I've always been fascinated by GPS.  I've got two handheld units - Garmin Vista Cx with Australian City Navigator, and a Garmin Geko 101 -  a very basic but small unit. 

The Garmin Vista CX is certainly an impressive unit for its size.  The screen is really excellent - easily readable in sunlight, and with vibrant backlighting and colours for low light.  With a microSD card, it can accommodate multiple maps.  With the right map sets (such as City Navigator), it will route with turn by turn directions.  I've got a 1GB microSD card in with Shonky Maps and contours, City Navigator, and the New Zealand OpenGPS routable mapset.  The 2 x AA batteries give good life.  I also like the fact it uses a standard mini-USB connector rather than some proprietary hard to get cable and connector for power and downloading.  The only thing that dissappoints is the inbuilt compass.  The accuracy can vary  by some 40 degrees depending on the angle you hold the unit.  I think I'll stick with my trusty compass.

I picked up the Geko 101 unit on Ebay becuase i needed a basic and easy to carry GPS for work and other activities.   With 250 waypoints and simple LCD screen it's perfect for finding the direction to various points.  Using AAA batteries gives it a small size.  Acquisition time seems quite fast.

Lastly in September 2010 my wife bought me a SPOT GPS Messenger - have a look at My SPOT Page.

Some of My Co-ordinates

Here's some GPS co-ordinates that I have in my handheld unit.  They are all in WGS84.  I wouldn't vouch for the altitude accuracy.

