I just googled "E70 X5 Tire Pressure Monitor" and come across this - it is written by somebody who seems to know what he is talking about, and seems to make sense.

This is the reason I believe that my M3 and this X5 are doing this in fundamentally different ways due to the equipment. IF this is accurate our vehicles are transmitting the actual data. My '08 X5 has the RDC system...

Subject: BMW Tire Pressure Monitor Retrofit From E39 M5
Author: Brian : member since January, 2005 : 4879 posts
Posted on: 2006-03-27 11:05:41

BWM used two different types of tire pressure monitoring systems the Tire Pressure Control (RDC) system, and the Tire Defect Indicator (RPA) system (formerly know as the “DWS” system). The RDC system uses pressure and temperature sensors installed in each wheel as part of the valve stem. The sensors transmit their individual tire data to antennas mounted in each wheel well that feed to the RDC controller. This system is technologically superior, because it can alert you to absolute pressure changes (like when all four wheels decrease pressure over time), it is more sensitive to slight changes, and it can react faster. The disadvantage to the RDC system is that it puts a few ounces of extra weight in each wheel, and the system is more expensive and labor-intensive to retrofit. The RPA system uses the speed pulses from the ABS controller to calculate their relative rolling diameters. The advantage of this system is that it is passive (does not add “stuff” to your wheels and lots of extra wiring) and less expensive and easier to retrofit. It’s down side is that it is not as sensitive to pressure changes.

BMW introduced the RDC system in the E38, and they may have used it in the E65/E66. However, the RPA system is clearly BMW’s preference, because they switched to the RPA system for all other models. I much prefer the RPA system too, since I do not want the weight and complexity of a sensor spinning mindlessly in each wheel. The RPA is better suited to a high performance vehicle, and I suspect that BMW has similar priorities, since they dropped their absolute pressure system for the ABS wheel speed version. According to the BMW WDS, the RDC system will detect about 5 PSI pressure change. The RPA system will detect pressure drops of about 30% (roughly 10 PSI, depending on tire pressure).