique. From 2018 on, 24 satellites will radiate the L5 signal.

   L5 waveform

- L1C signal: For interoperability with other GNSS. US and EU co-developed standards for GPS L1C and Galileo. Offer stable reception in blocked areas such and city building canyons. Frequency at 1575.42MHz. MBOC modulation. CNAV-2 message format. FEC error correction. From 2021 on, 24 satellites will transmit the L1C signal.

L1C waveform

 

[Acronyms]

CNAV(civil navigation)

FEC(forward error correction)

GPS-RTK(real time kinematic): also called CPGP. Using multiple GPS singnal's carrier phase (e.g., L1 at 1575.42MHz) horz. accuracy of 1cm and vert. accuracy of 2cm are realized. Carrier phase differences are broadcast from terrestrial stations.

GPS III: GPS moderniation program approved by the US Congress in 2000.

OCX(Operational Control System)

PPS(Precise Positioning Service): military-grade positition service

SA(Selective Availability): intentional degradation of accuracy for civilian GPS signal

SoL(safety-of-life): safey related positioning application such as aircraft takeoff and landing

SPS(Standard Positioning Service): civilian-grade positioning service with intentional degradation of accuracy. Accuracy is no longer degraded beginning in 2000.

[References]

D. Goldstein, "GPS modernization and GPS III", Proc. Southern California Section ION Meeting, 11 Mar. 2009.

US Government Official Site on GPS: http://www.gps.gov/systems/gps/

B. Gruber, "Status and modernization of the US Global Positioning System," Proc. Muich Satellite Navigation Summit 2012, 14 Mar. 2012.