Geoscience Reference
In-Depth Information
5.3.5
GPS modernization concept
In January 1999, the USA announced the GPS modernization concept, a
$400 million initiative. The key feature is the implementation of a new signal
structure in future satellites.
Future GPS satellites
The Block IIR satellites increase their presence in the GPS constellation. A
new effort will bring modernized functionality to IIR satellites. These mod-
ernized satellites, denoted as IIR-M (replenishment-modernization), will pro-
vide new services to military and civilian users. New signals and increased
L-band power will significantly improve the navigation performance (Mar-
quis 2001).
The Block IIF and the Block III satellites are the next generations.
These next generations of satellites will have many improvements over
the present satellites. It is planned to include the capability to transmit data
between satellites to make the system more independent. The autonomous
navigation (auto-nav) capability via intersatellite cross-link ranging will al-
low the satellites to essentially position themselves without extensive ground
tracking. In summary, the future satellites will have the following mainly mil-
itary advantages:
•
Navigation accuracy will be maintained for six months without ground
support and control.
•
Uplink jamming concerns will be minimized.
•
One upload per spacecraft per month instead of one or even more per
day will be performed.
•
Need for overseas stations to support navigation uploads will be re-
duced.
•
Improved navigation accuracy will be achieved.
New signal structure
Referring to codes, presently civil users have unlimited access only to the
C/A-code on the carrier L1. The modernization will provide new signals:
implementing military codes (M-codes) on L1 and on L2 and a civilian code
on L2 (abbreviated as L2c). The M-code will provide the authorized users
with more signal security, improved acquisition options, and more jamming
resistance. The new civilian L2c signal will provide nonauthorized users dual-
frequency operation to perform ionospheric error correction. In addition to
these codes, a new L5 frequency will be provided for civilian users to en-
hance aviation applications. The notation L5 is chosen because, actually,