  AHG Driving School WA                       S31.95009 E115.98307     23 m
Albany Stoney Hill Lookout WA S35.11292 E117.95153 200 m
Albany TV Tower WA S35.01786 E117.87050 133 m
Albany War Memorial WA S35.02508 E117.89506 155 m
Albany Whaling Station WA S35.09552 E117.95956
Albany Windfarm WA S35.06577 E117.79951 101 m
Argyle Diamond Mine WA S16.71753 E128.40226
Ascot Racecourse WA S31.93823 E115.92203
Augusta Lighthouse WA S34.37308 E115.13624 15 m
Bald Hill Avon National Park WA S31.60599 E116.22353 274 m
Barrow Island Airport WA S20.86594 E115.40608
Barrow Island Camp WA S20.82657 E115.44582
Bells Rapids Upper Swan WA S31.77305 E116.05994 17 m
Bentley Hospital WA S32.00576 E115.93003 11 m
Bhuddist Monastry WA S32.41375 E116.00963 240 m
Bibbulmum Track Abyssinia Camp WA S32.23369 E116.29084 311 m
Bibbulmum Track Ball Creek Camp WA S31.95015 E116.19825 158 m
Bibbulmum Track Beraking Camp WA S32.06176 E116.29609 283 m
Bibbulmum Track Brookton Hwy Crossing WA S32.20338 E116.29483
Bibbulmum Track Canning Camp WA S32.24159 E116.25460 288 m
Bibbulmum Track Chadoora Camp WA S32.75901 E116.23780 287 m
Bibbulmum Track Dookanelly Camp WA S32.95864 E116.21439
Bibbulmum Track Gringer Ck Camp WA S32.56919 E116.44576 330 m
Bibbulmum Track Hewitt Hill Camp WA S31.95853 E116.12888 221 m
Bibbulmum Track Monadnocks Camp WA S32.32913 E116.23883 446 m
Bibbulmum Track MtCooke Camp WA S32.40257 E116.29512 358 m
Bibbulmum Track MtDale Camp WA S32.13506 E116.29943 356 m
Bibbulmum Track MtWells Camp WA S32.69878 E116.33596 545 m
Bibbulmum Track Murray Camp WA S32.86804 E116.13902
Bibbulmum Track Nerang Camp WA S32.46198 E116.37719 334 m
Bibbulmum Track Noggerup Camp WA S33.56483 E116.09831 258 m
Bibbulmum Track Swamp Oak Camp WA S32.78152 E116.11755 190 m
Bibbulmum Track Waalegh Camp WA S32.01383 E116.25678 299 m
Bibbulmum Track White Horse Camp WA S32.60921 E116.33624 454 m
Bibbulmum Track Yabberup Camp WA S33.46368 E116.07739 281 m
Bickey TV Mast WA S32.01060 E116.08721 349 m
Big Brook Arboretum WA S34.40179 E116.00664 178 m
Bluff Knoll Stirling Ranges WA S34.37584 E118.25647 1002 m
Bobs Crossing Lane Poole Reserve WA S32.77385 E116.10226 161 m
Broadwalk Resturant WA S32.02611 E115.76565
Brookman Sawpits Rowes Rd WA S34.51590 E116.01700 117 m
Bunbury Forum Shopping Centre WA S33.33750 E115.65957 6 m
Bussellton Jetty WA S33.64437 E115.34442
Cambray Railway Siding WA S33.88070 E115.67820 94 m
Canebrook Pool Campsite WA S33.88011 E115.28334 102 m
Carousel Shopping Centre Cannington WA S32.02025 E115.93782 5 m
Caves House Hotel Yallingup WA S33.64556 E115.03141 59 m
Central Park Tower Perth WA S31.95367 E115.85558
Centre Rd Campsite WA S34.91203 E116.61080 56 m
Chapman Pool Campsite WA S34.09038 E115.20618 22 m
Christchurch International Airport NZ S43.48854 E172.53939 31 m
Claremont Showground Oval WA S31.97556 E115.78548 18 m
Cottesloe Beach WA S31.99507 E115.75131
Coastal Plain Trail Motic Camp WA S31.62365 E115.84728 64 m
Coastal Plain Trail Prick Camp WA S31.71331 E115.94962 71 m
Coastal Plain Trail Ridge Camp WA S31.54723 E115.73810 93 m
Coastal Plain Trail Yanchep Camp WA S31.51404 E115.66404 25 m
Curtin University Busport Bentley WA S32.00591 E115.89751
Deep Water Point WA S32.02352 E115.85433
DNA Tower Kings Park WA S31.96428 E115.83621 65 m
Draftys Campsite WA S34.51063 E115.94668 45 m
Dwellingup WA S32.71440 E116.06190
Edith Cowan Uni Joondalup Main Bldg WA S31.75052 E115.77104
Exmouth Lighthouse WA S21.80843 E114.10999 50 m
Fern Brook Falls WA S34.81875 E116.59408 61 m
Fremantle Hospital WA S32.05859 E115.75186 10 m
Galleria Shops WA S31.89681 E115.90124 31 m
Hawkesbury Heights Lookout NSW S33.66710 E150.65160 200 m
Hawkesbury Hospital Windsor NSW S33.61075 E150.82022 13 m
Hillary Boat Harbour Shops WA S31.82224 E115.74124
Honeymoon Pool WA S33.37775 E115.93628 104 m
Hut Pool Camp WA S34.08882 E115.29101 31 m
Jandakot Airport Entrance WA S32.09226 E115.87973 30 m
John Forrest National Park Ranger WA S31.88334 E116.09261 162 m
Kardinya Shopping WA S32.06825 E115.81649 35 m
Kent St Weir Bentley WA S32.02130 E115.92079
King Jarrah Campsite Lane Poole WA S32.85946 E116.11234
Kings Park Lookout WA S31.96086 E115.84467 33 m
Lane Poole Reserve Entry WA S32.76147 E116.07605
Lord Forrest Hotel Bunbury WA S33.32283 E115.63574 5 m
Mandurah Forum WA S32.53683 E115.74239
Martins Tank Campground WA S32.84524 E115.66841 10 m
Moons Crossing Campsite WA S34.50503 E116.14884 70 m
Mt Cooke Darling Range WA S32.41309 E116.30801 570 m
Mt Dale Darling Ranges WA S32.12661 E116.29950
Mt Hale Stirling Ranges WA S34.37719 E118.06873
Mt Wells Fire Tower WA S32.69892 E116.33625 546 m
Nanga Bridge Lane Pool Reserve WA S32.80128 E116.10446 172 m
Nepean Private Hospital NSW S33.75825 E150.71284 58 m
Newcrest Telfer Mining Camp WA S21.71186 E122.22575
North Blackwood Wals Campsite WA S34.07339 E115.35581 33 m
Observation City Scarborough WA S31.89383 E115.75700
Old Swan Brewery WA S31.96503 E115.84075
Pearce Airbase (Military) WA S31.66809 E116.01288
Perth Domestic Airport WA S31.93358 E115.96017
Perth International Airport WA S31.94183 E115.97428
Perth Zoo WA S31.97724 E115.85266
Point Rd Camp WA S34.09382 E115.02086 58 m
Prices Point WA S17.48487 E122.14779 4 m
Wireless Hill Perth WA S32.03052 E115.82791 43 m
Riverton Forum Shopping Centre WA S32.04039 E115.90445 11 m
Rottnest Island Jetty WA S31.99554 E115.54162
Royal Perth Yacht Club WA S31.98586 E115.82371
Rydges Hotel Melb Vic S37.81149 E144.97049 29 m
Shannon Campsite WA S34.59506 E116.41413 163 m
Standley Chasm NT S23.72164 E133.47024 751 m
Sues Bridge Campsite WA S34.07537 E115.38763 36 m
South Perth Mill St Jetty WA S31.97126 E115.85370
St John of God Hospital Murdoch WA S32.06768 E115.84512 34 m
Sydney Domestic Airport Entrance NSW S33.93381 E151.18563 7 m
Sydney International Airport Entr NSW S33.93243 E151.16475
Torbay Inlet WA S35.03615 E117.67918 3 m
Walyunga National Park Carpark WA S31.73204 E116.07389 33 m
Water Corporation John Tonkin centre WA S31.93968 E115.84403
Waychinicup National park WA S34.89243 E118.33312 1 m
Wellington Dam Campsite WA S33.41495 E116.00291
Westmead Public Hospital NSW S33.80497 E150.98810 32 m
Whitakers Mill Campsite S32.54792 E116.01356
Whitfords City Shopping WA S31.79570 E115.75113 33 m
Whiteman Park Entry1 WA S31.83432 E115.96511
Willie Creek Pearl farm Broome WA S17.76131 E122.21467 2 m
Yanchep National Park Visitor Center WA S31.54440 E115.68294
Yaegarup Lake Warren National Park WA S34.54227 E115.87129 81 m

