GPS Methodology

This web page is under construction. Please do not rely on this information in the field.

Background

In the spring of 1996, Yale University’s School of Forestry purchased two Magellan ProMark X global positioning systems (GPS). These GPS have the dual purpose of supporting the Roatan project as well as assisting other research activities of the School. During a three month research trip to Roatan during the summer of 1996 and a two week trip in March of 1997, the GPS units were used to determine the coordinates of sampling sites for soil analysis, ground control points for rectifying the remotely sensed data, and other points of interest.

Using GPS

In order to understand what GPS is and how it works, Trimble (a competitor of Magellan) has developed a tutorial that will provide a solid understanding of the technology. While the tutorial will provide general information, the following discussion page will provide information on how we use GPSs in Roatan and the details on how to use a Magellan Pro Mark X for determining a position. This web page has three sections: a discussion of how GPSs have been used in our project, a simplified explanation of the fundamentals of improving GPS accuracy, and a guide on how to use the Magellan GPS units.

GPS monitor low-frequency radio signals broadcast from satellites to determine their location. A single GPS unit can determine its location with an accuracy of approximately 100 meters. You can greatly improve this accuracy by utilizing two GPSs which are both capable of storing position data. In this case, one unit serves as the remote GPS and is taken into the field to record the location of sampling or ground control points. The second GPS serves as the base station and is left at a known location to record data concurrently with the remote. By comparing the position readings recorded by the base station over time to the known coordinates of the base station, we can determine the instantaneous error in the GCP readings. We then make the assumption that this offset error is consistent between all GCP readings taken at that particular time and in that particular area. This assumption has been shown to be valid so long as the position fixes are no more than a couple of hundred miles from one another. Using this assumption, we can correct the position fixes determined by the remote GPS on a second-by-second basis.

Depending on the GPS system, this correction can be performed automatically (called "real-time differential correction") or after the data has been collected (called "post-processed differential correction"). Our current Magellan GPSs only allow us to do post-processed differential correction. For our research in Roatan, we utilize two Magellan Pro Mark X GPS units in order to do post-processed differential correction. By averaging corrected position fixes taken over a 2 to 5 minute time interval, we insure position accuracy to within 2 to 5 meters.

For post processing, both the remote and the base station GPS units must determine their position using the same satellites. Since the choice of satellites available to the remote is often limited by dense forest cover, buildings, or hillsides, it is important to locate the base station where it can receive signals from almost all satellites in the sky. In order to insure that the base station records data from all satellites in the sky, we purchased an external antennae which strengthens the incoming radio signals from the satellites by amplifying all incoming signals. In addition, we mount the antennae on a wooden pole in the middle of the dolphin pen at the Institute for Marine Science. The antennae used at the base station amplifies the radio signals emitting from the satellite, and thus ensures a strong signal is recorded at the base By mounting the egg-shaped antennae on top of a PVC tube attached to the pole, the antennae has a nearly unobstructed view of the sky. This clear view is important since the base station and the remote must record information from the same satellites in order to perform differential correction. As opposed to the small antennae that mount on the side of the Magellan Pro Mark Xs, this antennae requires extra energy to use it.

A Step-by-Step Guide to Using the Magellan Pro Mark X

If you require accuracy to within 100 meters, then you need only a single Pro Mark X and need only perform steps 1 and 2 below. After these steps have been completed, pressing the POS key will display your position. If you would like to (somewhat) improve on this accuracy without doing differential corrections, see section on Averaging Data Values. On a practical note, I suggest using the GPS marked with yellow tape for single unit position fixes since it has a stronger signal reception than the red unit.

If you require accuracy to within 2-5 meters, then you will need to perform the following steps on both Magellan units. For the reason mentioned above, I would recommend using the yellow GPS as the remote and the red as the base station. The signal boost obtained from the external antennae compensates for the reduced signal quality of the red unit.

Please note that Magellan Operation Manuals are available in Benoit’s lab. While they leave much to be desired, they can help clarify issues or explain other functions of the GPS (like waypoints, routes, and navigation).

1. Power Supply

The Magellan Pro Mark Xs can be run using eight AA batteries, an external battery, a car lighter jack, or a wall outlet. Be warned that they burn through one clip of eight AA batteries very rapidly (supposedly within two hours). Thus, I recommend using the external battery which is stored with the units. The 12 amp-hr, 6 volt battery fits into the belt back, and

2. Setup Functions

You need to check and/or set the setup parameters before using the GPS.

3. Initialize the GPS

You need only initialize the GPS if the unit was last used in an area more than a couple hundred miles from where you intend to use it.

4. GPS Functions

Once

aux 1 and 3

Attributes and Descriptions

Improving Accuracy through Averaging

Improving Accuracy through Differential Correction

details on Using the gps on Roatan

Setting Up the Antennae at IMS

We have found an ideal location for the base station antennae on top of a wooden pole in the center of the dolphin pen at the Institute for Marine Sciences. The antennae For protection, the antennae cable is fed through the PVC tube before being attached to the GPS unit.

Setting Up the GPS at IMS

The placement of the base station GPS is not ideal. In past research trips, the GPS has been mounted on the wooden pole below the antennae using the supplied white mounting bracket. Leaving the GPS on the pole poses a security risk (No IMS staff are around on Wednesdays or after 5PM to keep an eye on the GPS. This is a concern since supplies have been stolen from the wet lab when the door has been left unlocked). Additionally, while the GPS is weather resistant, the power/computer cabling is not

The preferred spot would be inside the locked wet lab. However, the signal from the antennae becomes too weak when it is run through a long cable. To remedy this problem, a standard coaxial cable signal booster could be purchased and tested prior to the research trip. If this option is pursued, a coaxial cable sufficiently long enough to mount

GPS Power Supply

The power supply at AKR is generally good, but on occasion it has been known to shut off or get turned off. Loss of power will stop the base station from logging data, resulting in you being unable to post-process any of the remote files collected after the power outage. A solution to this problem is

Mounting the Antennae: Mounting the antennae is a bit tricky.