Wed., Apr. 25th

Today I brought the new hygrometer and the tape measure and went over to the Challenger Center.

Spoke with Reed about the detector arrangement.  Michelle Personette expressed a preference that the detectors be placed out in the exhibit area for easier public viewing.  Some good spots seem to be:  

1. Hanging from I-beam above double doors leading into big classroom.
2. Hanging from I-beam above exit sign near the rest rooms.
3. Inside awning above blank circle quadrant beside kiosk (near entrance to planetarium).  A notch may have to be cut out of the edge of the shelf to prevent overlap with awning support.

Using my tape measure, Reed and I confirmed that #1 and #2 are very close to 30' apart, and #1 and #3 are very close to 30' apart.  (We didn't measure #2 to #3, but eyeballing, it looks pretty close.)

The ME students (George & Brian) went to get more mounting hardware, hopefully they will come back later this afternoon and do the install of the detector shelves at least, at the locations we discussed with Reed.  Reed says he is staying pretty late tonight (8:30 or so).

We discussed mounting the main electronics platform sideways on the blank circle quadrant next to the kiosk.  This will allow for much easier public viewing.  However, this will require new planning as to exactly how it will be mounted.  Also, I doubt that the copper cooling rod would be stable in that configuration - the thermal paste probably isn't strong enough by itself to keep it from falling.  This location for the electronics platform will probably also necessitate buying new, longer coaxial cables to feed to the three detectors, due to the paths along which some of the cables will have to be routed.  (This will cut our sensitivity slightly due to cable attenuation.)

Regarding the GPS antenna cable:  After considering a couple of possible paths, a consensus emerged that the easiest, shortest, and least-likely-to-run-afoul-of-building-codes solution was the following:  

• GPS antenna on roof of planetarium (perhaps at center for easy cross-referencing to building blueprints), with a lightning protection device connected to the grounding cables strung along the roof.  (See Spectracom's technical note on lightning surge protection.)
• Antenna cable enters building through unused exhaust vent on roof of planetarium.  Protection device is installed where cable enters vent; ground runs from there to building's grounding cable which goes nearby.  See picture below.  (Damn, I should have taken iPhone photos while we were up there!  Go back tomorrow...)

• Antenna cable goes into crawl space/attic space above planetarium, and then down through area between walls, coming out ceiling tiles just inside the planetarium interest, then going over to the main electronics platform mounted there on that quarter-circle.

The length of the antenna cable in this configuration would be fairly short; perhaps even short enough that we wouldn't need a repeater, or at most just one repeater.

Let's do a calculation.  Suppose our total antenna cable length is 100'.  If we use RG213 (KX4) cable, that attenuates at 0.35 dB/m, so total cable attenuation would be 35.5 dB, including a 0.5 dB connector, plus another 1 dB for the Lightning Protection device gives 36.5 dB total attenuation.  This could be mostly compensated for by a high-gain antenna, e.g., SpectraCom's ANT-35 is +35 dB.

Interesting; Spectracom provides an OEM module that integrates a GPS receiver and an OCXO.  This might have simplified our CTU design a bit.  However, they only claim 25 ns accuracy, which isn't an enormous improvement over the 68 ns of the DeLorme board.  We might do better with averaging.

Regarding the dewpoint measurement:  I took a measurement near the kiosk, and got temp = 22.4 C, rel. humidity 48%, which imply dewpoint = 11 C - same as I got at home last night.  However, Reed warned that there could be significant seasonal fluctuations in temperature, and humidity might vary when people are coming in & out.

The hygrometer actually has a "MIN/MAX" feature to take minimum/maximum readings over an extended period; I could place the hygrometer at the approximate location of the electronics board and leave it there for a while; the maximum temperature/humidity values could then be used to calculate an upper bound on the dewpoint over that period.  I just need a little double-sided sticky-tape thingy to affix the hygrometer to the quarter-circle about where the electronics board will be.