Our narratives here correlates the GPS signal loss as reported in the special OIG report on Phillip Garrido and Alyssa's menstrual cycle.

The GPS device that Phil Garrido wore around his ankle has 2 systems built-in. It has the GPS receiver which provide works out its location from the constellation of satellite signals. On a good day out in an open field, the GPS receiver would be able to locate about 12 satellites. It just need 4 satellites in order for it to work out its own location.

For civilian use, the GPS receiver uses only the signal at 1575.42 MHz.

The other system that is built into device would most likely be a SMS transmitter which transmit its own GPS location. How often it transmit is programmable. The SMS transmission would be sent to a pre-defined cellular (mobile phone) number or numbers and presumably are routed to the parole office management system. The system collates the data it receives. This system would function like a fleet management system used by transport company or even the police force to track their vehicle whereabout.

GPS signal blocking
There are active low powered GPS blocking devices that transmit a noise signal at the same GPS satellite frequency. This works by transmitting a low power random noise signal at 1575.42 MHz. Under this environment, the GPS signal are swamped by the noise signal and thus is unable to decode it position.

Another passive technique and probably a more practical technique for Phillip Garrido would be to use some form of metallic foil. Effectively, he would have wrapped the metallic foil such as a aluminium foil around the GPS device he wore around his leg. The metallic foil would attenuate the received GPS signal sufficiently such that it is unable to decode its location. If he is inside a metallic enclosure, effectively, that enclosure becomes a radio frequency shield.

According to the OIG report, during a 32-day period between July 23, 2009 and August 23, 2009, the department lost the GPS signal from Garrido’s ankle monitoring device almost every night for prolonged periods of time, typically nine or more hours.

Within reason and by imaginative deduction, we will assume that he was within a metallic enclosure instead of the tent.

In Antioch, July 2009 had an average temperature of 90.70 degrees Fahrenheit (32.6 degree Celsius), while the coldest month of the year is December with an average minimum temperature of 37.10 degrees Fahrenheit (2.83 Celsius). The peak summer daytime temperature was about 102 degree Fahrenheit (38.0 degreee Celsius).

Prior to June 2009, the GPS management system sends an alert to the parole office after a GPS signal was lost for six or more hours. Since then, a new system has been implemented that sends an alert after signal has been lost for 24 hours or more.



From OIG report: Between April 2008 and June 2009, the GPS system alerted parole agents 335 times that Garrido’s GPS monitoring device lost a signal for prolonged periods of time. This was almost a nightly occurrence.

During a 32-day period between July 23, 2009 and August 23, 2009, the department lost the GPS signal from Garrido’s ankle monitoring device almost every night for prolonged periods of time, typically nine hours or more. We will use this 32 days as a reference point where we think Alyssa and Phillip Garrido slept together in that air-conditioned shed.

The report stated that the GPS monitoring started on April 14 2008 but was not specific on the June 2009 date. Assuming it was April 14 2008 to 30 June 2009, there are (16+31+365) 412 days. The 'normal' menstrual cycle for women would be about 28 days although some women can have it as long as 35 days.

The 335 times when Garrido’s GPS monitoring device lost a signal for prolonged periods of time happened from April 14 2008 and June 30 2009. Those 335 times were nightly occurrences over 412 days. By taking a 28 day menstrual cycle, we assumed Alyssa has an average of 24 menses-free days and 5 days of menstruation. At least we assume those were the 5 days Phillip Garrido would not dip his penis into her.

In total, we assume Alyssa menstrual cycle is about 28 days. To get the number of times she had gone through her menstrual cycle, we divide 412 by 28 which gives us about 14.7 times. Of course we cannot have odd number of cycles so we round that up to 15 cycles.

To get the number of days in total between that period when she was menstruating, we multiply 15 x 5 = 75 days. She had a total of 75 days which we think Phillip Garrido did not sleep with her because she was menstruating.

To get the total number of days she slept with Phillip Garrido, we then subtract 75 from 412 which gives us 337 days.

The OIG report mentioned the 335 alerts? How do we justify the discrepancy? There is a slight discrepancy of (337-335) 2 days which is not significant due to some variation in a woman's menstrual cycle.

Those 335 alerts were the 335 nights he spent with Alyssa in the backyard compound with his GPS receiver disabled. Those were the 335 nights when Alyssa was menses-free so that he would dip his penis into her. The man has his standard.

During a 32-day period between July 23, 2009 and August 23, 2009, the department lost the GPS signal from Garrido’s ankle monitoring device almost every night for prolonged periods of time, typically 9 hours or more. Her period might have been late which would have provided them the opportunity to prolong their copulation in the July-August summer holidays.



Sources
Special Report on Phillip Garrido from the Office of the Inspector General of California.