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GPS receiver modules for OEMs
| AN01 Indoor GPS
- Increased Sensitivity
- CW25 Performance
- Test Results
- Tree Foliage
- Car Trunk
- Internal Office
In order for a GPS receiver to acquire and track satellite signals, it must perform a two dimensional signal replication process of the received satellite signals. This two dimensional process consists first of a search for the phase of the desired satellite signal, and then a search for the carrier frequency plus Doppler of the satellite. The phase search is performed by shifting the phase of the replica PRN code generated by the receiver until it correlates with the received satellite PRN code. The carrier frequency search is performed by adjusting the rate of the PRN code generator on the receiver until it correlates with the received satellite carrier frequency plus Doppler. These correlations are undertaken with multiple time/frequency bins and normal GPS receivers contain typically a few hundred to a few thousand correlators. This means that for a GPS receiver with a few hundred correlators, a full sky search can take some significant time to perform which directly affects the Time to First Fix (TTFF) as well as the sensitivity of the receiver. This becomes even more evident as the received satellite signals reduce in strength, and with limited numbers of correlators the receiver sensitivity is rarely very much below –140 to –145dBm
The CW25 overcomes the issues of increased sensitivity and rapid TTFF by massively increasing the number of correlators applied to each receiver channel with a maximum number of 12,288 correlators.
This means that the CW25 receiver module can perform very rapid sky searches for satellite signals even under very poor signal conditions. This is achieved by allocating large numbers of correlators to each receiver channel to allow the receiver to search time/frequency bins in parallel rather than sequentially as in normal GPS receivers. In addition, the sensitivity of the CW25 receiver module is boosted to signal levels as low as –155dBm. This is also achieved by the use of large numbers of correlators allocated to each receiver channel, allowing the receiver to achieve a correlation peak with an extra 20 to 30dB of gain in a short period of time. In summary therefore, the CW25 performance is governed by the very large number of correlators available within the baseband processor ASIC, and allows the CW25 to achieve very high sensitivity (-155dBm) without compromising TTFF or introducing any real time lag in positional updates.
The sensitivity performance of the CW25 receiver was measured with the test set up shown in Figure 1
As GPS signals fluctuate in level considerably in poor signal strength locations due to fading of the satellite signals, it can be hard to establish the exact sensitivity of the GPS receiver under test. To overcome this, a GPS simulator was used in the test set up so that precise signal strengths can be set on the simulator to allow the tracking software to display the measured signal strength achieved by the GPS receiver
The GPS simulator used in this test was the STR4500 from Spirent, with a 20dB attenuator applied to the output of the simulator to allow the signal levels applied to the receiver to be able to be lowered right down to –155dBm and below. Once the CW25 receiver was calibrated in terms of sensitivity with the use of the STR4500 simulator, the CW25 was connected to a GPS antenna so that the sensitivity performance of the CW25 could be assessed with live satellite signals in different locations
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