There are lots of great internet resources on the technical side of GPS.  Here's a few notes I keep to remind me of the critical parameters:

How GPS Works

  • 24 satellites orbit every 12 hrs - about 20,000kms above surface
  • Transmit at 1.575Ghz - 50W power typical
  • Data rate 50bps
  • Course Acquisition (CA) code transmiited as 1 Mhz subtone of 1024 bits and repeats pattern every millisec. There is a different CA code for each satellite.
  • Navigation Information Message (NIM) comprises 25 frames - each frame is 1500 bits and takes 30 secs, Total NIM takes 12.5 minutes
  • Each frame comprises 5 subframes. Subframe 1 is status and clock corrections. Subframe 2 & 3 is ephemeris data for that satellite (exact position and clock offset), Ephemeris data is valid for about 30 mins. Subframes 4 & 5 contain almanac data for all the satellites. Almanac data is orbital parameters allowing calculation of approx satellite positions - valid for about 3 months. It takes the full NIM to gather all the almanac data, but ephemeris data is transmitted every 30 secs
  • The clock in each satellite is identical, and frame transmission starts at exactly the same time.  Since the signal travels at the speed of light, the receiver can "see" different clocks from the satellites.  The further the satellite is away the more behind it's clock appears.  Using this, the receiver can estimate the distance to each satellite and the deduce it's own location by triangulation.
The following diagram from may explain things a little better:

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