GPS_SKY is a utility that
will create a picture (as shown) of GPS reception at your
location, showing signal strength and your GPS Horizon. It is
FREEWARE software that will decode an existing ASCII NMEA log
file, or your GPSr if it can output the NMEA GSV sentence at
GPS_SKY if left connected to your GPSr for 24 hours will show the GPS Satellite trails across your sky as tracked by your GPSr. It can also decode an existing ASCII NMEA log file (assuming it has GSV and you have at least 24 hrs worth). Looking at the above output of the program you can see the effect of large trees on GPS reception at my place (esp in the NW). The horizontal lines are at 0,10, 20 degrees elevation, and the vertical lines are 0, 90,180, 270 degrees azimuth from left to right in the picture. The big "polar hole" at 180 (due South) shows that my GPS is in the Southern Hemisphere. The false colour denotes signal strength (Red - hottest, Yellow - medium, Cyan - lowest).
What does it do? GPS_SKY can show how "low" your GPSr can see in all directions, and allow you to equate "holes" in reception with local obstacles like trees and terrain. GPS_SKY outputs its data in two forms: first, graphical data (in either BMP or PSD form) that can be pulled straight into a graphics program for you to add comments or additional information; second, data in TEXT form (using comma separated values) that can be pulled straight into EXCEL etc, to graph and analyse. You can also use the data in the HORIZON.CSV file to manually load your Mission Planning Software with your GPS horizon curtain.
Why sample for 24 Hours? Although the GPS satellites
orbit the earth every 12 hours, our earth is also spinning
beneath them, so it takes 24 hours for all the satellites (that
are healthy) to complete all tracks across your sky. You can
sample for less time but you will only get less sky tracked.
GPS_SKY can be stopped and started (if you need to use the PC).
If you sample for longer than 24 hours the picture will not fill
out. Explanations are given in the _README_ file.
The reason for the "polar hole" is because the inclination of the GPS orbits is about 55 degrees to the equator. The satellites do not go over the polar regions. If you are in the Northern Hemisphere the polar hole is split in half either side of 0 degrees, in the Southern Hemisphere the hole is at 180 degree Az on the GPS_SKY graphic (see examples below).
Experiments - only limited by your imagination
- Find the winter and summer difference in signal strength due to effects of deciduous foliage. Can you see the sap rise?
- Does metal foil placed over the aerial block all microwave from the Satellites? (I was surprised!)
- Does metal foil block certain directions, can you use that to determine static direction to a 1 degree accuracy ?
- Place a shallow bowl of water over the aerial and see how much water attenuates the signal.
- Measure the height of tall objects by noting their GPS shadow and applying a little trig.
No doubt you can think of many more!
Requirements: The program was compiled as a 16bit DOS application, it should run using DOS 3.0 or above, or as a DOS window in the various Windows environments. It has been tested on plain DOS machines, W98 and a Power Mac (running SoftWindows 3.0). It is ideal for any old slow DOS PC or laptop. The Program is PASSIVE. It transmits no data on the Serial Port - only receives, so it can be used in parallel with other RS232 devices (if you just use RX and GND). You need to know how to get your GPS to transmit the NMEA GSV sentence (others are allowed but ignored) at 4800 baud - see your GPSr manual on how to do this. The GPS must connect to either COM 1 or COM 2 in live mode, or you have an existing ASCII NMEA log file to process.
GPS_SKY written by Geoff Hitchcox (Kiwi Geoff), Christchurch, New Zealand, South Pacific.
LINKS related to GPS_SKY
A few more pictures from GPS_SKY
Above: Output from GPS_SKY running on a MAC (using SoftWindows 3.0) in Georgia USA. Note the Polar Hole position is centred on 0 Az (GPSr in the Northern Hemisphere). Breaks in the satellite trails are due to the program being stopped and started (Larry needed the MAC for other work at the time). The satellite trails come down to the Horizon very well up to due south, then you will see they climb to about 20 degrees in the SW, this is because Larry has Sweat Mountain near his property. You can perhaps appreciate how you can use your GPS_SKY data to make a crude theodolite to measure distance or height. Nice picture, Larry!
Above: GPS aerial mounted on Geoff's roof. Note the Polar hole is centred on the South Pole (GPSr in Southern Hemisphere). Compare the nice signal level (lots of RED) in the sky compared with the poorer indoors signal below. The reason the trails do not approach the horizon (except in the SE) is because I am surrounded by large trees. To those who know Christchurch, the Port Hills are part of the Polar Hole, which is most convenient.
Above: GPS aerial inside Geoff's house, sitting on a window ledge. You can see that the best signal is to the South East, which is the direction the window is facing. Breaks in the satellite trails are due to obstructions inside house and trees outside - the program was running continuously. Note: GPS signals indoors may be due to multipath rather than direct through your roof etc.
Thanks to:Larry Knight from Marietta (GA USA) for his testing (on a MAC) and his ideas, suggestions and sense of humour. It was fun working with Larry on this project from half way round the world, taking advantage of time zone differences to hasten development.