Replacement Civil (R/C) Code for L2
The GPS Joint Program Office (JPO) is currently investigating a number of options regarding the addition of a second general-purpose civil-signal at L2. This new civil signal was called for in the Vice Presidential announcement as an upgrade to GPS in addition to the new civil signal at L5. Although the announcement identified this signal as the (Course/Acquisition) C/A code, the addition of a new signal at L2 presents an opportunity to replace the C/A code bit sequence with another, modernized code, with minimal impact to the space vehicle and future (upgraded) receivers. The rationale for adopting a replacement/civil (R/C) code in lieu of implementing C/A code and an assessment of its implications on existing and planned satellite programs, are described below.
Recent C/A code Findings
The C/A code was originally used primarily for rapid acquisition on a single frequency (L1-only) leading to P(Y) code track on L1 and L2. C/A code was designed to accommodate the needs of receivers using 70’s technology, that is, performing a cold start with no or poor a priori time or position information, and uses a short (1 millisecond) 1,023-bit sequence. Today, the C/A code has become the mainstay of the civil (and international) uses of GPS and augmentations that have outgrown the original military use.
Since the early 90’s, spectrum management-oriented studies have indicated that, under certain limited circumstances, the short C/A sequences have a lower margin than previously thought for co-existing with other C/A codes. A C/A code co-channel self-interference model was developed that demonstrates how the short length of the code heightens cross-correlation effects and how this can result in loss of acquisition capability and positioning. At present, the instances of co-channel interference are infrequent and of short duration but may still impact users that require high availability and accuracy. Thus, C/A code limits the total number of GPS or augmentation signals that may simultaneously use the same frequency and makes the signal more vulnerable to additional interference.
A new, longer code was developed for L5, the second civil signal for aviation, to overcome some of these limitations. This is not to say that current GPS C/A code users cannot meet the stringent requirements for safety-of-life use, just that current receiver technology permits use of a longer, more robust signal. The addition of a new signal on L2 provides an opportunity for its development.
R/C-code and the Block IIR and IIF Programs
The Air Force GPS Joint Program Office has determined the estimated cost of implementing a range of options for incorporating R/C code in the Block IIR and IIF modernization programs. The options are listed below:
Option 1: Generates a new, longer bit code sequence, instead of C/A code, on the new civil channel on L2. The new sequence is chipped at 1.023 MHz, which is the same bandwidth as the current C/A-code. It also adds the option to select either C/A code, R/C code, or no modulation. This signal is modulated by the standard (ICD-GPS-200) navigation message.
Option 2: Includes Option 1 and adds the option of turning off the navigation message on the civil channel on L2 independently of all other channels.
Option 3: Includes Option 2 and adds an additional R/C code signal on L1 in addition to the existing C/A code.
Option 1 simply allows for the replacement of the C/A code on L2 with the R/C code. Option 2 adds to Option 1 the ability to transmit either the C/A code or R/C code on L2 without the navigation message modulation. This feature allows for more robust track of the signal and improves the resistance of the channel to interference. The lack of navigation message allows for longer integration time in the receiver and thereby reduces the tracking noise and potentially increases accuracy. Option 3 is significantly more complex than Options 1 and 2 since it generates a total of 4 signals on L1: C/A code, R/C code, P(Y) code and M code. At this time there are no active plans to pursue Option 3 further.
For the other two options, no new code has been formally approved for the R/C code bit stream. A code has been identified for the purpose of determining the cost of the upgrade; namely, to use the L5 code sequences, albeit chipped at the C/A code rate. Initial analysis indicates that a longer code will improve cross correlation performance and thereby add some resistance to interference from external emitters or from large constellations of GPS satellites, augmentation satellites or other satellite navigation systems all sharing common frequencies.
Benefits and Risks of the R/C Code Option
The potential benefits of R/C code are:
- More robust civil signals for greater availability under interference
- Greater capacity for new (increased power) signals
- Competitive with potential future RNSS users in the band
- Option to restore or maintain current C/A-code and navigation message capability
- Earlier opportunity (Block IIR) to test the performance of the L5 code on L2
The potential risks of R/C code are:
- Additional testing required to validate noise improvement and acquisition performance
- Increased cost of modernization to the space and control segments (small increment expected)
- Increased cost to future user equipment for adding extra code option (small increment expected)
- Minor impact to precise ionospheric measurement, due to dissimilar codes on L1 and L2 (i.e., C/A and R/C respectively)
GPS frequency sharing and interference studies indicate that there are limits to the future expansion of C/A code use. Moreover, current receiver technology can support more robust acquisition and ranging signal options. The addition of a new civil signal at L2 presents an opportunity to adopt a more robust civil signal design, at relatively low marginal cost to the system. The code to be used on L5 has been identified as a promising candidate for the signal. It provides higher resistance to interference and greater capacity for adding additional signals on L2. The performance and potential risks and additional costs to the user segment for R/C code are being analyzed. Nevertheless, the anticipated changes to the Block IIR and IIF satellites would add the flexibility to broadcast either this new mode or to revert to a traditional C/A code for contingencies and testing.
Steve Lazar/The Aerospace Corporation/ 310-336-1938/22 November 2000