EPOS-N - European Plate Observing System
Seismometers / vault construction
Stations: Upgrade/install notes
Ideas for improvements
Installation / Inventory
JNE/Ulla, new solar panel August 2018.
, from this source:
North orientation is required for three reasons:
There are different ways in establishing North reference line.
A North reference line can be established by walking due South some hundred meters from a starting point, by keeping longitude (E/W) GPS coordinate fixed while walking.
Determine at what time the sun is due South at a particular location, and use the shadow cast by a pole as reference line. According to web app SunCalc: http://suncalc.net/#/70.9889,-8.2934,10/2018.07.18/08:52 - the sun should be due South at 14:40 local (Norwegian) time at Ulla location, on 18 July 2018. Check : Since Ulla is at longitude 8.29242W, and the sun moves 1/4 degrees per minute (hence 1/4 degree * 60 * 24 = 360 degrees per day), it should be due South (8.29242 / 0.25) 33 minutes later, compared to zenith time at the prime meridian. The same web app tells that seen from Greenwich-observatory, the zenith time is 14:07 Norwegian time (two hours after UTC) on the same date -- that is (14:40 - 14:07) 33 minutes earlier then at Ulla zenith time --- just as expected (a detail: geodetic zero meridian on a geocentric reference ellipsoid -- which is what GPS positioning yields --- is 102.5 meters east of Greenwich meridian).
Link to online apps that might help for North orientation:
We used this instrument to establish the "official" North referance. NOTE: FOG heading accuracy is 1.53° RMS at 71°N latitude (JNE / Ulla coordinates: 70.98842N, 8.29242W) - see below.
CTBTO/IMS uses this optical gyro. However, it has a specified operating latitude of ± 70° (ref datasheet). All new EPOS-N/SVALBARD stations will be above 76 deg N - how will the gyro operate at such latitudes?
Perhaps this limitation is a consequence of the physics behind optical gyros, which then has to be studied in more detail.
Model CDL-TOGS is intended for subsea use, with 3000 meter depth rating in basic version. Presumably such features drives price upwards. Aren't there any optical gyros for surface use, that could be suitable - and much less expensive? We have to search ...
When heading accuracy of gyros is specified to be "0.5° secant latitude" - which is the case for "CDL TOGS-S" - ref datasheet - it means it will be accurate to 0.5 degrees at the equator, and show reduced accuracy according to the secant of latitude, which is inverse of the cosine of latitude.
A plot of heading accuracy vs. latitude for this model, with some geographic locations added:
A North reference line can be established by walking due South some hundred meters from a starting point, by keeping longitude (E/W) GPS coordinate fixed while walking. This North reference line was marked with black on sensor pit concrete. Compared to "official" North line, measured by Teledyne Marine Fiber-optic gyro (FOG) model CDL-TOGS-S --- marked with red on concrete ---- there is a difference of 2.49 ° counter-clock wise. NOTE: FOG heading accuracy is 1.53° RMS at 71°N latitude (JNE / Ulla coordinates: 70.98842N, 8.29242W). Also, the photo used for the illustration might distort lines slightly, depending on viewing angle from camera lens. This means that handheld GPS method is most likely acceptable, in order to get North reference for similar shallow sensor pits. (We have some deep sensor pits, and some sensors located inside buildings, where the FOG is required.)
JNE/Ulla seismometer sensor pit. Photos shows GPS North reference line (black, to the left) and multiple FOG north reference lines (red). Seismometer is Trillium 120QA. Click to enlarge.
We had a theory that sensor pit and instrument cabin was North oriented when they were built and installed in 1984 and 1989, respectively. In 1984, the magnetic declination at JNE/Ulla coordinates (70.98842N, 8.29242W) was 16.33 deg W (ref: https://www.ngdc.noaa.gov/geomag-web/, selecting IGRF12 model for magnetic variation). And it seems sensor pit and old instrument cabin was oriented towards compass North, without correction for magnetic declination. Ref attachment.
Current solar panel azimuth. Its position is better then the alternate location. Click to enlarge.
Plot showing sun position, seen fram sensor site JNE/Ulla, at various dates:
Solar panels should from this figure have 40 degree vertical angle - ref figure below.
Assuming 40 degree vertical angle, how long should these struts be? ("Strut" = "Stag")
Click for PDF version.
We consider using solar panels of brand "MAX POWER", from distributor SunWind (Gylling): https://www.sunwind.no/search/?q=max+power